Effect of randomly distributed fibers on plastic shrinkage cracking of cement composites.

Plastic shrinkage cracking due to restrained shrinkage takes place within the first few hours after the concrete is placed and before it attains any significant strength. These premature cracks become critical weak points which allow aggressive substances from surroundings to penetrate into the internal portion of concrete leading to the acceleration of other detrimental forms of concrete deterioration. Consequently, the performance, serviceability, durability, and aesthetic of concrete structures are reduced. Controlling plastic shrinkage cracking is thus essential for developing more durable concrete structures at a minimum life-cycle cost. The main objectives of this study are (1) to develop durable fiber reinforced cementitious materials which are able to withstand restrained shrinkage tensile stresses under adverse environmental conditions, (2) to understand the mechanisms of fibers involved in controlling plastic shrinkage cracking, and (3) to provide some design recommendations for controlling plastic shrinkage cracking. To achieve the main objectives, the study consists of three main phases. The first phase includes an extensive experimental evaluation of the effect of randomly distributed fibers on controlling plastic shrinkage cracking. Four synthetic fibers; polypropylene, PVA, Spectra, and carbon, and one metallic fiber; Fibraflex, were investigated at volume fractions varied from 0.05% to 0.4%. The second phase involves the influence of randomly distributed fibers on properties influenced by plastic shrinkage cracking, namely, compressive strength, tensile strength, bond strength, free unrestrained shrinkage, and water evaporation. The third phase focuses on developing design charts using a statistical approach and recommendations based on the experimental findings. The most important parameters were found to be the volume fraction of fibers and their diameter. It was observed that, for all practical purposes, the addition of any fiber of diameter smaller than 40 microns, with aspect ratio about 200, and volume fractions exceeding 0.4% should effectively prevent plastic shrinkage cracking in concrete. Moreover fibers with poor bond (polypropylene and carbon fibers) are as effective as fibers with high bond (PVA fiber) in minimizing plastic shrinkage cracking. Generally all fibers had little influence on free shrinkage and water evaporation. It was concluded that, bond strength of fibers, water evaporation, and free shrinkage reduction do not seem to be key factors in the control of plastic shrinkage cracking. Finally, design recommendations for plastic shrinkage cracking control are provided.PhDApplied SciencesCivil engineeringUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttp://deepblue.lib.umich.edu/bitstream/2027.42/124980/2/3163971.pd