concrete durability
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CONCRETE DURABILITY
Seminar #2
Physical Properties and Causes of Deterioration of Construction Materials
90-553
by:
Alvin Olar
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OVERVIEW
Define durability Exposure conditions
Chemical and Physical
Durability factors w/c ratio and degree of hydration
Components and testing of mass transfer Mechanisms and tests of deterioration Recommendations and Q&A
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WHAT IS DURABILITY?
No material itself is durable or non-durable: it is the interaction of the material with its in-service environment that determines its durability. L. Masters
Durable concrete is concrete that will withstand the conditions for which it has been designed, without deterioration, over a period of years
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EXPOSURE CONDITIONS
Chemical attack: Leaching and efflorescence (P)
Sulfate attack (P)
Alkali-aggregate reaction (A)
Acids (P)
Corrosion of reinforcement (R)
A = Aggregate, P = Paste, R = Reinforcement
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EXPOSURE CONDITIONS
Physical attack: Freezing and thawing (P,A)
Wetting and drying (P)
Temperature Changes (P,A)
Wear and abrasion (P,A)
A = Aggregate, P = Paste, R = Reinforcement
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DETERIORATION
Can be caused by the adverse performance of the A, P, or R and can be due to chemical and/or physical causes listed
Common defects; Scaling / spalling / popouts
Cracking / disintegration
Efflorescence / incrustation
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DURABILITY FACTORS
Single parameter having the largest influence on durability is the W/C ratio W/C ratio dictates the porosity and permeability
of the paste Permeability determines the vulnerability of
concrete to external agencies,
in order to be durable, concrete must be relatively impervious
Lower W/C ratio increases the strength of concrete and resistance to internal stresses
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w/c = 0.475
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CAPILLARY PORES
W/C ratios > 0.38, all the cement can hydrate but capillary pores will also be present
Capillary pores represent that part of the gross volume not filled by the products of hydration
The volume of the capillary system is reduced with the progress of hydration
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CAPILLARY PORES
Interconnected Pores mainly responsible for the permeability of the
hardened cement paste and its vulnerability to deterioration
Discontinuous Pores as paste matures, the additional products of
hydration fill the water filled capillary voids producing discontinuity
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Cementitious Materials
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MASS TRANSFER
The permeability of concrete to liquids, gases, and ions is of direct relevance to durability
The rate at which water and aggressive agents penetrate into the pores of concrete is directly related to its resistance to both chemical and physical attack
Higher permeability results in quicker and more severe deterioration
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MASS TRANSFER
Three stages of moisture front propagation in concrete can be defined;
for partial vapor pressures below 0.45, before a meniscus is formed, the moisture movement is controlled by adsorption and surface diffusion
for partial vapor pressures between 0.45 and close to 1, moisture transfer is achieved through vapor diffusion and capillary tension
in saturated or nearly saturated material, moisture transfer is mainly due to laminar flow, controlled by viscosity and defined by Darcys Law
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ADSORPTION
At low relative vapor pressures (p/ps < 0.45) mass transfer of water vapor occurs by molecular migration rather than coherent flow
This type of flow does not contribute significantly to mass transfer of water through cement paste
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SURFACE DIFFUSION
Loosely bound top molecular layers of the adsorbed water may slide over the underlying layer, thus wetting bounding surfaces and hence causing mass transfer
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VAPOR DIFFUSION
At p/ps > 0.45 when a meniscus can form, vapor diffusion occurs in accordance with Ficks Law;
Q = Dp dp/dx
where Q = mass transport rateDp = diffusion coefficient
dp/dx = vapor pressure potential gradient
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VAPOR DIFFUSION
The continuous film of moisture in a partially saturated sample of hardened cement paste produces a short circuit allowing rapid moisture transfer without coherent flow
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BULK FLOW
Movement of liquid through an unsaturated of saturated matrix, or both
In unsaturated material with one end exposed to water the other to the atmosphere, capillary tension (absorption) creates mass transfer
Once pores are saturated, the curvature of the meniscus is zero and a pressure head is required to drive water through the matrix
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BULK FLOW
Pressure driven flow is governed by Darcys Law;
< = Kp *h/*lwhere < is the velocity of flow, *h is the head loss
over a flow path of length *l and Kp is the permeability coefficient
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PERMEABILITY TESTING
Gas Flow pressure induced gas flow follows Darcys Law
directly measure gas permeability through a field or laboratory specimen
Gas Diffusion measures the concentration of a chosen gas
(usually oxygen) across the sample, due to exposure at the upstream face
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PERMEABILITY TESTING
Water-Vapor Diffusion measures the movement of water vapor across
the sample driven by a difference in partial pressure between the upstream and downstream faces
placing a sample on a saturated sand bed with a desiccant on the downstream (top) face and periodically weighing the sample and desiccant
