Charateristics Of Normal-strength High-performance Concrete Exposed To High Temperature

This thesis presents an experimental investigation on residual mechanical properties and permeability of normal-strength high-performance concrete (NS/HPC) and high-strength high-performance concrete (HS/HPC) after exposure to high temperatures. First, experiments of explosive spalling and crack growth of HPC of different water/binder ratios (W/B=0.26, 0.32, 0.36, and0.45), were conducted on specimens incorporating none or different kind and dasage of fiber (polypropylene fiber or PVA fiber) at different moisture contents. Based on the experimental results above, residual compressive strength, tensile splitting strength, and fracture energy of HPC exposed to different temperatures were measured. Further more, residual mechanical properties and permeability of control concrete and fiber-toughened concrete exposed to high temperatures and then subjected to different cooling regimes were measured also.The experimental results show that NS/HPC may spall under high temperatures, which is different from the conclusion reported previously that normal-strength concrete could not spall under high temperatures. Moisture content and dense microstructure of NS/HPC are two main factors governing spalling occurrence. The higher the moisture content, the greater the frequency of explosive spalling and the degree of damage. W/B has an obvious influence on the explosive spalling. The lower the W/B, the higher the possibility of exlosive spalling. NS/HPC can be prevented from explosive spalling through incorporating PP fiber or PVA fiber with a relatively low melting point. With the increase of the dosage of fiber, the damage to NS/HPC is decreased.Tensile splitting strength and fracture energy of HPC at normal temperature are increased by incorporating PP fiber or PVA fiber of 0.05%～0.2% dosage. The dosage of 0.2% has the most obvious effect. But after exposure to high temperature, residual splitting strength and residual fracture energy of concrete incorporating fiber are increased slightly compared with that of plain concrete. Explosive spalling can be prevented through incorporating PP fiber or PVA fiber at a dosage range from 0.05% to 0.2%, and the fiber of such a dosage has little effect on residual compressive strength. The dosages from 0.05% to 0.2% are appropriate to improve the fire-resistance of HPC. A relatively regular order pore structure was found in concrete incorporating PP fiber after exposure to high temperature, while an inregular pore structure was left in concrete incorporating PVA fiber. The experimental results show that fracture energy of HPC with relatively regular pore structure is higher. PP fiber would melt when exposed to 200°C and PVA fiber keeps the original behaviour. The relative strength of HPC incorporating PVA fiber is higher exposed to 200℃.Cooling regimes has an obvious influence on residual mechanical properties and permeability (near-surface permeability and Cl~- -permeability) of NS/HPC. The residual mechanical properties of NS/HPC after exposure to high temperature and then subjected to water quenching or water spraying over 30 min, are relatively lower than those of concrete subjected to natural cooling. The permeability of NS/HPC subjected to water spraying over 30 min is higher than that of naturally cooled NS/HPC, while lower than that of NS/HPC subjected to water quenching. Compared with other cooling regimes, water quenching can cause the most severe thermal shock damage to concrete. It is concluded that hybrid fiber (steel fiber and PP fiber) can improve the permeability of NS/HPC exposed to high temperatures.