Control of Plastic Shrinkage Cracking of Concrete with Fibers

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CONTROL OF PLASTIC SHRINKAGE CRACKING OF CONCRETEWITH FIBERSAntoine E. NaamanUniversity of Michigan, Ann ArborProject Summary:The largest single application of synthetic fibers in concrete is in the~control of plastic shrinkage cracking which occurs during the first twenty four hours of age, mostly in slabs and similar restrained structures, while the concrete is still plastic or semi-plastic and before it has attained any significant strength. While numerous studies have dealt with prediction models for the tensile, bending, and fracture properties of fiber reinforced concrete, there is so far no "theoretical" explanation on why a small amount (0.1% to 0.3% by volume) of fine fibers helps control plastic shrinkage cracking in concrete. This study offers to provide a rational answer. An extensive experimental program is proposed and a limited analytical program suggested. The main objectives are: 1) to evaluate the effect of a number of fibers, covering a wide range of properties, on the plastic shrinkage cracking characteristics of concrete; 2) to understand the basic mechanisms by which fibers control plastic shrinkage cracking; 3) to develop an analytical model accounting for such mechanisms; 4) to carry out a parametric evaluation of the effectiveness of various fibers and fiber parameters; 5) to provide some recommendations on the most desirable fiber properties; and, 6) to recommend, for a given fiber, the minimum volume fraction of fibers needed to effectively control plastic shrinkage cracking under most adverse conditions. The results will help compare fiber properties and parameters that are important for control of plastic shrinkage cracking with those properties that are important for improving the mechanical properties of the composite. This study is expected to have a significant economic impact on the future use of fibers, particularly synthetic types, in concrete.