Combined effects of sulfate attack under drying–wetting cycles and loading on the fatigue behavior of concrete

Abstract

Concrete structures often undergo both fatigue loading and environmental impacts during their useful lifetime. This study aims to explore the fatigue properties of concrete subjected to sulfate attacks under drying–wetting cycles and loading. The coupled influences of major cycle number and sodium sulfate solution on the residual deformation, elastic modulus, and damage variable were investigated by uniaxial cyclic loading tests. Moreover, the phase composition of concrete samples was examined by X-ray diffraction. Results indicate that the concrete residual deformation and damage variable could be classified into initial and stable stages, while the elastic modulus fluctuated within a certain range. The fatigue strength of concrete was found to increase with an increase in the major cycle number and sodium sulfate concentration in the early stages, whereas the fatigue performance of concrete decreased as the major cycle number and sodium sulfate concentration increased in the later stage. The degree of influence of major cycle number and sodium sulfate concentration on the fatigue properties of concrete differed in each stage. These findings can contribute to understand the variation pattern of concrete properties in complicated environments and provide an important reference for associated construction projects.

Keywords

concrete drying–wetting cycles sulfate attack cyclic loading fatigue properties

Introduction

Concrete is the most extensively studied material in engineering structures, which has been widely used in the bridge engineering (Gagg et al., 2014). A large number of concrete bridge projects have been built in the eastern coastal areas and western saline soil areas of China. However, the soil and groundwater contain sulfate, a large number of concrete bridge facilities, including bridge pier and bridge decks, which are suffered from severe sulfate attack. Meanwhile, the concrete bridge structures are subjected to both drying–wetting cycles and sulfate attack due to water level changes or water splash action. In actual engineering, concrete bridge concrete structures are not only subjected to both drying–wetting cycles and sulfate attack, but also to fatigue loading, such as vehicle loads on bridges. The internal structure of concrete will be damaged under the action of drying–wetting cycles, sulfate attack, and loading (Tang et al., 2015; Pei et al., 2020). Therefore, it is necessary to investigate the fatigue characteristics of concrete under complicated conditions, to understand the failure mechanism of concrete better and evaluate the useful life of concrete structures exactly.

In the past decades, key parameters influencing the fatigue properties of concrete have been extensively investigated, such as loading level, wave form, frequency, amplitude, and confining pressure. Taliercio and Gobbit (1996) studied the effect of confining pressure and amplitude on the fatigue life of concrete under triaxial compression. It was concluded that the fatigue life of concrete increased with decreased amplitude but confining pressure showed a reversely trend. Zhang et al. (1996) performed a series of fatigue tests on concrete and derived a prediction method for fatigue life based on frequency. Sparks and Menzies (1973) pointed out that increasing the loading rate can enhance the fatigue strength of concrete based on cyclic compression. Chen et al. (2015) conducted an experimental study of concrete fatigue life subjected to cyclic loading with different frequency and stress level. They found that fatigue life increased with loading frequency but decreased with stress level. Moreover, a new damage model considering the loading frequency and stress level was proposed. Oneschkow (2016) studied the influence of the loading level, frequency, and waveform on the fatigue behavior of high-strength concrete. Fan et al. (2018) analyzed the fatigue performance of concrete subjected to continuous and discontinuous loading and found that the discontinuous loading of concrete can promote plastic deformation. Song et al. (2020) investigated the mechanical property of concrete subjected to monotonic and cyclic loading and showed that two loading manners of damage patterns, namely, lateral strain and energy dissipation, can lead to different fatigue behavior.

In addition, the relationship between concrete energy and fatigue loading was investigated. Tepfers et al. (1984) tested the energy change in concrete under cyclic and static loadings and found that the absorbed energy at failure under cyclic loading was similar to that under static loading. Lei et al. (2017) obtained the energy dissipation variation of concrete under cyclic loading, and a new method of concrete fatigue life prediction based on the energy dissipation was proposed. Song et al. (2018a) studied the hysteresis characteristics and dissipated energy and proposed important indicators, such as hysteresis occurrence ratio and hysteresis energy ratio under cyclic loading. Song et al. (2018b) suggested that the cumulative speed of energy dissipation is linked to the cyclic loading level.

