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- Numerical modeling of concrete beams under serviceability conditions with a discrete crack approach and noniterative solution-finding algorithmsPublication . Dias-da-Costa, Daniel; do Carmo, Ricardo N. F.; Graça-e-Costa, R.This paper describes the development and validation of a comprehensive numerical model enabling the simulation of reinforced concrete beams under serviceability conditions using a discrete crack approach. The highly nonlinear behavior introduced by the different material models and the many localized cracks propagating within the member pose a challenge to classic iterative solvers, which often fail to converge. This limitation is solved using a noniterative solution-finding algorithm to overcome critical bifurcation points. The finite element model was validated using experimental data on lightweight aggregate concrete beams under flexural loading. It was shown that the model properly simulates both the overall and localized features of the structural response, including curvature, crack openings, and crack patterns. The model was used to carry out a numerical study of the role of longitudinal reinforcement ratios and crack widths in reinforced concrete beams. It was observed that the total crack openings along a member seem to remain nearly independent of the tensile reinforcement for ratios >2.5% and the same level of strength.
- Partial safety factors for prestressed concrete girders 2 strengthened with CFRP laminatesPublication . Dias-da-Costa, D.; Neves, L. A. C.; Gomes, S.; Graça-e-Costa, R.; Hadigheh, S. A.; Fernandes, P.This paper provides a framework for the calibration of partial safety factors in prestressed concrete (PC) girders strengthened in flexure with carbon fiber-reinforced polymer (CFRP) laminates. A hybrid approach was proposed to take advantage of comprehensive nonlinear numerical models in reliability analysis using a first-order reliability method (FORM) in conjunction with the response surface method (RSM). The PC girders selected for analyses were taken from real structures designed and built in the 1980s based on old standards that now require strengthening and upgrade due to partial corrosion of the prestressing strands. Using the proposed approach, a sensitivity analysis was performed to identify the most relevant variables and assess the area of CFRP laminates needed to restore capacity up to new design standards. A partial safety factor was proposed for strengthening PC girders using CFRP laminates. Sensitivity analysis showed that traffic loads and model uncertainties are the most important variables for calibration. (C) 2019 American Society of Civil Engineers.
- O reforço de vigas de betão armado com chapas metálicas coladas com resinaPublication . Alfaiate, J.; Graça-e-Costa, R.Neste trabalho efectua-se a simulação do reforço com chapas metálicas coladas com resinas epoxy, em vigas de betão armado. Cada viga é inicialmente carregada à flexão e submetida a um elevado nível de dano. Posteriormente, sob carga constante, as fendas são reparadas com resina epoxy e aplica-se o reforço, colando a chapa metálica na face traccionada da viga. Conclui-se o teste aumentando a carga até se obter a rotura da viga reforçada. O nível de dano pré-reforço é modelado recorrendo a uma abordagem discreta com fendas embebidas e a ligação betão-resina-chapa é modelada em modo II de fractura através de elementos de interface com espessura inicial nula. Os resultados numéricos são comparados com os resultados obtidos em vigas ensaiadas experimentalmente de acordo com os procedimentos referidos.
- Mesh-independent framework for the bidimensional analysis of CFRP–concrete debonding shear tests with discrete fracturePublication . Graça-e-Costa, Rui; Mukhtar, Faisal M.; Dias-da-Costa, DanielThe performance of concrete structures strengthened with carbon fiber-reinforced polymer (CFRP) systems can depend heavily on the bond strength of the interface between the concrete and the reinforced polymer. Even though experimental testing can be used to derive suitable constitutive models, their interpretation and analysis is often limited by the reliability of available numerical/analytical models. The debonding in shear tests can be controlled by the highly nonlinear interaction of the bonded interface with the microcracks developing in the substrate. This process cannot be efficiently predicted by simplifying assumptions, which is why robust models accounting for those features, while relying only on material parameters that can be easily measured and interpreted, need to be developed. This paper introduces a framework for developing such models based on the discrete representation of fracture that can be easily deployed into existing finite-element codes. The substrate bond failure, in addition to the interface bond failure and any combination thereof, are automatically accounted for, and the cracks are not prespecified to the underlying finite-element mesh, which means that the results are mesh-insensitive and discretization-independent. A validation of the proposed framework was performed using modified double-shear bond tests between CFRP and concrete. An in-depth analysis was carried out to assess the influence of bond length and CFRP reinforcement area on the debonding behavior and ductility of the connection. (C) 2022 American Society of Civil Engineers.