Transient high-temperature stress relaxation of prestressing tendons in unbonded construction

Unbonded post-tensioned (UPT) flat plate concrete slabs are popular for modern continuous multiple bay floor assemblies due to economic and sustainability benefits (reductions in slab thickness and building self-weight) and structural advantages (decreased deflections over larger spans). Only limited research has been conducted on the performance of UPT flat plate slabs under fire conditions, yet the inherent fire endurance of these systems is sometimes quoted as a benefit of this type of construction. One concern for these structures in fire is that high-temperature stress relaxation of the unbonded prestressed reinforcement may cause considerable and irrecoverable prestress loss, with subsequent structural consequences. This paper uses a computational model which has been developed to predict the transient high-temperature stress relaxation (i.e., prestress loss) for typical UPT multiple span flat plate slabs in fire, to study the potential prestress relaxation behaviour under various plausible temperature conditions as might occur during exposure to a standard fire. The model is validated using experimental data from relaxation tests performed on locally heated unbonded seven-wire prestressing stand. The initial prestress level, concrete cover to the prestressed reinforcement, and ratio of heated length to overall tendon length are varied to investigate the potential implications for prestress loss, and subsequently for flexural and punching shear capacity. The results highlight the need for particular care in the construction of UPT slabs to ensure adequate concrete cover for structural fire safety.     

Whole-building behaviour of bonded post-tensioned concrete floor plates exposed to fire

This paper discusses the whole-building behaviour of post-tensioned concrete floor plates under fire conditions. Based on the results of eight fire tests on one-way spanning bonded post-tensioned concrete slab strips, recently conducted by the authors, a nonlinear finite element model was developed to model an entire typical concrete floor plate. The considered floor plate was post-tensioned using bonded tendons and was supported on traditional reinforced concrete columns. The overall objective of the study was to provide an understanding of the structural response and modes of failure of these floor plates under fire conditions. The mechanical and thermal material nonlinearities of the floor’s components, consisting of the concrete, grout, prestressing tendon, and the anchorages, as well as the reinforced concrete columns, have been considered in the model. The interface between the tendon and grout was also considered, allowing the bond behaviour to be modelled and the tendon to retain its profile shape during the deformation of the floor. The model has been validated against published finite element results on a floor plate under normal temperature conditions. The temperature distribution throughout the floor slab and supporting columns, together with the developed displacement and stress due to heating, and the overall fire resistance of the floor were predicted by the model. Furthermore, the variables that influence the structural behaviour comprising different natural fires, applied static load during fire, use of non-tensioned reinforcement, as well the difference between unbraced and braced frames were investigated in a parametric study. The study has shown that the failure mode of the floor under fire conditions is mainly due to tensile splitting along the tendons that extended to the top surface, while at ambient conditions the mode of failure is punching shear. The restraint provided by shear walls in the considered braced frame and the use of non-tensioned flexural reinforcement affected the vertical displacement behaviour under fire conditions, but did not affect the fire resistance due to the predicted tensile splitting failure mode. From the studies presented it is concluded that the design fire resistance of the floor specified in Eurocode BSEN1992-1-2 is acceptable, while that in the UK code BS8110 is unconservative and should be modified.     

Fire tests on bonded post-tensioned concrete slabs

The results from eight fire tests conducted on bonded post-tensioned one-way spanning concrete slabs are presented in this paper. The fire tests were augmented with two additional tests at ambient temperature, carried out to failure on slabs with identical geometry and prestressing tendons. The different structural response between using plastic and metallic ducts, Limestone and Thames Gravel aggregates, and different axial restraint conditions to longitudinal thermal expansion, have been highlighted. Slabs with Thames Gravel aggregates were shown to have a much higher deflection compared to slabs with Limestone aggregates, with restrained slabs having a lower vertical deflection compared to equivalent unrestrained slabs. In all the fire tests, cracks directly inline and parallel to the tendons occurred due to thermal stresses at relatively low tendon temperatures, which were not observed in the ambient tests. It is shown that the use of plastic ducts resulted in slightly higher tendon temperatures due to the ease at which water migrated from the grout once the duct had melted. The fire tests have shown that the fire resistance specified in current codes of practice are generally conservative for bonded post-tensioned one-way spanning concrete slabs. The tests have provided detailed experimental data in the form of temperature distributions within the slab, vertical and horizontal displacements and strains in the tendons, which will allow validation of future computer models to predict the behaviour of bonded post-tensioned concrete slabs under fire conditions.     

