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Durability of concrete exposed to sulfate attack under flexural loading and drying–wetting cycles
Construction and Building Materials 39 (2013) 33–38
Contents lists available at SciVerse ScienceDirect
Construction and Building Materials journal homepage: www.elsevier.com/locate/conbuildmat
Durability of concrete exposed to sulfate attack under flexural loading and drying–wetting cycles Jianming Gao ⇑, Zhenxin Yu, Luguang Song, Tingxiu Wang, Sun Wei Department of Materials Science and Engineering, Southeast University, Nanjing 211189, China Jiangsu Key Laboratory of Construction Materials, Southeast University, Nanjing 211189, China
h i g h l i g h t s " Durability of concrete exposed to sulfate under drying–wetting cycles and load is studied. " The microstructure of interior concrete are determined by using ESEM, XRD, and MIP. " Load and drying–wetting cycles can both accelerate the sulfate corrosion of concrete. " The effect of load on the damage of concrete under combined actions depends on the ratio.
a r t i c l e
i n f o
Article history: Available online 21 June 2012 Keywords: Flexural loading Drying–wetting cycles Sulfate attack Relative dynamic elastic modulus Microstructure
a b s t r a c t The damage process of concrete exposed to sulfate attack under flexural loading and drying–wetting cycles are investigated in this paper. ESEM, MIP, and XRD were used to investigate the changing of microstructure and corrosion products of interior concrete. The results indicate that compared with the single damage process of sulfate attack, flexural loading and drying–wetting cycles can both accelerate the damage process of concrete subjected to sulfate attack. While compared with sulfate attack and drying–wetting cycles, the effect of flexural loading on the deterioration of concrete depends on the stress level. It was also found that the addition of admixtures can improve the ability of sulfate resistance of concrete when it is also subjected to mechanical loading and drying–wetting cycles. Ó 2012 Elsevier Ltd. All rights reserved.
1. Introduction Sulfate attack is one of the most aggressive environmental deteriorations that affect the long-term durability of concrete structures and can cause huge economic loss. For concrete in marine environment, drying–wetting cycles can accelerate the deterioration of concrete such as in the environment like splash and tidal zone. Furthermore, concrete in marine environment endures multifarious loading all the time. In conclusion, the concrete in the splash and tidal zone of marine environment suffers from the deterioration of the coupling function of salt solution, drying–wetting cycles and mechanical loading. There have been many reports on the damage process of concrete exposed to sulfate attack [1–6] and the coupling function of sulfate attack and drying–wetting cycles [7,8] or flexural loading
[9–12]. However, few of them were related to the experimental results about the damage process of concrete under the combined actions of sulfate attack, drying–wetting cycles and mechanical loading. In this paper, the damage process of concrete exposed to sulfate attack under drying–wetting cycles and flexural loading is investigated. Relative dynamic elastic modulus (Erd) is determined using the ultrasonic method. The microstructure and corrosion products of interior concrete are determined using the environmental scanning electron microscopy (ESEM) and X-ray diffraction (XRD). The porosity and pore size distribution of concrete at different ages are determined using the mercury intrusion porosimetry (MIP).
2. Experimental details 2.1. Materials and mix proportions
⇑ Corresponding author. Address: Room 210, School of Materials Science, Southeast University, Jiangning District, Nanjing 211189, China. Tel./fax: +86 025 52090639. E-mail addresses: [email protected] (J. Gao), [email protected] (Z. Yu), [email protected] (L. Song), [email protected] (T. Wang), [email protected] (S. Wei). 0950-0618/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.conbuildmat.2012.05.033
Chinese standard 52.5 R (I) Portland cement equivalent to ASTM Type I Ordinary Portland cement supplied by Wuhan huaxin Cement Corporation in China was used. Fly ash (FA) and grounded blast furnace slag (GBFS), river sand with fineness modulus of 2.6 and coarse aggregate of crushed limestone with a maximum size of 20 mm were used. A polycarboxylate-type super-plasticizer with a water-reducing rate of 25% by weight was used. The chemical composition and properties of the ce-
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J. Gao et al. / Construction and Building Materials 39 (2013) 33–38
ment, FA and GBFS are shown in Table 1. The mix proportion is given in Table 2. A 30% FA and 50% GBFS were used as replacement of cement on weight-to-weight basis in the C50F30 and C50K50 concrete mixtures, respectively. Concrete specimens were cast in steel molds of 70 70 280 mm, removed from the molds 24 h after casting, and cured in the condition of 20 ± 3 °C and 95% of relative humidity for 60 d. Except for the two opposite surfaces (70 280), other four surfaces of the specimen were covered with epoxy resin before the exposure experiments. 2.2. Experiment programs In the present study, sodium-sulfate solution (5% by weight) was used in the experiment. Concrete specimens were first dried at a temperature of 60 °C for 45 h, then cooled down in air at a room temperature for 3 h. Then, they were immersed in the sodium-sulfate solution at room temperature for 21 h, followed by 3 h drying in air, which represents a drying–wetting cycle. This drying–wetting cycle was repeated until it reached a specified time. The mechanical loading was applied by using the four-point bending loading equipment, as shown in Fig. 1. 2.3. Testing parameter The dynamic modulus of elasticity (Erd) of the specimen was measured by using a high-accuracy nonmetal ultrasonic analyzer, which was used to detect the internal defect and damage degree of the concrete specimen[13].
