Experimental Study On Mechanical Properties Of Recycled Coarse Aggregate Concrete Strengthened With HCl

Since the 21st century,with the rapid development of urbanization,the contradiction between resources and environment is becoming more and more prominent,the performance in the coHRtruction industry is:on the one hand,more and more waste concrete is produced by demolishing buildings,if the waste concrete is not properly treated,it will cause an environmental damage that is hard to recover;on the other hand,the demand for concrete is increasing,natural aggregate is exploited in large scale,natural aggregates belong to non renewable resources,unrestrained mining will inevitably result in a shortage of natural aggregates.The recycling of waste concrete into recycled aggregate can not only solve the environmental pollution problem of waste concrete,but also save resources.Ordinary recycled aggregate is generated by crushing,sorting and sieving waste concrete.Since ordinary recycled aggregates are simply crushed,the resulting aggregate has a lot of mortar attached to its surface.Compared with natural aggregate,ordinary recycled aggregate has higher water absorption rate and higher crushing index.The mortar on the surface of the ordinary recycled aggregate also hinders the bond between the aggregate and the new cement mortar.Therefore,removing mortar from the surface of aggregate can strengthen recycled aggregate.And it is also an important meaHR to enable recycled concrete to be widely used.Hydration of portland cement will generate many Ca?OH?₂,Ca?OH?₂ is alkaline and adheres to the surface of recycled aggregate in crystal form,which provides feasibility for strengthening waste aggregate with hydrochloric acid.Hydrochloric acid can react with Ca?OH?₂ on the surface of aggregate to form CaCl₂.CaCl₂ is soluble in water.The mortar adhering to the aggregate surface was eroded by reaction,and the crushing index and water absorption of recycled aggregate were reduced.After the reaction,the bonding between fresh cement and recycled aggregate is more firm.Natural aggregates and ordinary recycled aggregates were used in this experiment.Ordinary recycled aggregate obtained from granite concrete test blocks in experimental waste disposal center.The natural aggregate and ordinary recycled aggregate were screened and their physical properties were measured.Through simple data comparison,it is proved that the recycled aggregate has the disadvantages of high porosity,high water absorption,low apparent deHRity,small packing deHRity and high crushing index.Four kinds of hydrochloric acid solutioHR with mass fraction of 1%,2%,3%and 4%were prepared respectively.12 parts of regenerated aggregate were weighed with 3 kg each.Immerse the aggregate in each concentration of hydrochloric acid for 1h,2h,3h.By testing the physical properties of the aggregate after immersion,the optimum immersion time of 2 h and the optimum concentration of 3%HCl were determined.The water absorption and crushing index of recycled aggregate were reduced by 20.3%and 16.1%respectively after soaking in 3%hydrochloric acid for 2hours.The compressive strength of concrete made of natural aggregate,ordinary recycled aggregate and recycled aggregate strengthened by hydrochloric acid was compared and analyzed.The results show that the replacement ratio,water-binder ratio and curing age are the same.The early compressive strength of recycled concrete with aggregate reinforced is higher than that of ordinary recycled concrete.With the same water binder ratio and curing age,the compressive strength gradually decreases with the increase of replacement rate.The compressive strength of recycled concrete strengthened by hydrochloric acid is obviously improved,but it is still lower than that of natural concrete.The linear relatioHRhip between the coefficient k of strength reduction and the replacement rate of recycled coarse aggregate Rv for two kinds of recycled concrete is summarized.The formula for calculating the 28d compressive strength of two kinds of recycled concrete is derived.Uniaxial compression tests were carried out on each test block,and three kinds of stress-strain curves of concrete were obtained.Comparative analysis found that:The failure process of the two recycled concrete specimeHR is similar to that of the natural concrete specimeHR.They all undergo elastic stage,elastic-plastic stage,plastic stage and failure stage.The ultimate failure of test block is oblique shear failure and oblique splitting failure.The effect of hydrochloric acid on the elastic modulus of recycled aggregate concrete is more obvious when the ratio of water to binder is 0.55.When the water binder ratio is the same,the rising section of stress-strain curves of the two kinds of recycled concrete coincide with each other.The average value of the ratio of axial compressive strength to cube compressive strength of the specimen with water-binder ratio of 0.55 is 0.78.The average value of the ratio of axial compressive strength to cube compressive strength of the specimen with water-binder ratio of 0.45 is 0.80.Both ratios are within the limits specified in the code for the ratio of axial compressive strength to cubic compressive strength of natural concrete.Stress-strain curves obtained from experiments treated by dimeHRionless treatment.Fitting the coHRtitutive relation curve with the existing stress-strain curve equation,the fitting curve is compared with the dimeHRionless curve.the results show that the two-stage equation proposed by Zhenhai Guo has the highest fitting degree.All the stress-strain curves are fitted by the two-stage equation proposed by Zhenhai Guo,and the coefficients of all the curves are calculated,and the arithmetic formulas of all the curves are obtained.