Use of nanomaterials for concrete pipe protection
Apr 10, 2007
Microbial induced corrosion (MIC) in concrete sewer conveyance systems is one of the most common types of deterioration encountered in such structures. This corrosion involves a chemical reaction between hydration products in the hardened concrete and biologically produced sulfuric acid, which alter the concrete chemical composition leading to early deterioration, loss of strength and, in extreme cases, pipe collapse due to inability to resist external earth and live loads.
H2S(g) + 2O2 → H2SO4 (1)
A freshly placed concrete has a pH of approximately 11 – 13, depending upon the mix design. This pH is due to the formation of a stable compound, calcium hydroxide Ca(OH)2, a common by-product produced during the hydration of cement. This initial high pH on the surface of the concrete will not allow any bacterial growth; however, this high pH state lasts for only a short period of time and the pH level slowly declines over time, due to interaction with carbon dioxide (CO2) and hydrogen sulfide (H2S) gases. These gases form weak acidic solutions when dissolved in water (carbon dioxide forms carbonic acid and hydrogen sulfide forms thiosulfuric acid and polythionic acid), that lower the pH of the concrete surface to 9 or 9.5. Thiobacillus bacteria that have a unique ability to convert hydrogen sulfide to sulfuric acid in the presence of oxygen start colonizing at a pH of about 9. The first colony of bacteria reduces the surface pH value from 9 to 6.5 by excreting sulfuric acid. These bacteria will not be able to survive at pH values lower than 6.5, and thus the colony dies and the residence is then taken over by another species that is capable of surviving below a pH of 6.5. This newly formed species of Thiobacillus further reduces the pH value from 6.5 to 4 before dying off. This process of colonizing and dying of bacteria continues, until the pH at the surface of the concrete can be brought to a value as low as 1 or 0.5 (Islander et al., 1991). The sulfuric acid produced by the bacteria interacts chemically with the hydration products in the hardened concrete paste. The chemical reactions involved in corrosion of concrete are:
H2SO4 + CaSi ↔ CaSo4 + 2H+ (2)
H2SO4 + CaCO3 ↔ CaSo4 + H2CO3 (3)
H2SO4 + Ca(OH)2 ↔ CaSo4 + 2H2O (4)
A large number of methods have been developed to mitigate MIC in wastewater collection systems. The following discussion will consider only approaches that involve applying a protective coat to the host pipe internal surface. Spray lining is the most widely used coating technique. However this method suffers from several disadvantages. It is often difficult to create proper adhesion between the coating material and the internal surface of the pipe. Also, the “rebound” effect from hurling the coating material at the pipe’s wall can result in uneven coating thickness. Combined with a non-uniform application, this uneven layer thickness tends to result in an uneven stress loading and subsequent premature failure. In summary, since the coating material does not penetrate the pipe surface, it tends to flake and fall down with time under loading from shrinkage, mechanical loading from the transported media and/or cracks induced by differential settlement of the pipe (Dietrich, 1989; James, 2003).
Controlling Sewer Crown Corrosion Using the Crown Spray Process (James, 2003):
Originally proposed by Esfandi (1986) and later adopted and further developed by the sanitation group of Los Angles County, California, the “crown spray” process involve a spray system mounted on a floating platform that sprays the crown region of the sewer pipe with magnesium hydroxide. The purpose of crown spray process is to neutralize the acid that has been generated by the Thiobacillus bacteria at the crown region of the pipe by spraying it with a high pH material (magnesium hydroxide), thus elevating the region’s alkalinity. The spray process needs to be repeated on the regular basis (time between applications approximately 6-9 months) due to depletion and washout of the magnesium hydroxide.
A method of treating concrete pipes against MIC by employing 100% solid polyurethane instead of PVC and coal tar. The 100% solid polyurethane coating was developed at Madison Chemicals, and is claimed to be more economical than PVC and more reliable than coal tar. Test results are provided for pressure, performance and bond strength. The solid polyurethane coating was tested under laboratory and fullscale conditions for a 5-month period. During the tests, no defects such as blistering, cracking, spalling or sticking were noticed. The product was also reported to perform satisfactorily when tested for chemical resistance (vapor and liquid phase tests) and bond strength (ASTM D4541 and ASTM C321).
Conshild is an antimicrobial chemical which is applied to the internal wall of the pipe to inhibit the growth of bacteria on the pipe surface. It is typically applied by mixing it with concrete and spraying it as shotcrete. Other application modes include mixing it with an epoxy, with the mixture applied to the concrete surface using spraying equipment.
