Strength Reduction of Concrete Subjected to High Temperatures: Effects of Various Aggregates

Wonchang Kim; Keesin Jeong; Hyeonggil Choi; Taegyu Lee

doi:10.7731/KIFSE.f5c2b512

Int J Fire Sci Eng > Volume 37(1); 2023 > Article

Kim, Jeong, Choi, and Lee: Strength Reduction of Concrete Subjected to High Temperatures: Effects of Various Aggregates

Research Paper

International Journal of Fire Science and Engineering 2023;37(1):45-52.

Published online: March 31, 2023

DOI: https://doi.org/10.7731/KIFSE.f5c2b512

Strength Reduction of Concrete Subjected to High Temperatures: Effects of Various Aggregates

Wonchang Kim¹, Keesin Jeong¹, Hyeonggil Choi^2,^†, Taegyu Lee^1,^†

¹Department of Fire and Disaster Prevention, Semyung University, 65 Semyung-ro, Jacheon-si, Chungcheongbuk-do 27136, Republic of Korea

²School of Architecture and Civil Engineering, Kyungpook National University, 80 Daehak-ro, Buk-gu, Daegu 41566, Republic of Korea

^†Corresponding Author, TEL: +82-43-649-1315, FAX: +82-43-649-1787, E-Mail: ltg777@semyung.ac.kr, hgchoi@knu.ac.kr

Received January 4, 2023 Revised January 31, 2023 Accepted February 2, 2023

This is an Open Access article distributed under the terms of the Creative Commons Attribution Non-Commercial License (http://creativecommons.org/licenses/by-nc/4.0/) which permits unrestricted non-commercial use, distribution, and reproduction in any medium, provided the original work is properly cited.

Abstract

Previous studies on the strength degradation of concrete subjected to high temperatures were analyzed. To analyze the effect of the coarse-aggregate type on strength degradation, data from previous studies were collected, and the coarse aggregate used, physical properties of the aggregate, and heating conditions were analyzed. The concrete types were classified into normal, heavyweight, and lightweight concrete. Their high-temperature characteristics were analyzed and evaluated according to the mixed coarse aggregate. Finally, the correlations derived from the analysis results were compared with the CEB Code. The analysis results were different for different concrete and coarse-aggregate types, and different tendencies from the CEB Code were observed.

Keywords: High temperature; Concrete; Residual compressive strength; Coarse aggregate; Trendline

1. Introduction

Cement-based materials have been continuously produced and developed as they are recognized as excellent construction materials with regard to workability, safety, and economic feasibility [1]. In particular, concrete, which is produced by mixing cement with a large amount of aggregate, has been used as a material with excellent fire resistance performance that maintains structural and material stability when subjected to high temperatures from fires inside and outside buildings [2]. Concrete can maintain structural safety when exposed to high temperatures over a relatively short period; however, its durability rapidly decreases with long-term exposure [3].

When concrete is subjected to high temperatures, strength degradation occurs owing to the high-temperature characteristics of its material constituents, followed by changes in the internal structure. The free water and bound water remaining in concrete evaporate in the temperature range of 100-200 ℃, and calcium hydroxide (Ca(OH)₂) is decomposed at a temperature of approximately 300 ℃ or higher. The higher temperature range represents a chemical decomposition stage, in which C-S-H gel and calcium carbonate are decomposed. At a temperature of approximately 1,000 ℃ or higher, concrete melts. Concrete is also significantly affected by the aggregate that it contains. When concrete is subjected to high temperatures, the aggregate exhibits thermal expansion, whereas cement-based materials exhibit thermal contraction (see Table 1) [4]. Owing to such different thermal properties, microcracks and macrocracks occur in concrete.

Through the analysis of existing experimental data on the high-temperature characteristics of concrete, various associations and committees, such as CEB/FIP, CEN, ACI, and ASCE, have proposed standard codes (see Figure 1) [5-10]. For these codes, experiments have been performed by various researchers because it is necessary to fully examine high-temperature characteristics along with the development of various materials on a regular basis.

Therefore, in this study, the high-temperature characteristics of concrete were analyzed with respect to different variables through the collection and classification of experimental data from studies conducted in the past 10 years. The studies were classified according to the type of coarse aggregate contained in the concrete, and the analysis results were compared with the CEB/FIP Code.

2. Analysis Plan and Method

2.1 Analysis plan and scope

Table 2 presents the analysis scope and plan of this study [11-26]. The concrete types for analysis were set as normal, heavyweight, and lightweight concrete. Classification was then performed according to the type of coarse aggregate. For normal concrete, the aggregate types were set as granite and limestone. In the case of lightweight and heavyweight aggregates, various types of aggregates were mixed in previous studies. Because few studies were conducted on a single aggregate type, an integrated analysis was conducted. The residual compressive strength was set as an analysis item. The residual compressive strength was analyzed together with the CEB Code (Siliceous) for normal and heavyweight concrete and the CEB (Lightweight) Code for lightweight concrete.

