基于粘结-滑移的FRP筋钢纤维轻骨料混凝土梁裂缝宽度计算方法

将纤维增强筋(FRP筋)混凝土梁裂缝的开展过程视作FRP筋由两侧混凝土中拔出的过程,建立了基于粘结-滑移的FRP筋轻骨料混凝土梁裂缝宽度微分方程。根据规范给出的裂缝宽度限值,合理确定滑移量上限,提出并引入了适用于梁正常使用阶段的FRP筋钢纤维轻骨料混凝土“低滑移”阶段粘结-滑移本构模型。进而,在明确最大裂缝间距l_max、裂缝宽度放大系数h₂/h₁与裂缝截面纤维混凝土残余应力σ_fib等特征参数的基础上,利用迭代算法建立了FRP筋钢纤维轻骨料混凝土梁最大裂缝宽度计算模型。基于正常使用阶段裂缝宽度实测数据对建议模型的适用性进行评估,结果表明:裂缝宽度限值0.5 mm内,建议模型能够准确预测FRP筋钢纤维轻骨料混凝土梁的最大裂缝宽度。     

玻璃纤维筋锚具研制及其力学性能试验研究

将纤维增强筋(FRP筋)混凝土梁裂缝的开展过程视作FRP筋由两侧混凝土中拔出的过程,建立了基于粘结-滑移的FRP筋轻骨料混凝土梁裂缝宽度微分方程。根据规范给出的裂缝宽度限值,合理确定滑移量上限,提出并引入了适用于梁正常使用阶段的FRP筋钢纤维轻骨料混凝土“低滑移”阶段粘结-滑移本构模型。进而,在明确最大裂缝间距l_max、裂缝宽度放大系数h₂/h₁与裂缝截面纤维混凝土残余应力σ_fib等特征参数的基础上,利用迭代算法建立了FRP筋钢纤维轻骨料混凝土梁最大裂缝宽度计算模型。基于正常使用阶段裂缝宽度实测数据对建议模型的适用性进行评估,结果表明：裂缝宽度限值0.5 mm内,建议模型能够准确预测FRP筋钢纤维轻骨料混凝土梁的最大裂缝宽度。     

Increasing durability of lightweight concrete through FRP wrap

The purpose of this study was to investigate the effects of Fiber Reinforced Polymer (FRP) wraps on reducing chloride ingress and increasing the durability of lightweight concrete. Several prior studies have dealt with FRP application on normal weight concrete. However, the literature on externally bonded FRP application on lightweight concrete is sparse. This study involved an accelerated testing to determine concrete degradation and decrease in chloride ingress and associated increased durability that FRP wrapping can provide to lightweight concrete. Forty two cylinders were subjected to electricity induced accelerated testing in a saline solution for 50 days. Samples were removed from the immersion tank after specific failures and analyzed for chloride content and failure modes. Both lightweight and normal weight concrete greatly benefited from FRP wrapping in terms of increased time to failure and reduced concrete chloride. Lightweight concrete generally performed better with Carbon FRP (CFRP), and normal weight concrete with Glass FRP (GFRP). CFRP wraps produced the best overall protection. Multiple wraps were more effective at reducing chloride ingress than single wraps. Lightweight concrete had greater chloride permeability reductions, whereas normal weight concrete benefited more from FRP confinement effects. The majority of sample failures were caused by cracking in the concrete substrate.     

A review on the driving performance of FRP composite piles

Fibre composites have been a viable option in replacing traditional pile materials such as concrete, steel and timber in harsh environmental conditions. However, driving composite piles require more careful consideration due to their relatively low stiffness. Currently, there are no specific guidelines on the installation of composite piles which limits their acceptance in load-bearing applications. There is a need therefore to understand their behaviour during driving in order for composite piles to be safely and economically driven into the ground. This paper presents an overview on composite pile technologies and an examination on the different factors that affects their driving performance. Emphasis on the potential use of hollow fibre reinforced polymer (FRP) piles and the need for further study on their impact behaviour is highlighted. It is expected that the information provided in this paper will help researchers and engineers to develop efficient techniques and guidelines in driving composites piles.     