预应力筋张拉伸长值的实用计算方法 --逐跨折减系数法

针对多跨预应力混凝土结构中常用的预应力筋线形,分别推导出其平均有效预应力的单跨折减系数,以此为基础建立了计算预应力筋理论伸长值的逐跨折减系数法,可供工程技术人员参考.     

Structural performance of a post-tensioned concrete floor during horizontally travelling fires

This paper highlights the structural performance of a bonded post-tensioned concrete floor subject to fires that travel horizontally between zones within the floorplate. The floorplate was previously analysed by the authors based on experimental and numerical investigations on one-way spanning bonded post-tensioned concrete slab strips. In the previous studies, a nonlinear finite element model was developed for the floor that considered the mechanical and thermal material nonlinearities of the floor’s components, interfaces between the components, different natural fire severities, different applied static load during the fire and different restraint conditions. The previous studies highlighted the importance of investigating the whole-building behaviour and provided a useful insight into the temperature distribution throughout the floor slab, failure modes, comparisons with current design rules and time-displacement behaviour of the floor under fire conditions. This paper extends the previous studies and uses the validated finite element model to investigate different horizontal travelling fire scenarios between zones and different inter-zone time delays to represent fire travelling time. The time-temperature distribution throughout the floor slab was predicted at different locations in the floor subject to travelling fires. Furthermore, the time-deflection and time-axial displacement relationships were predicted at different locations in the floor. The current study has shown that horizontally travelling fire scenarios and the inter-zone time delay affect the time-deflection behaviour considerably. The change in heating/cooling scenarios between zones has resulted in a cyclic deflection pattern, which has previously not been considered when designing post-tensioned concrete floors against fire. Based on the analysis of the results presented, it is shown that the worst case in terms of maximum vertical defection or maximum residual deflection, at a given point in the floorplate, could occur either under the assumption of a uniform fire or a travelling fire. It is therefore recommended that designers should consider the integrity of floorplates using various travelling fires.     

Modelling of unbonded post-tensioned concrete slabs under fire conditions

This paper investigates the structural behaviour of unbonded post-tensioned one-way spanning concrete slabs in fire conditions. The slabs were simply supported and reinforced with 15.7 mm nominal diameter seven-wire mono-strand tendons. A nonlinear finite element model for the analysis of post-tensioned unbonded concrete slabs at elevated temperatures was developed. The mechanical and thermal material nonlinearities of the concrete, prestressing tendon and anchorages have been carefully inserted into the model. The interface between the tendon and surrounding concrete was also modelled, allowing the tendon to retain its profile shape during the deformation of the slab. The temperature distribution throughout the slab, time–deflection behaviour, time–longitudinal expansion, time–stress behaviour in the tendon, and the failure modes were predicted by the model and verified against test data. The study has shown that the coefficients of thermal expansion currently used in the European Code for calcareous and siliceous concrete can lead to inaccurate predictions of the structural behaviour. A parametric study was conducted to investigate the effects on the global structural behaviour due to the change in the aggregate type, load ratio and boundary conditions. It was shown that by varying the boundary conditions the fire resistance was greatly affected. Although changing the aggregate type and load ratio affected the time-displacement response, the fire resistance defined by failure of the slab was not affected due to the splitting mode of failure above the tendon locations not being affected by these parameters. Comparison with the codes shows that the UK code BS8110 is generally unconservative, whereas the Eurocode EN1992-1-2 provides reasonable design rules.     

Anisotropic Curvature and Damage of Unbonded Post-tensioned Concrete Slabs During Fire Testing

Two furnace tests, using two different fire exposures, on unbonded post-tensioned concrete slabs (1700 × 1200 mm) are reported. Local curvature is measured along two lines approximately in the middle of the slabs both parallel (longitudinal) and orthogonal (transverse) to the prestressing direction. More pronounced curvature in the transverse direction is accompanied by the formation of cracks running predominantly in the longitudinal direction. While the transverse curvature relaxes back to the original state after the cooling phase the curvature in the longitudinal direction ultimately exhibits upward deflection due to the hogging moment caused by the prestress in the tendons acting on a cross section with temperature reduced mechanical properties at the fire exposed side. The effect on crack formation due to the prestressing can additionally be detected by ultrasonic pulse velocity measurements in the different directions through the depth of the slab, where a reduction of 5–25% is observed in the transverse direction compared to the longitudinal direction. The phenomenological mechanical behaviour of the slabs is captured in a finite element model which describes the evolution of stress in the prestressing tendons. This model additionally suggests that the curvature in the transverse direction is independent of the prestressing in the longitudinal direction.     

