Research On Impact Resistance Of PVA-BF Fiber Reinforced Cementitious Composites After High Temperature

Engineered cementitious composite(ECC)is a new type of building material developed to effectively improve the brittleness and crack propagation characteristics of traditional concrete.This material usually uses synthetic fibers to achieve multi-crack development and strain hardening characteristics.Due to its prominent ductility,toughness and energy dissipation,it is widely used in the field of structural repair and impact resistance.However,the fibers used in ECC materials usually have a low melting point.When the temperature in the matrix reaches about 200℃,the fibers will soften or even melt,and the dynamic mechanical properties after high temperature will inevitably deteriorate.In this paper,the dynamic impact test of ECC Brazilian discs before and after high temperature was carried out by using φ80mm split Hopkinson pressure bar(SHPB)device.Firstly,the effects of PVA fiber content,temperature and fly ash content on the dynamic peak stress-strain,dynamic growth factor and dynamic toughness of ECC under different strain rates were quantitatively analyzed.Secondly,based on the reference PVA fiber content,the effects of basalt fiber(Basalt fiber,’BF’)superposition content and temperature on the dynamic stress-strain,dynamic growth factor and dynamic toughness of hybrid fiber system(Hybrid-ECC,’Hy-ECC’)under different strain rates were analyzed by the same test method.At the same time,the dynamic residual mechanical properties of PVA-ECC and Hy-ECC fiber systems after high temperature were compared and analyzed.It provides a feasible method and theoretical basis for the optimization of dynamic residual mechanical properties of ECC materials after high temperature.The final test results show that:For PVA-ECC samples with single fiber,when the content of PVA fiber increases from1% to 1.8% and 2.5%,the dynamic peak stress and peak strain at room temperature show a trend of increasing first and then decreasing.When the temperature is greater than 200℃,the higher the PVA fiber content,the lower the dynamic peak stress,and the dynamic peak strain gradually increases.Among them,when the reference group P1.8 samples experienced 200℃,400℃,600℃ and 800℃,the average dynamic strength residual rates were 98.36%,83.54%,58.75% and 37.36%,respectively.The dynamic peak stress of PVA-ECC after high temperature decreases with the increase of fly ash content,and the dynamic peak strain increases gradually.When the mass ratio of fly ash to cement FA/C increases from 1.5 to 2.0 and 2.5,the average dynamic peak stress at200℃ decreases by 9.36% and 12.44% respectively compared with FA/C= 1.5.It decreased by 4.70% and 10.73% after 400℃.It decreased by 4.93% and 20.61% after 600℃.It decreased by 13.57% and 13.07% after 800℃.Compared with PVA-ECC samples,the addition of basalt fiber is beneficial to slow down the degradation of dynamic mechanical properties of ECC samples after high temperature.The dynamic peak stress of Hy-ECC after high temperature increases first and then decreases with the increase of basalt fiber content.When the basalt fiber content increases to 0.8 %,the dynamic peak stress reaches the peak.Among them,when the basalt superposition content is 0.8 %,the average dynamic strength residual rate of Hy-ECC samples after 200℃,400℃,600℃ and 800℃ is 97.99%,82.75%,72.22% and 49.51%,respectively.The dynamic peak strain law is not significant when the basalt superposition content is 0.4%.When it is increased to 0.8% and 1.2%,the dynamic peak strain begins to increase gradually.The dynamic toughness index and strain rate characteristics of PVA-ECC and Hy-ECC specimens before and after high temperature were compared and analyzed.Similarly,the dynamic toughness index of both samples gradually decreases with the increase of temperature.On the contrary,the dynamic toughness index of Hy-ECC samples at room temperature is lower than that of single-doped PVA fiber samples,but after the temperature is greater than 200℃,the dynamic toughness of Hy-ECC samples is better than that of PVA-ECC samples.In addition,the dynamic peak stress-strain,growth factor and toughness index of ECC specimens of two different systems increase with the increase of strain rate after each temperature,showing a significant strain rate sensitivity,but the strain rate sensitivity of dynamic elastic modulus is not significant.The influence coefficient of fiber and temperature is introduced to establish the prediction formula of dynamic increase factor and dynamic constitutive relation model of PVA-ECC and Hy-ECC after high temperature.Then,the experimental results and prediction results are compared respectively,and it is found that the two are in good agreement,which can provide some reference for further analysis and discussion of the dynamic characteristics of ECC materials after high temperature.