Study On Size Effect Of Basic Mechanical Properties Of Ultra-High Performance Concrete

Ultra high performance concrete（UHPC）removes the coarse aggregate in concrete,adds fiber,and uses admixtures to achieve low water binder ratio so that this can enhance the physical properties of concrete.Because of its good toughness and durability,high compressive strength,wonderful wear resistance and impact resistance,it is widely used in super high-rise buildings,large span structures,offshore platforms and other structures in bad environments,as well as military structures.At present,ultra high performance concrete has become a research hotspot for experts and scholars.Because UHPC is broadly used in long-span structures,it should be considered that the influence of size on the mechanical properties of UHPC.Therefore,the research of size effect has great engineering significance and practical value.Based on this,this paper presents an experimental study and theoretical analysis of the size effects of UHPC’s,splitting tensile strength and bending toughness,using specimen size,steel fibre volume admixture and length as variables,and a fine-scale simulation of a four-point flexural prism.The main conclusions are as follows:According to UHPC cube compression and splitting tensile size effect test,with the increase of steel fiber volume content,the size effect of UHPC cube compression and splitting tensile becomes more pronounced,and compared with the addition of long steel fiber,the size effect of UHPC cube compression strength and splitting tensile strength is lower than that with the addition of short steel fiber.Weibull’s strength theory was also used to derive formulas for calculating the compressive and splitting tensile strengths of specimens with different sizes,lengths and volumes of steel fibres incorporated into the specimens for a range of specimen sizes from 70.7 mm to 150 mm.The size effect test of UHPC bending performance shows that the volume fraction and length of steel fiber have strong influence on the initial crack strength P_LOPand initial crack deflectionδ_LOP.When the specimen size is fixed,with increasing volume content and length of steel fibres,the increase of P_LOPandδ_LOPis extremely large.Through analyzing the equivalent bending stress f _LOP,equivalent deflectionδ_LOP/L and equivalent bending toughness T_f-δ/Lof UHPC,it is indicated that the steel fiber volume content and length has little effect on the size effect of UHPC bending toughness.When the specimen is initially cracked,the size effect of f _LOPand T_f-δ/Lis consistent with that of UHPC matrix.By comparing the fine view simulation with the experimental results,the two are in good agreement,which verifies the validity of the finite element simulation and provides a certain numerical simulation basis for future simulation studies.Based on Bazant theory and steel fiber drawing mechanism,the size effect mechanism was analyzed from the aspects of UHPC matrix fracture,steel fiber drawing and torsion.The analysis shows that the essence of UHPC matrix size effect is the imbalance between energy release rate and crack energy consumption rate in the process of cracking.The nature of steel fiber drawing size effect is caused by the unbalanced fracture energy of UHPC matrix interface;the nature of the kink size effect in steel fibres is due to the size of the kink band formed by the steel fibres in terms of their ability to transmit stress.