1,721,081 research outputs found
Characterization and Design of Multilayer PBO FRCM Composite Reinforcements for Concrete Structures
No full textFiber-reinforced cementitious matrix (FRCM) composites can be preferred to other techniques to strengthen reinforced concrete (RC) members. Due to the limited cross-sectional area of textiles in FRCM, multilayer composites are often needed to provide adequate strength increase. Although multilayer FRCM composites have been already employed, quite limited research is available regarding the bond behavior of multilayer FRCM-concrete joints. Direct shear tests of FRCM-concrete joints and bending tests of FRCM-strengthened RC beams showed that when one or two textile layers are employed, debonding at the matrix-fiber interface generally occurs. When more than two layers are employed, detachment of the FRCM strip was observed in FRCM-strengthened RC beams, while limited studies of corresponding FRCM bond behavior were performed. The bond behavior of a multilayer bidirectional polyparaphenylene benzobisoxazole (PBO) FRCM composite applied onto a concrete substrate is investigated in this paper. Single-lap direct shear tests of PBO FRCM-concrete joints comprising two or four textile layers are performed, along with tensile tests of bare textile strips and single-layer FRCM coupons. Tests were conducted in both textile directions. The results obtained provide fundamental information on the effect of the textile direction and number of layers to be considered when designing the reinforcing system
Modeling the behavior of externally bonded reinforcement using a rigid-trilinear cohesive material law
No full textExternally bonded reinforcement (EBR) represents a valid solution for strengthening and retrofitting existing structures. Among possible reinforcements, fiber-reinforced composites gained large popularity in the construction industry mainly due to their high strength-to-weight ratio and durability. Due to their high strength, failure of externally bonded fiber-reinforced composites is usually due to debonding, which makes understanding the bond behavior of these materials of paramount importance for the effectiveness of the strengthening application. Within this framework, bond tests are generally employed to obtain information on the stress-transfer mechanism between the composite and the substrate. The results of these bond tests can be employed to derive the interface cohesive material law (CML), which describes the relationship between the shear stress and corresponding slip at the interface where debonding occurs. In this paper, a rigid-trilinear CML is proposed to describe the stress-transfer mechanism of externally bonded composites that show the presence of frictional stresses at the debonding interface. The analytical solution of the full range behavior of composite-substrate joints with long and short bonded lengths and with free and fixed far end is provided using the proposed CML. Then, the results of thirty-five single-lap direct shear tests of PBO fiber-reinforced cementitious matrix- (FRCM-) concrete joints are presented and employed to calibrate the CML. Good agreement is found between the analytical and corresponding experimental direct shear test load responses
Tensile and compressive behavior of thermoset and thermoplastic GFRP bars
No full textFiber-reinforced polymer (FRP) bars are increasingly adopted to reinforce concrete members due to their physical–mechanical properties and durability. Usually, glass FRP (GFRP) bars comprising thermosetting resin are employed. However, bars comprising thermoplastic resin are also available in the market, although quite limited research was done to investigate their behavior. The contribution to the member capacity of GFRP bars in compression is generally neglected due to the limited research regarding their compressive behavior and to the absence of a shared procedure to obtain reliable indications of GFRP bar compressive properties. In this paper, the tensile and compressive behavior of thermoset and thermoplastic bars is experimentally studied. Three different diameters were considered for thermoset bars, while five diameters were considered for thermoplastic bars. The tensile test was performed according to ISO 10406-1, while a set-up previously adopted in the literature is used for the compressive test. For each bar, the tensile and compressive strength and tensile and compressive elastic modulus are measured. The bar axial strain is obtained with an extensometer or with digital image correlation (DIC). The results obtained show limited differences of tensile and compressive strength with varying the diameter for both thermoset and thermoplastic bars, while the tensile and compressive elastic moduli are approximately equal and independent from the diameter for the same type of bar. These are used to put forward the constitutive law for thermoset and thermoplastic bars
