Study On The Deterioration Mechanism Of Recycled Aggregate Concrete Under Multi-Factor Coupling

With the gradual expansion of the scale of urban development in China,a large number of concrete buildings have been demolished,resulting in a huge amount of waste concrete.Under the background of "double carbon" in water industry,how to turn the large amount of waste concrete generated after the demolition of old buildings into treasure is an important issue for environmental protection.The preparation of recycled aggregate concrete by crushing waste concrete instead of traditional natural aggregates can not only solve the environmental pollution problem caused by old buildings,but also solve the problem of natural resource scarcity.However,there are few research results on the durability of recycled aggregate concrete under multi-factor coupling,it is of great practical significance to study the durability of recycled aggregate concrete under multi-factor coupling for its wide application in practical engineering.In this paper,the main environmental factors(freeze-thaw damage,sulfate erosion,dry and wet cycles)affecting the durability of recycled aggregate concrete in the northwest China were selected,and a combination of accelerated indoor tests and computational analysis was used to design concrete specimens with different recycled aggregate admixtures(0%,30%,60%)and different water-cement ratios(0.35,0.45,0.55),to carry out freeze-thaw cycle tests,freeze-thaw-sulfate erosion tests,and freeze-thaw-sulfate-dry-wet coupling tests on recycled aggregate concrete.The mechanical indexes such as mass loss and relative dynamic elastic modulus were measured,and the durability damage process and microstructure development of recycled aggregate concrete were investigated by using nuclear magnetic resonance(NMR),scanning electron microscope(SEM)and X-ray diffraction(XRD)techniques.On this basis,the optimal admixture of recycled aggregates and the optimal water-cement ratio were investigated with the objective of improving the durability of recycled aggregate concrete.The main research contents of this paper include.(1)The influence law of different recycled aggregate admixture and water-cement ratio on the mechanical properties of recycled aggregate concrete during the curing period was studied.It was found that the compressive strength of concrete specimens decreased with the increase of recycled aggregate admixture,and the larger the admixture of recycled aggregate,the faster the rate of reduction of its compressive strength.The larger the water-cement ratio,the lower the compressive strength of recycled concrete.(2)The test results show that the apparent morphological changes of the recycled aggregate concrete are divided into three stages.At the early stage of the cycle test,the surface of the recycled aggregate concrete is flat.At the middle of the cycle test,a small amount of mortar is lost from the surface and defects appear at the corners.At the later stage of the cycle test,the loss of mortar and fine aggregate from the surface is intensified,the coarse aggregate is exposed,and the surface gel becomes crispy and crumbly.From the macro level,the mass loss rate and relative dynamic elastic modulus damage size of concrete specimens according to the aggregate admixture: 30%>60%>0%,according to the water-cement ratio: 0.55>0.45>0.35,and according to the test conditions: freeze-thaw-sulfate-dry-wet coupling action>freeze-thawsulfate erosion>freeze-thaw cycle.(3)The effects of recycled aggregate admixture and water-cement ratio on the evolution of pore structure of recycled aggregate concrete were analyzed by NMR test.The incorporation of recycled aggregate increases the number of pores in small,medium and large pores,the small and medium pores contain less water inside and are basically not damaged by freeze-thaw cycles,and the large pores can effectively release the frost pressure in the pores,which improves the frost resistance of recycled aggregate concrete to a large extent,and the pore structure evolution law of recycled aggregate concrete is more similar when the recycled aggregate incorporation is 0% and 60%.With the increase of water-cement ratio,the original pore space inside the recycled aggregate concrete increases and the internal damage increases,and the durability performance of recycled aggregate concrete is optimal when the water-cement ratio is 0.35.(4)The internal structural evolution of recycled aggregate concrete,morphology of erosion products and analysis of erosion product composition were observed by SEM electron microscopy scanning and XRD physical phase analysis tests.The amount of recycled aggregate admixture and the size of water-cement ratio both affect the crack development and thus the macro mechanical properties.The amount of recycled aggregate admixture of 60% increases the number of tiny pores inside the concrete,which can effectively release the freezing and swelling pressure generated by freeze-thaw cycles.The larger the water-cement ratio,the weaker the interface transition zone.The concrete specimens with 30% recycled aggregates and0.55 water-cement ratio have the highest calcium alumina crystal and gypsum crystal wave crests and the greatest internal damage.(5)Based on the damage mechanics theory,the damage model of recycled aggregate concrete under multi-factor coupling is fitted using a two-stage damage model with the relative dynamic elastic modulus as the damage variable.The first stage is a linear relationship,and the second stage is a parabolic relationship.The relative dynamic elastic modulus damage evolution equation of recycled aggregate concrete is established,the regression coefficients of different damage evolution equations are processed to obtain the specific parameters of the evolution equation,and the effects of recycled aggregate admixture and water-cement ratio on the damage evolution process of recycled aggregate concrete are quantitatively analyzed.