Hewayde et al. studied coating a concrete pipe with copper oxide (CO) and silver oxide (SO) to provide bio-toxic surfaces. The experiment consisted of three concrete pipes, out of which two were internally coated with CO and SO, while the third served as a control. The heavy metal oxides were mixed with epoxy and sprayed on the internal surface of the pipe to a thickness of 350 microns. Specimens were then filled to a two third depth, with a nutrient solution that has a sulfate concentration of 350 mg/L and dissolved oxygen concentration of 0.5 gm/L. Next, six liters of bacterial solution that had Desulfovibro Desulfuricans bacteria were added to each of the tanks. The pipes were then filled to the brim with a nutrient solution. The flow inside the pipe was maintained at 26 L/min, simulating a worse case condition in gravity sewer pipes. At the conclusion of the test period the bacterial growth in the coated pipes was found to be significantly lower (by up to 99%) than that of the control pipe. Also the formation of slime layer was almost absent in the coated pipes. This experiment proved that heavy metal oxides such as CO and SO at low concentrations have a considerable toxic effect on the bacteria species responsible for generation of the hydrogen sulfide and sulfuric acid. A potential problem identified during the study was the leaching of silver oxide into the nutrient solution due to poor bonding between silver oxide and epoxy, which led to SO flaking and mixing with the nutrient solution.
Electrokinetics is a technique that uses a low DC voltage across the electrodes to move particles or contaminants in a porous medium. Electrokinetics comprises three major processes – electrophoresis, electro-osmosis and electro-migration. Electrophoresis is the movement of colloids, or charged particles, under the influence of an electrical field; electro-osmosis is the movement of fluid, or, in this case, coating solution, through the capillaries and pores from the anode to the cathode. Electromigration is the movement of ions or ionic complexes.
- Removal of heavy metals from mine tailings (Kim et al, 2002).
- A permeation technique for injecting grouting material into silty soils to enhance their strength properties (Thevanayagam, 2003)
- Chloride extrication and re-alkalization of reinforced concrete (Velivasakies et al., 1998).
This paper describes a innovative technique that can be used to coat a concrete pipe using electrokinetics. The process was developed to overcome problems such as poor adhesion, flaking and shrinkage of a coating material that were encountered in traditional spray-on coating processes. The benefits of coating a pipe with heavy metal oxides were demonstrated by Hewayde et al (2005). The present work aims to address the adhesion problem, and focuses on the development of a new coating process in which nano-scale particles of cuprous oxide are driven into the concrete under the influence of a weak electric field.
The experimental design involves a total of 35 mock concrete pipes. The results reported in this paper focus on the testing and results from a pilot study of three specimens, intended to provide an indication of the effectiveness of the proposed coating procedure for new concrete pipes, moderately deteriorated concrete pipes and severely deteriorated concrete pipes. Of the three specimens, one serves as a control (e.g., newly installed pipe), the second specimen was degraded to represent a partly corroded pipe (i.e., exposure of the coarse aggregates), while the third specimen was degraded to represent a moderate-to-severe corroded pipe (loss of coarse aggregates; random exposure of steel reinforcement). The degradation of the specimens was accomplished by filling a cavity in their center with sulfuric acid solution (pH = 0.7), which was refreshed regularly until the desired effect was reached. Following the surface preparation treatment, the three specimens were electrokinetically coated using a cuprous oxide solution. Following the coating process, a series of optical and chemical tests were undertaken to evaluate the effectiveness of this coating process in terms of the extent of penetration of the nanoparticles into the concrete and the concentration of the heavy metal as a function of depth.
Unaltered Specimen (control; fresh concrete pipe): Following the electrokinetics coating process, the solution remaining in each of the specimen was removed and the inner wall of the “pipe” exposed by cutting ‘slices’ using a masonry diamond saw. The slices were inspected using a powerful optical microscope for evidence of copper particles deposited in the concrete matrix. The images from the optical microscopy examination revealed what appeared to be copper oxide particles deposition inside the concrete matrix. Figure 4 presents selected snap shots from the microscopy examination. Copper depositions can be identified as green color areas on the gray-white background of the hardened cement matrix. Images 4a and 4b display distinct green areas within the gray-white background typical for hardened Portland cement paste. These areas are suspected to be traces of copper oxide driven into the concrete matrix via the electrokinetics process. Images 4a and 4b were taken from the non-corroded and partially corroded specimens, respectively. In contrast, image 4c displays a uniform gray color, with no traces of green areas. This suggests that the copper-oxide did not penetrate into this specimen during the electrokinetics process. A possible explanation is that the weak electrical field is present between the anode and cathode only. Exposing the reinforcement resulted in a “shortcircuit” with the electrical current gradient flowing primarily to the partially exposed reinforcement rather than moving through the less conductive concrete material.
