The Study On Mechanical Properties And Microcosmic Of Fiber-recycled Concrete After High Temperature Or Freeze-thaw

With the goal of "carbon neutrality" and "carbon peaking",the recycling of construction waste is the most socially and economically effective research topic in order to better implement the "Double Carbon Plan".As the tensile and crack resistance of recycled concrete is weak,adding fiber can optimize its internal structure and improve its performance.Moreover,fires are frequent in China and the northern regions are cold.Therefore,it is great practical engineering importance to study fiber recycled concrete under high temperature and freeze-thaw conditions.In this paper,a combination of experimental and theoretical analysis was used for four different recycled coarse aggregate replacement percentages(0%,30%,60%,100%)and six fiber contents(basalt fiber: 0%,0.15%,0.3%;polypropylene fiber: 0%,0.1%,0.2%)of concrete with 100×100×100mm,150mm× 150mm×150mm,150mm×150mm×300mm Compression test,splitting tensile test,axial tensile test of the dumbbell type and scanning electron microscope test.The specific conclusions are as follows:1.The effects of different aggregate replacement percentages and different fibre contents on the strength and microcosmic of the concrete were investigated by mechanical properties and SEM tests.The results showed that at constant fiber content,the cubic compressive strength,axial compressive strength and axial tensile strength all increased and then decreased with the increase of replacement percentages(30%,60% and 100%)with recycled coarse aggregate,and its various mechanical properties are optimal at 30% aggregate substitution percentage.The mechanical properties show a trend of increasing and then decreasing with the increase of fibre content when the aggregate substitution percentage is constant,i.e.at 30% aggregate substitution rate,the 0.15% of basalt fiber and 30% of aggregate substitution rate,0.1% of polypropylene fiber can be used as the best formulation solution with the best strength performance;the porosity,average diameter and roundness are 12.56%,3.42 μm,1.2126 and 12.08%,3.61 μm,1.2335,respectively.2.The effects of different temperature grades on the macroscopic properties and microcosmic of BFRAC were investigated.Compressive and splitting tensile tests and SEM tests were carried out on optimally proportioned BFRAC after the high temperature.The results show that the compressive strength increases and then decreases with increasing temperature grade,reaching the maximum strength at200℃.Compared with the strength at room temperature,the strength increased by15.18% and 6.04% at 200℃ and 400℃,respectively,and decreased by 20.08% and55.99% at 600℃ and 800℃.The microscopic performance is that at 200℃,the internal cementing particles of concrete further hydrate,generating a gel-like C-S-H that fills the pores and makes the structure more compact.After 400℃,the internal hydration products of concrete begin to decompose;At 600℃,a large number of decomposition,and at 800℃,the deterioration is obvious,the structural denseness decreases quickly,the cracks in the matrix are centred on the pores and spread in all directions,and the cracks coarse and large.At different high temperatures,vapour pressure is generated inside the concrete,which has a greater impact on the interface transition zone and is more likely to produce cracks between the new mortar and the old aggregate interface transition zone,so the splitting strength is reduced to different degrees.Compared to room temperature,the 200℃,400℃,600℃ and 800℃splitting strengths are reduced by 11.46%,42.54%,63.95% and 77.62%,respectively.3.The effects of different freeze-thawing cycles on the macroscopic properties and microcosmic of PPRAC were investigated.Compressive and splitting tensile tests and SEM tests were carried out on the optimally proportioned PPRAC after freeze-thawing.The results showed that the compressive and splitting tensile strengths decreased gradually with the increase of the number of freeze-thawing cycles,and the relationship was almost linear.The compressive and splitting tensile strengths decreased by 1.36%,2.65%,4.51% and 1.61%,4.19% and 6.37% for 25,50 and 75 freeze-thaw cycles respectively,compared to the unfreeze-thaw cycle.From a microscopic point of view,the alternation of water-ice pressure within the structure after saturation leads to freeze injury within the structure,rapid development of cracks until penetration and a gradual increase in crack width,a gradual increase in the number of pores within the concrete,and a gradual increase in microcosmic and their gradual connection and widening,eventually leading to structural deterioration.4.This paper analyses the macro and micro performance relationships of fibre-recycled concrete with the aid of Grey correlation theory.The results show that the stability of the pore structure,the size of the pores and the distribution of the pores are the dominant factors affecting the compressive,axial tensile and splitting strengths,with the roundness values correlating to a greater extent with the strength.Using the macro and micro parameters,a regression analysis of the mechanical property strength and pore structure parameters was carried out to establish a macro and micro performance prediction model.