Experimental Study On Flexural Performance Of Near-surface Mounted SMA Strengthening Of Concrete Beams

Reinforced concrete beams are commonly used in bridge engineering.Due to the low tensile strength of concrete,various types of cracks are prone to occur in the tension zone of beams and other key parts,which seriously affects the safety and durability of bridges.Therefore,how to reinforce and repair damaged concrete beams is an urgent problem to be solved.Based on the frontier development of concrete bridge reinforcement research at home and abroad,aiming at the limitations of existing reinforcement methods,this paper proposes an active reinforcement method without mechanical tension by using the unique shape memory characteristics of shape memory alloy(SMA)wire.In this method,the SMA wire embedded in the protective layer of the structure can be pre-stressed by thermal excitation,and then the concrete structure can be actively strengthened.Based on domestic nickel-titanium-niobium-based shape memory alloys(NiTiNb-SMA)wires,the following aspects are studied in this paper.(1)The stress-strain relationship of SMA wire at room temperature was studied by tensile tests,and the physical and mechanical properties of the alloy wire were determined.(2)The influence of initial pre-strain and excitation mode of SMA on the recovery force was discussed through the recovery tests.It is found that 10% pre-deformed SMA can produce the maximum permanent recovery stress through thermal excitation,and the direct current excitation method is more feasible in practical engineering.(3)In order to solve the problem of poor bonding effect between smooth SMA wire and concrete,a near-surface mounted anchorage system for SMA reinforced concrete beams is developed.Four kinds of small anchors are selected and designed in this system to improve the anchorage performance between SMA and concrete.The pull-out tests between SMA and anchor and the recovery tests of SMA embedded in mortar prove that the improved block anchor with the hex socket bolts can ensure that the alloy wire does not slip during the process of thermal excitation SMA and can provide effective prestressing force.(4)Considering the number of SMA,the unbonded length of SMA and the damage of the original structure,the bending tests of reinforced concrete beams strengthened with NiTiNb-SMA wires embedded in the surface were carried out by three-point loading method under reverse loading.The test results show that the yield load and ultimate load of concrete beams can be increased by the reinforcement with SMA wires embedded in the surface;the longer the unbonded length is,the more obvious the recovery effect is.