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Understanding Concrete Overlays

Urban arterial Rural primary/interstate

Urban freeway/interstate Intersections Rural secondary roads

ACPA WEBINAR- SEPTEMBER 4, 2014

Typical Questions

• What are Concrete Overlays and their history?

• How often do you use Concrete Overlays?

• What functions do they serve? • What are the repair strategies

used prior to overlay placement? • When and where are concrete

overlays used? • What are the evaluation methods

used to determine if a pavement is a good candidate for an overlay?

11-2

Introduction

• Definition: Concrete Overlays are part of the

resurfacing family and are defined as a new or fresh surface on a existing pavement.

5-3

Types of Concrete Overlays

Concrete Overlays

Bonded Concrete Overlay of Concrete

Pavements

Bonded Concrete Overlay of

Asphalt Pavements

Bonded Concrete Overlay of Composite Pavements

Bonded Overlay System

Unbonded Concrete Overlay of Concrete

Pavements

Unbonded Concrete Overlay of

Asphalt Pavements

Unbonded Concrete Overlay of Composite Pavements

Unbonded Overlay System

Thinner Thicker

Bond is integral to design Old pavement is subbase

Bonded vs. Unbonded Overlay Considerations

Concrete Overlay

Bonded Unbonded

Purpose Primarily a preventive maintenance or minor rehabilitation strategy to improve surface characteristics and/or load-carrying capacity

Primarily a minor and major rehabilitation strategy

Condition of Existing Pavement

Pavements in good to fair structural condition or made into that condition.

The underlying pavement can be poor to deteriorated but must be, along with the base and/or subgrade, stable and uniform

Resulting Improvements to the Pavement

Long-term wearing surface added Surface defects eliminated Surface characteristics like

smoothness, friction, and/or noise improved

Load-carrying capacity added Pavement life extended

Load-carrying capacity restored and increased

Pavement life extended Surface defects eliminated Surface characteristics like

smoothness, friction, and/or noise improved

11-5

Benefits of Concrete Overlays

• Consistently provide cost-effective solutions.

• Can be constructed quickly and conveniently and are easy to maintain.

• Are easy to maintain. • Can serve, in and of themselves, as

complete preservation solution (preventive maintenance or minor rehabilitation).

• 11-6

Benefits of Concrete Overlays

• Concrete overlays cost effectively extends pavement life and improves both functional and structural characteristics.

• Are an effective means to enhance pavement sustainability by improving – surface reflectance (albedo), – increasing structural longevity, – enhancing surface profile stability, and

maintaining ride quality.

5-7

Asset Management Through Concrete Resurfacing

11-8

Preservation = Preventative Maintenance + Minor Rehabilitation

A VERY LONG History of Performance and Cost

Effectiveness

Number of Concrete Overlays from 1900 through 1969 and from 1970 through 2010

1/2

3/9

26/6 1/42

6/0

MA: 2/0

RI: 1/0

CT: 2/4

NJ: 1/3

DE: 2/2

MD: 3/5

0/5

1/11

1/2

16/85 10/13

32/277

4/22 4/12

6/10

47/12

4/24

1/10

13/25

0/3

14/19 0/14

1/1

17/9

7/3

0/8

7/12

4/21

1/8

4/40

2/2

1/1

1/2

9/28

1/16 6/9

3/1

3/2

5/30

0/15

11-10

• 1,152 concrete overlays in the U.S., dating from to 1901 through 2012

• Concrete overlays have been successfully constructed in 45 different states

Adoption of Bonded vs. Unbonded

Based on over 1,000 concrete overlays from NCHRP Synthesis 99, NCHRP Synthesis 204, and ACPA’s National Overlay Explorer

… but Bonded is Increasingly Common!

Historically, Mostly on Concrete

57%

41%

2%

… but More and More on Asphalt

7,000 450,0001,200,000

5,456,100

3,226,700

Total by 1993

Total by 1999

Total by 2004

2009 2010

Square Yards, Thin Overlays

Thin (< 6 in.) Concrete Overlays in the U.S.

Avg. Thickness: Bonded Overlays

Avg. Thickness: Unbonded Overlays

EVALUATION OF EXISITNG PAVEMENT TO DETERMINE IF CANDIDATE FOR

OVERLAY

5-18

Evaluation of Existing Pavement

• The evaluation of the existing pavement is paramount to determine if uniform support and movement control exists, or if it can be cost-effectively achieved.

• Premature overlay failure can be traced to some violation “picking the wrong project” to overlay.

