External sulfate attack on concrete under combined effects of flexural fatigue loading and drying-wetting cycles

Construction and Building Materials 249 (2020) 118224

Contents lists available at ScienceDirect

Construction and Building Materials journal homepage: www.elsevier.com/locate/conbuildmat

External sulfate attack on concrete under combined effects of flexural fatigue loading and drying-wetting cycles Fang Liu a,b,⇑, Zhanping You c, Aboelkasim Diab d, Zhuangzhuang Liu b, Chao Zhang f, Shuaicheng Guo e,f,⇑ a

Faculty of Transportation Engineering, Huaiyin Institute of Technology, Huai’an, Jiangsu 223003, China School of Highway, Chang’an University, South Erhuan Middle Section, Xi’an, Shaanxi 710064, China c Department of Civil and Environmental Engineering, Michigan Technological University, 1400 Townsend Drive, Houghton, MI 49931, USA d Department of Civil Engineering, Aswan University, 81542, Egypt e Key Laboratory for Green & Advanced Civil Engineering Materials and Application Technology of Hunan Province, College of Civil Engineering, Hunan University, Changsha 410082, China f Key Laboratory of Building Safety and Energy Efficiency of the Ministry of Education, Hunan University, Changsha 410082, China b

h i g h l i g h t s
Sulfate attack on concrete under flexural fatigue loading and drying-wetting cycles simultaneously was investigated.
Flexural fatigue loading incorporated with drying-wetting cycles can accelerate the sulfate corrosion of concrete.
The stress level of flexural fatigue loading acts an important role in combined actions.
The phase composition of corrosion products was analyzed using XRD.

a r t i c l e

i n f o

Article history: Received 19 July 2019 Received in revised form 17 December 2019 Accepted 18 January 2020

Keywords: Sulfate attack Flexural fatigue loading Drying-wetting cycles Relative dynamic elastic modulus Mass loss rate

a b s t r a c t The concrete transportation infrastructures will undergo both fatigue traffic loading and environmental impacts during the whole service life, including rigid pavements, bridges decks, airfield runways, railway bridges, even high-speed railways and concrete structures in the ocean. This study aims to investigate the effects of external sulfate attack on concrete under flexural fatigue loading and drying-wetting cycles. The changes of mass loss rate and relative dynamic elastic modulus were measured to indicate the influence of the coupled effects on the integrity and mechanical performance of concrete, also the sulfate content inside concrete was determined to indicate the permeability of sulfate ion under different experimental conditions. Moreover, the phase composition of samples was analyzed using X-ray diffraction (XRD). In addition, the effect of fly ash on sulfate attack was duly studied. Fatigue loading causes cracking in the interfaces of various phases and forms cracks in the concrete. Within the influence depth of drying-wetting cycles, concrete is subjected to both water convection due to capillary action and diffusion due to concentration gradients. The results indicate the fatigue loading and drying-wetting cycles can accelerate the transportation of sulfate ion inside concrete and the deterioration degree of concrete subjected to sulfate. The findings in this study can help to understand the influence of fatigue loading and drying-wetting cycles on the development of sulfate attack. Ó 2020 Published by Elsevier Ltd.

1. Introduction

⇑ Corresponding authors at: Faculty of Transportation Engineering, Huaiyin Institute of Technology, Huai’an, Jiangsu 223003, China (F. Liu), Key Laboratory for Green & Advanced Civil Engineering Materials and Application Technology of Hunan Province, College of Civil Engineering, Hunan University, Changsha 410082, China (S. Guo). E-mail addresses: [email protected] (F. Liu), [email protected] (S. Guo). https://doi.org/10.1016/j.conbuildmat.2020.118224 0950-0618/Ó 2020 Published by Elsevier Ltd.

Sulfate attack is one of the most severe threats to the long-term performance of the concrete infrastructure [1]. The previous study has well understood the material damage mechanism [2–4], damage characterization [5–7], phase formation [8,9], damage evaluation [10–12], expansion mechanism of sulfate erosion, and chemo-mechanical modeling [13–15]. However, the knowledge gap on the development of sulfate attack under field conditions still exit. The former studies indicated that, besides the traffic

2

F. Liu et al. / Construction and Building Materials 249 (2020) 118224

loading, the environmental factors can also deteriorate the transportation structure [16,17]. Brown and Taylor [18] pointed out that field-like conditions should be considered in the experimental work and be related to field observations. In recent years, sulfate attack with existing environmental and mechanical factors have attracted the attention of scholars, such as sulfate attack under drying-wetting and heating–cooling environments [19,20], frost action [21–23], static flexural loading [24,25], flexural fatigue loading [26], static flexural loading and drying-wetting cycles [27–30], static flexural loading and freeze– thaw cycles [31]. Specifically, the study by Steindl et al. [32] build a novel powder protocol to examine the sulfate resistance of ecoconcrete. These studies manifest that environmental and mechanical factors have different degrees of effects on sulfate attack. Some concrete structures in service, such as structures under earthquake, pavement slabs, bridges decks, airfield runways, railway bridges, even high-speed railways and concrete structures in the ocean have to undergo fatigue loading [33] and environmental impacts simultaneously. Recently, some studies on fatigue loading and environmental factors are reported [26,34,35]. Currently the chemical sequences of sulfate attack in concrete has been widely studied and its reaction mechanism has been thoroughly understood [36]. Meanwhile, the current understanding on the correlation between external loading and sulfate attack is still under investigation, especially the fatigue loading. Specifically, the study by Yan et al. [37] indicated compressive fatigue loading at the stress level larger than 0.7 could promote the entrance of sulfate ions into the concrete. Besides that, the study by You et al. [38] Zhang et al. [39] and Long et al. [40] focused on the combined effects of loading and sulfate attack on the expanded polystyrene concrete, magnesium sulfate cement and fly-ash based geopolymer concrete, respectively. At present the understanding on the coupling effects between flexural fatigue loading and sulfate attack is still quite limited. This study aims to partially fill the knowledge gap by in-situ examining the concrete material with the combined actions of fatigue loading, drying-wetting cycles and sulfate attack simultaneously. Also to enhance the sulfate resistance of concrete, supplementary cementitious materials has been added into concrete to improve its durability performance. The study by Ghafoori et al. [41] indicated that, compared to the nanoscale silica, the microscale silica is more efficient to improve the sulfate resistance of mortar samples. Also the sulfate resistance of the nanoscale silica modified concrete highly depend on the dispersion effect [42]. The study by Jin et al. [43] indicated the added fly ash can help to prohibit the entrance of sulfate ions into concrete and then enhance its resistance to sulfate attack. Similar results were also reported in the study of Ghafoori et al [44]. The application of class F fly ash has been proved to be an efficient and cost-effective method to enhance the sulfate resistance of concrete [45]. To in-situ study the combined effects of sulfate attack and drying–wetting cycles, various protocols have been proposed in former studies. Yuan et al. [19] adopted a drying-wetting cycle regime consisted of 8 h drying with 70 ℃ in the oven and keeping 2 h cooling, next soaking in the sulfate solution for 16 h, to investigating the failure process of concrete under the coupled actions between sulfate attack and drying-wetting cycles by using X-ray CT. Gao et al. [27] adopted a similar regime consisted of drying at a temperature of 60℃ for 45 h and cooling down in the air at a room temperature for 3 h, then immersing in the sulfate solution at room temperature for 21 h, following by 3 h drying in air. Previous studies have shown that the erosive product ettringite will become unstable and the erosion mechanism will change when the heating temperature exceeds 70 °C, therefore, in this study, the drying temperature with 60 °C is taken. Chaube et al. [46] pointed out that the evaporation process of water in concrete sig-

