D. Dias-da-Costa

Generalisation of non-iterative methods for the modelling of structures under non-proportional loading

Abstract Localisation of initially distributed cracking is a numerical challenging task, which is difficult to accomplish with conventional iterative methods, e.g. the Newton–Raphson method (Crisfield in Comput Aided Anal Des Concr Struct (1):331–358, 1984). A total approach, such as the sequentially linear approach (SLA), has been used to overcome convergence problems. However, the use of a total approach in combination with non-proportional loading raises important difficulties due to: (1) an incomplete description of the material loading history; and (2) incremental nonlinear behaviour due to the rotation of the principal stress directions. In this manuscript, two existing methods adopting combined total and incremental approaches are extended to non-proportional loading conditions. In these methods, preferential use of the incremental approach is made and the total approach is adopted only when critical bifurcations points are found. Comparison with a purely total non-iterative method (SLA) is performed, and the numerical results are validated using experimental tests available in literature.

Automatic concrete health monitoring: assessment and monitoring of concrete surfaces

To predict the degradation of concrete structures is extremely challenging. The typical approach combines periodic visual inspections with required non-destructive tests. However, this methodology only discretely evaluates few areas of the structure, being also time consuming and subject to human error. Therefore, a new method designated ‘automatic concrete health monitoring’ is herein presented which aims at automatically characterising and monitoring the state of conservation of concrete surfaces by combining photogrammetry, image processing and multi-spectral analysis. The method was designed to (i) characterise crack pattern, displacement and strain fields; (ii) map damages and (iii) assess and define restoration tasks.

Numerical modeling of concrete beams under serviceability conditions with a discrete crack approach and noniterative solution-finding algorithms

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.

Assessment of the behavior of FRP-strengthened RC slabs using a discrete crack model

AbstractThis paper presents a new discrete crack model that can simulate the complex behavior of fiber-reinforced polymer (FRP)-strengthened reinforced concrete (RC) slabs. The model approximates t...

Modelling complex cracks with finite elements: a kinematically enriched constitutive model

A continuum constitutive framework with embedded cohesive interface model is presented to describe the failure of quasi-brittle materials. Both cohesive behaviour for cracking inside the fracture process zone and elastic bulk behaviour are treated at integration points making implementation straightforward. In this sense, the proposed approach is simpler than existing ones that focus on element enrichments, such as the extended finite element method, while share similarities with smeared crack models, and offers the capability to correctly model quasi-brittle failure in post-peak regime at constitutive level. In this work, the formulation is introduced, numerical algorithms described and static and dynamic fracture simulations with complex crack patterns are conducted to demonstrate the capability and advantage of the proposed approach.

Plastic rotation and tension stiffening effect analysis in beams using photogrammetry

Innovative procedures for monitoring experimental tests using photogrammetry and image processing have been recently proposed. This manuscript aims at providing a practical demonstration of the advantages of using these new techniques in experimental tests up to failure. In this scope, focus is given to the curvature, rotation and to the tension stiffening effect, i.e., the contribution of the concrete between cracks to increase the bending stiffness. These parameters are crucial to describe the structural behavior of reinforced concrete beams both in serviceability and in ultimate limit states. These new techniques allow monitoring a high number of points, enhancing the traditional monitoring methods and providing more information which would be impossible to obtain using the traditional methods.

Fracture toughness testing using photogrammetry and digital image correlation

Graphical abstract Measuring the crack mouth opening displacement of notched metallic specimens subjected to three-point bending using a combination of photogrammetry and digital image correlation.

Image Processing in the Characterization of Crack Propagation in Cold-Formed Steel Samples

This paper focuses on a technique developed to monitor and track automatically the crack propagation at two sides of the corner of a coupon to investigate the corner effect on the fracture toughness of the high strength cold-formed steel channel sections. The coupons taken longitudinally along the corners of channel sections were initially pre-cracked at both sides under fatigue loading and then loaded monotonically under tension until failure. When assessing the fracture toughness, it was particularly important to identify the precise moment of initiation and propagation of the pre-existing cracks. The technique was based on image processing with a grid of circular targets attached on the surface prior to the monotonic test and used for scaling and orientating all acquired images. A digital correlation procedure was applied to track both edges of the notch of each crack and quantify its opening during the test. By correlating the changes in the rate of crack mouth opening, the different stages of propagation could be identified together with the corresponding critical loads. The fracture toughness is determined based on these critical loads which could not be captured directly from the load versus axial displacement curves.

VALIDATION OF A DISCRETE CRACK MODEL FOR LIGHTWEIGHT AGGREGATE CONCRETE BEAMS

The development of crack models capable of simulating the discrete nature of fracture is of interest to many different areas of research. For example, the structural analysis innovative designs now made possible by new ultra-high performance concrete mixtures would certainly benefit from such improved predictive capabilities. Currently, there are numerous numerical approaches available in the literature, for instance based on nodal or element enrichment techniques, or even on remeshing strategies. Typically, the validation of such approaches was achieved using benchmark tests that contained few cracks and where the overall displacements were compared until failure. Having this into account, this paper describes a detailed validation of a discrete crack model based on embedded discontinuities for predicting the behaviour of lightweight aggregate concrete. The model itself includes the rigid body movements associated with the opening of cracks and relies on a robust noniterative algorithm to overcome convergence difficulties typically found with numerous cracks and material non-linearities. Validation was achieved using experimental data from tests performed on lightweight concrete beams (LWAC) under flexural load, where displacements, curvatures and cracks width were properly monitored. This data include, not only overall displacements, but also the complex crack patterns produced during the tests. The model was shown to predict well the overall crack patterns and openings, and was used to perform extrapolations on crack widths for different reinforcement ratios. D. Dias-da-Costa, R. Graça-e-Costa and R.N.F. Carmo

Probabilistic Analysis of High Strength Concrete Girders Strengthened with CFRP

The strengthening or upgrading of reinforced concrete girders with carbon fiber reinforced polymer (CFRP) composites has becoming 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.