Shear strength contribution provided by inorganic-matrix composites fully wrapped around reinforced concrete beams

Fiber-reinforced cementitious matrix (FRCM) composites have been increasingly employed as externally bonded (EB) reinforcement for existing structures. FRCMs are effective in increasing the shear strength of existing concrete members. When the fully-wrapped configuration is employed, the composite tensile failure could be attained. In this paper, an analytical approach previously proposed to describe FRCM U-wrapped RC beams is extended to fully-wrapped members. This approach provides an accurate description of the stress-transfer mechanism between the FRCM bridging the shear crack and the substrate, thus allowing for an in-depth study of the composite contribution to the member shear strength

Influence of severe thermal preconditioning on the bond between carbon FRCM and masonry substrate: Effect of textile pre-impregnation

Fabric-reinforced cementitious matrix (FRCM) composites often include polymer-impregnated bundles to improve the exploitation of the textile mechanical properties. However, organic components may degrade when exposed to elevated temperature. In this paper, the bond behavior of a carbon FRCM applied to a masonry substrate and exposed to a thermal preconditioning up to 300 °C for 250 min is investigated. Tensile tests on the textile and flexural and compression tests on the mortar matrix, as well as single-lap direct shear tests of FRCM-masonry joints with bare and impregnated textiles, are performed. Results show that the polymeric impregnation improves the mechanical properties of the FRCM even after thermal preconditioning

Analytical solution of the full-range behavior of adhesively bonded FRP-steel joints made with toughened adhesives

Fiber-reinforced polymer (FRP) composites represent an effective solution to strengthen and retrofit existing steel members. Namely, bonded or unbonded carbon FRP (CFRP) plates have been employed to improve the strength, fatigue behavior, and durability of steel bridges. In bonded solutions, the effectiveness of the CFRP reinforcement strongly depends on the adhesive employed to bond the plate, as failure usually occurs due to debonding. Within this framework, the use of toughened adhesives is particularly attractive since they may improve the load carrying capacity of the CFRP-steel interface, also providing a certain ductility. Debonding in CFRP-steel joints was previously studied using a cohesive approach. However, solutions able to describe the full-range behavior of joints with toughened adhesives and finite bonded length are not available in the literature. In this paper, a trapezoidal (trilinear) cohesive material law (CML) is employed to model the bond behavior of pultruded carbon FRP-steel joints made with a rubber-toughened epoxy adhesive, which showed cohesive debonding within the adhesive layer. The analytical solution provided is employed to study the experimental response of nine CFRP-steel joints tested using a single-lap direct shear set-up. Comparisons of analytical and experimental results of joints with three different bonded lengths confirm the effectiveness of the solution proposed

Long-Term Behavior of PBO FRCM and Comparison with Other Inorganic-Matrix Composites

Fabric-reinforced cementitious matrix (FRCM) composites, comprising high-strength fiber textiles embedded within inorganic matrices, represent an effective, cost-efficient, and low-invasive solution for strengthening and retrofitting existing masonry and reinforced concrete structures. Among different textiles employed in FRCM composites, polyparaphenylene benzo-bisoxazole (PBO) textiles are adopted due to their high tensile strength and good adhesion with the matrix. Although several experimental, numerical, and analytical works were performed to investigate the mechanical properties of PBO FRCM composites, limited information is available on their long-term behavior, as well as in the case of exposure to aggressive environments. This paper presents and discusses the results of a wide experimental campaign aimed at investigating the effect of different environmental conditions on the long-term tensile behavior of a PBO FRCM composite. Tests are performed using a clamping-grip tensile test set-up. The effect of various aggressive environments on the composite matrix cracking stress, composite tensile strength, ultimate strain, and fully cracked stage slope is investigated by comparing the results of nominally equal conditioned and unconditioned (control) specimens. These results are also compared with those of other FRCM composites comprising glass and carbon textiles subjected to the same conditionings, collected from the literature. The results show only limited reductions in the tensile properties, even after long exposure to aggressive environments

Bending and shear behavior of historic walls strengthened with composite reinforced mortar