11-19

Evaluation of Existing Pavement

• Will a bonded concrete overlay act as a monolithic unit with the underlying pavement?

• Or will an unbonded overlay be necessary to meet the same criteria but with the added burden of meeting critical elevation constraints?

• To have a successful overlay, the good and poor characteristics of the existing pavement must be understood.

11-20

Evaluations of Existing Pavements

• Evaluation is also used to determine: Required repairs where needed Establish the concrete overlay design

thickness When combined with an overlay can the

existing pavement help carry anticipated traffic as:

-an integrated part of the pavement (bonded) -or serve as a base or subbase (unbonded)

What are We Putting Overlays On?

• Age of Different Thickness Layers

• Estimate Remaining Life

• Mixture materials,

• Design & construction date and method,

• Performance Grades of Lifts (records)

• Type and Amount of Traffic Now and in the Future

• Pavement Management Records

• Desired Design Life

• Elevations and Grade Restrictions

Coring

• Layer confirmation • Layer thicknesses

– Variability – Minimum requirements

for thin overlays • Subsurface conditions

– Stripping – Delaminations

• Samples for laboratory testing – Material properties

Selecting the Appropriate Concrete Overlay Solution

5-24

• Desired level of service? • Desired pavement life? • What overlay will achieve

these objectives?

After Before

Bonded Concrete Overlay Over Asphalt

Before After

Bonded Concrete Overlay Over Composite

Concrete Overlay of Existing Asphalt or Composite in “Good” to “Fair” Condition

Pavement is structurally sound but has minor surface distresses such as potholes, block cracking, or random thermal cracking.

Pre-Overlay Question Can milling and minor spot repairs cost effectively solve deficiencies, bring the pavement to “Good Condition” and meet other constraints (i.e., vertical clearance, shoulders, safety rails, foreslopes, etc.) to allow for bonded overlay?

NO

Yes

Note: Concrete overlay thickness must be appropriately designed considering estimated traffic, desired design life, and budget

Bonded Concrete Overlay Over Asphalt Pavement

Concrete Unbonded Overlay of HMA or Composites

Before After

Concrete Overlay of Existing Asphalt or Composite in “Poor” or “Deteriorated” Condition

Pavement has measurable distresses such as alligator cracking, rutting, delamination, shoving, slippage, stripping, raveling thermal expansion, cracking and structural distresses.

Pre-Overlay Question Can milling and/or structural repairs cost effectively solve deficiencies, bring the existing pavement to a condition that will provide uniform support as a subbase, meet other constraints (i.e., vertical clearance, shoulders, safety rails, foreslopes, etc.), that allow for an unbonded overlay?

NO

Yes Unbonded Concrete Overlay Over Composite Asphalt Pavement

Stripping Delamination

Stripping

• Stripping occurs in the asphalt interlayer of an unbonded concrete overlay pavement when the interlayer contains trapped water and under repeated heavy truck traffic, results in the water stripping the asphalt binder from the aggregate.

•Prevention oDrainage of the interlayer oOpen-graded interlayer oAnti-strip additives oSeal Joints

11-30

31

Asphalt Pavements Which Are Not Candidates For An Unbonded Overlay

Recommended: Milled asphalt, lower grade, stabilize millings with cement and place concrete overlay :

• Following milling, repair any subgrade issues, and/or lower the profile of the sub grade to meet vertical clearances,

• Create a cement treated base by adding cement to the millings using full depth reclamation methods

• Place a concrete overlay. An options is to placed a concrete pavement over recycle asphalt material.

Mill Asphalt Surface (approx. 1” remains)

Lower Existing Subgrade

Existing Subbase

Portland Cement treated asphalt milling

4”-6”

3”-6” Concrete Overlays

Subdrain added if necessary

Before Overlay Overlay After 19 years service

Bonded Concrete Overlay of Concrete

2”–5” thickness

Pavement is structurally sound but has minor surface distresses such as random cracking, and joint spalling.

NO

Concrete Overlay Selection of Existing Concrete in “Good” to “Fair” Condition

Repairs Note: Concrete overlay thickness to be designed considering: estimated traffic, desired design life, and budget.

Bonded Concrete Overlay Over Concrete Pavement

Yes

Pre-Repair Question Can spot surface repairs and/or spot structural repairs cost effectively solve deficiencies, bring the pavement to “Good Condition,” and meet other constraints (i.e., vertical clearance?