Lam et al. (2012) conducted experiments on concrete subjected to cyclic loading at elevated temperatures and stated that the variation in the unloading strain ratio was sensitive to temperature change. Sinaie et al. (2016) performed cyclic loading tests on concrete at high temperatures to determine the relationship between concrete damage and temperature. Combrinck and Boshoff (2019) analyzed the influence of temperature and cyclic loading on the tensile properties of plastic concrete. Their results showed that properly raising the temperature could improve the tensile strength and Young’s modulus. Zhou et al. (2019) carried out the triaxial cyclic loading tests under different temperatures and showed that the residual strain and secant modulus of concrete increased with temperature. Furthermore, Bassuoni and Nehdi (2009) analyzed the combined effects of concrete subjected to external sulfate attack and flexural loading. You et al. (2019) conducted long-term cyclic loading experiments and wetting–drying cyclic experiments with 5% sodium sulfate solution and explored the fatigue performance and sulfate resistance of concrete. According to previous research, the aforementioned investigations mainly focus on the influences of loading conditions, energy, and temperature, as well as simple combination effects, on concrete fatigue performance. However, the fatigue behavior of concrete under the coupled effects of drying–wetting cycles and sulfate attack with various concentrations remains unclear.

In this study, fatigue tests were conducted on concrete samples with different drying–wetting cycles and sodium sulfate concentrations. The residual deformation, elastic modulus, and damage variable of concrete were comprehensively analyzed. Furthermore, the corrosion products of concrete were analyzed using XRD. The corrosion mechanism of concrete subjected to combined cyclic loading under drying–wetting cycles and a sulfate attack was investigated.

Experimental procedure

Experimental equipment

Fatigue tests were performed on the triaxial solid–liquid coupling test system, which consists of a loading frame, an axial loading system, and a data acquisition system. The data acquisition system is composed of signal conditioning and an acquisition unit, which is interfaced with a computer. The system has a maximum loading capacity of 400 kN and a maximum loading velocity of 100 mm/min. It can perform two load patterns and includes load and displacement control modes.

Materials and sample preparation

A Chinese-standard P.O 42.5 ordinary Portland cement produced by the Yunnan Ziyan Cement Co. Ltd was adopted, and the chemical compositions and physical properties of the cement are shown in Table 1 and Table 2, respectively. The fine aggregate was river sand with a fineness modulus of 2.9, and the coarse aggregate was gavel with a diameter of 5–40 mm. Chinese standard grade-II fly ash was obtained from the Pannan Power Station in Guizhou. A polycarboxylate superplasticizer with a water-reducing rate of 25% was used as the admixture. The concrete mix proportion is listed in Table 3. The water-to-binder ratio of the concrete was 0.36, and the slump of the concrete was 140–180 mm.

Table 1.

Chemical compositions of the cement.

Chemical compositions	CaO	SiO₂	Al₂O₃	MgO	SO₃	Fe₂O₃	Na₂O	K₂O	Loss
Content (%)	62.8	20.4	5.14	1.31	2.48	4.40	0.42	0.28	2.39

Table 2.

Physical properties of the cement.

Standard consistency water consumption (%)	Fineness (%)	Setting time (min)		Flexural strength (MPa)		Compressive strength (MPa)
Standard consistency water consumption (%)	Fineness (%)	Initial	Final	3d	28d	3d	28d
25.9	4.2	172	231	4.8	7.4	29.1	52.2

Table 3.

Concrete mix proportion.