钢筋混凝土结构抗火设计述评

最近几十年来,研究结构在火灾中的反应,从而对结构进行合理的抗火设计,即结构抗火工程设计,在国内外得到了发展。现就钢筋混凝土结构抗火性能的研究进行了综合述评,分析了火灾下钢筋混凝土力学性能劣化过程。针对现阶段的研究状况,对需要深入探讨课题提出了建议。     

预应力混凝土结构非线性有限元分析

提出了能反映预应力钢筋与混凝土间相对滑移和相互作用的单元模型,此模型可模拟预应力钢筋的张拉和用于无粘结预应力混凝土结构分析。采用这一单元模型建立了预应力混凝土结构的非线性分析有限元模型,非线性方面考虑了材料和几何非线性、预应力摩擦损失、钢筋松弛、混凝土收缩和徐变等因素。该有限元模型可用于有粘结和无粘结预应力混凝土结构张拉施工和工作阶段的全过程分析,分析内容包括弹塑性变形、混凝土开裂和破坏等。最后给出了有粘结和无粘结预应力混凝土连续梁的分析实例,受力变形、破坏形态等的分析结果与试验结果吻合良好,验证了该模型的合理性和有效性。     

Failure behavior of unbonded bi-directional prestressed concrete panels under RABT fire loading

Although the number of terror-and explosion-related incidents associated with military and terrorist activities is increasing globally, the existing design procedure for civil infrastructures does not consider a protective design for extreme loading scenarios such as blast, impact, and fire loading. Major infrastructure, for example bridges, tunnels, prestressed concrete containment vessels (PCCVs), and liquefied natural gas (LNG) storage tanks are often constructed using prestressed concrete, because it enhances the structural capacity. Concrete is often used as a construction material because of its low thermal conductivity, which makes it a good fire resistant material. However, the fire-resistant behavior of the high-strength concrete (HSC) and prestressing (PS) tendons used in prestressed concrete (PSC) is different than that of ordinary reinforced concrete (RC). Also, there has been limited research comparing PSC to RC under extreme loading conditions. This study presents experimental testing of unbonded bi-directionally prestressed concrete panels with dimensions 1000×1400×300 mm³ that were tested under RABT fire loading to simulate a jet aircraft crash-fire accident. A prestressing force of 430 kN was applied to the PSC specimens using unbonded threaded bars. After a RABT fire test, residual flexural strength tests were performed on the fire-damaged PSC and on RC specimens for comparison. Results of the RABT fire and residual flexural strength tests indicated that the fire-damaged PSC specimens showed severe thermal spalling damage induced by PS relaxation and deterioration of strength/stiffness, respectively. These study results can be used as basic research data for future research in numerical simulation of fire and the design of PSC structures under the fire scenario.     

后张有粘结预应力施工技术及应用

以滨西物流中心后张有粘结预应力框架梁施工为基础,介绍了后张有粘结预应力波纹管穿管固定、预应力筋锚具安装、预应力筋穿线、混凝土浇筑、张拉以及灌浆封锚等施工工艺,并提出了后张有粘结预应力梁施工过程中应注意的事项。     

预应力作用下组合梁混凝土法向应力分析

针对在混凝土翼板中设置后张预应力的连续组合梁,提出由分担预加轴力、主弯矩和次弯矩产生的混凝土法向应力计算公式,主要分析施加于混凝土翼板的预应力通过剪力键向钢梁传递的力学模型,建立混凝土翼板承担的预压力沿梁纵轴的分布函数和理论解,求得考虑剪力键柔性的混凝土法向应力计算公式,作为组合梁抗裂设计依据。     

Time-dependent behaviour of concrete beams pretensioned by carbon fibre-reinforced polymers (CFRP) tendons

Due to its non-corrosive and high tensile strength properties, carbon fibre-reinforced polymers (CFRP) has attracted researchers’ interests worldwide to investigate the feasibility and effectiveness of using CFRP as reinforcements or prestressing tendons in concrete structures. However until now these studies have been mainly in the form of experimental tests and there is lack of theoretical framework provided. This paper first proposes an analytical method to predict the time-dependent behaviour, including concrete strains, curvature and deflection as well as loss of prestress, of concrete beams prestressed by CFRP tendons, under sustained service load condition. The paper then presents three illustrative examples of how to use this analytical method to calculate the time-dependent concrete strains, curvature and deflection of concrete beams pretensioned by CFRP tendons. It is concluded that the proposed theoretical framework is suitable for analysis of the time-dependent performance of the concrete beams pretensioned by CFRP tendons and the behaviour of such beams are comparable to those with steel and therefore CFRP can be effectively used as prestressing tendons in concrete beams.     