Strengthening slabs using externally-bonded strip composites: Analysis of concrete covers on the strengthening

This study pertains to the experimental and theoretical behaviour of slabs strengthened by fibre reinforced polymer (FRP). The experimental results show that FRP significantly increases punching failure stress, resulting in a reduction of slab rotation around the loading column. The theoretical investigation presents a finite element model for the bending of strengthened slabs. The developed model considers the concrete as a 3D multi-layered non-linear material and explicitly takes into account the steel reinforcement and the FRP strips. The proposed model is then used to analyse the effects of a concrete cover on the reinforcement and repairs. In the analysed cases, the results show that an average reduction in the concrete shear modulus, between steel rod and FRP, of more than 30% leads to significant reductions of stress and slab stiffness.     

Strengthening of RC beams with epoxy-bonded fibre-composite materials

Strengthening of concrete beams with externally bonded fibre-reinforced plastic (FRP) materials appears to be a feasible way of increasing the load-carrying capacity and stiffness characteristics of existing structures. FRP-strengthened concrete beams can fail in several ways when loaded in bending. The following collapse mechanisms are identified and analysed in this study: steel yield-FRP rupture, steel yield-concrete crushing, compressive failure, and debonding. Here we obtain equations describing each failure mechanism using the strain compatibility method, concepts of fracture mechanics and a simple model for the FRP peeling-off debonding mechanism due to the development of shear cracks. We then produce diagrams showing the beam designs for which each failure mechanism is dominant, examine the effect of FRP sheets on the ductility and stiffness of strengthened components, and give results of four-point bending tests confirming our analysis. The analytical results obtained can be used in establishing an FRP selection procedure for external strengthening of reinforced concrete members with lightweight and durable materials.     

预应力FRP加固工程结构技术研究进展

围绕国内外预应力FRP加固技术的研究现状以及最新进展,从预应力FRP加固混凝土结构、预应力FRP加固钢结构、预应力FRP加固中关键技术的研究等方面进行了综述。试验研究表明：预应力FRP加固混凝土能显著提高构件的开裂荷载、屈服荷载和极限荷载,改善受弯构件在长期荷载的力学性能,提高构件的疲劳寿命;预应力CFRP加固钢梁后,其屈服荷载和极限荷载相对于对比梁都有明显的提高,其提高的程度随着预应力CFRP的用量和预应力水平的提高而增大;预应力CFRP加固对钢梁的刚度提高作用也比较明显,对低强度的钢材,提高效果更明显;采用预应力FRP加固工程结构的关键问题在于预应力的施加体系、预应力控制值、预应力损失和端部的锚固。     

Influence of FRP anchor fan configuration and dowel angle on anchoring FRP plates

To enhance the strain capacity of fibre-reinforced polymer (FRP) plates which have been bonded onto reinforced concrete (RC) members for strengthening purposes, FRP anchors can be utilised. Research on the characterisation of FRP anchors is still quite limited though despite the increasing use of FRP anchors in practice. In order to reduce such a knowledge gap, this paper reports the results of 30 single-shear FRP-to-concrete joint tests of which 26 joints were anchored with FRP anchors of differing geometric configurations and four joints were unanchored controls. More specifically for the anchored joints, the connection of the FRP anchor to the FRP plate via so called fan fibres was varied in addition to the angle of anchor insertion and these test parameters represent fundamentally important anchor components which have not been researched to date. Failure modes, joint strengths, load–slip responses as well as FRP plate strain distributions are reported and a relationship relating the influence of anchor insertion angle to joint strength is provided. A maximum increase in joint strength of 160% on average above the unanchored control joints was achieved. In addition, the maximum strain resisted by the FRP plate, relative to its elongation capacity, was increased on average from 25% for unanchored control joints to 67% for some anchored joints.     

