Experimental investigation of reinforced concrete and hybrid fibre reinforced concrete shield tunnel segments subjected to elevated temperature

This paper presents a comprehensive experimental study on the comparative behaviour of the reinforced concrete (RC) and the hybrid fibre reinforced concrete (HFRC) shield TBM (Tunnel Boring Machine) tunnel lining segments exposed to fire. The tests were conducted using a newly developed test facility, which is capable of accommodating different mechanical loading and boundary conditions under different fire scenarios. Six RC segments and six HFRC segments were tested to the standard Eurocode HC (Hydrocarbon) curve, while two reference specimens, one for each type, were tested in ambient environment to provide benchmark data. Apart from the spalling resistance, the fire effects on the structural behaviour were investigated under different boundary conditions at the segment ends, including free sliding (no horizontal constraint), total horizontal restraint and controlled horizontal reaction. The vertical load capacities were investigated for both under-fire and post-fire scenarios. The experimental results revealed excellent spalling resistance in the HFRC segments under thermo-mechanical loading, while the RC segments exhibited better structural performance. A combination of RC design (with flexural reinforcement) and the use of hybrid fibres is deemed to be effective in providing good spalling resistance while at the same time ensuring a robust structural behaviour.     

Efficiency of polypropylene and metallic fibres on control of shrinkage and cracking of recycled aggregate mortars

The use of recycled aggregates in mortar and concrete induces a large shrinkage due to the capacity of these aggregates to absorb a great quantity of water. The influence of polypropylene and metallic fibres (Fibraflex) has been studied in different proportioning to reduce shrinkage and cracking. The efficiency of polypropylene and metallic fibres to reduce restrained shrinkage cracking is evaluated by using a ring mortar cast around a stiff steel ring. By using fibres, the crack width decreases significantly. Free shrinkage results are equally investigated. Finally, crack width in reinforced fibre mortars is calculated analytically by using the model of Grzybowski and Shah. Results are compared with experimental values. The study demonstrates that theoretical values of crack widths correspond reasonably to experimental values.     

Effect of embedment length of untreated natural fibres on the bond behaviour in cement mortar

The present investigation is focused on studying the effects of various matrices with 1:3, 1:4 and 1:5 mortars and fibre types of sisal and coir on the bond behavior at various ages of curing, i.e., 24 h, 3 d, 7 d and 28 d. The other parameters included in the investigation are water/cement (w/c) ratio, sand gradation and embedment length of fibres. In addition, the type of failure of sisal and coir fibres for different mixes of mortars at various curing ages is also reported. From the results, it is seen that the bond strength is improving with respect to age of curing in case of sisal fibres, but decreases in case of coir fibres. The failure of fibres due to fibre fracture is observed in sisal fibres and fibre pullout is observed in coir fibres. The other varying parameters such as mortar mixes, sand gradation, w/c ratio and embedded length also showed significant effect on bond behaviour of sisal and coir fibre with the cement mortar mixes.     

混杂纤维对客运专线早强型支座灌浆料的增韧作用

通过测试掺入纤维的早强型支座砂浆的抗压强度、抗折强度、劈裂抗拉强度、弹性模量和韧性指数,研究了混杂纤维对早强型支座砂浆的增韧效果.结果表明:掺入纤维可以提高砂浆的抗压强度,显著提高了砂浆的抗折强度、劈裂抗拉强度和弹性模量.混杂纤维的最佳掺入比例为0.6％钢纤维+0.2％玻璃纤维+0.2％聚丙烯纤维,使砂浆28 d的抗压强度、抗折强度、劈裂抗拉强度和弹性模量分别提高7％、68％、44％和53％,压折比从空白样的6.35降低到4.03,大大提高了砂浆的韧性指数,增强了砂浆的韧性.     

Inelastic behaviour of tubular column-to-flat slab connections

This paper is concerned with the behaviour of ductile shearheads for connecting reinforced concrete flat slabs to internal tubular steel columns. A novel detail is presented in which a gap is left around the column to enable the shearhead to yield in shear prior to the slab failing in punching shear. The proposed connection could serve as a primary lateral resisting system in low-rise buildings in regions of low to moderate seismicity or as a secondary system in areas of significant seismicity. Test results and numerical analyses are presented for a fully embedded ACI-type shearhead as well as the proposed partially embedded connection. The latter is shown to offer enhanced ductility compared with traditional forms. Experimental and numerical results are used to demonstrate the favourable inelastic performance of the proposed detail.     

