CC BY-NC-ND
0Accepted Manuscript
Title: Physical sulfate attack on concrete lining-A field case analysis
Authors: Zanqun Liu, Zhang Fengyan, Dehua Deng, Youjun Xie, Guangcheng Long, Xuguang Tang
PII: DOI:
Reference:
S2214-5095(17)30036-0 http://dx.doi.Org/doi:10.1016/j.cscm.2017.04.002 CSCM 90
To appear in:
Received date: Revised date: Accepted date:
14-2-2017 28-3-2017 10-4-2017
Please cite this article as: Liu Zanqun, Fengyan Zhang, Deng Dehua, Xie Youjun, Long Guangcheng, Tang Xuguang.Physical sulfate attack on concrete lining-A field case analysis. Case Studies in Construction Materials http://dx.doi.org/10.1016Zj.cscm.2017.04.002
This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Physical sulfate attack on concrete lining - a field case analysis
Zanqun Liu a»*, Zhang Fengyan a, Dehua Deng a, Youjun Xie a, Guangcheng Long a, Xuguang Tanga,b
a School of Civil Engineering, Central South University, Changsha, Hunan 410075, PR China b School of Civil Engineering, Hunan University of Science and Technology, Xiangtan 411201, China
Abstract: In the tunnels of the Cheng-Kun Railway in China, concrete failure can usually be observed from the effect of sulfates. A field investigation was conducted to analyze the concrete damage mechanism by means of XRD, IC, AAS and EPMA. The results showed that (1) large amounts of Na2SO4 were generated on the surface layer of the concrete lining, causing concrete detachment layer by layer; (2) the concrete lining in contact with air had been neutralized to more than 50 mm in depth, and the cement hydration products had been converted into CaCO3; (3) ettringite and gypsum, the products of chemical sulfate attack, were also detected in the neutralized concrete lining. This analysis of a field case concluded that Na2SO4 crystallization caused the detachment of the surface layer of the concrete lining, and the chemical sulfate attack occurred in the inner part of the concrete lining.
Keywords: field case, physical sulfate attack, chemical sulfate attack, concrete
1. Introduction
Salt weathering is one of the most important degradation mechanisms that porous materials, such as stone and masonry, undergo at and near the surface of the earth [1]. When the porous material is in contact with the soil, salt (especially sulfates) containing ground water can enter the pores through capillary sorption. Evaporation, occurring in the part that is in contact with relatively dry air near the surface of the earth (evaporation zone), will lead to an increase in the salt concentration of the pore liquid in the evaporation zone and, ultimately, in supersaturation formation. Then, salt crystallization in pores causes the failure of the porous material in the evaporation zone, but with the sound part buried in the salt environment. A similar phenomenon can be found in Portland cement (PC) concrete elements partially exposed to sulfate environments. Researchers attribute the failure of concrete to sulfate salt weathering, sulfate salt crystallization or physical sulfate attack [2-3]. However, this viewpoint was questioned in the review paper [4 ] in detail because the indoor and field test results were opposed to the classic theories of salt weathering of porous materials: (1) Concrete cylinders partially exposed to a Na2SO4 solution under a high relative humidity (RH) condition show a larger damage area; (2) the sulfate salt crystallization does not occur at the portion of the sample cylinders where the SO3 content is the highest;(3) the change of pore structure of concrete due to air entraining agent did not show significant effect on concrete damage; (4) concrete with a higher water/cement (W/C) ratio is more resistant to sulfate attack, which is in contrast to one of the basic principles of salt crystallization: i.e., finer pores in porous materials cause more significant deterioration. According to
the discussion on the theory of crystallization based on the relationship between interfacial tension and growth pressure [5-6], a conclusion was deduced that the physical sulfate attack could not occur in the Portland cement concrete due to the chemical reactions between sulfates and cement hydration products, and the chemical sulfate attack may still be the main mechanism of concrete damage.
However, a large amount of white sulfate crystals on the surface of concrete is also a real observation in the field cases [3]. This phenomenon cannot reasonably be explained by the chemical sulfate attack. Therefore, what is the real fact of the concrete deterioration in the field cases?
A field investigation is quite significant and necessary to disclose the damage mechanism. In the southwestern regions of China, the groundwater contains a large amount of SO42- [7]. The concrete lining in many tunnels of Cheng-Kun Railway built in 1950s was found to be severely damaged due to the effect of sulfates (shown in Fig. 1). According to the guidance of Kunming Railway Administration, a field investigation was carried out and different micro tests were performed in the Repair Engineering Institute of SHO-BOND Corp., Japan to analyze the damage mechanism. The question will be discussed and answered according to the test results.
