Cracking Behaviour of Strain-Hardening Cementitious Composites (SHCC) Under Practical Creep Conditions

Strain-Hardening Cementitious Composites (SHCC) have become increasingly popular as a material which could be adopted for enhancing the durability of structures. Studies have shown that the cracks formed during short-term tensile loading in SHCCs can be controlled in the range of 50–100 µm as compared to conventional reinforced concrete, where cracks are designed to be around 200–300 µm at serviceability state. Thereby, applying SHCC as an additional protective layer (e.g. permanent formwork) on the concrete tensile face can effectively control the ingress of water and harmful chemicals due to the fine crack size, thus improving the durability of the structure. Although the crack widths are small under short-term loading, imposing a constant sustained load on SHCC over a longer period has shown to increase the crack size beyond acceptable levels (to over 200 µm). In real structures, however, the deformation is limited by design as well as load sharing between structural elements, therefore the crack patterns in such cases will follow a different propagation mechanism to that under constant loading tests. Specifically, for a reinforced concrete member with a SHCC layer on the surface, any tensile creep deformation in the SHCC will redistribute load to the steel reinforcement and/or concrete, whereby the stress on SHCC gradually reduces with deformation. This study aims to investigate the creep of SHCC under realistic conditions by the experimental simulation of different cases. A novel testing set-up is developed to impose a constant moment that is shared between reinforcement and an SHCC layer, so the ensuing crack formation and opening in SHCC under restrained creep conditions is studied. It was observed that the creep strain increase was predominantly due to widening of pre-existing cracks while the number of cracks stays the same. The crack widths increased by about 10 µm on average (20% increase) which is far less than the value reported for constant stress test and stabilised after 1000 h. Such a marginal increase would have a very low impact on the durability performance reported in short-term tests.