Stéphane Multon

Structural behavior of concrete beams affected by alkali-silica reaction

This article reports on an alkali-silica reaction (ASR) study that was carried out on beam specimens at the Laboratoire Central des Ponts et Chaussees (LCPC), with Electricite de France (EDF) as a partner. In the study, three-dimensional deformations were measured on five 3 m-long plain or reinforced concrete beams undergoing partial drying over a period of 14 months. The authors discuss the effect of a moisture gradient over the depth of the beams and the influence of reinforcement on the development of ASR-induced expansions. Tests on companion specimens (cylinders and prisms) were carried out to measure the material expansion and mechanical characteristics, including longitudinal strains and beam deflections, transverse strains, vertical strains, and cracking. The authors make three main conclusions: the anisotropy of ASR-induced expansions has been verified with vertical swellings twice as large as horizontal ones; significant ASR expansions (0.10%) can occur without an external water supply and can lead to significant structural degradation; and an increase in ASR expansions with an external water supply has been quantified by this work. The authors note that once models are validated in this framework, they will be used to assess the dimensional stability and the residual bearing capacity of real ASR-affected structures (e.g., bridges and dams).

Combination of Structural Monitoring and Laboratory Tests for Assessment of Alkali-Aggregate Reaction Swelling: Application to Gate Structure Dam

Southwestern Frances Temple-sur-Lot Dam, a gate structure built in 1948, has been subjected to continuous alkali-aggregate reaction (AAR)-induced displacements since 1964, despite nonsignificant residual swelling test results and low and relatively constant alkali content in the concrete. It has been assumed that this long-term behavior could be explained through a substitution process between calcium and alkali in the alkali-aggregate reactive gel. The calcium substitution phenomenon cannot be detected through a conventional residual swelling test since it is very slow, so an original AAR kinetics and residual swelling capability assessment method is proposed. A laboratory test dealing with silica consumption kinetics is first involved in this methods, and a numerical finite element inverse dam analysis, including laboratory measured consumption kinetics, is involved as a second step in this method. At a given period, only one observed structural displacement rate fit the final swelling amplitude. Comparison between instrument point displacement predicted by the calculations (not used for the fitting) and dam measured variations provided model prediction capability validation. Finally, future dam displacement and damage field prediction calculations were performed.

DEF modelling based on thermodynamic equilibria and ionic transfers for structural analysis

Delayed ettringite formation (DEF) is a process which can lead to swelling and cracking of concrete. This paper proposes a chemical model to predict the kinetics and the amount of DEF in concretes subjected to high-temperature curing. The modelling considers several types of phenomena: the thermodynamic equilibria of hydrate crystallisation, the binding of ionic species to hydrated calcium silicates and the mass balance equations, which include the diffusion mechanisms. All the constitutive equations are provided and the thermodynamic constants found from a wide-ranging literature review are given in particular detail. The model has been implemented in a finite element code. The numerical results give the amount of ettringite and monosulphates, and ionic concentration fields in the simulated structure. They are compared with experimentation in which the early-age thermal cycle and long-term alkali release combine to cause DEF.

Effets structuraux de l'alcali reaction : Apports d'une experimentation sur elements de structures a la validation de modeles

ABSTRACT Assessment of ASR-damaged structures is a major concern for bridge and dam owners in France. Thus, validated models are needed in order to predict the behavior and residual bearing capacity of such works. With this aim, a large experimental program was carried out at the LCPC with EDF as a partner. Measurements were taken from the behavior of structures and specimens placed in various moisture and mechanical environments in order to realize a complete data bank. The mechanical analysis of the measurements showed the significance of ASR-induced strains anisotropy due to stresses, and moisture effect on ASR-induced expansion amplitude, on predicted behavior. They appear to be the main parameters to be accounted for in order to obtain good predictive models.

Comparative study of compressive and tensile basic creep behavior of concrete

Due to its poor strain capacity and a low tensile strength, concrete is brittle and highly sensitive to cracking detrimental to application sustainability. Despite this well-established knowledge, the irony today is that investigations on concrete are usually limited to the compression behavior. The creep behavior that is a major concern for concrete structures is no exception to this observation. Only one reason can explain this aberration: the difficulty to perform a tensile test on cement-based materials, particularly their fixture to the loading device. This paper describes the experimental setup developed to achieve direct tensile and bending creeps. The precautions taken to obtain relevant data are described. For comparison, tensile, flexural and compressive basic creep test were conducted in parallel. Although the approach is still controversial, the basic creep strain was determined by subtracting the shrinkage strain and instantaneous strain from the total strain. Results available for specimens subjected to 50% of the strength in tension or in compression are presented. The final discussion compares the basic creep under the different types of loading.

Requirements for the Modeling of Medium-Term Behavior of Nuclear Containment Concrete for a “Loss of Coolant Accident” Analysis

The objective of this research is to understand the behavior of concrete subjected to temperatures up to 180°C and to gas absolute pressures up to 5 bars applied during the two weeks envisioned in the “loss of coolant accident” (LOCA) scenario. Previous studies about delayed mechanical behavior of concrete have pointed out an increase of delayed strains with the temperature rise: the basic creep can be multiplied by a factor 10 at 80°C, and coupling between creep and heating can lead to damage and to transient thermal creep. These phenomena could be predominant if the LOCA induced conditions are maintained several days and more probably several weeks. So, a model able to predict the cracking and the gas leakages has to be developed. It has to consider these phenomena and their coupling with other possible causes of concrete damage previous to the LOCA. In fact, if the LOCA occurs on structure already damaged by early age cracking or endogenous chemical reactions, such as AAR or ettringite, the leakage risk could be increased. The paper will focus on some important aspects of these phenomena (creep rate dependency on temperature, scale effects at early age, damage induced by swelling reactions), and on their coupling in a finite element model.