Influence of water on bond behavior between CFRP sheet and natural calcareous stones

In this paper the effect of a long term immersion in water on bond durability is analyzed when FRPs (Fiber Reinforced Plastic) are externally applied to a masonry substrate. In the performed research a substrate made by natural calcareous stones, strengthened by CFRP (Carbon Fiber Reinforced Plastic) sheets has been analyzed. For a better comprehension of water effect on the adhesive bond between stone and CFRP, the same treatments were performed to the constituent materials, namely epoxy resins, CFRP sheets and stones. To this aim mechanical tests were carried out on stone, composite materials and epoxy resins before and after their immersion in water, evaluating the effects of this agent on the properties of the materials. The influence of the aging in water on the interface stone-reinforcement was analyzed in terms of bond strength, maximum bond stress, optimal bond length, slip-bond stress relationship and mode of failure. In addition the possibility of calibrating design relationships, taking into account the influence of environmental conditions is discussed. Detailed results on adhesives and composites aged in water have been reported in a previous paper while in the present work the significant decay of the mechanical properties of the stone is specifically investigated. With regard to the conditioning treatment a reduction of the bond strength has been observed (up to 26%) as well as a similar decrease of the maximum bond stress; in addition the aged specimens have shown a more fragile behavior. On the basis of the obtained results the empirical coefficient, reported in the available Italian Guidelines, to determine the FRP-masonry bond strength seems still effective when the system FRP-masonry is aged in water once the properties of the aged materials are considered in the provided relationships.     

Dynamic bond strength between CFRP sheet and steel

The efficiency of the application of adhesively-bonded CFRP materials to rehabilitate and/or strengthen steel structures is completely dependent on the bond between the CFRP composites and the steel. As the repaired and/or upgraded structural elements are likely to be exposed to impact tensile loads during service, this necessitates the importance of understanding the bond strength between CFRP materials and steel under dynamic loadings. This paper presents the experimental procedure, the equipment and the test results for double strap joints at four speeds of loading to highlight the effect of loading rate on the bond strength. Two different CFRP layouts with various bond lengths were examined. It was found that the loading rate has significant influence on bond strength and failure modes, although it has little influence on the effective bond length.     

Effect of fatigue loading on the bond behaviour between UHM CFRP plates and steel plates

Extensive research has been conducted on static bond behaviour between CFRP and steel. However, very limited research is available on the effect of fatigue loading on the bond behaviour between CFRP and steel. This paper attempts to fill the knowledge gap in this area. A series of static and fatigue tests on UHM (ultra high modulus) CFRP plate and steel plate double strap joints were conducted. Five specimens were tensioned to failure under static loading as control specimens. The other 12 specimens were tested under fatigue loading with load ratios ranging from 0.2 to 0.6 (defined as the ratio of the maximum fatigue load to the average static bond strength of control specimens). After going through pre-set number of fatigue cycles, the specimens were tensioned to failure under static loading. The failure modes, residual bond strength and residual bond stiffness of such specimens were compared with those of control specimens, to facilitate the investigation of the effect of fatigue loading on the bond behaviour. Microscopic investigation was also performed to reveal the underlying failure mechanism.     

Influence of Aggregate Structure on Mode-III Interfacial Fracture between Concrete and CFRP

One of the crucial issues in the use of fiber reinforced polymers for civil engineering applications is the interfacial bonding between the different materials used. As a load transfer from the concrete to the composite components occurs via shear stresses in the interfacial region, studies of the interfacial bond quality should concentrate on load situations in which primarily shear stresses are induced. In this study, a 3-Point Bending Test was modified to initiate shear failure under Mode-III conditions in the interface between a composite and a concrete component. In particular it was the objective to study the interfacial shear strength between three different concretes and an unidirectional carbon fiber reinforced epoxy matrix system. The three concretes had the same compressive strength (B35 KP, German Standard), but differed in the type of filler material: (a) heavy concrete, filled with granite (Diorit) particles, (b) normal concrete, filled with gravel grains (Pyrite), and (c) light weight concrete, filled with vopourtone (Liapur). The mechanical test results showed that both the type of filler exposed to the joint surface as well as the type of adhesive used clearly influence the interfacial shear strength. Best values were achieved with Diorit-fillers, and the Sikadur-adhesive was in all cases superior to the Dywipox-adhesive. Reasons for these differences were demonstrated by light optical and scanning electron micrographs of the fractured surfaces.     