2.2 Analysis method

Figure 2 presents the analysis method of this study. First, previous domestic and overseas studies on the hightemperature characteristics of concrete were investigated and collected. They were classified by the concrete type (normal, lightweight, or heavyweight) and then by the coarse-aggregate type. From the classified studies, residual compressive strength data were analyzed and summarized. Finally, strength degradation models for concrete subjected to high temperature were proposed according to the coarse-aggregate type.

3. Analysis Results

3.1 Strength degradation characteristics of concrete according to coarse-aggregate type

Figure 3 shows the results for concrete mixed with granite aggregates. Overall, slight strength degradation was observed at approximately 100 ℃. This is a stage in which the formation of voids begins as the free water and surface water inside concrete evaporate [11-14,23]. Subsequently, a slight increase in strength was observed at 200 ℃. This phenomenon was also reported in previous studies on the high-temperature characteristics of concrete. In the 200-300 ℃ range, the effects of the forced hydration reaction, aggregate expansion, and water vapor pressure due to water evaporation were reported [27-32].

Thereafter, the strength continuously decreased, and most of the experimental results exhibited a tendency similar to that of the CEB Code. However, H. Mohammadbosseini et al. [11] reported a very low residual strength compared with the CEB Code. This appears to be due to the high heating rate (20 ℃/min) compared with other experiments. Conversely, A. K. Saha and J. B. da Silva et al. [12,13] reported higher residual strengths compared with the CEB Code and other experiments. E. Hwang et al. [14] reported a relatively low residual strength despite the water/binder (W/B) ratio being similar to those of the two aforementioned studies. This appears to be due to the difference in aggregate density. However, the residual strength was higher than those of the CEB Code and the remaining experiments.

Figure 4 shows the results for concrete mixed with limestone aggregates. A tendency similar to that of Figure 3 was observed in the temperature range below approximately 200 ℃, but the overall results exceeded the CEB Code in the higher temperature range. M. Saridemir et al. [16] reported a higher residual strength compared with the CEB Code and other studies. This appears to be due to the distribution and fineness modulus of the aggregates in various sizes. Figure

Figure 5 shows the results for concrete mixed with heavyweight aggregates. Most of the results exceeded the CEB Code. This appears to be due to the high density of the heavyweight aggregate. I. Demir et al. [17] reported a low residual strength even though the W/B ratio was lower than those of other studies. The heating rate was not clearly reported, except in the case of I. Demir et al., and it is judged that different heating conditions among the laboratories caused differences. Fewer studies on concrete mixed with heavyweight aggregates were collected compared with the other aggregate types. Thus, additional experiments are required for accurately determining the high-temperature characteristics of heavyweight aggregates.

Figure 6 shows the results for concrete mixed with lightweight aggregates. Except for the studies of W. Yao and Ö. S. Bideci et al. [24,26], a lower residual strength compared with the CEB Code was observed in the temperature range below 300 ℃. W. Yao et al. [24] added shale ceramsite—a type of sedimentary rock—and reported a dense structure compared with an ordinary aggregate. Scanning electron microscopy results indicated few defects, such as surface cracks and voids, at room temperature (20 ℃). In addition, phenomena such as cracks and voids were not clearly observed until the temperature reached 400 ℃. They reported that the internal structure was maintained despite the presence of defects when the shale ceramsite was heated to a temperature higher than 1,000 ℃. Such hightemperature characteristics appear to have contributed to the higher residual strength compared with the experimental results for other lightweight aggregates.

K. kçaözoğlu et al. [23] reported a relatively low residual strength until the temperature reached 400 ℃. At higher temperatures, however, the residual strength was similar to that of the CEB Code and higher than other experimental results. The aggregate mixed by K. kçaözoğlu et al. had a density of approximately 430 kg/m³, which was the lowest aggregate density among the analyzed studies on lightweight aggregate concrete. It appears that a high residual strength was observed at a high temperature because the thermal expansion of the aggregate was relatively small owing to the relatively high porosity; thus, the interfacial transition zone (ITZ) of the aggregate and paste was improved. M. Z. Jummat et al. [25] added oil palm shell (OPS). They reported that a very low residual strength was observed under the influence of the severe thermal contraction of the aggregate.

3.2 Strength degradation trendline of concrete according to mixed coarse aggregate

Figure 7 shows the trendlines of concrete subjected to high temperatures for different aggregate types. For normal and heavyweight concrete, a low residual strength was observed compared with the CEB Code (Siliceous) up to approximately 200 ℃. For lightweight aggregates, however, a high residual strength was observed. At 300 ℃, except for concrete mixed with lightweight aggregates, a residual strength similar to or higher than that of the CEB Code (Siliceous) was observed. When limestone and heavyweight aggregate were mixed, approximately 6.8% and 7.1% higher residual strengths were observed. When lightweight aggregate was mixed, a residual strength approximately 4.3% lower than that of the CEB Code (Lightweight) was observed.