nificantly lagged behind the moisture process during the dryingwetting cycles, the ‘‘ink-bottle effect” is the main cause of this phenomenon. In order to speed up the test process, the drying time should be longer than the soaking time, at the same time, a buffering period is set between the immersion and the drying to prevent the temperature stress caused by subcooling and overheating. Currently, it is still difficult for the existing instrument to adequately in-situ analyze the combined effects of fatigue loading, dryingwetting cycles and sulfate attack on concrete materials. Motivated by current status, this research aims to evaluate the effects of flexural fatigue loading and drying-wetting cycles on the mortar specimens that are exposed to an external sulfate solution. In this study, an experimental system was improved based on existing instrument [47] to simulate the impact of flexural fatigue loading and environmental factors on the sulfate erosion of concrete. At regular time intervals, the mass loss rate, relative dynamic elastic modulus, and sulfate ion content in layer sample were measured. In addition, the corrosion products of interior concrete are examined using XRD. This research is beneficial to providing a reference to the durability evaluation of concrete structures in service suffering from sulfate attack and durability design of concrete structures to be built. 2. Materials, methods, and calculations 2.1. Materials Ordinary Portland cement with the strength grade 42.5 was used in this study, which was produced by Ji-dong Cement Company in Shaanxi Province, China. Class F fly ash produced by Sanmenxia Thermal Power Plant was utilized, which is selected based on the recommendation in reference [43]. The contents of main oxides in cement and fly ash are given in Table 1. The limestone with an apparent density of 2.714 g/cm3 was selected as coarse aggregate (size range is 5–25 mm). River sand with 2.74 fineness modulus was used in this study (apparent density is 2.630 g/cm3). The polycarboxylic acid water-reducing agent was used as the concrete admixture produced by a building scientific research institute. The above materials were all mixed with tap water and the mix proportion of concrete is shown in Table 2. The prismy specimens of 100 mm  100 mm  400 mm were prepared for the test. 2.2. Method and calculations 2.2.1. Flexural fatigue loading There has no existing equipment to consider the combined impacts of flexural fatigue loading and environmental factors with the presence of the sulfate attack on concrete. Therefore, an experimental system was improved based on existing instrument in this research. The experimental system of sulfate attack under flexural fatigue loading and drying-wetting cycles is shown in Fig. 1. The device uses brackets, beams, motors, pressure heads, force value sensors, etc. to form stress loading components. The maximum and minimum values of the alternating load and the frequency of the alternating load are controlled by electronically controlling the meter and the frequency converter. The simulation parts of drying-wetting cycles and sulfate attack are composed of environmental chambers, heaters, temperature sensors, etc., and the temperature of the heating is controlled by a temperature control apparatus. The specimens are stacked one to another in the experimental box. According to the basic mechanical properties of concrete, concrete subjected to bending stress, when the corresponding stress is about 60% of its ultimate strength, it is in an unstable state. In this study, three maximum stress levels were

3

F. Liu et al. / Construction and Building Materials 249 (2020) 118224 Table 1 Contents of main oxides in cement/fly ash based on XRF analysis (% by mass). Main oxide

CaO

SiO2

Al2O3

Fe2O3

MgO

Na2O

K2O

SO3

Cement (%) Fly ash (%)

60.28 3.93

21.7 53.25

5.66 28.75

3.12 4.94

1.52 1.59

0.37 0.54

0.55 1.92

2.1 1.7

Table 2 Mix proportion of concrete. Water-binder ratio

Cement kgm3

Fly ash kgm3

Water kgm3

Sand kgm3

Coarse aggregate kgm3

Water reducing agent kgm3

28d Flexural strength (MPa)

0.4

440 308

0 132

176 176

642 642

1192 1192

1.54 0.88

5.91 4.28

Fig. 1. Experimental system of sulfate attack under flexural fatigue loading and drying-wetting cycles: (a) loading diagram (b) loading scheme (c) field test device.

selected as 20%, 35%, and 50%; the minimum stress level was onetenth of the corresponding maximum stress level. During the stress loading procedure, the loading and unloading remained 1.5 min, and that was 3 min per one cycle and 480 cycles every day. In this study, the loading and unloading cycles were set as 86,400 during 180 d, which could reflect the field fatigues of pavement slabs, bridge decks, airfield runways, railway sleepers and bridges whose

service life fatigue cycle number was believed between 103 and 107 [48]. 2.2.2. Drying-wetting cycle regime At present, there is no uniform regulation for the dryingwetting cycle regime. With regard to anti-sulfate attack test in Standard GB/T 50082–2009 [49], heating temperature of

