Shrinkage and shrinkage cracking behavior of cement systems containing ground limestone, fly ash, and lightweight synthetic particles

Brooks E Bucher, Purdue University

Abstract

Low water-to-cement (w/c) ratio concrete with a high paste content experiences significant autogenous shrinkage that may amplify the risk of earlyage cracking in restrained elements. Further, the high cement content of these materials increases the carbon-footprint and environmental impact of these materials due to energy consumption and CO2 emissions associated with cement production. This thesis introduces strategies to produce concrete which reduces or displays similar cracking propensity to traditional cement-based materials, although with a smaller environmental impact. This can be achieved by: (1) replacement of a portion of cement with limestone, (2) fundamentally altering the binder composition by incorporation of alternative binders such as fly ash, or (3) by replacing a portion of the natural aggregate in concrete with synthetic inclusions which reduce the risk of cracking. First, this study evaluates early-age and long-term volume changes in pastes and mortars made with cements interground with limestone; for a variety of curing times and exposure conditions. The results show mortars made using cements interground with up to 10% limestone (by mass replacement of cement) demonstrate similar or less early-age and long-term shrinkage as compared to plain cement systems. Limestone systems showed a similar or reduced risk of early-age cracking. Further studies were performed using fine, intermediate, and coarse limestone particles to examine the effects of limestone particle size and dilution (due to cement replacement by limestone) on the shrinkage and cracking response of pastes and mortars. Finally, the chemical shrinkage and heat of hydration were measured to evaluate whether the presence of limestone alters the rate of reaction in these materials. Second, this study evaluates the shrinkage and cracking behavior of cement-based materials that contain fly ash. Two types of fly ash were examined. The chemical shrinkage of mixtures containing up to 20% fly ash (mass replacement of cement) was measured to quantify the early-age reactivity of fly ash in binders. Autogenous and total bulk shrinkage due to drying was studied from the time of set up to 28 days. The volume change and cracking susceptibility results varied depending upon type of fly ash and replacement amount. Third, this research includes the study of concrete and mortars that contain low stiffness synthetic aggregate inclusions as a method to mitigate early-age cracking. Restrained and plastic shrinkage of concrete and mortar containing lightweight synthetic particles (LSP) as aggregate replacement are studied. The results indicate a reduction in the potential for cracking with increased replacement of LSP. Furthermore the addition of LSP reduces the susceptibility for plastic shrinkage cracking.

Degree

M.S.C.E.

Advisors

Weiss, Purdue University.

Subject Area

Civil engineering

COinS