Materials	Cement	Sand	Gravel	Water	Fly ash	Admixture
Content (kg/m³)	355	746	1030	160	89	4.44

The casting and curing conditions of the concrete mixtures comply with the standard requirements (GB/T50081-2002). The concrete was cast in cubic molds (150, 150, 150 mm), and the surfaces of the concrete cubes were covered with a water-impermeable film. 1 day later, the concrete cubes were demolded and cured in a curing room under standard conditions (20 ± 2°C and ≥95% RH). After curing for 28 days, a high-speed coring machine was used to drill the cylindrical samples. In accordance with the ISRM testing procedure (Fairhurst and Hudson, 1999), the concrete samples were cut into cylindrical blocks of 50-mm diameter and 100-mm length. The samples were equally divided into four groups, and three samples were tested for each condition, as shown in Figure 1.

Figure 1.

The processed concrete samples.

Experimental methodology

Solution preparation

In a natural environment, the deterioration of concrete structures subjected to sulfate attack is a slow, long-term process. It is known that groundwater includes sodium, calcium, magnesium, chloride, and sulfate ions. In this study, to analyze the damage of concrete attacked by a sulfate solution, 0, 5, 10, and 15% (by mass) sodium sulfate solutions were prepared; they were poured into four containers, and each container contained three samples.

Cyclic loading tests

These tests involved cyclic loading with a velocity of 0.1 mm/min. The average uniaxial compressive strength (UCS) of concrete specimens was measured to be 50 MPa, from three processed specimens. The upper and lower limits of the cyclic stress are $σ \max$ = 30 MPa (which is 60% of UCS) and $σ \min$ = 15 MPa (which is 30% of UCS), respectively. Concrete specimens were cyclically loaded up to 120 cycles. The loading path is shown in Figure 2.

Figure 2.

The cyclic loading path curve.

Drying–wetting cycles

In order to obtain the corrosion state similar to the natural environment from a laboratory perspective in shorter time and shorten the experimental period, the accelerated deterioration experiments of concrete were designed. The accelerated deterioration process is shown in Figure 3. After cyclic loading, the concrete samples were immersed in sodium sulfate solutions for 16 h, according to the standard for test methods of long-term performance and durability of ordinary concrete (GB/T 50082–2009). After soaking, they were dried at a temperature of 80°C for 7 h in a drying box. After drying, the concrete samples were cooled at a temperature of 20°C for 1 h. The aforementioned process is referred to as the drying–wetting cycle (1 day). Moreover, a cyclic loading and five drying–wetting cycles were defined as a major cycle (Figure 3), which was repeated 12 times. To maintain the concentration of sodium sulfate solution, it was changed every 30 days. Meanwhile, each container of sodium sulfate solution was equipped with a lid.

Figure 3.

The major cycle flow chart.

X-ray diffraction (XRD)

To evaluate changes in the composition of concrete, crystalline compounds of concrete with different sodium sulfate concentrations under various major cycle numbers were examined using an X-ray diffraction test. After the major cycle was complete, the concrete specimens were crushed by crusher, and the concrete fragments were ground into particles by ball mill, and particles of 200 mesh were selected for testing.

Experimental results

Characteristics of axial stress–strain curve under cyclic loading

The axial stress–strain curves obtained from the uniaxial cyclic compression tests are shown in Figure 4. It is observed that the concrete fatigue test can be divided into the static loading stage (ab) and the fatigue stage (bc). The axial stress increases from 0 to the upper limit of the cyclic stress in the static loading stage, and the stress–strain curve corresponds to an approximately straight line. This is because of the compression of microcracks during the initial stage of cyclic loading. The fatigue stage is observed under repeated cyclic loading with constant upper and lower stresses. The hysteresis curves were found to be sparsely distributed at the beginning of cyclic loading, and they became denser with an increase in the cycle number. A “sparse–dense–sparse” phenomenon could be observed in the fatigue process.

Figure 4.

Axial stress–strain curve under cyclic loading.

