Browsing by Author "Dias-da-Costa, D."
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- Bond strength of textured micropiles grouted to concrete footingsPublication . Veludo, J.; Dias-da-Costa, D.; Júlio, E.N.B.S.; Pinto, P.L.In a previous research study the authors performed push-out tests with smooth micropile inserts grouted under varying confinement conditions. It was shown that: (i) failure always occurs at the steel-to-grout interface; and (ii) the connection capacity increases with the passive confinement. To increase the connection capacity, it is a common practice to weld steel rings on the surface of the micropile and execute grooves in the predrilled hole. Therefore, a new study is herein presented aiming to widen the conclusions already drawn by analysing the influence of most important parameters in the bond strength of textured micropiles grouted to concrete footings. Laboratory tests were specifically designed for assessing the effect on the connection capacity of the: (i) diameter of the predrilled hole; (ii) insert’s embedment length; (iii) active confinement of the footing; and (iv) treatment of the hole surface. Eighteen textured micropile inserts grouted in RC footings were submitted to monotonic push-out tests until failure. In brief, it can be stated that the capacity of the micropile-to-footing connection increases by increasing the insert’s embedment length and by decreasing the hole diameter. Moreover, an adequate active confinement must be provided to achieve the required capacity.
- An element enriched formulation for simulation of splitting failurePublication . Dias-da-Costa, D.; Veludo, J.; Alfaiate, J.; Júlio, E.Radial cracking propagation is often related to the bond transfer mechanism induced by slippage of a deformed bar. However, this failure pattern can also develop in other situations, namely: (i) concrete pipes submitted to an excessive inner pressure or (ii) concrete structures exposed to adverse environmental conditions under which corrosion or frost develops. In this paper a new contribution for the simulation of radial splitting failure is given. A discrete strong discontinuity formulation is presented which is fully capable of embedding radial discontinuities into axisymmetric finite elements. Numerical examples are used to show: (i) the capability of fully softening the applied inner pressure and (ii) mesh independence. Comparison with two published analytical approaches is performed for varying brittleness numbers. Finally, the model is applied to the simulation of both plain and reinforced concrete cylinders subjected to increasing inner pressure. A good agreement with experimental data is obtained.
- Partial Safety Factors for Prestressed Concrete Girders 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.
- Probabilistic Analysis of High Strength Concrete Girders Strengthened with CFRPPublication . Gomes, S.; Neves, L.; Dias-da-Costa, D.; Júlio, E.; Fernandes, PauloThe strengthening or upgrading of reinforced concrete girders with carbon fiber reinforced polymer (CFRP) composites has become a common procedure in the last years. However, the lack of standards or codes proposed within the same design philosophy of the Eurocode, and which are capable of assuring an adequate confidence level for the CFRP as for steel or concrete, often implies that the safety of prestressed girders strengthened with CFRP has to be evaluated using probabilistic tools. This manuscript presents a reliability study of a Portuguese prestressed high strength concrete (HSC) girder strengthened with CFRP. The selected bridge is representative of what has been constructed in Portuguese highways during the last two decades. First, the reliability level and the importance of design and load variables are computed using the following two codes: Portuguese RSA (1983) and European EC1 (2002). Next, partial safety factors are determined for the CFRP. © 2014 American Society of Civil Engineers.
- Probabilistic-based characterisation of the mechanical properties of CFRP laminatesPublication . Gomes, S.; Dias-da-Costa, D.; Neves, L.A.C.; Hadigheh, S.A.; Fernandes, P.; Júlio, E.Fibre reinforced polymer (FRP) composites have been increasingly used worldwide in the strengthening of civil engineering structures. As FRP becomes more common in structural strengthening, the development of probability-based limit state design codes will require accurate models for the prediction of the mechanical properties of the FRPs. Existing models, however, are based on small sample sizes and ignore the importance of the tail region for analyses and design. Addressing these limitations, this paper presents a probabilistic-based characterisation of the mechanical properties of carbon FRP (CFRP) laminates using a large batch of tension tests. The analysed specimens were pre-cured laminates of carbon fibres embedded in epoxy matrices, which is the most commonly used laminate for the strengthening concrete beams and slabs. Based on the existing data, probabilistic models and correlations were established for the Young's modulus, ultimate strain and tensile strength. Analyses demonstrate the suitability of the Weibull distribution for the estimation of CFRP properties. Results also show that the statistical characterisation of the mechanical properties should be performed with a focus on the tail region. The proposed distributions constitute a set of validated probabilistic models that can be used for performing reliability analyses of structures strengthened with CFRP laminates.
- Time-dependent reliability analyses of prestressed concrete girders strengthened with CFRP laminatesPublication . Dias-da-Costa, D.; Neves, L.A.C.; Gomes, S.; Hadigheh, S.A.; Fernandes, PauloThis paper presents a time-dependent reliability analysis of prestressed concrete girders subjected to degradation caused by pitting corrosion. The procedure presented includes the effects of both spatial and temporal pitting corrosions on prestressing steel, as well as the degradation of the strengthening CFRP laminate used for the rehabilitation of the member. Results indicate that the correlation of corrosion in different segments of the prestressing tendons impacts on the computed safety index for the deteriorated structure. Ditlevsen bounds are proposed for a better approximation of the correlation between failure modes in the spatial discretisation. Results show that such approach produces adequate estimates of the reliability index over the full range of analysis in comparison with other tested models. It is also observed that the degradation of the CFRP laminates does not affect the reliability as significantly as corrosion, and that traffic loads, models uncertainties, corrosion error and corrosion rate are the most relevant variables in the analysis, followed by prestressing strength and concrete cover. The significance of the variables changes with time: the corrosion rate, corrosion model error and concrete cover increase in importance with the development of corrosion, whereas traffic loads become gradually less important.