Evaluating the Performance of Early-Age Fly Ash Concrete Subjected to Steam and Autoclave Heat Treatments

In the fabrication of precast concrete components, rapid early-age strength development during the hardening process is crucial to facilitate formwork operation and production cost. In this study, heat treatment was adopted to boost the compressive strength of cement paste and concrete mixtures over a threshold of 40 MPa at 12 h by applying steam curing and 3 h of autoclave at 180°C sequentially. Cement paste samples were prepared using different fly ash (FA) contents and water-to-binder (w/b) ratios, and concrete samples produced in accordance with the densified mixture design algorithm (DMDA) method. The compressive strength of the paste samples with w/b ratios of 0.23 and 0.30 attained the threshold strength (40 MPa) at 12 h. Compressive strength in the cement paste after autoclaving was 41% to 102% higher than at 12 h and was significantly increased in the FA samples. The 12 h compressive strength of concrete mixtures with FA all attained in excess of 40 MPa, while the reference mixture exhibited below 40 MPa (w/b ratio $=0.23$; cement content $=480\mathrm{kg}/{\mathrm{m}}^3$). The FA content designed by DMDA method at an alpha ratio ($\alpha$) of 5% was identified as the optimum FA level in the concrete mixture. The higher w/b ratios and lower cement content exhibited twelfth hour compressive strengths below 40 MPa. The compressive strength of autoclave-treated concrete samples designed with cement content of $480\mathrm{kg}/{\mathrm{m}}^3$ surpassed 80 MPa, while the lower-cement-content mixtures showed lower-strength values. All of the concrete mixtures attained high ultrasonic pulse velocity results (over $\mathrm{3,660}\mathrm{m}/\mathrm{s}$), indicating good durability.