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Report on the Erosion of Concrete in Hydraulic Structures Reported by ACI Committee 207

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Report on the Erosion of Concrete in Hydraulic Structures Copyright by the American Concrete Institute, Farmington Hills, MI. All rights reserved. This material may not be reproduced or copied, in whole or part, in any printed, mechanical, electronic, film, or other distribution and storage media, without the written consent of ACI.

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ACI 207.6R-17

Report on the Erosion of Concrete in Hydraulic Structures

Reported by ACI Committee 207

John W. Gajda, Chair

Fares Y. Abdo Oscar R. Antommattei

Terrence E. Arnold Katie J. Bartojay'

Teck L. Chua Timothy P. Dolen

Darrell Elliott Barry D. Fehl Mario Garza

Melissa 0. Harrison Michael G. Hernandez

James K. Hicks

Christopher C. Ferraro, Secretary

Rodney E. Holderbaum Ronald L. Kozikowski

Tibor J. Pataky Jonathan L. Poole Henry B. Prenger Ernest A. Rogalla

Ernest K. Schrader Kuntay K. Talay

Nathaniel F. Tarbox Stephen B. Tatro

Michael A. Whisonant Fouad H. Yazbeck

Consulting Members Jeffrey C. Allen Randall P. Bass

Anthony A. Bombich

'Primary author of this report.

!Deceased.

Robert W. Cannon Eric J. Ditchey Brian A. Forbes

Allen J. Hulshizer Richard A. Kaden William F. Kepler

David E. Kiefer

Committee 207 would like to thank the following individuals for their contribution to this report: J. Ballentine, J. F. Best, G.

Mass, W. McEwen, M. Petrovsky, and M. Stegallo.

This report outlines the causes, control, maintenance, and repair

of erosion in hydraulic structures. Such erosion occurs from three

major causes: cavitation, abrasion, and chemical attack. Design

parameters, materials selection and quality, environmental

factors, and other issues affecting the performance of concrete are

discussed.

Evidence exists to suggest that, given the operating characteris

tics and conditions to which a hydraulic structure will be subjected,

the concrete can be designed to mitigate future erosion. However,

when operational factors change or are not clearly known and

erosion of concrete surfaces occurs, repairs should follow. This

report addresses the subject of concrete erosion, inspection tech

niques, and repair strategies, providing references to a more

detailed treatment of the subject.

ACI Committee Reports, Guides, and Commentaries are intended for guidance in planning, designing, executing, and inspecting construction. This document is intended for the use of individuals who are competent to evaluate the significance and limitations of its content and recommendations and who will accept responsibility for the application of the material it contains. The American Concrete Institute disclaims any and all responsibility for the stated principles. The Institute shall not be liable for any loss or damage arising therefrom.

Reference to this document shall not be made in contract documents. If items found in this document are desired by the Architect/Engineer to be a part of the contract documents, they shall be restated in mandatory language for incorporation by the Architect/Engineer.

Keywords: abrasion; aeration; cavitation; chemical attack; concrete dams; corrosion; erosion; hydraulic structures; spillways.

CO NTENTS

CHAPTER 1-I NTRO DUCTIO N AND SCOPE, p. 2

1 . 1-Introduction, p. 2

1 .2-Scope, p. 2

CHAPTER 2-NOTAT ION, p. 2

2 . 1 -Notation, p. 2

CHAPTER 3-EROSIO N BY CAVITATION, p. 3

3 . 1-Mechanism of cavitation, p. 3

3 .2-Cavitation index, p. 3

3 .3-Cavitation damage, p. 4

CHAPTER 4-EROSIO N BY ABRASIO N, p. 6 4 . 1 -General, p. 6 4.2-Stilling basin damage, p. 6

ACI 207.6R-17 supersedes ACI 21 OR-93(08) and was adopted and published

September 2017.

Copyright© 2017, American Concrete Institute.

All rights reserved including rights of reproduction and use in any form or by

any means, including the making of copies by any photo process, or by electronic

or mechanical device, printed, written, or oral, or recording for sound or visual

reproduction or for use in any knowledge or retrieval system or device, unless

permission in writing is obtained from the copyright proprietors.

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2 REPORT ON THE EROSION OF CONCRETE IN HYDRAULIC STRUCTURES (ACI 207.6R-17)

4.3-Power plant tailrace damage, p. 7 4.4-Navigation lock damage, p. 8 4.5-Tunnel lining damage, p. 8 4.6-Hydraulic jacking, p. 8

CHAPTER 5-EROSION BY CHEMICAL ATTACK, p. 9

5 .!-Sources of external chemical attack, p. 9 5.2-Erosion by mineral-free water, p. 9 5.3-Erosion by miscellaneous causes, p. 9

CHAPTER 6-CONTROL O F CAVITATION

EROSION, p. 1 0 6.1-Hydraulic design principles, p . 10 Example 1, p. 10 6.2-Cavitation indexes for damage and construction

tolerances, p. 11 Example 2, p. 11 6.3....::...Using aeration to control damage, p. 12 6.4....::...Materials, p. 13 6.5-l.Materials testing, p. 14 6.6TConstruction practices, p. 14

CHAPTER 7-CONTROL O F ABRASION EROSION, p. 15

7 .!-Hydraulic considerations, p. 15 7.2-Materials evaluation, p. 16 7.3-Materials, p. 16

CHAPTER 8-CONTROL O F EROSION BY

CHEMICAL ATTACK, p. 1 7 8.1-Control of erosion by mineral-free water, p. 17 8.2-Control of erosion from acid attack due to bacterial

action, p. 18 8.3-Control of erosion by miscellaneous chemical

causes, p. 18

CHAPTER 9-PERIO DIC INSPECTIONS AND

CORRECTIVE ACTION, p. 19 9.1-General, p. 19 9.2-Inspection program, p. 19 9.3-Inspection procedures, p. 19 9.4-Reporting and evaluation, p. 19

CHAPTER 10-REPA IR METHO DS AN D

MATERIALS, p. 20 1 0.1-Design considerations, p. 20 1 0.2-Methods and materials, p. 20

CHAPTER 11-REFERENCES, p. 22

Authored documents, p. 23

CHAPTER 1-INTRO DUCTION AN D SCOPE

1.1-lntroduction Erosion is the progressive disintegration of a solid by:

1) cavitation; 2) abrasion; or 3) chemical action. Although

concrete deteriorates for a variety of reasons, this report is concerned with specific factors that influence these three areas of erosion: 1) cavitation-erosion resulting from the collapse of vapor bubbles formed by pressure changes within a high-velocity water flow; 2) abrasion-erosion of concrete in hydraulic structures caused by water-transported silt, sand, gravel, ice, debris, or hydraulic jacking; and 3) chemical action-disintegration of the concrete in hydraulic structures by chemical attack.

Concrete in properly designed, constructed, used, and maintained hydraulic structures can provide 30 to 50 years of erosion-free service (Liu and Wang 2000). However, for reasons including inadequate design or construction, or operational and environmental changes, erosion does occur in hydraulic structures.

1.2-Scope

Concrete erosion in hydraulic structures caused by cavitation, abrasion, and chemical attack are included in this report. Options available to the designer and user to control concrete erosion in hydraulic structures are discussed, along with information on the inspection and evaluation of erosion problems. This report includes repair techniques, as well as a brief guide to methods and materials for repair. Other types of concrete deterioration are outside the scope of this report.

CHAPTER 2- NOTATION

2.1-Notation

F force l length of air space between the jet and the spillway

floor, l (l = length) p water pressure at a given point, Fll2 p0 absolute pressure at a given Point 0, F!l2 Pc absolute pressure at a given Point c, F!l2 Pv vapor pressure of water, Fll2 qa volume rate of air entrainment per unit width of jet,

l31T qd amount of air a turbulent jet will entrain along its

lower surface, l3 IT T time v average jet velocity at midpoint of trajectory, liT v0 average velocity at Section 0, liT Y0 offset into the flow, l

Zo elevation at Centerline Of pipe, l Zc elevation of the vapor bubble, l a width of jet coefficient based on turbulent intensity

of the jet !J.p change in pressure between two points, F!l2 y specific weight of water, F!l3 ( 62.4 lb/ft3 [9 .81 kN/

m3], temperature-dependent p mass density of water, FT2fe4 (1.94 lb·s2fft4 [1000

kg/m3], temperature-dependent) (J cavitation index

value of cavitation index at which cavitation initiates

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REPORT ON THE EROSION OF CONCRETE IN HYDRAULIC STRUCTURES (ACI 207.6R-17) 3

CHAPTER 3-EROSION BY CAVITATION

3.1- Mechanism of cavitation

Cavitation is the formation of bubbles or cav1t1es in a liquid. In hydraulic structures, the liquid is water, and the cavities are filled with water vapor and air. The cavities form where the local pressure drops to a value that will cause the water to vaporize at the prevailing fluid temperature. Figure 3.1a shows examples of concrete surface irregularities that can trigger formation of these cavities. The pressure drop caused by these irregularities is generally abrupt and is caused by local high velocities and curved streamlines. Cavities often begin to form near curves or offsets in a flow boundary or at the centers of vortexes.

When the geometry of flow boundaries causes streamlines to curve or converge, the pressure may drop in the direction toward the center of curvature or in the direction along the converging streamlines. For example, Fig. 3.1b shows a tunnel contraction in which a cloud of cavities could start to form at Point (c) and then collapse at Point (d). The velocity near Point (c) is much higher than the average velocity in the tunnel upstream, and the streamlines near Point (c) are curved. Thus, for proper values of flow rate and tunnel pressure at Point (0), the local pressure near Point (c) drops to the vapor pressure of water and cavities will occur. Cavitation damage is produced when the vapor cavities collapse. The collapses that occur near Point (d) produce high instantaneous pressures that impact on the boundary surfaces and cause pitting, noise, and vibration. Pitting by cavitation is readily distinguished from the worn appearance caused by abrasion because cavitation pits cut around the harder coarse aggregate particles and have irregular and rough edges.

3.2-Cavitation index

The cavitation index is a dimensionless measure used to characterize the susceptibility of a system to cavitate. Figure 3.2 illustrates the design principle of the cavitation index in a tunnel contraction. In such a system, the critical location (or point) for cavitation is at Point (c) (Fig. 3.1 b).

The static fluid pressure, where the velocity is essentially the same as the approach velocity, at Point (1) will be

P1 = Pc + y(zc- zo) (3.2a)

where Pc is the absolute static pressure at Point (c); y is the specific weight of the fluid (weight per unit volume); Zc is the elevation at Point (c); and z0 is the elevation at Point (0).

The pressure drop in the fluid as it moves along a streamline from the reference Point (0) to Point (1) will be

!J.p =Po- [pc + y(zc- zo)] (3.2b)

where p0 is the static pressure at Point (0). The cavitation index normalizes this pressure drop to

the dynamic pressure. Dynamic pressure is the difference between the total pressure (pressure at the point of stagnation) and the static pressure, 1/2pv02 (Eq. (3.2b )).

. . /Vapor CaVItieS

4�/ .. . ·. � A. OFFSET INTO FLOW B. OFFSET AWAY FROM FLOW

Vapor cavities

C. ABRUPT CURVATURE

AWAY FROM FLOW

. ·.'• ',

D. ABRUPT S LOPE

AWAY FROM FLOW

Vapor cavities

E. VOID OR TR A NSVERSE

GROOVE

F. ROUGHEN ED SURFACE �t;es

G. PROT RUDING JOINT

Fig. 3.1 a-Cavitation situations at surface irregularities

(Falvey 1990).

(c) (d)

� (O) --,---- --- (1}-t---

Fig. 3.1 b-Tunnel contraction.

(j = Po - [Pc + y(zc -Zo)J 1!2pvo 2

(3.2c)

where p is the density of the fluid (mass per unit volume), and v0 is the fluid velocity at Point (0).

Readers familiar with the field of fluid mechanics may recognize the cavitation index as a special form of the Euler number or pressure coefficient, a matter discussed in Rouse (1978).

If cavitation is just beginning and there is a bubble of vapor at Point (c), the pressure in the fluid adjacent to the bubble is approximately the pressure within the bubble, which is the vapor pressure Pv of the fluid at the fluid's temperature.

Therefore, the pressure drop along the flow from Point (0) to ( 1) required to produce cavitation at the crown is

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-----..._ ........ ....