超长预应力混凝土框架梁的布索方案

确定预应力筋布索方案是超长预应力混凝土结构设计的一项关键内容。结合若干工程成功经验,就满足规范构造要求、减少摩擦损失、预应力筋有效连接、张拉锚固节点处理和无粘结预应力筋的应用等问题进行讨论并提出相关的建议,可为设计及施工提供参考。     

Pathology,appraisal, repair and management of old prestressed concrete beam and slab bridges

The beam and slab bridges named in France VIPP are viaducts with multiple single spans made of prestressed concrete beams that are precast on site and post-tensioned, and then assembled transversely by prestress. A great number of those bridges were built in France at the beginning of the development of the prestressing technique. Some of them present strong losses of prestress related to the corrosion or the rupture of tendons. After a presentation of the pathology and its various causes, the paper presents the difficulties of the appraisal and the methodology that has been developed to assess their residual load-carrying capacity. The appraisal method is based on several levels of investigations and re-calculations. Then the paper addresses the problem encountered with the management of such structures by emphasising the interest of the risk analysis for helping owners to optimise the management of such bridges, with respect to safety. Finally, different solutions of repair that have been used with success to maintain or to strengthen the capacity of these structures are reviewed.     

张拉体外预应力筋的钢筋混凝土梁设计

在工程加固中钢筋混凝土梁与张拉其体外的预应力筋系统(可为折线形、元宝筋形式或其他可能的形式)形成一种特殊形式的预应力筋与该梁间无粘结力的预应力混凝土组合梁(文中简称为预应力梁),本文研究了其实际工作特性及计算方法,即将预应力筋的作用视为内力,通过理论分析和试验研究,论证了在不同受荷阶段这种无粘结力的预应力混凝土梁的设计计算原则,推导出了设计需要的各种计算公式。     

Factors governing the shear response of prestressed concrete hollowcore slabs under fire conditions

This paper presents results from numerical studies on the effect of critical factors governing the shear response on prestressed concrete (PC) hollowcore slabs exposed to fire. A validated three dimensional finite element model is applied for evaluating failure of fire exposed prestressed concrete (PC) hollowcore slabs under different limiting states, including through shear. This model accounts for temperature induced property degradation in concrete and prestressing strands, cracking in concrete, varying fire exposure, loading and restraint conditions. The factors varied in the parametric study include, slab depth, load level, loading pattern, axial restraint, level of prestressing, and fire scenario. Results from parametric studies show that slab depth, load level, loading pattern, axial restraint, level of prestressing and fire scenario have significant influence on the fire response of PC hollowcore slabs, and failure under these conditions can occur through shear limiting state prior to reaching flexural limiting state. Results from parametric studies are further utilized to propose a simplified approach for evaluating shear capacity PC hollowcore slabs under fire conditions.     

后张法预应力施工中常见的两个问题

介绍了后张法预应力钢筋混凝土桥梁工程中经常遇到的问题，通过工程实例，对产生问题的原因进行了分析．提出了相应的预防措施，以保证后张法预应力施工质量。     

后张预应力混凝土结构设计中的几个概念问题

针对后张预应力混凝土结构设计中有关最佳平衡荷载，平衡荷载效应，控制合理预应力钢筋用量的最主要因素，楼盖平板上扁梁的内力分析以及预应力钢筋形状等概念问题进行了讨论，并提出了设计建议     

采用CFRP筋施加体外预应力的分析

CFRP筋为非金属材料,抗拉强度高和耐腐蚀性好,可以用作体外预应力束来增强混凝土梁。这里探讨了CFRP筋的蠕变、温度膨胀等特性和筋束弯曲的问题及它们在结构设计中的影响;并就采用CFRP筋施加体外预应力的预应力损失做了细致的分析,给出了设计建议。