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1 Bonded Concrete Overlay (BCO) Training Module TxDOT Research Project 0-4893 “Performance of Old Concrete Under Thin Overlays” Center for Transportation.

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Presentation on theme: "1 Bonded Concrete Overlay (BCO) Training Module TxDOT Research Project 0-4893 “Performance of Old Concrete Under Thin Overlays” Center for Transportation."— Presentation transcript:

1 1 Bonded Concrete Overlay (BCO) Training Module TxDOT Research Project “Performance of Old Concrete Under Thin Overlays” Center for Transportation Research The University of Texas at Austin 1

2 2 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) 2

3 3 Training Module Contents BCO Design Module BCO Construction Module BCO in Texas – Lessons Learned 3

4 4 Scope Primarily, continuously reinforced concrete pavement (CRCP) overlay on CRCP CRCP overlay on JCP is not fully covered. 4

5 5 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. 5

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8 8 Bonded Concrete Overlay (BCO) Design Currently, the AASHTO 1993 Guide is the most widely used design method for bonded overlay. 8

9 9 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 9

10 10 Structural Deficiency Approach to Overlay Design Structural Capacity Loads Original Capacity Capacity after Rehabilitation Effective Capacity of Existing Pavement Capacity of Overlay 10

11 11 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. 11

12 12 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 Cost s Required Future Design Life of the Overlay 12

13 13 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 13

14 14 Important Considerations in Overlay Design (cont’d) Existing Pavement Condition & Future Traffic Pre-overlay Repair Recycling Existing Pavement (PCC & AC) Overlay Materials 14

15 15 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 15

16 16 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 16

17 17 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 17

18 18 AASHTO OVERLAY DESIGN Loadings OVERLAY TYPE PCC or AC PCC PCC or AC EXISTING PAVEMENT JPCP or JRCP CRCP AC COMPOSITE Rigid Note: Flexible ESALs 2/3 Rigid ESALs ESAL SELECTION 18

19 19 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 AASHTO OVERLAY DESIGN Procedure 3.Perform Condition Survey

20 20 AASHTO OVERLAY DESIGN Procedure 4.Perform Deflection Testing 20

21 21 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 AASHTO OVERLAY DESIGN 21

22 22 CRCP Deflections for Various Slab Thicknesses

23 23 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. 23

24 24 AASHTO OVERLAY DESIGN Procedure 6.Determine Future Structural Capacity D f = Slab Thickness Required to Carry Future Traffic Loadings Use TxDOT’s Pavement Design Procedures for New PCC Pavements 24

25 25 Determination of Required Thickness for Future Traffic Factors Required for Slab Thickness Serviceability (p o, p t ) Traffic (ESALs, E-18s) Load Transfer (J) Concrete Properties (S’ c, E c ) Subgrade Strength (k, LS) Drainage (C d ) Reliability (R, S o ) 25

26 26 AASHTO OVERLAY DESIGN Procedure 7.Determine Structural Capacity of Existing Pavement D eff = Effective Slab Thickness of the Existing Pavement 26

27 27 AASHTO OVERLAY DESIGN Structural Capacity Determination Proper Evaluation of Existing Pavement is Essential to Selecting Appropriate Overlay Designs 27

28 28 Structural Capacity of Existing Pavement is evaluated by two methods: 1.Visual Survey 2.Fatigue Damage Due to Traffic (Remaining Life Method) AASHTO OVERLAY DESIGN Structural Capacity Determination 28

29 29 A. Visual Survey Visual Survey - Deteriorated Transverse and Longitudinal Joints and Cracks Localized failing Areas Localized Punchouts in CRCP AASHTO OVERLAY DESIGN Structural Capacity Determination 29

30 30 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 30

31 EFFECTIVE SLAB THICKNESS (D eff ) D eff = F jc * F dur * F fat * D Where F jc = Joints and Cracks Adjustment Factor F dur =Durability Adjustment Factor F fat = Fatigue Adjustment Factor D = Thickness of Existing Slab, in. Effective Slab Thickness by Visual Survey Method 31

