Feasibility of concrete prestressed by shape memory alloy short fibers

Abstract

Mortar and concrete exhibit low tensile strengths. Hence, cracks develop easily due to shrinkage and external actions. They can be prevented by applying prestress, thus obtaining crack-free products. Such products exhibit a high bending and tensile strength, are leak proof and of high durability. Prestress can be realized using external or internal wires or cables. In thin walled products, however, this is not feasible. For this purpose, short fibers of shape memory alloy (SMA) wires were embedded in mortar. The wires had been shaped by inelastic elongation into loop-and star-shaped fibers. After hardening of the mortar, the specimens were heated up in order to activate the tensile stress in the fibers, thereby causing a prestress of the surrounding mortar. The effect was monitored by length measurements both on specimens with and without fibers. Compression stresses in the cement mortar were estimated by multiplying the difference in strain between fiber-reinforced and reference prisms by the Young's modulus. Thus, compression of some 7 MPa was reached in the experiments. For practical applications, alloys with suitable temperature domains of austenitic and martensitic transformation, most likely Fe-based, and efficient methods for the production of such fiber mortars are to be developed.