Study On Flexural Toughness Of Steel-polyvinyl Alcohol Hybrid Fiber RPC

Reactive powder concrete(RPC)has broad development prospects in various engineering fields because of its superior performance than ordinary concrete.Many scholars achieve the purpose of strengthening and toughening by adding a certain amount of steel fiber,but the overall effect is limited.With the large-scale preparation of PVA and other organic fibers,scholars began to mix different types and sizes of fibers into concrete materials in order to further improve its strength,toughness Burst resistance and other properties.Among them,flexural toughness is an important index of the mechanical properties of fiber reinforced concrete,but at present,most of the evaluation methods of flexural toughness are aimed at steel fiber reinforced concrete.The standard test and calculation methods of various countries are different,and most of them are based on the qualitative analysis of macro test,which can not get the stressstrain change process inside the material,and the research on the correlation between meso mechanical properties and macro mechanical behavior is rare.Therefore,through slump test,cube compressive strength test and four-point bending test,combined with finite element simulation and extended finite element simulation,this paper analyzes the toughening performance of fiber from the mesomechanics point of view,and draws the following conclusions:(1)The slump test and basic mechanical property test were carried out by adding steel fiber and PVA fiber with different volume fraction into RPC.The slump test showed that steel fiber had little effect on the working performance of RPC,but PVA fiber had a significant effect,and the mixed fiber was prone to agglomeration when its content was more than 1.5%.The compressive strength test and four-point bending test show that the bridging effect of PVA fiber mainly controls the micro cracks,and the steel fiber mainly inhibits the macro crack propagation.The performance is that the integrity of the specimen is good after failure,and the ductility characteristics appear.Both fibers can significantly improve the pressure resistance of RPC,and most of the mixed combinations can improve the flexural strength,but the combination gain effect of steel fiber is more significant.(2)Through the analysis of three methods: bending performance index,fiber toughening index and bending toughness evaluation improved according to Japanese JSCE-S4 standard and domestic JGT 472-2015 standard,it is found that steel fiber can greatly increase the peak load,have a more significant impact on energy absorption capacity and have a stronger toughening effect;The toughening effect can be improved by about 1% when the fiber is added with too much PVA.The two kinds of fibers have a significant reinforcing effect on fiber reinforced concrete before the peak load,and make a great contribution to the residual flexural strength and load maintaining capacity after the peak load.It is comprehensively obtained that the optimal mixed fiber combination is 2% of steel fiber volume fraction and 1% of PVA fiber volume fraction.(3)Through the four-point bending test of steel PVA mixed fiber RPC simulated by finite element method.It can be observed from the mesomechanics point of view that the fiber and matrix produce stress at the same time after the load is applied,the fiber stress at the crack is great after the crack,and the effect of strengthening and toughening the concrete can be achieved by transmitting the load to the surrounding through bridging.Through the simulated expansion test,the calculation formula of the effect of the volume content of steel fiber and PVA fiber on the flexural strength of steel PVA mixed fiber RPC is fitted.The mixing effect of flexural toughness was quantitatively analyzed,and the mixing effect function of steel-PVA blended fiber RPC was obtained by surface fitting.The optimal ratio was 2.08% of steel fiber and 1.21%of PVA fiber.(4)Through the extended finite element simulation,it can be observed that the failure mode of the model is similar to the test.From cracking to the later stage of loading,the matrix can still bear large stress due to the incorporation of fiber,the fiber at the crack bears great stress,and the opening displacement of the crack opening is greater under the same load conditions.