4

F. Liu et al. / Construction and Building Materials 249 (2020) 118224

(80 ± 5) °C was designed in drying phase. In order to speed up the test process, the drying time should be longer than the soaking time, at the same time, a buffering period is set between the immersion and the drying to prevent the temperature stress caused by subcooling and overheating. In this research, the drying-wetting alternate system was adopted and the procedure was as following: firstly, the standard curing specimens for 28 d were baked at 60 °C for 24 h in the oven then cooled to room temperature. The dried specimens were immersed in sulfate solution for 10 h followed by drying for 1 h at room temperature then were baked at 60 °C for 36 h in the environmental box and finally cooled for 1 h in the air. The newly prepared corrosion solution replaced the original solution every 20 d to ensure that a little change in concentration under the corrosion environment of the concrete specimens. 2.2.3. Testing scheme In order to show the deterioration of concrete under different exposure conditions subjected to sulfate, in this study, sulfate attack under continuous soaking was considered to compare with the combined effects of sulfate attack and drying-wetting cycles; also sulfate attack under flexural fatigue loading and dryingwetting cycles was investigated. The studied conditions of sulfate attack under different factors are shown in Table 3. 2.2.4. Mass loss rate Specimens with the size of 100 mm  100 mm  400 mm were used and three replicates were made to verify reproducibility of results in this experiment. The mass loss rate of the concrete specimen was calculated using equation (1), as following:

DW t ¼

W0  Wt  100% W0

ð1Þ

where DW t : mass loss rate of concrete specimens at corrosion age t; (which calculated as the average of the three specimens, %); W 0 : mass of concrete specimens before corrosion, g; and W t : mass of concrete specimens at corrosion age t, g.

whereErd : relative dynamic elastic modulus; Edt : dynamic elastic modulus of concrete specimens at corrosion age t; and Ed0 : dynamic elastic modulus of concrete specimens before corrosion 2.2.6. Sulfate ion content Concrete specimens at specific corrosion age were obtained by drilling cores firstly then cores were cut into layers at certain thicknesses. The thicknesses of each slice were 2 mm (first layer), 3 mm (second layer), and 5 mm (third layer and the subsequent). The naturally dry slices were put into the agate mortar, broken and repeated grinded then sieved through 0.08 mm size sieve. Sulfate ion content of powder samples will be measured via barium sulfate gravimetric method according to JTJ270-98 [50], and sulfate ion content of concrete is calculated using equation (3), as following:

W SO2 ¼ 4

1:2  0:343  ðm2  m1 Þ  100% m

ð3Þ

where W SO2 : sulfate ion content of concrete (accurate to 4

0.01%); m1 : mass of porcelain crucible; m2 : total mass of sediment and porcelain crucible; and m: mass of concrete powder samples; where 0.343 and 1.2 represent the converted coefficient from BaSO4 to SO3 and the converted coefficient from SO3 to SO24 - , respectively. Fig. 2 shows the measurement process of sulfate ion content in concrete. Here, the initial sulfate-ion contents of two kinds of concrete, listed in Table 2, are 0.859%, 0.801%, respectively. That is, the final value obtained from barium sulfate gravimetric method should be subtracted with 0.859% or 0.801%. 3. Results and discussion 3.1. Mass loss rate

2.2.5. Relative dynamic elastic modulus The dynamic elastic modulus can be used to characterize the degree of damage for concrete specimen under different exposure conditions subjected to sulfate, and to assess the durability of concrete. In this study, DT-16 concrete dynamic elastic modulus tester was used to measure the concrete samples (100 mm  100 mm  400 mm) at regular time intervals. Relative dynamic elastic modulus is calculated using equation (2), as following:

Erd ¼

Edt Ed0

ð2Þ

The variation of the mass loss rate of concrete specimens with erosion time, to a certain extent, reflects the deterioration rule of concrete due to erosion. The changes in mass loss rate under different exposure conditions subjected to sulfate are depicted in Fig. 3 and Fig. 4. From Fig. 3, negative growth in the mass loss rate of the specimens L10, G10, and G5 was noticed, among which, the mass gain of specimen L10 was the slowest while the specimen G10 had the maximum gain. The mass loss rate of the specimen G10F30 has negative growth firstly, then turns positive. The specimens L10 were continuously soaked in sulfate sodium solutions, mass gain

Table 3 Experimental conditions of sulfate attack under different factors. Specimen code

Days of continuous soaking

Times of drying-wetting cycles

Stress level of flexural fatigue loading

Concentration of sodium sulfate (mass fraction)

Content of fly ash (mass ratio)

L10 G10 G5 G10F30 G10DS20 G10DS35 G10DS50

180 0 0 0 0 0 0

0 90 90 90 90 90 90

0 0 0 0 20% 35% 50%

10% 10% 5% 10% 10% 10% 10%

0 0 0 30% 0 0 0

Note: L-full immersion, G-drying-wetting cycle, F-fly ash DS-stress level of flexural fatigue loading. The numbers after G, L stand for the sulfate concentration, the number after F stands for the content of fly ash, and the numbers after DS stand for the stress level.

