装配式预应力混凝土梁与高强钢筋约束混凝土柱连接节点抗震性能试验研究

提出了一种新型全装配式预应力混凝土梁与高强钢筋约束混凝土柱端板螺栓连接节点形式,在低周反复水平荷载作用下,进行了6个装配式预应力中间节点试件和1个现浇节点试件的对比试验,得到了试件的破坏形态、滞回曲线、骨架曲线、延性系数、刚度退化以及耗能能力等抗震指标,确定了该新型装配式梁-柱连接节点的抗震性能。试验结果表明,新型全装配式预应力混凝土梁与高强钢筋约束混凝土柱端板螺栓连接节点试件均实现了"强柱弱梁"的设计目标。试件的滞回曲线饱满,抗震性能良好,研究成果可为预制装配式框架在地震区的推广应用提供理论依据和技术支持。     

A comparison of engineered cementitious composites versus normal concrete in beam-column joints under reversed cyclic loading

Engineered cementitious composites (ECC) are a class of high-performance fiber reinforced cementitious composite with strain hardening and multiple cracking properties. For a reinforced concrete member, substitution of conventional concrete with ECC can significantly improve the deformation characteristics in terms of reinforced composite tensile or shear strength and energy dissipation ability. In this paper, a number of RC/ECC composite beam-column joints have been tested under reversed cyclic loading to study the effect of substitution of concrete with ECC in the joint zone on the seismic behaviors of composite members. The experimental parameters include shear reinforcement ratio in the joint zone, axial load level on the column and substitution of concrete with ECC or not. According to the test results, for the specimens without shear reinforcement in the joint zone, substitution of concrete with ECC in the joint zone cannot change the brittle shear failure in the joint zone, but can significantly increase the load capacity and ductility of the beam-column joint specimens, as well as the energy dissipation ability due to high ductility and shear strength of ECC material. For the specimens with insufficient or proper shear reinforcement ratio, substitution of concrete with ECC in the joint zone can lead to failure mode change from brittle shear failure in the joint zone to a more ductile failure mode, i.e. flexural failure at the base of the beam, with increased load capacity, ductility and energy dissipation ability. Increase of axial load on column and shear reinforcement in the joint zone have little effect on seismic behaviors of the members when they failed by flexural failure at the base of beam. In a word, the substitution of concrete with ECC in the joint zone was experimentally proved to be an effective method to increase the seismic resistance of beam-column joint specimens.     

Seismic performance and prediction equations of sandwich beam-column joints subjected to skew cyclic loads

To study the seismic performance of sandwich beam-column joints constructed with high strength concrete column and normal strength concrete floor system and joint regions, four specimens with different column to beam concrete strength ratios (α) were tested under skew cyclic loads. The performance indices of the specimens including the failure mode, ductility, energy dissipation were compared and analyzed. The results show that the failure modes of the sandwich joints are in form of joint shear failure after yielding of the beam, while the ductility coefficient is found to be greater than 3.0. Compared to the joints with low concrete strength ratios α, the specimen with a high concrete strength ratio α features larger deformation at the joint. Based on the softened strut-and-tie model, a set of shear strength prediction equations for the sandwich beam-column joint, taking into account the effects of the concrete strength ratio α, floor slabs and plastic region of beams, is proposed. Comparison of the present tests against the published literature confirms that the shear strength of the sandwich joints can be well predicted by the proposed model.     

Numerical investigation of concrete subjected to high rates of uniaxial tensile loading

