Effect of different volume fraction of steel fiber/graphite on thermal conductivity and compressive properties of concrete

Abstract

The enhancement of the thermal conductivity can be utilized to reduce the temperature difference between the internal and external structural surfaces and thus the thermal induced stresses in concrete. This study investigated the compressive strength and thermal conductivity in concrete structures enhanced by the steel fiber and graphite at different volume fractions. First, the cubic and the cylinder concrete samples were fabricated by adding different volume fractions of the steel fibers, such as 0%, 0.5%, 1.0%, 1.5%, 2.0%, and 2.5%, to the concrete mixture. The fiber enhanced samples were tested following the standard procedures, and the optimal compressive strength and thermal conductivity can be obtained in the sample with the volume fraction of steel fiber at 1.5%. Moreover, graphite powders with different volume proportions at 5%, 10%, and 15% were added to the fiber enhanced concrete samples with identical size. The compressive strength and thermal conductivity tests were also performed and the optimal content of steel fiber was 1.5%, and the graphite was 5%. These results could not only meet the design requirements of the compressive strength of the concrete, but also improve the thermal conductivity of the concrete. The research could provide some reference for the design of the high thermal conductivity concrete.

1 Introduction

Large-span bridge structures are usually pivotal projects in transportation systems. Particularly, cable-bearing bridges, such as suspension bridges and cable-stayed bridges are the most common types of long-span bridges. The bridge pylon is an important structural component, most of which are hollow, thin-walled concrete structures. Under the natural environmental conditions, like sunlight radiation, daily temperature changes, cold currents and wind speed, the pylons will experience a significant temperature difference across the wall thickness due to the relatively poor thermal conductivity of concrete, thereby leading to considerable thermal stress in the concrete wall of the pylon. It was acknowledged that the thermal stress is the principal reason for the development of cracking in concrete pylon walls, which further affects their structural performance and durability (Liu, 1991; Zhang et al., 2017; Chen, 2018; Lei et al., 2019; Liu et al., 2019; Gu et al., 2020; Ren et al., 2021; Lin and Dou, 2022).

Over the years, extensive studies have been conducted on the thermal conductivity of concrete (Hamilton and Crosser, 1962; Campbell-Allen and Thorne, 1963; Harmathy, 1970). Tang et al. (2012a) investigated the effect of mesoscopic and macroscopic structures on the effective thermal conductivity of concrete. Based on the Weibull distribution assumption (Zhang et al., 2018), the non-homogeneity of the concrete can be considered from a mesoscopic viewpoint. Simulations of several non-homogeneous samples showed that the effective thermal conductivity was primarily dependent on the degree of non-homogeneity (Zhang et al., 2012; Xiao et al., 2013). The higher the homogeneity leads to the greater effective thermal conductivity (Du et al., 2014). From the macroscopic viewpoint, it was observed that the volume proportion of coarse aggregates had the most remarkable effect on the thermal conductivity of concrete (Kim et al., 2003; Tang et al., 2012b; Chang and Wang, 2017). Xiao et al. (2010) studied the effect of recycled coarse aggregate replacement rate on the thermal conductivity of recycled concrete. Deng (2008) found that the thermal conductivity of concrete increased with the increase of the aggregate content and the volume fraction of reinforcement.

Liu et al. (2012) and Sun et al. (2009) tested and analyzed the thermal conductivity of the steel fiber concrete and carbon fiber concrete. Li et al. (2007) investigated the effect of admixture on the thermal conductivity of the concrete using the heat transfer meter. The results showed that the fiber and admixture played a dominant role in the thermal conductivity of concrete. Zhang et al. (2015a) and Gu et al. (Zhang et al., 2015b) analyzed the influence of aggregate content and water-cement ratio on the thermal conductivity of concrete by experiments using the protective thermal plate method. The results showed that the thermal conductivity of concrete increased with the increase of the saturation, aggregate content, and volume the ratio.

To study the thermal conductivity of concrete in practical structures, Ren et al. (Ren et al., 2011a; Ren et al., 2011b; Ren and HuangHan, 2012) carried out on-site investigations on the bridge pylons of Yi Chang Jiang Bridge under the influence of sunshine temperature. They established an analytical model to estimate the maximum principal tensile stress under the loading of the temperature difference across the wall thickness of the pylon. Liu and Geng (2005) studied the effect of thermal parameters on the temperature difference for the concrete bridge in Shanghai, and they concluded that the enhanced thermal conductivity was beneficial in reducing the temperature difference within the concrete. Qu et al. (2019) improved the thermal conductivity of concrete by adding different graphite admixture and obtained an optimal admixture of graphite.

