Impact properties of Ultra High Performance Concrete (UHPC) cured by steam curing and standard curing

In order to study the effects of steam curing system, silica fume and steel fibre on the impact resistance of UHPC, the split Hopkinson pressure bar tests were used in the study. Two curing regimes were considered in the test, steam curing at 90 degrees C for 3 days and standard curing. The amount of silica fume replacement of cement was 0 % and 20 %, and the volume content of steel fibre was 0 % and 1 %. In order to study the crystal and pore structure of UHPC, XRD and MIP tests were utilised. The test results demonstrate that the failure mode of UHPC after SHPB test can be studied by the concept of the fineness modulus of residue O. Silica fume has little influence on the integrity of UHPC after impact, while steam curing and steel fibre improves the integrity of the UHPC after impact. The increasing of M-r value of UHPC under steam curing at strain rates around 80 s(-1) is 26.0 %. The M-r value of UHPC with the addition of steel fibre was greater than 1.25 times that of plain concrete. Steel fibre can lead to the redistribution of the internal stress of UHPC after impact, and change the propagation paths of cracks, so as to improve the energy absorption capacity of UHPC. Under dynamic load, the ultimate strength of UHPC increased by 10.6 %, 15.7 % and 25 % after steam curing at 90 degrees C for 3 days, silica fume (under steam curing condition) and steel fibre, respectively. E, DIF, and EA of UHPC all have a significant strain rate effect. Strain rate sensitivity of UHPC prepared by steam curing for 3 d is stronger than that prepared by standard curing for 28 d, especially for the concrete mixed with silica fume and steel fibres. The hydration degree of UHPC after steam curing for 3 d using the steam curing system in this study was slightly higher than that of standard curing for 28 d. Steam curing can reduce the porosity by 34.4 %, while silica fume can reduce the porosity by 15.3 % compared to benchmark group under steam curing. The addition of silica fume changes the probable diameter of UHPC while steam curing reduces it.