Assessing the compressive strength of concrete under sustained actions: From refined models to simple design expressions

Abstract

The detrimental influence of high levels of sustained load on the compressive strength of concrete has been acknowledged and investigated since the 1950s. Despite the potential significance of this phenomenon in many situations, current design codes still provide limited guidance on how to account for this effect at ultimate limit state. In addition, the practical case of a significant permanent stress followed by stress increases due to variable loads is not addressed in most codes of practice. The aim of this paper is to present a general but simple design approach to account for the influence of sustained loading on the uniaxial behavior of concrete in compression, both in terms of its strength and deformation capacity. This approach is based on a previously developed mechanical model accounting for nonlinear creep and material damage development on the long‐term response of concrete. On that basis, a design approach is formulated in a simple code‐like manner and used to investigate several practical design situations involving both sustained and variable actions. This approach is shown to be consistent and applicable to any general loading pattern, accurately reproducing the results of the general mechanical model and of available experimental results.

The influence of casting position and disturbance induced by reinforcement on the structural concrete strength

“…It has to be noted that the cylinder strength refers to rapid loading conditions (1–2 min before failure) while the series CK1 and CM1 have been performed with lower loading rates (typical test durations of approximately 30 min). For such loading durations, a reduction can be expected on the compressive strength of the material (approximately 4% for the investigated cases according to Tasevski et al 42 ). This effect is considered for series CK1 and CM1 (see Table 7 and Figure 11) by correcting the concrete strength measured in cylinders under rapid loading ( f c,cyl ) with the factor k t (equal to 0.96).…”

“…The structural resistance can in this case be determined in the following manner (where the concrete compressive strength is reduced by the strength reduction factor accounting for its brittleness): where factor k t , as previously discussed, accounts for the difference of test duration compared to the control specimens 42 . More refined estimates can also be obtained when considering the confinement provided by the stirrups.…”

Concrete compressive strength: From material characterization to a structural value

“… η t (time‐dependence factor): Two effects are normally considered with respect to the influence of time on the concrete strength: the increase of the compressive strength with time due to continuous cement hydration and the decrease of the compressive strength for high levels of sustained loads 17 . For conventional design of new structures, it is normally assumed that both phenomena compensate (although this may be arguable for early applications of permanent loads 17,18 ). As a consequence, it is normally assumed that η t = 1.0 if the concrete strength is measured at a reference time of about 28 days 8,9 .…”

“…As a consequence, it is normally assumed that η t = 1.0 if the concrete strength is measured at a reference time of about 28 days 8,9 . For assessment of existing structures, the concrete strength is measured on extracted cores when most of cement hydration has occurred; thus, more detailed considerations are needed accounting for the level of permanent loading and age of the structure 18 η D (disturbance factor): The presence of significant disturbances (as inserts or post‐tensioning ducts with a duct‐to‐member thickness ratio typically larger than 1/8 8,9 ) has also been reported to reduce the compressive strength.…”

“…In addition, the response is highly dependent on the concrete cylinder strength, with a steeper postpeak response (refer to Figure 3e), the less‐than‐proportional increase of the tensile resistance (Figure 3f) and the higher detrimental influence of transverse tensile stress on the compressive strength for increasing material strength 28 (Figure 3g). It shall also be noted that the deformation capacity of concrete is sensitive to the speed of loading, with higher loading times associated to larger deformations at uniaxial concrete failure (typically, loading times higher than 30–60 min are usually associated to yielding of the longitudinal reinforcement prior to spalling of the cover) 18,29 The concrete core behaves under confined conditions due to the presence of the hoops opposing the lateral dilatancy of the concrete.…”