Numerical modelling of carbon fibre‐reinforced polymer and hybrid reinforced concrete beams in fire

Investigation on the fire resistance of fibre‐reinforced polymer (FRP) reinforced concrete (RC) is essential for increased application of FRP bars in the construction industry. Experimental tests for determining the fire resistance of RC elements tend to be expensive and time‐consuming. Although numerical models provide an effective alternative to these tests, their use in case of FRP RC structures is hindered because of the insufficient constitutive laws for FRP bars at elevated temperatures. This paper presents the details of a numerical modelling work that was carried out for simply supported carbon FRP (CFRP) and hybrid (steel‐FRP) bar RC beams at elevated temperatures. Constitutive laws for determining temperature‐dependent strength and stiffness properties of CFRP bars are proposed. Numerical models based on finite element modelling were employed for the rational analysis of beams using the proposed constitutive laws. The behaviour of concrete was simulated by means of a smeared crack model based on the tangent stiffness solution algorithm. The employed numerical models were validated against previous experimental results. The theoretical rebar stresses were calculated in both the FRP and steel bars, and the differences are discussed. Copyright © 2012 John Wiley & Sons, Ltd.     

Load ratio effects on the structural fire behaviour of glass fibre-reinforced polymer-reinforced concrete beams with mid-span bar lap splices – Experimental study

Fibre-reinforced polymer (FRP)-reinforced concrete members have been gaining attention as an alternative to conventional steel-reinforced concrete members due to their advantageous characteristics. A few to mention are excellent resistance to corrosion, high strength-to-weight ratio, and reduced maintenance cost in the long term. Nevertheless, FRPs are often limited in use mainly due to fire safety considerations since they can encounter significant deprivation of strength and bond with concrete in fire conditions. This paper presents the results of an experimental study aimed to investigate the effects of the applied load level on the structural behaviour of glass fibre-reinforced polymer (GFRP)-reinforced concrete beams having mid-span bar lap splices when exposed to standard fire. Two 2750-mm long beams with cross-sectional dimensions of 300 mm wide × 350 mm high were exposed to elevated temperatures that followed the CAN/ULC-S101 standard fire time–temperature curve while being subjected to a load level equivalent to 85% of the beam ultimate design load. The experimental results of the two beams were compared to those obtained for identical beams but subjected to a lower load level that is equivalent to only 40% of the beam's ultimate design load. Results show that the increased load level (slightly more than double the load ratio) unexpectedly did not impact the fire resistance time of the GFRP-reinforced concrete beams but affected other structural responses of the beam, such as its midspan deflections and cracking patterns.     

The bond and flexural strengthening of reinforced concrete elements strengthened with near surface mounted prestressing steel (PS) bars

Abstract

The main objective of this work is to study the performance of prestressing steel (PS) bars as reinforcements in the reinforced concrete (RC) elements strengthened by the near-surface mounted method (NSM). The work includes two parts. In the first part, direct pull-out tests are performed in order to study the bond performance between PS reinforcement and concrete. The influences of groove sizes and PS surface conditions (smooth and sand coated) are evaluated. The results show that the sand coated PS (PS-Sc) reinforcement has the best adhesion behavior compared with the smooth bar, and its pull-out force is increased by 48%. For this reason, the PS-Sc bars are used in the second part of this work as NSM reinforcement to strengthen RC beams subjected to bending forces. Then, four-point bending tests are carried out to understand the flexural behavior of strengthened RC beams with PS-Sc reinforcements of different lengths and ratios. The obtained results demonstrate that the use of NSM-PS-Sc bars strengthening technique leads to important enhancement in the load carrying capacity of the RC beams. The first crack load and ultimate load of the strengthened RC beams attain 71.41 and 65.67%, respectively, which are higher than those of the control beam. Furthermore, the experimental values show a good agreement with the analytical values in both the ultimate deflection and the ultimate load. This proves that the NSM-PS-Sc bars studied in this work are promising reinforcement of the RC beams.     