F. Liu et al. / Construction and Building Materials 249 (2020) 118224

5

Fig. 2. Measurement process for sulfate ion content in concrete:(a) drilling core (b) core samples (c)slicing (d) slicing samples (e) grinding (f) weighing before titration (g) dissolving (h) filtering (i) adding 10% BaCl2 solution (j) static setting (k) filtering and washing (l) weighing empty crucible (m) ashing (n) burning in high temperature furnace (o) weighing precipitate + crucible.

depends on two reasons: one is on account of water absorption and further hydration, the other is that expansive erosion products fill micro pores and cracks in matrix. The study by Ikumi et al. [10] indicated the deterioration at microscale level can be generated by both external chemical attack [51] and external loadings. In comparison with drying-wetting cycles, the mass gain of specimen L10 is less than that of specimen G10. Drying and wetting cycles accelerate sulfate diffusion to concrete through humidity gradient of cycles and aggravate the deterioration of concrete. The mass loss rate of

the specimens L10, G10 for 180 d was 1.313% and 2.078%, respectively. The specimen G5 in the mass loss rate exhibited a similar trend to the specimen G10, while the degree of change is different. After 180 d, the mass loss rate of the specimen G5 was 1.273%. Under high-concentration Na2 SO4 solution (mass fraction), the speed of generating expansive products is faster; the mass gain is also faster, so the absolute value of mass loss rate is higher. The mass loss rate of the specimen G10F30 for 180 d was 0.619%, negative growth at first, then the forming surface of

6

F. Liu et al. / Construction and Building Materials 249 (2020) 118224

3.2. Relative dynamic elastic modulus

Fig. 3. Changes of mass loss rate for concrete under drying-wetting cycles and sulfate.

The changes in relative dynamic elastic modulus with different experimental conditions are shown in Figs. 5 and 6. Dynamic elastic modulus of concrete specimens will change under sulfate environment. In this study, relative dynamic elastic modulus was used to characterize the damage inside the concrete. From Fig. 5, relative dynamic elastic modulus of the concrete specimens L10 first has a significant increasing then rises slowly. After 180 d, relative dynamic elastic modulus of specimens L10 was 1.058, thus it could be seen that the damage process of concrete caused by continuous soaking in 10%Na2 SO4 solution is very slow. Relative dynamic elastic modulus of the specimens G10 was 0.797 after 180 d. The reduction is caused by the sulfate accumulated during drying-wetting cycles and the generated crystallized expansion [52]. The crystallization of sulfate salt will lead to crystallization pressure [53,54] and thus accelerated the damage process obviously with the number of cycles [55]. Relative dynamic elastic modulus of the specimens G5 was 0.854 after 180 d of drying-wetting cycles. There is a similar trend in relative dynamic elastic modulus between the specimens G10 and G5, increases firstly then decreases, however, the degree of loss is different. The rise and fall of the specimen G10 are larger than G5. Under high-quality fraction ofNa2 SO4 solution, the speed of generating expansive products is faster, so the expansion stress is also larger, crack propagation is faster, and the degree of degradation is more obvious. Relative dynamic elastic modulus of the specimens G10F30 was 0.943 after 180 d of drying-wetting cycles and the decreasing degree is smaller than that of the specimen G10. It was clear that the fly ash concrete has the better performance in terms of the resistance of concrete to sulfate attack. This is probably due to the reduction of pore sizes, filling and the pozzolanic effect, which results in an improvement in the resistance to sulfate attack of concrete, to some extent [56–58]. From Fig. 6, relative dynamic elastic modulus of the specimens G10DS50 declined at the beginning after 70 d and the value of relative dynamic elastic modulus was 0.919. A lot of specimens G10DS50 were broken, thus the longest time that the specimen can bear for the experiment was 70 d. Among these broken concrete specimens, it was found that its relative elastic modulus declines sharply before the brittle fracture. This also proves that relative dynamic elastic modulus could characterize the damage inside the concrete. The mechanical damage caused by high stress

Fig. 4. Changes of mass loss rate for concrete exposed to sulfate attack under drying-wetting cycles and flexural fatigue loading.

concrete specimen emerging peeling, the mass gain was not enough to resist the mass loss due to forming surface peeling, so the mass loss rate was from negative to positive. From Fig. 4, the mass loss rate of the specimens G10DS20 and G10DS35 both have negative growth firstly, then mass gain become smaller after reaching the extreme value, the magnitude of mass gain is greater than that of G10. A lot of specimens G10DS50 were broken and the longest time that the specimen can bear for the experiment was 70 d, the mass gain of the specimen G10DS50 was the largest. After 180 d, the mass loss rate of the specimens G10DS35 and G10DS20 were 0.458%, 1.345%, respectively. Compared with that of the specimen G10, the specimens G10DS35 and G10DS20 firstly showed a negative maximum, then increased, and the negative maximum of the specimen G10DS35 is earlier than the specimen G10DS20. The fatigue loading generates cracks and accelerates the damage process of concrete under drying-wetting cycles and sulfate attack, so the negative growth rate of the mass loss for the specimens was sooner than that of G10.

Fig. 5. Changes of relative dynamic elastic modulus for concrete under dryingwetting cycles and sulfate.

F. Liu et al. / Construction and Building Materials 249 (2020) 118224

Fig. 6. Changes of relative dynamic elastic modulus for concrete exposed to sulfate attack under drying-wetting cycles and flexural fatigue loading.

7

Fig. 7. SO2 4 Concentration depends on the depth for concrete under drying-wetting cycles and sulfate.

level of fatigue loading (50%) was the main factor leading to the deterioration of the concrete. Relative dynamic elastic modulus of the specimens G10DS35 was 0.732 after 180 d and the variation curve with time was going down all the time. Relative dynamic elastic modulus of the specimens G10DS20 increased with corrosion time in the early state, decreased afterwards, and relative dynamic elastic modulus was 0.773 after 180 d. Relative dynamic elastic modulus of the specimens G10DS20 and G10DS35 was smaller than that of the specimen G10. The higher the stress level of flexural fatigue loading, the faster the decrease in relative dynamic elastic modulus will be. The flexural fatigue loading accelerates the propagation of micro-cracks within the concrete, thereby expanding the diffusion of sulfate into the interior of the concrete. The generation of micro-cracks together with the sulfate attack and drying-wetting cycle action made concrete deteriorate rather quickly. 3.3. SO2 4 concentration depending on depth Figs. 7 and 8 depict the changes in SO2 4 concentration with the depth for the specimens after 180 d. The transport of sulfate ions from the erosion solution to the interior of the concrete is a gradual transfer process. From Fig. 7, with the increase of erosion depth, the content of sulfate ions reduced significantly. With the growth of erosion age, sulfate ion from the erosion solution into the shallow layer of concrete increases, which would produce a chemical reaction with cement hydration products in concrete, thus generating gypsum, ettringite, and other erosion products [59–61]. Due to the filling and compaction effects of erosion products, hindering the further transport of sulfate ions into the interior of the concrete, there had a certain enrichment of the surface sulfate ion distribution; concrete with larger depth was relatively lagged by the sulfate ion attack, so that the content of sulfate ions reduced significantly with the increase of erosion depth. Fig. 7 also shows that each specimen has different erosion depth; the maximum erosion depth of the specimen L10 was 7.5 mm, and the content of sulfate ion in the first three layers of L10 concrete was 3.524%, 1.695%, and 1.254%, respectively. The maximum erosion depth of the specimen G10 was 12.5 mm, and the content of sulfate ion in the first four layers of G10 was 4.282%, 4.558%, 1.988%, and 0.558%, respectively. Under dryingwetting cycles, the transmission of sulfate ions in concrete can

