Bonded Concrete Overlay (BCO) Training Module

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Bonded Concrete Overlay (BCO) Training Module

• TxDOT Research Project 0-4893
• Performance of Old Concrete Under Thin Overlays
• Center for Transportation Research
• The University of Texas at Austin

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Acknowledgement

• PC Charles Gaskin, P.E. (HOU)
• PD German Claros, Ph.D., P.E. (RTI)
• PA Joe Leidy, P.E. (CSTMP)
• Darlene Goehl, P.E. (BRY)
• Hua Chen, P.E. (CSTMP)

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Training Module Contents

• BCO Design Module
• BCO Construction Module
• BCO in Texas Lessons Learned

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Scope

• Primarily, continuously reinforced concrete
  pavement (CRCP) overlay on CRCP
• CRCP overlay on JCP is not fully covered.

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Bonded Concrete Overlay- Overview -

• Consists of concrete layer (2 to 8 inches) on top
  of an existing concrete surface.
• One of the most cost-effective way of enhancing
  structural capacity of under-designed pavements
• Specific steps are taken to bond the new concrete
  overlay to the existing concrete.
• Increases structural capacity of the pavement
  system by reducing deflections.

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Bonded Concrete Overlay (BCO) Design

• Currently, the AASHTO 1993 Guide is the most
  widely used design method for bonded overlay.

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AASHTO DESIGNRevisions in the 93 Guide

• Overlay Design was Completely Revised
• New Procedure consists of 7 Overlay Design
  Procedures
• Uses the Concept of Structural Deficiency
• Used for Structural Overlay Design

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Structural Deficiency Approach to Overlay Design
Original Capacity
Structural Capacity
Loads
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Pavement Evaluation for Overlay Design

• Functional Evaluation of Existing Pavement
• Surface Friction Problems/Polishing
• Use Diamond Grinding or Grooving to Restore Skid
  Resistance
• Surface Roughness
• Use CPR and Diamond Grinding.

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Overlay Type Feasibility

• Availability of Adequate Funds
• Construction Feasibility
• Traffic Control
• Materials and Equipment
• Climatic Conditions
• Construction Problems (noise, pollution,
  subsurface utilities, overhead clearance)
• Traffic Disruptions and User Delay Costs
• Required Future Design Life of the Overlay

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Important Considerations in Overlay Design

• Shoulders
• Existing PCC Slab Durability
• PCC Overlay Joints
• PCC Overlay Reinforcement
• PCC Overlays Bonding
• Overlay Design Reliability Level Overall
  Standard Deviation
• Pavement Widening
• Traffic Disruptions and User Delay Costs

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Important Considerations in Overlay Design
(contd)

• Existing Pavement Condition Future Traffic
• Pre-overlay Repair
• Recycling Existing Pavement (PCC AC)
• Overlay Materials

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AASHTO Bonded Concrete Overlay Design Procedure

• 1. Collect Existing Pavement Information.
• 2. Predict Future ESALs
• 3. Perform Condition Survey
• 4. Perform Deflection Testing (Recommended)
• 5. Perform Coring / Materials Testing
  (Recommended)
• 6. Determine Future Structural Capacity (TxDOT
  Design Procedures for New PCC Pavement)
• 7. Determine Existing Structural Capacity
• 8. Determine Overlay Structural Capacity
  and Thicknesses

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AASHTO OVERLAY DESIGN Procedure

• 1. Collect Existing Pavement Information
• Existing Slab or Layer Thicknesses
• Type of Load Transfer Mechanism
• Type of Shoulder
• Base/Subbase information
• Soils Information

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AASHTO OVERLAY DESIGN Procedure

• 2. Predict Future ESALs
• Predicted Future 18K ESAL's in the Design Lane
  over the Design Period
• Past ESAL's if the Remaining Life Method is used
  to determine Structural Capacity of the Existing
  Pavement

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AASHTO OVERLAY DESIGNLoadings
ESAL SELECTION
OVERLAY
EXISTING
TYPE
PAVEMENT
PCC or AC
JPCP or JRCP
Rigid
PCC or AC
CRCP
Rigid
PCC
AC
Rigid
PCC or AC
COMPOSITE
Rigid
Note Flexible ESALs 2/3 Rigid ESALs
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AASHTO OVERLAY DESIGN Procedure
3. Perform Condition Survey

• Number of punchouts per mile
• Number of deteriorated transverse cracks per mile
• Number of existing and new repairs prior to
  overlay per mile
• Presence and general severity of PCC durability
  problems (D-cracking or ASR)
• Evidence of pumping of fines or water

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AASHTO OVERLAY DESIGN Procedure

• 4. Perform Deflection Testing

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AASHTO OVERLAY DESIGN

• Nondestructive Deflection Testing (NDT)
• Estimate Effective k-value
• Examine Load Transfer Efficiency at Joints and
  Cracks
• Examine Resilient Modulus of Pavement Layers
• Quantify Variability Along the Project

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CRCP Deflections for Various Slab Thicknesses
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AASHTO OVERLAY DESIGN Procedure

• 5. Perform Coring Materials Testing
• The surveys and testing are used to estimate the
  in-situ material properties and the condition of
  the pavement and underlying layers.