Effectiveness of cement and plaster layers in protection of FRP confined concrete exposed to high temperatures

In recent years, fiber-reinforced polymers (FRPs) materials have shown great potential as materials for repair and reinforced concrete structures such as beams or columns by externally bonding FRP sheet(s) onto the surface of substrate concrete structures. However, the performance of FRP systems exposed to fire is a serious concern due to the combustibility of FRPs. This study introduces the results of an experimental investigation on the behavior of the circular columns of concrete under a load of axial compression, confined by an envelope of composite materials (carbon fiber and glass fiber) and protected by a layer of mortar cement or plaster coating, after they have been subjected at various temperature (23, 120, and 350 °C). The specific objectives of this study are verifying the applicability and the effectiveness of the proposed technique to improve the behavior of concrete in fire resistance and evaluate the effect of composite materials and the layer coating type used. The results indicated that protecting heat circular confined columns, with a layer of mortar cement or plaster has a significant effect on the axial strength and the ductility. It was shown that the ultimate load and axial strain of heated columns can be restored up to the original level or greater than those of unheated columns. However, the effect of a layer of plaster is more significant than a layer of mortar cement. So this coating system would enhance fire resistance of the FRP, safety and reliability of FRP reinforced concrete structures.     

混凝土梁嵌粘混合FRP筋弯曲性能试验研究

分别在钢筋混凝土简支梁和连续梁的受弯区、负弯矩区表层嵌粘不同FRP筋材进行加固,进行单调加载弯曲试验,研究不同初始荷载、FRP筋种类对简支梁、连续梁嵌粘FRP筋后的特征荷载、应变、变形等的影响程度,记录加载过程中梁体裂缝扩展情况。结果表明:不同FRP筋混合嵌入加固混凝土梁,可显著提高简支梁、连续梁的抗弯承载力,改善使用阶段性能,与单一FRP筋材嵌粘加固相比,裂缝分布特征相似,延性更佳;在初始荷载未完全卸除时,混凝土梁表层嵌粘不同FRP筋的加固效果略逊于无初始荷载作用下的加固梁,不过效果仍比较理想,建议实际工程中尽可能卸除外部荷载后再进行嵌粘混合FRP筋加固。     

Rheological modeling and finite element simulation of epoxy adhesive creep in FRP-strengthened RC beams

We have shown that a significant creep occurs at the concrete–fiber reinforced polymer (FRP) interface based on double shear long-term test. The primary test parameters were the shear stress to ultimate shear strength ratio, the epoxy curing time before loading as well as the epoxy thickness. The test results showed that when the epoxy curing time before loading was earlier than seven days the shear stress level significantly affected the long-term behavior of epoxy at the interfaces, and in particular the combined effect of high shear stress and thick epoxy adhesive can result in interfacial failure if subjected to high-sustained stresses. In this paper, based on the previous experimental observations, an improved rheological model was developed to simulate the long-term behavior of epoxy adhesive at the concrete–FRP interfaces. Furthermore, the newly developed rheological creep model was incorporated in finite element (FE) modeling of a reinforced concrete (RC) beam strengthened with FRP sheets. The use of rheological model in FE setting provides the opportunity to conduct a parametric investigation on the behavior of RC beams strengthened with FRP. It is demonstrated that creep of epoxy at the concrete–FRP interfaces increases the beam deflection. It is also shown that consideration of creep of epoxy is essential if part or the entire load supported by FRP is to be sustained.     

基于细观力学的延性构件的设计

本文阐述了依据细观力学原理设计高性能纤维增强水泥基复合材料(HPFRCC)的原则。普通钢筋或FRP筋混凝土构件需设置大量的箍筋保证延性。HPFRCC强度较高，受拉和受剪时延性较好，作为基体配置钢／FRP筋后，与相同配筋条件的混凝土梁相比，承载力和延性均提高。     

Experimental study on bond strength of fiber reinforced polymer rebars in normal strength concrete

The bond behavior of reinforcing bars is an important issue in the design of reinforced concrete structures and the use of fiber reinforced polymer (FRP) rebars is a promising solution to handle the problems of steel reinforcement corrosion. This study investigates the bond characteristics of carbon and aramid FRP (CFRP and AFRP) bars embedded in normal strength concrete. A pullout test was performed on 63 normal strength concrete specimens reinforced with FRP and steel rebars with different embedment lengths and bar diameters. The average bond stress versus slip curve is plotted for all specimens and their failure modes are identified. The effects of the embedment length and diameter of an FRP rebar on its bond strength are examined in this work. The bond strengths obtained from the test results are compared with the predictions by the bond strength equation proposed by Okelo and Yuan (2005), and its validity is evaluated.     

