Influence Of Combined Curing On The Properties Under Room Temperature And Explosive Spalling Behavior Of Ultra-high Performance Concrete Incorporating Coarse Aggregate

The influence of combined curing on the properties under room temperature and explosive spalling behavior of ultra-high performance concrete (UHPC) was studied in this thesis. Three significant parameters of thermal curing have been investigated: combination modes of different curing conditions, temperature, fiber types incorporated.The optimum curing mode for strength and spalling resistance of UHPC was identified via the first-stage experiment, which was a basic curing mode for further research. The influence of varing temperature on compressive strength, tensile splitting strength, fracture energy, and spalling behavior of UHPC was studied in the second stage. Three types of UHPC were prepared, including plain concrete, steel fiber reinforced UHPC (S-UHPC), polypropylene fiber and steel fiber reinforced UHPC (PS-UHPC). Finally, the fracture Surfaces were observed with optical microscope to analyse the relationship between the micro-mechanism and macroscopical behaviors.The mechanical test results reveal that hot water curing condition at preliminary stage was suitable for achieving the optimal combination effect. The strength of plain concrete and S-UHPC both increased with thermal curing temperature increasing, reaching maximum compressive strength of 209.7 MPa and 10.1 MPa for tensile splitting strength. Some of the combined cured specimens exhibited flexural hardening behavior, resulting in higher load-bearing capacity and multiple-cracking performance. The peak load tested was 17.3% greater than first cracking load, which means an excellent improvement in toughness. The reason why combined curing can improve the mechanical properties of UHPC might be attributed to the motivated pozzolanic reactivity and the formation of crystalline phases at high curing temperature. The C-S-H gel generated through pozzolanic reaction can decrease the pore volume and improve the density and strength. Furthermore, the amorphous C-S-H phases would be partly transformed into crystalline calcium silicate hydrate phases, such as tobermorite (C5S6H5) and xonolite (C6S6H) under the high thermal curing temperature.The results from the explosive spalling test reveal that spalling resistance of UHPC could be greatly improved by combined curing, and the improvement magnitude was mainly affected by thermal curing temperature level. Spalling occurrence in UHPC could be eliminated under higher thermal curing temperature. Under lower thermal curing temperature, explosive spalling did occur, but the spalling frequency decreased greatly. The incorporated steel fiber and polypropylene fiber could alleviate spalling to some degree. The reason for reduction of spalling occurrence in UHPC after combined curing is that the moisture content is decreased significantly after being exposed to the thermal curing environment, thus, the vapor pressure build-up is not enough to cause spalling.The results from the microscope observation show that combined curing could lead to micro cracks on the specimen surface, which was caused by the dehydration and shrinkage of surface layer in hot drying environment. The higher the thermal curing temperature, the more water loss, the severer cracking occurred. The incorporated steel fiber and polypropylene fiber were beneficial to alleviating surface cracking. The interface zone (ITZ) around coarse aggregate and steel fiber might be not so strong in UHPC when cured only in hot water, which were prone to causing spalling occurence in the heating process to 800℃. However, the ITZ could be reinforced in UHPC after combined curing, remarkably decreasing the possibility of spalling occurrence at ITZ. As a result, most exposed aggregates on the fracture surface experienced trans-aggregate fracture due to the spalling process. It is obvious to find macrocracks on the exposed motar. There exist cracks around the coarse aggregates along the radial direction, which may be induced by the deformation incompatibility caused by the thermal expansion difference between aggregate and the surrounding motar.The results from freeze-thaw cycle test reveal that the frost resistance of UHPC is excellent. Cracking may occurred on the specimen surface after combined curing, however, the micro cracks would not lead to freezing-thawing damage. The UHPC maintains good frost resistance after combined curing.The research carried out in this thesis indicates that combined curing can greatly increase the early age strength of UHPC and effectively improve its mechanical properties and spalling resistance. Strength of UHPC was affected by several curing parameters such as preliminary curing condition, thermal curing temperature level and duration, among which the thermal curing temperature level was the key factor to influence the properties under room temperature and determine whether explosive spalling can be eliminated. Under combined curing condition, the steel fiber and polypropylene fiber incorporated in UHPC could alleviate spalling to some degree. The UHPC maintains good frost resistance after combined curing.