the diffusion coefficient is based on Ficks law
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PERMEABILITY TESTING
Absorption and Rate of Absorption measure the weight gain of a sample, volume of
water entering the sample, depth of penetration, or a combination thereof
ASTM C642 - Test Method for Specific Gravity, Absorption, and Voids in Hardened Concrete, is a popular test that gives reliable results
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r = (4::/Po) .d/%%tr = mean pore radius (m)
:= viscosity of water (Pa.s)
Po= atm. pressure (Pa)
d = depth of penetration (m)
t = time (s)
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PERMEABILITY TESTING
Water Penetration measuring the depth of penetration or volume
of water entering the sample when one end of the unsaturated sample is subjected to a pressure head In situ test
an in-situ test is the Figgs test a 10mm dia. by 40mm deep hole is drilled into the
surface being tested and the time to disperse a known volume of water is measured
Initial moisture conditions effect results
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PERMEABILITY TESTING
Pressure induced water flow through a saturated sample follows Darcys Law
The test is performed on a water-saturated specimen and involves subjecting one end of the sample to a pressure head
The measurement of outflow determines the permeability coefficient
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PERMEABILITY TESTING
Ionic Diffusion because of the extent of reinforcement
corrosion damage and freeze thaw deterioration due to de-icing and marine salts, the main concern with ionic diffusion is chloride ion migration into concrete
testing includes establishing chloride ion concentration profiles of in-situ concrete by analyzing cores at successive depths
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PERMEABILITY TESTING
use of a concrete slice as a membrane between two salt solutions
the ion concentration change on the lower concentration side is monitored and correlated to a diffusion coefficient using Ficks Law
may take months to establish steady state
ASTM C1202 - Test Method for Electrical Indication of Concretes Ability to Resist Chloride Ion Penetration
measure total charged passed in 6h across a sample
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PERMEABILITY TESTING
sandwiched between NaOH and NaCl solutions.
the driving force is a 60-V dc current
measures the effect of all dissolved ions in the concrete, without emphasis on a particular ion
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CHEMICAL ATTACK
Leaching and Efflorescence salts left behind on the surface by evaporation
of water or interaction with CO2 in the atmosphere leads to efflorescence
typical salts are sulfates and carbonates of Na, K, and Ca
an aesthetic rather than a durability problem, however, efflorescence indicates substantial leaching is occurring
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CHEMICAL ATTACK
Sulfate Attack External - caused by ingress of sulfate from
soils, groundwater, and acid rain
Internal - caused by extensive sulfate content of cement, aggregates, mineral or chemical admixtures, and mix water
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CHEMICAL ATTACK
External Sulfate Attack results in microstructural changes leading to
deterioration of mechanical properties
common type is MgSO4
C - S - H
Mg - S - H - has no cementing properties
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CHEMICAL ATTACK
Internal Sulfate Attack Delayed Ettringite Formation - DEF
caused by decomposition and reformation of ettringite due to improper curing procedures
reformed ettringite is expansive and results in the formation of peripheral cracks/gaps around the aggregate
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CHEMICAL ATTACK
Tests for Sulfate Attack the resistance of concrete to sulfate attack can
be tested in the laboratory by storing specimens in a solution of sodium or magnesium sulfate, or in a mixture of the two
the effects of exposure can be estimated by; the loss in strength
changes in dynamic modulus
its expansion
loss of weight
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CHEMICAL ATTACK
Alkali-Aggregate Reaction - AAR reactions taking place in the presence of water
between the alkalis in Portland cement and certain rocks and minerals containing active siliceous or carbonate materials
the reactions result in large volume increases producing cracking, spalling, and popouts
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CHEMICAL ATTACK
Tests for AAR ASTM C 227 - Test Method for Potential
Alkali Reactivity of Cement-Aggregate Combinations (Mortar-Bar Method)
measures physical reactivity of aggregate
ASTM C 289 - Test Method for Potential Alkali-Silica Reactivity o f Aggregates (Chemical Method)
measures the silica content of the aggregate
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CHEMICAL ATTACK
Acid Attack in damp conditions SO2, C02, and other acid
fumes present in the atmosphere attack concrete by dissolving and removing part of the set cement, leaving behind a soft, mushy mass
various physical and chemical tests on the resistance of concrete to acids have been developed, but there are no standard procedures
results should be carefully interpreted
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CHEMICAL ATTACK
Corrosion of Reinforcement for corrosion of steel embedded in concrete to
occur, the following conditions must all be met; the provision of an anode-cathode couple with at
least part of the steel acting as an anode
the maintenance of an electrical current
the presence of moisture
the presence of oxygen
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CHEMICAL ATTACK
High alkaline conditions present within concrete (ph of 12 to 12.5) cause a passive oxide film to form on the surface of the iron and prevent corrosion
Atmospheric Carbonation reduces the ph by converting calcium hydroxide in cement paste to calcium carbonate
Chloride ions have the ability to destroy the passive oxide film of steel at high alkalinities