The present article is concerned with the response of structural concrete prisms to high rates of uniaxial tensile loading. The numerical investigation carried out is based on a finite-element (FE) program capable of carrying out three-dimensional (3D) nonlinear static and dynamic analyses. This program is known to yield realistic predictions to the response of a wide range of plain- and reinforced-concrete structural forms subjected to arbitrary static and earthquake actions. Furthermore, its application has recently been successfully extended in predicting the response of plain-concrete prism elements under high rates of uniaxial compressive loading. The main feature of the FE program is that it incorporates a 3D material model which is characterized by both its simplicity and its attention to the actual physical behaviour of concrete in a structure. Its analytical formulation is based on the assumption that the material properties of concrete are independent of the applied loading rate (strain rate) thus attributing the effect of the applied loading rate on the prism's response to inertia. The validation of this assumption is based on a comparative study between numerical and experimental data which reveals good agreement. This constitutes a major departure from current thinking as regards material modelling of concrete under high-rate loading. In addition, the available data (numerical and experimental) show that the response of the concrete prism elements depends on a number of parameters linked to geometry and material properties of the structural forms under investigation as well as the testing method adopted. This dependence explains, to a significant extent, the scatter that characterizes the available experimental data, and it also suggests that both experimental and numerical results describe structural rather than material behaviour thus raising questions regarding the validity of the use of such data in the constitutive modelling of concrete-material behaviour under high-rate loading conditions.     

A damage-plasticity interface approach to the meso-scale modelling of concrete subjected to cyclic compressive loading

Concrete is characterised by stiff inclusions in a soft matrix separated by weak interfacial transition zones (ITZs). Subjected to cyclic loading, this material exhibits a strongly nonlinear response, which is characterised by the occurrence of hysteresis loops. Furthermore, for cyclic loading, failure may occur before the equivalent strength for monotonic loading is reached. The present work investigates, whether the occurrence of permanent displacements in different phases of the meso-structure of quasi-brittle heterogeneous materials, such as concrete, leads to damage evolution during repeated loading.A new three-dimensional interface model based on a combination of damage mechanics and the theory of plasticity is proposed, which allows one to control the ratio of permanent and total inelastic displacements. The model is based on only a few material parameters, which can be directly determined by experiments.The interface model is applied to the plane-stress analysis of an idealised heterogeneous material with cylindrical inclusions and ITZs subjected to cyclic compressive stresses.     

Cyclic behavior of HPFRC-repaired reinforced concrete interior beam-column joints

A large number of old buildings have been identified as having potentially critical detailing to resist earthquakes. The main reinforcement of lap-spliced columns just above the joint region, discontinuous bottom beam reinforcement, and little or no joint transverse reinforcement are the most critical details of interior beam column joints in such buildings. Five 1/3-scale interior beam column joints representing these critical reinforcement details under seismic loads were constructed and tested under cyclic lateral loading simulating seismic excitation. These specimens behaved weakly such that they attained low load carrying capacity, small energy dissipation, and failed in diagonal shear in the joint region. After testing, the specimens were repaired using high performance fiber reinforced concrete (HPFRC) jacket, all around the joint column regions, and retested up to failure. Higher load levels were attained, more ductile behavior was achieved, substantial energy dissipation was observed, slower stiffness degradation was also noted. Failure modes were transformed from brittle shear in the joint diagonals to a ductile one in the beam region through plastic hinge formation in the beam maximum moment sections. Based on the findings of this study, the strengthening technique used promises to provide a cost effective, long-term repair and retrofit solution that can be implemented in the field.

---

Résumé Un grand nombre de constructions anciennes a été identifié comme ayant partiellement une situation critique vis-à-vis de la résistance aux tremblements de terre. Pour de telles structures, les détails les plus critiques des joints de colonnes de poutres intérieures sont l'armature principale des colonnes assemblées les unes au dessus des autres, directement au dessus de la zone des joints, le renforcement discontinu de la partie inférieure de la poutre, ou encore peu ou pas d'armatures transversales au niveau des joints. Des joints de colonnes de poutres intérieures à échelle 1/3 représentant ces éléments critiques liés au renforcement sous des charges sismiques ont été construits et testés sous un chargement cyclique latéral simulant une excitation sismique. Ces spécimens ont révélé un comportement si faible qu'ils ont atteint une basse capacité de charge porteuse, peu de dissipation d'énergie et qu'ils ont subi une défaillance du cisaillement diagonal dans la zone des joints. à l'issue de l'essai, ces échantillons ont été réparés en utilisant du béton de hautes performances renforcé par des fibres (HPFRC) tout autour de la zone des joints. Puis ils ont à nouveau été testés jusqu'à leur rupture. Des niveaux de charge plus élevés ont été atteints et on a observé un comportement plus ductile et une dissipation substantielle d'énergie, ainsi qu'une dégradation de la ténacité moins rapide. Les modes de rupture sont passés d'un cisaillement fragile au niveau des diagonales des joints à un cisaillement ductile au niveau des sections de poutre au moment maximal. S'appuyant sur les résultats de cette étude, la technique de reforcement utilisée propose une réparation à long terme pour un bon rapport qualité-prix et offre une solution qui peut être appliquée avantageusement sur le terrain.