2. Field Investigation
Fig. 1 presents several typical pictures of damaged concrete in several tunnels.Based on the visual observations shown in Fig. 1, the concrete damage process could be described as follows:
• A surface layer of concrete approximately 5 mm thickness was detached from the concrete lining due to the sub-efflorescence of a large amount of white crystals (shown in Fig. 1 (a) (b)).
• The aggregates of the concrete could be observed after detachment of the surface layer, and a large amount of white crystals appeared on the concrete surface (shown in Fig. 1 (c));
• Due to the continuous detachment of concrete layers the steel bars were exposed to air and rusted (shown in Fig. 1(d));
• It could be reasonably induced that the concrete lining would detach layer by layer on the surface and finally break down.
In order to identify the damage mechanism, two concrete cores ($30) were drilled with water from the lining of Yangjiuhe Tunnel and Bagele Tunnel (the red line circle shown in Fig.1(c)), respectively, and detached concrete pieces from the two tunnels were also collected. Another sample of air-contacting concrete pieces was broken from the outer part of the core of Bagele Tunnel and Yangjiuhe Tunnel. The samples were analyzed by means of a Carbonation Depth (CD) test, Electron Probe Micro-Analysis (EPMA), X-Ray Diffraction (XRD), Ion Chromatography (IC), and Atomic Absorption Spectrophotometry (AAS). The samples of concrete cores and pieces, and test methods are shown in Fig. 2 and Table 1.
3. Experimental Methods (1) Sample preparation for EPMA and CD
(1) The concrete cores were reinforced by intrusion with epoxy resin;
(2) Two sliced specimens were cut and ground with kerosene from the cores as shown in Fig. 3;
(3) The sliced specimens were cleaned with isopropyl alcohol and vacuum dried (Specimen for CD test);
(4) A layer of carbon was deposited on the surface of the specimens prepared for EPMA. EPMA-1600 (made by Shimadzu Scientific Instruments) was used. For example, using an electron
beam with a diameter (probe diameter) of 50 ^m, a step width of 100 ^m, and a specimen size of 52 x 52 mm, a set of 520 x 520 pixels is obtained. The quantity of the objective element is initially expressed as an intensity of characteristic X-ray. The intensity of the X-ray is converted to mass% by using a calibration curve separately prepared based on standard samples. The concentration is expressed in a color scale and a 2D image of element distribution is obtained [8].(2) Sample preparation for XRD, IC and AAS
(1) The concrete pieces were ground and coarse aggerates were removed through a sieve of 90^m and vacuum dried for XRD analysis (MultiFlex made by Neo-confucianism motor industrial co., LTD);
(2) Ten grams of powder were sieved to 150 ^m and immersed in 50°C water. After shaking for 2 hours, the solution was removed for the IC and AAS tests.
4. Results
(1) CD test
The carbonation depth of the concrete core was measured as shown in Table2. The concrete lining in the two tunnels was almost neutralized for more than 55 mm after 60 years' exposure to the atmosphere.
(2) EPMA
The S and Na concentration in the two cores detected by EPMA is shown in Fig. 4. The S and Na were concentrated on the surface layer of the concrete core in contact with air (the white color meaning the high concentration), and the depth of high concentration was approximately 3 mm. Obviously, the high concentration of Na and S means a large amount of Na2SO4 was accumulated on the surface of the concrete lining due to the evaporation. The resulting super-saturation generated Na2SO4 crystallization. (3) IC and AAS
The concentrations of different ions in the detached concrete pieces were quantitatively analyzed by means of IC and AAS. According to the results shown in Table 3, the detached concrete pieces contained a large amount of SO42- and Na+. The Na2SO4 concentrations were quite high and had already reached 11.8% and 24.7% versus the Na+ concentrations in No.3 and No.4, respectively. It would be notably easy to reach super-saturation and generate crystallization distress.
(4) XRD analysis
The products in the concrete were further identified by XRD analysis. The patterns shown in Fig,5 indicate that (1) the detached concrete pieces were totally carbonated, and CaCO3 became the only components of the pieces; (2) Na2SO4 crystals were clearly detected not only in the detached pieces
but also in the concrete lining; (3) the identification of ettringite and gypsum means that the chemical sulfate attack also occurred in the inner part of the concrete lining; and (4) Ca(OH)2 was not observed in the detached pieces and pieces broken from the concrete core.
