Durability directly impacts the service life of infrastructure such as bridges, highways, and hydraulic engineering. It remains the most critical challenge for concrete structures, especially the widespread early cracking and poor frost resistance. Traditional expansion agents often fail to perform their intended function of compensating shrinkage in the early age. The tiny bubbles generated by traditional air-entraining agents in fresh concrete are significantly lost during transportation and vibration molding. This study investigates the application of a novel plastic expansion agent (PEA), primarily composed of azodicarbonamide, to address these limitations. PEA generates uniformly distributed and tightly packed microbubbles in concrete during the setting process. These bubbles induce slight volume expansion, and help to enhance cracking resistance and antifreezing durability of concrete. A series of indoor studies were conducted to assess the effects of PEA on the volume expansion, compressive strength, air bubbles spacing factor, frost resistance, and pores characterization. Experimental results demonstrated that a PEA dosage of 0.04%–0.08% effectively compensated for cement paste shrinkage, with 0.06% PEA achieving optimal performance. At this dosage, the bubble spacing factor in concrete was reduced to 157 μm, enabling medium-strength concrete to withstand over 500 freeze-thaw cycles. Notably, the appropriate amount of PEA had little impact on the compressive strength. MIP and SEM confirmed that PEA generated bubbles delayed crack propagation under freezing stresses. The volume stability and antifreezing durability of the concrete can be comprehensively improved by adding PEA. These findings provide a practical solution to simultaneously improve volume stability and frost resistance, offering significant potential for enhancing the durability of concrete.
Wang et al. (Thu,) studied this question.