Behavior of SCC confined in short GFRP tubes

While there is abundant research information on ordinary concrete confined in FRP tubes, there is little data on the behavior of self-consolidating concrete (SCC) under such condition. Because of the usually higher total shrinkage and lower coarse aggregate content of SCC compared to that of ordinary concrete, its composite performance under confined conditions needs special investigation. SCC confined in FRP tubes can have unique structural applications. For instance, cast-in-place deep foundations such as drilled shaft piles are often subjected to two sources of problems. First, the integrity and uniformity of the cross-sectional area of these structural elements cannot be assured using normal concrete because of limited accessibility and visibility during construction. Cavities and soil encroachments leading to soil and air pockets can jeopardize the load-bearing capacity of such piles. Second, corrosion problems of steel reinforcement in deep foundations have been costly, requiring annual repair costs of more than $2 billion in the US alone. SCC, which is able to consolidate under its self-weight without vibration, can be cast into GFRP envelopes that act as corrosion-resistant reinforcement to offer an alternative pile construction method that addresses both challenges cited above. To demonstrate the concept, this paper presents results of a laboratory investigation on the behavior of SCC confined in short GFRP tubes and subjected to axial and transverse load, including the effect of using expansive cement and shrinkage-reducing admixtures to enhance the GFRP tube-SCC interfacial contact.     

Probabilistic modeling of hybrid transition structures

Within lightweight structures, often Fiber Reinforced Plastics (FRP) are used in combination with metallic materials. Most of these hybrid structures are manufactured by established methods like riveting, bolting or adhesive bonding. In order to avoid disadvantages like drilled FRP or large bond areas, the development of hybrid transition structures compatible to loads and material properties is required. To fulfill the requirements for enhanced lightweight design, novel, integral joint concepts are currently designed, dimensioned and produced by using textile, welding and casting techniques.Three concepts are under investigation which consist of different materials (titanium and Ti-alloys, glass fibers), manufacturing methods (casting, welding, textile techniques) and geometries.Various phase boundaries, materials and influences of the manufacturing processes have to be investigated that influence the structural behavior and its failure. Based on the results of Finite Element Models on the meso scale, further modeling is performed to include effects like material uncertainties and/or process influences.In this paper, a probabilistic computation procedure based on local survival probabilities and distribution functions is proposed and investigated. This approach allows to model the complex global failure behavior for each component or its interfaces as well as the whole hybrid transition zone. It also shows the interactions and consequences of certain component changes within the hybrid transition zone. First computations are carried out and compared with experiments.     

Load response due to local and global indeterminacies of FRP-deck bridges

In a cellular FRP deck-on-beam bridge, the deck’s load response is influenced by the cellular frame’s local indeterminacy and by the global indeterminacy due to multiple beams at each support. Thus, the stiffness distributions for both levels of indeterminacy are important. This paper reports concentrated load tests and FE analyses (FEA) of an 8 m long bridge comprising cellular FRP decking bonded to and spanning across three pretensioned concrete beams. Another specimen comprising a rigidly supported, narrow width of decking where only local strains developed under load and which enabled strain measurements near the loads on external and internal surfaces of the deck, is also presented. Loads were applied via steel plates and elastomeric pads. Pressure-sensitive films enabled deduction of pad-to-deck contact zones. The FEA used contact elements at pad–deck interfaces, solid elements for the deck and beams, and included deck material and geometry anisotropies. The bridge’s test data show a 25% asymmetry of the deck’s local longitudinal strains and rapid transverse attenuation of these strains away from the loads. The FE results – including the strain asymmetry when FRP moduli are locally altered – are comparable to the experimental data. This suggests that FEA can reliably represent indeterminacy effects for these complex structures.     

Flexural behavior of a hybrid FRP and lightweight concrete sandwich bridge deck

This paper presents a new concept for a lightweight hybrid-FRP bridge deck. The sandwich construction consists of three layers: a fiber-reinforced polymer composite (FRP) sheet with T-upstands for the tensile skin, lightweight concrete (LC) for the core and a thin layer of ultra high performance reinforced concrete (UHPFRC) as a compression skin. Mechanical tests on eight hybrid beams were performed with two types of LC and two types of FRP/LC interface: unbonded (only mechanical interlocking of LC between T-upstands) and bonded with an epoxy adhesive. The ultimate loads of the beams increased by 104% on average due to bonding. However, the beam failure mode changed from ductile to brittle. The beams using a LC of 44% higher density exhibited an 81% increase in the ultimate load. The manufacturing of the beams proved to be economic in that epoxy and concrete layers were rapidly and easily applied wet-in-wet without intermediate curing times. The experimental results showed positive results regarding the feasibility of the suggested hybrid bridge deck.     