5. Discussion
According to the test results: (1) the concrete detachment was caused by the Na2SO4 crystallization, but the primary product of the detached concrete pieces was CaCO3; (2) ettringite and gypsum were identified in the concrete lining, meaning the chemical sulfate attack also occurred within the concrete; (3) Na2SO4 crystals could be detected in the concrete lining, however, the concrete was also carbonated without Ca(OH)2.
The abovementioned results indicate the significance of the effect of carbonation on Na2SO4 crystallization in concrete. The cement hydration products from the detached concrete pieces had been totally converted to CaCO3 due carbonation. It seems more reasonable to say that Na2SO4 crystallization caused the CaCO3 detachment from the concrete lining rather than the cement hydration products the same as for limestone. Na2SO4 could not be detected in the concrete in the tests [9-11], and this was attributed the water washing during the coring operation. However, the designed tests results without the influence of water showed sulfate crystals still could not be identified by means ESEM and XRD in spite of the damage in the concrete or pure paste under constant or fluctuating exposure conditions [12-13]. In this investigation, the Na2SO4 could also exist in the neutralized concrete shown in Fig. 6 even with the water washing during concrete cores drilling as the same as the investigation on foundation damage [14]. Therefore, it seems that Na2SO4 can just crystallize in carbonated areas where the chemical interaction between sulfate ions and cement hydration product is restrained.
According to the definition, physical attack or salt weathering [2] does not involve a chemical reaction. It should also be noted that the lack of chemical reaction between the core wall and crystallization salt is also a precondition for crystal growth according to surface physicochemical theory [5]. In a test designed to study the damage of calcium sulfoaluminate (CSA) cement paste partially exposed to Na2SO4 [6], it was very interesting to find the CSA cement paste was quietly detached into successive layers after just 7 days' exposure, and Na2SO4 crystals were identified in the layers. However, a similar test of Portland cement paste did not show successive layer detachment, and ettringite and gypsum were found to cause damage without Na2SO4 crystals. The CSA cement hydration products contain the same components as Portland cement except for Ca(OH)2, and they are inert to Na2SO4. Therefore, it can be deduced that Ca(OH)2 is the source of the chemical reaction between sulfate and Portland cement hydration products because the physical sulfate attack would occur without Ca(OH)2. Another interesting test under alternate conditions showed that Na2SO4 crystallization distress, not Na2CO3, caused the damage of CSA cement paste partially immersed in Na2CO3 solution [15]. Na2SO4 is the chemical reaction product between ettringite (the main hydration product of CSA cement) and Na2CO3. This result means that chemical reaction was the first step, although the exposure conditions met the requirement for salt crystallization. Regarding the occurrence of sulfate crystals in concrete, they formed after the concrete neutralization.
Finally, the question asked in the introduction may be answered: physical sulfate attack occurs on the surface of neutralized concrete, and chemical sulfate attack occurring in the inner part of the concrete lining can also contribute to the concrete damage.
6. Conclusions
Based on the field investigation and micro-analysis results of damaged concrete lining from tunnels of the Cheng-Kun Railway in China:
(1) Physical sulfate attack occurred on the surface layer of the concrete lining that had been neutralized; chemical sulfate attack occurred in the inner part of the concrete lining;
(2) Physical sulfate attack on concrete in the field case was a notably complicated process combining concrete carbonation, chemical reactions between sulfates and hydration products, and sulfate crystallization in the core;
(3) The effect of carbonation on the failure of concrete partially exposed to a sulfate environment warrants further study.
Acknowledgments
This work was financially supported by the National Science Foundation of China (Grant No. 51378508), the Major Technology Program of the Railways Ministry of China (Grant No. 2008G025-C). The authors wish to thank the Kunming Railway Administration for providing the test results.
Funding: This study was funded by the National Science Foundation of China (Grant No. 51378508). Conflict of Interest: Zanqun Liu, Zhang Fengyan and Dehua Deng have received research grants from the National Science Foundation of China.
No conflict of interest: Youjun Xie, Guangcheng Long and Xuguang Tang declares that he has no conflict of interest.
Funding: This study was funded by the Major Technology Program of the Railways Ministry of China (Grant No. 2008G025-C).