Experimental investigation into bond behavior of CFRP sheets attached to concrete using EBR and EBROG techniques

Over the last decade, an extreme increase in the application of fiber reinforced polymers (FRPs) for strengthening of reinforced concrete (RC) structures has been observed. The most common technique for strengthening of RC members utilizing FRP reinforcements is externally bonded reinforcement (EBR) technique. Despite certain benefits of the technique such as simple and rapid installation, the main problem which has greatly hampered the use of EBR method is premature debonding of FRP composite from concrete substrate. Recently, grooving method (GM) has been introduced as an alternative to conventional EBR technique. Grooving with the special technique of externally bonded reinforcement on grooves (EBROG) has yielded promising results in postponing or, in some cases, completely elimination of undesirable debonding failure in flexural/shear strengthened RC beams. Consequently, the main intention of the current study is to make a comparison between FRP-to-concrete bond behavior of EBR and EBROG techniques by means of single-shear bond tests. To do so, CFRP sheets were adhered to 16 concrete prism specimens using EBR and EBROG techniques. The specimens were then subjected to single-shear bond test and the results were compared. A non-contact, full field deformation measurement technique, i.e. particle image velocimetry (PIV) was utilized to investigate the bond behavior of the strengthened specimens. Successive digital images were taken from each specimen undergoing deformation during the test process. Images were then analyzed utilizing PIV method and load–slip behavior as well as slip and strain profiles along the strengthening CFRP strips were reported. Experimental results of the current study strongly verify the capability of GM for strengthening RC members to completely eliminate the debonding failure.     

Some mechanisms of microstructure weakening in high-porous calcareous stones

Certain kinds of stone used in buildings of archaeological, historic and architectural significance resemble aggregate of micrometre size, of weakly cemented, essentially calcite particles, containing numerous voids. Below the results are presented of an experimental investigation conducted for the purpose of determining the water-induced weakening effects on such stone. A calcarenite of the Sardinian Miocene (Cagliari pietra cantone) has been selected as model material for the investigation. Its microstructure is characterized by means of XRD, ESEM and MIP techniques. Compression strength, ultrasound propagation velocity and elongation provide an indication of the role played by various fluids (water, aqueous sodium chloride solution, kerosene) in weakening the interparticle bonding forces of the calcite grains. Permeability, pH and calcium ions concentration in the permeating water, on the other hand, point to the dissolution behaviour and hence weakening effects of water permeating through this type of microstructure.     

PVA纤维水泥基复合材料与钢筋粘结性能研究

PVA纤维水泥基复合材料与钢筋之间的粘结性能,是二者安全、稳定、耐久工作的前提和保证。通过中心拉拔实验,探讨了PVA纤维水泥基复合材料和钢筋粘结滑移研究中影响因素及本构关系问题,测试得到了粘结滑移曲线,通过对加载到破坏全过程的受力分析及基材中纤维的特性分析,在已有模型的基础上,根据振动阻尼的理论提出一种新的粘结-滑移本构关系模型。并与实验结果和已有模型比较。新构建的粘结-滑移本构关系能较好地反映PVA纤维水泥基复合材料和钢筋的受力全过程,与实验结果比较吻合。可为PVA纤维水泥基复合材料与钢筋性能的非线性有限元法分析提供参考和依据;为有关规程的修订提供了依据。     

Experimental investigation of bond characteristics between CFRP fabrics and steel plate joints under impact tensile loads