Possible estimation of resilient modulus of fine-grained soils using a dynamic lightweight cone penetrometer

Resilient modulus is an important parameter to characterise the resilient behaviour of pavement materials. Resilient modulus can be determined in the laboratory from repeated load triaxial test and is defined as the ratio of deviator stress to recoverable strain. Inherently, it is a challenge to perform repeated load triaxial tests as a routine basic test due to its complicated, time-consuming and expensive procedure; hence, several empirical approaches to estimate the resilient modulus from other soil mechanical properties – California bearing ratio, unconfined compressive strength or physical properties – have been proposed. This study has investigated the application of a dynamic lightweight cone penetrometer for the estimation of the resilient modulus in the laboratory and field conditions for some Victorian fine-grained subgrade soils. The results show the possibility to estimate the resilient modulus of fine-grained soils using the dynamic lightweight penetration index at any moisture content (MC) from optimum MC to soaked conditions.     

Standardised double-shear test for determining bond of FRP to concrete and corresponding model development

Bond stress – slip characteristics play a major role in the behaviour of EBR FRP strengthened RC beams. These characteristics are normally determined by using experimental data from small-scale bond tests. However, the research community is yet to agree on a unified experimental set-up and testing procedure for these tests. The lack of standard tests leads to high variability in published results and hinders the development of reliable design models. The testing programme presented herein was part of an international Round Robin Test (iRRT) exercise, aimed at assessing the suitability of double shear tests to characterise the bond behaviour of FRP strengthening systems. A total of 20 tests were performed on different FRP plates. Recommendations on how to improve the iRRT methodology are given. Based on the results, an improved capacity model that accounts for concrete surface preparation is proposed and validated against an extensive database of published results. The outcome of the current research is expected to provide engineers with more confidence in designing safely strengthening applications whilst making the best use of the FRP materials.     

Design of Emergency Aircraft Repairs Using Composite Patches

The increasing demand for fatigue life extension of both military and civilian aircraft has led to advances in repair technology for cracked metallic structures. Conventional structural repairs may significantly degrade the aircraft fatigue life and lower its aerodynamic performance. Adhesively bonded composite reinforcement is a new technology of great importance due to the remarkable advantages obtained, such as mechanical efficiency and repair time and cost reduction. In this article, bonded composite patch repairs were designed for quick application to aircraft under emergency conditions, such as aircraft battle damage repair (ABDR). A formulated method was developed, to be applied when damage has to be restored quickly, without restrictions to safety of flight. Different damage cases were investigated using finite-element analysis (FEA), taking into account specific parameters of the structure under repair. Based on the FEA results, a quick design procedure using composite patch repairs for the most frequent damage cases is proposed.     

Computer-aided strengthening of masonry walls using fibre-reinforced polymer strips

Studies on the use of fibre-reinforced polymers (FRP) as strengthening materials of masonry walls have been numerous. FRP materials, particularly in the form of unidirectional strips, provide a highly effective method of structural intervention in masonry walls. However, the selection of the reinforcing pattern (positioning of the strips) and the calculation of respective cross sectional FRP areas still remains a problem, which is solved in this study through the development of a methodology that relies on strut-and-tie modelling. This methodology has been implemented in a computer programme that enables the definition of the locations where FRP strips should be placed in masonry walls subjected to in-plane loading. Moreover, the tensile forces in the ties can be used to calculate the required FRP cross section areas, hence the number of strips in each location. Hence, a valuable tool for the dimensioning of interventions in masonry walls using FRP materials in the form of strips has been developed. Applications of the tool in simple case studies (masonry walls with openings subjected to both vertical and horizontal loads) are also provided. Finally, a comparison between results predicted by this strut-and-tie model and those from an experimental study found in the literature adds some confidence to the model.     

Festwalzen – eine Technologie für effizienten Leichtbau

Deep Rolling – a technology for efficient lightweight design Deep rolling is a proven method for increasing the fatigue strength of dynamically loaded parts. Although this application has been used for many years, procedures that induce work hardening and residual stresses are not well known. This article concerns both deep rolling and the potential for considering the resultant residual stresses in the design phase of components to achieve efficient lightweight design. In addition, the article includes new research results regarding the numerical modelling of deep rolling.     

