Study On Mechanical Properties Of Hybrid Fiber Reinforced Engineered Cementitous Composites

In recent years,Engineered Cementitious Composites(ECC)has been widely used in civil engineering.Most of the ECC materials are single fiber system.The ductility of ECC materials can be achieved by adding PVA fibers or PE fibers.However,due to the complexity of the service environment of engineering structures,there are many requirements for material properties(high strength,high toughness,impact resistance,etc.).In order to overcome the limitations of the mechanical properties of the single fiber system,different mechanical properties,different sizes and different functions of fibers can be combined(i.e.hybrid fiber method)to optimize the mechanical properties of engineered cementitious composites to meet the multiple requirements of engineering structures for material properties.In this paper,hybrid fibers are used to improve the tensile performance of engineered cementitious composites,in which the static and dynamic tensile performance of the improved engineering cementitious composites are tested and theoretically studied.The research work is as follows:1.The tensile performance of the improved engineered cementitious composites was investigated..The effects of fiber types(PE fiber,PP fiber and steel fiber),fiber fraction by volumn(0.5%,1%,1.5%and 2%)and laoding rate(0.1 mm/min,1 mm/min,10 mm/min and100 mm/min)on the tensile performance of the modified engineered cementitious composites were studied.2.Based on the theory of fracture mechanics,the mechanical model of static tension of improved engineered cementitious composites was established to describe the toughening and strengthening effects of fiber on the modified engineered cementitious composites.3.The ratio of dynamic and static ultimate tensile strength of the improved engineered cementitious composites was obtained to describe the effect of loading rates on the ultimate tensile strength of the modified engineered cementititous compoistes.Based on the experimental and theortical results,the following conclusions can be obtained:1.Under the same loading rate,for the PE-ECC and PP-ECC,the higher the volume fraction of PE and PP fiber,the greater the peak stress and the greater the ultimate strain.Whereas for the ES-ECC and PS-ECC,the higher the volume fraction of steel fiber,the greater the peak stress,At the same time,it was found that the higher the content of PE fiber or PP fiber,the greater the ultimate strain.The effect of PE and PP fibers on the development of ductility of ECC was better than that of steel fiber.The effect of steel fiber on the development of tensile strength of ECC was better than that of PE and PP fibers.2.PP-ECC and PS-ECC performed the strain hardening behavior under the vaired loading rates.It can be found that the PP fiber can effectively increased the ductility of the cementitious composite,and then the hybrid of PP and steel fibers can be used to prevent the brittle failure of the cementitious composite and achieve the high tensile strength.Under the different loading rates,the PE-ECC and ES-ECC also performed the strain hardening behavior.The strain hardening performance of the PE-ECC and ES-ECC was not influenced by the loading rates significantly,which demonstrated that the PE fiber was superior to the PP fiber.3.Based on the results of static and dynamic load tests,the optimal ratio of hybrid fiber engineered cementitious composites with high tensile strength and energy absorption capacity under dynamic load was the E1.5S0.5(1.5%PE fiber+0.5%steel fiber),and the P1S1(1%PP fiber+1%steel fiber).At the loading rate of 100 mm/min,the peak stress,the ultimate strain and the energy absorption of the E1.5S0.5 was 4.58 MPa,3.4%,and 13.8 MJ/m~2,respectively;while for the P1S1,its peak stress,ultimate strain and energy absorption was found to be 6.32 MPa,1.6%and 7.5 MJ/m~2,respectively.The E1.5S0.5 had better toughness whereas the P1S1 had higher tensile strength.However,it should be noticed that the cost of the PE fiber was higher than that of the PE fiber,and hence it is suggested to adopt the PP fiber in the hybrid fiber engineered cemetitious composite.4.Based on the theory of fracture mechanics,the devived tensile model of hybrid fiber engineered cementitious composites can well describe the ultimate tensile strength of cementitious composites under static loading.The dynamic ultimate tensile strength of hybrid fiber engineered cementitious composites can be futher obtained through the product of dynamic strength coefficient and the static ultimate tensile strength...