3. Results and discussion 3.1. Erd variation of concrete exposed to sulfate attack Fig. 2 shows the variation of Erd with time for C50 concrete specimens immersed in 5% Na2SO4 solution. It is clear that under sulfate attack alone Erd increases with time at the initial stage, followed by a stabilizing stage and then decreases slowly with the immersion time in the later stage. The peak value of Erd was about 1.035 occurring at 80 d. With further increased immersion time Erd decreases, but the rate of decrease is very slow. For example, after 174 d immersion, Erd reduced to 1.024. This indicates that the damage process of concrete under sulfate attack alone is extremely slow. 3.2. Erd variation of concrete exposed to sulfate attack under flexural loading or drying–wetting cycles Fig. 3 shows the variations of Erd with time for C50 concrete specimens immersed in 5% Na2SO4 solution as well as subjected to the drying–wetting cycles or flexural loading. It can be seen from Fig. 3 that the addition of 40% flexural loading did not change the variation of Erd very much. In contrast, the drying–wetting cycles have a significant influence on the variation of Erd and they can speed up the damage process of concrete. For instance, after 174 d immersion, under combined sulfate attack and 40% flexural loading Erd value was 1.03, while under combined sulfate and drying–wetting cycles it dropped to 0.935. This can be explained by the two different mechanisms of accelerating damage of drying–wetting cycles and flexural loading. The former made sulfate accumulated
Fig. 1. The schematic diagram of loading equipment.
and crystallized in concrete by cycled moisture gradient and capillarity absorption, which accelerated the damage process obviously with number of cycles. However, the latter accelerated the damage process of concrete by extending micro-crack of interior concrete, which enlarged the transmission path of sulfate. The results proved that the diffusion effect depending on concentration gradient was not clear as the capillarity absorption depending on moisture gradient.
3.3. Erd variation of concrete exposed to sulfate attack under flexural loading and drying–wetting cycles Fig. 4 shows the variations of Erd with time for C50 concrete specimens immersed in 5% Na2SO4 solution as well as subjected to combined drying–wetting cycles and flexural loading. For the purpose of comparisons, the two curves shown in Fig. 3 for combined sulfate attack and 40% flexural loading and combined sulfate attack and drying–wetting cycles are also superimposed in the figure. It can be seen from the figure that the two curves involving drying–wetting cycles are very similar. This demonstrates that the drying–wetting cycles are much more dominant than the 40% flexural loading in terms of the reduction of Erd. This again explains the capillarity absorption effect is stronger than the diffusion effect. Fig. 5 shows the variations of Erd with time for C50 concrete specimens subjected to combined sulfate attack, drying–wetting cycles and various different levels of flexural loading. It is found from the figure that the variation curves of Erd with time are very close when the flexural loading levels are smaller than 40% of the maximum flexural load. However, when the flexural loading level
Table 1 Property of cement, FA, and GBFS. Parameters
Cement
FA
GBFS
SiO2 (%) Al2O3 (%) Fe2O3 (%) CaO (%) MgO (%) Na2O (%) SO3 (%) K2O (%) Loss Specific surface (m2/kg) Specific gravity (g/cm3)
21.38 4.71 3.68 65.03 2.53 – 0.53 – 0.67 362.2 3.15
52.42 33.25 5.53 3.49 1.07 – – – 1.45 400.0 2.35
33.48 12.21 1.40 36.35 10.60 1.27 0.66 0.56 0.36 460.0 2.82
Fig. 2. Erd vs. time for C50 concrete exposed to sulfate attack.