Fig. 8. SO2 Concentration depends on the depth for concrete exposed to sulfate 4 attack under drying-wetting cycles and flexural fatigue loading.

be regarded as the coupling of convection caused by capillary action and the diffusion caused by concentration gradient, while under continuous soaking, the transmission of sulfate ions in concrete is dominated by diffusion. The maximum erosion depth of the specimen G5 was also 12.5 mm while the amount of sulfate ions per erosion depth was less than that of G10. The maximum erosion depth of G10F30 was 7.5 mm, and its sulfate ions content decreased the fastest with erosion depth. Compared with G10, it was also confirmed that the addition of admixtures can improve the ability of sulfate resistance of concrete when it is subjected to drying-wetting cycles. The effect of compressive stress on the sulfate ion transport properties is not significant as it is basically the same as the nonstress state. Compared with the acceleration effect of tensile stress in tensile zone on the ion transmission, the compressive stress of the compression zone has a smaller effect on the ion transport, so in this research, only the distribution of sulfate ion in the tension zone is analyzed. As the stress level of fatigue loading was relatively high, the fracture of the specimen G10DS50 was earlier than that of the

8

F. Liu et al. / Construction and Building Materials 249 (2020) 118224

experimental design, thus there has no curve of sulfate ion evolution for the specimen G10DS50 in Fig. 8. The maximum erosion depth of the specimen G10DS20 was 12.5 mm and the content of sulfate ion in the first four layers of G10DS20 concrete was 5.289%, 4.743%, 2.257%, and 0.837%, respectively. The maximum erosion depth of the specimen G10DS35 was also 12.5 mm and the content of sulfate ion in the first four layers of G10DS35 concrete was 5.809%, 5.078%, 3.056%, and 1.469%, respectively. The erosion depth is also shown in Fig. 8. The content of sulfate ions in Fig. 8 can be arranged as G10DS35 > G10DS20 > G10. Without flexural fatigue loading, the sulfate ion contents of the specimen G10 were much lower than those of specimens G10DS35 and G10DS20. The stress level had a positive effect on sulfate ion in layers. The greater the stress level, the higher the sulfate ion content will be. More discrete small cracks generated under fatigue loading at first. With the increase in the number of cycles, generated new cracks and initiated the expansion of the original crack, which provided a channel for the entry of sulfate ions into the interior of the concrete. On the same time, drying and wetting cycles accelerated sulfate ion diffusion into concrete through humidity gradient of cycles, so more sulfate ions reacted with hydration products of cement and generated expansive erosion products. When the internal stress of expansion exceeded the tensile strength of concrete, cracks generated. The fatigue stress accelerated the corrosion, and the corrosion exacerbated the fatigue damage of the material. 3.4. The corrosion products of interior concrete X-ray diffraction analysis of ground powder samples at different depth was carried out using a D8 ADVANCE X-ray diffractometer manufactured by Bruker AXS Co., Ltd. The XRD pattern obtained from the experiment was compared with the standard diffraction pattern in the database to determine the phase of the crystal in the sample. XRD patterns at different depth for specimen G10DS35 under the combined actions of sulfate attack, dryingwetting cycles, and flexural fatigue loading simultaneously after 180 d are shown in Fig. 9. As can be seen from Fig. 9, the diffraction peak of ettringite appeared in the corrosion product of concrete surface layer under the actions of 35% flexural fatigue load after erosion for 180 days. A diffraction peak of ettringite and gypsum appeared in the corrosion product of the second layer (1 cm), wherein the diffraction peak of