Variation in residual deformation

Generally, rock deformation can be divided into reversible and irreversible deformation under cyclic loading. Reversible deformation is caused by rock elastic properties, while irreversible deformation, also known as the residual deformation, is mainly induced by dislocation sliding and microcrack initiation and propagation (Chen et al., 2015). Residual deformation is the primary cause of fatigue damage and the best way to describe fatigue failure (Xiao et al., 2010). The residual deformation is the difference between the strain at point e and point d in Figure 4.

Figures 5(a), (b), and (c) show the residual deformation–loading cycle number relations of concrete at 2nd, 6th, and 12th major cycles, respectively. The residual deformation exhibits two distinct stages: initial deformation and stable deformation. During the initial deformation stage, the residual deformation increased rapidly due to the compression of microcracks, which accounts for 12–20% of the total cycle number. In the stable deformation stage, which takes up the majority of the cycle, the crack is in a stable growth phase and residual deformation shows a slow-growth trend. The specimens will be not crushed during cyclic loading, so residual deformation of concrete is lack of the acceleration stage. Nevertheless, the residual deformation follows an inverted S-shape proposed by Xiao et al. (2009).

Figure 5.

The relationship between residual deformation and loading cycle number at 0, 5, 10, and 15% sodium sulfate solution: (a) second major cycle, (b) sixth major cycle, (c) 12th major cycle.

As the stable deformation stage occupies the majority of the entire cycle, the maximum value of the stable deformation stage has been selected as a standard value. In the 2nd major cycle, the residual deformation of concrete decreased with increasing sodium sulfate concentration and was reduced by 46, 25, and 29%, respectively (Figure 5(a)). The residual deformation appears to be irregular with different sodium sulfate concentration in the sixth major cycle; this may be due to the physicochemical reaction between sodium sulfate and concrete (Figure 5(b)). However, the residual deformation of concrete increased by 41, 18, and 41%, respectively, with the increase in sulfate solution concentration in the 12th major cycle (Figure 5(c)).

Similarly, the maximum value of each major cycle could be defined as a standard value, which is illustrated in Figure 6. The residual deformation of concrete first decreased and then increased with increasing major cycle number, and the sixth major cycle becomes the inflection point. The concentration of sodium sulfate solution has a greater impact on residual deformation, sorted in the descending order as 15, 10, 5, and 0%, respectively.

Figure 6.

The relationship between residual deformation and major cycle number at 0, 5, 10, and 15% sodium sulfate solution.

Variation in elastic modulus

The elastic modulus reflects the relationship between stress and strain and contributes to the understanding of concrete deformation. In this work, elastic modulus could be represented by the secant modulus.

Figure 7 depicts the relationship between residual deformation and major cycle numbers at different sodium sulfate concentration. From a global perspective, the elastic modulus fluctuated within the range of 0–0.6 GPa. The elastic modulus of concrete increases slowly with the loading cycle number at 2nd, 6th, and 12th major cycles. In the initial loading stage, the elastic modulus of concrete increases rapidly because of pore compression and microcrack closure; thus, concrete enters the linear elastic phase (Yang et al., 2017).

Figure 7.

Elastic modulus-loading cycle number curves at 0, 5, 10, and 15% sodium sulfate solution: (a) second major cycle, (b) sixth major cycle, (c) 12th major cycle.

The elastic modulus of concrete increased with increasing sodium sulfate concentration during the second and sixth major cycles. At the second major cycle, the elastic modulus at the initial cycle stage increased by 0.96, 0.85, and 0.48 GPa, respectively (Figure 7(a)). Meanwhile, the value of the sixth major cycle increased by 0.92, 0.46, and 0.54 GPa, respectively (Figure 7(b)). However, a significant decrement in the elastic modulus of concrete was observed when the sodium sulfate concentration increased at the 12th major cycle, and it decreased by 0.61, 0.70, and 0.89 GPa, respectively (Figure 7(c)).