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AASHTO OVERLAY DESIGN Procedure

• 6. Determine Future Structural Capacity
• Df Slab Thickness Required to Carry Future
  Traffic Loadings
• Use TxDOTs Pavement Design Procedures for New
  PCC Pavements

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Determination of Required Thickness for Future
Traffic

• Factors Required for Slab Thickness
• Serviceability (po, pt) Traffic (ESALs,
  E-18s) Load Transfer (J) Concrete Properties
  (Sc, Ec) Subgrade Strength (k, LS) Drainage
  (Cd) Reliability (R, So)

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AASHTO OVERLAY DESIGN Procedure

• 7. Determine Structural Capacity of Existing
  Pavement
• Deff Effective Slab Thickness of the Existing
  Pavement

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AASHTO OVERLAY DESIGN Structural Capacity
Determination

• Proper Evaluation of Existing Pavement is
  Essential to Selecting Appropriate Overlay Designs

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AASHTO OVERLAY DESIGN Structural Capacity
Determination

• Structural Capacity of Existing Pavement is
  evaluated by two methods
• 1. Visual Survey
• 2. Fatigue Damage Due to Traffic (Remaining
  Life Method)

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AASHTO OVERLAY DESIGN Structural Capacity
Determination

• A. Visual Survey
• Visual Survey - Deteriorated Transverse and
  Longitudinal Joints and Cracks Localized
  failing Areas Localized Punchouts in CRCP

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AASHTO OVERLAY DESIGN Structural Capacity
Determination

• B. Fatigue Damage Due to Traffic (Remaining Life)
• Uses Estimate of Past Traffic to Determine
  Existing Damage
• Remaining Life Determined from Past Traffic and
  Expected Future Traffic

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Effective Slab Thicknessby Visual Survey Method

• EFFECTIVE SLAB THICKNESS (Deff)
• Deff Fjc Fdur Ffat D
• Where
• Fjc Joints and Cracks Adjustment Factor Fdur
  Durability Adjustment Factor Ffat Fatigue
  Adjustment Factor D Thickness of Existing
  Slab, in.

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Bonded Concrete Overlay Joints Cracks
Adjustment Factor, (Fjc)

• Adjusts for PSI loss due to unrepaired joints,
  cracks, and other discontinuities
• Pavements with no D cracking or reactive
  aggregates
• Number of deteriorated transverse joints per mile
• Number of deteriorated transverse cracks per mile
• Number of existing expansion joints,
  exceptionally wide joints (gt1 in.), or AC
  full-depth patches
• Do not include joints or cracks with D cracking
  or reactive aggregate deterioration

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Bonded Concrete Overlay Joints Cracks
Adjustment Factor, (Fjc)
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Bonded Concrete Overlay

• THICKNESS DESIGN Dol Df - Deff
• Where
• Dol Required Slab Thickness of Overlay,
  in. Df Slab Thickness to Carry Future
  Traffic, in. Deff Thickness of Existing Slab,
  in.

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BCO Design Procedures
Thickness Needed for Future Traffic
(13-in)
Effective Thickness of Existing Pavement
(10-in 8-in)
Determine Overlay Thickness
(13 8 5 in for BCO)
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Bonded Concrete Overlay Joints Cracks
Adjustment Factor, (Fjc)
Fjc 1.0
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Bonded Concrete Overlay Durability Adjustment
Factor, (Fdur)

• Adjusts for PSI loss due to durability problems,
  such as D cracking and reactive aggregates
• 1.00 No durability problems
• 0.96-0.99 Durability cracking exists, no spalling
• 0.88-0.95 Substantial cracking, some spalling
• 0.80-0.87 Substantial cracking, Severe spalling

Fdur 1.0 (no durability problems)
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Bonded Concrete Overlay Fatigue Adjustment
Factor, (Ffat)

• Adjusts for PSI loss due to fatigue damage in the
  slab
• 0.97-1.00 Few Cracks / punchouts JPCP lt5
  Slabs cracked JRCP lt25 working
  cracks/mile CRCP lt 4 punchouts/mile
• 0.94-0.96 Significant cracking /
  punchouts JPCP 5-15 Slabs cracked JRCP
  26-75 working cracks/mile CRCP 4-12
  punchouts/mile
• 0.90-0.94 Extensive cracking /
  punchouts JPCP gt15 Slabs cracked JRCP
  gt75 working cracks/mile CRCP gt12 punchouts/mile

Ffat 0.97
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DETERMINATION OF EFFECTIVE SLAB THICKNESS
(Deff) Deff Fjc Fdur Ffat
D Where Fjc Joints and Cracks Adjustment
Factor Fdur Durability Adjustment Factor Ffat
Fatigue Adjustment Factor D Effective
Thickness of Existing Slab, in.
Deff 1.0 1.0 0.97 8 7.75-in
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Bonded Concrete Overlay
THICKNESS DESIGN Dol Df - Deff Where Dol
Required Slab Thickness of Overlay, in. Df
Slab Thickness to Carry Future Traffic, in. Deff
Thickness of Existing Slab, in.
Dol 12.5 7.75 4.75 in Use 5-in for BCO.
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Reinforcement

• The amount of longitudinal reinforcement about
  0.6 of concrete cross-sectional area.

