A Macro Model of Seismic Coupling Beams with Small Aspect Ratio Adopting New Reinforcement Arrangement Scheme

A new reinforcement arrangement scheme was suggested to provide satisfactory seismic performance and good workmanship for the coupling beams with small aspect ratio. Coupling beams with small aspect ratio are counter flexural deep beams with both ends restrained, in which the Bernoulli hypothesis are not valid. Although their load bearing capacity can be predicted with the strut-and-tie model developed recently, how to simulate this kind of beams adopting new reinforcement arrangement is still a problem. A macroscopic strut-and-tie model is thus presented for these coupling beams. Good agreement with experimental results is achieved.     

Analysis of Fiber Renforced Concrete Beam-Column Joints Model

Due to the large stirrup ratio and reinforcement congest in beam-column joints, the ordinary reinforced concrete beam-column joints take inconvenience to construction. Cracked fiber renforced concrete (FRC) has strong bridge ability and better tensile performance so that it can replace part or all of the stirrups. Based on previous researches on resistance mechanism of reinforced concrete joints, a new model using FRC materials in the core zone of beam-column joints is presented. It is a kind of model in which horizontal shear supported by the diagonal strut mechanism and softening truss mechanism with a certain percentage. The calculation results of the model is compared with the existing test results. It is a bit conservative to specimens with low axial load ratio. However, the results are in line with the specimens with high axial load ratio. Therefore, the results totally demonstrate the rationality of the proposed model in this paper. Meanwhile, according to the proposed model, the shear capacity of beam-column joints can be not only calculated, it also check whether FRC compressive strength in core zone of joints and horizontal stirrup ratio meets design requirements, which has a higher practicability.     

Calculation Method of Shear Behavior of Hybrid Fiber Reinforced High Performance Concrete Deep Beams

The static tests on hybrid fiber (steel fiber and polypropylene fiber) reinforced high performance concrete deep beams according to the orthogonal experimental design were conducted. The shear capacity and calculation method of deep beams were discussed as well. The contributory factors such as the characteristic parameters of steel fiber (types, volume fraction, aspect ratio), the volume fraction of polypropylene fiber, the ratio of web horizontal reinforcement and the ratio of web vertical reinforcement were analyzed. Results show that the shear failure mode of deep beams is changed with adding a reasonable volume of hybrid fibers, and hybrid fiber can greatly increase the diagonal cracking strength and shear strength of HPC deep beams. The diagonal cracking strength is increased by 45.2% averagely while the shear strength is increased by 25.6% averagely. A satisfied result is obtained when the plasticity theory is used to analyze shear behavior of hybrid fiber reinforced HPC deep beams. The contribution of web horizontal reinforcement and web vertical reinforcement to shear strength of deep beams is not obvious but the former plays a major role. After analyzing the strengthening mechanism of hybrid fiber, a formula to calculate the shear capacity of hybrid fiber reinforced HPC deep beams is presented based on spatial strut-and-tie mode and splitting failure.     

Experimental Analysis of Pre-cracked RC Beams Strengthened with EB-FRP and HB-FRP

On the basis of the crack characteristics of aging RC beam bridges, the cross-section of RC simple beams with different pre-cracking levels were strengthened by the externally bonded FRP（EB-FRP）and the FRP-bolt hybrid bonding（HB-FRP）respectively to check the strengthening characteristics of HB-FRP. The flexural test with symmetrical loading on two points was conducted to study the mechanical behavior and failure mode of pre-crack beams strengthened with EB-FRP and HB-FRP. The influence of pre-cracking level on the FRP initial debonding load and FRP utilization ratio was assessed respectively. The test results show that the flexural capacity of pre-cracking beam strengthened with HB-FRP can be improved by 30%~44% compared with the one with EB-FRP. The failure mode has been changed from FRP brittleness debonding of beam strengthened with EB-FRP to obvious ductility failure of beam strengthened with HB-FRP. Furthermore, the crack distribution of RC beam has obvious influence on the FRP initial debonding load, the FRP utilization ratio and the failure mode of strengthened pre-crack beams.     

Bond properties of CFRP-high performance concrete subjected to freeze-thaw cycles

通过双面剪切试验,研究了冻融环境下CFRP-高性能混凝土界面粘结性能的发展规律。对比分析了未经冻融和经历25、50、100、150、200及300次冻融循环作用试件的破坏特征、剪应变分布、荷载滑移曲线、粘结承载力以及粘结破坏机理。结果表明,所有试件的界面破坏均发生在混凝土表层内,但随着冻融循环次数的增加,破坏界面有向胶层发展的趋势;经受冻融循环次数较少时(25、50次),界面的粘结强度、刚度及开裂荷载的变化不明显,甚至略微提高;但随着冻融循环次数的进一步增加,界面粘结性能有明显的变化,界面粘结强度、端部滑移量减小,刚度退化,初始开裂荷载水平降低,非线性特征增强。粘结极限承载力与混凝土立方体抗压强度均随冻融循环次数的增长存在先提高后下降的趋势,混凝土强度变化是界面粘结性能变化的最重要因素。