| This research is one of the main content of project, named as properties of fiberand nanosized materials reinforced concrete at and after high temperature, whichfinancially supported by the National Science Foundation of China(NO.51178434).Experiments were carried out to investigate the mechanical property, axialcompression stress-strain, chloride ion penetration performance after exposure tohigh temperature. Scanning Electron Microscope(SEM) examinations wereperformed to observe the microstructure and mechanism of concrete. Somecorresponding models were proposed. The main research results are as follows:1. By the experiments of slump, penetration resistance and compressive strength, splitting strength of 294 cube specimens with the size of 150mm×150mm× 150 mm and flexural strength of 147 beam specimens with the size of 100mm×100mm×400mm, and combining the micro analysis and macro properties experiments, it was investigated the effect of fiber volume fraction and nanosized materials content on the micro mechanism and physical and mechanical properties of fiber and nanosized materials reinforced concrete. Test data indicates that the suitable combined usage of nanosized materials with steel fiber improves the microstructure of concrete, increases its densification and enhances the physical and mechanical properties. With the increase of steel fiber volume fraction from 0 to 1.5%, the mixture slump decreases from 40 mm to 25 mm, and the compressive strength, splitting tensile strength and flexural strength of FNMRC increase, respectively, by 12%, 32% and 12.5%. With the increase of nano-Si O2 content from 0 to 2%, the mixture slump reduce 95 mm, and the initial and final setting time reduce 52.3% and 35.9% respectively, and the compressive strength, splitting tensile strength and flexural strength of FNMRC increase 9%, 24% and 14.7% respectively. With the increase of nano-Ca CO3 content from 0 to 2%, the mixture slump reduce 50 mm, and the initial and final setting time reduce 35.2% and 3.8% respectively, and the compressive strength, splitting tensile strength and flexural strength of FNMRC increase 8%, 20% and 8.8% respectively. Based on the theory of composite materials and the regression analysis of test data in this paper and related literature, a model for strength calculation of FNMRC is proposed, which considers the effect of nanosized materials and steel fiber.2. By the experiments of morphology observation and mass loss test of 150 cube specimens with the size of 150mm×150mm×150mm, it was investigated the appearance characteristics, relationship between mass loss rate and temperature of FNMRC before and after exposed to high temperature. Test data indicates that with the increase of temperature from 25℃ to 800℃, apparent degradation is more serious, mass loss rate increase from 1% to about 10%. Based on the analysis of test data, the models for mass loss rate after exposed to high temperature are proposed.3. By the experiments of compressive strength, splitting strength of 450 cube specimens with the size of 150mm×150mm×150mm and flexural strength of 300 beam specimens with the size of 100mm×100mm×400mm of FNMRC after exposed to high temperature, it was investigated the effect of fiber volume fraction, nanosized materials and temperature on compressive strength, splitting strength and splitting deformation, flexural strength and bending-deflection. Test data indicates that with the increase of the steel fiber volume fraction from 0 to 1.5%, compressive, splitting and flexural strength of FNMRC showed a trend of increase. After being exposed to 600℃, residual compressive strength, residual relative splitting strength and residual flexural strength increase 43.91%, 47.2% and 84.8% respectively. The deformation of splitting accordingly increases, the area under the splitting deformation curve gradually increase. Bending load-deflection curve is more and more plump. The peak load, the corresponding deflection and area of bending deflection curve, and the flexural toughness index and rate showed a trend of increase. As adding nanosized materials into concrete, strength after exposed to high temperature increase in certain degree. After being exposed to 600℃, residual compressive strength, residual relative splitting strength and residual flexural strength with 1% NS or NC increase 45.5% and 38.2%, 63.9% and 28.1%, 78.7% and46.3% respectively. With the increase of nanosized materials content, the peak load of splitting and flexural showed a trend of increase, area under the bending-deflection curve and flexural toughness index increase in certain degree. With the increase of temperature from 25℃ to 800℃, strength of FNMRC decrease, the peak load and area under the splitting-deformation curve gradually decrease. Bending-deflection curve is more and more flat, the peak load of the curve