FRP筋异强混凝土叠浇梁挠度与延性研究

为提高纤维增强聚合物(FRP)筋异强混凝土叠浇梁的抗弯性能与延性,研究了钢纤维掺量、钢纤维混凝土叠浇层厚度对FRP筋异强混凝土叠浇梁的影响。以钢纤维体积掺量(0%、0.5%、1.0%、1.5%)与钢纤维混凝土叠浇层厚度(0 mm、180 mm、210 mm、300 mm)为变量,对6根FRP筋异强混凝土叠浇梁进行三分点弯曲试验,并对试验梁的破坏过程、破坏形态、裂缝宽度以及跨中挠度进行分析。研究结果表明：钢纤维的掺入改善了FRP筋异强混凝土叠浇梁的受力性能,使其由脆性破坏向延性破坏发展;随着钢纤维掺量、钢纤维混凝土叠浇层厚度的增加,FRP筋异强混凝土叠浇梁的极限承载力提高了9%～33%,抗弯性能提升了4%～21%,延性提升了22%～89%。基于试验与理论分析,建立了钢纤维作用下的FRP筋异强混凝土叠浇梁挠度计算公式与延性评价方法。     

An Investigation of Interfacial Stresses in Reinforced Concrete Beams Using FRP Laminates

Reinforcement of reinforced concrete (RC) beams against bending through utilization of bonded fibre-reinforced plastic (FRP) laminates has been accepted as an effective method of strengthening. In this study, the effects of FRP reinforcement over the parameters of interfacial stresses in reinforced concrete beams were examined both experimentally and numerically. Essentially, the main goal of the study was to investigate quantitatively the behaviour of the RC beams strengthened with adhesively bonded FRP. In order to achieve this goal, an experimental study was initially carried out. Afterwards, the ANSYS® WB finite element program was employed to model and analyze the RC beams externally bonded to FRP. The obtained results are expected to demonstrate the main characteristics of interfacial stress distributions inside beams strengthened with FRP. The evaluation of interfacial stresses provides the basis for understanding the main characteristics in such beams and for developing suitable design rules.     

Residual response of fire‐damaged high‐strength concrete beams

This paper presents results from an experimental study on residual capacity of fire‐damaged high‐strength concrete (HSC) beams. Four reinforced concrete (RC) beams, fabricated with HSC, were first subject to structural loading and fire exposure with a distinct cooling phase and then loaded to failure upon cooldown to ambient conditions to evaluate residual capacity. Temperatures, deflections, and spalling in the beams were monitored during heating and cooling phases of fire exposure. Further, residual capacity, strains at critical section, and crack patterns (failure mode) of fire‐damaged beams were recorded during residual capacity tests. Results from experiments indicate that the load level during fire exposure, duration of heating phase, rate of cooling, extent (type) of spalling, and duration of postcooling storage influence residual deformations and also residual capacity of RC beams. Further, fire‐damaged HSC beams can recover 40% to 70% of their flexural capacity with respect to their room temperature design capacity provided they survive the entire duration of fire exposure.     

