Abstract
The paper presents a new healing system that uses pre-tensioned hybrid tendons to close cracks in cementitious structural elements. The tendons comprise an inner core, formed from aramid fibre ropes, and an outer sleeve made from a shape memory PET. During the manufacturing process, the inner core of a tendon is put into tension and the outer sleeve into compression, such that the tendon is in equilibrium. A set of tendons are then cast in a cementitious structural element and heat activated once cracking occurs. This triggers the shrinkage potential of the PET sleeve, which in turn releases the stored strain energy in the inner core. The tensile force thereby released applies a compressive force to the cementitious element, in which the tendons are embedded, that acts to close any cracks that have formed perpendicular to the axis of the tendons. Details of the component materials used to form the tendon are given along with the tendon manufacturing process. A set of experiments are then reported that explore the performance of three different tendon configurations in prismatic mortar beams. The results from these experiments show that the tendons can completely close 0.3 mm cracks in the mortar beams and act as effective reinforcement both before and after activation. A nonlinear hinge-based numerical model is also described, which is shown to be able to reproduce the experimental behaviour with reasonable accuracy. The model is used to help interpret the results of the experiments and, in particular, to explore the effects of slip at the tendon anchorages and the amount of prestress force that remains after activation. It is shown that, with two of the tendon configurations tested, over 75% of the prestress potential of the tendon remains after crack closure.
Highlights
It has long been known that cracking can reduce the service life of concrete structures and such cracking remains a problem, even in new structures [19]
It may be seen that the final stress in the Kevlar core was, in all cases, well below the strength of the ma terial, i.e.740 MPa (See Table 3)
The tendons act as effective reinforcement both before and after activation and, when the relative core area is 0.6% or greater, all but eliminates post-crack softening behaviour even before the tendons are activated
Summary
It has long been known that cracking can reduce the service life of concrete structures and such cracking remains a problem, even in new structures [19]. A potential solution to this problem is to introduce engineered (autonomic) self-repair mechanisms into structural ele ments, which are able to heal cracks as they form [31]. A much fuller ac count of previous work on self-healing cementitious materials may be found in several literature review articles [60,11,18,51,65]. A very different approach to crack ‘healing’ is to embed shape memory bars or tendons into structural elements that, when activated, release a shrinkage potential that provides a crack-closure mechanism. Some investigators [48,30] have used shape memory alloy (SMA) bars for this purpose, sometimes in combination with fibre-reinforced poly mers [3,33,64,66], whilst others have employed shape memory polymer (SMP) tendons [26,16,25,21,56,57]
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