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�l o, 2 3 4 ::. 678910 2 3 4 5 • 189100 2 3 4 5 I 7 II 9 1000 2 3 4 5. 711910000 HOURS OF OPERATION

Fig. 3.2-Cavitation erosion experience in spillways (Falvey 1990). (Note: 1 m/s = 3.28ft/s.)

and the cavitation index at the condition of incipient cavitation is

cr =Po-Pv + y(zc -zo) c 1/2pv�

(3.2d)

It can be deduced from fluid mechanics considerations (Knapp et a!. 1970), and confirmed experimentally, that in a given system, cavitation will begin at a specific ac, no matter which combination of pressure and velocity yields that a c.

If the system operates at a a above ac, the system does not cavitate. If a is below ac, the lower the value of a, the more severe the cavitation action in a given system. Therefore, the designer should ensure that the operating a is safely above ac for the system's critical location (refer to Chapter 6).

Actual values of ac for different systems differ markedly, depending on the shape of flow passages, the shape of objects fixed in the flow, and the location where reference pressure and velocity are measured.

For a smooth surface with slight changes of slope in the direction of flow, the value of ac can be below 0.2. For systems that produce strong vortexes, ac could exceed 10. Values of ac for various geometries are given in Chapter 6. Falvey (1982) provides additional information on predicting cavitation in spillways.

A system having a given geometry will have a certain ac; despite differences in scale, ac is a useful concept in model studies. Tullis (1981) describes modeling of cavitation in closed circuit flow. Cavitation considerations (such as ,surface tension) in scaling from model to prototype are discussed in Knapp et a!. (1970) and Arndt (1981).

3.3-Cavitation damage

Cavitation bubbles will grow and travel with the flowing water to an area where the pressure field will cause bubbles to collapse. Cavitation damage can begin at that point. When a cavitation bubble collapses or implodes close to or against a solid surface, a high pressure is generated, which acts on an infinitesimal area of the surface for a short time. A succession of these high-energy impacts will damage almost any solid material. Tests on soft metal show initial cavitation damage in the form of tiny craters. Advanced stages of damage show a rough honeycomb texture with some holes that penetrate the thickness of the metal. This type of pitting often occurs in pump impellers and marine propellers.

The progression of cavitation erosion in concrete is not as well documented as it is in metals. Work by Falvey (1990), however, indicates that the rate at which damage progresses from minor to major is dependent on the cavitation index. The time to major cavitation damage can be approximated by summing the rate of progression over time for all operations. The time of operation to major cavitation damage depends on the cavitation index and can vary from hours to years. It may be possible to adjust flows to avoid conditions leading to rapid cavitation damage. For both concrete and metals, however, the erosion progresses rapidly after an initial period of cavitation exposure slightly roughens the surface with tiny craters or pits. Figure 3.3b shows a tendency for the erosion to follow the mortar matrix and undermine the aggregate.

Roughness does not necessarily have to be caused by cavitation. The presence of increased roughness by whatever cause is enough to accelerate cavitation damage. At Glen Canyon Dam, Arizona, the cavitation damage initiated at

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Fig. 3.3a-Christmas tree configuration of cavitation

damage on a high-head tunnel surface, Glen Canyon Dam,

Arizona (U.S. Bureau of Reclamation 2015).

locations where there was a buildup of calcite or other flaws in the concrete surface. When significant roughness exists, it may shorten the time to major damage as if the operation time to minor damage had already passed. Severe cavitation damage will typically form a Christmas tree configuration on spillway chute surfaces downstream from the point of origin, as shown in the damage pattern of Glen Canyon Dam left spillway tunnel in 1983 (Fig. 3.3a) (U.S. Bureau of Reclamation 20 15).

Once erosion has begun, its rate may be expected to increase because protruding pieces of aggregate and other damage caused by the initial cavitation become new generators of vapor cavities. In fact, a cavitation cloud often is caused by the change in direction of the boundary at the downstream rim of an eroded depression. Collapse of this cloud farther downstream starts a new depression, as indicated in Fig. 3.3a.

Microcracks in the interfacial transition zone (ITZ), the region between the mortar and coarse aggregate, are believed to contribute to cavitation damage. Compression waves in the water that fills such interstices can produce tensile stresses that cause microcracks to propagate. Subsequent compression waves can then loosen pieces of the material. The simultaneous collapse of all cavities in a large cloud, or the supposedly slower collapse of a large vortex, is capable of exerting more than 100 atmospheres of pressure. Loud noise and structural vibration attest to the violence of collapsing cavitation bubbles. The elastic rebounds from continuous collapsing over time could initiate and propagate cracks, causing chunks of material to break loose.

Fig. 3.3b-Concrete test slab featuring cavitation producing

devices.

Fig. 3.3c-Cavitation erosion pattern after 47 hours of

testing at a 240 ft (7 3 m) velocity head.

Figure 3.3b shows the progress of erosion of concrete downstream from two protruding bolts used to generate cavitation in a test slab. The tests were made at a test facility located at Detroit Dam in Oregon (Houghton et a!. 1978). Figure 3.3c shows cavitation damage on test panels after 47 hours of exposure to high-velocity flows in excess of 100 ft/s (30 m/s). A large amount of cavitation erosion caused by a small offset at the upstream edge of the test slab is evident.

Figure 3.3d shows severe cavitation damage that occurred to the flip bucket and training walls of an outlet structure at Lucky Peak Dam, Idaho. In this case, water velocities of 120 ft/s (37 m/s) passed through a gate structure into an open outlet manifold (Jansen 1988). Figure 3.3e shows cavitation damage to the side of a baffle block and the floor in the stilling basin at Yellowtail Afterbay Dam, Montana (U.S. Bureau of Reclamation 1981).

Once cavitation damage has substantially altered the flow regime, other forces then begin to act on the surface, causing fatigue due to vibrations of the element. High water velocities striking the irregular surface can lead to mechanical failure due to vibrating reinforcing steel. Significant amounts of material could be removed by these added forces, thereby accelerating failure of the structure. This sequence of ca�itati?n da111age f�llowed by high-impact damage from

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Fig. 3.3d-Cavitation erosion of discharge outlet training

wall and flip bucket at Lucky Peak Dam, Idaho.

Fig. 3.3e-Cavitation erosion of baffle block and floor in

stilling basin (U.S. Bureau of Reclamation 1981).

the moving water was evident in the 1983 spillway tunnel failure at Glen Canyon Dam, Arizona (Burgi et al. 1984).

CHAPTER 4-EROSION BY ABRASION

4.1-General Abrasion erosion damage results from the abrasive

effects of waterborne silt, sand, gravel, rocks, ice, and other debris impinging on a concrete surface during operation of a hydraulic structure. These particles move around in a cascading motion, then impact the concrete surface, similar to the ball-milling action seen in mechanical grinders. Abrasion erosion is readily recognized by the smooth, worn-appearing concrete surface, which is distinguished from the small holes and pits formed by cavitation erosion, as can be compared in Fig. 3.3e, 4.1a, and 4.1b. Spillway aprons, stilling basins, sluiceways, drainage conduits or culverts, and tunnel linings are particularly susceptible to abrasion erosion.

The rate of erosion is dependent on many factors, including size, shape, quantity, and hardness of particles being transported; water velocity; and concrete quality. While highquality concrete can resist high water velocities for many years with little or no damage, concrete cannot withstand the abrasive action of debris grinding or repeatedly impacting on its surface. In such cases, abrasion erosion ranging in depth

Fig. 4.1 a-Abrasion damage to concrete baffle blocks and

floor area in Yellowtail Diversion Dam sluiceway, Montana

(U.S. Bureau of Reclamation 1981).

Fig. 4.1 b-Close-up of the abrasion damage to invert of the

Hoover Dam, Nevada Spillway (Falvey 1990).

from a few inches (millimeters) to several feet (meters) could result, depending on flow conditions. An erosion and sedimentation manual by the U.S. Bureau of Reclamation (2006) is a good reference for evaluating bed movement and sediment transport in hydraulic structures.

4.2-Stilling basin damage A typical stilling basin design includes a downstream sill

from 3 to 20 ft (1 to 6 m) high intended to create a permanent pool to aid in energy dissipation of high-velocity flows.

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Fig. 4.2a-Typical debris resulting from abrasion erosion of

concrete.

Unfortunately, in many cases, these pools also trap rocks and debris (Fig. 4.2a) (Hanna 2010). Material that becomes trapped in the stilling basin is typically sand, gravel, cobbles, or boulders. Figure 4.2b shows recirculating flow patterns produced over the basin end sill create a turbulent flow that continually moves the materials against the concrete surfaces. This ball mill-type action causes severe damage due to a repetitive grinding process. Flows during normal operation of a hydraulic jump energy dissipation basin are not capable of washing the particles out of the basin.

The stilling basins at Libby and Dworshak Dams-highhead hydroelectric structures-were eroded to maximum depths of approximately 6 and 10 ft (2 and 3 m), respectively (Schrader and Tatro 1987). In the latter case, nearly 2000 yd3 (1530 m3) of concrete and bedrock were eroded from the stilling basin (Fig. 4.2c). This was also a problem at Grand Coulee Dam, Washington, that resulted in a massive cleanup of the river channel to remove loose material. Impact forces associated with turbulent flows carrying large rocks and boulders at high velocity contribute to the surface damage of concrete (Price 1947).

There are many cases where the concrete in outlet works stilling basins of low-head structures also exhibited abrasion erosion. Chute blocks and baffles within the basin are particularly susceptible to abrasion erosion by direct impact of waterborne materials. There also have been several cases where baffle blocks connected to the basin training walls have generated eddy currents behind these baffles, resulting in significant localized damage to the stilling basin walls and floor slab of Nolin Dam, as shown in Fig. 4.2d (McDonald and Liu 1987).

In most cases, abrasion erosion damage in stilling basins has been the result of one or more of the following:

a) Construction diversion flows through constricted portions of the stilling basin

b) Eddy currents created by diversion flows or powerhouse discharges adjacent to the basin

c) Construction activities in the vicinity of the basin, particularly those involving cofferdams

d) Nonsymmetrical discharges into the basin

Fig. 4.2b-Stilling basing abrasion damage from recircu

lating flows (Hanna 2010).

Fig. 4.2c-Erosion of stilling basin floor slab, Dworshak

Dam, Idaho (Schrader and Kaden 1976b).

e) Separation of flow and eddy action within the basin sufficient to transport riprap from the exit channel into the basin

f) Recirculating flow in hydraulic jump stilling basins (ball milling)

g) Failure to clean basins after completion of construction work

h) Topography of the outflow channel i) Rockfall from canyon walls above (McDonald 1980). Unlike cavitation damage, abrasion damage in stilling

basins is generally slow to develop. Damage generally requires several flood events or long sustained operations with materials present such as rocks that can cause damage. Regular inspection and cleaning can help minimize damage.

4.3-Power plant tailrace damage Abrasion erosion damage can also occur in the tailrace

of a power plant where water is discharged into the river channel. At the Buffalo Bill Powerplant in Wyoming, the draft tubes exit the plant at a lower elevation than the river channel. At high flow rates through the powerplant, river water is pulled back into the tailrace, trapping bed material and riprap from the river and upstream dike. Erosion holes were found occupied by large boulders that closely match the size and shape of the hole. Beneath the boulder, smaller rounded rocks were supporting much of the weight of the boulder (Fig. 4.3), resulting in point loads that greatly accelerate the erosion process (Bartoj ay 2011 ).

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Fig. 4.2d-Abrasion erosion damage to stilling basin, Nolin

Dam (McDonald and Liu 1987).

Reinforcing steel Bulkheod seol with rounded edges ) 0 . . ..

· Exposed onchor ·_. ·_ .. :·()" � .· . . o

.. . o .· . .. . o . l·.o W .. . ·. :o. : -·1 ... _.11· .. . o· . . - ·o I. Jl . · ·

. · ·1o· l l '? 0 . . J . 1J '? • • • 0 - -

Fig. 4.3-Abrasion erosion damage in tailrace, Buffalo Bill

Powerplant, Wj;oming (Bartojay 2011).

4.4-Navigation lock damage Hydraulic structures other than spillways are also subject

to abrasion erosion damage. When the Upper St. Anthony Falls Lock, Minnesota, was unwatered to repair a damaged miter gate, an examination of the filling and emptying laterals and discharge laterals revealed considerable abrasion erosion (Fig. 4.4) (McDonald and Liu 1987). This erosion of the concrete to maximum depths of 23 in. (580 mm) was caused by rocks up to 18 in. (460 mm) in diameter that had entered the laterals, apparently during discharge of the flood of record through the lock chamber. Subsequent filling and emptying of the lock during normal operation agitated those rocks, causing them to erode the concrete by grinding.

