Study On Stress-strain Behavior Of GFRP-confined Geopolymer Concrete With Recycled Aggregate

Using building waste to produce Recycled aggregate concrete(RAC)has great significance for environmental protection and sustainable development of building resources.However,due to its poor mechanical properties and durability,RAC can not be widely used in building structures directly.Geopolymer,as a kind of green high performance inorganic gelled material,with the RAC to polymer composed of recycled aggregate concrete(Geopolymeric recycled aggregate concrete,referred to as GRAC)not only can improve the defects of recycled aggregates(recycled aggregate,hereinafter referred to as RA),but also to open up a new idea of harmless treatment of industrial solid wastes such as granulated blast furnace slag and fly ash in China.Therefore,from the point of view of resource conservation and environmental protection,the geopolymer prepared from industrial solid waste is combined with RAC,and a new composite compression component-GFRP confined GRAC column is developed by taking advantage of the high strength of glass fiber reinforced composite material(GFRP).On the one hand,this kind of green components reduce the use of cement and alleviate the carbon emissions caused by the production of cement.On the other hand,it can realize the resource utilization of construction waste and industrial solid waste and promote the development of green building materials.At the same time,the confined effect of GFRP can improve the strength and ductility of GRAC,and as the protective shell of GRAC,improve the durability of the components,which will promote its extensive application prospects in the infrastructure construction of port,dock platform,coastal buildings and other complex environments.Although the scholars at home and abroad have done a lot of research on GFRP confined natural aggregate concrete,from the perspective of the literature of reported at home and abroad,on the one hand,the GFRP confined GRAC test database is currently missing,on the other hand,the lack of model which suitable for predicting ultimate condition of GFRP confined GRAC and stress-strain constitutive,greatly limits the development and application of GFRP confined GRAC components.Therefore,this study sets parameters such as RA replacement rate,thickness of GFRP tube and size as variables,by adopting the combination of experimental study and theoretical analysis method,explore mechanical behavior of GFRP confined GRAC in static axial compression,axial dynamic impact and dynamic splitting.Analyzing the stress-strain relationship of the new compression member under dynamic and static load,and establish the ultimate strength model and the ultimate axial strain model of the GFRP confined GRAC.The main contents and partial conclusions are as follows:(1)The static compressive performance of GFRP confined GRAC was studied.a)The effects of RA replacement rate,GFRP thickness and size of specimens on the compressive strength,stress-train curve and failure mode of GFRP-confined GRAC were investigated.The results show that FRP has a better confinement effect on GRAC with higher RA replacement rate.Taking M-size 6-layer confined specimens as an example,the ultimate compressive strength of R0,R50 and R100 confined specimens is 1.92 times,2.27 times and 2.76 times of that of unconstrained specimens,respectively,and when the number of confinement layers is up to 10,The increase rate was 2.49 times,2.99 times and 3.91 times,respectively.However,under the same confinement thickness,the increase of specimen size will reduce the confinement ratio of FRP,which will lead to a significant decrease in the ultimate compressive strength.b)Considering the effects of RA replacement rate,GFRP tube thickness and GRAC size,combined with the above test results,the ultimate strength model and ultimate axial strain model of GRAC confined by GFRP are established.The results show that,compared with the existing GFRP-confined ordinary concrete model,the strength enhancement factor of GFRP confined GRAC is similar,and the existing strength model can also predict the ultimate strength of GFRP confined GRAC more accurately.However,the existing axial ultimate strain model overestimated ultimate strain for the specimens in this study,and the ultimate axial strain data in this study has obvious size effect.Therefore,the ultimate axial strain model established in this study takes the influence of specimen size into comprehensive consideration.(2)The dynamic compressive performance of GFRP confined GRAC was studied.Based on the dynamic impact test of split Hopkinson pressure bars,the effects of RA replacement rate and GFRP thickness on the dynamic mechanical properties of GRAC under different strain rates were investigated.The results show that based on the energy consumption analysis of GFRP-confined GRAC by stress-strain curves,the impact toughness of both unconfined and confined GRAC has obvious strain rate sensitivity.RA has a significant effect on the impact toughness of unconstrained specimens.The impact toughness of the specimens with RA replacement rate of 0% is about 1.3 times that of 100%.For the confined specimens with 6and 8 layers,the impact toughness of the confined specimens with 6 and 8 layers increases by2.3 to 3.1 times and 2.6 to 3.4 times,respectively,compared with the specimens without confinement.With the increase of strain rate and the decrease of RA replacement rate,the dynamic compressive strength of the confined specimens were significantly increased,for RA replacement rate were 0%,50% and 100% of the confined specimen,with the loading pressure value increased from 1.0 MPa to 1.3 MPa,can make the dynamic compressive strength increased by 33%,32% and 42% respectively,but RA replacement rate has not changed the influence law of strain rate on dynamic compressive strength.(3)The dynamic splitting performance of GFRP confined GRAC was studied.A series of SHPB impact splitting tests were carried out to investigate the effects of the replacement rate of RA,confinement thickness and strain rate on the failure mode,dynamic splitting tensile strength and dynamic increase factor of GFRP-confined GRAC.The results show that,the failure degree of the confined specimens generally increases with the increase of strain rate,and with the increase of the replacement rate of RA,the crushing degree of the concrete fragments generated after the failure increases.In the process of dynamic impact loading,the time-stress curve of the confined specimens presents a double "peak" shape.With the increase of strain rate,the slope of the ascending stage gradually increases,and the dynamic splitting tensile strength of the confined specimens also increases.Compared with R0,the dynamic compressive strength of R100 can be reduced by 0% to 9.3%(6 layers),11% to 25.5%(8 layers)and 12.5% to 20.8%(10 layers).With the increase of strain rate,DIF showed an obvious upward trend,and increased with the increase of RA replacement rate.For the test specimens with constraint thickness of 6 and 8 layers,the DIF sensitivity of the test specimens is more affected by RA.