In recent years, the use of adhesively-bonded fibre-reinforced composite materials has attracted widespread attention as a viable alternative for the retrofitting of civil infrastructure such as buildings and bridges. This has been particularly the case for concrete structures. The retrofitting of metallic bridges and buildings with FRP materials, however, is still in its early stages. In real life, these structures are subjected to dynamic loads. Therefore, it is necessary to understand the bond behaviour between steel and the strengthening materials for both static and dynamic loads. To examine the bond between steel plates and carbon fibre-reinforced polymers (CFRP) fabrics, this paper describes the experimental procedures and results of double strap steel joints loaded at different loading rates (2 mm/min, 3.35, 4.43 and 5 m/s). In this test program, ultimate load-carrying capacity, effective bond length, failure mechanism and strain distribution were examined for all loading rates. Different numbers of CFRP layers with different bond lengths were investigated. Experimental findings reveal that the maximum improvement in joint capacity occurs at a rate of 3.35 m/s. It was observed that the effective bond length is insensitive to loading rate for both joints. The failure modes and strain distributions, however, exhibit little difference between static and dynamic conditions.     

Damage evolution behavior of CFRP laminates under post-impact fatigue with water absorption environment

In this study, the damage evolution behavior was evaluated considering the effect of the textile structure and water absorption. Damage observation was conducted by the integration of non-destructive and direct observation methods. Candidate textile reinforcements were T300-3k plain woven fabric (PW) and T700S-12k multi-axial knitted fabric (MA). The effect of water absorption on the performances of compression after impact (CAI) and PIF were small in PW CFRP laminates. Conversely, PIF properties of water-absorbed MA CFRP laminates drastically decreased than that of dry ones. CAI strength was not affected by water absorption. PIF performance of dry MA CFRP was fairly higher than that of the others. From the precise observation, some evidences of interfacial deterioration caused by water absorption were confirmed in both PW and MA CFRP laminates.     

CFRP筋的疲劳性能

进行了不同循环应力下CFRP筋的常温疲劳试验,选取CFRP筋70％的极限拉伸应力作为最大循环应力,在R=0．5（最小循环应力与最大循环应力的比）和R=0的应力率下,测量了CFRP筋的疲劳寿命曲线,研究了CFRP筋的疲劳性能．结果表明：CFRP筋的最大循环应力应控制在70％极限拉伸应力以下;在R=0．5应力率下,最大循环应力下降5％的极限拉伸应力时,CFRP筋的疲劳寿命增长10倍左右;最大循环应力分别为60％和50％极限拉伸应力时,R=0应力率下CFRP筋的疲劳寿命分别为R=0．5应力率下疲劳寿命的百分之一和十分之一．这说明CFRP筋具有很大的脆性,需要有足够的强度安全系数,才能发挥作用;根据试验拟合的疲劳寿命曲线,CFRP筋的疲劳性能远高于Q235光圆钢筋．     

Interventions to historic masonries: Investigation of the bond mechanism between stones or bricks and grouts

Within the present work, the mechanism of bond is studied in composite grout/substrate specimens. Three types of tripartite (lime-pozzolan-cement) grouts are examined, combined with three substrates (two types of limestone and bricks). The interfaces between grout and substrate are characterized by means of mechanical tests in direct tension and shear. The in-time development of the tensile and shear bond strength is also investigated. In order to correlate the characteristics of the substrates and the obtained strengths of the interfaces, their surfaces and porosity are examined. The main conclusions of this study are that the studied tripartite grouts can develop tensile and shear bond strength comparable to Portland cement-based grouts, and that the value of the reached bond strength is governed mainly by the substrate characteristics and the binding properties of the grouts. The results of this project confirm the efficiency of tripartite lime-pozzolan-cement grouts with reduced Portland cement content for repair and strengthening of historic masonries.     