     

Crack propagation in concrete subjected to flexuralcyclic loading

Fatigue tests were carried out on notched microconcrete beams subjected to three-point bending tests in a saturated atmosphere. The feature of the crack growth was studied by means of the replica technique associated with scanning electron microscopy, in conjunction with the measurement of the crack mouth opening displacement (CMOD). Two domains must be distinguished in which the fatigue can be considered, depending on the upper boundary of the cyclic load. Beyond a given threshold for the upper boundary of the cyclic load, the first cycle induces an actual crack at the tip of the artificial notch. Observations of fatigue crack propagation are presented in that case and compared with previous investigations in static tests. The influence of the load level on the crack growth rate is discussed.

---

Résumé Des essais de fatigue ont été réalisés sur des éprouvettes encochées de microbéton, en flexion trois points sous atmosphère saturée. La topographie de la croissance de fissure a été observée au microsope électronique à balayage par la technique de la réplique, couplée avec la mesure de l'ouverture de la fissure. Deux domaines de comportement à la fatigue, dépendant du niveau maximal de charge doivent être distingués dans de tels essais de fatigue. Au delà d'un seuil de charge, le premier cycle provoque déjà l'ouverture d'une fissure réelle à la pointe de l'encoche artificielle. Les observations de la propagation de fissure par fatigue sont menées dans ce cas. L'influence du niveau de charge sur la vitesse de propagation est aussi étudiée.

---

Editorial Note Prof. Alain Bascoul is a RILEM Senior Member and a member of Technical Committee 148-SSC, “Test Methods for the Strain Softening Response of Concrete”.     

Fly ash concrete subjected to thermal cyclic loads

The present study describes the behaviour of concrete as well as fly ash concrete when subjected to varying number of high temperature heating cycles. A Concrete mix (1:2.37:2.98) with 340 kg/m³ cement and w/cm ratio 0.45 was prepared. Cement was replaced by varying percentages (0%, 20%, 40%, 50% and 60%) of fly ash by weight of cement. The concrete was subjected to a constant temperature of 200°C for 7, 14, 21 and 28 heating cycles. One heating cycle corresponds to 8 h heating and subsequent cooling in 24 h. Subsequently the effect of temperature on the properties of the concrete was investigated and compared with that of the properties of unheated concrete. The compressive strength of plain as well as fly ash concrete increased when it was subjected to thermal cyclic loads. Moreover, the compressive strength increased with an increase in number of heating cycles. Thermal conductivity of concrete was found to decrease with an increase in the fly ash content.     

复合加固方形木柱的抗震性能分析

基于性能抗震设计思想的600 MPa 级高强钢筋混凝土柱位移角限值研究,对于推广高强钢筋混凝土柱的应用至关重要。通过17根HTRB630高强钢筋混凝土柱试件和3根HRB400钢筋混凝土柱试件的拟静力试验研究了其抗震性能。与HRB400钢筋混凝土柱相比,HTRB630高强钢筋混凝土柱的滞回曲线的形状没有发生明显改变,试件具有较好的承载能力和延性。将基于 Kunnath 损伤模型计算的损伤指数与试验中测量的损伤指数范围进行了比较,结果表明,Kunnath 损伤模型可以准确计算HTRB630高强钢筋混凝土柱的损伤指标。根据HTRB630高强钢筋混凝土柱的破坏特点,以屈服点、峰值点和极限点作为高强钢筋混凝土柱的性能水平控制点。基于性能抗震设计的思想,将600 MPa 级高强钢筋混凝土柱的性能水平划分为5个性能水平,即正常使用、暂时使用、修复后使用、生命安全和接近倒塌性能水平。结合参考文献中600 MPa 级高强钢筋混凝土柱的试验数据,对65个600 MPa 级高强钢筋混凝土柱各特征点的位移角进行了相对频率统计分析,得到了600 MPa级高强钢筋混凝土柱在暂时使用、修复后使用和接近倒塌3个性能水平下具有超过90%安全保证率的位移角限值分别为1/150、1/80和1/60。