Concrete Unbonded Overlay of Concrete

35

Concrete Overlay of Existing Concrete in “Poor” Condition

Pavement can exhibits significant surface deterioration and structural distresses

Pre-overlay Questions Can milling and/or structural repairs, retrofit subdrains, slab stabilization, etc. cost effectively providing uniform base?

NO

Yes Unbonded Concrete Overlay Over Concrete Pavement

Replaced deteriorated joints with lower quality concrete (lean concrete) with no joint sawing.

36

There are situations where an overlay is not appropriate. When this occurs, Reconstruction is an alternative.

Concrete Pavements Which Are Not Candidates For An Unbonded Overlay

• Recommended: In-place recycling of the existing pavement and use as a base for new pavement or shoulder material.

• Permissible in specific� situations: Rubblizing the existing concrete pavement to serve as a base for new concrete pavement.

• Not recommended: Crack-and-seat to serve as a base for new pavement

OVERLAY DESIGN

5-37

Concrete Overlay Design

11-38

• Today, we have data-driven methods to design major elements of concrete pavements oTraffic Loading (ESALs)

oCondition of Existing Pavement

oPre-overlay Repair and Crack Control

oThickness

oJoint Spacing Required Future Design Life of the Overlay

oLoad Transfer and Shoulders

oSubdrainage

How Are Pavements (and Overlays) Designed

Most Often Influence Cost & Selection of Projects

Cost

• Geometrics

• Thickness

• Joint Systems

• Materials

The Principal Factors of Concrete (Overlay) Pavement Design

Thickness Design

Unbonded:

Bonded:

Effects of AC Thickness

NA

Asphalt

Concrete

Comp.

Tension

NA

Asphalt

Concrete

Tension Comp.

x 2x

Guide for the Design of Concrete Overlays Using Existing Methodologies

• Not a new design procedure!

• Background of recommended overlay design techniques

• Detailed examples of how to use the existing design methodology

• Learn by example – then apply for your situation!

Example Software - Bonded Overlay Of Asphalt (BCOA)

Street Pave • StreetPave Software:

– Concrete Thickness Design

– Asphalt Thickness Design (Asphalt Institute)

– Life Cycle Cost Analysis • Updated IS184 Publication

1993 AASHTO Guide

• Based on mathematical models derived from empirical data collected during the AASHO Road Test in the late 1950’s.

• Procedure provides suitable bonded and unbonded concrete overlay designs.

• A number of agencies and State Departments of Transportation have developed custom software and spreadsheets to apply this procedure.

M-E PDG

• M-E PDG combines a mechanistic-based approach with field performance data in order to enable the engineer to confidently predict the performance of pavement systems

• Method adopts an integrated pavement design approach which allows

- Designer to determine the overlay thickness based on the interaction between the pavement geometry (slab size, shoulder type, load transfer, steel reinforcement)

- Support conditions, local climatic factors, and concrete material and support layer properties.

Bonded Concrete Overlay Software Programs

5-48

• ACPA-BCOA • BCOA-ME

• AASHTO 93 • AASHTO Ware

ME Design Guide

• ACPA-BCOA • BCOA-ME

Unbonded Concrete Overlay Software Programs

5-49

• AASHTO 93 • AASHTO Ware

ME Design Guide

• StreetPave

• AASHTO 93 • AASHTO Ware

ME Design Guide

• StreetPave

• AASHTO 93 • AASHTO Ware

ME Design Guide

• StreetPave

• Geometrics

• Thickness

• Joint Systems

• Materials

Most Often Influence Real-world Performance

PERFORMANCE

The Principal Factors of Concrete (Overlay) Pavement Design

Bonded on Asphalt Effects of Joint Spacing

10.0 ft 3.0 ft 3.0 ft 3.0 ft

Short Slabs Deflect Very little flexural stress

Standard Slabs Bend Higher flexural stress

Bonded on Asphalt Longitudinal Joint Layout

2 ft x 2 ft

3 ft x 3 ft

12 ft

6 ft x 6 ft

4 ft x 4 ft

12 ft

Out

er S

houl

der

Out

er S

houl

der

Traffic

Bonded over Concrete Coefficient of Thermal Expansion (CTE)

