4.4 Influence of free water and strain-rate on the confined strength of ordinary concrete and high-strength concrete

strength of OC in saturated condition whereas both concretes present similar

OC and HSC concretes. (d) Dynamic deviatoric behavior of saturated OC and HSC concretes.

Quasi-oedometric compression tests applied to ordinary concrete (R30A7) and high-strength concrete in dry and saturated conditions [34]. (a) Quasi-static deviatoric behavior of dry OC and HSC concretes. (b) Dynamic deviatoric behavior of dry OC and HSC concretes. (c) Quasi-static deviatoric behavior of saturated

Investigation of the Quasi-Static and Dynamic Confined Strength of Concretes by Means…

DOI: http://dx.doi.org/10.5772/intechopen.89660

The quasi-oedometric compression testing technique constitutes one of the most convenient and efficient testing methods to characterize the quasi-static and dynamic confined behavior of concrete and rock-like materials. However, precautionary measures need to be considered in the mounting procedure, to fill the gap between ring and the confining cell, and in the data processing to take into account for the barreling deformation of the cell, the shortening of the sample and, when necessary, for the plastic behavior of the cell, the possible effects of friction at each interface and the axial dissymmetry between the sample and the confining cell. Thus, it is possible to determine the hydrostatic and deviatoric behaviors of these materials under pressure ranging from few tens to a thousand of MPa in quasi-static loading conditions with large capacity hydraulic press or at high strain-rates with a Split-Hopkinson Pressure Bar apparatus. The main obtained results illustrate the beneficial effect of the presence of strong particles added in mortars, a strong influence of free water content on both hydrostatic and deviatoric behaviors of concretes especially in the case of microconcrete and ordinary concrete. Finally, it is concluded that concrete composition and water content have a strong influence on the concrete behavior

strengths in dry condition.

5. Conclusion

83

Figure 12.

The experimental results of quasi-static and dynamic QOC tests performed on dried and saturated ordinary concrete (OC, in this case, R30A7 concrete) and high-strength concrete (HSC) were compared in [34]. The main results obtained regarding their quasi-static and dynamic deviatoric responses are reported in Figure 12. First, under dry conditions, both OC and HSC concretes exhibit similar strength with a slightly higher strength of HSC at low pressure but with a more reduced increase of strength with pressure as compared to OC (Figure 12(a, b)). In addition, it can be noted that the dynamic strength of dried OC and HSC is significantly higher than their quasi-static strength whatever the considered level of pressure. Finally, the influence of strain-rate on the confined strength of dried concretes cannot be neglected.

On the other hand, under saturated conditions, both concretes exhibit completely different strength. In quasi-static conditions, a saturation of strength around 70 MPa was observed with OC, whereas the strength of HSC goes up to 270 MPa. In dynamic loading conditions, higher strengths are observed with both concretes compared with their quasi-static strengths and the influence of strain-rate cannot be neglected (Figure 12(c, d)). As in quasi-static loading, the dynamic confined strength of saturated HSC is much higher than that of saturated OC. Now, if dried and saturated concretes are compared, it can be concluded that the confined strength of dried samples is higher than that of saturated samples for both concretes. As concluded with microconcrete, it is supposed that interstitial waterpressure inside saturated concretes reduces their confined strength. In addition, this effect is even more pronounced in OC compared with HSC due to the higher level of porosity (11.8%) in OC as compared to HSC (8.8%), which explains the much lower Investigation of the Quasi-Static and Dynamic Confined Strength of Concretes by Means… DOI: http://dx.doi.org/10.5772/intechopen.89660

#### Figure 12.

cement paste. In addition, a correlation was noted between the large porosities of millimeter class and the compaction of the concretes under high level of pressure. Finally, it was demonstrated that the experimental data provided by QOC experiments could be used to simulate numerically impact tests involving high levels of