     

Joint investigation of concrete at high rates of loading

Uniaxial tensile tests have been carried out on a micro-concrete with strain rates between 0.5 and 1.25 s⁻¹. The investigation is described and the results are discussed with respect to the influence of water content on the strain-rate sensitivity.     

Mechanical behavior of reinforced concrete subjected to impact loading

Understanding the behavior of concrete and reinforced concrete at high strain rates is of critical importance in a range of application. The behavior of concrete and reinforced concrete at strain rates of the order of 10⁴/s and pressure up to 1.5 GPa are studied experimentally. The concrete analyzed has the same composition and processing conditions as the matrix phase in the reinforced concrete. The dynamic compression experiments of reinforced concrete are carried out by one-stage light gas gun apparatus which subjects the reinforced concrete to deformation at strain rates of the order of 10⁴/s with confining pressures of 1–1.5 GPa. The voltage–time signals are recorded by the manganin pressure gauges embedded in the target. The stress–strain curves of reinforced concrete with different impact velocities are obtained using Lagrangian analysis, from which the distribution regulations of other mechanical parameters such as specific internal energy and specific volume in the flow field are acquired. Experimental results indicate that the load-carrying capacities of concrete and reinforced concrete increase significantly with strain rate. The concrete and reinforced concrete are non-linear, rate-sensitive and pressure-dependent.     

Damage-induced property changes in composites subjected to cyclic thermal loading

Damage in the form of transverse cracks resulting from thermal loading is studied as it relates to the dimensional stability of flat laminates and stiffness changes in cylindrical tubes. Graphite-epoxy specimens were subjected to cyclic thermal loading in the temperature range −250 to +250°F. It is shown that transverse cracking is the dominant damage mechanism in both types of structural elements. Fiber splitting is also quite common at the low test temperatures. Experimental results indicate that damage significantly reduces the inplane coefficients of thermal expansion of flat laminates and the torsional stiffness of the tubes. Theoretical predictions for coefficients of thermal expansion as a function of crack spacing in flat laminates followed the same trend as experimental results.     

Pull-out tests of steel-encased specimens subjected to reversed cyclic loading

When deformed bars are anchored in concrete, this gives rise not only to bond stresses but also to splitting stresses. To measure the splitting stresses, tests were carried out in which a reinforcement bar was pulled out of a concrete cylinder surrounded by a thin steel tube. The tangential strains in the steel tube were measured, together with the applied load and slip. In five tests, specimens were loaded by monotonically increasing the load, while nine other tests were subjected to reversed cyclic loading. All of the tests resulted in pull-out failures. The results from the monotonic tests indicate that the splitting stresses decreased after the maximum load had been obtained, however not as much as the load decreased. The results from the cyclic tests show a typical response for bond in cyclic loading. When there was almost no bond capacity left the measured strain in the steel tubes stabilised and remained more or less unaffected by the last load cycles. The test results give valuable information about the splitting stresses that result from anchorage of reinforcement bars in concrete. These test results can be useful as a reference when calibrating models of the bond mechanism, and give a better understanding of the bond mechanism.     