the gypsum was very low, which was related to the low gypsum content in the corrosion product. There was no diffraction peak of ettringite or gypsum in the corrosion product of the inner layer (2 cm), indicating that sulfate ion did not erode to the inner layer. 3.5. Discussion This research aims to study the durability performance of the concrete transportation structures under the combined environmental and mechanical loadings. During its service life, the transportation structures will be impacted by the repeated dynamic traffic loading, which will lead to fatigue damage of the concrete infrastructure. Specifically, the flexural fatigue loading can crack the concrete and also lead to the entrance of the sulfate ions into concrete. But the study between the coupling effects of fatigue loading and sulfate attack is still quite limited. The results in this study support that the flexural fatigue loading can accelerate the development of sulfate attack by enhancing the transportation of the sulfate ion. Also the drying-wetting cycles are to simulate the drying condition in the western China, where drying-wetting cycles can also deteriorate concrete materials, especially the structures exposed to sulfate condition. The reduction effect of the flyash used in this study can be generated by three mechanisms. The reaction mechanism of the sulfate attack is now clear that the sulfate ions entered into concrete will first react with the Ca2+ to form gypsum. Then the gypsum will react with the C3A to form the expansive ettringite material [36]. Hence, the mitigation mechanism of the fly-ash can be explained in these aspects. Firstly the fly-ash can consume the generated lime due to cement hydration through the pozzolanic reaction [62]. After the entrance of sulfate ions, the generation of gypsum in concrete can then be prohibited due to reduced lime content. Secondly, the fly ash material can lead to a denser internal structure and prohibited the entrance of sulfate ions [63]. Finally, replacing Portland cement with fly ash can reduce the amount of C3A [64] and then the reaction between C3A and generated gypsum can be diminished. The reduction mechanism for other Supplementary Cementitious Materials (SCMs) is similar to that of the fly ash. The further studies also indicated better sulfate resistance can be achieved by adding silica fume [65]. The former studies indicate that the MgSO4 can lead to severer threat to the durability performance of concrete compared to that of Na2SO4 [66] used in this study. Besides the sulfate ion, the Mg2+ will also react the lime content and the magnesium hydroxide can be generated during the process [67]. Besides that, the Mg2+ can also lead to the destabilization of the C-S-H phase [68] by replacing the contained calcium content. Hence the future study will try to study the coupling effects between flexural fatigue loading and the explosion to MgSO4 environment. Furthermore, the former study the silica fume is more effective to enhance the resistance to MgSO4 compared to that of other SCMs [65]. 4. Conclusions In this paper, external sulfate attack under flexural fatigue loading and drying-wetting cycles was investigated. The mass loss rate, relative dynamic elastic modulus, and sulfate ion content were measured under sulfate attack incorporating with flexural fatigue loading and drying-wetting cycles. Moreover, the phase composition of attached samples was analyzed using XRD. The conclusions are summarized as following:

Fig. 9. XRD patterns at different depth for specimen G10DS35.

(1) The reduction on relative dynamic modulus is more obvious for specimens under drying-wetting cycles and sulfate attack compared to the specimens under sulfate attack.

F. Liu et al. / Construction and Building Materials 249 (2020) 118224

Drying-wetting cycles accelerate the sulfate accumulation in concrete and aggravate the damage degree of concrete subjected to sulfate attack. However, it should be cautious to choose appropriate drying-wetting cycle regime. (2) Concrete with the addition of 30% fly ash had the better performance in terms of the resistance to sulfate attack and drying-wetting cycles. The degree of concrete degradation was more obvious under high-concentration sodium sulfate solution (mass fraction) and drying-wetting cycles. (3) The fatigue loading can accelerate the development of sulfate attack by increasing the cracks and transportation of the sulfate ion, which then can lead larger amount of sulfate ion in the pore solution and then higher expansion force. The acceleration effect can be further examined with higher stress level. CRedit authorship contribution statement Fang Liu: Conceptualization, Methodology, Software, Formal analysis, Investigation, Resources, Data curation, Visualization. Zhanping You: Conceptualization, Methodology, Resources, Supervision, Project administration, Funding acquisition. Aboelkasim Diab: Software, Formal analysis, Investigation. Zhuangzhuang Liu: Software, Validation, Formal analysis, Investigation, Resources, Data curation. Chao Zhang: . Shuaicheng Guo: Conceptualization, Methodology, Visualization, Supervision, Project administration, Funding acquisition. Acknowledgements This study was sponsored by the Construction System Science and Technology Project of Jiangsu Province (Grant Number: 2019ZD001229) and National Key R&D Program of China. Shuaicheng Guo acknowledges the financial support from the China Scholarship Council under No. 201406370141. References [1] B. Ma, X. Gao, E.A. Byars, Q. Zhou, Thaumasite formation in a tunnel of Bapanxia Dam in Western China, Cem. Concr. Res. 36 (4) (2006) 716–722. [2] P.K. Mehta, Mechanism of expansion associated with ettringite formation, Cem. Concr. Res. 3 (1) (1973) 1–6. [3] P.K. Mehta, Mechanism of sulfate attack on portland cement concrete — another look, Cem. Concr. Res. 13 (3) (1983) 401–406. [4] Z. Liu, X. Li, D. Deng, G. De Schutter, L. Hou, The role of Ca(OH)2 in sulfate salt weathering of ordinary concrete, Constr. Build. Mater. 123 (2016) 127–134. [5] Z. Zhang, X. Jin, W. Luo, Long-term behaviors of concrete under lowconcentration sulfate attack subjected to natural variation of environmental climate conditions, Cem. Concr. Res. 116 (2019) 217–230. [6] Z. Liu, D. Deng, G.D. Schutter, Z. Yu, Chemical sulfate attack performance of partially exposed cement and cement+fly ash paste, Constr. Build. Mater. 28 (1) (2012) 230–237. [7] M.T. Bassuoni, M.L. Nehdi, Resistance of self-consolidating concrete to ammonium sulphate attack, Mater. Struct. 45 (7) (2012) 977–994. [8] F. Mittermayr, A. Baldermann, C. Baldermann, G.H. Grathoff, D. Klammer, S.J. Köhler, A. Leis, L.N. Warr, M. Dietzel, Environmental controls and reaction pathways of coupled de-dolomitization and thaumasite formation, Cem. Concr. Res. 95 (2017) 282–293. [9] Z. Liu, D. Deng, G. De Schutter, Z. Yu, The effect of MgSO4 on thaumasite formation, Cem. Concr. Compos. 35 (1) (2013) 102–108. [10] T. Ikumi, S.H.P. Cavalaro, I. Segura, A. de la Fuente, A. Aguado, Simplified methodology to evaluate the external sulfate attack in concrete structures, Mater. Des. 89 (2016) 1147–1160. [11] S.-T. Lee, Performance deterioration of Portland cement matrix due to magnesium sulfate attack, KSCE J. Civ. Eng. 11 (3) (2007) 157–163. [12] Z. Liu, G. De Schutter, D. Deng, Z. Yu, Micro-analysis of the role of interfacial transition zone in ‘‘salt weathering” on concrete, Constr. Build. Mater. 24 (11) (2010) 2052–2059. [13] A. Campos, C.M. López, A. Aguado, Diffusion–reaction model for the internal sulfate attack in concrete, Constr. Build. Mater. 102 (2016) 531–540. [14] A.E. Idiart, C.M. López, I. Carol, Chemo-mechanical analysis of concrete cracking and degradation due to external sulfate attack: a meso-scale model, Cem. Concr. Compos. 33 (3) (2011) 411–423.