Durability and degradation mechanism of GFRP bars embedded in concrete beams with cracks

This paper reports an experimental study on the durability performance of glass fibre-reinforced polymer (GFRP) bars embedded in concrete beams with cracks. Accelerated tests were performed by immersing the beams in an alkaline solution at 60°C and in tap water at 23°C. Tensile tests were conducted to evaluate the residual tensile properties of the aged GFRP bars. Moreover, scanning electron microscopy, Fourier transform infra-red spectroscopy and differential scanning calorimetry were employed to observe and analyse potential deterioration of the fibres, matrix and fibre/matrix interface. The results showed that the tensile strength of the GFRP bars embedded in concrete beams decreased in all the different environments tested, and most of the tensile strength losses occurred during the initial stage. Actually, compared to the accelerated aging environment, the degradation of the tensile strength of GFRP bars embedded in concrete in the real-world environment is minor because the concrete, which has superior barrier properties, acts like a protective cover for the GFRP bar. Based on the test results, it can be concluded that a crack in concrete may increase the environment effect on the durability performance of the GFRP bar, but cannot change the mechanisms of mechanical degradation of the GFRP bar. In addition, the degradation rate of the GFRP bar was accelerated by sustained flexural loading. Consequently, it is important to restrain and alleviate the combined interaction of a harsh environment and sustained loading in practical engineering to enhance the durability of GFRP bars.     

锈蚀钢筋混凝土梁中钢筋与混凝土间粘结性能退化分析

钢筋与混凝土之间的粘结强度下降是导致锈蚀钢筋混凝土构件承载力退化的主要原因之一.本文在锈蚀钢筋混凝土梁试验数据的基础上,建立ANSYS有限元模型,其中混凝土采用Solid65单元、钢筋采用Link8单元、钢筋与混凝土之间的粘结作用采用Combin39弹簧单元来模拟.根据试验梁荷载-位移曲线进行模型修正,获得合适的粘结强度退化修正系数,并利用曲线拟合技术得到锈蚀钢筋混凝土梁中钢筋与混凝土粘结强度退化修正公式.该公式能够用于实际锈蚀钢筋混凝土结构承载力的计算以及结构安全评估工作.     

玄武岩纤维增强塑料筋混凝土黏结性能的梁式试验研究

设计制作了14个玄武岩纤维增强塑料筋（BFRP筋）混凝土梁式试件和2个钢筋混凝土梁式试件,通过梁式试验分析了影响BFRP筋混凝土黏结性能的主要因素。结果表明,（1）BFRP筋的受力过程可分为微滑移段、正常滑移段、加速滑移段和下降段;（2）当BFRP筋的锚固长度相同时,随着混凝土强度的提高,黏结强度随之增大;（3）当混凝土强度相同时,随着BFRP筋锚固长度的增加,黏结强度明显减小,并且试件的破坏模式也发生了改变;（4）BFRP筋直径的大小对黏结强度的影响不明显;（5）当筋直径、锚固长度和混凝土强度相同时,BFRP筋混凝土的黏结强度与钢筋混凝土基本相当;（6）BFRP筋的外形对BFRP筋混凝土的黏结性能有着较大的影响。     

FRP筋和钢筋混合配筋混凝土梁受弯性能数值模拟及理论分析

近年来,FRP筋和钢筋混合配筋混凝土结构已逐渐成为土木工程界的研究热点问题之一。在试验研究基础上,应用有限元分析软件ANSYS建立了GFRP筋和钢筋混合配筋混凝土梁的数值模型,对其抗弯性能进行了有限元分析,提出了FRP筋和钢筋混合配筋混凝土梁名义配筋面积的概念,并结合中国《纤维增强复合材料建设工程应用技术规范》建立理论模型,对混合配筋混凝土梁的抗弯承载力和挠度进行了理论分析。通过与试验结果对比,证明了有限元模型的精确性以及基于《纤维增强复合材料建设工程应用技术规范》提出的理论计算模型对混合配筋混凝土梁的有效指导性。     

Interfacial stresses in FRP-plated RC beams: Effect of adherend shear deformations

A recently popular method for retrofitting reinforced concrete (RC) beams is to bond fibre reinforced polymer (FRP) plates to their tensile faces. An important failure mode of such plated beams is the debonding of the FRP plates from the concrete due to high level of stress concentration in the adhesive at the ends of the FRP plate. This paper presents an improved solution for interfacial stresses in a concrete beam bonded with the FRP plate by including the effect of the adherend shear deformations. The analysis is based on the deformation compatibility approach where both the shear and normal stresses are assumed to be invariant across the adhesive layer thickness. In the present theoretical analysis, the adherend shear deformations are taken into account by assuming a parabolic shear stress through the thickness of both the concrete beam and the bonded plate. Numerical results from the present analysis are presented both to demonstrate the advantages of the present solution over existing ones and to illustrate the main characteristics of interfacial stress distributions.     