In order to improve the performance of concrete, many scholars at home and abroad have done research on the single doping of steel fiber or graphite and other materials Up to now, howerver, there are few reports on quantitative research on the thermal conductivity and compressive behavior of concrete under the action of steel fiber/graphite. This work investigates the thermal conductivity and compressive performance of the concrete after adding steel fiber or steel fiber/graphite materials by experiments. The results suggested that the optimal addition proportions for the steel fiber and the mixed steel fiber/graphite materials was the premise to meet the compressive strength requirement of the bridge pylon. The outcome also provides a guideline for the application of concrete enhanced by the steel fiber or the mixed steel fiber/graphite in concrete thin-walled box bridge pylons.

2 Specimen preparation and experimental setup

2.1 Materials

FIGURE 1

2.2 Preparation of specimens made of fiber-enhanced concrete

2.2.1 Mix proportion of the ordinary concrete

According to Chinese specifications (JGJ55-2011) (Specification, 2011), the concrete specimens were prepared and optimized under the following considerations.

2.2.1.1 Compressive strength

The target concrete for experiments is C40 with standard cubic compressive strength of 40 MPa. With the addition of steel fiber or steel fiber/graphite, the concrete compressive strength was enhanced, denoted as the concrete preparation strength here.

As the compressive strength of concrete is less than 60 MPa, the concrete preparation strength can be calculated by the equation (Specification, 2011)f_cu,o----- Concrete preparation strength;f_cu,k----- Cube compressive strength;σ----- Standard deviation of the concrete strength.

Standard deviation (σ) of the concrete strength as shown in Table 5, is calculated as:

f_cu,o = f_cu,k + 1.645·σ = 40 + 1.645×5 = 48.23 MPa

2.2.1.2 Water binder ratio

W/b-----Water binder ratio of the concrete; γ_c ----- Redundancy coefficient of cement strength grade value; α_a, α_b----- Regression coefficient; f_ce,g----- Cement strength grade value (MPa). Redundancy coefficient of cement strength grade value (γ_c) as shown in Table 6. Regression coefficient α_a α_b value as shown in Table 7.

The parameters were derived by taking them into Eq. 2.2:

The water binder ratio was 0.49 according to the mixing effect.

2.2.1.3 Water consumption per unit weight (kg)

m_w0 ----- Water consumption per unitm_w ----- Water consumption of the concreteβ----- Water reducing rate of water reducing agent

Crushed stones were used as coarse aggregate with diameters from 5 to 15 mm, the slump was 60 mm m_w= 220 kg/m³ The water reducing rate of water reducing agent was 25%, derived from the Eq. 2.4: m_w0 = 165 kg/m³, as listed in Table 8.

2.2.1.4 Unit cement and additive quantity

m_bo----Amount of cementitious material per cubic meter (kg/m³);m_co ----- Amount of cement per cubic meter (kg/m³);m_ao ----- Amount of water reducing agent per cubic meter (kg/m³);m_fo ----- Amount of mineral adhesives per cubic meter (kg/m³);β_a ----- Water reducing agent in cement mass fraction (%).

According to Eq. 2.5, m_co is 440 kg/m³ and m_ao is 6.195 kg/m³.

2.2.1.5 Sand ratio and aggregate consumption

Sand ratio of the concrete (%) as shown in Table 9. β_s ----- Sand ratio(%);_so ----- Amount of sand per cubic meter of concrete (kg/m³);_go ----- Amount of crushed stone per cubic meter of concrete (kg/m³).

According to Eq. 2.6, is 749 kg/m³, and is 1,123 kg/m³.

2.2.1.6 Trial and adjustment

Through test and trial adjustment, the basic concrete mix proportion is shown in Table 10.

2.2.2 Mix proportion design for high thermal conductivity

2.2.2.1 Mix proportion for steel fibers in steel fiber enhanced concrete

Six groups of the steel fiber enhanced concrete (SFC) with different volume fractions of the steel fiber (Table 11) were designed to investigate the influence of varying volume fractions of the steel fiber on the compressive strength of the SFC.

In this paper, the compressive strength of the SFC was obtained by changing the volume ratio of steel fiber in concrete.

Concrete cube compressive strength conversion formula:F----- Ultimate load(N);A----- Compression area(mm²);0.95----- Size effect coefficient.

2.2.2.2 Mix proportion of steel fiber/graphite concrete with different volume fraction

Based on the test results of SFC, six groups of the steel fiber/graphite concrete (SFGC) with different steel fiber/graphite volume fraction was designed by adding three different graphite powders on the basis of 1.5% and 2.5% steel fiber volume fraction (Table 12).

Graphite was doped according to the internal adding law (Jia et al., 2010).----- Graphite amount;----- Cement amount;----- Percentage of graphite doping.

