Influence of the curing period of encapsulated polyurethane precursor on the capillary water absorption of cracked mortar with selfhealing properties

Cracks serve as preferential pathways for aggressive substances (H2O, O2, Cl-, CO2, etc.) that reduce the durability and service life of steel reinforced cementitious materials. The implementation of an autonomous crack healing mechanism counts as a solution to this problem. A very promising way to achieve that goal consists of incorporating capsules filled with polyurethane (PU) precursor in the mortar/concrete. Upon crack occurrence the capsules break, the PU precursor flows into the crack, reacts with surrounding moisture and seals the crack again once hardened. Now, depending on the specific exposure conditions of the structure (very humid versus dry), the reaction kinetics of the PU can vary. In this research it has been investigated by means of neutron radiography whether the crack healing efficiency is the same regardless of the curing period, i.e., when having direct exposure to water upon release of the healing agent (after 15 min to 3 h) versus when the PU is first given enough time (at least 24 hours) to harden completely. Preferably, the healing agent works under all circumstances. However, it was found that the first scenario resulted in the highest resistance to capillary water absorption for singular cracks, 300 μm in width. Neutron radiography images and water profiles extracted from those images during a two-hour water uptake experiment clearly show that this was the case for both a high viscosity PU precursor that was developed in-house and a commercially available low viscosity PU-based healing agent.