Effect Of Aggregate Content And Modification On Drying Shrinkage Of Recycled Aggregate Thermal Insulation Concrete

Recycled aggregate thermal insulation concrete(RATIC),mixed with glazed hollow beads(GHBs)as fine aggregates,is considered as a new type of building material to meet the demand of thermal insulation and energy saving on building structure.The application of recycled aggregate concrete(RAC)alleviates the consumption of natural sand and gravel resources and the large stacking of building waste concrete.However,the inherent physical and mechanical properties of recycled aggregate always restrict the development of long-term deformation on RAC.The shrinkage performance of RAC,as an important long-term deformation,has a significant impact on the normal use of concrete structure,while the dry shrinkage is the most significant and lasts the longest among all types of shrinkages.The drying shrinkage of RATIC is closely related to the evolution of the internal relative humidity.The combined action of recycled coarse aggregate(RCA)and thermal insulation aggregate on the development of drying shrinkage and internal relative humidity of concrete.The complicated shrinkage performance of RATIC is attributed to the unique internal structure and basic properties of the two aggregates.Based on the above development characteristics of drying shrinkage of concrete,this research carries out experimental studies on the evolution of internal relative humidity and drying shrinkage development of RATIC with different aggregate content to reveal the time-varying development and internal mechanism of drying shrinkage of RATIC under the influence of aggregate.By modification of RCA,this research also studies and analyzes the development of drying shrinkage of RATIC after modification.The study establishes the theoretical prediction model of internal relative humidity evolution and drying shrinkage of RATIC and the interaction between them,and explores a reasonable method to optimize the drying shrinkage performance of RATIC.The main research contents are as follows:(1)Studies on internal relative humidity of recycled aggregate thermal insulation concrete with different content of aggregates.The internal relative humidity tests of RATIC and normal concrete(NC)with different content of thermal insulation aggregate and replacement rate of RCA(0%,30%,50%,70%and100%)were carried out under sealed and unsealed conditions.The effects of self-desiccation and moisture diffusion on the internal relative humidity and the attenuation rate of internal relative humidity were studied and analyzed which concluded the time-varying attenuation law of internal relative humidity of RATIC.With the increase of the content of thermal insulation aggregate,the internal relative humidity of gradually increased.While the internal relative humidity of concrete decreased as the replacement rate of RCA increased.The action effects of the two aggregates gradually weakened with time.Compared with 100-0,the internal relative humidity of 100-100 increased by 11.68%at 180 days;compared with 0-100,the internal relative humidity of 100-100 decreased by 10.35%.Based on the results of internal relative humidity test,the time-varying development of the curing effect in GHBs and the deterioration effect of RCA was introduced.Taking different aggregate content as the main parameters,the theoretical model of internal relative humidity of RATIC was established through regression analysis.Comparing the theoretical model with the test results,it was found that the linear regression coefficient R2 is more than 0.93 which indicated that the fitting of theoretical model was well.(2)Studies on drying shrinkage of recycled aggregate thermal insulation concrete with different content of aggregates.The drying shrinkage tests of RATIC and NC with different content of thermal insulation aggregate and replacement rate of RCA(0%,30%,50%,70%and 100%)were conducted.The drying shrinkage and the shrinkage reduction or shrinkage amplification efficiency of RATIC with different aggregates were studied.The test showed that with the increase of the content of GHBs,the drying shrinkage of RATIC increased.While with the increase of the replacement rate of RCA,the drying shrinkage decreased.Compared with 100-0,the drying shrinkage of100-100 decreased by 15.99%at 180 days.Compared with 0-100,the drying shrinkage of 100-100 at 180 days increased by 18.35%.Moreover,the correlation between internal relative humidity and drying shrinkage of RATIC was analyzed and a logarithmic curve relationship was proposed.Through linear regression analysis,it is found that the linear regression coefficients(R~2)are above 0.96 which shows that there is a good correlation between internal humidity and drying shrinkage of RATIC.The development of drying shrinkage can be evaluated in accordance with the variation of humidity in concrete.Based on the drying shrinkage test results of RATIC under different aggregates content and AFREM shrinkage model of the French building code,the internal curing effect of GHBs and the shrinkage amplification effect of RCA are considered in the modified model.Taking the aggregate contents as the main influence parameters,the influence coefficient of GHBs and RCA are introduced.Through regression analysis,the theoretical model of drying shrinkage of RATIC is obtained.Through the comparison between the test results and the theoretical model,it is found that the consistency is excellent.The theoretical model can be used to predict the development of drying shrinkage of RATIC.In addition,based on the capillary mechanism and internal curing effect,the internal mechanism of drying shrinkage of RATIC is analyzed.It is found that the addition of GHBs compensates the adverse effects of RCA,maintains the relative stability of internal relative humidity and effectively inhibits the shrinkage cracking of concrete.(3)Studies on drying shrinkage of recycled aggregate thermal insulation concrete after strengthening treatment.In the research,it is found that the internal defects of RCA weaken the ability of RATIC to resist shrinkage deformation.Therefore,the drying shrinkage performance of RATIC are promoted by strengthening RCA to compensate the internal defects.Dilute hydrochloric acid,water glass and PVA solution were used to strengthen RCA,and the improvement effect on the physical properties of RCA before and after strengthening was compared.RATIC specimens with 100%replacement rate of strengthened RCA were used to measure the compressive strength and dry shrinkage in contrast to the untreated RATIC.The test results show that the compressive strength and drying shrinkage of RATIC are effectively improved by strengthening treatment of chemical solution.Among the strengthening treatment methods,PVA solution has the most significant strengthening effect on RCA.The compressive strength increased by 7.46%and the drying shrinkage of RATIC reduced by 14.08%.