Experimental Study On Influence Of Water Saturation On Mechanical Properties Of Reclaimed Concrete

This paper will explore the feasibility of using RAC in hydraulic buildings,and study the influence of different water saturation(0%,50%,100%)on the mechanical properties of concrete with different replacement rates of RA(0%,30%,50%,70%).The static and dynamic mechanical properties of the test object were investigated,that is,the uniaxial compression and splitting tensile properties of the reclaimed concrete with different water saturation were investigated at the strain rate of 10-5/s.The dynamic mechanical properties are mainly to explore the dynamic uniaxial compression and dynamic splitting tensile properties of reclaimed concrete with different water saturation under the seismic strain rates of 10-4/s,10-3/s and10-2/s,to obtain the influence of water saturation on the static and dynamic mechanical properties of reclaimed concrete,and to conduct a qualitative analysis of its action mechanism.Finally,the effect mechanism of water saturation on reclaimed concrete is quantitatively analyzed at mesoscopic level.The research content of this paper is as follows:(1)The slump test is used to test and evaluate the working performance of the mixture.It is found that with the increase of the replacement rate of recycled aggregate,the slump of concrete gradually decreases.The reclaimed concrete specimens with different water saturation were prepared by vacuum water saturation equipment,and the empirical curve of reclaimed concrete water saturation change with the saturation time was obtained.The prediction model of water saturation change with time was obtained by fitting it,which proved that the prediction model could accurately guide the preparation of reclaimed concrete with different water saturation.It provides scientific basis for preparing specimens with different water saturation.(2)The static mechanical test of recycled concrete with different water saturation was carried out.It was found through the test that: when the uniaxial compression test was carried out at the quasi-static loading rate,the compression damage of the specimen was more obvious with the increase of water saturation,and the fragments under spallation were relatively crisp,which was particularly significant with the increase of the replacement rate of recycled aggregate.The compressive strength and peak strain of the specimen decrease with the increase of water saturation,while the elastic modulus increases.When the splitting tensile test was carried out at the static loading rate,it was found that with the increase of water saturation,the splitting tensile strength of the specimen would decrease,and the greater the replacement rate of recycled aggregate was,the greater the decrease was.The splitting tensile strength of the specimen with 70% replacement rate of recycled aggregate decreased by 24.42% under 100%water saturation.The mechanism of free water in pores and cracks is analyzed from the perspective of energy.It is considered that free water reduces the surface energy between concrete particles and the van der Waals forces between molecules.Under static load,free water acts as a "wedge",which accelerates the failure of specimens and weakens the strength of concrete.(3)The dynamic uniaxial compression and dynamic splitting tensile tests of reclaimed concrete specimens with different water saturation were carried out.The results showed that:under high strain rate,the specimens would produce obvious strain rate effect.With the increase of strain rate,the compressive strength and splitting tensile strength of the specimens would increase,and the rate sensitivity of reclaimed concrete was stronger than that of ordinary concrete.When the strain rate is large,the free water in the specimen will produce viscosity effect and Stefan effect,which can curb the development of cracks to some extent and enhance the strength of the specimen.However,with the increase of the replacement rate of recycled aggregate,the strength increases greatly at high strain rate because the content of free water in the specimen is higher than that in other specimens.Based on fracture mechanics,Newton’s law of viscosity and Stefan effect,the action mechanism of free water in concrete under dynamic load is analyzed.(4)The concrete with different regenerated aggregate substitution rate was analyzed by NMR and SEM.Through the quantitative analysis of the pores inside the specimen by nuclear magnetic resonance instrument,the porosity and pore size of the specimen with different aggregate replacement rate were explored.It was found that with the increase of the replacement rate of recycled aggregate,the porosity inside the concrete also increased,and the number of pores with small pore size(0～0.1um)accounted for the largest proportion.By scanning electron microscopy,specimens with different regenerated aggregate replacement rates were scanned to observe the changes of the interfacial transition layer and matrix of concrete with different regenerated aggregate replacement rates.It was found that with the increase of the replacement rate of regenerated aggregate,the interfacial transition layer of new and old mortar did not bond well,and there would be gaps.When the specimen was full of water,these cracks would be filled by free water infiltration.It is easy to produce stress concentration when bearing static load,thus accelerating the failure of the specimen.It is also found that with the increase of the replacement rate of recycled aggregate,the matrix compactness of concrete will decrease,and it is not as dense as ordinary concrete,which is easy to be permeated by free water,thus affecting the performance of concrete.By exploring the influence of water saturation on the mechanical properties of recycled concrete,the research results obtained have certain guiding significance for the application of recycled concrete in hydraulic buildings or wet environment.