We choose the elastic modulus of the first cycle at each major cycle, as shown in Figure 8. With the increase in major cycle number, the elastic modulus first increased and then decreased sharply. The influence of 0% sodium sulfate solution on elastic modulus is the smallest, while the influence of 15% sodium sulfate solution on elastic modulus is the highest. The reason for the above results could be as follows: In the initial period, debris fills cracks, enhancing the elasticity modulus of concrete. However, as the major cycle number increases, the plastic accumulation of concrete reduces elastic modulus.

Figure 8.

Elastic modulus-major cycle number curves at 0, 5, 10, and 15% sodium sulfate solution.

Variation in damage variable

Damage is an irreversible degradation process under environmental erosion of external loads. From the perspective of damage mechanics, the concept of continuum damage mechanics was proposed by Lemaitre (Guo et al., 2012) Based on the theory of strain equivalence hypothesis, the damage constitutive equation can be expressed as

σ = E (1-D) ε

(1)

where

σ

is the stress of the undamaged material,

ε

is the strain of the undamaged material,

E

is the elastic modulus, and

D

is the damage variable.

Xiao et al. (2010) analyzed the advantages and disadvantages of six damage variables, which mainly include elastic modulus, energy dissipation, maximum strain, residual deformation, ultrasonic wave velocity, and acoustic emission cumulative counts. The results indicated that the residual deformation could be a better option to reflect damage due to the initial damage. In this study, based on elastic modulus and residual strain, the damage equation of elastic plastic materials under one-dimensional conditions is given by the following equation

D = 1 - \frac{ε - ε i}{ε} \frac{E i}{E}

(2)

where

E i

is the unloading elastic modulus for the loading cycle

i

E

is the initial elastic modulus,

ε

is the axial strain,

ε i

is the unloading residual deformation. A similar expression as equation (2) has been proposed by Munoz and Taheri (2017).

Figures 9(a), (b), and (c) illustrate the damage variable-loading cycle number relationship at 2nd, 6th, 12th major cycles, respectively. The damage variable of concrete can be classified as the initial damage and stable damage. In the initial damage stage, it accumulates large deformation and accounts for 30–40% of the total damage variable. However, the change rate of the damage variable becomes comparatively stable during the stable damage stage, and this stage presents a slow increase trend.

Figure 9.

Damage variable evolution curves at 0, 5, 10, and 15% sodium sulfate solution: (a) second major cycle, (b) sixth major cycle, (c) 12th major cycle.

At the second major cycle, the damage variable of concrete decreases with increased sodium sulfate concentration, which has been reduced by 43, 34, and 23%, respectively (Figure 9(a)). The damage variable of concrete is almost similar to the trend observed for the residual deformation at the sixth major cycle (Figure 9(b)). However, the damage variable was enhanced with the increase in the sodium sulfate concentration at the 12th major cycle, and the rate of increase was 46, 27, and 27%, respectively (Figure 9(c)).

The damage variable of the stable stage in each major cycle is presented in Figure 10, which shows that the damage variable first decreases and then increases. Sodium sulfate plays an inhibiting effect on the concrete damage from the first major cycle to the sixth major cycle. Inversely, sodium sulfate accelerates the damage of concrete after the sixth major cycle (Chen et al., 2016). In addition, the damage variable and residual deformation are strongly linked.

Figure 10.

Damage variable-major cycle number curves at 0, 5, 10, and 15% sodium sulfate solution.

Discussion

X-ray diffraction characterization

Figure 11 depicts the XRD patterns of concrete with different sodium sulfate concentrations under various major cycle numbers. According to Figure 11, crystalline compounds of concrete specimens mainly include calcium hydroxide (CH), quartz (Q), gypsum (G), ettringite (E), and calcium carbonate (CC). The CH peak for concrete specimens subjected to the sulfate attack were evidently lower than those for concrete specimens soaked in 0% sodium sulfate solution. At the 2nd, 6th, and 12th major cycles, the CH peak decreased with increasing sodium sulfate concentration, whereas diffraction peaks of Q and G increased with the sodium sulfate concentration. The comprehensive diffraction peaks of Q and G increased with major cycle numbers at the same concentration.

