Relationship between Nonlinear Creep and Cracking of Concrete under Uniaxial Compression

Abstract

This paper investigates the nonlinear creep behaviour of concrete in compression and its relationship with cracking under uniaxial compression (cracks developing parallel to the loading direction). A physical model explaining the nature and the role of linear and nonlinear creep strains is presented, together with a failure criterion for concrete under sustained loads. The model assumes that all nonlinear creep strains are due to concrete micro-cracking. The soundness of this assumption is checked against the experimental results obtained by the authors and by other researchers. The proposed model is shown to fit quite well the experimental results, for various load patterns and concrete ages. The model also proves that the affinity hypothesis between linear and nonlinear creep strains (usually taken for granted in the design for stress levels below 70% of concrete strength in compression) is no longer valid when concrete fails under a sustained load, because of the unstable growth of cracking. Concrete response in these cases is analyzed in detail and a simplified but realistic approach for the evaluation of the failure envelope in compression is proposed.

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

“…Due to its practical relevance, the topic of the long‐term strength of concrete structures has attracted many research efforts after the works of Rüsch, with a number of investigations conducted both for normal strength concrete and high‐strength concrete . The same phenomenon has been also investigated for the tensile and flexural behavior under sustained load (see for instance References ).…”

“…Within this frame, most experimental evidence with respect to the effect of sustained loading on the compressive strength has been obtained by considering a constant stress level applied to the member (refer to the load pattern shown in Figure c). However, some experimental programs and researches have also reported this phenomenon for low loading rates . Several authors have also investigated the origin of the phenomenon, relating the delayed failure of concrete to microcracking and to material damage development and progression .…”

“…However, some experimental programs and researches have also reported this phenomenon for low loading rates . Several authors have also investigated the origin of the phenomenon, relating the delayed failure of concrete to microcracking and to material damage development and progression . According to these research works, if the level of sustained stress is higher than the threshold at which microcrack propagation is likely to occur ( σ c / f c > 0.4, typically associated to the development of nonlinear creep strains), delayed damage may develop in the material.…”

Nonlinear creep response of reinforced concrete beams

“…In general, the creep rupture failure envelope in compression is a descending curve from the peak static strength to a horizontal line at about (0.7-0.8) f cm [Carol and Murcia 1989;Omar et al 2009]. Here, it is assumed that this descending curve is similar to the descending (strain softening) part of the stress-strain curve given in (30), indicating that also under compression, creep rupture occurs once the viscoelastic strain intersects the stress-strain diagram as shown in Figure 2 [Fernández Ruiz et al 2007]. Such similarity between the modeling of damage due to creep and material nonlinearity under short term loading is owned to the similarities in their sources, which correspond to interfacial bond microcracks between the aggregates and the mortar, and microcracking of the mortar material itself [Bažant and Asghari 1977;Li and Qian 1989].…”

“…Note that in this simplified formulation, the nonlinearity between stresses and creep strains is modeled through the constitutive law only [Bockhold and Petryna 2008] and not through a stress-dependent creep coefficient [CEB-FIP 1990;Fernández Ruiz et al 2007]. Following the procedure outlined above, different potential stress histories (σ x x (t) = σ h (t)) need to be introduced into (34).…”