35
J. Gao et al. / Construction and Building Materials 39 (2013) 33–38 Table 2 Proportions of concrete mixture prepared. Mix
Water (kg/m3)
C50 C50F30 C50K50
157 157 157
Binder (kg/m3) Cement
FA
GFS
449 314 225
– 135 –
– – 225
Fine aggregate (kg/m3)
Coarse aggregate (kg/m3)
Super-plasticizer (kg/m3)
w/b
674 674 674
1123 1123 1123
4.04 4.04 4.04
0.35 0.35 0.35
Fig. 3. Erd vs. time for concrete exposed to sulfate attack under flexural loading or drying–wetting cycles.
Fig. 4. Erd vs. time for C50 concrete exposed to sulfate attack under flexural loading and drying–wetting cycles.
is greater than 40% of the maximum flexural load, Erd dropped very quickly with time. This indicates that the mechanical effect may also be important if the stress level is higher. The reason for this is likely due to the micro-cracks, which can be only generated and propagated in a certain stress level. The generation of microcracks together with the sulfate attack and drying–wetting cycle action makes concrete deteriorate rather quickly. Fig. 6 shows the influence of added FA and GBFS in concrete on the variation of Erd with time. It is clear from the figure that the concrete with added FA has the best performance in terms of the resistance of concrete to sulfate attack. In contrast, the concrete with added GBFS has slightly better performance than the concrete without mineral admixture. This is probably due to the reduction of pore sizes, filling and the pozzolanic effect. It is believed that the concrete with mineral admixture has denser porosity, and
Fig. 5. Erd vs. time for C50 concrete exposed to sulfate attack under different flexural loading and drying–wetting cycles.
Fig. 6. Erd vs. time of C50/C50F30/C50K50 concrete exposed to sulfate attack under flexural loading and drying–wetting cycles.
the smaller the size of the additives, the smaller the pore sizes. So it is an effective method to improve the resistance of concrete to sulfate attack by adding some mineral admixtures of small sizes. 3.4. Microstructure of concrete exposed to sulfate under drying– wetting cycles and flexural loading ESEM was used to examine the microstructure of concrete under the combined actions of sulfate attack, drying–wetting cycles, and flexural loading. It is conceived that the microstructure evolution mainly depends on the sulfate attack as both the drying–wetting cycles and flexural loading can only speed up the process of the attack effect. Sarkar [14] developed a numerical model of cementitious materials under external sulfate attack and explained
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J. Gao et al. / Construction and Building Materials 39 (2013) 33–38
Fig. 8. Pore size distribution of C50 concrete exposed to sulfate attack under drying–wetting cycles and flexural loading at different ages (MIP). (a) Cumulative distribution and (b) differential distribution.
Fig. 7. ESEM images of microstructure of C50 concrete exposed to sulfate attack under drying–wetting cycles and flexural loading at different ages. (a) 0 d, (b) 110 d, and (c) 180 d.
the process, which contained two stages of filling and expanding in the long term. Microstructure of C50 concrete specimens with various different immersion times (0 d, 110 d, 180 d) under the combined actions of sulfate attack, drying–wetting cycles, and flexural
loading are shown in Fig. 7. A few reaction products were observed in concrete pores after 110 d (see Fig. 7b). These products were further developed after 180 d (see Fig. 7c), which almost filled the pores. The experimental result of MIP shown in Fig. 8a revealed that the porosity decreases with the immersion time. This is further demonstrated by Fig. 8b, which shows that the most probable pore sizes at different immersion times have almost no change, but the ratio of pore size around the most probable pores decreases apparently. Fig. 9 shows the growth of expansion products in the specimens under the combined actions of sulfate attack, drying–wetting cycles, and flexural loading after 110 d. It can be seen from these photos that needle-like ettringite (a) and flake gypsum (b) grow along the surface of coarse aggregates. Fig. 10a shows the XRD patterns of concrete exposed to sulfate attack under the combined actions of drying–wetting cycles and flexural loading at three different times. The results show that, after 180 d immersion of specimens in sodium-sulfate solution there are clear diffraction peaks of ettringite and gypsum, which demonstrates that ettringite and gypsum indeed formed in the concrete. XRD patterns for C50 specimens with FA and GBFS, and C50 specimen without admixture are shown in Fig. 10b. Compared with XRD pattern of C50 concrete, the diffraction peaks of ettringite and gypsum of concrete with FA and GBFS were hardly observed. This may be explained as FA and GBFS delayed the transport process of sulfate ions and reduced the amount of expanding products caused by the chemical reactions between sulfate and hydration products. In addition, the results of differential
J. Gao et al. / Construction and Building Materials 39 (2013) 33–38
37
Fig. 10. XRD patterns of concrete exposed to sulfate under drying–wetting cycles and flexural loading. (a) C50 (0 d/110 d/180 d) and (b) C50/C50F30/C50K50 (180 d).
