Mechanical And Water Absorption Properties And Microstructure Of High Temperature Damaged SHCC

Cement-based materials are widely used in the field of construction engineering and their service environments are complex and varied,with cement-based materials undergoing a complex range of physical and chemical changes when exposed to fire or other types of high-temperature effects.This will lead to microstructure deterioration of the material and seriously affect the safety and durability of the structure.Strain Hardening Cementitious Composites(SHCC)is a new building material.Cement slurry or mortar as the matrix,with a lower content of fiber(generally not more than 2% of the total volume of the cementing material),so that the material has strain-hardening characteristics.Due to the addition of fly ash and fiber,the composition of SHCC is complex,and its microstructure,mechanics and water absorption degradation law may be significantly different from that of ordinary cement-based materials at high temperature.This paper studies the variation rules of mechanical properties,water transport performance and microstructure of SHCC at different heating temperatures,and considers the influence of fiber and fly ash content:(1)By analyzing the effect of temperature,it is found that with the increase of heating temperature,the mass loss rate of SHCC sample(the ratio of mass change after high temperature to mass before high temperature)shows an increasing trend,and the dynamic elastic modulus decreases gradually.and the compressive strength and flexural strength increased first and then decreased.By comparing the effects of different fiber content and different fly ash content,it is found that the mass loss rate of SHCC samples decreases with the increase of fiber content and fly ash content.With the increase of temperature,SHCC samples with higher fiber content and lower fly ash content show better flexural strength.The compressive strength increases with the increase of fiber content and decreases with the increase of fly ash content.(2)The water absorption law of SHCC samples was quantitatively characterized by weighing method.Found that with the increase of heating temperature,the initial capillary water absorption coefficient of SHCC samples increased.By comparing the relationship between initial capillary water absorption coefficient and square root of water absorption time,it is found that SHCC samples exhibit bilinear water absorption when the heating temperature is below 600℃,after 800℃,SHCC samples are unilinear in water absorption.By comparing the effects of different fiber content and fly ash content,it is found that when the heating temperature is lower than 200℃,the initial capillary water absorption coefficient of SHCC increases with the increase of fiber content and fly ash content.The permeability of SHCC samples increases with the increase of temperature,and there is a obvious correlation between the initial capillary water absorption coefficient and permeability of SHCC samples.(3)Observe the surface morphology of SHCC samples with fiber content of 2%volume fraction of cementing material.Low field nuclear magnetic resonance technology was used to test the microstructure deterioration of SHCC damaged by high temperature.By comparing the influence of heating temperature,it was found that at 105℃ and 200℃,the surface morphology of SHCC has little change.With the increase of temperature,microcracks form and expand gradually,and porosity increases gradually.By comparing the influence of fiber content and fly ash content,it was found that at 105℃,porosity increases with the increase of fiber content and fly ash content.At 200℃,the samples with 2.0% fiber content and 1.8 mass ratio showed better microstructure.(4)When the temperature is less than 200℃,the microstructure of SHCC samples is relatively dense,and the mechanical and water absorption properties do not change significantly.At 400℃,the pore size distribution changes significantly,the volume fraction of large pore(crack)increases,water absorption improves,and mechanical properties deteriorate.With the increase of temperature to 600℃ and800℃,the hydration products were decomposed by heat,and the number of microcracks on the sample surface increased and gradually expanded.The microstructure degradation was more and more significant,the mechanical properties of the sample deteriorated sharply,and the water absorption performance was significantly improved.By comparing the relationship between initial capillary water absorption coefficient and porosity,it can be found that the initial capillary water absorption coefficient of SHCC samples increases with the increase of porosity.When the acting temperature is less than 200℃,the initial capillary water absorption coefficient changes little with the increase of porosity.When the temperature rises to400℃ and above,the initial capillary water absorption coefficient increases rapidly with the increase of porosity.It is also found that the change of pore volume fraction has a limited effect on permeability,and the microcrack is the main reason for the increase of permeability.