Self-induced stress in highperformance concrete treated with a combination of shrinkage-reducing and expansive admixtures

The use of high-performance concrete (HPC) in reinforced concrete structures and various infrastructures has spectacularly increased in the last two decades. In fact, HPC offers a combination of mechanical performance and durability enhancement that cannot always be achieved by using conventional concrete materials. On the other hand, this type of concrete is known by its high sensitivity to early-age cracking as a result of the large magnitude of early-autogenous shrinkage exhibited which is strongly related to the two main requirements of HPC: a low water-tobinder mass ratio (w/b) and the inclusion of ultrafine supplemental cementitious materials such as silica fume. It has been proven that internal capillary stress induced during cement hydration reactions progress is the major cause of cracking and consequently, premature damage in HPC. Therefore, HPC requires a special curing practice and mix proportions, particularly at early-age when concrete is still weaker and not yet developed an adequate tensil e strength to counteract the internal tensile stresses exhibited. In this paper, self-stress induced during the evolution of autogenous shrinkage strains, especially at early-age was investigated on three different HPC types. Silica fume HPCs made with three different low w/b of 0.15, 0.23, and 0.30. Shrinkage reducing agent (SRA) and expansive additive (EXA) were combined and added to HPC mixtures to reduce autogenous shrinkage magnitude. The results have shown that the larger the magnitude of autogenous shrinkage developed, the higher the induced internal capillary stress. It has been found that for the reference concretes; more than 90% of the ultimate capillary tensile stress was developed during the first 24 hours. However, adding a combination of SRA and EXA results in a significant reduce of bath autogenous shrinkage and the induced stress. Furthermore, the use of a combination of SRA and EXA has revealed that the internal stress develops gradually compared to the fast development observed with the reference concrete mixtures. Moreover, the inclusion of SRA and EXA has resulted in a slight decrease of the compressive strength and young's modulus bath at early and later ages. It can be stated that using a proper content of this combination of (SRA+EXA) could result in a SF-HP concretes with very low-autogenous shrinkage even non-shrinking HPC and consequently, low tensile stress and low tendency to earl-age cracking which results in a durable concrete structure.