Review: Residual Compressive Strength Analysis of Fiber-Mixed Concrete Exposed to High Temperatures

Younga Jang; Youngjin Nam; Hajun Im; Wonchang Kim; Taegyu Lee

doi:10.7731/KIFSE.6d2b06f6

Int J Fire Sci Eng > Volume 38(3); 2024 > Article

Jang, Nam, Im, Kim, and Lee: Review: Residual Compressive Strength Analysis of Fiber-Mixed Concrete Exposed to High Temperatures

Research Paper

International Journal of Fire Science and Engineering 2024;38(3):16-22.

Published online: September 30, 2024

DOI: https://doi.org/10.7731/KIFSE.6d2b06f6

Review: Residual Compressive Strength Analysis of Fiber-Mixed Concrete Exposed to High Temperatures

Younga Jang, Youngjin Nam, Hajun Im, Wonchang Kim, Taegyu Lee^†

Department of Fire and Disaster Prevention, Semyung University, Choongbuk 27136, Republic of Korea

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

Received August 12, 2024 Revised August 22, 2024 Accepted August 23, 2024

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

This study analyzed the high-temperature properties of concrete mixed with polypropylene (PP) and steel (ST) fibers, based on previous research cases. To examine the effects of different fiber types on concrete under high temperatures, fiber-mixed concrete was categorized into PP, ST, and PP+ST fibers, and the target concrete strengths were set at three levels: 40-60 MPa, 60-80 MPa, and 80-100 MPa. The temperature range for analysis was set at 20-900 °C, and the heating rate range was set at 0.5-20 °C/min. The residual strength of concrete exposed to high temperatures was analyzed based on target strengths and fiber types, and the results were compared with the CEB code. Based on the results of thermogravimetric analysis and differential scanning calorimetry, PP fibers were observed to melt at approximately 170 °C and vaporize at 341 °C, forming pores in the concrete matrix, whereas ST fibers underwent decarburization and carbonation at approximately 700 °C. Fiber-mixed concrete categorized by target strength showed that concrete with a target strength of 80-100 MPa had approximately 8% higher residual compressive strength at 600 °C compared with other target strength levels. The residual compressive strength analysis revealed that most concrete samples exhibited residual strength approximately 10%-31% higher compared with the CEB code above 600 °C.

Keywords: Concrete; High temperature; Fiber; Compressive strength; Review

1. Introduction

With the increasing urban population density and the growing demand for high-rise and large-scale buildings to accommodate more people, the use of high-strength concrete is increasing. However, high-strength concrete has a denser internal structure compared with normal-strength concrete, making it prone to explosive spalling when exposed to high temperatures owing to internal vapor pressure. Explosive spalling is correlated with the increase in vapor pressure within the concrete. As the temperature of the concrete increases, internal water turns into steam, which may have limited mobility as the strength of the concrete increases. Consequently, the trapped steam accumulates internally, causing an increase in pressure and forming a pressure gradient inside the concrete. When this pressure exceeds the tensile strength of the concrete, explosive spalling occurs. To prevent such explosive spalling, a method of adding fibers with a low melting point into the concrete has been proposed to alleviate vapor pressure. In particular, polypropylene (PP) fibers melt at approximately 161 °C, forming pores that allow the vapor to escape. Additionally, using PP fibers together with steel (ST) fibers has been reported to reduce spalling [1].

Figure 1 shows the changes in concrete cracks after heating, categorized by fiber type. The inclusion of fibers prevents spalling, and subsequently, PP fibers melt, becoming absorbed into the concrete matrix and leaving behind pores. ST fibers, owing to their differing thermal expansion coefficients compared with concrete, cause cracks to form in the concrete.

These pores and cracks negatively affect the residual mechanical properties of the concrete. To investigate the effects of these pores and cracks, a study was conducted by N. Yermak and colleagues to evaluate the mechanical properties of fiber-mixed concrete containing 0.75-1.5 kg/m³ PP fibers and 30-60 kg/m³ ST fibers, after heating at a rate of 0.5 °C/min. Microanalysis confirmed that concrete without ST fibers exhibited more cracks compared with concrete with ST fibers [2].