4.5- Tunnel lining damage Concrete tunnel linings are susceptible to abrasion erosion

damage, particularly when the water carries large quantiti�s of sand, gravel, rocks, and other debris. There have been many instances where the concrete in both temporary and permanent diversion tunnels has experienced abrasion erdsion damage. Generally, the tunnel floor or invert is the mo,st heavily damaged. The diversion tunnels of the Glen Canyon Dam in Arizona experienced moderate abrasion damage (less than I ft [800 mm]) of the invert of the tunnels

Fig. 4.4-Abrasion erosion damage to discharge lateral,

Upper St. Anthony Falls Lock, Minnesota.

Fig 4.6a-Initial damage due to uplift generally occurs at

the construction joints (Frizell 2007).

during construction, after passing almost 3 million acre-ft of water (3.7 billion cubic meters). The exposed aggregate, concrete matrix, and exposed reinforcing steel bars had a smooth, worn appearance (Wagner 1967).

4.6-Hydraulic jacking

Typically, the stability of reinforced-concrete-lined chutes in spillways depends on the overall concrete design, including joint and waterstop details; reinforcement; anchorage; and a functioning, filtered underdrain system. Damage resulting from hydrodynamic uplift on slabs can begin at a joint, where offsets or spalling has occurred (Fig. 4.6a). Spillway flows over these offsets can introduce water into the foundation, which can lead to structural damage due to uplift or erosion of the foundation material. If the leading edge has a crack or begins to separate, this creates a stagnation point where jacking pressures can be induced (Frizell 2007). Hepler and Johnson (1988) described typical analysis of spillway failures due to uplift and discussed case studies.

Frizell (2007) determined that a considerable flow is possible to induce through the gap into the subsurface drainage system. Most drainage designs are not meant to provide for this amount of inflow and could be undersized



Fig 4. 6b-Structural collapse when enough undermining

has occurred to cause loss of support (Frizell 2007).

and pressurized by the incoming flow. This can result in elevated uplift pressure, undermining of the foundation, and structural collapse (Fig. 4.6b ).

CHAPTER 5-EROSION BY CHEMICAL ATTACK

5.1-Sources of external chemical attack

The compounds present in hardened portland cement are attacked by water and many salt and acid solutions. Fortunately, in most hydraulic structures, the deleterious action on a mass of hardened portland-cement concrete with a low permeability is so slow that it is not a concern. There are, however, situations where chemical attack can become serious, accelerating deterioration and concrete erosion.

Acidic environments can cause the deterioration of exposed concrete surfaces. The acidic environments can range from low acid concentrations found in mineralfree water to high acid concentrations found in many processing plants. One example is the Spring Creek Debris Dam in California that was designed to control the flow of acid mine drainage into the Sacramento River. Before treatment of water flowing into the reservoir began, the reservoir water had a pH typically between 2 and 3 (acid), which attacked the cement paste in the exposed concrete. However, after 30 years of this exposure, the reinforced concrete in the outlet works intake structure was mostly intact with only approximately 112 to 1 in. (10 to 25 mm) concrete loss (Smoak 1997).

Soil or groundwater conditions can also cause concrete deterioration. In the presence of moisture, alkali soils or water-containing sulfates of magnesium, sodium, calcium, potassium, or ammonium can attack concrete, forming chemical compounds that imbibe water and swell, causing concrete damage (Mehta and Monteiro 2006).

Hydrogen sulfide corrosion, a form of acid attack, is common in septic sanitary systems. Under certain conditions, this corrosion can be severe and cause early failure of a sanitary system.

5.2-Erosion by mineral-free water

Hydrated lime is one of the compounds formed when cement and water combine. It is readily dissolved by water and more aggressively dissolved by pure, mineral-free water, found in some mountain streams and desalinization plants and other facilities using demineralized (distilled) water. Dissolved carbon dioxide is contained in some fresh waters in sufficient quantity to make the water slightly acidic and add to its aggressiveness. It has been reported that serious attacks by fresh water on exposed concrete surfaces has led to deterioration (Popovics and McDonald 1989). In the United States, there are many instances where the surface of the concrete has been etched by fresh water flowing over it, although serious damage from this cause is uncommon (Holland et a!. 1980). This etching is particularly evident at hydraulic structures carrying runoff from high mountain streams in the Rocky Mountains and the Cascade Mountains of the central and western United States. A survey (I COLD 1951) of the chemical composition of raw water in many reservoirs throughout the United States indicates a nearly neutral acid-alkaline balance (pH) for most of these waters.

5.3-Erosion by miscellaneous causes 5.3.1 Acidic environments-Decaying vegetation is the

most frequent source of acidity in natural waters. Decomposition of certain minerals may be a source of acidity in some localities. Running water that has a pH as low as 6.5 will leach lime from concrete, reducing its strength and making it more porous and less resistant to freezing and thawing and other chemical attack. The amount of lime leached from concrete is a function of the area exposed and the volume of concrete.

Waters flowing from peat beds may have a pH as low as 5. The presence of acid of this strength will result in severe attack of concrete (Neville 2009). For this reason, when conveyances for groundwater are being designed, the corrosiveness of water should be determined using standard water quality tests (pH, acidity, and ion composition) to determine its aggressiveness on the concrete.

5.3.2 Bacterial action-Most of the literature addressing the problem of deterioration of concrete resulting from bacterial action has evolved because of the great impact of this corrosive mechanism on concrete sewer systems. This is a serious problem that, as Rigdon and Beardsley (1958) observed, occurs more readily in warm climates such as California, Australia, and South Africa. This problem also occurs at the terminus of long-pumped sewage force mains in the northern climates (Pomeroy 1974).

Sulfur-reducing bacteria belong to the genus of bacteria that derives the energy for its life processes from the reduction of some element other than carbon, such as nitrogen, sulfur, or iron (Rigdon and Beardsley 1958). Some of these bacteria reduce the sulfates that are present in natural waters and produce hydrogen sulfide as a waste product. These bacteria are anaerobic.

Another group of bacteria takes the reduced sulfur and oxidizes it back so that sulfuric acid is formed. The genus

. Thiobacillus is the sulfur-oxidizing bacteria that is most American Concrete Institute Provided by IHS Markit under license with ACI No reproduction or networking permitted without license from IHS

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1 0 REPORT O N THE EROSION OF CONCRETE IN HYDRAULIC STRUCTURES (ACI 207.6R-17)

Fig. 5.3.2-Acid attack in crown of waste-water conduit

showing exposed reinforcement, Denver Metro Waste Water

Treatment Plant (photo courtesy of URS).

destructive to concrete. It has a remarkable tolerance to acid. Concentrations of sulfuric acid as great as 5 percent by volume do not completely inhibit its activity.

Sulfur-oxidizing bacteria are likely to be found wherever wannth, moisture, and reduced compounds of sulfur are present. Generally, a free water surface is required in combinat\on with low dissolved oxygen in sewage and low velocities that permit the buildup of scum on the walls of a pipe in which the anaerobic sulfur-reducing bacteria can thrive. Certain conditions should prevail before the bacteria can produce hydrogen sulfide from sulfate-rich water. Sufficient moisture should be present to prevent the desiccation of the bacteria. There should be adequate supplies of hydrogen sulfide, carbon dioxide, nitrogen compounds, and oxygen. In addition, soluble compounds of phosphorus, iron, and other trace elements should be present in the moisture film.

Newly made concrete is strongly alkaline with a pH of approximately 12. No species of sulfur bacteria can live in such a strong alkaline environment. Therefore, the concrete is temporarily free from bacterially-induced corrosion. Natural carbonation of the free lime by the carbon dioxide in the air slowly reduces the pH of the concrete surface to 9 or less. At this level of alkalinity, the sulfur bacteria Thiobacillus thioparus, using hydrogen sulfide as the substrate, generate thiosulfuric and polythionic acid. The pH of the surface moisture steadily declines, and at a pH of approximately 5, Thiobacillus concretivorus begins to proliferate and produce high concentrations of sulfuric acid, dropping the pH to a level of 2 or less. The destructive mechanism in the corrosion of the concrete is the aggressive effect of the sulfate ions on the calcium aluminates in the cement paste.

The main concrete corrosion problem in a sewer, therefore, is chemical attack by this sulfuric acid, which accumulates in the crown of the sewer (Fig. 5.3.2). Information is available that enables the designer to design, construct, and operate a sewer that could reduce the development of sulfuric acid (Pomeroy 1974; ASCE-WPCF Joint Task Force 1982; American Concrete Pipe Association 1980).

CHAPTER 6-CO NTROL OF CAVITAT IO N EROSIO N

6.1-Hydraulic design principles In 3.2, the cavitation index a was defined by Eq. (3.2b).

When the value of a at which cavitation begins (a c) is known, a designer can calculate velocity and pressure combinations that will minimize potential damage. The object of a safe design is to assure that the actual operating pressures and velocities will produce a value of a greater than the value at which cavitation begins (Falvey 1990). Note this is where cavitation begins; there is little, if any, information on when actual damage begins. However, it is possible that damage occurs at the inception of cavitation, but it is so minor as to not be detected at first.

One good way to minimize cavitation erosion is to make a large by keeping the pressure p0 high and the velocity v0 low. For example, deeply submerged baffle blocks in a stilling basin downstream from a spillway chute are unlikely to be damaged by cavitation during normal operations if both these conditions are satisfied. This situation is illustrated in Fig. 6.1a. The following example illustrates how a is calculated for this case.

Example 1

From model studies, the following information is given: a) The mean prototype velocity at Point (0), immediately

upstream from the baffle block, is 30 ft/s (9.1 m/s). b) The minimum prototype gauge pressure, exceeded 90

percent of the time, is 7.1 psi (49 kPa). c) The barometric pressure for the prototype location is

estimated to be 13.9 psi (95.8 kPa). d) The vapor pressure of water (pv) is 0.3 psi (2.1 kPa) and

the density of water (p) = specific weight divided by gravity (y/g) = 1.94 lb·s2fft4 (103 kg/m3), from standard tables at a given temperature (Haynes 2016).

Therefore, the absolute pressure at Point (0), 6.6 ft (2.0 m) above Point (1 ), becomes

Po = gauge pressure + barometric pressure + y(zc - z1), from Eq. (3.2a)

Po = 7.1 lb/in.2 + 13.9 lb/in.2

+ 62.4 lb/fe co - 6.6 ft) 18.1 lb/in.2

(12 in./ft)2

and because Points (c) and (0) are on the sample plane, zc = z0, it follows that

18.1 lb/in.2 - 0.3 lb/in.2 cr = = 2 9 �(1.94 1�:2 ) (30 ft/s)

.


777777 6 .6 ft

( I ) {2 m) (01 �

7/7/ 77/// // /'lJ(Pressure �:ck transducer

Fig. 6. 1 a-Baffle block downstream from a low spillway.

In SI units

Po = 49 kPa + 95 .8 kPa

+ 9.8 1 kN/m2 (0 - 2.0 m) = 125 kN/m2 , or 1 25 kPa

and because Points (c) and (0) are on the sample plane, Zc =

z0, it follows that

(125 kPa - 2. 1 kPa)( l OOO Pa/kPa) 0 = = 2.9

1/2(1 000 kg/m2)(9. 1 m/s)2

because 1 Pa = 1 kg/(m· s2), This value of a is well above the a of 1 .4 and 2 .3 for

beginning of cavitation for this baffle block with sharp edges (Fig. 6. l b) (Galperin et a!. 1 977). Hence, cavitation damage is unlikely in the prototype.

A second, equally effective procedure to minimize potential cavitation damage is to use boundary shapes and tolerances characterized by low values of a for incipient damage. For example, a carefully designed gate slot, with an offset and rounded downstream comer, may have damage at a calculated a value of 0.2. Unfortunately, the lowest value of a a designer can use may be fixed by unintentional surface imperfections in concrete, the need for small abrupt expansions in flow passages, or the likelihood that vortexes will be generated by obstructions such as partially open sluice gates. Boundary geometry and construction techniques influence the potential for cavitation damage.

A third choice, often inevitable, is to expect cavities to form at predetermined locations. In this case, the designer can: a) supply air to the flow; or b) use damage-resistant materials such as stainless steel or polymer concrete systems.

Using damage-resistant materials will not eliminate damage, but could extend the useful life of a surface. This alternative is particularly attractive, for example, for constructing or repairing outlet works that will be used infrequently or abandoned after their purpose has been served.