9

[15] N. Cefis, C. Comi, Chemo-mechanical modelling of the external sulfate attack in concrete, Cem. Concr. Res. 93 (2017) 57–70. [16] K. Liu, D. Dai, C. Fu, W. Li, S. Li, Structural investigation of the snow-melting heated bridge deck based on the thermal field distribution, Appl. Therm. Eng. (2019). 114132. [17] K. Liu, H. Xie, P. Xu, Z. Wang, H. Bai, F. Wang, The thermal and damage characteristics of an insulated-conductive composite structure for the heated bridge deck for snow-melting, Constr. Build. Mater. 216 (2019) 176–187. [18] P.W. Brown, H.F.W.T., The role of ettringite in external sulfate attack, in Sulfate Attack Mechanisms, Materials Science of Concrete, American Ceramic Society, 1999, Ohio. [19] J. Yuan, Y. Liu, Z. Tan, B. Zhang, Investigating the failure process of concrete under the coupled actions between sulfate attack and drying–wetting cycles by using X-ray CT, Constr. Build. Mater. 108 (2016) 129–138. [20] M. Sahmaran, T.K. Erdem, I.O. Yaman, Sulfate resistance of plain and blended cements exposed to wetting–drying and heating–cooling environments, Constr. Build. Mater. 21 (8) (2007) 1771–1778. [21] D. Wang, X. Zhou, Y. Meng, Z. Chen, Durability of concrete containing fly ash and silica fume against combined freezing-thawing and sulfate attack, Constr. Build. Mater. 147 (2017) 398–406. [22] M.L. Nehdi, M.T. Bassuoni, Durability of self-consolidating concrete to combined effects of sulphate attack and frost action, Mater. Struct. 41 (10) (2008) 1657–1679. [23] Q.H. Xiao, Q. Li, Z.Y. Cao, W.Y. Tian, The deterioration law of recycled concrete under the combined effects of freeze-thaw and sulfate attack, Constr. Build. Mater. 200 (2019) 344–355. [24] K.-C. Werner, Y. Chen, I. Odler, Investigations on stress corrosion of hardened cement pastes, Cem. Concr. Res. 30 (9) (2000) 1443–1451. [25] Jin, Z.S., Wei; Jiang, Jinyang; Zhao, Tiejun., Damage of concrete attacked by sulfate and sustained loading. J. Southeast Univers. (English Edition), 2008(1): p. 15. [26] D. Yu, B. Guan, R. He, R. Xiong, Z. Liu, Sulfate attack of Portland cement concrete under dynamic flexural loading: a coupling function, Constr. Build. Mater. 115 (2016) 478–485. [27] J. Gao, Z. Yu, L. Song, T. Wang, S. Wei, Durability of concrete exposed to sulfate attack under flexural loading and drying–wetting cycles, Constr. Build. Mater. 39 (2013) 33–38. [28] R. Gao, Q. Li, S. Zhao, Concrete deterioration mechanisms under combined sulfate attack and flexural loading, J. Mater. Civ. Eng. 25 (1) (2013) 39–44. [29] Y. Geng, Z. Jin, B. Hou, T. Zhao, S. Gao, Long-term behavior of fiber reinforced concrete exposed to sulfate solution cycling in drying-immersion, J. Wuhan Univers. Technol.-Mater. Sci. Ed. 32 (4) (2017) 875–881. [30] Wu, X.F., Z.Q. Jin, T.J. Zhao, and S. Gao. Damage of cement paste in sulfate environment with different temperature and drying-immersion cycles, Appl. Mechan. Mater. 2013. Trans Tech Publ. [31] Y.H. Zhang Yunqing, Sun Wei, Jianye Zhang, Stress corrosion of concrete exposed to the action of freezing-thawing cycles, J. Civil, Architect. Environ. Eng. 32 (6) (2010) 147–152. [32] F.R. Steindl, A. Baldermann, I. Galan, M. Sakoparnig, L. Briendl, M. Dietzel, F. Mittermayr, Chemical resistance of eco-concrete–Experimental approach on Ca-leaching and sulphate attack, Constr. Build. Mater. 223 (2019) 55–68. [33] M. Rodriguez, D.L.J.H.G.Y.G.Z.R.E.K., H. L. Graves, III, Dynamic behavior of tensile anchors to concrete. Struct. J. 98(4). [34] W. Li, W. Sun, J. Jiang, Damage of concrete experiencing flexural fatigue load and closed freeze/thaw cycles simultaneously, Constr. Build. Mater. 25 (5) (2011) 2604–2610. [35] Y. Qiao, W. Sun, J. Jiang, D. Pan, Coupling mechanism of saturated concrete subjected to simultaneous fatigue loading and freeze-thaw cycles, J. Wuhan Univers. Technol.-Mater. Sci. Ed. 33 (5) (2018) 1121–1128. [36] P.K. Mehta, Mechanism of sulfate attack on portland cement concrete— Another look, Cem. Concr. Res. 13 (3) (1983) 401–406. [37] X. Yan, G. Yang, L. Jiang, Z. Song, M. Guo, Y. Chen, Influence of compressive fatigue on the sulfate resistance of slag contained concrete under steam curing, Struct. Concr. (2019). [38] Q. You, L. Miao, C. Li, H. Fang, X. Liang, Study on the fatigue and durability behavior of structural expanded polystyrene concretes, Materials 12 (18) (2019) 2882. [39] Zhang, J.Y. Wen, L. Chen, Fatigue properties of basic magnesium sulfate cement reinforced concrete beams: based on response surface methodology. febfresenius environmental. Bulletin, 2019, p. 7655. [40] T. Long, H. Zhang, Y. Chen, Z. Li, J. Xu, X. Shi, Q. Wang, Effect of sulphate attack on the flexural fatigue behaviour of fly ash–based geopolymer concrete, J. Strain Anal. Eng. Design 53 (8) (2018) 711–718. [41] N. Ghafoori, I. Batilov, M. Najimi, M. Sharbaf, Sodium sulfate resistance of mortars containing combined nanosilica and microsilica, J. Mater. Civ. Eng. 30 (7) (2018) 04018135. [42] N. Ghafoori, I. Batilov, M. Najimi, Influence of dispersion methods on sulfate resistance of nanosilica-contained mortars, J. Mater. Civ. Eng. 29 (7) (2017) 04017038. [43] J. Zuquan, S. Wei, Z. Yunsheng, J. Jinyang, L. Jianzhong, Interaction between sulfate and chloride solution attack of concretes with and without fly ash, Cem. Concr. Res. 37 (8) (2007) 1223–1232. [44] N. Ghafoori, M. Najimi, H. Diawara, M.S. Islam, Effects of class F fly ash on sulfate resistance of Type V Portland cement concretes under continuous and interrupted sulfate exposures, Constr. Build. Mater. 78 (2015) 85–91.

