Effects of temperature and temperature gradient on concrete performance at elevated temperatures

To assure adequate fire performance of concrete structures, appropriate knowledge of and models for performance of concrete at elevated temperatures are crucial yet currently lacking, prompting further research. This article first highlights the limitations of inconsistent thermal boundary conditions in conventional fire testing and of using constitutive models developed based on empirical data obtained through testing concrete under minimised temperature gradients in modelling of concrete structures with significant temperature gradients. On that basis, this article outlines key features of a new test setup using radiant panels to ensure well-defined and reproducible thermal and mechanical loadings on concrete specimens. The good repeatability, consistency and uniformity of the thermal boundary conditions are demonstrated using measurements of heat flux and in-depth temperature of test specimens. The initial collected data appear to indicate that the compressive strength and failure mode of test specimens are influenced by both temperature and temperature gradient. More research is thus required to further quantify such effect and also to effectively account for it in rational performance-based fire design and analysis of concrete structures. The new test setup reported in this article, which enables reliable thermal/mechanical loadings and deformation capturing of concrete surface at elevated temperatures using digital image correlation, would be highly beneficial for such further research

Appraisal of MC2010 shear resistance approaches coupled with a residual flexural strength prediction model

In the present work the predictive performance of the two approaches proposed by Model Code 2010 for the evaluation of the shear capacity of fiber reinforced concrete (FRC) elements flexurally reinforced with conventional steel bars is assessed considering a database (DBs) constituted by 80 FRC beams do not including conventional transverse reinforcements. The accuracy of these shear models is evaluated by statistical analysis of the prediction ratio between the experimental and estimated shear capacity of the beams of the DBs, and applying the Demerit Points Classification approach for further information about the reliability of the two approaches in design context. Due to the absence of the post-cracking experimental characterization of the FRC used in several beams considered in the DBs, an approach was developed for estimating the residual flexural strength parameters from the most relevant known variables of steel fiber reinforcement mechanisms for concrete, namely the fiber volume and aspect ratio, and the concrete compressive and tensile strength. The residual flexural strength prediction model is assessed and its influence on the performance of the shear resistance models is evaluatedSFRH/BDE/96381/2013 co-funded by CiviTest - Pesquisa de Novos Materiais para a Engenharia Civil, Lda. and by FCT - Portuguese Foundation for Science and Technology. The authors also acknowledge the support provided by the FCT project PTDC/ECM-EST/2635/201