Effect of heating rate on the free expansion deformation of concrete during the heating process
Introduction
Concrete is a thermally inert material, which can maintain better fire resistance than other building materials in fire. However, high temperature gradients were caused by the different thermos physical properties of the various components in concrete. The inconsistency of the temperature inside and outside of the concrete was prone to cracks in the weak links with it, which further weakened the cohesion between the various components and reduced the structural bearing capacity [[1], [2], [3], [4]].
Concrete is a heterogeneous composite material composed of various phases such as cement, aggregate, and porosity. The solid phase, liquid phase, and gas phase had different thermal deformation characteristics from each other [5]. The thermal expansion characteristics of the hardened cement paste and aggregate in concrete were different because the changes in temperature and their various thermal physical properties [6,7]. The thermal stress was generated in the concrete due to temperature changes which caused the temperature gradients. In addition, the compressive stresses due to thermal deformation between the aggregate and C–S–H could cause crack in concrete [8,9]. Under high temperature, the thermal stress generated by the thermal expansion and deformation between the various constituent materials inside the concrete. The bonding properties between the steel bar and the concrete were influenced by the micro-cracks generated in concrete [10].
Hu et al. studied the free expansion strain, temperature strain under constant compressive stress and transient thermal strain of high-strength concrete at high temperature [11]. It is found that in the free stress state, the deformation growth was rapid and nonlinear below 500 °C. While, the deformation growth was slow above 500 °C. The higher the initial stress, the smaller the temperature deformation of the concrete. The transient thermal strain increased with the initial stress of concrete. Compared with the ordinary strength concrete, the free expansion deformation of high strength concrete was higher than that of ordinary strength concrete at the same temperature. At the same initial stress, the deformation of high strength concrete was higher than that of the ordinary concrete. However, the transient thermal strain was lower than that of the ordinary concrete. Ni et al. [12] studied the effect of heating rate (2, 5, 7 °C/min) on the free expansion deformation of concrete and found that the free expansion deformation of concrete decreased gradually with the increase of heating rate. The effect of heating rate on the free expansion deformation was small below 300 °C. The effect of heating rate on the free expansion deformation of concrete was obvious at 550 °C. It was also found that when the concrete was heated at 2 °C/min, the effect of the strength of the concrete on the free expansion deformation was small. Grassley et al. [13] found that the thermal expansion deformation of the hardened cement slurry could be divided into three parts. The first part was the expansion of the hydration product. The second part was the thermal deformation caused by free water in concrete. The third part was the volume change caused by the pressure of solid particles. Uygunoglu et al. [14] investigated the expansion properties of concrete with limestone aggregate and lightweight aggregate with a heating rate of 5 °C/min. The results showed that the thermal expansion coefficient of concrete with lightweight aggregate was lower than that of the concrete with limestone aggregate.
The expansion deformation of different concrete elements in at room temperature was different from that of the concrete at high temperature. The deformation of concrete at high temperature mainly includes free expansion deformation and transient thermal strain. Free expansion deformation occupied a basic position in the research of expansion deformation. The characteristics of free expansion deformation of concrete could be used to predict the value of free expansion deformation of structural members at high temperature. And it can be used to evaluate the fire resistance of components [15].
The temperature rising rate of concrete structure during fires is different, which causes a large temperature gradient inside and outside of the concrete. The large temperature gradient in concrete will lead to steam pressure and thermal stress of concrete [16]. The effect of heating rate on the free expansion deformation of concrete was significant. Therefore, it was necessary to study the expansion deformation of concrete at high temperatures under different heating rates. Many studies have been focused on the effects of high temperature on the transient thermal expansion deformation of concrete [[17], [18], [19], [20]]. The free expansion deformation of large-volume concrete due to temperature occurred in the hydration process was also investigated. In addition, the thermal deformation differences between the constituent materials of concrete were analyzed. However, there are few studies focused on the increasing rate of the free expansion deformation of concrete under different heating rate. The reasons for the deterioration in the mechanical properties of concrete after high temperature were analyzed considering the effects of heating rate on the free expansion deformation of concrete and the differences in the thermal expansion properties of the various components of the concrete after fire. Which will provide a reliable theoretical basis for the evaluation, appraisal, repair, and reinforcement of the concrete structure exposed to the fire [21,22].
Based on the above researches, in order to study the influence of heating rate on the free expansion deformation of concrete in the heating process, the concrete specimens were heated from room temperature to the target temperature. Then the temperature in the furnace were maintained at the target temperature until the central temperature of the specimen also reached to the target temperature. Then the specimens were cooled down from the target temperature to room temperature in the furnace. The main research contents are listed as following.
- (1)
The effect of heating rate on the weight loss of concrete.
- (2)
The effects of heating rate on the LER and TEC of concrete at different temperatures.
- (3)
The effect of heating rate on the residual LER of concrete at different temperatures.
- (4)
The effects of heating rate on the LER and TEC of concrete before constant temperature.
- (5)
The effects of temperature and holding time on the LER and TEC of concrete.
Section snippets
Raw materials
The materials are shown in Table 1, and the chemical composition and properties of cement are shown in Table 2. The concrete mix is shown in Table 3.
Preparation and curing of specimens
The mix proportion of concrete have been listed in Table 3. The size of the test specimens was 100 mm × 100 mm × 100 mm. The concrete mixture was poured into the standard mold of the specimen. All specimens were compacted by the vibration table and smoothed on its surface. After being casted for 24 h, the specimens were removed from molds and then
Effect of heating rate on mass loss of concrete
The effects of heating rate on the mass loss of concrete heated at high temperature are illustrated in Fig. 4.
As shown in Fig. 4, after being heated at high temperature with different heating rates, the change regulation of mass loss of concrete was similar to each other. The mass loss increased clearly in the initial phase from room temperature to 200 °C. From 200 to 600 °C, the mass loss increased slowly. However, there was a rapid increment from 600 to 800 °C, which was similar to the law of
Discussion and analysis
The test results were compared with the changes in the high-temperature expansion deformation of concrete with temperature in the specifications of EN 1992-1-2: 2004 (EC2) [37] and ASCE No. 78 (ASCE) [38]. The law of concrete expansion strain at different heating rates before constant temperature was shown in Fig. 16. The test results were similar to the change law of the high-temperature expansion strain of concrete with temperature in the code. And it shown an overall upward trend with
Conclusions
In this paper, three kinds of heating rates were used to conduct a test for the high-temperature deformation of concrete. The specimens was heated from room temperature to the target temperature, being held at the constant temperature until the center temperature of the specimens also reached the target temperature. Then the temperature was reduced from the target temperature to room temperature. The effect of heating rate on the LER and TEC of concrete were analyzed till the specimens were
CRediT authorship contribution statement
Qingtao Li: Conceptualization, Resources, Funding acquisition. Minghua Wang: Writing - original draft, Test. Hufei Sun: Test, Data curation. Guangyun Yu: Writing - review & editing, Validation, Investigation.
Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
The authors wish to thank the financial support by the National Natural Science Foundation of China (Grant No. 51208504).
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