|% COPPER IN CONCRETE
The objective of this paper was to report on the study of a potential new coating process for pre-cast concrete pipes (dry-cast mix design; low w/c ratio; high compaction effort) using electrokinetics. The results demonstrated the feasibility of depositing copper oxide particles inside a dry-cast concrete matrix using a weak electric field. Optical microscopy images appeared to reveal the presence of copper oxide inside the concrete matrix in the case of new and partially deteriorated concrete pipe cylinders treated using the proposed coating process. No penetration of copper-oxide was detected in the fully deteriorated specimen, potentially due to the development of a “short-circuit” via the exposed reinforcement mesh that came into contact with the cuprous oxide lactate solution. Atomic absorption spectroscopy analysis proved that copper was deposited in the concrete matrix of all 3 specimens. However, in the case of the non-corroded and partially corroded specimens the percentage of copper was considerably higher than in the fully-corroded specimens. The treatment of the partially corroded concrete was shown to be the most successful than the non-corroded specimen that had added wall thickness and higher electrical resistance. Possible explanations are the larger surface area due to the corrosion process and the reduced wall thickness resulted in a higher concentration per unit volume between the pipe surface and the reinforcement. Further studies will include the durability of the coating under physical, thermal and chemical loadings, and a half-scale performance assessment where treated specimens will be exposed for simulated sewer and colonized with bacteria cultures. Research will also continue in terms of increasing the transport of copper into the concrete matrix and determining the level of copper in the matrix that will inhibit microbial induced corrosion.
The authors would like to acknowledge Dr. Sven Eckland for his assistance with AAS and Dr. Ray Sterling for his continuous support.
Acar, Y.B. and Alshawabkeh, A.N. 1993. ‘Principles of electrokinetic remediation’, Envir. Sci. and Technol., 27(13), 2638-2647.
EPA, 1992, “Detection, Control and Corrosion of Hydrogen Sulfide Corrosion”, EPA/1992-312- 014/40169.
Hewayde, E., Nakhla, G.F and Allouche, E.N. (2005). ‘Beneficial Impact of Coatings on Biological Generation of Sulfides in Concrete Sewer Pipes’, Journal of Structures and Infrastructure Engineering, Taylor and Francis (in press).
Islander, R.L., Devinny, J.S., Mansfeld, F., Postyn, A., Shih, H, 1991. ‘Microbial Ecology of Crown Corrosion in Sewers’. J. Environ. Engg, Vol., 117, No.6, pp 751-770.
James K.M., 1970. ‘In place Treatment of Foundation Soils’, J. of the Soil Mechanics and Foundations Division, 96(1), 73-110.
James, J. 2003. ‘Controlling Sewer Crown Corrosion Using The Crown Spray Process with Magnesium Hydroxide’, Underground Construction Technology Conference, Houston, TX.
Kim, S., Kyoung, W. and Stuben, D. 2002. ‘Evaluation of Electrokinetic Removal of Heavy Metals from Tailing Soils’, J. of Environ. Engrg, 128(8), 705-715.
Parker, C. D., 1946, “The Isolation of Species of Bacterium Associated with the Corrosion of Concrete Exposed to Atmospheres Containing Hydrogen Sulfide”, Australian Journal of Experimental Biology and Medical Science, Vol. 23, pp. 81-90.
Jongh, P.E., Vanmaekelbergh, D., and Kelly J., 1999. ‘Cu2O: Electrodeposition and Characterization’, J. Chem. Mater, 11, 3512- 3517.
Shiwei, G., 2006. Synergistic Protection against Microbiologically Induced Corrosion Using a 100% Solid Polyurethane Incorporated with Antimicrobial agents”, http://www.madisonchemical.com/pdf_tech_papers/Synergistic_Protection_Against_MIC.pdf
Thevanayagam S. and Jia., W. 2003. ‘Electro-osmotic Grouting for Liquefaction Mitigation in Silty Soils’, 3rd International Specialty Conference on Grouting and Ground Treatment, February 10–12, New Orleans, Louisiana, USA, pp. 1507-1517
William, E. Shook, L. and Bell, W., 1998. ‘Corrosion Control in Concrete Pipe and Manholes’, Water Environmental Federation, Orlando, Florida. http://www.conshield.com/corrosionControl.htm
Velivasakis, E., Henriksen, S/K. and Whitmore, D., 1998. ‘Chloride Extraction and Re-alkalization of Reinforced Concrete to Stop Corrosion”, J. of Performance of Constructed Facilities, 12( 2), 77-84.
Esfandi, E. (1986). “Proposal to Conduct Research for Control of Sewer Corrosion”, Internal Memorandum, County Sanitaion Districts of Los Angles County.
Dietrich Stein, (1989). “Rehabilitation and Maintenance of Drains and Sewers”, John Wiley & Sons Inc. ISBN: 3-433-01316-0.
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