| Elevated temperature and freeze-thaw cycle may reduce the strength and durability of structural concrete,and affect the safety and service life of concrete structures.Ultra-high performance concrete（UHPC）is a cement-based material with new constitutive relations and structural life,which is characterized by "Super Strength","High Toughness" and "High Durability".At present,UHPC is considered to have significant application advantages and good application prospects in national defense engineering,nuclear industry,marine engineering,protection engineering,traffic engineering,bridge engineering and construction engineering.Taking UHPC roads,bridges and slabs as examples,which may be affected by the compound action of elevated temperatures of fire and freeze-thaw cycles during their service lives.For elevated temperatures and freeze-thaw cycles testing,432 UHPC specimens were designed.The strength degradation law and frost resistance durability performance of UHPC materials under the elevated temperature,and the compound action of elevated temperatures and freeze-thaw cycle were studied systematically.The research results can be used to provide technical support for the safety and durability evaluation of UHPC structures affected by fire elevated temperature and freeze-thaw cycles,which has certain practical theoretical significance and engineering application value.The main work of this study is listed as follows:（1）From the point of view of economy and practicability,the UHPC mix proportion of 3kinds of steel fiber content was designed,and quartz sand was replaced by natural river sand with particles larger than 5mm removed.The effects of curing methods and steel fiber content on the compressive strength,flexural strength and explosive spalling of UHPC specimens were studied.The test results show that the compressive strength of UHPC can be effectively improved by hot water-dry air combined curing methods,which was increased by 36.6%compared to the standard cured specimen.With steel fiber content of 2.0% and 2.5%,the cubic compressive strength of UHPC specimens after combined curing was 127.4 MPa and 136.3MPa,respectively.Meanwhile,it is calculated that the H-2.0 UHPC designed in this thesis has a good economical cost of about 2,000 yuan per square material under the condition that the strength meets the requirements.（2）144 UHPC specimens with 3 kinds of steel fiber content were designed and manufactured for elevated temperature testing at different temperatures（200℃,250℃,300℃,350℃,400℃,450℃,500℃）.The failure patterns,residual compressive strength,residual flexural strength and mass loss of UHPC specimens were explored,and the change law of strength and mass of the UHPC specimens to different elevated temperature environments were studied.The results show that with the increase of temperature,the compressive and flexural strengths of UHPC specimens first increased and then decreased.The compressive strength of UHPC with a steel fiber content of 1.5% was reduced to lower than the initial value after being subjected to a elevated temperature of 400℃.However,the compressive strength of UHPC with steel fiber content of 2.0% and 2.5% still increased by 3.0% and 8.7% after experiencing elevated temperature at 500℃.Compared with compressive strength,the flexural strength and ductility of UHPC specimens are more affected by steel fiber content and elevated temperature.The flexural strengths of UHPC specimens with steel fiber content of1.5%,2.0% and 2.5% were reduced to 54.5%,34.4% and 34.2% of the initial flexural strength after experiencing elevated temperature at 500℃.According to the test data,the calculation formulas of compressive strength and flexural strength of UHPC specimens with elevated temperature were fitted.（3）Based on the elevated temperature tests,288 UHPC specimens with 3 kinds of steel fiber content were designed for compound test of elevated temperature and freeze-thaw cycles.The failure patterns,residual compressive strength,residual flexural strength and mass loss of UHPC specimens were carried out,and the change law of strength and mass of UHPC specimens under the combined action of elevated temperature and freeze-thaw were studied.The results show that the under the influence of freeze-thaw cycles,the compressive strength and flexural strength of UHPC specimens were gradually reduced,but the mass of UHPC specimens remained basically unchanged.Moreover,the compressive strength and flexural strength of UHPC specimens after freeze-thaw cycles were further reduced after being subjected to elevated temperature.And the less steel fiber content and the higher the elevated temperature,the more the compressive strength and flexural strength after freeze-thaw cycles were reduced.The compressive strength of UHPC specimens with steel fiber content of 1.5%,2.0% and 2.5% was reduced to 68.7%,90.1% and 91.3% of the initial compressive strength after being subjected to a elevated temperature of 500℃ and 300 times of freeze-thaw cycles,and the flexural strength was reduced to 39.1%,58.7% and 58.8% of the initial flexural strength.According to the test data,the calculation formulas of the compressive strength and flexural strength of UHPC specimens with the number of freeze-thaw cycles after experiencing different elevated temperatures were fitted.（4）Based on the above experiments,the test results of three UHPC specimens with different steel fiber contents were compared and analyzed.The results show that the mass loss and compressive strength loss of UHPC specimens with steel fiber content of 2.0% and 2.5%not more than 5% and 25% after being subjected to a elevated temperature of 500℃ and 300 times of freeze-thaw cycles,which meet the requirements of F300 frost resistance grade.Specially,the UHPC specimen with steel fiber content of 2.0% showed excellent mechanical properties and durability,and also had lower production cost and higher application value. |