2.3 Experimental setups and fabrication of the test specimens

2.3.1 Experimental setups

2.3.1.1 Concrete mixer

HJW-60 single-horizontal axle forced concrete mixer, manufactured in China, was mainly used for concrete mixing. The equipment parameters were shown in Table13 and the picture was shown in Figure 2A.

FIGURE 2

2.3.1.2 HZJ-1 Concrete vibration table

The table surface of the concrete shaking table was welded on the base frame, and the center of the table was fixed with the shaker and installed with the hanger coupling. The main parameters of the equipment were shown in Table 14 and the picture was shown in Figure 2B.

2.3.1.3 Strength testing equipment

YAS series servo press was used. The main machine of YAS series (four-pillar) servo press was a moving beam structure with adjustable compression space. The pressing plate was made into a trolley which could move along the guide rail and was convenient for loading and unloading large blocks. The main technical indexes were shown in Table 15 and the picture was shown in Figure 2C.

2.3.1.4 Thermal conductivity testing equipment

Hot Disk TPS2500S thermal parameter analyzer was used, which was manufactured by Hot Disk, Sweden. The equipment consisted of test host, probe and sample fixture, which measured the thermal parameters (thermal conductivity, specific heat capacity, thermal diffusivity) of solids, powders and other materials under different temperature and humidity conditions. The main technical parameters were shown in Table 16, and the picture was shown in Figure 2D.

2.3.2 Fabrication of the test block

According to the Chinese Standard (GB/T 50080-2016) (Standard for test method, 2016), the cubic concrete specimens with the dimensions of 100 mm × 100 mm × 100 mm were fabricated, as shown in Figure 3A.

FIGURE 3

The cylinder concrete blocks for thermal conductivity testing: A cylinder with a diameter of 5 cm and a height of 2.5 cm was required by this test instrument.

The cubic concrete blocks were placed into the core machine.

The cylindrical specimens with adiameter of 5 cm were drilled out of the concrete plates with a thickness of 10 cm.

Put the cylinder into the cutting machine and cut it into a cylinder 5 cm in diameter and 3 cm in height.

Put them into the grinder and ground them into a cylinder with a diameter of 5 cm and a height of 2.5cm, as shown in Figure 3B.

2.4 Test method

YAS-5000 servo press was used to test the compressive strength of the cubic concrete blocks, as shown in Figure 4A. Before the test, the press plate of the experiment machine needed to be raised through the operating handle, and then the cubic concrete blocks were put the at center of the pressure plate, and finally lowered the pressure plate to full and uniform contact with the cubic concrete blocks. The loading mode adopted displacement controls. First, a force of 5 kN was set to preload the cubic concrete blocks, so that the pressure surface of the experiment machine was in full contact with the surface of the cubic concrete blocks, and then adjusted the loading displacement rate that it was 0.005 mm/s. The data acquisition system interface was shown in Figure 4B. After the cubic concrete blocks were broken, the test machine would automatically stop and automatically record the load value at the failure moment.

FIGURE 4

Hot Disk TPS2500S thermal parameter analyzer was used to test the thermal conductivity of the cylinder concrete blocks, which was based on the transient plate heat source method, easy to operate, high test accuracy and fast time. The instrument was produced by Sweden’s Hot Disk and consisted of a test host, a probe and a sample fixing device, as shown in Figure 5.

FIGURE 5

3 Test results and discussion

3.1 The failure behaviors of the specimens

The failure behaviors of different concrete blocks were showed in Figure 6. The blocks mixed with steel fiber or steel fiber/graphite, had a long stress time. It was concluded that the compressive strength could be improved by adding steel fiber or steel fiber/graphite. When the compressive load exceeded the ultimate value, the ordinary concrete (OC) blocks were completely crushed as soon as cracks occurred. However, the OC blocks doped with steel fiber or steel fiber/graphite could delay the fracture process. When the compressive load exceeded the ultimate strength, multiple cracks nucleate simultaneously, propagate with load increasing, and crash in brittle manner. The results indicated that adding steel fiber material could not only improve the compressive strength of concrete, but also delay the failure time of the concrete. On the one hand, the SFC and SFGC are limited by the tension of steel fibers compared with OC. On the other hand, with the addition of steel fiber material, the overall density of concrete increases, and the overall filling ability of concrete is satisfied, thus improving its compressive strength.

FIGURE 6

3.2 Effect of steel fiber on the compressive strength of the concrete

Table 17 shows the measured compressive strengths of the concrete specimens with different volume fractions of steel fibers.

As shown in Table 17, with the increase in steel fiber volume fraction, the strength of the SFC increased and then decreased. When the volume fraction of steel fiber was 1.5%, the strength of the test specimens reached the maximum value of around 32% higher than that of the OC (specimens: SJ-0). Although the strengths of the specimens decrease with the increase of the volume fraction of steel fibers from 2.0% to 2.5%, they are higher than that of OC.

