Predicting the Shear Behaviour of Solid Slab Bridges using Nonlinear Finite Elements

Catastrophic collapse of Concorde Bridge in 2006 near Montreal raised the issue shear stress concentration in straight and skewed concrete concrete slab bridges. These bridge have been traditionally designed without shear reinforcement. Questions were raised concerning the reliability of existing bridges and also on the design guidelines of new bridges. Straight and skewed solid concrete slab bridges are modeled using linear and nonlinear finite element models to highlight shear forces that develop in slab bridges, particularly at the corners. Various types of loads are applied in order to understand the elastic and plastic behavior of the corner force in service conditions and near ultimate. A parametric study is carried out on several configurations of simply supported slabs with geometries covering one to four lane bridges of 3 to 20 m spans and with skew angles ranging from 0° to 60°. The analyses show that comer forces develop locally and the actual design of straight and skew concrete slab bridges is not influenced by the comer forces. A redistribution of corner forces is observed near ultimate limit states with apparition of shear crack bringing ductility at this failure mode. The analyses indicate that these corner forces appear along the free edge with depth about a half-thickness of the concrete slab. This justifies reinforcing the free edge by stirrup, when the design propose a reinforcement. For existing bridges, the analyses show that strength obtained using a beam type behavior is too conservative for estimating the actual shear strength

Effet du microclimat sur l'initiation de la corrosion des aciers d'armature dans les ouvrages en béton armé

The deterioration of reinforced concrete structures and the associated rehabilitation work represent an economic cost of premier importance. In Switzerland, the structural maintenance expenditures amount to 40% to 65% of the initial construction cost (over a 100 year life-span). Among the maintenance and rehabilitation activities, those dedicated to combating corrosion constitute the largest portion of invested financial resources. For example, the curbs of each highway bridge need to be repaired every 20 to 30 years, resulting in extensive maintenance and user costs. The corrosion phenomenon is initiated by the presence of oxygen and water acting upon steel reinforcement with a compromised protective oxide film (a natural protective layer surrounding the steel). This protective layer is eliminated by carbonization of the concrete (the reaction between carbondioxide and concrete calcium-hydroxide paste), a sufficient chloride ion concentration around the steel reinforcement (introduced by the spreading of de-icing salts on the roads during winter), or a combination of the two phenomena. The water diffusion in the concrete cover facilitates the chloride ion movement, reducing the corrosion initiation time, and likewise, increasing deterioration rate. This research proposes a model, "TransChlor," to evaluate the duration of the corrosion initiation processes. TransChlor can therefore serve as a decision aid during the structural maintenance plan selection process and during the design of new structural elements to mitigate the steel reinforcement corrosion problem. The model can also complement and elaborate upon the existing codes. TransChlor is based on a structural analysis approach that groups the elements of a structure into distinct groups. Each group is defined by similar functional and structural qualities influencing the corrosion phenomenon (concrete cover permeability, degree of exposure to aggressive corrosive agents, local defects). The model in particular distinguishes the elements exposed to mist and liquid water. This model is based on in-situ measurements (structural monitoring) and inspections performed by trained inspectors. TransChlor employs a deterministic approach to simulate water diffusion, chloride ion diffusion, gas penetration, and thermal conduction. The penetration of liquid water into the concrete pours, called capillary suction, was experimentally studied in the laboratory at low temperatures to match in-situ conditions. These experiments formed the calibration base of this model. TransChlor also incorporates a probabilistic model to consider the variable and indeterminate aspects of the corrosion problem, such as the permeability space, the concrete cover thickness, and the reinforcing steel physical-chemical resistance to chloride ion attack. The deterministic model is refined by the probabilistic approach and together TransChlor offers a more exact evaluation of the corrosion deterioration risks and can facilitate the objective dialogue and discussion between owners and maintenance specialists. For locations exposed to liquid water and brine, the standards should impose additional protective provisions such as periodic hydrophobic impregnations, implementation of stainless steel reinforcing bars, etc. For other locations (exposed to salt spay and carbonated concrete not exposed to chloride ions), the codes should require complementary construction and maintenance provisions