Fig. 9. ESEM images of expansion products of C50 concrete exposed to sulfate attack under drying–wetting cycles and flexural loading (110 d).
pore size distribution shown in Fig. 11 revealed that the most probable pore sizes of C50F30 and C50K50 are smaller than that of C50, which demonstrates that FA and GBFS could improve the poremicrostructure of concrete. Based on the macroscopic and microscopic evidence, it has been revealed that there was a dense process of concrete subjected to sulfate attack because sulfate ions transported into the pore and reacted with hydration products, especially C3A, to form ettringite and gypsum, which could block the pore, micro-cracks and interfacial transition zone (ITZ). After filling the pore, these products continued to form and produce expanding pressure. When the pressure exceeded the concrete’s tensile strength, cracks produced and Erd dropped sharply.
4. Conclusions Drying–wetting cycles and flexural loading can accelerate the sulfate accumulation in concrete and aggravated the damage degree of concrete subject to sulfate attack. For case where the flexural loading is below 40% of the maximum flexural load, drying– wetting cycles play a main role in the damage process of concrete.
Fig. 11. Differential pore size distribution of C50 concrete exposed to sulfate attack under drying–wetting cycles and flexural loading at different ages (0 d).
Significant effect of flexural loading on the deterioration of concrete under combined actions of sulfate attack, drying–wetting cycles and flexural loading was found when the flexural loading exceeds 40% of the maximum flexural load.
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J. Gao et al. / Construction and Building Materials 39 (2013) 33–38
Mineral admixture can increase chemical resistance of concrete subjected to sulfate attack under loading and drying–wetting cycles, which can be explained by reducing pore size in concrete, filling and pozzolanic effect and making microstructure denser than concrete with no admixture. There was a dense process of concrete subjected to sulfate under flexural loading and drying–wetting cycles. Sulfate ions transported into the pore and reacted with hydration products to form ettringite and gypsum, which could block the pore, micro-crack and interfacial transition zone (ITZ). Acknowledgments This work was part of project ‘‘Foundation Research of Modern Environmental Friendly Concrete’’ supported by Chinese national key basic research and development plans (973 Plan) No. 2009CB623203. The authors gratefully acknowledge the financial support received from 973 Foundation. References [1] Adam Neville. The confused world of sulfate attack on concrete. Cem Concr Res 2004;34:1275–96. [2] Al-Akhras Nabil M. Durability of metakaolin concrete to sulfate attack. Cem Concr Res 2006;36:1727–34.
