Shrinkage and cracking behavior of HPC used for bridge deck overlays

High Performance Concrete (HPC) overlays have been used as a cheap and effective method of repair for bridge deck structures from wearing from traffic loadings. These HPC mixes usually consist of high amounts of cementitious materials and tend to have high tendency towards cracking induced by shrinkage. Accelerated corrosion of reinforcing steel and deterioration of deck surface are potential threats in bridge decks where cracks have occured. Cracking potential of a mix under restraint is currently evaluated by ASTM C1581. The method looks into the rate at which the strain develops in a restrained condition to evaluate cracking potential. But a mix that develops shrinkage at a high rate may also develop strength at a higher rate, compensating the potential to cracking. This study involves investigating the use of simpler shrinkage measurements and strength characteristics to determine the cracking potential of a concrete mix. For this investigation 11 HPC mixtures selected by the Iowa DOT which were composed of 3 cements, Type I, I/II and IP. Supplementary cementitious materials class c fly ash, slag and metakaolin were replaced by 20%, 15% and 5.6% respectively. Limestone coarse aggregate was used in 10 mixes and 1 mix with quartzite. Two gradations of coarse aggregate were used for limestone aggregates. The HPC mixes were investigated for free drying, restrained ring, elastic modulus, compressive and split tensile strength for a duration of 28 days. Average creep coefficient was calculated using the B-3 and AASHTO Report 496 (2009) models. Restrained shrinkage and elastic modulus measured was used to calculate induced stress in full restraint which was then adjusted for creep. The stress calculated the restrained specimens were compared to the split tensile stress developed in time to check whether the stress level was above or below the strength of the mix. The results obtained showed close relationship to observed cracking in ring specimens and split tensile strength. Stress induced by free drying shrinkage under restrained conditions and restrained shrinkage samples showed a good correlation. This aids us in obtaining an understanding of restrained shrinkage through measuring free drying shrinkage, which is a relatively simple experiment to perform