M.R.A. van Vliet

Strain-softening of concrete in uniaxial compression

0025-5432/97

Influence of microstructure of concrete on size/scale effects in tensile fracture

Abstract Size/scale effects on the fracture of concrete subjected to uniaxial tension are studied by means of analyses with the Delft lattice beam model and compared to recent experimental results. Using a simple local elastic–purely brittle material global softening behaviour is calculated. The effect of deterministic and statistical contributions to size effect is studied by implementing different degrees of heterogeneity to the lattices. They vary from ‘homogeneous’ regular triangular lattices to lattices with randomly varying beam length. Computer generated particle overlays are used to improve resemblance to real concretes. Trends in size effect on nominal strength and fracture energy are in close agreement with the experiments. The type of approach can be used for tuning macroscopic size/scale laws for concrete and related materials.

Experimental investigation of concrete fracture under uniaxial compression

Localization of deformations has been investigated in a series of displacement controlled uniaxial compression experiments. Of main interest are the effects of specimen slenderness and friction between loading platen and specimen. Both effects have a direct influence on the development of localized fracture zones in the specimen. The results indicate that the use of a double layer of teflon with an intermediate layer of grease yields size-independent results as far as the pre-peak stress–strain behaviour and the peak strength are concerned. However, in terms of stress and strain, a significant influence of both the specimen slenderness and the amount of boundary restraint has been observed in the post-peak regime. It is found that the post-peak curves become almost completely identical when they are plotted in terms of nominal stress and post-peak displacement. For any type of loading platen used, the post-peak relative stress-displacement curves are found to be independent of the specimen height. Furthermore, since during post-peak localization relative sliding and movements of larger parts of the specimen are observed, the definition of a unique Poissons ratio is virtually impossible.

Effect of strain gradients on the size effect of concrete in uniaxial tension

A series of uniaxial tension experiments has been conducted to investigate the size effect on strength and fracture energy of quasi brittle materials like concrete and sandstone. This paper focuses on the results of the concrete tests, and specifically deals with the variation of the nominal strength for specimens of six different sizes in a scale range of 1:32. It was found that under given experimental conditions, the nominal strength strongly depended on the specimen size. More important however, is the fact that most of this size effect could be attributed to strain gradients which were present in the cross section of the specimens. These strain gradients were caused by the specimen shape, load eccentricity and material inhomogeneity. Through a combination of experimental data and a simple linear elastic analysis, the importance of strain gradients with respect to the ultimate load level could be visualized. This leads to the conclusion that studying a material size effect is not possible without taking into account structural stress/strain gradients.

Experimentation, numerical simulation and the role of engineering judgement in the fracture mechanics of concrete and concrete structures

Fracture mechanics plays a role in both structural engineering and materials engineering. The aim here is to improve understanding of the behaviour of structures and materials in the limit state. The use of numerical models can help improve the accuracy of our designs, but only if the certainty about material models improves. The models tend to become more detailed as the performance of computers increases. However, the question is, will this increased amount of detail help to improve our understanding, and improve the reliability of the numerical models. These questions are addressed in this paper. It is shown that through increasing the amount of detail, certain phenomena may be observed that seem to correspond to limits that are reached in practice as well. The example given is the limit reached when trying to fill a plane with circular aggregates. Next it is shown that certain fracture behaviours of concrete can be simulated, be it that a virtual world is created. The role of the experiment is evident. Another role for the experiment is in the development of bench-mark problems in structural engineering. These benchmarks also serve to improve the quality of numerical models.

Numerical Characterization Of The ElasticProperties Of Heterogeneous Materials With A 3DLattice Model

A 3D lattice model is used to predict the elastic behaviour of homogeneous or heterogeneous materials. In this paper, for the heterogeneous case, it is shown that such a model gives the same results as the analytical model proposed by Hashin and Shtrikman. These results are also compared with one set of experiments.

Size Effect Of Concrete In Uniaxial Tension

In the Stevin laboratory a new experimental program has been started to investigate size effect on strength and fracture energy of concrete under uniaxial tension. The starting-points of the experiments are outlined and both a description of the test set-up and some preliminary results are given. Furthermore, the results of additional simulations with the lattice model are presented, which are used to better understand the experimental outcome. The simulations focus on size effect on strength within a lattice model, and the prediction of experimentally observed specimen behaviour.