[3] Wee TH, Suyavanshi AK, Wong SF, Anisur Rahman AKM. Sulfate resistance of concrete containing mineral admixtures. ACI Mater J 2000;97:536–49. [4] Ouyang C, Nanni A, Chang WF. Internal and external sources of sulfate ions in Portland cement mortar: two types of chemical attack. Cem Concr Res 1988;18:699–709. [5] António Costa, Júlio Appleton. Case studies of concrete deterioration in a marine environment in Portugal. Cem Concr Compos 2002;24:169–79. [6] Roziere E, Loukili A, El Hachem R, et al. Durability of concrete exposed to leaching and external sulphate attacks. Cem Concr Res 2009;39:1188–98. [7] Bassuoni MT, Nehdi ML. Durability of self-consolidating concrete to sulfate attack under combined cyclic environments and flexural loading. Cem Concr Res 2009;39(3):206–26. [8] Bassuoni MT, Nehdi ML. Durability of self-consolidating concrete to different exposure regimes of sodium sulfate attack. Mater Struct 2009;42(8):1039–57. [9] Jin Zuquan, Sun Wei, Jiang Jinyang, et al. Damage of concrete attacked by sulfate and sustained loading. J Southeast Univ 2008;24:69–73. [10] Schneider U, Nagele E, Dumat F. Stress corrosion initiated cracking of concrete. Cem Concr Res 1986;16:535–44. [11] Klaus-Christian Werner, Yaoxing Chen, Ivan Odler. Investigations on stress corrosion of hardened cement pastes. Cem Concr Res 2000;30:1443–51. [12] Schneider U, Chen SW. The chemomechanical effect and the mechanochemical on high-performance concrete subjected to stress corrosion. Cem Concr Res 1998;28:509–22. [13] Jin Zuquan, Sun Wei, Zhang Yunsheng, et al. Interaction between sulfate and chloride solution attack of concrete with and without fly ash. Cem Concr Res 2007;37:1223–32. [14] Sarkar S, Mahadevan S, Meeussen JCL, et al. Numerical simulation of cementitious materials degradation under external sulfate attack. Cem Concr Compos 2010;32:241–52.
Contents lists available at SciVerse ScienceDirect
Construction and Building Materials journal homepage: www.elsevier.com/locate/conbuildmat
Durability of concrete exposed to sulfate attack under flexural loading and drying–wetting cycles Jianming Gao ⇑, Zhenxin Yu, Luguang Song, Tingxiu Wang, Sun Wei Department of Materials Science and Engineering, Southeast University, Nanjing 211189, China Jiangsu Key Laboratory of Construction Materials, Southeast University, Nanjing 211189, China
h i g h l i g h t s " Durability of concrete exposed to sulfate under drying–wetting cycles and load is studied. " The microstructure of interior concrete are determined by using ESEM, XRD, and MIP. " Load and drying–wetting cycles can both accelerate the sulfate corrosion of concrete. " The effect of load on the damage of concrete under combined actions depends on the ratio.
a r t i c l e
i n f o
Article history: Available online 21 June 2012 Keywords: Flexural loading Drying–wetting cycles Sulfate attack Relative dynamic elastic modulus Microstructure
a b s t r a c t The damage process of concrete exposed to sulfate attack under flexural loading and drying–wetting cycles are investigated in this paper. ESEM, MIP, and XRD were used to investigate the changing of microstructure and corrosion products of interior concrete. The results indicate that compared with the single damage process of sulfate attack, flexural loading and drying–wetting cycles can both accelerate the damage process of concrete subjected to sulfate attack. While compared with sulfate attack and drying–wetting cycles, the effect of flexural loading on the deterioration of concrete depends on the stress level. It was also found that the addition of admixtures can improve the ability of sulfate resistance of concrete when it is also subjected to mechanical loading and drying–wetting cycles. Ó 2012 Elsevier Ltd. All rights reserved.
1. Introduction Sulfate attack is one of the most aggressive environmental deteriorations that affect the long-term durability of concrete structures and can cause huge economic loss. For concrete in marine environment, drying–wetting cycles can accelerate the deterioration of concrete such as in the environment like splash and tidal zone. Furthermore, concrete in marine environment endures multifarious loading all the time. In conclusion, the concrete in the splash and tidal zone of marine environment suffers from the deterioration of the coupling function of salt solution, drying–wetting cycles and mechanical loading. There have been many reports on the damage process of concrete exposed to sulfate attack [1–6] and the coupling function of sulfate attack and drying–wetting cycles [7,8] or flexural loading
[9–12]. However, few of them were related to the experimental results about the damage process of concrete under the combined actions of sulfate attack, drying–wetting cycles and mechanical loading. In this paper, the damage process of concrete exposed to sulfate attack under drying–wetting cycles and flexural loading is investigated. Relative dynamic elastic modulus (Erd) is determined using the ultrasonic method. The microstructure and corrosion products of interior concrete are determined using the environmental scanning electron microscopy (ESEM) and X-ray diffraction (XRD). The porosity and pore size distribution of concrete at different ages are determined using the mercury intrusion porosimetry (MIP).
