Method to evaluate the residual strength in concrete elements exposed to fire using physic-chemical and microstructural parameters

Abstract

After the exposition of concrete to fires, it is necessary to evaluate the residual strength of the concrete affected elements. Some methods to evaluate the residual capacity are based on the knowing of the depth of concrete affected by temperatures higher than 500ºC. The layer of concrete that have been exposed to temperatures upper than 500ºC must be not taken into account to do the structural recalculation. In the present work different physic-chemical parameters are used in order to define the alteration zones from the surface exposed to high temperature to the interior. Different layers of concrete exposed to fire are analyzed, from the surface, determining different physic-chemical parameters related with thermal behavior, using DTA-TG, and determining the main crystalline compounds, using XRD. Also, the microstructural analysis by BSE-EDX permits to corroborate the situation of concrete exposed to high temperature.the crystallization water of the hydration products disappears gradually between 100ºC and 1,000ºC. It is considered that the total dehydration of the CSH comes about from the exposure of pastes to 1,000ºC for 15 minutes, in small-sized samples (Taylor, H.F.W. 1997, Mehta, P.K. 1986). Other transformations are related with the carbonates that can be formed due to the natural carbonation or by recarbonation processes associated with the CO 2 or CO formation during the fire (Menéndez, E. et al 2012a). Different instrumental techniques are used with the aim of analyzing the behavior of the concrete exposed to a real fire or laboratory tests. Specially, thermal analysis, scanning electron microscopy and X-ray diffraction are useful to analyze the concrete exposed to fire. It is possible to identify the temperature intervals in which the decomposition of certain compounds present in the cement paste takes place (Sarvaranta, L. et al 1993, Menéndez, E. & Vega, L. 2010. The behavior of the pastes in the presence of temperature allows the monitoring of the existence of certain products, when these pastes are tested by means of simultaneous analysis by Differential Thermal Analysis and Thermogravimetry (DTA-TG). Analysis through X-ray diffraction (XRD) also allows the crystalline phases present in the original or burnt concretes to be identified. Also, microstructural and compositional changes can be