Delamination tests on CFRP-reinforced masonry pillars: Optical monitoring and mechanical modeling

In this methodological study, delamination phenomena of Carbon Fiber Reinforced Polymer (CFRP) strips from masonry pillars were investigated on the basis of single-lap shear tests. The masonry pillar considered in this case was constituted of three historical bricks, derived from a XVII century rural building. The bricks were bonded by a high strength mortar recently proposed for the restoration process. To follow the specimen response up to collapse, the test was controlled by a clip gauge located at the anchorage of the reinforcement strip. The conventional experimental information, concerning the overall reaction force and relative displacements provided by point sensors (LVDTs and clip gauge), were herein enriched with no-contact, kinematic full-field measurements provided by 2D Digital Image Correlation (2D DIC). Special care was devoted to improve the precision of the optical measurement, which included correction of the effects owing to the lens distortion and to lack of coplanarity between the camera sensor and the monitored flat surface. The overall accuracy was assessed through laboratory benchmarking tests. Moreover, an advanced three-dimensional mechanical model based on nonlinear finite elements was developed under the simplifying assumption of perfect adhesion. This model was adopted to describe bulk damage inside the heterogeneous specimen and to correlate it to the macroscopic response and surface measurements. Coherence between the above mentioned sources of information and the predictions provided by the advanced finite element model were critically discussed.     

Mechanical response of solid clay brickwork under eccentric loading. Part II: CFRP reinforced masonry

The mechanical response of solid clay brickwork in presence of CFRP near-surface reinforcement and under eccentric loads is addressed in this paper. Experimental tests on CFRP reinforced masonry (CFRP-RM) prisms under eccentric loads are compared to similar tests on URM specimens in order to give a preliminary insight in the collapse mechanisms of CFRP-reinforced masonry. While URM is found to exhibit a quasi-brittle response, so raising objections to several analytical procedures for masonry bridge assessment, CFRP reinforcement may be responsible of a significant increase in the brickwork compressive strength and ductility. Some considerations on the cracking processes inside reinforced masonry prisms are discussed.     

The effect of CFRP and CFRP/concrete interface models when modelling retrofitted RC beams with FEM

Concrete structures retrofitted with fibre reinforced plastic (FRP) applications have become widespread in the last decade due to the economic benefit from it. This paper presents a finite element analysis which is validated against laboratory tests of eight beams. All beams had the same rectangular cross-section geometry and were loaded under four point bending, but differed in the length of the carbon fibre reinforced plastic (CFRP) plate. The commercial numerical analysis tool Abaqus was used, and different material models were evaluated with respect to their ability to describe the behaviour of the beams. Linear elastic isotropic and orthotropic models were used for the CFRP and a perfect bond model and a cohesive bond model was used for the concrete–CFRP interface. A plastic damage model was used for the concrete. The analyses results show good agreement with the experimental data regarding load–displacement response, crack pattern and debonding failure mode when the cohesive bond model is used. The perfect bond model failed to capture the softening behaviour of the beams. There is no significant difference between the elastic isotropic and orthotropic models for the CFRP.     

Modelling of the bond behaviour of tuff elements externally bonded with FRP sheets

The performance of fibre reinforced plastic (FRP) materials used for external strengthening depends strongly on the bond behaviour at the FRP-substrate interface. In this paper, the results of an analytical model and of two Finite Element (FE) models (bi-and three-dimensional) for simulating bond behaviour in FRP-strengthened masonry elements using zero-thickness interface elements are presented. The primary parameters of bilinear and nonlinear bond-slip laws were determined from experimental results of single shear bond tests that the authors conducted on masonry blocks of yellow tuff bonded with FRP carbon and glass fabrics. Several parametric analyses were conducted to estimate the effect of the primary bond law parameters on the global behaviour of the specimens and to determine the effective bonded length for the investigated masonry support.     

