Experimental Study On Mechanical Properties Of Fiber Reinforced Geopolymer Concrete With Recycled Aggregate

The surface of recycled aggregate often contains some old mortar,and it will also produce some damage in the process of crushing and screening.Therefore,the material performance indexes of recycled aggregate are different from those of natural aggregate,which will also affect the mechanical properties of recycled aggregate concrete.Geopolymer is a new type of cementitious material obtained by alkali-excited silicate aluminate minerals.Compared with ordinary Portland cement,geopolymer has better mechanical properties,bonding strength,and durability.However,the spatial structure of geopolymer with high cross-connection makes it more brittle than ordinary Portland cement.Based on this,this article uses geopolymer to replace ordinary Portland cement,and incorporates steel fibers to prepare steel fiber reinforced geopolymer recycled concrete(SFGRC).A series of studies on the mechanical properties of this material have been carried out.The main research contents and conclusions are as follows:(1)The optimal mix proportion of fly ash and slag based geopolymer cementitious material was determined through orthogonal test,and the mix proportion of SFGRC was determined on this basis.The test results show that the slag content is the most important factor affecting its mechanical properties.From the perspective of optimal compressive strength,the optimal mix proportion of fly ash and slag based geopolymer is as follows: slag content is 70%,activator modulus is 1.4,alkali content is 8%.(2)The compressive properties of 21 groups of concrete specimens were tested.The test results show that the compressive failure mode of geopolymer recycled concrete is typical brittle failure,and the failure mode of specimens change from brittle failure to ductile failure after adding steel fiber.With the increase of the replacement rate of recycled coarse aggregate,the compressive strength,elastic modulus,and compressive toughness of SFGRC decrease gradually,but the peak strain increases.With the increase of steel fiber volume fraction,the compressive strength,peak strain,postpeak ductility,and compressive toughness of SFGRC increase gradually,but the steel fiber volume fraction has no obvious effect on the elastic modulus of SFGRC.(3)The flexural properties of 21 groups of concrete specimens were tested.The test results show that the replacement rate of recycled coarse aggregate mainly affects the flexural strength of SFGRC,but has little effect on the flexural-compression ratio and flexural toughness.With the increase of steel fiber volume fraction,the flexural strength,flexural-compression ratio and flexural toughness of SFGRC all show an increasing trend.Based on the analysis of flexural toughness,the steel fiber volume fraction is suggested to be 1.0%.(4)The splitting tensile properties of 21 groups of concrete specimens were tested.The test results show that the replacement rate of recycled coarse aggregate mainly affects the splitting tensile strength of SFGRC,but has little effect on the tension-compression ratio.With the increase of steel fiber volume fraction,the splitting tensile strength and tension-compression ratio of SFGRC increase.(5)Based on the test results,considering the influence of steel fiber volume fraction and the replacement rate of recycled coarse aggregate,the strength formulas and uniaxial compression constitutive model suitable for SFGRC are proposed.The calculation results of the proposed formulas are in good agreement with the test results,which provide a theoretical basis for the engineering application of this kind of concrete.