As the target strength of concrete increases, its structure becomes denser, and the effects of strength on the concrete need to be considered. Although several researchers have conducted experimental studies on fiber ratios and types in fiber-mixed concrete exposed to high temperatures across various high-strength ranges, there is a need for quantitative analysis regarding the strength reduction of high-strength concrete with fiber inclusion, considering its strength range. G. Kim and colleagues observed that, as the target strength increases, the rate of strength reduction under high temperatures also increases. Therefore, analyzing the residual mechanical properties of fiber-mixed concrete at high temperatures, based on target strength, is considered necessary [3]. This case study analyzed the residual strength properties of high-strength concrete mixed with PP, ST, and PP+ST fibers exposed to high temperatures, across various target strength ranges, to evaluate the residual mechanical properties based on target strength.

2. Analysis Plan and Methods

2.1 Analysis plan

This study reviewed research on concrete mixed with PP and ST fibers, which are commonly used in fiber-mixed concrete, to analyze the residual mechanical properties of high-strength concrete exposed to high temperatures. The factors analyzed include fiber type, fiber length, fiber diameter, mixing ratio, and heating rate. The details are summarized in Table 1 [1,2,4-16]. The mechanical properties of each fiber type from the reviewed studies are presented in Table 2 [1,2,4-16]. As concrete in reinforced concrete members typically bears compressive forces while the reinforcement bears tensile forces, only 6 out of 15 studies included tensile strength data. Owing to the small sample size and lack of reliability, tensile strength data were excluded from this study. The melting point of PP fibers is 160-180 °C, with a density of 0.91 g/cm³, a flexural strength of 250-800 MPa, and an elastic modulus of 1.3-8 GPa. ST fibers are generally used to improve the mechanical properties, such as compressive strength, flexural strength, and tensile strength. The melting point of ST fibers ranges from 1410-1540 °C, with a density of 7.85 g/cm³, and their flexural strength and elastic modulus are 250-800 MPa and 20-21 GPa, respectively.

2.2 Analysis methods

In this study, the effects of fibers on concrete with varying target strengths were analyzed by categorizing the compressive strength into 40-60 MPa, 60-80 MPa, and 80-100 MPa. The fibers analyzed were PP and ST fibers, which are commonly used in domestic high-strength concrete. Other types of fibers were excluded from the scope of this study owing to the limited availability of samples. The temperature range for analysis was set from room temperature (20 °C) to 900 °C, and the heating rate was set in the range 0.5-20 °C/min. The residual compressive strength of concrete at the target temperatures was calculated using Equation (1), and the calculated values were analyzed by comparing them with the CEB code, based on the target strength and fiber type.

fcre=fcTfcR

where fc_re is the residual compressive strength, fc_T is the compressive strength at the target temperature, and fc_R is the room temperature (20 °C).

3. Results

3.1 Analysis of thermal properties of fibers

Figure 2 summarizes the results of thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC) conducted on PP and ST fibers in previous studies [17,18]. PP fibers melt at approximately 160-170 °C, vaporize at 350 °C, and combust at 447 °C. The melted PP fibers are partially absorbed into the cement matrix, leaving gas pathways [2,8]. These gas pathways create microcracks in the concrete and form a network that facilitates the release of steam to reduce vapor pressure and prevent spalling.

Consequently, previous researchers have conducted various studies on incorporating PP and ST fibers to prevent spalling in high-strength concrete. M. Mubarabak and colleagues reported that PP fibers mixed into concrete at high temperatures melt, forming a network that creates pathways for gas movement, contributing to the reduction of pore pressure [9].

ST fibers are generally used to improve mechanical properties, such as compressive strength, flexural strength, and tensile strength. Owing to their high melting point and thermal conductivity, they uniformly distribute heat throughout the concrete compared with concrete with added PP fibers, potentially improving the residual mechanical properties at high temperatures [10]. ST fibers exhibit a melting peak at approximately 700 °C. This is attributed to the decarburization and carbonation of steel, which prevents it from providing a bridging effect and instead causes cracks that can negatively affect the concrete.

