Abstract
The use of encapsulating healing agents that allow the self-healing of concrete has emerged as a potential solution to the current decay and costly maintenance of concrete infrastructure; however, many healing agents are expensive. The objectives of this study were to develop a procedure for the production of urea–formaldehyde microcapsules of calcium nitrate and to evaluate the effects of the microcapsules on self-healing efficiency in concrete. Calcium nitrate was selected for microencapsulation, given its low cost and its effect on acceleration of the setting of unhydrated cement. The results indicated that the agitation rate and the temperature had a linear correlation with the microcapsule diameter and the shell wall thickness, respectively. A higher agitation rate resulted in a smaller microcapsule diameter, whereas a higher temperature resulted in a thinner shell wall. The morphology of all microcapsules synthesized was virtually the same throughout the experimental matrix, with the shell walls of all the microcapsules observed having a smooth exterior surface and a rough interior surface. All microcapsules produced were also observed to have a broad particle size distribution. This characteristic was attributed to the type of surfactant used in the process. Analysis of the effects of the microcapsules on the self-healing efficiency of concrete showed that the modulus of elasticity increased after healing for all concrete specimens prepared with self-healing agents. The largest increase in the modulus of elasticity was observed at a microcapsule content of 0.50%. The results also showed that concrete specimens into which microcapsules were incorporated at any concentration had greater surface resistivities than control specimens.