In any case, values of a at which cavitation erosion begins are needed for all sorts of boundary geometries. Sometimes critical values of a can be estimated by theory, but they usually come from model or prototype tests.

6.2-Cavitation indexes for damage and

construction tolerances Figure 6 . 1 b lists values of a at which cavitation begins

and the references from which these values came. A designer

Structure or irregularity cr Reference Tunne l inlet 1 . 5 Tullis ( 19 8 1 1

Sudden expansion in tunne l 1 . 0 * Russell and Ball ( 1967 1 0 . 1 9 Rouse and Jezdinsky ( 19 6 6 1

Baffle blocks

Gate and gate slots

Abraded concrete

3 / 4 in . max . depth

of roughness

_i..".o OJ;;;;;; 7_22](?))/ /?777

1 . 4 &

2 . 3

0 . 2 to

3 . 0

0 . 6

0 . 2

0 . 2

1 . 6

1 . 0

Galperin e t al .

Galperin et a l .

Ball ( 1959 1 Wagner 1 1967 1

Ball 1 1976 1

Ball ( 19 7 6 1 Arndt ( 1977 ) Falvey ( 1982 )

�unusual definition of a .

Fig. 6.1 b-Values of a at beginning of cavitation.

( 1977 1

( 1977 1

should not use these numbers without studying the references. Some reasons for this are:

a) The exact geometry and test circumstances should be understood.

b) Authors use different locations for determining the reference parameters of Eq. (3.2b ), although the general form of Eq. (3 .2b) is accepted by practitioners in the field.

c) Uniformity in the model is difficult to achieve. d) Smooth, uniform concrete surfaces may be difficult to

achieve, maintain, or both, in the field. Many of the essential details involved in the original refer

ences are explained in Hamilton ( 1 983a,b; 1 984). The values of <Jc listed in Fig. 6. 1 b show the importance of good formwork and concrete finishing.

Example 2

The following information is given: a) A 1 /4 in. (6 mm) offset (Yo) into the flow caused by

mismatched forms has a ac of 1 .6. b) A 1 :40 chamfer has a <Jc of0.2 (only one-eighth as large)

cr - - a chamfer - g offsel

From Eq. (3.2d),

By the definition of <Jc, the allowable velocity past the chamfer, if (p0 - Pv)ll l2p is constant, becomes

vchamfer _1_1 __ = J8 . voflset = 2.83 . voflset [ 2 - · v 8 offset

Thus, on a spillway or chute where p0 - Pv might be 1 7.4 psi ( 120 kPa), cavitation would begin behind the offset when

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the local velocity reached 40 ft/s ( 1 2 m/s), but the flow past the chamfer would cause no trouble until the velocity reached approximately 2.83 times 40 ft/s ( 1 2 m/s), which equals 1 13 ft/s (35 m/s).

When forms are required, as on walls, ceilings, and steep slopes, skilled workers may produce a nearly smooth and only slightly wavy surface for which a may be as low as 0.4. Whereas on plane, nearly horizontal surfaces using a stiff screed controlled by steel wheels running on rails and hand floating and troweling, a a value of 0.2 may be achieved.

Construction tolerances should be included in all contract documents. These establish permissible variation in dimension and location giving both the designer and the contractor parameters within which the work is to be performed. ACI 1 1 7 provides guidance in establishing practical tolerances. It is sometimes necessary that the specifications for concrete surfaces in high-velocity flow areas, or more specifically, areas characterized by low values of a, be even more demanding. However, achieving and maintaining more restrictive tolerances for hydraulic surfaces than those recommended by ACI 1 1 7 can become costly or even impractical. The final specification requirements require judgment on the part of the designer (Schrader 1981 ).

The U.S. Bureau of Reclamation (20 1 4) specifies surface roughness tolerances (Table 6.2) that define the limits of allowable surface irregularity such as bulges, depressions, and offsets as measured by the roughness slope ratio shown in Fig. 6.2.

Joints can cause problems in meeting tolerances, even with the best workmanship. Some designers prefer to saw and break out areas where small offsets occur rather than to grind the offsets that are greater than specified. The trough or hole is then patched and hand-finished to produce a surface more resistant to erosion than a ground surface would be. In some cases, grinding to achieve alignment and smoothness is adequate. However, to help prevent the occurrence of aggregate popouts, a general rule of thumb is to limit the depth of grinding to one-half the maximum diameter of the coarse aggregate. Ground surfaces can also be protected by applying a low-viscosity, penetrating phenol epoxy-resin sealer (Borden et a!. 1 97 1). However, the smooth, polished texture of the ground surface or the smoothness of a resin sealer creates a different boundary condition that could affect the flow characteristics. Cavitation damage has been observed downstream of such conditions in high-velocity flow areas (in excess of 80 ft/s [24 m/s]) where there was no change in geometry or shape (Popovics and McDonald 1 989).

The difficulty of achieving a near-perfect surface and the doubt that such a surface would remain smooth during years of use have led to designs that permit the introduction of air into the water to cushion the collapse of cavities when low pressures and high velocities prevail.

6.3-Using aeration to control damage Laboratory and field tests have shown that surface irregu

larities will not cause cavitation damage if the air-water ratio in the layers of water near the solid boundary is approximately 8 percent by volume (Peterka 1 953). The air. in H

Table 6.2-Examples of surface roughness tolerances

Cavitation index <J Abrupt offsets limits Gradual offset limits

0 2 0.5 < 1 /4 in . (6 mm) 1 : 1 6 or flatter

0.5 > 0 > 0.2 :S 1 /4 in. (6 mm)

1 1 6 to 1 :32 or flatter to :S 1 18 in. (3 mm)

Construct an aeration ramp or slot for existing 0 ::: 0.2 spillways

Redesign (realign) for new spillways.

Finished Surface

Roughness Slope Ratio= Roughness Height Roughness Length

CASE I - Offset on the Surface

J Roughness Length

Finished Surface � Roughness Depth

------"------:...

Roughness Slope Ratio= Roughness Depth Roughness Length

CASE 2 - Offset into the Surface

Fig. 6.2-Measuring surface roughness of an offset.

the water should be distributed rather uniformly in small bubbles.

When calculations show that flow without aeration is likely to cause damage, or when damage to a structure has occurred and aeration appears to be a remedy, the problem is dual: 1 ) the air should be introduced into the flowing water; and 2) a portion of that air should remain near the flow/ concrete boundary where it will be useful.

The migration of air bubbles involves two principles: I ) bubbles in water move in a direction of decreasing water pressure; and 2) turbulence disperses bubbles from regions of high air concentration toward regions of low concentration.

Attention should be paid to the motion of bubbles due to pressure gradients. A flow of water surrounded by atmospheric pressure is called a free jet. In a free jet, there are no gradients except possibly weak local ones generated by residual turbulence, and the bubbles move with the water. There is no buoyant force. On a vertical curve that is convex, the bubble motion may have a component toward the bottom. In a flip bucket, which is concave, the bottom pressure is large and the bubbles move rapidly toward the free surface. When aeration is required, air usually should be introduced at

the bottom of the flow. These bubbles gradually move away A · C t 1 Licensee=ZHEJIANG INST OF STANDARDIZATION 5956617 p��c:d

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F l ow net Q; 76,270 ft 3/s 12160 m 3/ol

13ft (4 m l . o

Fig. 6.3a-Aeration ramps at King Tala! Spillway, Jordan (Wei and DeFazio 1982).

Table 6.3-Examples of use of air to prevent cavitation damage

Structure or description References

Palisades Dam, Idaho outlet sluices Beichley and King ( 1 975)

Yellowtail Dam, Montana spillway Borden et al. ( 1 97 1 ); Colgate tunnel ( 1 97 1 )

Glen Canyon Dam, Arizona Burgi et al. ( 1 984)

spillway tunnel

Ust-IIimsk Dam, Russia spillway Galperin et al. ( 1 977)

Bratsk Dam, Russia spillway Semenkov and Lentyaev ( 1 973)

Foz do Areia, Brazil spillway Pinto ( 1 982)

General Galperin et al. ( 1 977)

Comprehensive Hamilton ( 1 983a,b, 1984);

Quintela ( 1 980)

from the floor despite the tendency for turbulent dispersion to hold them down. At the point where insufficient air is in the flow to protect the concrete from damage, a subsequent source of bottom air should be provided.

Aeration data, shown in Table 6.3 and measured on Bratsk Dam in Russia, which has a spillway approximately 295 ft (90 m) high and an aeration device, are discussed by Semenkov and Lentyaev ( 1 973). Downstream from the aeration ramp, measurements showed that the air-water ratio in a 6 in. ( 1 50 mm) layer next to the concrete declined from 85 to 35 percent as the mixture flowed down the spillway a distance of 1 74 ft (53 m). If an exponential type of decay is assumed, the loss per 1 ft ( 1 /3 m) was a little less than 2 percent of the local air-water ratio.

It is usually not feasible to supply air to flowing water by pumping or compressing the air because the volumes involved are too large. Instead, the flow is projected from a ramp or step as a free jet, and air is introduced at the airwater interfaces. Then the turbulence within the jet disperses the air entrained at the interfaces into the main body of the jet. Figure 6.3a shows typical aeration ramps for introducing air into the flow (Wei and DeFazio 1 982).

To judge whether sufficient air will remain adjacent to the floor of a spillway, the amount of air that a turbulent jet will entrain should be estimated. Equation (6.3) for entrainment by the lower surface has been proposed (Hamilton 1 983a,b, 1 984)

(6.3)

Model and prototype measurements indicate that the value of the coefficient a lies between 0 .01 and 0.04, depending on velocity and upstream roughness.

The length of cavity, l (Fig. 6.3a), is difficult to measure in prototypes and large models. Instead, the upper and lower profiles of the nappe can be estimated from two-dimensional irrotational flow theory. One method is to use a finite element technique for calculating nappe traj ectories.

As indicated previously, ramps and down-steps are used to induce the flow in a spillway or tunnel to spring free from the floor. A ramp is a wedge anchored to, or integral with, the floor and usually spans the tunnel or spillway bay. Wall and corner wedges and wall offsets away from the flow also are used to cause the water to leave the sides of a conduit. The objective is to provide a sudden expansion of the solid boundaries. Such devices, often referred to as aerators, are schematically depicted in Fig. 6.3b and 6.3c. (Ball 1 959; DeFazio and Wei 1 983 ; Vischer et a!. 1 982; Russell and Ball 1 967). Air is allowed to flow into a cavity beside or under a jet by providing passages as simple as the layout of the project will permit.

Although offsets, slots, and ramps in conduits can introduce air into high-velocity flow to effectively control cavitation, if improperly designed, they can accentuate the cavitation problem. For this reason, it is advisable to conduct physical hydraulic model studies to ensure the adequacy of a proposed aeration device.

6.4-Materials Proper material selection can increase the cavitation

resistance of concrete. However, the only effective solution is to reduce or eliminate the factors that trigger cavitation, because even the strongest materials cannot withstand the forces of cavitation indefinitely. The difficulty is that in the repair of damaged structures, the reduction or elimination of cavitation may be difficult and costly. The next best solution is to replace the damaged concrete with more erosionresistant materials.

In areas of new design where cavitation is expected to occur, designers may include the higher-quality materials

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, - -1 I I I I

I I I I L _

Fig. 6.3b-Types of aerators (Vischer et al. 1982).

during the initial construction or include provisions for subsequent repairs in service. For example, in many installations, stainless steel liners are installed on the concrete perimeter downstream of slide gates to resist the damaging effects of cavitation. These liners, although quite durable, could pit and eventually need to be replaced.

The cavitation resistance of concrete where abrasion is not a factor can be increased by using a properly designed low water-cementitious material ratio (w/cm), higher-strength concrete, placed and cured properly to reduce cracking. Careful selection of aggregate size and gradation and the use of water-reducing admixtures has proven beneficial, as has lowering the placement temperature to reduce thermal cracking potential (ACI 207 . 1 R). Hard, dense aggregate and good bond between aggregate and mortar are essential to achieving increased cavitation resistance (May 1 987).

The use of polymer impregnation, steel fiber reinforcement, and polymers as a matrix binder or a surface binder has been found to substantially improve the cavitation resistance of conventional concrete, but could be costly (Schrader 1 978, 1983 ; DePuy and Dikeou 1 973).

