Rapid-Hardening Concrete Mixes

Results of experimental investigation of the properties of rapid-hardening concrete are presented. Three types of cements that can achieve $14\mathrm{MPa}$ compressive strength and $2.4\mathrm{MPa}$ modulus of rupture after $3\mathrm{h}$ at $22°\mathrm{C}$ and remain workable for $10\text{to}15\min$ were selected based on an extensive literature review. The variables in this study were (1) cement type, (2) cement content, (3) water-cement ratio, (4) use of admixtures, and (5) temperature. The measured parameters were (1) strength gain with time, (2) slump loss with time, (3) workability, (4) plastic and drying shrinkage, and (5) relationship between compressive strength and modulus of rupture. Test results showed that for those selected cements it is possible to achieve a workable concrete having $14\mathrm{MPa}$ compressive strength and $2.4\mathrm{MPa}$ modulus of rupture after $3\mathrm{h}$ . The strength gain in flexure was more rapid than the strength gain in compression. The concrete had good flowable characteristics with vibration, and the use of retarding admixtures increased workability time up to $25\min$ . Plastic and drying shrinkage test results showed that rapid-hardening cement is more susceptible to plastic shrinkage than ASTM Type I cement, and care should be taken that exposed surfaces are protected by curing to avoid water loss.