SUMMARYA thermo-mechanical model for concrete under transient high temperatures is presented. Particular emphasis is placed on the transient thermal creep model, which can be seen as an extension of Thelandersson's formulation, and the gradient-enhanced damage model that has been extended here to include temperature dependency. Degradation of the material stiffness due to exposure to elevated temperatures is included via a simple thermal damage model. The model's performance is illustrated on several benchmark problems under both transient and steady state heating conditions.
The article presents an overview of the environmental actions of the Danish cement and concrete industry over the last ten years. The areas include reduced Portland clinker content which means improved CO 2 footprint of the concrete. It is described how carbonation of concrete after demolition and crushing may improve the CO 2 footprint even further by taking into account the absorption of CO 2 from the atmosphere. Recently there has been a 3-year project initiated by the Danish cement and concrete industry. This project has succeeded in promoting the image of concrete as a sustainable building material in the Danish public. It is the result of several scientific investigations for instance determining the effect of concrete emissions on the indoor air quality and the solution to hydrocarbon pollution in concrete slurry at the concrete plant. Finally the article contains examples of how to improve the sustainability of concrete production and how to produce green concrete. Green concrete is the term used in Denmark for environmentally friendly concrete production and structures.
Test data on the residual fracture energy of two significantly different concrete types are presented. About 80 beams of high performance basalt concrete and ordinary gravel concrete have been tested in accordance with the RILEM work of fracture method. The bemns are heated at 1~ per minute up to a certain maximum temperature and kept at this temperature tbr 8 hours before cooling them back to room temperature and testing in three-point bending.The tests show that the two concretes behave almost identical when the fracture energy GF is considered as a function of maximum temperature. It is found that the damage introduced by a maximum temperature of 300 to 400~ increases the fracture energy by 50 % compared with the reference tests at room temperature. A more tortuous crack surface is one plausible explanation for the significant increase in GF.The article also presents temperature and weight loss recordings from the heating scenarios and finally, the characteristic length and the cohesive tensile softening curve are shown to depend on the maximum temperature. Basically it is demonstrated that the temperature exposure makes the concrete significantly more ductile.
RESUM
An analytical solution to a uniaxial benchmark problem of a concrete member restrained by an elastic spring and subject to heating and subsequent cooling is given. The model demonstrates the effect of load-induced thermal strain, which has so far only been described through experiments. Heating a concrete specimen under constant compressive load shows a significant contraction rather than expansion, which would be the case if the specimen were heated without any constraints. The magnitude of this contraction is beyond that of conventional creep and mechanical strain even though the stiffness degradation from high temperature exposure is taken into account. Thus, it is attributed to a special thermo-mechanical coupling, taking place during the first transient heating process. Furthermore, the analytical model describes the tensile stresses, developing during cooling, including non-linear material behaviour arising from fictitious cracking. Through several examples the effect of the various parameters is evaluated.
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