Composite reinforced mortar (CRM) is a relatively new solution for the strengthening of existing masonry members that comprises fiber-reinforced polymer (FRP) grids reinforcing inorganic mortar overlays. CRMs were proven to be effective in strengthening masonry members against in- and out-of- plane loads. In this paper, a glass FRP-CRM is employed to strengthen 5-leaf historic masonry walls cut from an existing building located in Milan, Italy. The walls were strengthened and then subjected to three-point bending and diagonal compression tests. Results were compared with those of corresponding non-strengthened walls and showed the CRM effectiveness also in the case of thick masonry members

Fracture Behavior and Digital Image Analysis of GFRP Reinforced Concrete Notched Beams

This study presents three-point bending fracture tests on glass fiber-reinforced polymer (GFRP) reinforced concrete notched beams. Few studies have been conducted to date to understand the fracture behavior of this type of specimens. The specimens have nominal depth, width, and length equal to 150 mm, 150 mm, and 550 mm. Plain concrete notched beams with the same dimensions are cast from the same batch of concrete to compare the responses with GFRP reinforced concrete notched beams. The notch of the plain concrete specimens is either saw cut or cast. These two notch fabrication methods are compared based on the load responses. The peak load, crack mouth opening displacement (CMOD), GFRP bar slip at two ends, and load point displacement are used to discuss the results of the fracture tests. In addition, digital image analysis is performed to identify the fracture process zone (FPZ) and the location of the neutral axis, which are used to determine the force in the GFRP bar via cross-sectional analysis. Finally, the GFRP bar force versus slip responses are compared with those from the pull-out tests performed on the same bar to show that the bond of the bar in the pull-out tests represents an upper bound limit compared to the behavior in bending

Effect of cyclic load on the tensile behavior of a PBO FRCM composite

The use of externally bonded fiber-reinforced cementitious matrix (FRCM) composites represents a valid alternative to traditional techniques for the strengthening and retrofitting of existing reinforced concrete and masonry structures. FRCM composites are comprised of high strength textiles embedded within inorganic matrices and can be directly applied to the external surface of the existing structural element to increase its displacement and load capacity (i.e., axial, flexural, and shear strength). Thus, FRCM have a low invasiveness and a high strength-to-weight ratio. Recently, investigations on the bond behavior of FRCM composites showed that the presence of friction between the textile and matrix can induce damage to the fiber, which in turn determines possible reductions in the strengthened element capacity. This effect appears particularly critical in the case of cyclic and dynamic loads. In this paper, the cyclic behavior of a PBO FRCM composite is experimentally investigated using low-cycle tensile tests on composite specimens. Namely, FRCM rectangular coupons are subjected to clamping- and clevis-grip tensile tests. These tests provide important information on the effect of low-frequency dynamic loading on the composite tensile properties under different test configurations

Saúde mental e violência entre estudantes da sexta série de um município paulista

OBJECTIVE: To analyze risk and protective factors for mental health problems among adolescents. METHODS: Cross-sectional study with a random sample (N=327; attrition rate=6.9%) of sixth grade students from all public and private schools in the city of Barretos, Southeastern Brazil, conducted in 2004. The factors studied were: exposure to intrafamilial and urban violence, family socioeconomic level, sex, motherless household, participation in social activities (protective factor). All the independent risk and protective factors were included in the initial logistic regression model. Only the variable with a significance level of pOBJETIVO: Analisar fatores de proteção e de risco para problemas de saúde mental entre adolescentes. MÉTODOS: Estudo transversal realizado com amostra aleatória (N=327; perda=6,9%) de estudantes da sexta série de todas as escolas públicas e privadas de Barretos, SP, em 2004. Os fatores examinados foram: exposição à violência doméstica e urbana, nível socioeconômico familiar, sexo, morar sem a mãe, participar de atividades sociais (fator de proteção). As associações entre esses fatores e problemas de saúde mental foram analisadas por meio de modelos de regressão logística. Todos os fatores de risco e proteção independentes foram incluídos no modelo inicial de regressão logística, permanecendo no modelo final apenas a variável com nível de significância inferior a 0,05. RESULTADOS: Verificou-se que apenas exposição à violência permaneceu no modelo final como fator associado a problemas de saúde mental (p=0,02; IC 95%: 1,12;4,22). Crianças expostas à violência doméstica tinham três vezes mais chances de apresentarem estes problemas do que aquelas expostas à violência urbana (p=0,04; IC 95%: 1,03;7,55). CONCLUSÕES: A violência doméstica associou-se a problemas de saúde mental nos adolescentes do estudo, podendo ser mais importante que a violência urbana em cidades de médio porte