Experimental study of natural self-healing of hydraulic concrete at different pre-damage ages

Given the large number of unhydrated cement particles in C30 two-graded concrete of typical Chinese dam concrete and that the age of cracks appearing in hydraulic concrete structures often exceeds 28 days, we designed and implemented indoor self-healing tests at various concrete pre-damage ages (3, 7, 14, 28, 45, 60, and 90 days) to investigate the natural self-healing of hydraulic concrete as a function of pre-damage age. The percent growth R sh in compressive strength and the crack-healing rate ξ after 28 and 60 days of continuous standard curing (20±2 °C, relative humidity ≥95%) or non-flowing water curing were measured by using cube specimens with different pre-damage ages. In addition, X-ray diffraction (XRD) and nitrogen adsorption were used to elucidate the evolution of the macro-micro properties of pre-damaged self-healed hydraulic concrete. The results show that, when the pre-damage age is less than 28 days, the compressive strength growth rate and crack width healing of the hydraulic concrete specimens gradually decrease after self-healing with increasing pre-damage age; When the pre-damage age exceeds 28 days, the self-healing does not change significantly. After secondary curing of pre-damaged concrete specimens, the characteristic parameters of microscopic pores are reduced when the pre-damage age does not exceed 28 days. When the pre-damage age exceeds 28 days, the pore characteristics remain constant upon secondary curing. Water curing is a more effective way of promoting self-healing than standard curing. Unlike standard curing, the fractal dimension increases after secondary water curing. Surface cracks self-heal better than internal cracks due to more frequent exchange of substances with the environment. Quartz and calcite (CaCO₃) are the main crystalline phases of self-healed hydraulic concrete. The characteristic XRD peaks increase in magnitude with increasing pre-damage age, and higher calcite diffraction peaks appear near cracks. This work has important engineering value for safety evaluation of temperature cracks in concrete placement bays during construction interval periods.