Conflict of Interest: Zanqun Liu, Dehua Deng, Youjun Xie, Guangcheng Long and Xuguang Tang have received research grants from the National Basic Research Program of China (Grant No. 2013CB036201 ).
No conflict of interest: Zhang Fengyan declares that he has no conflict of interest. References
[1] Benavente D, Garcia Del Cura MA, Bernabeu A, Ordonez S. Quantification of salt weathering in porous stones using an experimental continuous partial immersion method. Eng. Geolm. 2001;59(3-4):313-25.
[2] ACI 201.2R-16 Guide to Durable Concrete, Chapter 8 Physical Salt Attack, 2016.
[3] Haynes H., Bassuoni M.T., Physical salt attack on concrete, Concr. Int. 2011;33(11): 38-42.
[4] Liu Z., Deng D., De Schutter G., Does concrete suffer sulfate salt weathering? Constr. Build. Mater. 2014, 66(1): 692-701.
[5] Flatt R. J., Steiger M., Scherer G. W., A commented translation of the paper by C.W. Correns and W. Steinborn on crystallization pressure, Environ. Geol. 2007, 52(2): 187-203.
[6] Liu Z., Li X., Deng D., De Schutter G., The role of Ca(OH)2 in sulfate salt weathering of ordinary concrete, Constr. Build. Mater. 2016, 123: 127-134.
[7] Hang T, Evaluation of the corrosive concrete in Baijialing Tunnel, China Railway, 1999 (8): 29-31.
[8] Daisuke M., Kazuo Y., A Review of recent applications of EPMA to evaluate the durability of concrete, J Adv. Concr. Technol. 2007;6(3):258-298.
[9] Haynes H, O'Neill R, Mehta PK. Concrete deterioration from physical attack by salts. Concr. Int., 1996;18(1):63-8.
[10] Haynes H., O'Neill R, Neff M, Mehta PK. Salt weathering distress on concrete exposed to sodium sulfate environment. J. ACI. Mater. 2008;105(1):35-43.
[11] Irassar E.F, Di Maio A, Batic OR. Sulfate attack on concrete with mineral admixtures. Cem Concr Res 1996;26(1):113-23.
[12] Liu Z., De Schutter G., Deng D., Yu Z., Micro-analysis of the role of interfacial transition zone in "salt weathering" on concrete, Constr. Build. Mater. 2010, 24(11): 2052-2059.
[13] Liu Z., Deng D., De Schutter G., Yu Z., Micro-analysis of "salt weathering" on cement paste, Cem. Concr. Compo. 2011, 2011; 33(1):179-191.
[14] Yoshida N., Matsunami Y., Nagayama M., Sakai E., Salt weathering in residential concrete foundations exposed to sulfate-bearing ground. J. Adv. Concr. Technol. 2010;8(2):121-34.
[15] Liu Z., Hou L., Deng D., De Schutter G., Damage of calcium sulphoaluminate cement paste partially exposed to Na2CO3 Solution, accepted by J. Chin. Chem. Soc.
Fig. 1 Damaged concrete in the tunnels
Fig. 3 Sliced specimen for CD test and EPMA test
Fig. 4. S and Na concentration distribution of samples No.1 and No.2
No.3 • • ■ A Na2SOj s;o2 CaCOj
• k i
ii A I] A 1 k-JLu A jJ ^
10 20 3P0 4B 5C t m 70
29 (deg)
Fig. 5 XRD analysis patterns of concrete pieces
Table 1 Test samples and micro-analysis methods
Sample Location Test methods
No.1 Concrete Core in Yangjiuhe Tunnel CD, EPMA
No.2 Concrete Core in Bagele Tunnel CD, EPMA
No.3 Detached pieces in Yangjiuhe Tunnel XRD, IC, AAS
No.4 Detached pieces in Bagele Tunnel XRD, IC, AAS
No.5 Concrete pieces contacting air broken from core No.2 in Bag' ele Tunnel XRD
Table 2 CD test results
Sample Depth ( mm )
1 2 3 Mean value
No.1 57.8 55.9 55.7 56.5
No.2 59.5 53.7 56.5 56.6
Table 3 SO4 2- and Na + concentration
IC results No.3 No.4 AAS results No.3 No.4
Cl - (%) 0.20 0.75 Na + (%) 3.83 7.99
NO32- (%) 0.13 - K+ (%) 0.45 0.09
SO42- (%) 12.06 16.52 Ca2+ (%) 0.47 0.43