Analysis and Design of Straight and Skewed Slab Bridges

Results of an investigation aimed at determining bending moments and shear forces, required to design skewed concrete slab bridges using the equivalent-beam method are presented in this paper. Straight and skewed slab bridges were modeled using grillage and finite-element models to characterize their behavior under uniform and moving loads with the objective of determining the most appropriate modeling approach for design. A parametric study was carried out on 390 simply supported slabs with geometries covering one to four lane bridges of 3- to 20-m spans and with skew angles ranging from 0 to 60°. The analyses showed that nonorthogonal grillages satisfactorily predict the amplitude and the transverse distribution of longitudinal bending moments and shear forces, and can be used for the analysis of skewed slab bridges. Results of the parametric study indicated that shear forces and secondary bending moments increase with increasing skew angle while longitudinal bending moments diminish. Equations are proposed to include, as part of the equivalent-beam method for skew angles up to 60°, the increase of shear forces and the reduction of longitudinal bending moments. Equations are also given for computing secondary bending moments. A simplified approach aimed at determining the corner forces for straight and skewed bridges is proposed as an alternative to a more-refined analysis. The analyses indicated the presence of high vertical shear stresses in the vicinity of free edges that justifies suggesting to provide shear reinforcement along the slab free edges

Markov chain-based stochastic modeling of chloride ion transport in concrete bridges

Over the last decade, there has been an increasing interest in models for the evaluation and prediction of the condition of bridges in Canada due to their large number in an advanced state of deterioration. The models are used to develop optimal maintenance and replacement strategies to extend service life and optimally allocate financial and technical resources. The main process of deterioration of concrete bridges in Canada is corrosion of the reinforcing steel due to the widespread use of de-icing salts. In this article, numerical models of the diffusion process and chemical reactions of chloride ions in concrete are used to estimate the time to initiation of corrosion and for the progression of corrosion. The analyses are performed for a range of typical concrete properties, exposure and climatic conditions. The results from these simulations are used to develop parametric surrogate Markov chain models of increasing states of deterioration. The surrogate models are more efficient than physical models for the portfolio analysis of a large number of structures. The procedure provides an alternative to Markov models derived from condition ratings when historical inspection data is limited

Updating of chloride ingress models using core data measurements

A large proportion of concrete bridges in North America are reaching the end of their service life. An important factor that accelerates deterioration is the amount of salts used during winter months. The chloride ions from the salt enter the concrete and destroy the passive-oxide layer that protects the reinforcing steel from corrosion. The TransChlor® software is used to simulate the time history of chloride ion ingress into a typical concrete bridge. A climate model is used to simulate the exposure of the structure to winter conditions. A probabilistic method is used to propagate the uncertainty on material properties in the model and to obtain the distribution of chloride ion concentrations. A procedure for updating the distribution is proposed using data from core samples at two depths (25/50 mm) and for the two types of exposure (splash/direct). The procedure is shown to be very efficient in reducing bias and variance from model predictions

Optical fiber sensors implementation for monitoring the early-age behavior of full-scale timber-concrete composite slabs

The study of the early-age behavior of Timber-Concrete Composite (TCC) structures is of great interest as it provides valuable information for manufacturing specification development, quality control, and optimization of the formwork design. In this study, the results of the continuous monitoring of the short-term behavior of TCC slabs using Brillouin Distributed Optical Fiber Sensors (DOFS) are reported. Two TCC slabs with 8.5 m of length were monitored. The composite elements are constituted of Cross-Laminated Timber (CLT) connected to a High-Performance Concrete (HPC) slab. During a monitoring period of about 30 days, the early-age temperature/strain variation in the fresh concrete and in the CLT slab was measured in great details by DOFS. From the presented results, the significant influence of the curing conditions on the early-age shrinkage was highlighted. It was also observed that creep and the daily hygrometric variations of environment affect considerably the composite action between the timber and the concrete. In addition, it was experimentally demonstrated that such mechanisms generate considerable structural changes in the composite elements even before their entry into service