Some coatings, such as neoprene or polyurethane, have effectively reduced cavitation damage to concrete, but because near-perfect adhesion to the concrete is mandatory, their use is not common. Once there is a tear or a chip in the coating, the entire coating is soon peeled off.

6.5- Materials testing

Because of the massive size of most hydraulic structures, full-scale prototype testing is usually not possible. Model

DUCT THROUGH SIDEWALL

"" '" ""'""' �

"''"' ""'""' ~ DUCT UNDER OFFSET

RAMP ON SIDEWALL

Fig. 6.3c-Air supply to aerators (Falvey 1990).

.

' '

'

testing can identify many potential problem areas, but determining the ultimate effect of hydraulic forces on the structure requires some judgment. In some cases, it is desirable to evaluate a material after it has been subjected for a reasonable time to flows of a magnitude approaching that expected during operation of the facility.

The U.S. Army Corps of Engineers evaluated erosion resistance of materials at the Detroit Dam in Oregon (Houghton et al. 1 978). Erosion testing at the facility consisted of preparing test slabs 2 1 in. (530 mm) wide by 1 0 ft (3 m) long using the desired material, coating, or overlay. High-velocity water exceeding velocities of 80 ft/s (24 m/s) was passed over the slabs for various durations, and the performance of the material evaluated. Cavitation erosion resistance was studied by embedding small obstacles in the test slabs that protrude into the flow (Fig. 3 .3b ) .

Materials and coating systems evaluated for repairs to the Tarbela Dam in Pakistan were tested at the Detroit Dam facility. They included various concrete mixtures, fiberreinforced concrete (FRC), roller-compacted concrete, polymer-impregnated concrete, polymer-impregnated FRC, and several concrete coatings (Houghton et al. 1 978). Figure 6.5 shows the performance of several of these materials subjected to flows with velocities of 1 20 ft/s (37 m/s).

6.6-Construction practices Construction practices are of paramount importance

when hydraulic surfaces may be exposed to high-velocity flow, particularly if aeration devices are not incorporated in design. Such surfaces should be as smooth as can be practically obtained (Schrader 1 983). Surface imperfections and deficiencies have been known to cause cavitation damage at



0 TEST SLAB NO. 1 -- CONVENTIONAL CONCRETE -- Cement 600 1b/yd3 (356 kg(m3J; MSA 1 - 1 /2 in. (3B mm)

0 TEST SLAB NO. 2 -- STEEL FRC -- Cement 690 lb/yd3 (409 kg/m3); MSA 3/4 in. ( 1 9 mm)

e TEST SLAB NO. 3 -- POLYMERIZED CONVENTIONAL -- Cement 600 lb/yd3 (356 kg/m3); MSA 1 - 1 /2 in. (38 mm)

• TEST SLAB NO. 4 -- POLYMERIZED FRC -- Cement 690 lb/yd3 (409 kg/m3); MSA 3/4 in . ( 1 9 mm)

MSA-- Maximum size aggregate

100

75

E .£ E � .r: 0. 0. ., 50 ., 0 0 c c 0 0

·;;; ·;;; e e w w 25

o L------L------�------L-----�------�-------L ______ L_ __ L_� o 0 20 40 60 8 0 I 0 0 I 20 I 40 20 0

Test Time . h

Fig. 6. 5--Erosion depth versus time, Tarbela Dam, Pakistan concrete mixtures (Houghton

et al. 1978).

flow velocities as low as 26 ft/s (8 m/s). Offsets no greater than 1/8 in. (3 mm) in height have been known to cause cavitation damage at flow velocities as low as 82 ft/s (25 m/s). Patching repairs improperly made at the time of construction have been known to fail under the stress of water flow, thereby providing the surface imperfections that triggered cavitation damage to the concrete farther downstream. This phenomenon occurred in the high head spillway tunnel at Yellowtail Dam in Montana, ultimately resulting in major cavitation and structural damage to the concrete lining (Borden et a!. 1 97 1 ; Colgate 1 97 1). Accordingly, good construction practices, as recommended in ACI 1 1 7, 302. 1 R, 304R, 308 . 1 , 309R, and 347R should be maintained both for new construction and repair. Formed and unformed surfaces should be checked during each construction operation to confirm that they are within specified tolerances.

Thinner placements bonded to underlying substrate concrete should be avoided. Thin concrete placements could have different thermal and mechanical properties from the substrate material if different aggregates or mixture proportions are used, possibly causing them to debond during normal temperature cycles. In addition, the bond surface will typically be weaker than other regions, making the interface more susceptible to damage, and stresses closer to the surfaces may be larger. Any debonding or delamination at the leading edge can be a source of further damage by induced jacking pressures from high-velocity flow or damage from standing water that freezes. Thicker repairs should be constructed with similar aggregates and mixture proportions with similar strengths for similar properties as the substrate material. They should also be mechanically fastened to the substrate material with reinforcement and anchors.

If the potential for cavitation damage exists, care should be taken in placing the reinforcement. The bars closest to the surface, if practical for structural purposes, should be placed

parallel to the direction of flow to offer the least resistance to flow if erosion reaches the depth of the reinforcement, as well as to provide transverse crack control. Extensive damage has been experienced where the reinforcement near the surface is perpendicular to the direction of flow. Additionally, increasing the concrete cover should be evaluated by design experts when erosion due to high flows or abrasive material is a concern. Consideration should be made to the potential for larger drying shrinkage cracks in the surface of the concrete as a result of having the reinforcement at a greater depth (U.S. Bureau of Reclamation 20 1 5).

Where possible, transverse joints in concrete conduits or chutes should be minimized. Preplanned, more closely spaced joints are generally preferable to uncontrolled cracking. One construction technique that has proven satisfactory in placement of reasonably smooth hydraulic surfaces is the traveling slipform screed. This technique can be applied to tunnel inverts and to spillway chute slabs but requires a specialized or experienced contractor. Information on the slipform screed is found in Hurd (2005). Proper mixture design (Neville 1 999), construction techniques, and curing of these surfaces is essential because the development of surface hardness improves cavitation resistance.

CHAPTER 7--CONTROL O F ABRASION EROSIO N

7. 1 --Hydraulic considerations Under appropriate flow conditions and transport of

debris, all the construction materials currently being used in hydraulic structures are, to some degree, susceptible to abrasion. While improvements in materials should reduce the rate of damage, these alone will not solve the problem. Until the adverse hydraulic conditions, which can cause abrasion erosion damage, are minimized or eliminated, it is difficult for any current construction materials to perform without


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1 6 REPORT O N T H E EROSION O F CONCRETE IN HYDRAULIC STRUCTURES (ACI 207.6R-17)

damage. Before construction or repair of major structures, conduct hydraulic model studies of the structure to identify potential causes of erosion damage and evaluate the effectiveness of various modifications in eliminating those undesirable hydraulic conditions. If the model test results indicate it is impractical to eliminate the undesirable hydraulic conditions, provisions should be made in design to minimize future damage. For example, good design practices should consider the following measures in the construction or repair of stilling basins:

a) Include provisions, such as debris traps or low division walls, to minimize circulation of debris; design end sills and chute blocks so that they do not create a back roller below the flow, trapping debris in the area.

b) Design downstream materials for stability at the design flows.

c) Use model tests for design and detailing of the terminus of the stilling basin and the exit channel to maximize flushing of the stilling basin and to minimize chances of debris from the exit channel entering the basin.

d) For stilling basins less than 25 ft (7.6 m) wide, flow deflectors should be installed over a stilling basin end sill, as shown in Fig. 7. l a, to redirect the current responsible for carrying abrasive materials into the still basin (Hanna 201 0).

Flow defectors were installed at Choke Canyon (Fig. 7. 1b) and Mason Dams to improve flow conditions to minimize the potential for carrying downstream materials back into the basin. Hanna (201 0) shows that a single mobile deflector or two stationary staggered deflectors, staggered in position both vertically and horizontally, are effective at sites where large ranges of operations need be considered. This does not, however, prevent materials from entering from above or increase the effectiveness of flushing material from the basin.

Balanced flows into the basins of existing structures should be maintained using all gates to avoid discharge conditions where flow separation and eddy action are prevalent. Substantial discharges that can provide a good hydraulic jump \vithout creating eddy action should be released periodically in an attempt to flush debris from the stilling basin. Guidance as to discharge and tailwater relations required for fl�shing should be developed through model or prototype tests, or both. Periodic inspections should be required to det¢rmine the presence of debris in the stilling basin and the extent of erosion. If the debris cannot be removed by flushing operations, water releases should be shut down and the basin cleaned by other means.

In locations where abrasion damage cannot be avoided, consider measures to avoid major damage leading to failure of the structure. Some materials, as discussed in the following, can slow the progress of abrasion. However, for existing structures subject to abrasion damage, the cost of repairs may be considerable. Expensive repairs could be delayed or avoided if a program to regularly clean debris from the surface is implemented along with one to periodically measure damage. Typically, ball-milling abrasion does not lead to failure during a single event. Surface damage increases over time with each subsequent event. While there may be a range of flows that can move debris onto the

Fig. 7. 1 a-Flow deflector and desired flow pattern produced

(Hanna 20 10).

Fig. 7.1 b-Choke Canyon stilling basin deflectors (Hanna

2010).

concrete surfaces, once there, other flows could continue the ball-milling damage. By periodically removing debris accumulations, the events that could cause ball-milling but that do not move abrasive materials onto the concrete surfaces will not cause damage with the debris removed. Periodic clearing could also provide opportunity for periodic measurements of surface damage. For example, if a stilling basin is pumped dry to remove rocks, the surface damage should be surveyed at the same time. Periodic surveys can help predict the rate of damage, which is useful for approximating when damage might reach a critical point that requires repair.

7.2-Materials evaluation Materials, mixtures, and construction practices should be

evaluated prior to use in hydraulic structures subjected to abrasion-erosion damage. ASTM C 1 1 38M covers a procedure for determining the relative resistance of concrete to abrasion under water. This procedure simulates the abrasive action of waterborne particles (silt, sand, gravel, and other solid objects). Development of the test procedure and data from tests on a wide variety of materials and techniques have been described by Liu ( 1 980) and Causey ( 1 985).

7.3-Materials Many materials and techniques have been used in the

construction and repair of structures subjected to abrasion erosion damage, with varying degrees of success. The degree of success is inversely proportional to the degree of

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REPORT ON THE EROSION OF CONCRETE IN HYDRAULIC STRUCTURES (ACI 207.6R-17) 1 7

exposure to those conditions conducive to erosion damage (McDonald 1 980). No single material has shown consistently superior performance when compared to others. Improvements in materials are expected to reduce the rate of concrete damage due to abrasion erosion.

Abrasion-resistant concrete should include the maximum amount of the hardest available coarse aggregate and the lowest practical w!cm. Concrete mixtures should be designed for workability and the capability of consolidation and finish to meet the required tolerances. Good placement practices and curing are also key. In laboratory studies, a small increase in abrasion-erosion resistance has been shown when using larger aggregates ( 1 - 1/2 in. [38 mm] versus 3/4 in. [ 1 9 mm]) (White 201 1 ) . The abrasion-erosion resistance of concrete containing chert aggregate has been shown to be approximately twice that of concrete containing limestone (Fig. 7.3). Given a good, hard aggregate, any practice that produces a stronger paste structure will increase abrasionerosion resistance. Loss due to abrasion is directly proportional to w/cm (White 20 1 1 ) . In some cases where hard aggregate was not available, high-range water-reducing admixtures and silica fume were used to develop a strong concrete with high compressive strength-approximately 15 ,000 psi ( 1 00 MPa)-to overcome proble�s with unsa�isfactory aggregate (Holland 1 983). At these h1gh compress1ve strengths, the hardened cement paste assumes a greater role in resisting abrasion-erosion damage, and the aggregate quality becomes correspondingly less important. However, thermal and shrinkage stresses also will likely be higher, increasing the risk of cracking from early temperatures and shrinkage. Concrete, when produced with a shrinkage-reducing admixture or concrete designed to have low shrinkage potential, can be beneficial when properly proportioned and cured.

In laboratory tests, the abrasion loss of a range of steel fiberreinforced concrete (FRC) mixtures was consistently higher than that of conventional concrete mixtures with the same w/ em and aggregate type (Liu and McDonald 198 1 ). However, the improved impact strength of FRC (Schrader 1981) can be expected to reduce concrete spalling where large debris is being transported by high-velocity flow (ACI 544. 1R).