Steel-fibre-reinforcement and hydration coupled effects on concrete tensile behaviour

This paper presents the effect of hydration development on the tensile behaviour of steel-fibre-reinforced concrete. Tensile tests were performed on plain and fibre-reinforced concretes at 2, 7 and 28 days in order to determine the response of the composites, in particular to establish the post-peak behaviour and the evolution of the residual post-peak strength with hydration development and the associated improvement in the fibre–matrix bond.From the laws governing the tensile behaviour of plain concrete on the one hand and the residual strength capacity due to fibre reinforcement on the other, parameters fitting an analytical model of the uniaxial tensile response of fibre-reinforced concrete were determined. The proposed model takes into account the tensile damage to the concrete and the development of hydration. An original aspect of the model is that it also integrates the damage to the fibre–matrix bond.     

拱坝横缝钢筋与混凝土相互作用研究

本文在总结了钢筋与混凝土相互作用研究现状的基础上，建议了一个计算钢筋滑移量、钢筋粘结受力段长度的数值模型，该模型考虑了混凝土保护层厚度、截面混凝土应力分布的影响，并引用有关试验结果进行验证。文末在假定截面应力分布型态和临界混凝土保护层厚度条件下，将模型用于计算小湾拱坝横缝钢筋的滑移量、钢筋临界应力、钢筋受力段长度，计算结果表明用横缝钢筋控制横缝开度的这一工程措施基本可行，且建议的保护层厚度、横缝钢筋布置等均有一定工程参考价值。     

Three dimensional nonlinear model of beam tests for bond of near-surface mounted FRP rods in concrete

The use of near-surface mounted FRP reinforcement in reinforced concrete structures has seen a considerable increase in recent years as a strengthening method. Beam pull-out tests for near-surface reinforcement allow obtaining the local bond–slip behavior of a bonded joint. The current paper deals with the three-dimensional modeling of this kind of test with the purpose of suitably characterizing the mechanics of the bond between FRP rods and concrete. Different alternatives to represent the FRP bar – concrete interface have been evaluated. Furthermore, to do this modeling, a PID controller has also been designed to conduct the numerical tests correctly in order to properly capture the softening branch of the load-slip behavior. The numerical FE simulations were compared with previously published experimental measurements. Load-slip predictions compare well with the corresponding experimental data. The proposed model is also able to predict the failure mode at the FRP-concrete interface. Some parametric studies have also been carried out.     

Transfer zone of prestressed CFRP reinforcement applied according to NSM technique for strengthening of RC structures

This study presents an experimental program to assess the tensile strain distribution along prestressed carbon fiber reinforced polymer (CFRP) reinforcement flexurally applied on the tensile surface of RC beams according to near surface mounted (NSM) technique. Moreover, the current study aims to propose an analytical formulation, with a design framework, for the prediction of distribution of CFRP tensile strain and bond shear stress and, additionally, the prestress transfer length. After demonstration the good predictive performance of the proposed analytical approach, parametric studies were carried out to analytically evaluate the influence of the main material properties, and CFRP and groove cross section on the distribution of the CFRP tensile strain and bond shear stress, and on the prestress transfer length. The proposed analytical approach can also predict the evolution of the prestress transfer length during the curing time of the adhesive by considering the variation of its elasticity modulus during this period.     

Effect of temperature on RC elements strengthened with CFRP

The strengthening of RC elements with CFRP is a technique that has been acquiring more and more potential. The bond between the CFRP reinforcement and the concrete support is usually made with epoxy adhesives. However, it is here that the integrity of the system can be affected, namely by exposure to high temperatures. In order to study the effect of an increase of air temperature on the behaviour of the epoxy adhesive, CFRP strengthened RC and reference RC specimens were tested. After cyclical thermal exposures, with temperatures rising between 20 and 80°C, specimens were subjected either to compressive shear tests or bending tests. The results demonstrated that epoxy adhesive exhibits poor behaviour when subjected to increased temperatures, causing important bond deterioration. The improvement achieved with the CFRP reinforcement tends to disappear with an increase of the environment temperature. So, the thermal resistance of this strengthened system cannot be considered very high. However, the inclusion of insulating materials can be a good solution to protect the strengthened RC elements. Among tested materials, the foamed polyurethane showed the best behaviour.     

