Research On High-Temperature Performance And Degradation Mechanism Of Ultra-High Performance Concrete With Coarse Aggregates

As the country continuously promotes sustainable development and the "dualcarbon" strategy goals,promoting and using high-performance materials and structures has become an important means for the construction industry to achieve these goals.Among them,ultra-high performance concrete with coarse aggregates(CA-UHPC)has become a low-carbon,high-performance building material with broad application prospects due to its advantages of ultra-high strength,high toughness,good durability,and relatively low cost.However,such structures are highly likely to be exposed to hightemperature effects such as fire exposure during their service life.Therefore,it is necessary to study the thermal and mechanical properties of CA-UHPC at high temperatures,which is also the basis for the research and application of CA-UHPC structures.In view of the shortcomings of existing research,this paper systematically studied the thermal and mechanical properties and degradation mechanism of CA-UHPC at high temperatures,aiming to provide basic data and theoretical support for the hightemperature design and application of CA-UHPC structures.The main research work and results are as follows:(1)The high-temperature thermal performance tests of CA-UHPC have been completed,focusing on the factors such as coarse aggregate content,steel fiber content,and temperature,etc.A total of 196 specimens were tested to obtain experimental results on the changes in macro and micro morphology before and after exposure to high temperatures,as well as the thermal conductivity,mass loss,specific heat,and thermal expansion under high temperature.The impact of coarse aggregate content and steel fiber content on the thermal properties at different temperatures was qualitatively and quantitatively analyzed.The results showed that temperature has a significant impact on the thermal properties of CA-UHPC,and the coarse aggregate content also has a relatively noticeable effect,while the effect of steel fiber content is relatively small and can be ignored.Based on the classical parallel model and experimental data,a calculation method for the high-temperature thermal parameters of CA-UHPC considering the coarse aggregate content was established.(2)The strength tests on 336 CA-UHPC specimens under and after high temperatures has been completed,with a focus on the influence factors such as the coarse aggregate content and temperature.The typical failure modes,cubic compressive strength and axial compressive strength of CA-UHPC under and after different high temperatures were obtained.The results showed that both CA-UHPC under and after high temperatures exhibited shear failure,and the shear failure angle is between 60° and85°,increasing with the increase of temperature.The shape of the specimens after failure is intact,and the coarse aggregate leads to more distributed cracks in the damaged specimens,resulting in a higher compressive strength.The compressive strength of CAUHPC under and after high temperatures is different,and in general,the compressive strength of CA-UHPC after high temperature was higher than that under high temperature.Based on the above test data,the formulas for calculating the cubic compressive strength and axial compressive strength reduction coefficient of CA-UHPC under and after different high temperatures were established.(3)An auxiliary device was designed for the axial compression test of quasi-brittle materials under high temperatures,and then uniaxial compressive stress-strain tests of CA-UHPC were conducted under and after high temperatures,obtaining corresponding complete stress-strain curves.The results showed that temperature had a significant effect on the shape of the stress-strain curves,and the higher the temperature,the flatter the curves.Adding coarse aggregates can increase the slope of the rising segment,peak stress,and peak strain of the stress-strain curves under and after high temperatures,but has little effect on the shape of the curves.Based on the above experimental data,formulas for calculating the elastic modulus and peak strain of CA-UHPC under and after high temperatures were respectively established,as well as the mathematical expressions of the uniaxial compressive stress-strain curves of CA-UHPC.(4)With the help of thermal analysis,scanning electron microscopy,phase detection,mercury intrusion porosimetry,and X-ray computed tomography(X-CT)tests,the hightemperature degradation characteristics and characterization of the microstructure of CAUHPC were analyzed.The results showed that 300°C and 700°C were critical temperature points for the changes in macro and micro performance of CA-UHPC.Before 300℃,the microstructure degradation of CA-UHPC was not significant,and the porosity did not change much.In fact,the internal structure is even compacted by the newly produced hydration products,and the median pore size decreased.The above changes corresponded to the increase in compressive strength and elastic modulus after high temperatures in this temperature range.After 700℃,the microstructure degradation of CA-UHPC was severe,most hydration products decomposed,and the porosity and the proportion of large pores increased sharply.The changes described previously caused significant changes in the macroscopic thermal properties and substantial decreases in compressive strength of CA-UHPC.(5)Based on the X-CT scanning data,the geometric model of CA-UHPC mesostructure with different temperature grades was reconstructed,and the meso-numerical model of CA-UHPC with real coarse aggregates and steel fibers was established by using the background grid technique.The validity of the model is verified by the comparison with the high-temperature thermal and mechanical tests.Based on the above work,the whole compression process of CA-UHPC with different content of coarse aggregates and steel fibers under and after high temperatures was simulated and analyzed.The results showed that the damage of CA-UHPC all originated from the interface between the coarse aggregates and the matrix,and the direction was parallel to the loading direction.The more the content of coarse aggregates,the longer the damage path,and the higher the compressive strength.This meso-numerical model can well reflect the influence of temperature and coarse aggregate content on thermal and mechanical properties,thereby revealing the compression failure process and degradation mechanism of CA-UHPC at different temperature levels and modes.The research work provides scientific basis for the engineering application and relevant specification formulation of CA-UHPC materials and structures under and after high temperature or fire,and also lays the foundation for the study of related materials at normal and high temperatures.