Piezoresistive effect of plain-weave CFRP fabric subjected to cyclic loading

The present study deals with electrical resistance changes in woven-fabric CFRP during loading. Four kinds of plain weave woven-fabric CFRP laminated specimens are prepared and subjected to cyclic tensile loading that does not cause any damages, and the electrical resistance changes of the specimens are measured experimentally by the four-probe method. As a result, the present study shows that the electrical resistance of a specimen comprised of six ±45° plies decreases remarkably with increasing number of loading cycles. The decrease is caused by shear plastic deformation of ±45° plies. The thickness shrinkage caused by shear plastic deformation increases the number of fiber contacts, and this decreases the interlaminar contact resistance between the plies. For a single ±45° ply, the same electrical resistance decrease caused by the shear plastic deformation is observed, and the magnitude of the decrease is smaller than that of the six-ply laminate tested. This is because the effect of interlaminar contact resistance decrease does not exist for a single ±45° ply. For the six 0°/90° plies, the present study shows that electrical resistance in the through-thickness direction is decreased by out-of-plane plastic deformation of carbon fiber and misalignment of the plies.     

Fracture energy of concrete at high loading rates in tension

The effect of high loading rates in tension on the failure energy and strength of concrete is reported in this paper. High loading rates exceeding 5000 GPa/s corresponding to strain rates higher than ∼120 s⁻¹ can be applied by use of Hopkinson bar set-up designed to produce spall. Tension tests were performed on cylindrical specimens made of micro-concrete. At high loading rates, or strain rates, the failure energy of micro-concrete, as well as the strength, was found to substantially increase.     

Adhesively bonded joints under cyclic loading spectra

Abstract Current designs which involve the use of composite materials in primary aircraft structures are often conservative. This, in turn, significantly lowers the weight advantage that composites have over established metallic airframe materials. Strain restrictions are often applied because the failure mechanism(s) in (fibre) composite joints and stiffener runouts where the stress state is often complex, are not fully understood. Nevertheless, from the airworthiness perspective it is essential that both the static strength and the fatigue behaviour of the components subjected to complex multiaxial stress conditions are both understood and predicted. This topic is extremely complex, and numerous criteria ranging from the purely empirical to the theoretical have been proposed. In both cases, it is necessary to know the localised stress–strain history. One common design methodology is to keep the stresses so low that fatigue will not be an issue. However, this can lead to an overly conservative design. On the other hand, while a detailed (nonlinear) finite element analysis can be performed it is often both resource‐intensive and time‐consuming. The present paper shows that Glinka's hypothesis can be used in order to calculate the localised stresses and strains for a bonded joint subjected to cyclic loading. This is a new result and has not previously been noted. It has the potential to extend the Hart‐Smith design methodology to the adhesively bonded joints in order to encompass durability considerations. This formulation also raises the possibility of enabling the degree of conservatism inherent in traditional joint design to be relaxed provided that failure occurs in the adhesive. This paper also addresses the problem of variable adhesive thickness. We show that while variable adhesive thickness can change the stress and the energy fields, the peak in the strain energy density is relatively insensitive to the stress–strain relationship for the adhesive and that Glinka's hypothesis still appears to be true. This means that, for the present class of problems, even if there is variability in the thickness of the adhesive bond the energy field and, hence, the strength of the joint can be estimated from a purely linear elastic analysis of the joint, provided that failure occurs in the adhesive.     

钢管混凝土柱-双面组合作用梁框架节点抗震性能试验研究

梁柱节点受剪承载力计算是混凝土框架结构节点抗震设计的关键点之一,由于节点剪力传递机理与构造的复杂性,目前尚未形成完善的节点受剪承载力计算理论。该文阐述了预应力混凝土梁-型钢混凝土柱新型框架节点的受力机理,并基于桁架模型和斜压杆模型,提出了一种同时考虑预应力筋与型钢贡献的节点受剪承载力计算方法。开展了新型节点受力性能拟静力试验,分别选取了梁端破坏与节点剪切破坏两种破坏形式的节点试验数据,对所建立的新型节点受剪承载力计算方法进行了验证评估,结果表明,该文提出的预应力混凝土梁-型钢混凝土柱框架节点承载力计算方法具有较好的精度。