Microstructure of concrete subjected to elevated temperatures: physico-chemical changes and analysis techniquesMicroestrutura do concreto submetido a altas temperaturas: alterações físico-químicas e técnicas de análise Abstract ResumoThe exposure of concrete to high temperatures, such as in a fire, leads to physical and chemical changes, which may cause deterioration of mechanical properties, cracking and spalling. This paper presents a critical review of microstructural changes in concrete exposed to high temperature. The transformations developed in the cement paste, aggregates and interfacial transition zone were studied, as well as the experimental techniques of microanalysis presented in recent related researches. Lastly, a critical analysis of experimental results from literature was performed. It was verified that microstructural changes are related to concrete properties and the heating process. The experimental techniques has a potential use for assessment of thermally damaged concrete, however, these techniques must be applied simultaneously and specific methods must be established.Keywords: microstructure, concrete, concrete structures, high temperatures.A exposição a altas temperaturas, como as de um incêndio, promove alterações físicas e químicas no concreto, provocando deterioração das propriedades mecânicas nas estruturas, fissuração e desplacamento. O presente trabalho consiste em uma revisão crítica das alterações microestruturais que incidem no concreto submetido a altas temperaturas. As transformações desenvolvidas na pasta de cimento hidratada, nos agregados e na zona de transição foram estudadas, bem como as técnicas experimentais de microanálise utilizadas em recentes pesquisas desenvolvidas na área. Por fim, uma análise crítica dos resultados de estudos experimentais apresentados pela literatura foi realizada. Verifica-se que as alterações da microestrutura estão relacionadas com as características constitutivas do concreto e com processo do aquecimento. Constata-se a potencialidade das técnicas microestruturais para as etapas de na inspeção e recuperação de estruturas incendiadas, entretanto verifica-se a necessidade de combinação de técnicas e o estabelecimento de métodos padronizados.Palavras-chave: microestrutura, concreto, estruturas de concreto, altas temperaturas.
Among the processes that involve the degradation of concrete structures subject to the high temperatures of a fire there is the spalling phenomenon. Its mechanisms are related to the thermal stress of the materials dilatations and pore pressure the process of vaporization of water during heating. The factors that influences in its occurrence are related to concrete properties, structural member characteristics or the exposure conditions, and their parameters are not clearly known yet. This paper aimed to study the influence of three concrete mixtures, four coating thicknesses and two bars diameters of longitudinal reinforcement in the spalling phenomena exposed to ISO 834 fire curve. The characterization of concrete were performed either of the axial compression strength tests, water absorption by capillary and mercury intrusion porosimetry, besides the fire resistance tests in real-scale specimens. It was concluded that the diameter of the bar does not have influence, while the mixture and the concrete cover thickness does. More spalling was recorded for the columns with thicker concrete cover and concrete compressive strength at 61,9 MPa, and although higher strength concrete have less permeability, this characteristic can be balanced with the higher tensile strength of this type of concrete.
There is still a concern regarding concrete structures' fire safety, mostly due to the occurrence of concrete spalling. Although many tests have already been carried out, there is no clear definition about the parameters of the factors that influence its occurrence. This paper aimed to compare three different types of concrete panels, with dimensions of 300 x 315 x 10 cm (124.0 x 39.4 x 3.9 in.), composed of reinforced concrete (RC), prestressed concrete, and polypropylene microfiber RC. The panels were exposed to the standard fire curve based on ISO 834, aged 28 days, measuring the temperatures in panels' surfaces. Prestressed concrete panels experienced explosive spalling 18 minutes after the test began. RC panels and the panels with polypropylene microfiber addition maintained their integrity and structural stability for 240 minutes, failing in the thermal insulation criteria at 210 and 140 minutes, respectively. Although polypropylene microfiber concrete panels presented no spalling of concrete, conventional concrete panels attended the standardized criteria for a longer period due to its better thermal insulation.
When exposed to elevated temperatures, concrete suffers physicochemical changes, resulting in reduction of mechanical properties, cracking and spalling. These macroscopic changes are related to changes in the microstructure of concrete. In this paper, the microstructure of thermally damaged concrete from real-scale reinforced concrete columns was studied, using scanning electron microscopy (SEM) and x-ray diffraction (XRD). Two real-scale reinforced concrete columns, with different concrete mixtures, were exposed to ISO 834 firestandard curve for 4 hours. After heating, concrete core samples were drilled, sectioned and analyzed in different column section depths.Core samples from reference column not exposed to high temperatures, were also obtained. Macroscopic observations showed a change inthe color of concrete exposed to high temperatures. SEM micrographs presented distinct changes in morphology, like coarsening, and thepresence of cracks and voids. XRD diagrams showed a reduction of portlandite and presence of larnite as depth increases.
In Brazil, reinforced concrete structures designed based on NBR 6118 [1] must be assessed in fire conditions through NBR 15200 [2], which proposes multiple test methods. Concerning beams, this standard offers the tabular and simplified methods as possible choices. This study aimed to compare these two methods by applying their principles to isostatic and hyperstatic beams with section of 20x50 cm, designed at room temperature according to NBR 6118 [1]. The temperature distribution along sections was obtained by means of a software program. The results from this study indicated that the simplified method yielded higher fire-resistance ratings (FRR) in 75% of cases, although not surpassing the tabular method by 30 minutes. The simplified method allowed an optimization of the results, despite the longer design time.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.
hi@scite.ai
10624 S. Eastern Ave., Ste. A-614
Henderson, NV 89052, USA
Copyright © 2024 scite LLC. All rights reserved.
Made with 💙 for researchers
Part of the Research Solutions Family.