• Overlay CTE should be similar to underlying pavement • If not near the same, the overlay CTE should be lower

than existing pavement

• Key ☞ similar coarse aggregate type

Shear Shear Overlay

Existing Pavement

Tensile stress

11-53

Bonded Over Concrete Joint Design Full Depth Cut & Width of Cut

11-54

Mixture Design

• Use conventional concrete mixtures • Use high modulus synthetic structural fibers

under certain conditions • Well graded aggregates will reduce the water

and paste content • Maximum size aggregate should be one-third of

the overlay thickness

5-55

Thin Overlays Fiber-Reinforced Concrete

• Fiber reinforcement should be considered in any of the following situations: − The project has specific vertical

restrictions − If asphalt lift is very thin it may

not bond with the concrete − Subbase thickness and/or

condition is inadequate − Design thickness makes

conventional reinforcement difficult to use

Steel Synthetic Structural

11-56

OVERLAY CONSTRUCTION

5-57

Bonded on Asphalt Surface Cleaning

Power Sweeping

Water Blasting Not normally required

Air Blasting 11-58

PCC Placement and Finishing

• Same as conventional PCC paving

– Slipform

– Fixed form

• Avoid surface contamination

11-59

PCC Joint Sawing CRITICAL •Effective curing

•Timely joint sawing

11-60

Bonded over Asphalt/Composite Keys to Success

• Bonding is critical • Small square panels reduce curling,

warping, & shear stresses in bond (1.5 times thickness).

• Mill to remove surface distresses, or improve bonding.

• Be sure to leave at least 3” of HMA after milling.

• HMA surface temperature below 120 F before paving.

• Joints in the overlay should not be placed in wheel paths, if possible

• Application of curing compound is critical 11-61

Unbonded Over Asphalt/Composite Keys to Success

• Milling to eliminate surface distortions of 2 in. or more

• Complete repairs at isolated spots where structural integrity needs restoring

• Concrete patches in the existing pavement should be separated from the overlay

• Surface temperature of existing asphalt pavement should be maintained below 120ºF (48.9ºC) when placing overlay

• Partial bonding between the overlay and the existing asphalt pavement is acceptable and may even improve load-carrying capacity

11-62

Unbonded Overlay of Concrete Pavements: Keys to Success

• Full-depth repairs -only where structural integrity is lost at isolated spots.

• Separator layer (normally 1” asphalt or geotextile fabric)

• Use to restore structural capacity of the existing pavement and increase pavement life equivalent to full-depth pavement.

• Faulting of 3/8 in. or less in the existing concrete pavement • Shorter joint spacing helps minimize curling and warping stresses. • To not match joints with those of the underlying concrete pavement.

11-63

Spot Repairs for Unbonded Overlays of Concrete

Joint Patching

11-64

Separation Layer

• Required for good performance - Isolate overlay from existing

distress Prevent reflective cracking Prevent bonding/mechanical

interlocking - Provides a bidding cushion for

the unbonded overlay - A good drainage system to drain

the interlayer • Recommended separation layer

material: - 1 in HMA - Geotextile fabric

11-65

• Concrete Bonding is important • Concrete aggregate used in overlay should have thermal properties similar to that of existing pavement

• Matching joints with underlying pavement allows structure to move monolithically.

• Existing joints must be in fair condition or be repaired • Timing of joint sawing is important • Cut transverse joints full depth +1/2” and longitudinal joints at T/2.

• Width of transverse joint of overlay to be equal to or greater than underlying crack width of the existing pavement.

• Curing should be timely and adequate

Bonded Overlay on Concrete: Keys to Success

11-66

Surface Preparation for Bonded Overlay Bonding is Critical

Shotblaster Shot Blasted Pavement

Cleaning the Surface to Prepare for Bonding

• Sweeping surface followed by compressed air cleaning in front of the paver.

• Air blasting or water blasting is only necessary to remove material that cannot removed any other way.

• No standing water should not be on the surface prior to paving or de-bonding can occur.

Page 13

Stringless Paving for PCC Overlay

Traffic Shift

US 18 Single Lane Paving With Through Traffic

Pilot Car

Typically 10% loss of traffic after pilot car starts

Centerline Safety Wedge

Vertical and Horizontal Thickness Transitions

• Variable profile &/or cross slopes depths

• Paver follows string line or sensors

• Maintain surface ride quality

• Concrete yield

• Overlay depth (Minimum/average/maximum)

Overlay Costs

Where Should You Go From Here With PCC Overlays

• Consider PCC overlays as an alternative for every rehabilitation concept

• Establish agency PCC overlay goals

• Consider the two lane with closed road,& single lane with pilot car operation for each PCC overlay concept

• Standardize the PCC planning & design process

• Build & enjoy the performance of PCC overlays

Thank You National Concrete Pavement

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