Bond performance of fiber reinforced polymer rebars in different casting positions

In the last two decades, fiber reinforced polymer (FRP) rebars have become attractive to civil engineers, due to their corrosion resistance and high strength‐to‐weight ratio. To take advantage of such properties, many research studies have been carried out on the mechanical and physical properties of FRP rebars in concrete, and critical issues have been discovered regarding the bond behavior of FRP rebars. In this study, an effort was made to investigate the effects of FRP rebar types, locations and concrete casting positions on the bond performance of FRP rebars embedded in concrete members. Experimental investigations were conducted using a total of 63 specimens, with seven types of reinforcing bars, and three different casting positions. According to the test results, the bond performance of FRP reinforced concrete was greatly influenced by FRP type and surface condition; while vertical casting showed higher bond strength than horizontal casting. In addition, the bond strength reduction ratio of FRP rebars cast in the horizontal direction was smaller than that of steel bars. POLYM. COMPOS., 37:2098–2108, 2016. © 2015 Society of Plastics Engineers     

先张法预应力玄武岩纤维增强塑料筋混凝土梁受弯性能试验研究

采用先张法工艺设计制作了1根全预应力玄武岩纤维增强塑料筋(BFRP筋)混凝十梁,2根部分预应力BFRP筋混凝土梁和1根普通BFRP筋混凝土梁,对其进行三分点加载试验,主要测试了构件的开裂荷载、裂缝和挠度发展情况、屈服荷载和极限荷载等性能。结果表明,对BFRP筋施加预应力,可以提高梁的杭裂度,有效减小梁的挠度和裂缝宽度;非预应力钢筋的配筋率越大,梁的极限抗弯承载力越大,在BFRP筋配筋率相同的情况下,全预应力梁和非预应力梁的极限抗弯承载力相当;在预应力梁中采用非预应力钢筋,可以减小裂缝宽度间距,并且提高梁的延性;全预应力梁和非预应力梁在纯弯段上的裂缝数量和裂缝分布基本相同,部分预应力梁的裂缝数量明显多于全预应力梁和非预应力梁。     

Tensile and fatigue investigations of timber joints with glued-in FRP rods

Glued-in rods are an effective method to form timber connections that are increasingly in the focus of research. Compared to steel rods, fibre-reinforced polymer (FRP) bars provide higher resistance against corrosion, reduced weight, and lower heat conductivity. Despite excellent mechanical performance, high fire resistance, and improved aesthetics, they are, however, not yet widely used due to lack of design regulation. This is particularly true for cyclic loads, where determination of fatigue characteristics depends upon time-intensive experimental procedures. In this research, 50 glued-in FRP rod specimens with different embedment lengths were manufactured and tested in uniaxial tension: a first set under quasi-static load and a second set under cyclic load. For the fatigue tests, a new approach based on a stepwise load increase was used to estimate fatigue strength, a method that aims at reducing the experimental effort usually associated with Woehler curves. Experimental results indicated that quasi-static and fatigue strength increased with the overlap length up to an apparent maximum. The estimated fatigue strength from the load increase tests (LITs) was approx. 41% of the quasi-static strength. Additionally, constant-amplitude tests were performed and the obtained data points fit well into established Woehler curves for timber.     

玻璃钢筋增强水泥基复合材料的试验研究

本文采用对裸玻璃钢筋进行表面处理的方法提高水泥对玻璃钢筋的握裹力；研究了挤拉玻璃钢筋增强水泥基复合材料的结合性能，制备出抗压强度49.6MPa、抗折强度25．8MPa的优质玻璃钢筋增强水泥基复合材料；初步探讨了用玻璃钢筋增强水泥的可行性。