Cracking control of concretes modified with short AR-glass fibers at early age. Experimental results on standard concrete and SCC

At early ages (less than 24 h), cracking can occur in concrete because it can be subjected to dimensional changes, due to shrinkage, can generate loads which are greater than the low strength capacity of the material at this age. This is especially the case in members with highly exposed surfaces, such as floor slabs or precast panels.As any other cement based composite, Self Compacting Concrete (SCC) shrinks at an early age and can crack when shrinkage is restrained.One possible solution to reduce the impact of early age shrinkage on concrete durability is to include low volumetric fractions of short fibers in order to control crack growth. To evaluate the cracking control ability of Alkali Resistant (AR) glass fibers in standard concrete and SCC, an experimental program, developed in accordance with the AR-glass fiber producer, was conducted. Two different types of AR-glass dispersible fibers, two concrete compositions and several volumetric fractions of fiber have been studied.The experimental program included a mechanical characterization of the different concrete compositions (compression and flexural strength tests), free shrinkage tests, with and without air flow over the samples, and double restrained slab cracking tests (Kraai slab modified test).The results obtained show that the inclusion of low volumetric fractions of the two types of AR-glass fiber under study can control the cracking produced due to very early age shrinkage on both standard concrete and SCC in two different ways: reducing the total cracked area and the maximum length of the cracks. Although, a non-linear dependence of cracked area on AR-glass fiber amount was found. A microscopic study of the cracked surface confirms the favorable effect of the presence of dispersed AR-glass fibers on cracking control.When standard concrete and SCC results were compared, it was observed that, although SCC drying shrinkage was larger, standard concrete with a similar performance in the hardened state produced equivalent cracking area.