Research On Preparation And Properties Of Ultra-high-toughness Cementitious Materials

With the development trend of engineering structure to large span,high rise and super large structures,higher performance concrete is needed.High strength has become the major direction of concrete development.Due to the relatively higher strength,high strength concrete structure has smaller cross-section and dead weight.And the durability of high strength concrete is better than that of normal concrete,due to the dense microstructure of high strength concrete resulted from low water-binder ratio and pozzolanic effect of mineral admixtures.However,with the increase of compression strength,the toughness of concrete decreases and the concrete becomes brittler,which blocks the development of high strength concrete.For example,in bridge engineering,many practices show that merely to improve the strength of concrete cannot meet the requirements of the crack resistance and durability of the bridge.High strength and toughness is the developing trend of the cementitious material used in civil construction and transportation field.In this study,high volume fraction steel fiber and calcium carbonate whisker were used to two-scale reinforce high strength cementitious materials to prepare ultra-high-toughness and high strength cementitious materials by technology of reactive powder concrete(RPC)and slurry infiltrating steel fibers concrete(SIFCON)and the mechanical properties of ultra-high-toughness cementitious materials were investigated.The results obtained in this study can be summarized as in the following:(1)Basic mechanical properties of specimens that are prepared by high volume steel fibers reinforcing RPC increase with the volume of steel fibers increasing.But when the volume of steel fibers exceeds 5%,the basic mechanical properties of RPC cannot be improved and even decline due to nonuniform distribution of high volume steel fibers.The volume fraction of steel fibers of SIFCON can reach 12.2% and 28 d flexural strength of SIFCON can be up to 53.1 MPa.When compressive strength of SIFCON specimens is tested,the strength of specimens cannot be got due to the ductility of specimens as high as metallic materials.(2)The compressive stress-strain curves of SIFCON did not appear to peak after the crack-stabilized expansion stage,but rather the straight line continuing to grow.At this stage,the steel fibers cross-linked with each other have strong mechanical bite force,and with the matrix have strong sliding resistance.Thus,specimens' bearing capacity continues to rise,and the specimens' ultimate strain exceeds 0.08.(3)After the compressive strength test,steel fiber reinforcing RPC will not be completely destroyed and still have a certain carrying capacity and good appearance quality.The appearance quality of the specimens after test increase with the increase of steel fiber content.The two times compressive strength ratio of steel fiber reinforcing RPC can reach 0.80 or more in the secondary compressive strength test,and the specimens have good secondary carrying capacity.(4)The flexural toughness of steel fiber reinforcing RPC increases with the increase of steel fiber content,and the flexural load-displacement curve of steel fiber reinforcing RPC starts to appear to false strain hardening after the steel fiber content reaching 4%.The flexural toughness of the slurry infiltrating steel fiber cementitious materials is significantly improved compared with that of the based group.There is obvious false strain hardening phenomenon,and the peak displacement is more than 5.5mm.Increasing the water-cement ratio will reduce the flexural strength of the slurry infiltrating steel fiber cement-based materials,but will increase its bending deformation ability and increase the peak displacement.But the steel fiber reinforcing RPC and slurry infiltrating steel fiber cement-based materials are unable to produce false strain hardening phenomenon in uniaxial tensile test.(5)The ability of steel fiber reinforcing RPC to resist impact load increases with the increase of steel fiber content,and the impact absorbing power of 5% steel fiber is 6.42 J.Low water-binder ratio is conducive to improve the ability of the specimen to resist the impact load,and the slurry infiltrating steel fiber cement-based materials with 0.16 water-binder ratio has the highest impact absorption energy 7.02 J.Aggregate can also improve the impact resistance of cement-based materials.(6)In this study,the optimum content of calcium carbonate whiskers is 1%.Moderate amount of calcium carbonate whiskers and steel fiber double-scale reinforcing effect is better than single steel fiber reinforcing effect.After adding calcium carbonate whiskers,The basic mechanical properties,bending properties,direct tensile properties and impact resistance of the specimens are improved.However,when the content of calcium carbonate whiskers is too high,the mechanical properties of the specimens are reduced due to nonuniform distribution of the whisker.In this paper,infiltration steel fiber technology and calcium carbonate whisker are used to prepared ultra-high-toughness cement-based materials which uniaxial tensile strength can be up to 20 MPa or more and ultimate tensile strain exceeds 1.3%,and that has excellent bending performance,impact resistance and compression deformation ability.It has broad application prospects in the underground engineering,seismic structure,large deformation structure,impact structure and other engineering structure,and even expected to replace some of the metal materials as mechanical engineering materials to expand cementitious materials application fields.