Durability of glass FRP reinforcing bars: A state of the art
No full textGlass fiber reinforced polymer (GFRP) bars represent a valid solution for internal reinforcement of concrete elements. With respect to steel reinforcing bars, GFRP bars have a good resistance to corrosion and a high tensile strength. However, durability of GFRP bars is still an open issue and design guidelines and recommendations provide environmental and long-term reduction factors that strongly limit the tensile stress in the bar. In this paper, a large database of GFRP reinforcing bars exposed to different environmental conditions with and without the application of a sustained load and subjected to tensile testing was collected from the literature. The variation of the bar tensile strength with respect to different exposure conditions, namely hot dry and humid air, different alkali environments, and salt solutions with various concentrations, was analyzed and discussed. Furthermore, the effect of sustained stress on the bar tensile strength was studied. A statistical analysis based on the design by testing approach provided by EN 1990 was performed on the results collected to calibrate characteristics and design values of the product between environmental and long-term reduction factors
A proposal to improve the effectiveness of the deflection control method provided by eurocodes for concrete, timber, and composite slabs
Full text linkLimited deflection of structural members represents an important requirement to guarantee proper functionality and appearance of building and infrastructures. According to Eurocodes, this requirement is ensured by limiting the maximum deflection of horizontal structural members to a fraction of their span. However, each Eurocode provides different maximum deflection limits, which are independent of the type of superstructures considered. Thus, the respect of these limits may not always guarantee the integrity of certain superstructures. In this paper, the reliability of the Eurocode deflection control methods, in guaranteeing the integrity of the superstructures, is assessed and discussed. First, different types of horizontal member, namely rib and clay (hollow) pot, composite steel–concrete, and timber beam slabs are designed to respect the deflection limit enforced by the Eurocodes. Then, the maximum curvature developed by these members is compared with the ultimate (limit) curvatures of various superstructures (e.g., ceramic and stone tile floorings). The results obtained show that the approach adopted by Eurocode 2 may provide non-conservative results, but also that the rules proposed by Eurocodes 4 and 5, albeit more reliable, do not always guarantee the integrity of the superstructure. Based on these results, an alternative method, based on the curvature control, is proposed and its advantages and limitations critically discussed. This method appears simpler and more reliable than the method currently adopted by the Eurocodes
Influence of sustained stress on the durability of glass FRP reinforcing bars
Full text linkGlass fiber reinforced polymer (GFRP) bars can be employed as an alternative to steel bars for internal
reinforcement of concrete elements. Although GFRP bars have been used to reinforce concrete for the last
two decades, their long-term behavior is still an open issue. The tensile strength of bars subjected to
aggressive exposure conditions has been investigated by different authors. However, clear and reliable
indications on the bar residual strength are still lacking. Furthermore, the effect of sustained stress on
the bar tensile strength is not completely clear and severe limitations on the stress level attained by
the bar are imposed by available design guidelines.
