Smart prestressing of concrete with shape memory alloy fibers

Concrete is currently the most important material in the building industry. However, it is very weak in tension,  compared to its strength in compression. To overcome this problem, prestressed concrete is usually used.  Prestressed concrete is plain concrete with reinforcement of steel, polymers or, in this case, shape memory alloys. The prestressing is usually introduced by applying tension to the reinforcement in the concrete members. Consequently, initial compressive stresses are transmitted to the concrete matrix; the application of permanent  compressive stress increases the apparent tensile strength of the concrete, since upon tensile loading, the compressive stresses must first be nullified.

In the case of thin walled members, and for small tension stresses, the use of shape memory alloy bars is not feasible. For this purpose, shape memory alloy fibers may be embedded in the cement matrix. The already stretched SMA fibers (with residual strain) are embedded in the concrete matrix at low temperature (which relates to the shape memory effect). Upon heating the composite, the shape memory alloy fibers regain their original shape, and compression stresses are transmitted to the concrete. This procedure of prestressing is referred to as activation.

In this study, a thermomechanically micromechanical model of prestressed concrete reinforced by shape memory alloys fibers is presented and examined for different reinforcement aspects. It was found that there is a strong relation between the activation temperature change and the behavior of the prestressed concrete. The relation between the fiber volume fraction and the composite response and the effect of the shape of the reinforcing fibers and residual strain orientations are examined in details.