Ceramics-Silikáty

Concrete is prone to cracking during hardening due to shrinkage, which significantly impairs its mechanical properties and durability, particularly under freeze-thaw cycles. Urea, with inherent hygroscopicity and recrystallisation characteristics, can effectively mitigate concrete shrinkage, thereby enhancing its mechanical performance and frost resistance. This study analysed urea’s effects on concrete properties, including the slump, hydration heat flow, mechanical strength, crack index, pore structure, water absorption, and freeze-thaw durability. The results show that the concrete performance was notably improved when urea content reached 0.01 wt.%. After 28 days, the cracking index, drying shrinkage restriction rate, restrained shrinkage stress, porosity, and water absorption were reduced by 37.90%, 27.97%, 27.72%, 27.96%, and 13.08%, respectively. Meanwhile, the compressive strength loss rate and mass loss rate after 200 freeze-thaw cycles decreased by 39.84% and 34.44%, respectively. Additionally, the flexural strength and splitting tensile strength of concrete with 0.01 wt.% urea reached 9.70 MPa and 6.20 MPa, representing increases of 38.57% and 16.98% compared to the control group. The enhanced crack resistance and mechanical properties are attributed to pore structure optimisation induced by the urea decomposition, while the improved frost resistance stems from the increased cumulative hydration heat and reduced water freezing point.