In the temperature range higher than 400 ℃, the residual strengths of normal and heavyweight concrete exceeded those of the CEB Code (Siliceous). For lightweight concrete, a lower residual strength compared with the CEB Code (Lightweight) was observed. As the temperature increased, the difference from the CEB Code tended to gradually increase for all levels. The results for the granite aggregates were the most similar to the CEB Code (Siliceous), with a residual strength approximately 5.3% higher. For the limestone aggregates, the residual strength was approximately 17.3% higher. The heavyweight aggregates exhibited the largest difference from the CEB Code (Siliceous), with a residual strength approximately 35.2% higher. For the lightweight aggregates, the residual strength was approximately 17.4% lower than that of the CEB Code (Lightweight).

In the analysis results of this study, normal concrete mostly exhibited a higher residual strength compared with the CEB Code at all temperatures. This appears to be due to the development of chemical and production technologies compared with previous cement. Concrete mixed with heavyweight aggregates exhibited a large difference from the CEB Code. It is deemed necessary to develop a strength degradation model for heavyweight concrete through additional experiments. In addition, concrete mixed with lightweight aggregates exhibited a lower residual strength compared with the CEB Code, and this needs to be considered during the design process.

4. Conclusion

Previous studies on the strength degradation characteristics of concrete subjected to high temperature for different types of coarse aggregates were analyzed. The analysis results are summarized as follows.

1) At all levels, slight strength degradation was observed until the temperature reached approximately 200 ℃, and the strength was lower than that of the CEB Code. A slight increase in strength was observed at approximately 300 ℃. Subsequently, continuous strength degradation was observed.

2) Concrete mixed with granite and limestone aggregates mostly exhibited higher residual strengths compared with the CEB Code in the temperature range higher than 300 ℃.

3) For concrete mixed with heavyweight aggregates, the residual strength exceeded the CEB Code and exhibited the largest difference. Because studies on the high-temperature characteristics of heavyweight aggregates are insufficient compared with the other aggregate types, continuous research is required. It is also deemed necessary to develop a strength degradation model for heavyweight aggregates.

4) For lightweight aggregates, the residual strength was lower than that of the CEB Code, indicating that conservative design is required considering the characteristics (e.g., low stiffness) and safety of lightweight aggregates.

Notes

Author Contributions

Conceptualization, H.C. and T.L.; methodology, H.C. ; software, W.K and K.J..; validation, H.C. and T.L. ; formal analysis, W.K.; investigation, W.K.; resources, H.C.; data curation, W.K.; writing―original draft preparation, W.K.; writing―review and editing, H.C. ; visualization, T.L.; supervision, H.C.; project administration, T.L.; funding acquisition, K.J. All authors have read and agreed to the published version of the manuscript.

Conflicts of Interest

The authors declare no conflict of interest.

Acknowledgments

This work was supported by a National Research Foundation of Korea (NRF) grant funded by the Korean government (MSIT) (No. 2022R1F1A1073333).

Figure 1

Residual compressive strength models of various standard codes.

Figure 2

Analysis method.

Figure 3

Granite aggregates.

Figure 4

Limestone aggregates.

Figure 5

Heavyweight aggregates.

Figure 6

Lightweight aggregates.

Figure 7

Correlation for various coarse-aggregate types.

Table 1

Coefficient of Thermal Expansion for Different Types of Rock

Type of rock	Coefficient of Thermal Expansion (× 10⁻⁶/°C)
Granite	3.6-8.1
Quartzite	10.1-14.4
Sandstone	6.1-11.7
Basalt	6.1-7.2
Limestone	2.2-9.5
Gneiss	6.5
Pelite	7.4
Feldspar	0.9-17.5
Gabbro	7.4

Table 2

Analysis Plan

Type of Concrete	Type of Coarse Aggregate	Density (kg/m³)	Absorption (%)	Maximum Size (mm)	Water/Binder	Temperature Range (°C)	Rate of Temperature (°C/min)	Specimen (mm)	Analysis Item
Normal Concrete	Granite	2,610-2,730	0.18-1.81	8-38	0.19-0.68	100-1,000	1-20	Cylinder Cube Prism	Residual Compressive Strength
Normal Concrete	Limestone	2,610-2,730	0.18-1.81
Heavyweight Concrete	Heavyweight Aggregate	2,870-4,050	0.62-2.67
Lightweight Concrete	Lightweight Aggregate	430-1,800	8.5-24.6

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