Study of the self-healing behaviour of early age cracks in concrete with crystalline admixtures under six environmental exposures

The search for self-healing materials is justified by the increasing sustainability and safety needs of structures. The presence of small cracks in structural concrete, which could accelerate the degradation process and diminish its service life, is unavoidable due to the heterogeneous nature of the material. A method to enhance self-healing in concrete is the use of crystalline admixtures (CA), which are hydrophilic products formed by chemical active substances, cement and sand. This research analyzes the self-healing properties of young concrete specimens for autogenous and CA-based healing under different exposure conditions. Two types of concrete were studied: a control mix, and a mix containing the CA (with a dosage of 4% by the weight of cement). Six exposure conditions were studied: water immersion at 15ºC, water immersion at 30ºC, water contact with a water head of 2 centimeters, wet/dry cycles, humidity chamber, and laboratory conditions. Self-healing capability was evaluated by measuring the permeability of concrete specimens before and after the healing period. Cylindrical specimens were pre-cracked by means of splitting test at the age of 2 days, and their permeability measured at the age of 3 days and after 42 days in the healing exposure. The range of studied crack widths was limited to 300 micrometers. From the tests, the self-healing reaction was confirmed for both mixtures for exposures in direct contact with water. Exposing just one surface to water contact increased healing rates, but only the crack in direct contact could be sealed. Finally, slight differences were measured between both mixtures, with a higher stability for concrete with CA. To sum up, only cracks in direct contact with water could heal, achieving higher healing rates when the specimens were immersed in water. Ambient humidity was not enough to produce healing either for control concrete or concrete with CA