纤维布加固砖砌体墙平面内受力性能有限元模型

考虑一顺一丁的砌筑形式,针对纤维布加固前后砖砌体墙的特点,利用ABAQUS提供的零厚度粘结单元,建立纤维布加固砖砌体墙有限元分离模型,通过前期拟静力试验数据和相关规范公式计算结果,验证所建模型在单调荷载和往复荷载作用下的适用性。结果表明,有限元模型能有效模拟纤维布加固砖砌体墙的抗震性能,分析纤维布的加固效果,计算纤维布加固前后砖砌体墙在单调荷载作用下的受剪承载力。     

Prediction of debonding strength for masonry elements retrofitted with FRP composites using neuro fuzzy and neural network approaches

This paper proposes application of neuro fuzzy and neural network for predicting debonding strength of retrofitted masonry elements. In order to achieve high-fidelity model, this study uses extensive experimental databases for bond test results between Fiber Reinforced Polymer (FRP) and masonry elements by collecting existing bond test subassemblage tests from the literature. Various influential parameters that affect debonding resistance including thickness of the FRP strip, width of the FRP strip, elastics modulus of the FRP, bonded length, tensile strength of the masonry block and width of the masonry block are considered as input parameters to the artificial neural network (ANN) and adaptive neuro fuzzy inference system (ANFIS). Test results of the ANN and ANFIS models were compared with multiple nonlinear regression, multiple linear regression and existing bond strength models. The accuracy of the optimal MNLR model was increased by 39% and 23% with respect to RMSE and MAE criteria using ANFIS. The comparison results indicated that the ANN and ANFIS models performed better than the other models and could be successfully used for prediction of debonding strength of retrofitted masonry elements.     

Modelling experimental bond fatigue failures of concrete beams strengthened with NSM CFRP rods

Near surface mounting (NSM) is a promising strengthening technique provided that the full bond between the strengthening material and the existing structure develops. Wahab et al. (2011) reported fatigue bond failures by de-bonding between the CFRP rod and the epoxy that started at mid span. As the load was cycled, the de-bonding propagated towards the support. The model presented here describes the failure of those beams. The de-bonding was modeled as a crack growing at the interface between the CFRP rod and the epoxy where the driving force was the interfacial shear stresses between the CFRP rod and the epoxy. Once the stresses at the crack tip exceeded the allowable fatigue stresses between the CFRP rod and the epoxy, de-bonding occurred and the crack length increased. Ahead of the crack tip, the CFRP rod was fully bonded to the epoxy and the forces decayed exponentially. Behind the crack tip, the rod was partially de-bonded. The experimental and calculated number of cycles until excessive slipping between the CFRP rod and the epoxy occurred and the forces in the CFRP rod at all locations in the shear span at the onset of failure were in good agreement.     

An experimental analysis about the effects of mortar joints on the efficiency of anchored CFRP-to-masonry reinforcements

Fiber reinforced composite materials are widely used for structural rehabilitation and retrofitting of existing buildings; recent studies, devoted to Carbon Fiber Reinforced Polymers (CFRP) reinforcements of concrete structural elements [1–5], demonstrated that spike anchors are able to effectively increase the load carrying capacity and the ductility of CFRP bonded joints. However, application to masonry structures is disregarded by research since few experimental results are available. One of these, described in Refs. [6,7], compares the efficiency of a CFRP strengthening system provided with one or more CFRP spike anchors, also in dependence of some geometrical parameters; reinforcement sheets and spike anchors were applied only on the brick surface in order to evaluate the effects due to anchors only. In this paper the authors investigate the influence of mortar joints on the efficiency of anchored CFRP reinforcements on brick masonry. For this reason, an experimental campaign was planned on masonry pillars built with the same materials employed in Refs. [6,7], subjected to Near End Supported Single Shear Tests. Masonry pillars were built according to two different patterns, in order to detect the influence of both bed and perpend joints. The results are compared with results obtained from previous experimental campaign.     

