Transcript BCO Training Module - Library | Center for Transportation

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 underdesigned 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 DESIGN
Revisions 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 Capacity
Structural Deficiency Approach to
Overlay Design
Capacity after
Rehabilitation
Original
Capacity
Capacity
of Overlay
Effective Capacity
of Existing
Pavement
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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AASHTO Bonded Concrete
Overlay Design Procedure
1.
2.
3.
4.
5.
Collect Existing Pavement Information.
Predict Future ESALs
Perform Condition Survey
Perform Deflection Testing (Recommended)
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
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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 DESIGN
Loadings
EXISTING
PAVEMENT
OVERLAY
TYPE
ESAL
SELECTION
JPCP or JRCP
PCC or AC
Rigid
CRCP
PCC or AC
Rigid
AC
PCC
Rigid
COMPOSITE
PCC or AC
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 TxDOT’s 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 (S’c, 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
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 Thickness
by 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
(>1 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
Determine Overlay
Thickness
8-in)
(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
0.96-0.99
0.88-0.95
0.80-0.87
No durability problems
Durability cracking exists, no spalling
Substantial cracking, some spalling
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: <5% Slabs cracked
JRCP: <25 working cracks/mile
CRCP: < 4 punchouts/mile
Ffat = 0.97
• 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: >15% Slabs cracked
JRCP: >75 working cracks/mile
CRCP: >12 punchouts/mile
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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
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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 < 6-in : near the bottom of overlay slab
- D > 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
doesn’t 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
Small size specimen
Insert VWG in conc. 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 watercement 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 doesn’t 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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