Influence of pozzolan on the plastic shrinkage cracking of concrete

Concrete at its fresh state allows for the occurrence of plastic shrinkage cracks due to the excessive and rapid evaporation rate of water and the inability or lack of bleed water to replace the evaporating water. Plastic shrinkage cracks can occur in concrete slabs, pavements, and flatworks and has been known on occasion to penetrate the whole depth of a section. Unless such cracks are small and marrow, they could permit penetration of deleterious substances and weaken the structure if not controlled. Since the use of pozzolan in the concrete industry has been increasing due to its ability to improve concrete properties, then it would be possible that such materials could provide a means to control plastic shrinkage cracking in concrete. Pozzolan used as cement replacement were fly ash and pelletized silica fume. Fly ash contents of 0 to 50% and silica fume contents of 0, 3, and 5% were mixed with water/binder ratios of 0.30, 0.40, and 0.50. Specimens were prepared and tested in accordance with ASTM C 1579-06. Images of the specimens studied were taken and processed for crack quantification in terms of crack area, maximum crack width, and average crack width with the use of Image Analysis technique. The behavior of the cracks was also observed after 28 days of exposure to different curing conditions : moist curing by wet burlap and exposure to air curing conditions. Results show that with as the amount of fly ash as cement replacement was used, crack area decreased. However, maximum crack width and average crack width increased as fly ash content increased For silica fume concrete, crack area, maximum crack width, and average crack width decreased as the amount of silica fume increased. Both fly ash and silica fume concrete specimens had greater percent change in crack quantification values when exposed to moist cured conditions.