Marta Choinska

Permeability due to the Increase of Damage in Concrete: From Diffuse to Localized Damage Distributions

Experimental tests exhibit a strong interaction between material damage and transport properties of concrete. There are at least two asymptotic cases where some theoretical modeling exists: in the case of diffuse cracking, the material permeability should be controlled by damage, e.g., by the decrease of average stiffness due to microcracking. In the case of localized microcracking, and after a macrocrack has formed, permeability should be controlled by a power function of the crack opening (Poiseuille flow). For quasi-brittle materials with evolving microstructure due to mechanical loads, a transition regime on the evolution of permeability between these two asymptotic cases is expected. In this contribution, we define a relationship between permeability and damage that is consistent with the two above configurations. One of the key issues is to relate the crack opening to the state variables in the continuum approach, so that the two asymptotic cases are expressed in the same variable system and can be matched. A simplified approach is used for this purpose. The permeability law is then derived using a mixing formula that weights each asymptotic regime with damage. To illustrate the influence of the matching law on structural response, finite-element simulations of a Brazilian splitting test and a comparison with existing test data are presented.

Coupling between Progressive Damage, Temperature and Permeability of Concrete: Experimental and Numerical Study

A synthesis of work carried out for several years within our laboratory is presented. The first part includes an experimental study. The tests are performed on hollow cylindrical concrete specimens, subjected to compressive loading. At stress levels lower than 80 % of the peak stress, the variation of permeability is small and it is slightly influenced by the stress, but as the load exceeds 80 % of the peak stress, micro-cracking increases rapidly, causing an increase of the permeability and a greater sensitivity to the applied load. In the post-peak phase the increase of permeability is much larger due to significant crack width growth. The effects of the applied load on permeability are greater with temperature. Finally, the experimental results seem to agree with the format of coupled evolution of the permeability due to damage and temperature assumed by Gawin et al. [10]. The second part of this paper includes a numerical study. The lattice mechanics model is extended to the hydraulic problem and, for this case, it appears that permeability is the size independent variable. Additionally, the evolution of permeability with damage and with stress ratio in the pre-peak phase is compared with experimental results on different types of concrete.

Experimental Study of Aggregates Size Effect on Strain, Damage and Permeability of Concrete

Two approaches can be used to study the size effect: one based on the change in sample size, and the other based on the variation in the aggregates size. The main objective of this research was to study the second approach. We studied 6 various concrete mixes and each test was repeated three times. For each material, uniaxial cyclic compressive tests have been performed to investigate the behaviour of concrete in a partially damaged state. The specimens were submitted to three levels of loading corresponding to 30, 60 and 80% of the maximal compressive strength. The damage indicator chosen is the decrease of Young’s modulus and the tightness indicator is the gas permeability. Results show that the concrete composition and more particularly the aggregate size have an important influence on the mechanical and transfer properties of concretes.

Etude expérimentale de l'interaction endommagement-température-état de contrainte-perméabilité du béton

ABSTRACT The objective of this study is to investigate damage-temperature-permeability interactions in structural concrete. The tests are performed on hollow cylindrical concrete specimens, subjected to compressive loading and temperature up to 150°C. The results emphasize that at stress levels lower than 80–85% of the peak stress, the variation of permeability is small and it is slightly influenced by the stress. As the load exceeds 80–85% of the peak stress and approaches the peak, microcracking increases rapidly, causing an important increase of the permeability. In the post-peak phase the increase of permeability is much larger due to significant macrocrack width growth. The increase of permeability with the applied mechanical load seems to be greater with temperature, inducing the further alteration of concrete and dilation of the porous structure of the material. Finally, the obtained results reveal that the effects of damage and temperature may be decoupled for the estimation of the concrete permeability.

Détermination de la tension des câbles à partir des fréquences de vibration

ABSTRACT When the relationship between the tension force and the eigen frequencies is given by the model of the vibrating cord, the determination of the tension in stay and external prestressing cables from measured vibration frequencies may be relatively easy. This paper deals with simple mechanical models of anchorage conditions. The model updating method based on the sensitivity of the vibration frequencies is used to identify the boundary conditions and the bending rigidity of the cable. Thus, the method based on the vibrating cord model has been extended to cases where it was no longer applicable. Case studies of short stay-cables and external pre-stressing bars are presented.