In this paper, the effect of sustained stress on GFRP bars exposed to different environmental conditions,
namely air, alkaline environments, deionized water, and salt solutions, is studied by analyzing a database
comprising 127 GFRP bar tensile tests collected from the literature. The results are analyzed and
discussed to investigate the effect of sustained stress on the tensile strength of GFRP bars and to calibrate
characteristic and design values for tensile strength long-term reduction factors
General Analytical Model for the Bond Capacity of NSM FRP-Concrete Joints
No full textFiber-reinforced polymer (FRP) near-surface mounted (NSM) reinforcement represents an effective solution for strengthening and retrofitting existing concrete structures. As it is embedded into concrete, NSM reinforcement is protected from accidental impact, high temperature, and vandalism and it is less prone to debonding than externally bonded reinforcement. However, debonding of the NSM reinforcement remains the main issue associated with this strengthening technique. Numerous studies have focused on the bond behavior of NSM-concrete joints and in some of them analytical models for the prediction of NSM-concrete joint bond capacity were proposed. However, these models are often based on a few experimental results of a specific strengthening configuration. In this paper, a new analytical model to estimate the effective bond length and the bond capacity of NSM-concrete joints that fail due to cohesive debonding within concrete is proposed. The model is based on a pure fracture mechanics Mode-II loading condition and can be applied to either NSM strips, round bars, or rectangular bars. The accuracy of the model proposed and of existing analytical models was assessed by comparing analytical and experimental results of 117 NSM-concrete joints collated from the literature. The assessment showed that the model proposed provided accurate estimations of the NSM-concrete bond capacity for all types of reinforcement considered
Use of bending test to determine the tensile strength and elastic modulus of GFRP bars
No full textFiber-reinforced polymer (FRP) composite bars have been increasingly employed to reinforce concrete members
when the use of traditional steel bars would represent an issue for the durability of the structure. In fact, FRP bars
have high strength-to-weight ratio, do not suffer of corrosion, and do not conduct electricity. Mechanical
characterization of composite bars is complex due to the remarkable difference between bar properties in the
direction parallel and orthogonal to the fiber. Therefore, direct tensile testing of FRP bars requires proper
specimen preparation, which usually involves expensive bonding of metal pipes at the specimen ends. In this
paper, a 3-point bending test is proposed as a possible alternative to the direct tensile test to obtain a quick and
reliable estimate of the bar tensile strength and elastic modulus. An analytical model that accounts for bar shear
deformation and large displacements is used to describe the specimen bending behavior. Forty bending tests of
glass FRP (GFRP) bars with thermosetting or thermoplastic resin and four different diameters are presented and
used to validate the test set-up and analytical model proposed. The results are compared with those of corresponding
direct tensile tests showing the validity of the proposed bending test
The role of the fiber–matrix interfacial properties on the tensile behavior of FRCM coupons
No full textFiber-reinforced cementitious matrix (FRCM) composites are usually mechanically characterized by means of tensile and bond tests. The former, in the clevis-grip version, is referred to by the American guidelines ACI 549.4R (2013) to determine the tensile properties of the FRCM composite. The latter, in the single-lap version, is used in the Italian guidelines CNR-DT 215 (2018) to determine the effective strain. The effective strain is the strain at which debonding occurs and therefore composite action is lost. In this paper, the poliparafenilene benzobisoxazole (PBO) fiber–matrix stress transfer law, also known as cohesive material law (CML), is employed in an analytical model that describes clevis-grip tensile tests of PBO-FRCM composites. The CML was previously obtained by the authors from single-lap shear tests. The load responses provided by the model are compared with the results of tensile tests herein presented in addition to selected tests from the literature. The experimental cracking process, tensile strength, and deformation capacity can be accurately predicted by the analytical model. The comparison indicates that the knowledge of the CML of the fiber–matrix interface allows for an accurate prediction of the main tensile properties of the PBO-FRCM coupon
Some key aspects in the mechanics of stress transfer between srg and masonry
Full text linkThe use of composite materials to strengthen masonry structures has become common practice within the civil engineering community. Steel-reinforced grout (SRG), which comprises high-strength steel fibers embedded in a mortar matrix, is part of the family of the fiber-reinforced cementitious matrix (FRCM) composites that represent a suitable alternative to fiber-reinforced polymer (FRP) composites for strengthening existing structures. Although studies on FRCMs have already reached a certain level of maturity, some key issues remain open, such as the role of matrix type and layout, substrate properties, and test rate. This paper focuses on some of these issues. The results of single-lap direct shear tests on masonry blocks strengthened with SRGs are presented to analyze the bond behavior between the composite material and the substrate. Four aspects are considered: (1) the change in the width of the SRG mortar matrix while keeping the width of the fiber sheet fixed; (2) the type of mortar used for the SRG; (3) the influence of the test rate, and (4) the type of substrate (i.e., concrete vs. masonry). The results obtained indicate the active role of the matrix layout and the importance of the test rate, encouraging further investigations to clarify these aspects
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