Suitable structure of thermosetting CFRP sheet for cold/warm forming

The forming processes at room temperature and under a warm condition have been investigated for carbon-fiber-reinforced plastic (CFRP) sheets consisting of thermosetting resin and continuous fibers used for mass production. While CFRPs consisting of thermosetting resin have the advantage of high strength, to subject them to press forming, in contrast to carbon-fiber-reinforced thermoplastics (CFRTPs), which consist of thermoplastic resin. When CFRP sheets are formed into the desired shape through plastic deformation, a higher-strength structural material easily applicable to mass production is obtained. To improve the formability of thermosetting CFRP sheets while retaining their strength, a suitable structure allowing plastic deformation under warm condition is proposed. The tensile stress obtained by a tensile test and the bending properties obtained by a stretch-bending test indicate the strength and formability, respectively. A stretch-bending test is a bending test in which a tensile load is placed on a sheet, and it has the characteristics of a bending test and a deep drawing test. A suitable structure containing prepreg layers to allow plastic deformation based on the relationship between the bending load and the results of specimen observations is revealed.     

Influence of the properties of externally bonded CFRP on the shear behavior of concrete/composite adhesive joints

This paper concerns the strengthening of concrete structures with externally bonded composite reinforcement, and focuses mainly on the influence of the FRP characteristics on the mechanical behavior of the composite to concrete interface. An experimental investigation was conducted, based on the characterization of such bonded assemblies and using a double lap joint shear test. Twelve different series of specimens were studied in order to evaluate the influence of various parameters related to the FRP material (i.e. the use of carbon or aramid and of fiber reinforced systems, the type of manufacturing process, the values of the Young modulus, the thickness of the FRP and the bonded length) as well as several parameters related to the adhesive joint (i.e. the lap joint thickness, the curing conditions and the elastic modulus of the epoxy adhesive). Analyses of the strain and shear stress distributions along the lap joints emphasized significant effects of the FRP properties and epoxy curing conditions on the interfacial strength. In addition, a bond strength model is proposed in the last part of the study.     

Long-term tensile and bending strength of natural building stones

The results of experimental long-term tension and bending tests carried out on four natural stones formely widely used in Italian traditional building (a white-veined Carrara marble, the “pietra Serena” and “Pietraforte” sandstones, and the Piobbico-Cesana limestone) are presented. The tests consisted of applying continually different fractions of the “instantaneous” fracture loads and monitoring the strain evolution of the specimens until a time-delayed fracture was reached. A typical three-stage creep response was obtained in all the tests. At the same time, a 60% reduction in tensile and bending strength was observed in relation to testing periods not much longer than one year. A phenomenological constitutive model is proposed for analytically reproducing the experimental viscoplastic behaviour. Furthermore, a creep time-to-rupture function giving direct estimates of the strength decay effects is provided, with a view to technical applications. The results of the identification analyses carried out to calibrate the characteristic parameters of the above laws are also reported.     

Linear and nonlinear torsional behavior of unidirectional CFRP and GFRP

The helicopter bearingless rotor flexbeam is usually made of glass-fiber reinforced plastic composite (GFRP). Carbon-fiber composites (CFRP) are candidate for future flexbeam materials due to their superior tensile fatigue strength. This research examines the feasibility of CFRP as a future flexbeam material. The torsion behaviors of unidirectional CFRP and GFRP with the same matrix resin were investigated. As a result, it was confirmed that the behavior of both CFRP and GFRP is comprised of linear/nonlinear domains. The initial torsional rigidity of CFRP was almost the same as that of GFRP. The torsional rigidities calculated from Lekhnitskii’s equations agreed with the experimental results, and they are mainly determined by the shear stiffness of the materials. The nonlinear torsional behavior was observed above 0.5% of the shear strain, and it is due to plastic deformation of the matrix resin. A 3D plasticity model proposed by Sun et al. was applied to the plasticity parameters obtained from off-axis tensile tests. The numerical curves agree with the experimental data below 1.5% of the shear strain. The experimental result suggests that GFRP can be replaced by CFRP as torsional elements of a helicopter flex beam without an increase in torsional rigidity.     

