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 (regul
ar and ultrafine), five types of fibers, two fly ash contents, and two fibe
r contents. The samples were monitored under a lab drying condition for rat
e of water loss and time for crack initiation. Image analysis was used to m
easure the samples for maximum crack width and crack area, and a mercury-in
trusion porosimeter (MIP) was used to analyze the pore structure of selecte
d pastes. The results indicate that depending on the reactivity of cementit
ious 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 behav
ior of the paste. Depending on the fibers and their characteristics, the to
tal plastic shrinkage crack area can be reduced by 30 to 40% when 0.1% (by
volume) of fiber is used.