A modified accelerated chloride migration tests for UHPC and UHPFRC with PVA and steel fibers

Accelerated migration tests which are commonly used to measure chloride diffusion in ordinary cement-based materials cannot be directly applied to composite with very low permeability, such as Ultra High-Performance Fiber Reinforced concretes (UHPFRC). In order to assess the UHPFRC enhancement on the structural durability, there is a critical need to accurately assess the permeability level of the material to chloride ions. The objective of this work is to adapt an existing set-up of accelerated chloride migration test in order to (i) better characterize the resistance of chloride ion penetration in UHPFRC; and (ii) to compare the resistance of chloride ion penetration between UHPC and UHPFRC. The material characterization, the set-up modifications of the existing accelerated migration test, the results are presented. In conclusion, the modification of the test-set-up allowed to accurately measure chloride transport of very low permeability UHPFRC and to shed light on the effect of the fiber reinforcement

Sensitivity of chloride ingress modelling in concrete to input parameter variability

The Rosenblueth point estimator method is used to propagate uncertainty on stochastic parameters in a multi-ionic reactive transport model for concrete degradation. The degradation mechanism that was analysed is the ingress of chlorides into concrete. This point estimator method is used because it requires a minimum number of simulations to compute means, standard deviations and skewness for the solution of the transport model. Two exposure cases are considered in this article: the first is a saturated case, and the second is an unsaturated case with wetting (4 days) and drying (3 days at 50 % relative humidity) cycles. The sodium chloride exposure during the wetting cycle of both cases was 500 mmol/L. In both cases, predictions from the model were compared to experimental results. The Rosenblueth approach yielded results comparable to Monte Carlo simulations for both saturated and unsaturated cases. The relative sensitivity of model parameters on prediction results are investigated through a sensitivity analysis. The sensitivity results show the high importance of ionic diffusion parameters and of exposure conditions, while the calculated initial mineral phases in the hydrated paste have lower importance

Statistical analysis of concrete transport properties

Results of a statistical analysis of transport properties measurements carried out on a number of concrete mixtures are presented. Migration and drying experiments were performed to evaluate the ionic diffusion coefficients and permeability of concrete mixtures. Seven concrete mixtures were tested. The mixtures were either batched under laboratory conditions or sampled from construction sites. For each mixture, approximately eighty concrete samples from a single batch were tested. Ionic diffusion coefficients were calculated by analyzing the evolution of the electrical current passing through concrete samples during the migration tests. Calculations took into account the volume of permeable pores of the materials and the pore solution chemistry. Permeability was calculated from mass loss measured during drying tests. The study indicates that the coefficient of variation of ionic diffusion coefficients is 11.1 % on average, with a maximum of 23.5 %, and that the coefficient of variation of permeability is 20.6 % on average, with a maximum of 37.3 %

Comportement des ponts de type dalles pleines en béton armé

Un projet de recherche en cours à l'École Polytechnique de Montréal vise l'amélioration des connaissances sur le comportement des dalles épaisses de ponts avec ou sans biais dans le but d'en faire une évaluation et une conception plus juste. Des études par éléments finis linéaires et non-linéaires, ainsi qu'une étude paramétrique par la méthode de grillage ont été réalisées afin de bien cerner le comportement réel de ce type de pont. Cet article identifie les effets d'augmentation des contraintes dans la dalle dus à la géométrie de la dalle (les effets de biais) et également dus aux déformations du béton par flexion anticlastique (déformation par l'effet de la loi de Poisson). L'article présente les principaux résultats du projet de recherche dont l'objectif principal est de montrer la provenance et les raisons physiques de l'apparition de ces contraintes supplémentaires. En deuxième partie, le développement de ces contraintes est mis en exergue en fonction du développement de la fissuration dans la dalle sous charges variables