Performance Evolution And Degradation Mechanisms Of High Toughness Concrete Under High Temperature And Salt Attacks

Ultra-high toughness cementitious composites(UHTCC),which is prepared based on the design of fiber,matrix,and fiber-matrix interface properties via micromechanics theory,has significant multiple cracking and strain hardening characteristics under tensile load.The tensile strain capacity is more than 3% and the average crack width is less than 100 μm,which make UHTCC overcome the quasi-brittleness characteristic of normal concrete and have higher durability.The addition of large amounts of mineral admixtures and higher durability endow UHTCC higher sustainability compared with normal concrete.Currently,there have been some researches on the basic mechanical properties of UHTCC.However,to promote the practical application of UHTCC and ensure that the material can be safely served in the real environment for a long time,the research on its durability remains to be further carried out.In view of the characteristics of various harsh environments in practical engineering,this thesis studied the evolution law and mechanisms of mechanical properties of UHTCC under long-term exposure to high temperature and aggressive salt solutions.The main research contents are as follows:(1)In order to simulate the construction and service of the structure at high temperature,UHTCC was cast at 60 ℃ in this experiment,and its long-term thermal stability at 60 ℃ was studied.The results showed that the pore structure of UHTCC was coarsened at 60 ℃.However,the mechanical properties were significantly improved.After 180 days of exposure,the compressive strength,flexural strength,ultimate tensile stress,and strain capacity of UHTCC were 65.62 MPa,26.66 MPa,6.32 MPa,and 0.40%,respectively,accompanied with multiple cracking behavior,which indicates that UHTCC can still maintain high strength and good deformation ability after long-term exposure to the high-temperature environment.Good thermal stability of UHTCC was proved,revealing its potential application prospect in regions with hightemperature climates.(2)To fully understand the mechanism of salt attack on UHTCC,the resistance of UHTCC to chloride,sulfate,and seawater erosion was studied by full immersion method.According to the concentration ranges of aggressive media in the actual environments,multiple concentrations of each kind of salt solutions were selected for the study.The mechanical properties(compressive,flexural,direct tensile,and fracture properties)of UHTCC were measured to characterize the performance evolution under different salt attacks.Meanwhile,scanning electron microscopy,X-ray diffractometer,and mercury intrusion porosimeter microscopic testing techniques were used to characterize and analyze the changes in the microstructure of specimens after salt attack,as well as the morphology and content of the main erosion products.The degradation mechanism of UHTCC was revealed from the microscopic point of view.In addition,the single fiber pullout test was conducted to characterize the polyvinyl alcohol(PVA)fiber-matrix interface bonding properties during salt attack.Based on this,the constitutive relationship of fiber bridging was calculated and the mechanism of salt attack on the toughening and cracking control process of UHTCC was interpreted by micromechanics theory.(3)To estimate the effect of the incorporation of PVA fiber on the salt resistance of UHTCC and explore the difference of salt resistance between UHTCC and traditional Portland cementbased materials,the evolution law and mechanisms of the properties of UHTCC and UHTCC matrix,mortar,and concrete under different salt solution environments were comparatively studied.The degradation degree of different specimens was investigated by means of morphology observation,microstructure analysis,and mechanical properties tests.At the same time,the transformation properties of chloride and sulfate ions from different salt solutions in UHTCC and concrete were characterized and analyzed.It was found that the addition of PVA fiber could effectively improve the salt resistance of UHTCC,and the salt resistance of UHTCC was stronger than that of mortar and concrete.Magnesium salts showed a stronger corrosive effect on specimens than sodium salts.The test results also showed that when studying the effect of magnesium chloride attack on material properties,it was more reliable to use mechanical properties to characterize the deterioration degree of materials than microstructure or ion diffusion coefficient.(4)Notched beam test is proved to be a more sensitive method to characterize the salt resistance of materials than strength.In this study,the fracture properties of UHTCC matrix,UHTCC,and mortar specimens after salt attack were measured by notched three-point bending beam method and evaluated by the residual flexural strength,fracture energy,and double-K fracture criterion.The results showed that UHTCC still exhibited ductile fracture characteristics after salt attack.Long-term salt solutions exposure improved the fracture toughness of UHTCC to a certain extent,and enhanced the energy dissipation capacity of single crack.It was also found that the double-K fracture criterion could be used to characterize and analyze the properties of the regions with different degradation degrees under salt attack in specimens,which is difficult to be achieved by other macroscopic mechanical property characterization methods.(5)To explore the durability of UHTCC in the most severe marine environment,such as the water level variation zone,a wet-dry cycle test method with the coupling effect of high temperature exposure and seawater attack was adopted to simulate this environment to study the evolution law of mechanical properties and self-healing performance of UHTCC.Mortar specimens were also prepared for comparison.The results showed that the resistance to wet-dry cycle degradation of the compressive strength of UHTCC was better than that of mortar.After90 days of wet-dry cycles,UHTCC still maintained high strength,while the ultimate tensile strain decreased.Whereas,the strain capacity of UHTCC was still nearly 60 times higher than that of normal concrete.UHTCC possessed good self-healing ability in the water level variation zone of the marine environment,and the healing effect would be gradually improved with the increase of exposure time.