Serviceability of corrosion-affected RC beams after patch repairs and FRPs under load

Substantial resources continue to be used worldwide to repair corroding reinforced concrete (RC) structures with the intent to meet or extend their design service life. Experimental observations have shown that the rate of steel corrosion as well as the extent of corrosion damage on RC structures is very much influenced by the level of the sustained load during the corrosion process. It is therefore expected that the effectiveness of interventions to maintain the serviceability of corroded structures will also be influenced by the level of the sustained load. This paper presents experimental results and a discussion on the effectiveness of patch repairs and fibre reinforced polymer (FRP) bonded plates to restore the serviceability state of quasi-full-scale RC beams (153 × 254 × 3,000 mm) that were corroded and repaired whilst under sustained service loads. Steel corrosion was induced using two sequential corrosion processes; firstly accelerated corrosion by impressing an anodic current followed by natural corrosion. It was found that under natural steel corrosion; (1) no additional widening of corrosion cracks was observed when the load applied on test beams was reduced without repairs. However, increasing the load increased the rate of corrosion crack widening; (2) despite having a significantly lower rate of steel corrosion, the rate of widening of corrosion cracks was at times comparable to the corresponding rate under the accelerated corrosion process; and (3) strengthening with FRPs without patch repairs increased the load-bearing capacity of the beams but worsened their serviceability state in terms of crack widths. It was concluded that strengthening of corrosion-affected RC structures with FRPs should be done subsequent to patch repairs.     

喷射FRP加固震损钢筋混凝土柱抗震性能试验

通过12组72件喷射纤维/树脂复合材料(FRP)试样的拉伸强度试验,研究了纤维种类、树脂基体材料、纤维体积分数、纤维混杂比及纤维长度等因素对喷射FRP拉伸强度、弹性模量和断裂伸长率等性能的影响。通过8根钢筋混凝土(RC)柱试件的拟静力试验,研究了喷射玄武岩纤维/树脂复合材料(BFRP)和混杂玄武岩-碳纤维/树脂复合材料(BF-CFRP)加固震损RC柱的抗震性能,分析了喷射FRP层厚度、纤维混杂比、柱预损程度和柱轴压比等对加固试件的极限承载力、抗侧变形能力、刚度退化特征和滞回特性的影响。结果表明:玻璃纤维与乙烯基酯树脂基体的协同工作性能最优,而玄武岩纤维具有耐久性高、延性好、与乙烯基酯树脂基体协同工作性能好等优良性能,可以作为玻璃纤维的良好替代品;玄武岩纤维混杂少量比例的碳纤维作为树脂基体增强材料,可以有效提高喷射FRP的拉伸强度和变形性能;震损RC柱经喷射FRP加固后,可以基本恢复其震损前设计极限承载力,并有效提高其延性和耗能能力。该加固方法可以对地震区已震损RC柱进行快速加固,有效防止整体结构在余震中发生倒塌等严重破坏。      

Evaluation of corrosion activity in FRP repaired RC beams

This paper presents the results of an experimental study to evaluate the corrosion activity in reinforced concrete beams repaired with fibre reinforced polymer (FRP) sheets. Ten beam specimens (152 × 254 × 3200 mm) were constructed. One specimen was neither strengthened nor corroded to serve as a reference. Three specimens were corroded and not repaired. The remaining six beams were corroded and repaired with FRP sheets. The FRP sheets were applied after the main reinforcing bars were corroded to a 5.5% mass loss. Following the FRP repair, some specimens were subjected to further corrosion to investigate their post-repair performance. The corrosion activity was evaluated using non-destructive and destructive techniques. The non-destructive techniques included half-cell potential measurements. The destructive techniques included evaluation of the mass loss of the main reinforcing bars. The experimental results showed that the corrosion potential decreased with the progress of corrosion, and the FRP repair caused a higher rate of decrease in the corrosion potential with time than that observed when FRP was not provided. Results showed that mass loss of the main reinforcing bars due to corrosion was reduced by up to 16% because of FRP repair.