Presentation on theme: "SHRINKAGE AND CRACKING BEHAVIOR OF HPC USED FOR BRIDGE DECK OVERLAYS By Hasitha Seneviratne Iowa State University, 2013."— Presentation transcript:
SHRINKAGE AND CRACKING BEHAVIOR OF HPC USED FOR BRIDGE DECK OVERLAYS By Hasitha Seneviratne Iowa State University, 2013
Objective To examine the shrinkage and cracking potential of HPC concrete overlay mixes Different cements Supplementary materials Shrinkage cracking potential
Shrinkage and Cracking Behavior of HPC Used for Bridge Deck Overlays MaterialsProportions Experimental workModeling ε auto ε sh ε ring E, F c & F split ε sh Creep Stress analysis σ Split tensile σ t =E c ε sh σ t ring σ t creep Cracking Behavior
Types of shrinkage Chemical Autogenous Plastic Drying Effects of constituent materials Cementitious material Aggregates Admixtures Factors affecting restrained shrinkage behavior Creep prediction models B3 Modified NCHRP 496 model
Plastic shrinkage Chemical shrinkage pore Capillary Force Water evaporates Drying shrinkage Autogenous shrinkage
Factors affecting shrinkageInfluence on shrinkageReference Autogenous shrinkage Cement C 3 A, C 4 AF increases observed shrinkage Tazawa (1997) Fly AshReduces autogenous shrinkage Nakarai (2009) Slag No clear evidence of increasing or decreasing effect, depends on source and fineness Whiting (2000) Metakaolin Similar shrinkage to control up to 10% replacement, significant reduction at 15% replacement Brooks (2001) Free Shrinkage Cement Higher fineness cements increases shrinkage Deshpande (2007) Fly AshReduces free shrinkage Nakarai (2009) Slag Fineness influences performance. Fine ground slag reduces shrinkage Jianyong (2001), Miyazawa (2009) Metakaolin Both total and pure free shrinkage are reduced by metakaolin Brooks (2001)
Cement Type IP, I/II and I Fly Ash : Class C Fly Ash (Headwaters Resources) GGBFS (Holcim) Metakaolin : Davison Catalysts Coarse aggregates Crushed Limestone (2 gradations), Crushed Quartzite Fine aggregates – River Sand Admixtures Air Entraining Agent: Daravair 1000, Retarder: Daratard 17, Mid-range Water Reducer (MRWR): Mira 62, Standard Water Reducer (NRWR): WRDA 82
Test methods of concrete shrinkage Autogenous shrinkage (ASTM C157) Free Drying shrinkage (ASTM C157) Restrained ring shrinkage (ASTM C1581) Test methods of mechanical properties Elastic modulus (ASTM C469) Compressive strength (ASTM C39) Split tensile strength (ASTM C496)
Shrinkage displayed by cements were as follows Type IP < Type I/II < Type I Autogenous shrinkage has a high correlation to the amount of cementitious material Free shrinkage has a strong linear correlation to the mass loss Coarser coarse aggregate displayed lesser restrained shrinkage
Strength Parameters Concrete mixtures with supplementary cementitious material display late age strength development Elastic modulus is highly dependent on the amount of cementitious material used Fly ash improved the strength parameters Slag and combination of MK and fly ash had no significant impact on strength parameters Combination of fly ash and slag reduced the early age strength but the strength grew with time. Split tensile strength was greater with coarser aggregates while elastic modulus was greater with quartzite.
Mix Total cementitious material content/pcy Strain Rate α, (μstrain/day) Cracking time t r, (days) Stress Rate q, (psi/day) Average Stress Rate, S (psi/day) Rank ASTM C 1581 Cracking Potential Rating S1S2S3S1S2S3S1S2S Moderate-Low Moderate-Low Moderate-Low Moderate-High Moderate-High Moderate-High Moderate-High Moderate-High Moderate-High Moderate-High Moderate-Low
Mixes 4, 5 and 6 have high cracking potential, Mixes 1, 7, 8, 9 and 10 have medium cracking potential and Mixes 2, 3 and 11 have low cracking potential Mix No. σ free = E*ε free (psi) σ free /(1+φ), psi (σ free /1+φ)/F sp Cracking Potential Peak (σ ring /1+φ)/F s p, (psi/psi) Cracking Potential Average Stress Rate, S (psi/day) ASTM Cracking Potential Rating 14 day28day14 day28day14 day28dayRank Medium0.77Medium23.6Moderate-Low Low0.56Low19.675Moderate-Low Low0.55Low16.6Moderate-Low High1.00High31.9Moderate-High High0.85High24.9Moderate-High High0.89High37.3Moderate-High Medium0.69Medium35.6Moderate-High Medium0.77Medium24.5Moderate-High Low0.60Medium27.1Moderate-High Low0.76Medium27.7Moderate-High Medium0.52Low24.2Moderate-Low
Concrete mixes with high shrinkage values may not always crack first and it is the combined effect of shrinkage and mechanical properties (elastic modulus, creep, and strength) that determines concrete cracking potential. 20% fly ash which reduces shrinkage and 25% GGBFS which has little effect on the shrinkage and are recommended to be used in bridge deck overlay concrete either as singular replacements or in combination. Type I/II Cement may be preferred over Type I cement and Type IP is preferred over Type I/II cement for the consideration of the shrinkage cracking resistance. Type IP < Type I/II < Type I
Since free drying shrinkage and mass loss have a strong correlation, mass loss can be used as a good indicator for free drying shrinkage. Compressive strength is a good indicator to evaluate elastic modulus and split tensile strength. Controlling the paste volume in concrete to maintain minimum paste volume is highly recommended. Cautions shall be taken when total cementitious material content in concrete of over 700lb/ft 3 is used for bridge decks. Results of the finite element analysis reveals that the mixes would not display cracking within the 56 day period of study.