Effect of calcium nitrate healing microcapsules on concrete strength and air permeability

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Abstract
This study investigates the compressive and flexural strengths, flexural modulus and air permeability of concrete samples containing 0·75% by cement weight of modified calcium nitrate self-healing microcapsules. The compressive and flexural strengths of concrete mixes with microcapsules were determined before healing and compared with those of the control mix. Moreover, another set of samples were loaded up to 60% of their ultimate load and placed in a water bath to accelerate their healing. The samples' air permeability was measured before loading, after applying 60% of the ultimate load, and after 3 and 7 d of healing. The stress–strain curves were plotted to determine the flexural modulus. The results show that there is a significant statistical difference in the compressive strengths of mixes with and without microcapsules. However, the mix flexural strength did not show a significant statistical effect. Scanning electron micrograph images of the fracture surfaces of samples with microcapsules showed a good healing efficiency of calcium nitrate microcapsules. It was also found that the addition of microcapsules considerably decreased the samples' air permeability. Although the flexural moduli of samples containing microcapsules were found to be less than those of samples without microcapsules before healing, modulus recovery was reported after 7 d of self-healing.

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