Plastic shrinkage cracking in concrete materials - Influence of fly ash and fibers

In this paper, the study of plastic shrinkage behavior in a cement-fly ash-fiber system is described. The influences of fly ash and fibers on concrete pore structure as well as the influences of the pore structure on concrete plastic shrinkage cracking were investigated. Samples of cement paste were prepared with four water-binder ratios (w/b), two types of fly ash (regular and ultrafine), five types of fibers, two fly ash contents, and two fiber contents. The samples were monitored under a lab drying condition for rate of water loss and time for crack initiation. Image analysis was used to measure the samples for maximum crack width and crack area, and a mercury-intrusion porosimeter (MIP) was used to analyze the pore structure of selected pastes. The results indicate that depending on the reactivity of cementitious materials, adding fly ash influences paste pore structure and plastic shrinkage behavior in different ways. Adding fibers generally increases the amount of large pores in cement paste, thus Influencing the bleeding behavior of the paste. Depending on the fibers and their characteristics, the total plastic shrinkage crack area can be reduced by 30 to 40% when 0.1% (by volume) of fiber is used.