Figure 11.

The XRD patterns of the concrete specimens at 0, 5, 10, and 15% sodium sulfate solution: (a) second major cycle, (b) sixth major cycle, (c) 12th major cycle.

Corrosion mechanism of concrete under coupled effects of major cycle and sulfate attack

Concrete corrosion is a result of physical and chemical changes upon cyclic loading under drying–wetting cycles and sulfate attack (Haynes et al., 2008; Chen et al., 2017). The Portland cement mainly consists of 3CaO.SiO₂, 2CaO.SiO₂, 3CaO.Al₂O₃, and 4CaO.Al₂O₃.Fe₂O₃. The hydration products were generated by hydration reaction, which includes Ca(OH)₂, 3CaO.2SiO₂.3H₂0, 3CaO.Al₂O₃.6H₂0, and CaO.Fe₂O₃.H₂O (Geng et al., 2015). The corresponding chemical equations can be expressed as

2 (3 CaO .SiO 2) + 6 H 20 \to 3 CaO . 2 SiO 2.3 H 20 + 3 Ca (OH) 2

(3)

2 (2 CaO .SiO 2) + 4 H 20 \to 3 CaO . 2 SiO 2.3 H 20 + Ca (OH) 2

(4)

3 CaO .Al 2 O 3 + 6 H 20 \to 3 CaO .Al 2 O 3.6 H 20

(5)

4 CaO .Al 2 O 3 .Fe 2 O 3 + 7 H 20 \to 3 CaO .Al 2 O 3.6 H 20 + CaO .Fe 2 O 3 .H 20

(6)

A substantial amount of sulfate could diffuse into concrete and react with hydration products, generating gypsum (CaSO₄.2H₂0) and ettringite (3CaO.Al₂O₃.CaSO₄.32H₂0) (Jin et al., 2007). The reaction equations can be written as

Na 2 SO 4 + Ca (OH) 2 + 2 H 20 \to CaSO 4.2 H 20 + 2 NaOH

(7)

3 (CaSO 4.2 H 20) + 3 CaO .Al 2 O 3.6 H 20 + 20 H 20 \to 3 CaO .Al 2 O 3.3 CaSO 4.32 H 20

(8)

At the beginning of drying–wetting cycles, gypsum and ettringite were generated by the reaction of sulfate with hydration products. However, the unreacted sulfate could produce crystallites of sodium sulfate when the concrete dried. As a result, a small number of expansive substances (such as gypsum, ettringite, and the crystallites of sodium sulfate) could fill the inner pores of concrete, decreasing the porosity while improving strength (Ouyang et al., 2014). Therefore, these expansive substances enhance the performance of concrete in the early stage of drying–wetting cycles.

During the drying–wetting cycles, the temperature gradient of concrete varies considerably, which can accelerate the diffusion of sulfate into the concrete interior (Gao et al., 2013). As shown in Figure 11, quartz and gypsum content increased with the major cycle number. With an increased number of drying–wetting cycles, superabundant substances are generated in concrete. When the concrete interior has no space to hold the redundant substances, the expansive force could destroy the microstructure, forming new cracks. Subsequently, it could degrade the physical and mechanical properties of concrete (Chen et al., 2008). Meanwhile, the bonding force of concrete between cement and aggregate has been significantly reduced under cyclic loading. Microcracks in concrete quickly expand, improving the permeability of concrete and enlarging the transmission channel (Yu et al., 2016). Therefore, it can provide favorable conditions for sulfate diffusion.

In summary, the mechanism of concrete under major cycle and sulfate attack was indicated well by variations in crystalline compounds obtained by X-ray diffraction; these can well explain the variations in the fatigue behavior of concrete.