4.3 Influence of free water and strain-rate on the confined behavior of

microconcretes in [16]. The dynamic tests exhibited an important dissimilarity between dry and saturated specimens concerning both deviatoric and hydrostatic behaviors. First, dried microconcretes exhibited a continuous compaction whereas saturated specimens showed a non-linear (hardening) hydrostatic behavior (Figure 11(b)). Moreover, a strong and continuous increase of the strength with pressure was noted with dry samples whereas water-saturated specimens exhibited an almost-perfect saturation of the strength (Figure 11(a)). The quasi-static results allowed highlighting the reason of this dissimilarity (Figure 11(c, d)). On the one hand, dried specimens behave similarly in dynamic tests and no strain rate effect is observed. On the other hand, the behavior of saturated specimens gradually tends to that of dried specimens when the loading rate is decreased, and an expulsion of water during slow quasi-static tests was observed. Finally, it was concluded that, waterpressure inside saturated microconcrete plays a major role on their fast-quasi-static or

dynamic confined behavior by reducing drastically their shear strength.

concrete and high-strength concrete

concretes cannot be neglected.

82

4.4 Influence of free water and strain-rate on the confined strength of ordinary

The experimental results of quasi-static and dynamic QOC tests performed on dried and saturated ordinary concrete (OC, in this case, R30A7 concrete) and high-strength concrete (HSC) were compared in [34]. The main results obtained regarding their quasi-static and dynamic deviatoric responses are reported in Figure 12. First, under dry conditions, both OC and HSC concretes exhibit similar strength with a slightly higher strength of HSC at low pressure but with a more reduced increase of strength with pressure as compared to OC (Figure 12(a, b)). In addition, it can be noted that the dynamic strength of dried OC and HSC is significantly higher than their quasi-static strength whatever the considered level of pressure. Finally, the influence of strain-rate on the confined strength of dried

On the other hand, under saturated conditions, both concretes exhibit completely different strength. In quasi-static conditions, a saturation of strength around 70 MPa was observed with OC, whereas the strength of HSC goes up to 270 MPa. In dynamic loading conditions, higher strengths are observed with both concretes compared with their quasi-static strengths and the influence of strain-rate cannot be neglected (Figure 12(c, d)). As in quasi-static loading, the dynamic confined strength of saturated HSC is much higher than that of saturated OC. Now, if dried and saturated concretes are compared, it can be concluded that the confined strength of dried samples is higher than that of saturated samples for both concretes. As concluded with microconcrete, it is supposed that interstitial waterpressure inside saturated concretes reduces their confined strength. In addition, this effect is even more pronounced in OC compared with HSC due to the higher level of porosity (11.8%) in OC as compared to HSC (8.8%), which explains the much lower

Quasi-static and dynamic QOC tests were performed on dry and water-saturated

confining pressure in front of the projectile head [41].

microconcrete

Compressive Strength of Concrete

Quasi-oedometric compression tests applied to ordinary concrete (R30A7) and high-strength concrete in dry and saturated conditions [34]. (a) Quasi-static deviatoric behavior of dry OC and HSC concretes. (b) Dynamic deviatoric behavior of dry OC and HSC concretes. (c) Quasi-static deviatoric behavior of saturated OC and HSC concretes. (d) Dynamic deviatoric behavior of saturated OC and HSC concretes.

strength of OC in saturated condition whereas both concretes present similar strengths in dry condition.

### 5. Conclusion

The quasi-oedometric compression testing technique constitutes one of the most convenient and efficient testing methods to characterize the quasi-static and dynamic confined behavior of concrete and rock-like materials. However, precautionary measures need to be considered in the mounting procedure, to fill the gap between ring and the confining cell, and in the data processing to take into account for the barreling deformation of the cell, the shortening of the sample and, when necessary, for the plastic behavior of the cell, the possible effects of friction at each interface and the axial dissymmetry between the sample and the confining cell. Thus, it is possible to determine the hydrostatic and deviatoric behaviors of these materials under pressure ranging from few tens to a thousand of MPa in quasi-static loading conditions with large capacity hydraulic press or at high strain-rates with a Split-Hopkinson Pressure Bar apparatus. The main obtained results illustrate the beneficial effect of the presence of strong particles added in mortars, a strong influence of free water content on both hydrostatic and deviatoric behaviors of concretes especially in the case of microconcrete and ordinary concrete. Finally, it is concluded that concrete composition and water content have a strong influence on the concrete behavior

under high confinement and the strain-rate cannot be neglected whatever the concrete types and the water-saturation conditions.

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