The abrasion-erosion resistance of vacuum-treated concrete, polymer concrete, polymer-impregnated concrete, and polymer-portland-cement concrete can be significantly superior to that of comparable conventional concrete. This is attributed to a stronger cement matrix. The increased costs associated with materials, production, and placing ofthese and any other special concretes in comparison with conventional concrete should be considered during the evaluation process.

Several types of coatings have exhibited good abrasionerosion resistance in laboratory tests. These include polyurethanes, polyurea, ceramic-filled epoxy, epoxy-resin mortar, furan-resin mortar, acrylic mortar, and iron-aggregate toppings. Problems in field application of coatings have been reported (McDonald 1 980). These have been due primarily to improper surface preparation or thermal incompatibility between coatings and concrete. More recently, formulations have been developed that have coefficients of thermal expansion more similar to that of the concrete substrate.

1 0

/]/

L_ v ,......

v / i-� .......... _ . �

f.- -:'� r:-· �- - - - -1-/ --

/� ........ V·_:....- LEGEND

--- LIMESTONE - - QUARTZITE -

0 0.3 0.4 0.5 0.6 0.7 Water-Cement Ratio

--- TRAP ROCK --- - CHERT I I

0.8 0.9 I.C

Fig. 7.3-Relationships between water-cement ratio (w/c)

and abrasion-erosion loss.

Substrate and surface preparation are important in ensuring a long trouble-free coatings project. Concrete surfaces should be clean and dry before coating to establish strongest bond. Dirt, dust, oil, and all other contaminates should be removed and concrete surfaces prepared in accordance with ASTM D4258. Surface preparation should include abrasive blast or the equivalent to remove laitance and other loose concrete in accordance with ICRI 3 10.2.

CHAPTER 8-CO NTROL O F EROSIO N BY

CHEMICAL ATTACK

8. 1-Control of erosion by mineral-free water Pure water from glacial runoff or from condensation, as

in a desalination plant, can dissolve the calcium hydroxide in the cement matrix (Neville 2009). The mild acid attack possible with pure water rarely develops into deterioration that can cause severe structural damage. Generally, mineralfree water will leach mortar on surfaces exposed to this water. This can be seen on exposed surfaces and at joints and cracks in concrete sections. As the surface calcium is leached from the concrete, coarse aggregate is exposed, which naturally decreases the amount of mortar exposed. With less mortar exposed, less leaching occurs, resulting in less chance for major structural problems to occur. Roughened surfaces, however, could lead up to problems associated with cavitation if they are at a critical location. The gradual erosion of leached mortar is minimized by use 0f aluminous cements; partial substitution of cement with slag cement, fly ash, or both; or use of low-lime portland cements with less tricalcium silicate than dicalcium silicate (Tuthill 1 966). Protective coatings can also be effective when applied to concrete surfaces.

8.2-Control of erosion from acid attack due to bacterial action

The process of sulfide generation in a sanitary sewer when insufficient dissolved oxygen is present in the wastewater has been discussed and illustrated by an ASCE-WPCF




Joint Task Force ( 1 982). This original work was performed by Pomeroy ( 1 974). Continuing work by Pomeroy and Parkhurst ( 1 977) produced a quantitative method for sulfide prediction. Engineers involved with projects of this nature would be wise to also review the recommendations set forth in the American Concrete Pipe Association ( 1 980).

Concrete conduits have served in sewer systems for many years without serious damage in systems that were properly designed and operated. The minimum adequate velocity of flow in the sewer for the strength and temperature of the sewage is usually 2 ft/s (0.6 m/s). Providing this velocity without excessive turbulence and proper ventilation of the sewer will generally prevent erosion by bacterial action. Avoid turbulence because it is an H2S-releasing mechanism. Where conditions are such that generation of H2S cannot be eliminated by the system design, apply other means, including:

a) Using hydrogen peroxide or chlorine compounds that will convert the H2S (Water Pollution Control Federation 1 979):

i . H202 + H2S --->---> 2H20 + S ii.

( 1 ) Clz + H20 --->---> HOC! + W + CI-(2) HOC! + H2S --->---> S + HCI + H20 (low pH) (3) S2- + 4Ciz + 80H- --->---> SOi- + 8Cl + 4H20 (high pH)

b) Introducing compressed air into the water to keep sewage fresh and thereby preventing development of an anaerobic environment

c) Using an acid-resistant pipe such as vitrified clay or polyvinyl chloride (PVC) pipes

d) Using acid-resisting liners on the crown of sewers e) Increasing the concrete section to allow a sacrificial

thickness based on predicted erosion rates, at the risk of increased cracking

f) Using concrete with limestone aggregate in place of siliceous aggregates to give the acid more material to dissolve, slowing down the overall rate

Graphical methods have been published to determine sulfide buildup in sanitary sewers using the PomeroyParkhurst equations (Kienow et a!. 1 982).

Parker ( 1 95 1 ) lists the following remedial measures for the control of H2S attack in concrete sewers:

a) Reduction-potential-generation i. Inflow reduction ii. Partial purification iii. Chemical dosage to raise oxidation (but addition of

nitrates is impracticable) iv. Aeration v. Chlorination vi. Removal of slimes and silts vii. Velocity increase

b) Emissions i. Turbulence reduction ii. Treatment with heavy metal salts (Cu, Fe, Zn) iii. Treatment with alkalis iv. Full flow in sewer

c) H2S fixation on concrete i. Ventilation ii. Periodic wetting

iii. Use of resistant concrete iv. Ammoniation v. Use of protective coatings

The designer faced with reducing bacterial action should be aware that: 1 ) chlorination may, under certain circumstances, be in violation of local codes because it can produce trihalomethane, a known carcinogen; and 2) it could also be a violation of local codes to add heavy metal salts to wastewater.

Lining concrete pipe, walls, and conduit with PVC sheets, a plastic liner, or a chemical-resistant liquid-applied coating is an effective method of protecting the concrete and reducing surface roughness. This technique has been used commercially for many years. The designer should carefully determine which system is appropriate for the exposure conditions and structural requirements for each application. Further information on remedial measures for sanitary sewer systems is available (U.S. Environmental Protection Agency 1985).

8.3-Control of erosion by miscellaneous chemical causes

8.3.1 Acid environments-No portland-cement concrete, regardless of its other ingredients, will withstand attack from water of high acid concentration. Where strong acid corrosion is indicated, other construction materials or an appropriate surface covering or treatment should be used. This includes applications of sulfur-concrete toppings, epoxy coatings, polymer impregnation, linseed-oil treatments, or other processes, each of which affects acid resistance differently. Replacement of a portion of the portland cement by a suitable amount of slag cement or fly ash selected for that property can improve the resistance of concrete to weak acid attack. Also, limestone or dolomite aggregates have been found to be beneficial in extending the life of structures exposed to acid attack (Bicz6k 1 967).

Performance-based cements having reduced calcium, and when meeting ASTM C 1 1 57/C 1 1 57M or ASTM C 1 600/ C 1 600M, have proven to slow deterioration and be more resistant to acid attack. Some cements made primarily from fly ash particles and additives have no significant calcium and have been found to be denser and allow less acid past the surface. Laboratory studies have shown that only a small amount of material is lost initially when exposed to strong sulfuric acid solutions, with little or no deterioration at later ages (VanderWerf 201 1 ) .

In 1 994, laboratory and field evaluation of acid-resistant materials at Spring Creek Debris Dam, California, were evaluated in a pH 1 .7 test tank just outside the Richmond Mine portal. Ordinary portland-cement concrete, silica fume concrete, a polymer concrete overlay, a flexible epoxy coating, and a flexible polyurethane coating were tested (Smoak 1997) . Findings showed that silica fume concrete offered no improvement, and the concrete with the polyurethane coating showed superior performance. However, it was noted that the concrete coated with the epoxy and polymer overlay failed due to holidays or defects in the coating system, leaving access for acid attack to the underlying concrete. Additional



information on chemicals that attack concrete can be found in Portland Cement Association (2007).

8.3.2 Alkali-aggregate reaction and chloride admix

tures-Deterioration of concrete caused by alkali-aggregate reaction and by chloride admixtures in the concrete mixture is not included in this discussion. Extensive information on these topics can be found in ACI 22 1 . 1 R and ACI 2 1 2.3R.

8.3.3 Soils and groundwaters-Sulfates of sodium, magnesium, and calcium frequently encountered in the alkali soils and groundwaters of the western United States attack concrete aggressively. ACI 201 .2R discusses this in detail. Use ofType V sulfate-resisting cement, which is low in tricalcium aluminate (C3A), is recommended whenever the sulfate in the water is within the ranges shown in ACI 3 1 8-14 Table 19.3 . 1 . 1 . The subject of designing a sulfate-resistant concrete mixture is complex. It is generally agreed that limiting the C3A content of the cement to the 3 to 5 percent range, as in a Type V cement, is beneficial. Limits of C3A content also are established for Types I and II cements (ASTM C 1 50/C 1 50M). Additional issues are also important, including restricting the tetracalcium aluminoferrite content (C4AF) to 1 0 percent; providing air entrainment (an air-entrained mixture using Type II cement can be more sulfate resistant than a non-air-entrained mixture using Type V cement); replacing 20 to 30 percent of the cement content with a pozzolan or fly ash; and using a rich mixture, with the w/cm restricted to a maximum of 0.45 . The use of shrinkage-compensating cements, made with Type II or Type V portland-cement clinker and adequately sulfated, produces concrete having sulfate resistance equal to or greater than portland cement made of the same type of clinker (Mehta and Polivka 1975).

CHAPTER 9-PERIO DIC INSPECTIONS AND CORRECTIVE ACTION

9.1-General The regular, periodic inspection of completed and oper

ating hydraulic structures is important. Observance of concrete erosion should be included in these inspections. The frequency of inspections is usually a function of use and evidence of distress. Inspections provide a means of routinely examining structural features, as well as observing and discussing problems requiring remedial action. ACI 20 1 . 1R, ACI 207.3R, and EM- 1 1 1 0-2-2002 (U.S. Army Corps of Engineers 1 995) provide detailed instructions for conducting extensive investigations.

9.2-lnspection program

The inspection program should be tailored to the specific type of structure. Designers should provide input to the program and identify items of primary and secondary importance. The inspection team should be composed of qualified technical personnel who are experienced and can relate in common terminology. Team size is usually dependent on the number of technical disciplines required. The program should be established and monitored by an engineer who is experienced in design, construction, and operation of the project. The use of underwater dive inspections, climb team

rope inspection, or the use of remotely operated vehicles may be necessary.

9.3-lnspection procedures Before the on-site inspection, the team should thoroughly

evaluate all available records, reports, and other documentation on the condition of the structure and maintenance and repair, and become familiar with previous recommendations. Observations to make during an examination of hydraulic facilities include:

a) Identifying structural cracking, spalling, and displacements

b) Identifying surface irregularities where cavitation potential is a concern

i. Offset into or away from flow (including at joints or cracks)

ii. Abrupt curvature away from flow iii. Abrupt slope away from flow iv. Local slope changes along flow surface v. Void or transverse groove vi. Roughened or damaged surfaces that give evidence

of cavitation or abrasion erosion vii. Structural imperfections and calcite deposits

viii. Cracking, spalling, and rust stains from reinforcement c) Inspecting gate slots, sills, and seals, including identifi

cation of offsets into the flow d) Locating concrete erosion adjacent to embedded steel

frames and steel liners and in downstream water passages e) Finding vibration of gates and valves during operation '.

f) Observing defective welded connections and the pitting, cavitation, or both, of steel items

g) Observing equipment operation and maintenance : h) Making surveys and taking cross sections to determine :

the extent of damage . i) Investigating the condition of concrete by nondestruc- '

tive methods or by core drilling and sampling, if distressed conditions warrant

j) Noting the nature and extent of debris in water passages Observed conditions, the extent of the distress, and recom

mendations for action, should be recorded by the inspection team for future reference. High-quality photographs or videos of deficiencies are beneficial and provide a permanent record that will assist in identifying slow progressive failures. A report should be written for each inspection to record the condition of the project and justify funding for repairs. To-scale drawings should be created to show damaged areas.

9.4-Reporting and evaluation The inspection report can vary from a formal publication

to a trip report or letter report. The report should include the standard items: who, why, what, where, and when. A pre-established outline is usually of value. An inspection checklist of deficiencies and subsequent corrective actions should be established from prior inspections. Any special items of interest can be shown in sketches or photos. The report should address existing and potential problems and categorize deficiencies relative to the urgency of corrective action, as well as identify the extent of damage, probable

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cause of damage, and probable extent of damage if immediate repairs are not made. It is important that the owner or agency distribute the report in accordance with applicable U.S. federal or state safety regulations.

