Effects of different cooling methods on the resistance to chloride ion penetration in concrete produced with slag-modified cement and exposed to elevated temperatures

The deterioration of reinforced concrete elements is associated with the ingress of aggressive agents through the concrete capillary pores. The exposure to elevated temperatures can be a serious concrete degradation factor, depending on its intensity and the cooling procedures. This paper presents a pioneering quantification of the effects of slow cooling (SC) and fast cooling (FC) methods on the chloride-ions penetration into fire-damaged concretes produced with a slag-modified cement. A (2 x 4 x 2) factorial experimental program was designed to evaluate the diffusion of chloride ions within different types of concrete (characteristic compressive strengths of 20 MPa and 35 MPa) before (25 °C) and after exposure to different high temperatures (200 °C, 500 °C and 700 °C) and cooling procedures (SC and FC). Chloride diffusion coefficients were used to estimate the service life of the concretes and scanning electron microscopy was used to investigate the microstructural mechanisms behind the durability results. The exposure of concrete to high temperatures compromises its durability, as thermal cracking propagation and porosity increases tend to facilitate the penetration of aggressive agents and the diffusion of chloride ions. The use of the slag-modified cement did not provide significant benefits to the durability of concretes exposed to 200 ºC and the effects of different cooling methods on the chloride ions permeability were very noticeable. In contrast, statistical analyses did not detect significant difference between the effects of SC and FC for concretes exposed to 500 ºC or 700 ºC.