Hysteretic behaviour of tubular T-joints reinforced with doubler plates after fire exposure

The hysteretic behaviour of tubular T-joints reinforced with doubler plates was experimentally and numerically investigated in this study. Two specimens were tested to failure under cyclic axial load at the brace end, one under ambient temperature and the other after fire exposure. Specimens with identical material and geometric properties were compared. The failure modes of the specimens were cracking along the weld toe at the intersection of the plate and brace. The results indicated that the hysteretic behaviour and energy dissipation of the joint after fire exposure were smaller than those at ambient temperature. The finite element package ABAQUS was then used to simulate the joint specimens. In a comparison of the hysteretic curve, skeleton curve, energy dissipation and failure location, the simulation and experimental results were in good agreement. The finite element method was subsequently used to carry out a parametric study. Parameters τ and ε had little influence on the post-fire hysteretic behaviour of the joint, but joints with a large γ or small α, β, or ξ values had a low capacity for hysteretic behaviour after fire exposure.     

Impact of melting and burnout of polypropylene fibre on air permeability and mechanical properties of high-strength concrete

This study intends to investigate the impact of high temperature, melting and burnout of Polypropylene Fibre (PP fibre) on mechanical properties, pore size distribution and air permeability of high strength concrete. The specimens were high-strength concrete with 120 MPa strength produced with a water-binder ratio of 20%. To examine the effects of melting and burnout of the PP fibre, the experiment was conducted using two mixtures. One mixture contained 1.5 kg/m³ of PP fibre, while the other did not contain any PP fibre. Heating temperatures were set to room temperature (RT), 120, 200, 300 and 400 °C, considering the temperatures for the melting and burnout of the PP fibre. After heating and cooling, compression tests were carried out on the concrete specimens to measure the modulus of elasticity and Poisson's ratio. Pore size distribution was measured using the fragments created by the compression tests. Air permeability was estimated by measuring the pore size distribution. It was found that melting and burnout of the fibre did not affect the compressive strength and modulus of elasticity but the Poisson's ratio of the specimens containing fibres increased at 400 °C. The effect of melting and burnout of fibre on pore volume and air permeability is quite small. If it is assumed that micro-cracks affected the air permeability, it is expected that high strength concrete with a large fibre content should create many micro-cracks at high temperature, leading to an increase of air permeability.     

Effects of stirrup corrosion on shear behaviour of reinforced concrete beams

Corrosion of stirrups is always more severe than longitudinal steel bars, leading to possible shear failure instead of bending failure. This paper covers the authors’ investigation on the effects of stirrup corrosion on shear behaviour of reinforced concrete beams. Based on modified compression field theory, a method of the whole process analysis was developed showing shear behaviour of reinforced concrete beams with corroded stirrups. Of particular interest was the cracking damage of concrete cross sections, the loss of cross-sectional area and deterioration of mechanical properties of corroded stirrups. After comparing analytical results with available shear test results, further analysis was carried out to investigate the deterioration of shear behaviour and the effects of corrosion degree, concrete strength, stirrup ratio and shear-span ratio. Stirrup corrosion was found to have little impact on cracking load, but shear strength and ductility decreased with the increase in corrosion degree, and shear failure was determined possible for beams originally designed for bending failure. With the decrease of concrete strength or the increase of stirrup ratio and shear-span ratio, the contribution of corroded stirrups to the shear strength of beams increased, and the shear strength and ductility caused by stirrup corrosion decreased as expected.     

Experimental research on post-fire behaviour of reinforced concrete columns

This paper reports an experimental research into the effect of fire exposure time on the post-fire behaviour of reinforced concrete columns. Nine full-size reinforced concrete columns (45×30×300 cm) with two longitudinal reinforcement ratios (1.4% and 2.3%) were unexposed and exposed to the ISO 834 standard fire for 2 and 4 h with a constant preload. One month after cooling, the specimens were tested in axial load combined with uniaxial or biaxial bending. The test results show that the residual load-bearing capacity decreases with increase in fire exposure time. This deterioration in strength following an increase in fire exposure time can be slowed down by the strength recovery of hot rolled reinforcing bars after cooling. In addition, the reduction in residual stiffness is higher than that in ultimate load; consequently, much attention should be given to the deformation and stress redistribution of the reinforced concrete building subject to earthquakes after a fire.     