10

F. Liu et al. / Construction and Building Materials 249 (2020) 118224

[45] K. Torii, K. Taniguchi, M. Kawamura, Sulfate resistance of high fly ash content concrete, Cem. Concr. Res. 25 (4) (1995) 759–768. [46] R. Chaube, Multiphase water movement in concrete as a multi-component system. In: Proceedings of the Fifth Intn’l ConCreep RILEM Symp. 1993. [47] Chen Shuanfa, G.B., Li Zuzhong, Sheng Yanping, Xiong Rui. , A mechanical stress- corrosion coupling fatigue test device. 2011: China. [48] M.K. Lee, B.I.G. Barr, An overview of the fatigue behaviour of plain and fibre reinforced concrete, Cem. Concr. Compos. 26 (4) (2004) 299–305. [49] D. Code, Standard for test methods of long-term performance and durability of ordinary concrete. Chinese Standard GB/T50082-2009, Beijing, 2009. [50] Editors, Testing Code of Concrete for Port and Waterway Engineering (JTJ 27098). 1999, Beijing: China Communications Press. [51] T. Ikumi, S.H. Cavalaro, I. Segura, A. Aguado, Alternative methodology to consider damage and expansions in external sulfate attack modeling, Cem. Concr. Res. 63 (2014) 105–116. [52] A. Baldermann, M. Rezvani, T. Proske, C. Grengg, F. Steindl, M. Sakoparnig, C. Baldermann, I. Galan, F. Emmerich, F. Mittermayr, Effect of very high limestone content and quality on the sulfate resistance of blended cements, Constr. Build. Mater. 188 (2018) 1065–1076. [53] Z. Liu, W. Hu, M. Pei, D. Deng, The role of carbonation in the occurrence of MgSO4 crystallization distress on concrete, Constr. Build. Mater. 192 (2018) 167–178. [54] Z. Liu, W. Hu, L. Hou, D. Deng, Effect of carbonation on physical sulfate attack on concrete by Na2SO4, Constr. Build. Mater. 193 (2018) 211– 220. [55] F. Mittermayr, M. Rezvani, A. Baldermann, S. Hainer, P. Breitenbücher, J. Juhart, C.-A. Graubner, T. Proske, Sulfate resistance of cement-reduced eco-friendly concretes, Cem. Concr. Compos. 55 (2015) 364–373. [56] Wang Aiqin, Zhang Chengzhi, Sun Wei, Fly ash effects Ⅰ: the morphological effect of fly ash, Cem. Concr. Res. 33 (2003) 2023–2029.

[57] Wang Aiqin, Zhang Chengzhi, Sun Wei, Fly ash effects Ⅱ: the active effect of fly ash, Cem. Concr. Res. 34 (2004) 2027–2060. [58] Wang Aiqin, Zhang Chengzhi, Sun Wei, Fly ash effects III: the microaggregate effect of fly ash, Cem. Concr. Res. 34 (2004) 2061–2066. [59] R. Ragoug, O.O. Metalssi, F, Barberon, et al . Durability of cement pastes exposed to external sulfate attack and leaching: physical and chemical aspects, Cem. Concr. Res. 116 (2019) 134–145. [60] W. Müllauer, R.E. Beddoe, D. Heinz, DSulfate attack expansion mechanism, Cem. Concr. Res. 52 (2013) 208–215. [61] J.K. Chen, M.Q. Jiang, J. Zhu, Damage evolution in cement mortar due to erosion of sulphate, Corros. Sci. 50 (9) (2008) 2478–2483. [62] C. Ouyang, A. Nanni, W.F. Chang, Internal and external sources of sulfate ions in Portland cement mortar: two types of chemical attack, Cem. Concr. Res. 18 (5) (1988) 699–709. [63] C. Shi, Early microstructure development of activated lime-fly ash pastes, Cem. Concr. Res. 26 (9) (1996) 1351–1359. [64] C. Plowman, J. Cabrera, Mechanism and kinetics of hydration of C3A and C4AF. Extracted from cement, Cem. Concr. Res. 14 (2) (1984) 238–248. [65] T. Vuk, R. Gabrovšek, V. Kaucˇicˇ, The influence of mineral admixtures on sulfate resistance of limestone cement pastes aged in cold MgSO4 solution, Cem. Concr. Res. 32 (6) (2002) 943–948. [66] P.W. Brown, S. Badger, The distributions of bound sulfates and chlorides in concrete subjected to mixed NaCl, MgSO4, Na2SO4 attack, Cem. Concr. Res. 30 (10) (2000) 1535–1542. [67] M. Moukwa, Characteristics of the attack of cement paste by MgSO4 and MgCl2 from the pore structure measurements, Cem. Concr. Res. 20 (1) (1990) 148– 158. [68] O.S.B. Al-Amoudi, M. Maslehuddin, M.M. Saadi, Effect of magnesium sulfate and sodium sulfate on the durability performance of plain and blended cements, ACI Mater. J. 92 (1) (1995) 15–24.