When the inspection report indicates that remedial action is required, the next step is either a supplemental investigation or the actual corrective action. Deficiencies noted in the inspection should be evaluated and categorized as to minor, major, or potentially catastrophic. The scope of work should be defined to establish reliable budget estimates. Design of proper repair schemes sometimes requires model tests, redesign of portions of the structure, and materials investigations. Each of these items requires funding by the owner. The more details identified in the scope of work, the more accurate the cost estimate. Wherever possible, it is important to correct the probable cause so that the repairs will not have to be repeated in the near future.

CHAPTER 10-REPA IR METHO DS AN D MATERIALS

1 0.1- Design considerations 1 0.L1 General-Although it is always desirable to elimi

nate the cause of erosion, it is not always possible; therefore, a variety of materials and material combinations are used for concrete repair. Some materials are better-suited for certain repairs and the designer should use judgment in their proper selecti<?n. Also, consider the time available to make repairs, access points, logistics in material supply, ventilation, nature of the work, available equipment, and skill and experience of the local labor force. Underwater repair options can be evaluated using ACI 546.2R. Detailed descriptions of repair considerations and procedures are found in the Concrete

Repair Manual (American Concrete Institute 20 1 3). 1 0.1.2 Consideration of materials-A major factor that is

critical to the success of a repair is how well the repair material matches the strength and thermal expansion properties of the substrate concrete, as previously discussed. Adhesion of the repair material to the substrate concrete is also critical for a long-lasting repair. ICRI 2 1 0.3R discusses adequate bond strength at the interface between concrete repairs. Some specifiers allow acceptance of bond strengths for cementitious materials as low as 100 psi (0.69 MPa) when tested in accordance with ASTM C 1 583M.

Normal portland-cement concrete will generally match the characteristics of in-place concrete with regard to temperature change. Thermal stresses that could result in a thicker repair should be minimized. This can often be managed by control of temperatures, replacement of cement with supplementary cementitious materials, and precooling the repair materials. Care should be taken to minimize drying shrinkage.

10.2-Methods and materials 1 0.2.1 Steel plating-Installing stainless steel liner plates

on concrete surfaces subject to cavitation erosion has been a generally successful method of protecting the concrete against cavitation erosion. Colgate's ( 1977) studies show

stainless steel to be approximately four times more resistant to cavitation damage than ordinary concrete. The currently preferred stainless material is ASTM A240/ A240M S30403, from the standpoint of excellent corrosion and cavitation resistance, and weldability. The steel plates should be securely anchored in place and sufficiently stiff to minimize the effects of vibration. Vibration of the liner plate can lead to fracturing and eventual failure of the underlying concrete or failure of the anchors. Grouting behind the plates to prevent vibration is recommended. Unfortunately, the steel plating could hide early signs of concrete distress. The transition from steel plating to concrete should also be designed carefully so that the concrete surface does not cavitate due to an offset.

This repair method, like many others, treats only the symptom of erosion and eventually, if the cavitation is not reduced or eliminated, the steel itself can become damaged by pitting.

10.2.3 Fiber-reinforced concrete (FRC)-Laboratory

abrasion-erosion tests under conditions of low velocity carrying small-size particles have concluded that FRC should not be used for new construction or repair where abrasionerosion is of major concern (Liu and McDonald 1 98 1 ) .

10.2.4 Epoxy resins-Resins are natural or synthetic, solid or semisolid organic materials of high molecular weight. Epoxies are one type of resin. These materials are typically used in preparation of special coatings or adhesives or as binders in epoxy-resin mortars and concretes. Several varieties of resin systems are routinely used for the repair of concrete structures . ACI Committee 503 ( 1 973) describes the properties, uses, preparations, mixtures, application, and handling requirements for epoxy resin systems.

The most common use of epoxy compounds is in bonding adhesives. Epoxies will bond to most building materials, with the possible exception of some plastics. Typical applications include the bonding of fresh concrete to existing concrete. Epoxies can also be used for bonding dry-pack material, FRC, polymer concretes, and some latex-modified concretes to hardened concrete. Epoxy formulations have been developed that will bond to damp concrete and even concrete under water. There are case histories of successful uses of these materials in hydraulic structures. To help assure proper selection and use of materials, consultation with product representatives is advised before an epoxy is specified or procured. When using a bonding agent, the bonding agent should not set before the concrete or mortar is applied; otherwise, bond can be poor. ASTM C88 1 /C8 8 1 M is a specification for epoxy bonding systems useful in concrete repairs, and ACI 503.2 covers epoxy bonding in repair work.

Experience shows that the localized application of epoxies can create serious problems in areas of high-velocity flow. If the finished surface has a smooth or glassy texture, flow at the boundary can be disrupted and may have the effect of a geometric irregularity, which could trigger cavitation. This texture problem is minimized by using special finishing techniques, improving the surface texture of the patch with sand, or both. Sometimes the patch can be too resistant to damage, with the result that the abutting original material



erodes away, leaving an abrupt change in surface geometry and developing a condition worse than the original damage.

Epoxy mortars and epoxy concretes use epoxy resins for binder material instead of portland cement. These materials are ideal for repair of normally submerged concrete, where ambient temperatures are relatively constant. Beware that they are different from traditional concrete. Epoxies can generate more heat while hardening, are likely to have a different coefficient of thermal expansion than the substrate concrete, and hardened epoxies may soften when exposed to elevated temperatures. Mixed results have been observed in the epoxy-mortar repair of erosion of outlet surfaces, dentates, and baffle blocks (McDonald 1 980). Depending on the epoxy formulation, the presence of moisture, either on the surface or absorbed in the concrete, can be an important factor that affects the success of the repair. ACI 503.4 is a specification for epoxy mortar in repair work.

The concept of improving concrete by incorporating the epoxy directly into the mixture was encouraged by the successful latex modification of concrete (Murray and Schrader 1 979). Several commercial products have been developed and research is continuing. The epoxies generally enhance the concrete's resistance to freezing and thawing spalling, chemical attack, and mechanical wear. Epoxymodified concrete (Christie et a!. 1 98 1 ) has a curing agent that is retarded by the water in the mixture. As the water is used up by cement hydration and drying, the epoxy resin begins to gel. Accordingly, the mixture will not become sticky until the portland cement begins to set, and this greatly extends the pot life ofthe wet concrete. These materials have had limited use in hydraulic structures.

1 0.2.5 Acrylics and other polymer systems-There are three main ways in which polymers have been incorporated into concrete to produce a material with improved properties as compared to conventional portland cement concrete:

1 ) Polymer-impregnated concrete (PIC) 2) Polymer-portland-cement concrete (PPCC) 3) Polymer concrete (PC) PIC is a hydrated portland-cement concrete that has been

impregnated with a monomer that is subsequently polymerized in place. By effectively case hardening the concrete surface, impregnation protects structures against the forces of cavitation (Schrader 1 978) and abrasion erosion (Liu 1980). The depth of monomer penetration depends on the porosity of the concrete and the process and pressure under which the monomer is applied. In addition to noting that these materials are costly, the engineer is cautioned that some monomer systems can be hazardous and that monomer systems require care in handling and should be applied only by skilled workers experienced in their use (DePuy 1 975). Surface impregnation was used at Dworshak Dam in Idaho in the repair of cavitation and abrasion erosion damage to the regulating outlet tunnels (Schrader and Kaden 1 976a) and stilling basin (McDonald 1 980; Schrader and Kaden 1976b ). High-head erosion testing of PIC at Detroit Dam test facility has shown excellent performance (U.S. Army Corps of Engineers 1977).

PPCC is made by the addition of water-submersible polymers directly into the wet concrete mixture. PPCC, compared with conventional concrete, has higher strength, increased flexibility, improved adhesion, superior abrasion and impact resistance, and usually better freezing-and-thawing resistance and improved durability. These properties can vary considerably, depending on the type of polymer being used. The most commonly used PPCC is latex-modified concrete. Latex is a dispersion of organic polymer particles in water. Typically, the fine aggregate and cement contents are higher for PPCC than for normal concrete.

PC is a mixture of fine and coarse aggregate with a polymer used as the binder. This results in rapid-setting material with good chemical resistance and exceptional bonding characteristics. Polymer concrete has had limited use in large-scale repair of hydraulic structures because of the expense of large volumes of polymer for binder. Thermal compatibility with the parent concrete should be considered before using these materials.

Polymer concretes are finding application as concrete repair materials for patches and overlays, and as precast elements for repair of damaged surfaces (Fontana and Bartholomew 1 98 1 ; Scanlon 1981 ; Kuhlmann 1 98 1 ; Bhargava 1 98 1 ). Field test installations with precast PC have been made on parapet walls at Deadwood Dam in Idaho, and as a repair of cavitation and abrasion damage in the stilling basin of American Falls Dam, also in Idaho.

ACI 548 . 1 R provides an overview of the properties and use of polymers in concrete. Smoak ( 1 985) has described polymer impregnation and polymer concrete repairs at Grand Coulee Dam.

1 0.2.6 Silica fume concrete-Laboratory tests have shown that the addition of an appropriate amount of silica fume and a high-range water-reducing admixture to a concrete mixture will greatly increase compressive strength. This, in turn, increases abrasion-erosion resistance (Holland 1 983 , 1 986b; Holland et a!. 1 986). As a result of these tests, concretes containing silica fume were used by the U.S. Army Corps of Engineers to repair abrasion-erosion damage in the stilling basin at Kinzua Dam in Pennsylvania (Holland 1986a) and in the concrete lining of the low-flow channel for the Los Angeles River (Holland and Gutschow 1 987). Despite adverse exposure conditions, particularly at Kinzua Dam, the silica fume concrete continues to exhibit excellent resistance to abrasion erosion.

Silica fume offers potential for improving many properties of concrete. However, the high compressive strength and resulting increase in abrasion-erosion resistance are particularly beneficial in repair of hydraulic structures. Silica fume concrete should be considered in repair of abrasion-erosionsusceptible locations, particularly in those areas where available aggregate might not otherwise be acceptable. However, silica fume may not be the best option for thick placements where the surface is too dense to let bleed water escape (increasing the risk of plastic drying shrinkage), or costprohibitive when the volume of materials is large. Silica fume concrete also usually has a higher risk of cracking from early hydration temperatures and drying shrinkage (NCHRP


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2008; ACI 224R). Guidance on the use of silica fume in

concrete is given in ACI 234R.

A successful underwater repair of the stilling basing at

Canyon Ferry Dam in Montana was done with 1 2,000 psi

(82 MPa) silica fume concrete (U.S. Bureau of Reclamation

2005). A remotely operated hydro-demolition tool was used

to remove concrete and exposed reinforcing steel bars at a

water depth of 50 ft ( 1 5 m). From barges, divers assisted

the installation of custom form work and placement of self

consolidating high-strength concrete having a 24 in. (600

mm) spread, which was pumped through a slickline.

Underwater abrasion testing has also shown that concrete

with ultra-fine Class F fly ash as a partial replacement for

cement performs as well as concrete with silica fume at the

same replacement rates (White 20 1 1 ).

1 0.2.7 Shotcretes-Shotcrete has been used extensively

in the repair of hydraulic structures. This method permits

replacing concrete without the use of formwork, and the

repair can be made in restricted areas. Shotcrete, also known

as pneumatically applied mortar, can be an economical alter

native to other, more conventional systems of repair. ACI

506R provides guidance in the manufacture and application

of shotcrete. In addition to conventional shotcrete, modi

fied concretes such as fiber-reinforced shotcrete, polymer

shotcrete, and silica fume shotcrete have been applied by

the air-blown or shotcrete method. In areas where cavita

tion is possible, additional trowel finishing may be neces

sary to achieve acceptable tolerances and smoothness of the

concrete.

1 0.2.8 Coatings-High-head erosion tests have been

conducted using both polyurethane and neoprene coatings

(Houghton et al. 1 978). Both coatings exhibited good resis

tance to abrasion and cavitation. The problem with flexible

coatings like these is their bond to the concrete surfaces.

Once an edge or a portion of the coating is tom from the

surface, the entire coating can be peeled off rather quickly

by hydraulic force.

