Effect of Restraint in Plastic Shrinkage and Settlement of Repair Materials Based on Self-consolidating Concrete

Plastic shrinkage occurs in fresh concrete within few hours after mixing the concrete. When the plastic concrete is allowed to shrink freely, it never cracks. But, generally, removal of unsound material or damaged concrete is a part of repair operation. Thus usually the surface of the substrate is rough, the repair concrete will be under restrained condition, which will result in the development of tensile stresses in the plastic concrete and will create cracks on the surface of the concrete. Four types of repair materials were used in this work: plane selfconsolidating concrete (SSC), SCC containing silica fume (S.F.), SCC containing S.F. and latex (Styrene Butadiene Rubber), and SCC containing S.F., latex and fiber. To roughen the surface of the substrate base, dents with different degree, referring to restraint indices, RI, were used to provide restraints. RI is defined as the ratio of the surface area dents to smooth surface area of the slab. The panels were subjected to sever combination of wind, temperature and relative humidity. The test program includes: settlement strains, free and restrained shrinkage strains, evaporation and bleeding rates, and cracks characteristic measurements. The results showed that, while the rate of evaporation and bleeding affected the shrinkage properties, it was over shadowed by properties of the repair materials and restraints indices. The results also indicated that, for a given repair concrete, the restrained strain increasedas the R.I. increased. A significant improve was observed for the SCC with latex and fiber, exhibiting the lowest plastic settlement, shrinkage strains and crack area, and the highest time to the first crack.