Study On Ultra High Performance Concrete Shrinkage And Interfacial Stress In Repair Engineering

In the repairing project,the interface between the new and old concrete is a weak area of the new and old concrete composite structure.The early restraint shrinkage of the repair material will generate a stress field at the interface that is detrimental to the interface performance,which will affect the later use and life of the structure.In the early period,the restrained tensile stress distribution at the interface between UHPC and ordinary concrete,the growth of tensile stress with age and its laws need to be further understood.HCSA expansion agent is suitable for ultra-high performance concrete,which can effectively reduce its free shrinkage and restrained shrinkage,and reduce the restrained shrinkage stress at the interface.It is of great practical significance to study the stress and its distribution at the interface during the early UHPC restrained shrinkage,and the change of the interface stress with time to ensure non-unrestricted cracking of the repair material interface at an early stage and to understand the law of interface stress change.In this paper,six kinds of mechanical properties of UHPC under four kinds of HCSA expansion agents are combined.The combination of free shrinkage and constrained shrinkage is used to theoretically calculate the distribution of interfacial tensile stress and its early change with time and the results are verified by finite element software.The main work and research results are as follows:(1)The compressive strength,elastic modulus and tensile strength of UHPC on days 1,3,7,14,and 18 when the amount of HCSA expansion agent is 0%,4%,6%,and 8% are calculated according to the specifications.According to these three mechanical properties,the creep coefficient,the aging coefficient and the stress relaxation coefficient are calculated.Log function regression was used to derive the functional relationship of mechanical properties in the early 28 days for later stress calculations.The results show that the incorporation of HCSA expansion agent makes UHPC compressive strength decreases,elastic modulus increases,tensile strength decreases,stress relaxation coefficient becomes larger,creep coefficient becomes larger,and aging coefficient decreases.But the impact is not very obvious.(2)The free shrinkage and restrained shrinkage of UHPC during the early 28 days were measured using a non-contact concrete shrinkage gauge and a fiber Bragg grating sensor,respectively.The calculation considers the relative slip of the interface and does not consider the relative slip of the interface.The results show that the incorporation of HCSA expansion agent reduces the free shrinkage,the constrained shrinkage and the difference between free shrinkage and constrained shrinkage of UHPC.The constrained shrinkage strain of the interface is smaller than its free shrinkage,which is tensile stress;the constrained shrinkage strains of four points near the upper surface are larger than the free shrinkage strain,which is compressive stress.The constrained shrinkage tensile stress at the center of the interface is the largest,and the further away from the center of the interface,the smaller the tensile stress caused by the constrained shrinkage.The incorporation of the expanding agent reduces the likelihood of UHPC cracking.The ratio of the non-relative slip of the interface to be greater than the ratio of the relative slip of the interface.When considering the relative slippage of the interface,the ratio of tensile stress and tensile strength of the interface increases rapidly in the previous day,and the growth slows down during 2-7 days,and then the ratio gradually decreases.(3)The ABAQUS simulation was used to obtain the stress distribution of the interface between the old and new concrete,and the UHPC shrinkage was measured by the equivalent temperature difference method.The two cases of interface with or without relative slip use the interface binding and the interface bonding unit cohesive element.The results show that the interface tensile stress distribution changes little when the relative sliding of the interface is not considered.The simulated and theoretically calculated stresses are consistent with the time,but they are not in line with the real situation.When considering the relative slippage of the interface,the distribution of tensile stress along the length direction of the interface changes greatly and decreases sharply.The simulated value of the interface stress at the interface center point is basically consistent with the experimental calculation value.