1 0.2.9 Prep/aced-aggregate concrete-Preplaced-aggre

gate concrete, also referred to as prepacked concrete, is used

in the repair of large cavities and inaccessible areas. Clean,

well-graded coarse aggregate, generally of 0.5 to 1 . 5 in. ( 1 2

to 3 8 mm) maximum size, i s placed in the form. Neat cement

grout or a sanded grout, with or without admixtures, is then

pumped into the aggregate matrix through openings in the

bottom of the forms or through grout pipes embedded in

the aggregate. The grout is placed under pressure, and pres

sure is maintained until initial set. Concrete placed by this

method has a low volume change because of the point-to

point contact of the aggregate; there is high bond strength

to top bars for the same reason. The use of pozzolans,

water-reducing admixtures, and low water content is recom

mended to further reduce shrinkage and thermal volume

changes while maintaining the fluidity required for the grout

to completely fill the voids in the aggregate. Successful

installation is often difficult, and a mockup is recommended

before using. ACI Committee 706 (2005) provides details

and guidance for the use of prep laced-aggregate concrete.

10.2.10 Pipe inserts-For repair of small-diameter pipes,

many of the methods discussed in the previous sections of

this report are not applicable. A common construction prac

tice today is to obtain a jointless, structurally sound pipe

inside-a-pipe without excavating the existing unsound pipe.

One such method that has been used successfully is to insert

steel or plastic pipe inside the deteriorated concrete pipe and

then fill the annular space between the concrete and plastic

liner with grout. With the proper selection of material, a pipe

insert can provide a sound, chemically-resistant lining (U.S.

Department of Housing and Urban Development 1 985 ; U.S.

Army Corps of Engineers 1 995).

Another popular method is the installation of a resin-satu

rated fiberglass hose into the pipeline. The hose is inserted

into the pipeline using water pressure. After installation, the

hose is filled with hot water to initiate the chemical reaction

of the resin. The hardened resin forms a rigid pipe lining.

1 0.2.11 Linings-Tunnels, conduits, and pipes that have

surface damage due to abrasion erosion, bacterial action, or

chemical/acid attack can be protected from further damage

with a nonbonded, mechanically attached PVC lining.

Depending on the extent of the damage, some patching of

the concrete surface may be required before installation.

Carbon fiber-reinforced polymer (CFRP) has also been

an effective method for lining tunnels, conduits, and pipes.

CFRP systems were initially implemented for structural

repair of pipelines in 1 997 (Sleeper 2 0 1 0). Various struc

tural loading conditions can be designed for based on the

direction and number of layers of CFRP applied. Labora

tory studies have shown that some systems have excellent

cavitation resistant (Fyfe Company 2008). Inspection during

installation is recommended to ensure that substrate prepa

ration is adequate to obtain a good bond between the CFRP

and the concrete.

1 0.2.12 Aeration slots-The installation of an aeration

slot is not only a consideration in the design of a new facility

but often an appropriate remedial addition to a structure

experiencing cavitation erosion damage. Structural restora

tion and the addition of aeration slots have been used in the

repair of several structures . Refer to 6.3 for a more detailed

discussion ofthis method. The addition of aeration slots will

likely reduce the flow capacity of the structure significantly

because of the added volume of entrained air.

CHAPTER 11-REFERENCES

Committee documents are listed first by document number

and year of publication followed by authored documents

listed alphabetically.

A merican Concrete Institute

ACI 1 1 7- 1 0( 1 5)-Specification for Tolerances for

Concrete Construction and Materials and Commentary

ACI 20 1 . 1 R-08-Guide for Conducting a Visual Inspec-

tion of Concrete in Service

ACI 2 0 1 .2R- 1 6-Guide to Durable Concrete

ACI 207 . 1 R-05( 1 2)-Guide to Mass Concrete

ACI 207.3R-94(08)-Practices for Evaluation of Concrete

·· in Existing·Massive Structures for Service Conditions



ACI 2 1 2.3R-1 6-Report on Chemical Admixtures for

Concrete

ACI 22 1 . 1 R-98(08)-Report on Alkali-Aggregate

Reactivity

ACI 224R-0 1 (08)-Control of Cracking in Concrete

Structures

ACI 234R-06( 1 2)-Guide for the Use of Silica Fume in

Concrete

ACI 302. 1 R- 1 5-Guide for Concrete Floor and Slab

Construction

ACI 304R-00(09)-Guide for Measuring, Mixing, Trans-

porting, and Placing

ACI 308. 1 - 1 1-Specification for Curing Concrete

ACI 3 09R-05-Guide for Consolidation of Concrete

ACI 3 1 8- 1 4-Building Code Requirements for Structural

Concrete and Commentary

ACI 347R- 1 4-Guide to Formwork for Concrete

ACI 503 .2-92(03)-Standard Specification for Bonding

Plastic Concrete to Hardened Concrete with a Multi-Compo

nent Epoxy Adhesive

ACI 503 .4-92(03)-Standard Specification for Repairing

Concrete with Epoxy Mortars

ACI 506R- 1 6-Guide to Shotcrete

ACI 544. 1 R-96(09)-Report on Fiber Reinforced Concrete

ACI 546.2R-1 0-Guide to Underwater Repair of Concrete

ACI 548 . 1 R-09-Guide for Use of Polymers in Concrete

ASTM International

ASTM A240/A240M- 1 6-Standard Specification for

Chromium and Chromium-Nickel Stainless Steel Plate,

Sheet, and Strip for Pressure Vessels and for General

Applications

ASTM C 1 50/C 1 50M- 1 7-Standard Specification for

Portland Cement

ASTM C88 1 /C88 1 M- 1 5-Standard Specification for

Epoxy-Resin-Base Bonding Systems for Concrete

ASTM C 1 1 3 8M-1 2-Standard Test Method for Abrasion

Resistance of Concrete (Underwater Method)

ASTM C 1 1 57/C 1 1 57M- 1 1-Standard Performance Spec

ification for Hydraulic Cement

ASTM C 1 5 83M-1 3-Standard Test Method for Tensile

Strength of Concrete Surfaces and the Bond Strength or

Tensile Strength of Concrete Repair and Overlay Materials

by Direct Tension (Pull off Method)

ASTM C 1 600/C 1 600M- l l-Standard Specification for

Rapid Hardening Hydraulic Cement

ASTM D4258-05(20 12)-Standard Practice for Surface

Cleaning Concrete for Coating

International Concrete Repair Institute

ICRI 2 1 0.3R- 1 3-Guide for Using In-Situ Tensile Pulloff

Tests to Evaluate Bond of Concrete Surface Materials

ICRI 3 1 0.2-97-Selecting and Specifying Concrete

Surface Preparation for Sealers, Coatings, and Polymer

Overlays

Authored documents ACI Committee 503, 1 973, "Use of Epoxy Compounds

with Concrete," ACI Journal Proceedings , V. 70, No. 9,

Sept., pp. 6 1 4-648.

ACI Committee 706, 2005, "RAP-9 : Spall Repair by the

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F armington Hills, MI.

American Concrete Institute, 20 1 3 , Concrete Repair

Manual, fourth edition, American Concrete Institute, Farm

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American Concrete Pipe Association, 1 980, Concrete

Pipe Handbook, ACPA, Irving, TX.

Arndt, R. E. A., 198 1 , "Recent Advances in Cavitation

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ASCE-WPCF Joint Task Force, 1 982, "Gravity Sani

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Ball, J., 1 959, "Hydraulic Characteristics of Gate Slots,"

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8 1 - 1 14.

Barto j ay, K., 20 1 1 , "Buffalo Bill Powerplant Tailrace

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nical Memorandum," Technical Memorandum BBPP-

8 180-FEA-2001- 1 (MERL- 11-47), U.S. Bureau of Reclama

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Beichley, G. L., and King, D . L., 1 975, "Cavitation

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Hydraulics Division, V. 1 0 1 , No. 7, July, pp. 829-846.

Bhargava, J. K., 1 98 1 , "Polymer-Modified Concrete for

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Institute, Farmington Hills, Ml, pp. 205-2 1 8 .

Bicz6k, 1967, Concrete Corrosion and Concrete Protec

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Borden, R. C.; Colgate, D . ; Legas, J.; and Selander, C. E.,

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71-2 3, U.S. Bureau of Reclamation, Denver, May, 809 pp.

Burgi, P. H.; Moyes, B. M.; and Gamble, T. W., 1 984,

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Causey, F. E., 1985, "Preliminary Evaluation of a Test

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Colgate, D. , 1 977, "Cavitation Damage in Hydraulic Struc

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DePuy, G. W., and Dikeou, J. T., 1 973, "Development of

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American Concrete Institute, Farmington Hills, MI.

Falvey, H. T., 1 982, "Predicting Cavitation in Tunnel

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Falvey, H . T., 1 990, "Cavitation in Chutes and Spillways,"

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Fontana, J. J., and Bartholomew, J., 1 98 1 , "Use of Concrete

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Frizell, W., 2007, "Uplift and Crack Flow Resulting from

High Velocity Discharges over Open Offset Joints-Labora

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Fyfe Company, 2008, "Cavitation Resistance of PWC

Coating, Testing Performed at Metropolitan Water District

of Southern California," Test Report FTC-005, Fyfe Tech

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Galperin, R.; Oskolkov, A.; Seminkov, V.; and Tsedrov,

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Hamilton, W. S., 1 983b, "Preventing Cavitation Damage

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Hamilton, W. S. , 1 984, "Preventing Cavitation Damage to

Hydraulic Structures," International Water Power and Dam

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Hanna, L., 20 1 0, "Flow Deflectors for Mitigation of

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Report HL-201 0-03, U.S. Bureau of Reclamation, Denver,

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Haynes, W. M., ed., 2 0 1 6, CRC Handbook of Chemistry

and Physics, 97th edition, CRC Press, Boca Raton, FL,

2670 pp.

Hepler, T. E., and Johnson, P. L., 1 988, "Analysis of

Spillway Failures by Uplift Pressure," Proceedings of the

I 988 National Conference on Hydraulic Engineering, Amer

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Holland, T. C., 1 983, "Abrasion-Erosion Evaluation of

Concrete Mixtures for Stilling Basin Repairs, Kinzua Dam,

Pennsylvania," Miscellaneous Paper No. SL-83- 1 6, U.S.

Army Engineer Waterways Experiment Station, Vicksburg,

MS, 68 pp.

Holland, T. C., 1 986a, "Abrasion-Erosion Evaluation

of Concrete Mixtures for Repair of Low-Flow Channel,

Los Angeles River," Miscellaneous Paper SL-86- 1 2 , U.S.

Army Engineer Waterways Experiment Station, Vicksburg,

MS, 96 pp.

Holland, T. C. , 1 986b, "Abrasion-Erosion Evaluation of

Concrete Mixtures for Stilling Basin Repairs, Kinzua Dam,

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68 pp.

Holland, T. C . ; Krysa, A . ; Luther, M . D . ; and Liu, T. C . ,

1 9 86, "Use o f Silica-Fume Concrete t o Repair Erosion

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Silica Fume, Slag, and Natural Pozzolans in Concrete ,

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Holland, T. C., and Gutschow, R. A., 1 987, "Erosion

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Holland, T. C.; Husbands, T. B.; Buck, A. D.; and Wong,

G. S., 1 980, "Concrete Deterioration in Spillway Warm

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Houghton, D . L.; Borge, 0. E.; and Paxton, J. H., 1 978,

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Hurd, M. K., 2005, Formworkfor Concrete, SP-4, seventh

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Knapp, R. T.; Daily, J. W.; and Hammitt, F. G., 1 970,

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Liu, T. C., and McDonald, J. E. , 1 98 1 , "Abrasion

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McDonald, J. E., 1 980, "Maintenance and Preservation of

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U.S. Bureau of Reclamation, 20 14, "Design Standard No.

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As ACI begins its second century of advancing concrete knowledge, its original chartered purpose remains "to provide a comradeship in finding the best ways to do concrete work of all kinds and in spreading knowledge." In keeping with this purpose, ACI supports the following activities:

Technical committees that produce consensus reports, guides, specifications, and codes.

Spring and fall conventions to facilitate the work of its committees.

Educational seminars that disseminate reliable information on concrete.

Certification programs for personnel employed within the concrete industry.

Student programs such as scholarships, internships, and competitions.

Sponsoring and co-sponsoring international conferences and symposia.

Formal coordination with several international concrete related societies.

Periodicals: the ACI Structural Journal, Materials Journal, and Concrete International.

Benefits of membership include a subscription to Concrete International and to an ACI Journal. ACI members receive discounts of up to 40% on all ACI products and services, including documents, seminars and convention registration fees.

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