The use of iron-based shape memory alloy (Fe-SMA), so-called "memory steel" elements for strengthening of existing concrete structures has high potential for being a competitive option to established methods. The prestressing function is obtained by activating the shape memory effect - that is, the ability of the material to return to its original shape after deformation, when heated. If the SMA element is restrained on a structure when heated, stresses will developed in the element and consequently also in the structure.
The underlying idea of the current investigation is to mount ribbed Fe-SMA bars into grooves in the cover of concrete elements with a cementitious mortar, which is called near-surface mounted (NSM) strengthening technique. In the current study, the bond behaviour of NSM Fe-SMA bars was investigated by means of pull-out tests. The aim was to study the influence of the groove dimensions and to find an optimal configuration, which could be used in a real application. The obtained average bond stress - slip behaviour can furthermore be implemented in analytical and numerical calculations.
Specimens consisted of concrete cubes, where grooves were cut into one surface and Fe-SMA bars were embedded to, on a bond-length of five times the bar diameter. The Fe-SMA bars were then subjected to tensile loading, i.e. pull out load. The design of the experiments was based on RILEM specifications. Besides pull-out load and end-slip, full-field deformations were measured with a 3D digital image-correlation system (DIC). The tests delivered satisfying results in terms of load capacity and slip. Two main failure modes were identified, which were either based on shearing of the mortar substrate adjacent to the ribs, or splitting of the cover and surrounding concrete. It was also found that the groove depth and therefore the cover thickness highly affect the bond capacity and mode of failure. The groove width did not influence either the load capacity or the slip significantly. In general, near-surface mounted Fe-SMA bars show sufficient bond performance for the use as a structural strengthening method.