Near surface mounted CFRP laminates for shear strengthening of concrete beams

A Near Surface Mounted (NSM) strengthening technique was developed to increase the shear resistance of concrete beams. The NSM technique is based on fixing, by epoxy adhesive, Carbon Fiber Reinforced Polymer (CFRP) laminates into pre-cut slits opened in the concrete cover of lateral surfaces of the beams. To assess the efficacy of this technique, an experimental program of four-point bending tests was carried out with reinforced concrete beams failing in shear. Each of the four tested series was composed of five beams: without any shear reinforcement; reinforced with steel stirrups; strengthened with strips of wet lay-up CFRP sheets, applied according to the externally bonded reinforcement (EBR) technique; and two beams strengthened with NSM precured laminates of CFRP, one of them with laminates positioned at 90° and the other with laminates positioned at 45° in relation to the beam axis. Influences of the laminate inclination, beam depth and longitudinal tensile steel reinforcement ratio on the efficacy of the strengthening techniques were analyzed. Amongst the CFRP strengthening techniques, the NSM with laminates at 45° was the most effective, not only in terms of increasing beam shear resistance but also in assuring larger deformation capacity at beam failure. The NSM was also faster and easier to apply than the EBR technique. The performance of the ACI and fib analytical formulations for the EBR shear strengthening was appraised. In general, the contribution of the CFRP systems predicted by the analytical formulations was slightly larger than the values registered experimentally. Performance of the formulation by Nanni et al. for NSM strengthening technique was also appraised. Using bond stress and CFRP effective strain values obtained in pullout bending tests with NSM CFRP laminate system, the formulation by Nanni et al. predicted a contribution of this CFRP system for the beam shear resistance of 72% the experimentally recorded values.     

In-plane shear performance of masonry panels strengthened with FRP

The opportunities provided by the use of Fiber Reinforced Polymers (FRPs) composites for the shear strengthening of tuff masonry structures were assessed on full-scale panels subjected to in-plane shear-compression tests at the ENEL HYDRO S.p.A. laboratory, ITALY. Tuff masonry specimens have been arranged in order to simulate both mechanical and textural properties typical of buildings located in South-Central Italian historical centres. In this paper, the outcomes of the experimental tests are presented. The monotonic shear-compression tests were performed under displacement control and experimental data have provided information about in-plane behaviour of as-built and FRP strengthened tuff masonry walls. Failure modes, shear strength, displacement capacity and post-peak performance are discussed.     

胶膜连接碳纤维增强树脂复合材料板-钢搭接接头室温条件的力学性能试验

针对碳纤维增强树脂（CFRP）复合材料板-钢搭接接头连接的糊状胶黏剂粘层厚一致性控制较难、铅垂向成形可能不易等问题,将糊状胶黏剂换成胶膜,制作了胶膜连接的五种粘结长度共15个CFRP板-钢双搭接接头试件,并对该胶膜连接的CFRP板-钢搭接接头进行了室温条件下的破环模式、有效粘结长度、传力规律、粘结-滑移本构、承载力等的试验研究。结果表明:所用胶膜的连接强度略高于CFRP板层间强度（即碳纤维与树脂基体的黏聚强度）;室温下,所用胶膜连接的CFRP板-钢搭接接头有效粘结长度约为80 mm;加载初期,剪应力最大值位于接头钢板端;继续加载,其位置向接头CFRP板端移动;加载末期,其位置位于距接头钢板端20 mm （粘结长度不超过80 mm时）或者50 mm （粘结长度不小于120 mm时）处;胶膜连接的CFRP板-钢搭接接头界面粘结-滑移模型为近似梯形,不同于胶黏剂连接的CFRP板-钢搭接接头的近似三角形,胶膜连接接头的延性大为提升;所用胶膜连接接头界面峰值剪应力、断裂能、界面刚度等代表值（可视为准平均值）分别为四种典型商品胶黏剂连接接头的1.2~3.0倍、1.6~5.7倍和5.4~7.5倍;在粘结长度不小于有效粘结长度条件下,所用胶膜连接接头的抗拉承载力代表值为四种典型商品胶黏剂连接接头的1.25~2.39倍;胶膜连接接头的抗拉承载力、最大位移的变异系数与糊状胶黏剂连接接头相差不大。      

