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PART 1
Instantaneous or Time-Independent Models for Concrete
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Chapter 1
Test Techniques and Experimental Characterization 1
1.1. Introduction
Detailed experimental analysis of concrete material has always been a complex task. This complexity is due to the variability of the material, which depends on its composition, its conditioning, and on the mechanical, thermal, or chemical loads it is undergoing. For precise experimental characterization it is necessary to know the composition and “history” of the material. Accuracy and rigor, as always, are required for an experimental study on this type of material.
Moreover, the complexity of concrete characterization has amplified the need for experimental studies. The goal is to understand, explain, and model the behavior of concrete. Experimental studies of concrete will always have at least one of the following aims: characterization, modeling, or validation. Indeed it is necessary to characterize the behavior of material under different loadings experimentally. An understanding of the physical phenomena involved is then possible. Phenomenological or micro-mechanical modeling is improved using the observations made during tests, and the obtained models have to be validated via an experiment on a specimen or a structure.
In order to be concise, we will focus here on the experimental characterization of concrete under mutli-axial mechanical loading. Moreover, we will limit ourselves to the study of quasi-static loads, that is for deformation rates ranging from 10-5 to 10- 2 s-1. We can therefore suppose that long-term effects (mainly shrinkage and creep) and dynamic effects are negligible. We will consider the conditions to be isothermal and at room temperature. Indeed, thermal strains on concrete material leads to various physical and physicochemical processes (internal pressure due to water evaporation, aggregates setting off, phase changing, Calcium Silicate Hydrate CSH dehydration, etc.) which change the microstructure of the material, and therefore, its mechanical behavior (more information can be found in the following papers: [BAZ 96; HEI 98; NEV 00; NEC 00]). In addition, concretes undergoing chemical attack will not be considered in this study as microstructural modifications influence their mechanical behavior (see as examples [BAR 92; CAR 96; GER 96; TOR 99a]). The concretes and mortars studied herein are archetypal; specifically, their composition, assembly, and conservation modes are common in civil engineering, which allows us to show the general characteristics of their mechanical behaviors.
The initial focus of the chapter will be ways to consider the specificities of the materials towards a reliable and adequate experimental analysis. The interpretation of an experimental result of a concrete or a mortar cannot be performed out of this study’s context. Boundary conditions, curing conditions, or the maturation of the material have to be specified. In the second part, classical extensometry will be described. The influence of the boundary conditions will also be related to extensometry which can only be performed on the external surfaces of the specimen. Moreover, we will see that it is not always possible to obtain a “material” test, i.e. where all the characteristics and the applied stresses are homogenous. Depending on the observation scale, concrete can be considered as either a homogenous material (which we will assume in most of the interpretations) or as a composite or heterogenous material (implying an additional difficulty regarding the analysis of the physical phenomena explaining these types of behavior). In this chapter, we will consider the material as homogenous, even if microstructural observations cannot support our reasoning. The last section of this chapter will focus on the study of the main test techniques used to test a concrete material under multiaxial stresses. The results obtained for mortar or concrete will be given in parallel. Of course, these different techniques can be used to identify a mechanical behavior or validate a model.
1.2. Experimental specificities related to concrete material
1.2.1. Composition and variability of the material
“Classical” concretes (see [BAR 96a; DRE 98] for more information) that will be studied are made of a granular skeleton of a sand-lime type, with constant grain size distribution and with common cleanness [BER 96]. The binding agent is a commercially available cement without any particular properties [BAR 96b]. Workability is standard and concrete implementation is done as usual. We will not cover high-performance concretes (more information can be found in the following papers: [MAL 92; DEL 92; LAP 93; BAR 94; BEL 96; AIT 01]). Once mixed, concrete hardens and its mechanical performances increase with time. It then becomes a porous material, made of aggregates and of hydrated cement, which contains a fluid interlayer. The influences of this liquid interlayer can be delayed: it can modify the short-term behavior of concrete via evaporation; for example, leading to desiccation cracking, and therefore, the existence of defects located at the surface. Interactions between the microstructure, the fluid interlayer and the environment can also modify the mechanical behavior of concrete. The action of the environment can become a major factor and sometimes leads to a change of the poromechanical properties of concrete [COU 95].
Moreover, the aggregates and cement used to make the material are often of local origin, which implies a significant range from one production site to another. It should be noted that the experimental results obtained for a particular concrete or mortar are a function of all ingredients entering into the mix. Therefore, the comparison of two results must be done carefully. Nevertheless, it is possible to provide general characteristics of concrete behavior as long as it is assumed that concrete, even if made of sand, gravel, cement, and water, is a homogenous material at the considered mechanical test scale. In addition, a reliable comparison of various results can be performed by using a similar cement and Leucate’s normalized sand (complying with the EN 196-1 and ISO 679 rules), in the case of a mortar. This choice of cement and similar sand whatever the test, und...
