Experimental Investigation On Long-Term Performance Of FRP-Glulam-UHPC Composite Beam

In order to promote the application of Timber-Concrete composite structure in bridge engineering and promote the transformation of modern new Bridges to environmentally friendly Bridges,aiming at the problem that the design of traditional Timber-Concrete composite structure does not meet the long-term deformation limit due to the lack of major and long-term performance,In this paper,UHPC(Ultra-High Performance Concrete)and FRP(Fiber Reinforced Polymer)are introduced into conventional TimberConcrete composite beams.This paper proposes a new FRP-Glulam-UHPC composite structure,which can be integrated precast,with small dead weight,small long-term deformation of material and large bending rigidity,in order to solve the problem of long-term performance shortage of traditional Timber-Concrete beams,which provides a new idea and feasibility for the practical application and technological innovation of our country’s wood-concrete bridge.In this paper,experimental research,theoretical calculation and parameter analysis are combined to study the long-term performance of FRP-Glulam-UHPC composite beams affected by ambient temperature and humidity.The main work is as follows:(1)In order to reveal the time-varying performance of the new composite structure,taking the type of concrete slab,load level and FRP reinforcement ratio as the main parameters,two types of five test beams were designed and manufactured,one is the traditional Glulam-Concrete composite beam,the other is the new Glulam-UHPC composite beam(with or without FRP bars).A short-term static load test of a Glulam-UHPC composite beam was completed to determine the long-term load.At the same time,a 298d long-term loading test was carried out on four test beams.The specimens were constantly loaded in an uncontrolled indoor environment.During the test,the ambient temperature and humidity,the water content of glulam,the mid-span deflection,the section strain and the relative slip of interface were measured.Pu is the influence of failure load of test beam)on the long-term performance of test beam.(2)The test results show that compared with ordinary Glulam-Concrete composite beams,the initial stiffness of Glulam-UHPC composite beams and FRP-Glulam-UHPC composite beams increased by 11.8%and 25.7%,the creep coefficient of 298d decreased by 52%～116.0%and 23.8%～81.0%and the relative slip of the interface decreases by 25.7%and 32.8%,respectively,indicating that the application of UHPC and FRP can improve the initial stiffness and long-term performance of the composite beams.At 298d,the strain of the UHPC roof in the FRP-Glulam-UHPC composite beam is 0.496 times that of the ordinary concrete slab,and the strain amplitude is reduced by 34.0%.The bottom strain of glulam beams implanted with FRP bars is 0.564 times of that of ordinary glulam beams,and the increase of strain is reduced by 76.8%.The excellent long-term performance of UHPC and FRP makes them as the upper and lower structure of composite beams significantly improve the long-term performance of composite beams.For Glulam-UHPC composite beams under the same load level,the inhibition degree of long-term mid-span deflection of composite beams implanted with FRP with 1%reinforcement ratio is limited,but FRP has obvious inhibition effect on the bottom strain of Glulam-UHPC beams.The fluctuation of FRP-Glulam-UHPC composite beam under the influence of ambient temperature and humidity is obviously less than that of Glulam-UHPC composite beam.Therefore,the improvement effect of FRP on the long-term performance of Glulam-UHPC composite beams is mainly reflected in reducing the variation degree of long-term performance.When the load level is doubled(13%Pu→26%Pu).the initial deflection of the FRP-Glulam-UHPC composite beam will increase significantly,but the better long-term deformation resistance of the UHPC plate inhibits the overall long-term deformation of the composite beam,which makes it have a smaller creep coefficient compared with the same type of test beam at low load level and thus shows better long-term performance.(3)Based on the concrete shrinkage and creep principle,timber creep principle and considering the long-term deformation effects of shear connectors,three Timber-Concrete long-term deformation calculation models(γ method,reduced section method considering the relative slip effect,simplified method)were summarized and compared,and the applicability of the three calculation models was analyzed according to the fitting results.The results show that for traditional Glulam-Concrete composite beams,the three calculation methods can better characterize the long-term deformation of test beams.For Glulam-UHPC composite beams,the γ method and the reduced section method considering the effect of relative slip are more suitable,but there are some defects.For the test beams with 13%Pu,Due to the influence of low stress level and interfacial friction,the initial relative slip is small and it is difficult to make linear prediction,resulting in poor fitting accuracy of the calculation model in the early stage.For the test beam with 26%Pu,due to the poor sensitivity of the calculation model to ambient temperature and humidity,the fluctuation of the fitting curve is insufficient.(4)The main parameters were compared and the influencing factors were analyzed for the calculation models of the long-term deformation of the three Timber-Concrete composite beams,and the sensitivity of the parameters was analyzed for the calculation models of the γ method with good fitting effect and the converted section method considering the influence of relative slip,and the key parameters leading to the errors in the fitting between the two calculation models and the test beam mid-span deflection were obtained.Based on this,the Timber-Concrete long-term deformation calculation model is modified,and a more suitable long-term deformation calculation model for FRP-Glulam-UHPC composite beams is obtained.The fitting accuracy of the calculation model is improved greatly,and it can effectively simulate the long-term deformation trend of FRP-Glulam-UHPC composite beams under the influence of ambient temperature and humidity at different times.