3.2 Compressive strength

Figure 3 shows the analysis results of the residual compressive strength of concrete [1,2,4-16]. Figure 3(a) shows the results for concrete mixed with PP fibers, where PP_80-100 concrete exhibited the highest residual strength. The residual strength of PP_40-60 was approximately 4% lower than that of PP_80-100, whereas PP_60-80 showed approximately 5% lower residual strength compared with PP_80-100. Figure 3(b) shows the results for concrete mixed with ST fibers. Concrete with a target strength of 80-100 MPa and ST fibers was excluded owing to insufficient sample size. Similar to PP fiber concrete, ST_40-60 MPa exhibited higher residual strength among ST fiber concretes. At 600 °C, ST_60-80 exhibited approximately 7% lower residual strength compared with ST_40-60. In the case of concrete mixed with PP+ST fibers, PP+ST_80-100 exhibited 5% and 14% higher residual strength at 600 °C compared with PP+ST_40-60 and PP+ST_60-80, respectively (Figure 3(c)).

Figure 4 shows the residual strength analysis results of concrete mixed with different types of fibers [1,2,4-16]. When comparing concrete in the 40-60 MPa range with the CEB code, the residual compressive strength exceeded the CEB code at temperatures above approximately 400 °C, regardless of the fiber type (Figure 4(a)).

At 400 °C and 600 °C, PP fibers showed approximately 10% and 24% higher residual strength, ST fibers exhibited 7% and 19% higher residual strength, and PP+ST fibers displayed 0.05% and 12% higher residual strength, respectively. For concrete in the 60-80 MPa range, when compared with the CEB code, concrete mixed with PP fibers showed approximately 7% and 23% higher residual strength at 400 °C and 600 °C, respectively, whereas ST fibers showed 3% and 13% higher residual strength, respectively.

Concrete mixed with both PP and ST fibers showed approximately 5% lower residual strength at 400 °C. For concrete in the 80-100 MPa range, after approximately 400 °C, concrete mixed with PP fibers exhibited more than 10% higher residual compressive strength, and concrete mixed with PP+ST fibers showed more than 5% higher residual compressive strength.

While some previous studies reported that concrete mixed with PP+ST fibers performed better than concrete mixed with single fibers (PP or ST) [11], this study observed that, at 600 °C, concrete mixed with PP+ST fibers exhibited an average of 11.9% lower residual strength compared with concrete mixed with single fibers (PP, ST). This may be attributed to the combined effects of differing thermal expansion coefficients between ST fibers and concrete, the impact of oxidized ST fibers on the concrete, and the pores left by the evaporation of PP fibers at high temperatures. However, these results may also be influenced by differing experimental conditions, such as aggregate type, heating rate, mixing ratio, and aspect ratio [2,12,14].

4. Conclusion

This study collected and reviewed papers on concrete mixed with fibers and subjected to high-temperature exposure, categorizing them by fiber type. The residual compressive strength of concrete mixed with fibers was then compared and analyzed based on the target strength.

1) PP fibers melt at approximately 170 °C, vaporize at 341 °C, and combust at 447 °C. As PP fibers melt, they are absorbed into the cement matrix. ST fibers negatively affect the mechanical properties of concrete owing to decarburization and carbonation occurring at approximately 700 °C.

2) Fiber-mixed concrete categorized by target strength showed that concrete with a target strength of 80-100 MPa exhibited 4%-14% higher residual strength at 600 °C compared with fiber-mixed concrete with the target strengths of 40-60 MPa and 60-80 MPa. Concrete with a target strength of 60-80 MPa exhibited 1%-14% lower residual strength at 600 °C compared with other target strength levels.

3) Concrete mixed with single fibers (PP or ST) and combined fibers (PP+ST) showed higher residual strength than the CEB code at temperatures above approximately 400 °C. The residual strength was highest for concrete mixed with PP fibers (400 °C: approximately 10%, 600 °C: approximately 24%), followed by ST fibers (400 °C: approximately 7%, 600 °C: approximately 19%), and PP+ST fibers (400 °C: approximately 0.05%, 600 °C: approximately 12%).