Behavior of unreinforced masonry prisms and beams retrofitted with engineered cementitious composites

The impact of a thin layer of a ductile fiber-reinforced concrete referred to as engineered cementitious composites (ECC) on unreinforced masonry (URM) prisms and beams has been evaluated. The objective of the research was to characterize the performance and potential benefits of using ECC to retrofit URM with eventual application to masonry infill walls in non-ductile reinforced concrete frames. Compression tests of masonry prisms and flexural tests of masonry beams with different ECC retrofit schemes were conducted. The variables studied were the use of wall anchors to improve the ECC-masonry bond and alternate steel reinforcement ratios within the ECC layer in the form of welded wire fabric. The ECC retrofit was found to increase the strength and stiffness of URM prisms by 45 and 53 %, respectively compared to those of a plain specimen. When wall anchors were installed on the masonry specimens, the bond between the ECC layer and the masonry surface was improved. Four-point bending tests indicated that the strength and more importantly the ductility of an ECC retrofitted brick beam are increased significantly, especially when light reinforcement is added to the ECC layer, relative to an URM beam. Analytical models for estimating the strength and stiffness of ECC retrofitted masonry specimens are proposed and evaluated.     

Interaction of interface delamination and plasticity in tensile steel members reinforced by CFRP plates

This paper presents experimental and numerical results related to double shear lap tests performed on steel specimens reinforced using CFRP plates. These tests have been simulated taking into account the elastic–plastic behavior of the steel and the delamination between steel and CFRP by means of a cohesive approach. The results obtained are discussed in terms of: (i) maximum load bearing capacity, (ii) minimum bond length in order to exploit the maximum load bearing capacity, (iii) brittle to ductile transition of the interface failure (iv) combination of interface delamination and plastic deformation of the steel. Design formulas for the definition of the minimum bond length and the load bearing capacity of the joint are validated.     

CFRP加固砌体结构的力学性能分析

在分析碳纤维加固的砖砌体在水平周期反复荷载作用下试验结果的基础上,研究了碳纤维加固砖砌体的约束及抗倒塌机理;建立了墙体侧向位移与碳纤维应变、碳纤维应变与墙体抗震剪切强度的关系;讨论了不同加固方式、碳纤维面积百分率等在墙体不同受力阶段对墙体抗剪承载能力和变形性能的作用与影响;提出了计算碳纤维加固墙体承载能力和变形的计算方法。计算方法考虑了不同粘贴碳纤维角度、碳纤维面积百分率等对墙体承载能力和变形性能的影响;推导了最优粘贴加固角;提出了改进的粘贴加固方法以提高加固效果。     

CFRP筋在RPC中锚固性能的理论分析及试验研究

静载试验详细研究了碳纤维增强塑料CFRP筋在活性粉末混凝土RPC的锚固性能。试验结果表明:对于抗拉强度不大于3000MPa的表面压纹CFRP筋在抗压强度130MPa的RPC中的临界锚固长度为20倍CFRP筋直径;多根压纹CFRP筋的合理筋间距为1倍CFRP筋直径。平均粘结强度及其对应滑移量的公式具有较好的适用性。平均粘结应力与滑移之间的预测曲线与试验曲线吻合较好,验证了提出的粘结滑移本构关系。理论推导了锚固变量沿锚固长度分布的表达式,算例验证了有效性。分析表明:距离自由端约为0.6倍锚固长度位置处的粘结应力等于平均粘结应力。对于压纹CFRP筋,当锚长≤12.5倍筋材直径时,粘结应力沿埋长分布较为均匀,其不均匀性系数在1.02―1.05之间;当埋长>12.5倍且≤20倍筋材直径时,粘结应力沿埋长分布较为不均匀,其不均匀性系数在1.05―1.14之间。