Microstructural study of material removal mechanisms observed in abrasive waterjet cutting of calcareous stones

An experimental study was conducted to determine the influence of material properties on the cutting mechanisms involved in abrasive waterjet (AWJ) of calcareous stones. SEM analysis was performed to investigate the material removal mechanisms. Two Portuguese calcareous stones were tested with different genesis and thus structural and mechanical characteristics.

It was found that rock hardness and porosity have a significant effect on the cutting mechanisms investigated. In harder homogeneous rocks, like marbles, cutting occurs due to intergranular cracking and cleavage of calcitic grains. In limestones, the material removal process mostly involves intergranular cracking and material sliding along the argilous matrix and between this and calcitic grains. Plastic deformation of the matrix can also be observed.     

混合材水泥石微观结构对其声学特性的影响

固井质量评价主要依赖于水泥胶结测并结果；而声波水泥胶结测井结果受被测介质（套管-水泥环、水泥环-地层）的影响；文章以混合材水泥石为主要研究对象，利用超声波测试、扫描电镜实验，定性探索了混合材质对水泥石声学特性的影响，以及混合材水泥石微观结构对其声学特性的影响。实验表明：混合材质、养护时间、养护温度是影响水泥石声学特性的外部因素。决定水泥石声学特性不同的本质因素是水泥石的微观结构及强度性能；在进行固井质量评价时，必须考虑混合材水泥设计对声波测井结果的影响。     

Influence of reinforcing bar type on autogenous shrinkage stress and bond behavior of ultra high performance fiber reinforced concrete

This study investigated the effects of reinforcing bar type and reinforcement ratio on the restrained shrinkage behaviors of ultra high performance fiber reinforced concrete (UHPFRC), including autogenous shrinkage stress, degree of restraint, and cracking potential. In addition, the influence of the type and embedment length of reinforcing bars on the bond behavior of UHPFRC was evaluated by performing pullout test. Three different reinforcing bars (deformed steel bar, round steel bar, and GFRP bar) were investigated in the restrained shrinkage and pullout tests. The GFRP bar exhibited the best performance in relation to the autogenous shrinkage stress, degree of restraint, and cracking potential because of its low stiffness. The highest bond strength was obtained for the deformed steel bar, and the bar yielding was observed when the bar embedment length of l_b = 2d_b was used. The round steel bar exhibited the poorest behaviors for both of the restrained shrinkage and pullout.     

Temporary bond strength of partly cured epoxy adhesive for anchoring prestressed CFRP strips on concrete

Externally bonded Carbon Fiber Reinforced Polymer (CFRP) strips have been used for strengthening reinforced concrete structures. This paper presents an experimental study on the debonding of externally bonded CFRP strips anchored to a concrete substrate by a commercial epoxy adhesive. The study represents the basis for the characterization of an innovative ‘gradient method’, giving the possibility to anchor prestressed CFRP strips to concrete without the use any mechanical anchorage systems such as plates and bolts. Bond between the two components is achieved by an epoxy adhesive able to carry loads after an accelerated curing process under elevated temperatures. The effect of heating configuration/duration, strip thickness and bond length on the temporary bond resistance have been investigated using prestressed and non-prestressed CFRP-strips. Besides the optimization of the heating elements necessary for the curing process, curing parameters for an optimal temporary bond strength could be determined. Twenty-five minutes of heating and curing at 90 °C was found to be an optimum heating configuration, resulting in better short-term mechanical performances than after conventional curing at room temperature for several days. The main reason is a temporarily softer adhesive which allows the use of the full bond length by reducing shear force peaks.     

钨铜合金薄板坯烧结行为研究

以常规工业级钨粉和铜粉作为原料，制备了厚度均匀、密度较高的钨铜薄板坯，并对该薄板坯的烧结行为进行了系统地研究。结果表明，钨铜两相的润湿性对烧结致密化过程起主导作用，它在各温区所引起的致密化因素各异，使致密度变化程度不同。研究发现，钨铜在高温液相烧结过程中的动力学特征，与非晶体粘性流动烧结理论相吻合。