4) Concrete mixed with both PP and ST fibers exhibited lower residual compressive strength than concrete mixed with either PP or ST fibers. This was attributed to cracks caused by the differing thermal expansion coefficients between ST fibers and concrete, and the pores formed by the melting of PP fibers impacting the residual compressive strength.

Notes

Author Contributions

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

Conflicts of Interest

The authors declare no conflict of interest.

Acknowledgments

This study was supported by the National Research Foundation of Korea (NRF) grant funded by the Korea government (MSIT) (No. 2022R1F1A10733331330482048500103).

Figure 1.

Pore development process in fiber-mixed concrete.

Figure 2.

TGA, DSC: (a) polypropylene fiber; (b) steel fiber.

Figure 3.

Residual compressive strength: (a) PP; (b) ST; (c) PP+ST.

Figure 4.

Residual compressive strength: (a) 40-60 MPa; (b) 60-80 MPa; (c) 80-100 MPa.

Table 1.

Summary of Fiber Properties and Heating Rates

Type of concrete	Fiber type	Dimensions of fiber
Type of concrete	Fiber type	Length (mm)	Diameter (µm)	Target temperature (°C)	Item
40-60 (MPa)	PP¹⁾	PP (6-30)	PP (18-78)	20-900	Compressive Strength TGA, DSC
	ST²⁾	PP (6-30)	PP (18-78)
60-80 (MPa)
		ST (25-60)	ST (55-900)
80-100 (MPa)	PP+ST	ST (25-60)	ST (55-900)

¹⁾ PP: polypropylene;

²⁾ ST: Steel

Table 2.

Summary of Fiber Properties and Heating Rates

Fiber type	Density (g/cm³)	Tensile strength (MPa)	Elastic modulus (GPa)	Melting (°C)	Vaporization (°C)	Ignition (°C)	Heat transfer coefficient (W/m²°C)	Thermal expansion coefficient (m/m ⋅ °C)
PP	0.9-1.3	250-800	3.5-8	160-180	341-350	460-590	0.1-0.22	1.3×10^-7
ST	7.85	650-3070	20-21	1410-1540	-	-	43	-

References

1. A Ahmed M. Tahwia, Marwa Mokhles and Walid E. Elemam, “Optimizing Characteristics of High-Performance Concrete Incorporating Hybrid Polypropylene Fibers”, Innovative Infrastructure Solutions, Vol. 8, No. 297, (2023), https://doi.org/10.1007/s41062-023-01268-6.

2. N. Yermak, P. Pliya, A.-L. Beaucour, A. Simon and A. Noumowé, “Influence of Steel and/or Polypropylene Fibers on the Behaviour of Concrete at High Temperature: Spalling, Transfer and Mechanical Propertie”, Construction and Building Materials, Vol. 132, pp. 240-250 (2017), http://dx.doi.org/10.1016/j.conbuildmat.2016.11.120.

3. A. P. Naveen, R. S. Priyadarsini and D. Anupama Krishna, “Effect of High Temperature on the Compressive and Flexural Performance of Fibrous Concrete- An Experimental Investigation”, Materialstoday: Proceedings, Vol. 33, No. 4, pp. 239-259 (2023), https://doi.org/10.1016/j.matpr.2023.05.313.

4. J. H. Oh, J. H. Cheon, M. S. Lee and S. W. Yoo, “Evaluation of Spalling Characteristics and Fire Resistance Fiber-Entrained Mixed Cement Concrete at Ultra-High Temperatures”, Journal of the Korea Institute for Structural Maintenance and Inspection, Vol. 27, No. 5, pp. 23-29 (2023), https://doi.org/10.11112/jksmi.2023.27.5.23.

5. M. Tawfik, A. El-said, A. Deifalla and A. Awad, “Mechanical Properties of Hybrid Steel-Polypropylene Fiber Reinforced High Strength Concrete Exposed to Various Temperatures”, Fibers, Vol. 10, No. 6, (2022), https://doi.org/10.3390/fib10060053.