砌块专用砂浆与钢筋粘结性能试验与可靠性研究

进行了l2组试件的拉拔试验,给出了拉拔力与滑移的关系曲线,确定锚固钢筋受力破坏的各个阶段及特征点,通过统计回归提出了粘结强度的统计公式。并采用一次二阶矩法对钢筋在砌块专用砂浆中的锚固长度进行了可靠度分析,进一步验证了砌体结构设计规范给出锚固长度的可靠性。     

Interfacial shear strength of fiber reinforced soil

The interfacial mechanical interaction between the reinforcement and soil matrix is a key factor in controlling the engineering properties of reinforced soil. To evaluate the factors affecting the interfacial strength properties of polypropylene fiber (PP-fiber) reinforced soil, single fiber pull-out tests were performed by using a modified special apparatus. It has been found that the designed pull-out test is an efficient method to qualitatively obtain the interfacial peak strength (IPS) and interfacial residual strength (IRS) of fiber/soil. Both the IPS and IRS decrease with water content increase, while increase with increasing soil dry density. The cement inclusions dramatically improve the interfacial shear strength of fiber/soil, and the IPS and IRS increase with an increase in additives content and curing time. Finally, by using scanning electron microscopy (SEM), the micromechanical interaction behavior between soil particles and fiber reinforcement were discussed. The interfacial shear resistance of fiber/soil depends primarily on the rearrangement resistance of soil particles, effective interface contact area, fiber surface roughness and soil compositions, etc.     

Experimental study of tunnel segmental joints subjected to elevated temperature

Segmental joints present a weak link in the tunnel lining both structurally (due to its low stiffness) and non-structurally (high risk of water leakage); therefore the behaviour of the lining joints has a significant effect on the performance of the shield TBM tunnel lining. Segmental joints are thus a particular concern when the tunnel lining is exposed to high temperature in the case of a tunnel fire. This paper presents an experimental study on the behaviour of TBM tunnel joints in fire under different mechanical loading and boundary conditions, and with both the normal reinforced concrete (RC) segments and hybrid fibre reinforced concrete (HFRC) segments. Totally thirteen jointed specimens were constructed at a scale of 1:3 and tested. Eleven specimens were exposed to a HC (Hydrocarbon) curve and mechanically loaded to failure either under-fire or post-fire, while two specimens were tested in ambient temperature to provide benchmark data. The results demonstrate that the initial loading conditions have a significant effect on the jointed segments during and after fire, and this is closely related to different rate of degradation of concrete in different stress state under high temperature. In general, the resistance capacity of both RC and HFRC joints increased with axial force. The use of HFRC material provided good spalling resistance.     

Mechanical behaviour and fibre dispersion of hybrid steel fibre reinforced self-compacting concrete

In this study, mixture design, workability, fibre dispersion/orientation, mechanical properties and fracture behaviour of hybrid steel fibre reinforced self-compacting concretes (HSFRSCCs) were investigated. Three different types of steel fibres with and/or without hooked-ends were added to the mixtures in two different volume fractions (0.75 and 1.5% of the total volume of concrete). The results of slump flow, U-box, V-funnel and J-ring tests have shown that increasing the fibre content of the concretes slightly reduced the workability of HSFRSCC, and the main influencing factor on flowability is the geometry of fibres. The addition of fibres, although did not change the final flowability, decreased the rate of flowability. The results from the experimental tests showed that the flexural strengths increased slightly with increasing strength of long fibres, whereas the splitting tensile strength remained unchanged. The concretes with high strength, long steel fibres show behaviour of enhanced toughness and ductility compared to that with normal strength steel fibres. The orientation and distribution of fibres in concrete have been investigated by image analysis and it was observed that fibres dispersed homogeneously in all concrete series without any clumping. With increasing the amount of fibres, the fibres were more vertically orientated relative to the bending loading direction, resulting in enhancement in the mechanical properties of concrete.     

Investigations on fracture characteristics of geosynthetic reinforced asphalt concrete beams using single edge notch beam tests

Reflective cracking is a major cause for premature deterioration of asphalt pavements. Different varieties of geosynthetics are used at the interfaces of surface layers to control the reflective cracks. The significant factors influencing their efficiency are the flexural strength and interfacial bonding. Fracture energy that leads to development of cracks and their propagation can be investigated by single-edge notched beam (SENB) tests with sufficient accuracy. Double layered asphalt samples were extracted from pavement sections purposely built as part of this investigation for conducting quasi-static SENB tests. The goal of this paper is two-fold (a) to present a methodology for conducting SENB tests to measure the fracture properties of geosynthetic reinforced samples at temperatures of 10 °C, 20 °C and 30 °C and (b) evaluation of the flexural and the fracture characteristics of unreinforced and geosynthetic reinforced samples. The geosynthetic reinforcement did not show much improvement of the Asphalt Concrete (AC) in the pre-cracking phase but slowed down the crack propagation. The failure pattern of reinforced specimens has changed from quasi-brittle to ductile. An equation is proposed to predict the crack initiation force of SENB sample knowing the bond strength of the corresponding reinforced AC layers at their respective temperature.     