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End of Design Module
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BCO Construction Module

• Material Selection
• Pre-overlay repair
• Surface Preparation
• Reinforcement
• Concrete Placement
• Finishing
• Curing

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Material Selection

• Concrete material properties in new layer are
  critical for the good performance of BCO.
• More specifically, coarse aggregate type is of
  utmost importance.
• Coarse aggregate with low CTE and modulus of
  elasticity is most desirable.

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Material Selection

• Fiber or no fiber?
• For thin BCO, up to 3-in., fibers appear to
  improve performance.
• For thicker BCO, fibers do not seem to help.

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Pre-Overlay Repair

• Severe distresses need to be repaired.
• Repair punchouts, wide open transverse
  construction joints, working cracks
• Do not repair shallow and medium spalling,
  non-working transverse and longitudinal cracks

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Surface Preparation

• Needed for good bond of new concrete to old
  concrete
• Good bond is essential to good long-term
  performance of BCO.
• Making surface texture rough enough to provide
  enhanced physical bonding
• However, loose materials need to be removed.

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Bonding

• One of the most critical element in BCO
  construction
• Poor construction practices might result in poor
  bonding and premature pavement distresses (PPD).

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Bonding- Factors -

• Soundness/texture and cleanliness of the existing
  pavement surface
• Concrete materials low coefficient of thermal
  expansion and modulus of elasticity
• Curing
• Location of steel reinforcement

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Surface Preparation

• Shotblasting
• Milling

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Shotblasting

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Shotblasting
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Milling
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Surface Cleaning
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Bonding- Factors -

• Bonding grout? - Do not use.
• Existing surface dry or wet? Keep it wet before
  the concrete placement.

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Reinforcement

• If thickness is more than 3 inches, provide
  longitudinal reinforcement.
• Vertical location of reinforcement
• - D lt 6-in near the bottom of overlay slab
• - D gt 5-in middle of the overlay slab

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Reinforcement

• The amount of longitudinal reinforcement about
  0.6 of concrete cross-sectional area
• Follow Item 360 requirements for splicing and
  staggering.

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Concrete Placement

• Follow Item 360 requirements for the following
  items
• - temperature restrictions
• - sawing timing requirements
• Requirements might be different from Item 360
• - strength
• - slump
• - sawing depth

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Concrete Placement
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Finishing

• Follow Item 360 requirements.
• Do not over-finish as it increases potential for
  segregation.
• Surface will be made rough later by carpet drag
  and tining. Therefore, the surface doesnt have
  to be slick.

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Curing

• Follow Item 360 requirements.
• Uniformity of curing is quite important.
• Poor curing will result in plastic shrinkage
  cracks and de-bonding, as well as poor durability
  of concrete.

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Non-uniform curing
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plastic shrinkage crack after 6 hours of concrete
placement
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Curing

• Curing is critical to the performance of BCO.
• Good curing keeps moisture and reduces volume
  changes in concrete due to drying shrinkage and
  temperature variations.
• Reduced volume changes at early ages provide
  concrete to develop bond strength prior to the
  development of bond stress.

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CTE and Drying Shrinkage Measurements
Vibrating Wire Gage
Insert VWG in conc. specimen
Small size specimen
Spray curing compound
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BCO in Texas- Lessons Learned -

• A number of BCO projects have been placed in
  Texas.
• Most of them have provided good performance.
• However, problems were experienced in one project.

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One BCO Project with Premature Distress

• Too high strength was required.
• Contractor used concrete with low water-cement
  ratio, resulting in rather dry concrete produced.
• Dry concrete did not have enough moisture to
  develop bond.

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One BCO Project with Premature Distress

• As long as concrete meets durability and strength
  requirements, it doesnt have to be super strong.
• Currently, there is no minimum requirement for
  water cement ratio. However, pay attention to
  water cement ratio.

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CRCP BCO on JCP

• Georgia DOT placed CRCP BCO on JCP in 1971 on IH
  75 southbound between Atlanta and Macon.
• It has provided excellent performance over 30
  years.

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Concluding Remarks

• BCO is one of the most cost-effective options to
  extend the life of structurally deficient
  pavement.
• Proper design, materials selection, pre-overlay
  repairs, and proper construction will result in
  good long-term pavement system.

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