6. H. R. Moosaei, A. R. Zareei and N. Salemi, “Elevated Temperature Performance of Concrete Reinforced with Steel, Glass, and Polypropylene Fibers and Fire-proofed with Coating”, Internarional Journal of Engineering, Vol. 35, No. 5, pp. 917-930 (2022), https://doi.org/10.5829/ije.2022.35.05b.08.

7. H. H. Y. AL-Radi, S. Dejian and H. K. Sultan, “Performance of Fiber Self Compacting Concrete at High Temperatures”, Civil Engineering Journal, Vol. 7, No. 12, (2021), http://dx.doi.org/10.28991/cej-2021-03091779.

8. M. Mubarak, R. S. M. Rashid, M. Amran, R. Fediuk, N. Vatin and S. Klyuev, “Mechanical Properties of High-Performance Hybrid Fiber-Reinforced Concrete at Elevated Temperatures”, Sustainability, Vol. 13, No. 23, (2021), https://doi.org/10.3390/su132313392.

9. M. A. Moghadam and R. A. Izadifard, “Effects of Steel and Glass Fibers on Mechanical and Durability Properties of Concrete Exposed to High Temperatures”, Fire Safety Journal, Vol. 113, (2020), https://doi.org/10.1016/j.firesaf.2020.102978.

10. H. Caetano, G. Ferreira, J. P. C. Rodrigues and P. Pimienta, “Effect of the High Temperatures on the Microstructure and Compressive Strength of High Strength Fiber Concretes”, Construction and Building Materials, Vol. 199, pp. 717-736 (2019), https://doi.org/10.1016/j.conbuildmat.2018.12.074.

11. J. Eidan, I. Rasoolan, A. Rezaeian and D. Poorveis, “Residual Mechanical Properties of Polypropylene Fiber-Reinforced Concrete After Heating”, Construction and Building Materials, Vol. 198, pp. 195-206 (2019), https://doi.org/10.1016/j.conbuildmat.2018.11.209.

12. F. U. A. Shaikh and M. Taweel, “Compressive Strength and Failure behaviour of Fiber Reinforced Concrete at Elevated Temperatures”, Advances in Concrete Construction, Vol. 3, No. 4, pp. 283-293 (2015), https://doi.org/10.12989/acc.2015.3.4.283.

13. Y. Ding, C. Azevedo, J. B. Aguiar and S. Jalali, “Study on Residual Behaviour and Flexural Toughness of Fiber Cocktail Reinforced Self Compacting High Performance Concrete After Exposure to High Temperature”, Construction and Building Materials, Vol. 26, No. 1, pp. 21-31 (2012), https://doi.org/10.1016/j.conbuildmat.2011.04.058.

14. U. Sharma, K.. Zaidi and N. Bhandari, “Residual Compressive Stress-Strain Relationship for Concrete Subjected to Elevated Temperatures”, Journal of Structural Fire Engineering, Vol. 3, pp. 327-350 (2012), https://doi.org/10.1260/2040-2317.3.4.327.

15. P. Pliya, A. L. Beaucour and A. Noumowé, “Contribution of Cocktail of Polypropylene and Steel Fibers in Improving the Behaviour of High Strength Concrete Subjected to High Temperature”, Vol. 25, No. 4, pp. 1926-1934 (2011), https://doi.org/10.1016/j.conbuildmat.2010.11.064.

16. G. Y. Kim, S. H. Jung, T. G. Lee, Y. S. Kim and J. S. Nam, “Compressive Behavior of Concrete with Loading and Heating”, Journal of the Korea Institute for Structural Maintenance and Inspection, Vol. 14, No. 4, pp. 119-125 (2010), https://doi.org/10.11112/jksmi.2010.14.4.119.

17. M. Ozawa and H. Morimoto, “Effects of Various Fibers on High-Temperature Spalling in High-Performance Concrete”, Construction and Building Materials, Vol. 71, pp. 83-92 (2014), https://doi.org/10.1016/j.conbuildmat.2014.07.068.

18. D. Zhang and K. Hai Tan, “Fire Performance of Ultra-High Performance Concrete: Effect of Fine Aggregate Size and Fibers”, Archives of Civil and Mechanical Engineering, Vol. 22, No. 116, (2022), https://doi.org/10.1007/s43452-022-00430-8.