钢筋混凝土剪力墙板的剪切滞回性能试验研究

为提高低矮钢筋混凝土剪力墙板的数值模拟精度,完善混凝土在循环荷载作用下的剪切本构理论,进行6个钢筋混凝土剪力墙板的循环剪切试验,研究配筋率及钢筋角度对钢筋混凝土剪力墙板在循环剪切应力作用下滞回性能及累积耗能能力的影响,得到试件的滞回曲线、骨架曲线、延性、刚度退化、累积耗能能力等。试验结果表明：所有试件均发生剪切破坏模式。配筋率对钢筋混凝土剪力墙板的抗剪承载力有一定影响,对其变形能力影响不明显。钢筋角度对钢筋混凝土剪力墙板的滞回曲线有显著影响,分布钢筋与剪力墙剪切斜裂缝夹角越小,其滞回曲线越饱满,累积塑性剪切角、累积延性比、等效黏滞阻尼比及累积塑性耗能系数越大,但其初始剪切刚度反而越低。该文的试验结果可为完善混凝土的本构理论提供参考。     

组合连续梁桥高强砂浆填充群钉连接件抗剪承载力试验研究

以71m 110m 71m三跨连续钢-混凝土组合箱形梁桥为工程背景,采用足尺模型推出试验方法研究了高强砂浆填充群钉连接件的抗剪承载力和荷载滑移曲线。共制作了7个C65砂浆填充剪力连接件试件,其中4个为单排标准推出试件,另外3个为20根22×200的群钉试件。比较了单排钉和群钉连接件的抗剪承载力和荷载滑移曲线,推荐了相应的经验公式,并且讨论了公式的适用性。     

冷弯薄壁钢短柱在均匀高温下的性能

本篇章对常温及均匀高温条件下的冷成型薄壁短槽钢的轴向强度进行了初步研究。对常温及均匀高温下的一系列卷边的或非卷边的槽钢进行了试验研究。应用一系列的设计方法和商业化的有限元软件ABAQUS(1998)对试验进行了分析。本中涉及的设计方法包括：英国规范BS5950(1987)、欧洲规范Eurocode3Part1．3(CENl996)和美国规范AISI(1996)，在有限元分析中，考虑了几何非线性和材料非线性。高温下钢材的应力一应变曲线根据欧洲规范Eurocode3Part1．2(CENl995)和Qutinen(2001)采用。在某些试验中发现扭转屈曲，为了扩展针对这种破坏模式的设计方法，在这些规范中引用了Yong和Hancock(1992)考虑扭转屈曲的方法。对设计规范BS5950Part5(1987)、欧洲规范Eurocode3Part1．3(CENl996)和美国规范AISI(1996)中常温下薄壁柱的设计方法加以修改，来考虑钢材高温下的强度和刚度的变化。从试验结果、规范预期的结果和数值分析的比较可见，可很容易的通过上述修正当前规范的方法来分析薄壁型钢短柱高温下的性能。     

Seismic behaviour of concrete-filled steel tubular frame to RC shear wall high-rise mixed structures

Composite frames consisting of concrete-filled steel tubular (CFST) columns and steel beam are being used more and more popularly in building structures. In China, the composite frame structures are often mixed with reinforced concrete shear walls to form a high-rise building system. However, there was seldom information on the seismic performance of this kind of mixed construction. Shaking table tests on two building models with 30 storeys consisting of composite frames and RC shear walls were thus presented in this paper. CFST columns with circular and square sections were used in the composite frames respectively. Three kinds of real earthquake records, including Taft (EW), El Centro (NS) and Tianjin waves with peak accelerations of 0.2g, 0.4g, 0.6g, and 0.8g, were applied respectively to simulate different levels of earthquakes in the tests. It was found that the composite frames cooperated well with the core RC shear wall structure under earthquakes, and the two building models exhibited excellent seismic performance.