2. Experimental details 2.1. Materials and mix proportions
⇑ Corresponding author. Address: Room 210, School of Materials Science, Southeast University, Jiangning District, Nanjing 211189, China. Tel./fax: +86 025 52090639. E-mail addresses: [email protected] (J. Gao), [email protected] (Z. Yu), [email protected] (L. Song), [email protected] (T. Wang), [email protected] (S. Wei). 0950-0618/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.conbuildmat.2012.05.033
Chinese standard 52.5 R (I) Portland cement equivalent to ASTM Type I Ordinary Portland cement supplied by Wuhan huaxin Cement Corporation in China was used. Fly ash (FA) and grounded blast furnace slag (GBFS), river sand with fineness modulus of 2.6 and coarse aggregate of crushed limestone with a maximum size of 20 mm were used. A polycarboxylate-type super-plasticizer with a water-reducing rate of 25% by weight was used. The chemical composition and properties of the ce-
34
J. Gao et al. / Construction and Building Materials 39 (2013) 33–38
ment, FA and GBFS are shown in Table 1. The mix proportion is given in Table 2. A 30% FA and 50% GBFS were used as replacement of cement on weight-to-weight basis in the C50F30 and C50K50 concrete mixtures, respectively. Concrete specimens were cast in steel molds of 70 70 280 mm, removed from the molds 24 h after casting, and cured in the condition of 20 ± 3 °C and 95% of relative humidity for 60 d. Except for the two opposite surfaces (70 280), other four surfaces of the specimen were covered with epoxy resin before the exposure experiments. 2.2. Experiment programs In the present study, sodium-sulfate solution (5% by weight) was used in the experiment. Concrete specimens were first dried at a temperature of 60 °C for 45 h, then cooled down in air at a room temperature for 3 h. Then, they were immersed in the sodium-sulfate solution at room temperature for 21 h, followed by 3 h drying in air, which represents a drying–wetting cycle. This drying–wetting cycle was repeated until it reached a specified time. The mechanical loading was applied by using the four-point bending loading equipment, as shown in Fig. 1. 2.3. Testing parameter The dynamic modulus of elasticity (Erd) of the specimen was measured by using a high-accuracy nonmetal ultrasonic analyzer, which was used to detect the internal defect and damage degree of the concrete specimen[13].
3. Results and discussion 3.1. Erd variation of concrete exposed to sulfate attack Fig. 2 shows the variation of Erd with time for C50 concrete specimens immersed in 5% Na2SO4 solution. It is clear that under sulfate attack alone Erd increases with time at the initial stage, followed by a stabilizing stage and then decreases slowly with the immersion time in the later stage. The peak value of Erd was about 1.035 occurring at 80 d. With further increased immersion time Erd decreases, but the rate of decrease is very slow. For example, after 174 d immersion, Erd reduced to 1.024. This indicates that the damage process of concrete under sulfate attack alone is extremely slow. 3.2. Erd variation of concrete exposed to sulfate attack under flexural loading or drying–wetting cycles Fig. 3 shows the variations of Erd with time for C50 concrete specimens immersed in 5% Na2SO4 solution as well as subjected to the drying–wetting cycles or flexural loading. It can be seen from Fig. 3 that the addition of 40% flexural loading did not change the variation of Erd very much. In contrast, the drying–wetting cycles have a significant influence on the variation of Erd and they can speed up the damage process of concrete. For instance, after 174 d immersion, under combined sulfate attack and 40% flexural loading Erd value was 1.03, while under combined sulfate and drying–wetting cycles it dropped to 0.935. This can be explained by the two different mechanisms of accelerating damage of drying–wetting cycles and flexural loading. The former made sulfate accumulated
Fig. 1. The schematic diagram of loading equipment.
and crystallized in concrete by cycled moisture gradient and capillarity absorption, which accelerated the damage process obviously with number of cycles. However, the latter accelerated the damage process of concrete by extending micro-crack of interior concrete, which enlarged the transmission path of sulfate. The results proved that the diffusion effect depending on concentration gradient was not clear as the capillarity absorption depending on moisture gradient.