32 Bonded Concrete Overlay Joints & Cracks Adjustment Factor, (F jc ) 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 32

33 Bonded Concrete Overlay Joints & Cracks Adjustment Factor, (F jc ) 33

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36 Bonded Concrete Overlay THICKNESS DESIGN D ol = D f - D eff Where D ol = Required Slab Thickness of Overlay, in. D f = Slab Thickness to Carry Future Traffic, in. D eff = Thickness of Existing Slab, in. 36

37 BCO Design Procedures Thickness Needed for Future Traffic Effective Thickness of Existing Pavement Determine Overlay Thickness (13-in) (10-in 8-in) (13 – 8 = 5 in for BCO) 37

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43 43 Bonded Concrete Overlay Joints & Cracks Adjustment Factor, (F jc ) F jc = 1.0

44 44 Bonded Concrete Overlay Durability Adjustment Factor, (F dur ) Adjusts for PSI loss due to durability problems, such as “D” cracking and reactive aggregates 1.00No durability problems Durability cracking exists, no spalling Substantial cracking, some spalling Substantial cracking, Severe spalling F dur = 1.0 (no durability problems)

45 45 Bonded Concrete Overlay Fatigue Adjustment Factor, (F fat ) Adjusts for PSI loss due to fatigue damage in the slab Few Cracks / punchouts JPCP: <5% Slabs cracked JRCP: <25 working cracks/mile CRCP: < 4 punchouts/mile Significant cracking / punchouts JPCP: 5-15% Slabs cracked JRCP: working cracks/mile CRCP: 4-12 punchouts/mile Extensive cracking / punchouts JPCP: >15% Slabs cracked JRCP: >75 working cracks/mile CRCP: >12 punchouts/mile F fat = 0.97

46 46 DETERMINATION OF EFFECTIVE SLAB THICKNESS (D eff ) D eff = F jc * F dur * F fat * D Where F jc = Joints and Cracks Adjustment Factor F dur =Durability Adjustment Factor F fat = Fatigue Adjustment Factor D = Effective Thickness of Existing Slab, in. D eff = 1.0 * 1.0 * 0.97 * 8 = 7.75-in

47 47 Bonded Concrete Overlay THICKNESS DESIGN D ol = D f - D eff Where D ol = Required Slab Thickness of Overlay, in. D f = Slab Thickness to Carry Future Traffic, in. D eff = Thickness of Existing Slab, in. D ol = 12.5 – 7.75 = 4.75 in: Use 5-in for BCO.

48 48 Reinforcement The amount of longitudinal reinforcement: about 0.6 % of concrete cross-sectional area. 48

49 49

50 50 End of Design Module 50

51 51 BCO Construction Module Material Selection Pre-overlay repair Surface Preparation Reinforcement Concrete Placement Finishing Curing 51

52 52 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. 52

53 53 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. 53

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55 55 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 55

56 56 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. 56

57 57 Bonding One of the most critical element in BCO construction Poor construction practices might result in poor bonding and premature pavement distresses (PPD). 57

58 58 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 58

59 59 Surface Preparation Shotblasting Milling 59

60 60 Shotblasting 60

61 61 Shotblasting

62 62 Milling 62

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65 65 Surface Cleaning 65

66 66 Bonding - Factors - Bonding grout? - Do not use. Existing surface dry or wet? – Keep it wet before the concrete placement. 66

67 67 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 67

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69 69 Reinforcement The amount of longitudinal reinforcement: about 0.6 % of concrete cross-sectional area Follow Item 360 requirements for splicing and staggering. 69

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71 71 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 71

72 72 Concrete Placement 72

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76 76 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. 76

77 77 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. 77

78 78

79 79 Non-uniform curing 79

80 80 plastic shrinkage crack after 6 hours of concrete placement 80

81 81 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. 81

82 82 Vibrating Wire Gage Insert VWG in conc. specimen Small size specimenSpray curing compound CTE and Drying Shrinkage Measurements

83 83

84 84 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. 84

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86 86 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. 86

87 87 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. 87

88 88 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. 88

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90 90 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. 90


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