significantly decreases, the corresponding deflection increase. Based on the regression analysis of test data, the models for relative residual strength after exposed to high temperature are proposed.4. By the experiments of ultrasonic pulse velocity, rebound and compression of 150 cube specimens with the size of 150mm×150mm×150mm, it was investigated the effect of steel fiber and nanosized materials on ultrasonic pulse velocity(UPV) and rebound number(RN) of fiber & nanosized materials reinforced concrete(FNMRC) before and after expose to high temperature, and the correlationship of compressive strength of FNMRC, the highest exposure temperature with UPV & RN. Test data indicates that the suitable usage of steel fiber,polypropylene fiber and nanosized materials improves the microstructure of concrete, increases its UPV and RN in certain degree. After being exposed to 800℃, UPV and RN with 1% steel fiber increase 45.32% and 36.28% respectively, and UPV and RN with 1% NS or NC increase 21.66% and 22.25% respectively. There is a good regressive correlationship between compressive strength and UPV & RN, the ultrasonic-rebound combined method is suitable for predicting compressive strength and the highest exposure temperature of FNMRC. Based on the regression analysis of test data, the ultrasonic-rebound strength curve and the model for predicting the highest exposure temperature are proposed.5. The uniaxial compression experiments were conducted on 306 prism specimens with the size of 150mm×150mm×300mm for fiber and nanosized materials reinforced concrete(FNMRC) after being exposed to 25-800℃. Based on the experimental observation, the effects of steel fiber volume fraction and nanosized materials content and exposure temperature on the uniaxial compressive stress-strain curves of FNMRC were systematically analyzed. Test data indicated that the compressive stress-strain curve of FNMRC can be divided into the elastic stage, crack initiation stage, crack instability stage and failure stage. With the increasing of the steel fiber volume fraction and nanosized materials content, stress-strain curves gradually plump, both peak stress and strain at peak stress have a certain improved. With the increasing of exposure temperature, stress-strain curves gradually flatten, ascending liner portions of the stress-strain curve shorten, peak stress significantly decrease, and strain at peak stress show a trend of increase. High temperature causes significant deterioration to the nature of the stress-strain curve. A simple analytical model is proposed to generate both the ascending and descending portions of the stress-strain curve, which considers the effect of steel fiber, nanosized materials and exposure temperature.6. By the experiments of chloride solution soaking wet and dry cycle of 150 specimens with the size of 100mm×100mm×200mm, it was investigated the effect of steel fiber volume fraction, nanosized materials content and temperature on chloride ion penetration resistance of FNMRC. Test data indicates that with the increase of steel fiber volume fraction and nanosized materials content, chloride ion content of concrete showed a trend of decrease, chloride ion penetration resistance increase. After being exposed to 800℃, chlorine ion content at 7.5mm with 1% steel fiber is 64.2% of without steel fiber, and chlorine ion content at 7.5mm with 1% NS or NC is 72.9% and 68.1% of without nanomaterials repectively. With the increase of temperature, chloride ion content of concrete gradually increases, and chloride ion penetration resistance decrease. It showed the interrelation between chloride ion penetration resistance and compressive strength. The chloride ion penetration resistance increase with the increase of compressive strength. Based on the regression analysis of test data, the model for calculate the chloride ion content of concrete is proposed, which considers the effect of fiber volume fraction, nanosized materials content and temperature.7. By the Scanning Electron Microscope(SEM) examinations, it was investigated the effect of fiber, nanosized materials and temperature on microstructure of FNMRC and the interrelation between microstructure and mechanical property. Test data indicates that concrete matrix become denser with adding nanosized materials into concrete, and improve the microstructure and interface properties. Because polypropylene fibers melt at high temperature, internal pressure significantly reduce to prevent burst at high temperature. Temperature gradient and degradation significantly reduce because the effect of bridge and crack resistance of steel fiber. As a result, mechanical properties of FNMRC before and after high temperature are improved by adding appropriate content fiber and nanosized materials into concrete. |
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