3.3. Erd variation of concrete exposed to sulfate attack under flexural loading and drying–wetting cycles Fig. 4 shows the variations of Erd with time for C50 concrete specimens immersed in 5% Na2SO4 solution as well as subjected to combined drying–wetting cycles and flexural loading. For the purpose of comparisons, the two curves shown in Fig. 3 for combined sulfate attack and 40% flexural loading and combined sulfate attack and drying–wetting cycles are also superimposed in the figure. It can be seen from the figure that the two curves involving drying–wetting cycles are very similar. This demonstrates that the drying–wetting cycles are much more dominant than the 40% flexural loading in terms of the reduction of Erd. This again explains the capillarity absorption effect is stronger than the diffusion effect. Fig. 5 shows the variations of Erd with time for C50 concrete specimens subjected to combined sulfate attack, drying–wetting cycles and various different levels of flexural loading. It is found from the figure that the variation curves of Erd with time are very close when the flexural loading levels are smaller than 40% of the maximum flexural load. However, when the flexural loading level
Table 1 Property of cement, FA, and GBFS. Parameters
Cement
FA
GBFS
SiO2 (%) Al2O3 (%) Fe2O3 (%) CaO (%) MgO (%) Na2O (%) SO3 (%) K2O (%) Loss Specific surface (m2/kg) Specific gravity (g/cm3)
21.38 4.71 3.68 65.03 2.53 – 0.53 – 0.67 362.2 3.15
52.42 33.25 5.53 3.49 1.07 – – – 1.45 400.0 2.35
33.48 12.21 1.40 36.35 10.60 1.27 0.66 0.56 0.36 460.0 2.82
Fig. 2. Erd vs. time for C50 concrete exposed to sulfate attack.
35
J. Gao et al. / Construction and Building Materials 39 (2013) 33–38 Table 2 Proportions of concrete mixture prepared. Mix
Water (kg/m3)
C50 C50F30 C50K50
157 157 157
Binder (kg/m3) Cement
FA
GFS
449 314 225
– 135 –
– – 225
Fine aggregate (kg/m3)
Coarse aggregate (kg/m3)
Super-plasticizer (kg/m3)
w/b
674 674 674
1123 1123 1123
4.04 4.04 4.04
0.35 0.35 0.35
Fig. 3. Erd vs. time for concrete exposed to sulfate attack under flexural loading or drying–wetting cycles.
Fig. 4. Erd vs. time for C50 concrete exposed to sulfate attack under flexural loading and drying–wetting cycles.
is greater than 40% of the maximum flexural load, Erd dropped very quickly with time. This indicates that the mechanical effect may also be important if the stress level is higher. The reason for this is likely due to the micro-cracks, which can be only generated and propagated in a certain stress level. The generation of microcracks together with the sulfate attack and drying–wetting cycle action makes concrete deteriorate rather quickly. Fig. 6 shows the influence of added FA and GBFS in concrete on the variation of Erd with time. It is clear from the figure that the concrete with added FA has the best performance in terms of the resistance of concrete to sulfate attack. In contrast, the concrete with added GBFS has slightly better performance than the concrete without mineral admixture. This is probably due to the reduction of pore sizes, filling and the pozzolanic effect. It is believed that the concrete with mineral admixture has denser porosity, and
Fig. 5. Erd vs. time for C50 concrete exposed to sulfate attack under different flexural loading and drying–wetting cycles.
Fig. 6. Erd vs. time of C50/C50F30/C50K50 concrete exposed to sulfate attack under flexural loading and drying–wetting cycles.
the smaller the size of the additives, the smaller the pore sizes. So it is an effective method to improve the resistance of concrete to sulfate attack by adding some mineral admixtures of small sizes. 3.4. Microstructure of concrete exposed to sulfate under drying– wetting cycles and flexural loading ESEM was used to examine the microstructure of concrete under the combined actions of sulfate attack, drying–wetting cycles, and flexural loading. It is conceived that the microstructure evolution mainly depends on the sulfate attack as both the drying–wetting cycles and flexural loading can only speed up the process of the attack effect. Sarkar [14] developed a numerical model of cementitious materials under external sulfate attack and explained
36
J. Gao et al. / Construction and Building Materials 39 (2013) 33–38
Fig. 8. Pore size distribution of C50 concrete exposed to sulfate attack under drying–wetting cycles and flexural loading at different ages (MIP). (a) Cumulative distribution and (b) differential distribution.