Coupled effect of concrete fatigue properties under major cycle and sulfate attack

To investigate the superposition effect of concrete between major cycle number and sodium sulfate concentration, the concrete residual deformation, elastic modulus, and damage variable are fitted to obtain Figures 12(a), (b), and (c), respectively.

Figure 12.

Variation in the fatigue properties of concrete with different sodium sulfate concentration under various major cycle numbers: (a) residual deformation, (b) elastic modulus, (c) damage variable.

As can be seen from Figures 12(a), (b), and (c), as the major cycle number increases, the concrete residual deformation and damage variables slowly decrease first and then increase sharply, but the elastic modulus follows an opposite trend. In terms of sodium sulfate concentration, the residual deformation and damage variable decrease with increasing concentration of sodium sulfate solution in the initial stage of a major cycle, while it shows the reverse trend in the late stage of a major cycle.

Based on the above conclusions, the fatigue performance of concrete could be affected by the major cycle number and sodium sulfate concentration. Therefore, three areas are distinctly observed. (1) Area I (0–fifth major cycle, 0–15% sodium sulfate solution), the larger the major cycle number and sodium sulfate concentration, the greater the fatigue strength of concrete. In this region, the effects of the major cycle number and sodium sulfate concentration on improving the fatigue performance of concrete become similar. (2) Area II (5–ninth major cycle, 0–15% sodium sulfate solution), where the influence of sodium sulfate concentration on the fatigue performance of concrete is less obvious, although the major cycle number has a significant impact on reducing the fatigue performance of concrete. (3) Area III (9–12th major cycle, 0–15% sodium sulfate solution), where fatigue performance of concrete decreases considerably with the major cycle number and sodium sulfate concentration. Meanwhile, the effect of the major cycle number on the fatigue performance of concrete was slightly greater than the sodium sulfate concentration.

Results

In this study, the coupled effects of drying–wetting cycles and sulfate attack on concrete fatigue properties are investigated. The fatigue performance of concrete is analyzed from the aspects of residual deformation, elastic modulus, and damage variable. The following conclusions are obtained:

During the cyclic loading, both the concrete residual deformation and damage variable exhibit two distinct stages: initial phase and stable phase. There is no obvious partition of the elastic modulus of concrete, which fluctuated within the range of 0–0.6 GPa.

As the major cycle number increases from 0 to 12, both the concrete residual deformation and damage variable were first increased and then decreased, but the elastic modulus showed an inverse trend.

The fatigue performance of concrete increases with sodium sulfate concentration from the first major cycle to the sixth major cycle, while the fatigue performance of concrete decreases with increasing sodium sulfate concentration from the sixth major cycle to the 12th major cycle.

Three regions are notable as concrete is subjected to cyclic loading under drying–wetting cycles and sulfate attack. In the first region, the effects of the major cycle number and sodium sulfate concentration on enhancing the fatigue performance of concrete are similar. The major cycle number plays a dominant role in reducing the fatigue performance of concrete in the second region. The effect of the major cycle number on decreasing the fatigue performance of concrete was slightly greater than the sodium sulfate concentration in the third region.

Based on those results, it is suggested that the concrete can be coated with the acid-resistant and anticorrosive coatings in order to prevent physical–chemical reactions between concrete and the environment. Moreover, the mineral admixture can be added to fill the concrete pores and make the concrete structure more compact.

Footnotes

Acknowledgments

We would like to thank Editage () for English language editing.

Declaration of Conflicting Interests

The author(s) declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

Funding

The author(s) disclosed receipt of the following financial support for the research, authorship, and/or publication of this article: This work was financially supported by the National Natural Science Foundation of China (grant numbers 51774057, 52074048), Research Foundation of Chongqing University of Science and Technology (grant number CK181901004).

ORCID iDs

Song Ren

Ping Zhang

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, et al. (2019) Mechanical characteristics of well cement under cyclic loading and its influence on the integrity of shale gas wellbores. Fuel 250: 132–143.