Effect of PVA, glass and metallic fibers, and of an expansive admixture on the cracking tendency of ultrahigh strength mortar

Abstract Ultrahigh strength mortars (UHSM) are new materials with high compressive and tensile strengths. For such materials, autogenous shrinkage can sometimes induces cracks in restrained conditions without any external loading. In order to determine if a UHSM has a cracking tendency, the knowledge of the autogenous shrinkage, Young's modulus and tensile strength is not sufficient. The relaxation of the specimen must also be taking into account. Therefore, a cracking testing bench has been set up. It measures continuously the stress needed to maintain the strain in the range of ±4 μm/m. The ratio between induced tensile stress and direct tensile strength, both aged in restrained conditions, is a good indicator of the cracking tendency. Different possibilities to reduce the cracking tendency have been investigated: PVA fibers, glass fibers, metallic fibers and an expansive admixture. Results show that two ways are interesting to reduce the cracking tendency: reducing restrained tensile stress and/or increasing tensile strength. The first can be obtained by using moderate dosage of expansive admixtures and/or glass fibers, the second by using high dosage of metallic fibers.