Compared with the OC specimens, the transverse expansions in the SFC specimens under the compression are limited by the traction of the steel fiber. With the increase of the volume fraction of the steel fiber, the SFC specimens show multiple cracks after a certain compressive load. When the volume fraction of steel fiber exceeds 1.5%, the cement cannot completely wrap the steel fiber and aggregate and tend to reduce the bond force between the steel fiber and the matrix. Meanwhile, with the increase of the steel fiber volume fraction, steel fibers became more readily to overlap with each other and agglomerate, hindering the reinforcing effect of steel fiber. This phenomenon can explain why the compressive strength of the SFC decrease gradually.

3.3 Effect of steel fiber on the thermal conductivity of the concrete

Hot Disk TPS-2500S thermal parameter analyzer was used to test the thermal conductivity of the specimens. Three specimens of each group were fabricated to test the thermal conductivity in the same environment. The results showed that the measured values of the thermal conductivity with the same volume fraction of the steel fibers remained constant. The average value of three specimens was taken as the thermal conductivity of the SFC. The trend of the effective thermal conductivity and strength of the SFC with the volume fraction of the steel fiber was shown in Figure 7. Thermal conductivity test values of the SFC were shown in Table 18.

FIGURE 7

As shown in Figure 7, the thermal conductivity of the SFC increased gradually with the increase of the steel fiber volume fractions. The fact is that steel fibers are liable to form heat conduction connections. Meanwhile, the compressive strength with a 1.5% the volume fraction of the steel fiber was higher than the concrete preparation strength. The optimum volume fraction of the added steel fiber was decided to be 1.5% considering the balance between the thermal conduction coefficient and compressive strength of the SFC. In this case, the strength and thermal conductivity of the SFC specimens are about 31.8% and 8.3% higher than that of the OC specimens, respectively.

3.4 Effect of steel fiber/graphite on compressive strength of the concrete

Based on the previous results, the specimen with steel fiber volume fraction at 1.5% was used as a reference. The graphite powder with volume fractions at 5%, 10% and 15% were added to the basic specimens, respectively. The effect of steel fiber/graphite on the compressive strength of the concrete was listed in Table 19.

From Table 19, it is noted that the compressive strengths of the SFGC decrease with the increase of graphite volume fraction. The compressive strength is 48.15 MPa for the specimen with 5% graphite.

Compared with 1.5% of the steel fiber admixture fraction of the SFC, it was reduced by 19.2%, but reached the concrete preparation strength of the C40 grand and met the application requirements of the project.

When the graphite mixture fraction exceeded 5%, the compressive strength of the SFGC could not reach the concrete preparation strength of the C40 grand, especially the graphite mixture fraction was 10%, and 15%, the values of the compressive strength were 39.87 and 30.54 MPa respectively.

However, when the graphite volume fraction exceeded 5%, the compressive strength of the SFGC could not meet the service requirements, so the optimal volume fraction of the graphite was 5%.

Firstly, the graphite material had excellent lubricity, and its friction coefficient was small. When the graphite volume fraction became larger, it increased the contact area between the graphite particles and decreased the friction resistance, and thus reduced the compressive strength of the blocks. Secondly, due to the hydrophilic difference between the graphite powder and the cement material, when both of them were mixed, the interfacial bond force was weak with the increase of graphite volume fraction, the interfacial bond force decreased, which caused the compressive strength to decrease.

3.5 Effect of steel fiber/graphite on thermal conductivity of the concrete

The thermal conductivity of three SFGC specimens were tested under identical condition. The average thermal conductivity of the three SFGC specimens are shown in Table 20.

According to Figure 8, the thermal conductivity and compressive strength of the SFGC, with the volume fraction of steel fiber of 1.5%, had an opposite trend with the graphite volume fraction. The effective thermal conductivities of the SFGC increase with the increase of the graphite volume fraction, while their compressive strengths decrease. When the graphite volume fraction increases from 5% to 15%, the thermal conductivity of the SFGC specimen rises from 36.4% to 50.2%.

FIGURE 8

The graphite used in the SFGC was flake graphite powders with high thermal conductivity. When they are mixed with the cement and hydrated to form concrete, they will fill the pores of concrete due to the small particle size, and improve the bondage between the aggregate and the cement paste. This will improve the thermal conductivity of the SFGC (Guo et al., 2017). Moreover, as the graphite volume fraction increases, the heat conducting particles form an interconnected heat conducting network in the concrete which reduces the thermal resistance of the blocks and improves the thermal conductivity (Zheng, 2020). The experiments showed the thermal conductivity of the concrete can be significantly by mixing the graphite with the concrete.

4 Conclusion

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