Fig. 7. ESEM images of microstructure of C50 concrete exposed to sulfate attack under drying–wetting cycles and flexural loading at different ages. (a) 0 d, (b) 110 d, and (c) 180 d.
the process, which contained two stages of filling and expanding in the long term. Microstructure of C50 concrete specimens with various different immersion times (0 d, 110 d, 180 d) under the combined actions of sulfate attack, drying–wetting cycles, and flexural
loading are shown in Fig. 7. A few reaction products were observed in concrete pores after 110 d (see Fig. 7b). These products were further developed after 180 d (see Fig. 7c), which almost filled the pores. The experimental result of MIP shown in Fig. 8a revealed that the porosity decreases with the immersion time. This is further demonstrated by Fig. 8b, which shows that the most probable pore sizes at different immersion times have almost no change, but the ratio of pore size around the most probable pores decreases apparently. Fig. 9 shows the growth of expansion products in the specimens under the combined actions of sulfate attack, drying–wetting cycles, and flexural loading after 110 d. It can be seen from these photos that needle-like ettringite (a) and flake gypsum (b) grow along the surface of coarse aggregates. Fig. 10a shows the XRD patterns of concrete exposed to sulfate attack under the combined actions of drying–wetting cycles and flexural loading at three different times. The results show that, after 180 d immersion of specimens in sodium-sulfate solution there are clear diffraction peaks of ettringite and gypsum, which demonstrates that ettringite and gypsum indeed formed in the concrete. XRD patterns for C50 specimens with FA and GBFS, and C50 specimen without admixture are shown in Fig. 10b. Compared with XRD pattern of C50 concrete, the diffraction peaks of ettringite and gypsum of concrete with FA and GBFS were hardly observed. This may be explained as FA and GBFS delayed the transport process of sulfate ions and reduced the amount of expanding products caused by the chemical reactions between sulfate and hydration products. In addition, the results of differential
J. Gao et al. / Construction and Building Materials 39 (2013) 33–38
37
Fig. 10. XRD patterns of concrete exposed to sulfate under drying–wetting cycles and flexural loading. (a) C50 (0 d/110 d/180 d) and (b) C50/C50F30/C50K50 (180 d).
Fig. 9. ESEM images of expansion products of C50 concrete exposed to sulfate attack under drying–wetting cycles and flexural loading (110 d).
pore size distribution shown in Fig. 11 revealed that the most probable pore sizes of C50F30 and C50K50 are smaller than that of C50, which demonstrates that FA and GBFS could improve the poremicrostructure of concrete. Based on the macroscopic and microscopic evidence, it has been revealed that there was a dense process of concrete subjected to sulfate attack because sulfate ions transported into the pore and reacted with hydration products, especially C3A, to form ettringite and gypsum, which could block the pore, micro-cracks and interfacial transition zone (ITZ). After filling the pore, these products continued to form and produce expanding pressure. When the pressure exceeded the concrete’s tensile strength, cracks produced and Erd dropped sharply.
4. Conclusions Drying–wetting cycles and flexural loading can accelerate the sulfate accumulation in concrete and aggravated the damage degree of concrete subject to sulfate attack. For case where the flexural loading is below 40% of the maximum flexural load, drying– wetting cycles play a main role in the damage process of concrete.
Fig. 11. Differential pore size distribution of C50 concrete exposed to sulfate attack under drying–wetting cycles and flexural loading at different ages (0 d).
Significant effect of flexural loading on the deterioration of concrete under combined actions of sulfate attack, drying–wetting cycles and flexural loading was found when the flexural loading exceeds 40% of the maximum flexural load.
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Mineral admixture can increase chemical resistance of concrete subjected to sulfate attack under loading and drying–wetting cycles, which can be explained by reducing pore size in concrete, filling and pozzolanic effect and making microstructure denser than concrete with no admixture. There was a dense process of concrete subjected to sulfate under flexural loading and drying–wetting cycles. Sulfate ions transported into the pore and reacted with hydration products to form ettringite and gypsum, which could block the pore, micro-crack and interfacial transition zone (ITZ). Acknowledgments This work was part of project ‘‘Foundation Research of Modern Environmental Friendly Concrete’’ supported by Chinese national key basic research and development plans (973 Plan) No. 2009CB623203. The authors gratefully acknowledge the financial support received from 973 Foundation. References [1] Adam Neville. The confused world of sulfate attack on concrete. Cem Concr Res 2004;34:1275–96. [2] Al-Akhras Nabil M. Durability of metakaolin concrete to sulfate attack. Cem Concr Res 2006;36:1727–34.
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