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PHYSICAL ATTACK
Freezing and Thawing Porous materials containing moisture are
susceptible to damage under cycles of freezing and thawing (frost attack)
freeze-thaw depends on the moisture content and rate of replenishment
higher porosity means more freezable water
essential that substantial hydration takes place before exposure to frost - lower porosity and permeability
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PHYSICAL ATTACK
Mechanisms of frost attack 2 main dilating pressures exist - hydraulic and
diffusion pressures
Hydraulic pressure 9% increase in volume as water freezes
excess water in the cavity is expelled
hydraulic pressure developed depends on the resistance to flow- length of path and permeability of the paste between air void
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PHYSICAL ATTACK
if the concrete is at least 91% (critically) saturated hydraulic pressure can exist
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PHYSICAL ATTACK
Diffusion pressure caused by osmotic pressure brought about by
local increases in solute concentration due to the separation of frozen (pure) water from the solution
cations from deicing salts cause an increase in the number of pores occupied by adsorbed water and thus becoming osmotically active
deicing salts tend to accelerate deterioration
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PHYSICAL ATTACK
When dilating pressure in the concrete exceeds its tensile strength, damage occurs scaling, spalling, to complete disintegration
Freeze thaw deterioration is a function of the amount of adsorbed water relative to free void space
W/C ratio, degree of hydration, and moisture content
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PHYSICAL ATTACK
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PHYSICAL ATTACK
Tests for freeze thaw and salt scaling resistance ASTM C 666 - Test Method for Resistance of
Concrete to Rapid Freezing and Thawing A) Rapid freezing and thawing in water
B) Rapid freezing in air and thawing in water
cycle - 40oF - 0oF and 0oF to 40oF
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PHYSICAL ATTACK
Durability factor
(D.F.) = # of cycles x relative
at end of test dynamic modulus
# of cycles at which
exposure is to be determined
If the surface area of the cement paste can be determined the amount of adsorbed water can be determined
by nitrogen adsorption or X-ray scattering
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PHYSICAL ATTACK
ASTM C 457 - Test Method for Microscopical Determination of Parameters of the Air-Void System in Hardened Concrete
most significant parameter determined is the air void spacing factor (limit = 0.004 - 0.008in or 0.1 -0.2mm)
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PHYSICAL ATTACK
ASTM C 672 - Test Method for Scaling Resistance of Concrete Surfaces Exposed to Deicing Chemicals (ponding method)
sample is dried for 14 days at 23oC and 50% RH
test surface is covered with a salt solution and the specimen is submitted to a series of 50 daily freezing and thawing cycles
the deterioration is evaluated visually (drawback)
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PHYSICAL ATTACK
CSA CAN3 A23.1.2 - Brine Immersion Test sample is dried to a constant mass at 110oC
specimen is immersed in a 3% by mass NaCl solution for 24 hours
submitted to 50 daily cycles of freezing and thawing
weight loss of sample is monitored
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PHYSICAL ATTACK
Wetting and Drying deterioration due to wetting and drying cycling
is caused by volume changes and is an osmotic diffusion pressure dilation
depends on pore size, pore size distribution, and permeability - ions increase critical pore size
tests for total porosity, capillary rise, depth of penetration, and adsorption give indication of wetting and drying resistance
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PHYSICAL ATTACK
Temperature Changes; Differential coefficients of thermal expansion
between cement paste and aggregate may lead to cracking
Generally concretes with a higher coefficient of thermal expansion are less resistant to temperature changes
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Heated and cooled between 4 and 60oC at 2.2oC per minute
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PHYSICAL ATTACK
Wear and Abrasion; Under may circumstances, concrete surfaces are
subjected to wear - (pavements, sidewalks, hydraulic structures)
The test employed should match the cause of wear
Function of compressive strength of concrete and aggregate, and w/c ratio
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PHYSICAL ATTACK
ASTM C 779 - Test Method for Abrasion Resistance of Horizontal Concrete Surfaces
A) the revolving disk machine operates by sliding and scuffing of steel disks in conjunction with abrasive grit
B) the dressing wheel machine operates by impact and sliding friction of steel dressing wheels
C) the ball bearing machine operates by high contact stresses, impact, and sliding friction from steel balls
Measures depth of wear with respect to exposure time
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CONCLUSION
The mix proportions and degree of hydration play the largest role in concrete durability, irrespective of the type of attack
The rate of which moisture and ions penetrate into concrete give indication of its ability to withstand deterioration
Both chemical and physical attack has been cause for large amounts of damage
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RECOMMENDATIONS
Cement, aggregate, water, and admixtures are required to meet normal specifications for average use
W/C ratio should be not exceed 0.45 Good curing to obtain adequate hydration Air entraining should be used for severe
exposure Air drying after curing and before exposure
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RECOMMENDATIONS
Adequate drainage and run-off features in the design of concrete elements and structures
Q and A
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