Unrestrained early age shrinkage of concrete with polypropylene, PVA, and carbon fibers

Plastic shrinkage cracking due to restrained shrinkage is a primary problem that often occurs in concrete structures with a relatively large surface area, such as concrete walls, bridge decks, slabs, and overlays. These applications are susceptible to rapid changes in temperature and humidity resulting in high water evaporation and high potential for shrinkage cracking. Free shrinkage at early age is one of the most influential factors leading to tensile stresses in concrete structures. When the tensile stress is higher than the tensile strength, cracking occurs. The addition of micro-fibers in amounts as small as 0.1% by volume is an effective method to control plastic shrinkage cracking. However, the effect of fibers on the free shrinkage and water evaporation of concrete is not clearly understood. The main objective of the study described in this paper is to evaluate the influence of fibers on the free shrinkage and water evaporation of fiber reinforced cement composites during the first 24,h after mixing, with particular attention to fiber content, fiber bond, and fiber stiffness. Prismatic concrete specimens of 1000,mm in length and 100 x 60,mm in cross section were tested to measure their unrestrained shrinkage strain. Simultaneously, flat concrete specimens of dimensions 327 x 230 x 50,mm were tested to determine loss of water by evaporation. The tests were carried out under adverse environmental conditions known to encourage high shrinkage, namely high temperature, low relative humidity, and exposure to high volume and velocity of air flow. Three types of fibers, polypropylene, PVA, and carbon fibers were investigated at four different fiber volume fractions, namely: 0.1%, 0.2%, 0.3%, and 0.4%. Experimental results are presented and conclusions are drawn.