2. Principle and data processing of QOC tests

#### 2.1 Mounting procedure

The introduction of the sample into the confining ring constitutes a very delicate stage. Indeed, to be sure that the entire gap between the sample and the inner surface of the ring is filled, it is necessary to cover the inner volume of the ring prior to inserting the sample. In addition, the sample needs to be carefully introduced without any contact with the confining cell under pain to block the sample into the ring by bow buttress. For this purpose, a special procedure was developed, as detailed in [16], to align the ring, the sample and the two plugs. First, the concrete sample is scotch tape to the upper plug. A special device (Figure 3) is used to introduce the concrete specimen within the ring previously partially filled by the bi-components epoxy resin named "Chrysor® C6120". During this stage, this resin is slowly extruded out and the internal gap between the specimen and the ring is totally fulfilled by the Chrysor®, which hardens in less than 24 hours. Next, the lower and upper frames are disassembled and the assembly is ready for testing.

Figure 3. Schematic of the device to set the sample, the ring and the steel plugs [16].

and the specimen to fill the gap between the sample and the vessel, which allows for correcting any possible defects of cylindricality of the sample. The interface product was an epoxy bi-component resin, Chrysor® C6120, commonly used for structural applications, and once polymerized, eliminates any internal play. The radial stress in the specimen was deduced from the measurements provided by strain-gauges attached to the outer surface of the vessel [27] based on the well-known analytical solution of an elastic tube subjected to a uniform pressure applied against its whole inner surface. So, the 'barrel' deformation of the vessel was not taken into account in this analysis. Smaller confining cells, 30 or 50 mm as inner diameter and 50 or 70 mm as outer diameter, were used in [28] to test a micro-concrete under quasi-static and dynamic loadings (Figure 2(c)). A maximum axial strain up to 30% was reached before unloading. Later, a new processing method was proposed in [29] and applied to these experimental data to evaluate the level of radial stress in the specimen from the hoop strain measured on the outer surface of the confining cell, taking into account the sample shortening. Both deviatoric and hydrostatic responses of this microconcrete were obtained from the processed data, which showed a quite limited influence of the rate of loading on the strength, even at a

Quasi-oedometric compression testing devices applied to concrete. (a) Technique method developed by Bažant et al. [25]. (b) Instrumented vessel used in [27], (c) smaller confining cells used under quasi-static and

The testing procedure and data processing method were substantially improved in several works and applied to successively investigate the influence of particles size and shape [31, 32] and of the porosity [6] on the confined behavior of

strain-rate of 400 s<sup>1</sup> [30].

70

Figure 2.

dynamic loading conditions [28].

Compressive Strength of Concrete
