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The Basics of Deteriorating Concrete at Wastewater Plants: Tips On astewater treatment plants are among the harshest of environments for high-per- formance protective coat- ings. In a wastewater facil- ity, there is never only one cause for a structure to degrade; there are numer- ous causes, and because Wof the multiple causes, once a structure starts to degrade, it does so rapidly. This article gives a basic introduction to, and review of, the chemical and other aggressive exposures within wastewater facilities; how the exposures can cause degradation of a structure; how to diag- nose a corrosion or deterioration problem; The Basics of Deteriorating how to repair the structure; and how to prevent corrosion from recurring. The arti- Concrete at Wastewater Plants: cle will use an example of a wastewater clarifying tank made of hydrated Portland By Heather Bayne, SSPC cement concrete to illustrate the above. Tips on Causes, Repair, and Resources Corrosive Environments Assets,” October 2006, pp. 50–63; sulfate ions, forming sulfide ions, which in Wastewater Treatment Tanks and G. Hall,JPCL, “Out of Sight, Out of Mind, are released back into the wastewater. Wastewater clarifying treatment tanks and Often Out of Order,” October Through chemical reactions in the waste- need to be protected and maintained 2004, pp. 40–48.) JPCL, water system, the sulfide ions combine because their environment exposes with hydrogen to form hydrogen sulfide, them, on a daily basis, to chemical which further reacts and forms hydro- attack, abrasion erosion, chloride ion- SewageChemical in Attack a wastewater storage tank gen sulfide gas. The gas reduces the induced corrosion, and freeze-thaw con- must receive chemical treatment, biologi- pH of the concrete. ditions, summarized below. (For detailed cal treatment, or both. The chemicals Once the concrete pH is reduced discussions of causes of deterioration in used are manufactured acids, which, from approximately 12 to 9.5, sulfuric wastewater treatment structures, see R. when discharged into the wastewater, acid can be formed. Its formation A. Nixon, “Deterioration of Wastewater lower its pH, causing acid attack of the occurs because at wastewater facilities, Treatment and Collection System hydrated Portland cement concrete. the atmosphere around the concrete Sewage contains sulfate ions. Sewage structures contains moisture and ample Photos, top, left to right: Oil contamination (left); traveling through the wastewater systems oxygen. With the combination of the coating delamination (center); bare concrete with leaves behind a layer of sludge. The moisture, oxygen, and lower pH, sulfur bugholes exposed (right). Courtesy of SSPC. oxidizing bacteria (SOB) can colonize Photo, bottom: Wastewater clarifying tank sludge contains sulfate-reducing bacteria (underground). Courtesy of Sauereisen, Inc. (SRB). The SRB react with oxygen in the on the concrete substrate. The SOB www.paintsquare.com JPCL September 2009 47 use the oxygen and hydrogen gas pre- film forms over the surface to protect it history of operations may reveal periods sent to form sulfuric acid, which will from further corrosion, provided the film of high operating temperatures or other cause acid attack of Portland cement remains intact. The highly alkaline envi- aggressive conditions. Answers to ques- concrete. ronment of hydrated Portland cement in tions such as the following may help you Concrete wastewater storage tanks the concrete can maintain the passive diagnose problems . also undergo sulfate attack and carbon- protection film. But the protective film will • When was the tank built? ation of the concrete in the head- be destroyed if moisture, chloride ions, • What surface preparation method and spaces. In sulfate attack, the sulfates and oxygen penetrate pores or cracks to coating system were used (if any) and react with the hydrated Portland cement reach the reinforcing steel surface. Local when? paste and form a by-product, which pro- corrosion cells are then established, and • Was there a third party QA inspector motes expansion of the concrete rust forms on the surface of the reinforced during construction or coating of the through solid volume increases. steel, increasing the volume of steel, tank, and is documentation from the Carbonation is a natural occurrence in which in turn creates tensile forces within inspection available? concrete exposed to the atmosphere. the concrete. Because the tensile strength If the history of the tank is not avail- Carbon dioxide in the atmosphere of concrete is relatively weak, the con- able, proceed directly to the next part of reacts with the hydroxide in the hydrat- crete will crack to relieve the tensile assessing the structure, performing a ed Portland cement paste to form a car- stresses. Once the concrete begins to condition survey. bonate, which causes shrinkage of the crack, water, oxygen and aggressive Condition surveys are typically con- cement paste. Expansion from sulfate chemicals can freely enter the concrete ducted before writing a repair specifica- attack and contraction from carbonation and further attack the embedded rebar, tion. They include comprehensive and weaken concrete tanks. escalating the deterioration. systematic visual and analytical analyses of the existing conditions. There is no one single method or for- Freeze-Thaw Deterioration WastewaterAbrasion and can Erosion also contain suspended It is inevitable that concrete will degrade mat that may apply for every structure or solid material, such as sand, rocks, ice, or over time when exposed to differential job; however, the more detailed the sur- silt. The solid materials impinge on the thermal conditions (cold and hot) and vey and the more experienced the sur- surface of a concrete clarifying tank dur- humidity cycling (wet and dry) on oppos- veyor, the more reliable will be the bud- ing turbulent water flow conditions, caus- ing sides of the structure or to intermittent get and specification prepared from the ing an abrasive breakdown of the con- water immersion along with freezing tem- survey. Some facility owners have their crete and leaving a smooth wear pattern peratures. own qualifications and requirements for on the substrate. performing the survey. (Reminder: When Corrosion Occurs—Steps for Repairing Though concrete coating assessments a Wastewater Clarifying Tank and surveys are normally conducted by WhenChloride-Induced concrete is placed Corrosion around reinforc- There are four basic steps in the repair of experienced concrete surveyors, this arti- ing bars (rebar), the steel surface initially a deteriorated wastewater clarifying tank. cle is written for those who desire an corrodes. Then, a tightly adherent oxide 1. Assess the condition of the concrete introduction to, or review of, the survey structure. process.) Heather Bayne is a 2. Diagnose the problem. The following are two main reasons for protective coatings 3. Develop the repair specification. periodically conducting surveys of struc- professional in 4. Develop an inspection and mainte- tures in a wastewater facility. SSPC’s Product nance program. • Detecting early concrete or coating Development deterioration and gauging its progress Department, where • Determining what maintenance/repair she is responsible for Step 1—Assess the Condition of the technical writing, actions may become necessaryj Concrete Structure contributing articles to JPCL, responding to A basic understanding of causes of con- In general, two basic types of surveys scientific inquiries, and generally acting as a crete corrosion is essential to performing are used to determine the condition of technical resource for SSPC members. She a successful repair of a concrete tank and existing concrete structures to formulate is a member of SSPC, the American its lining system. In addition, if available, plans for maintenance actions: the visual Chemical Society, ASTM, and NACE the history of the wastewater treatment survey and the detailed survey. Each International. Ms. Bayne is a graduate of the tank may provide clues to causes of the survey type has its own purpose and lim- University of Pittsburgh. present condition. A review of the plant’s itations. In a visual survey (described in www.paintsquare.com 48 JPCL September 2009 this section), a surveyor gathers informa- through (visual survey) are valid for all con- sure, chemical exposure, and tempera- tion through observation, with no testing crete substrates but are especially impor- ture or sampling. In a detailed survey (see tant for wastewater treatment tanks and • Determining if the structure has aes- Step 2 on p. 51), a surveyor incorporates other structures in severe service. thetic value (rarely a concern with waste- the information gathered through the A minimum walk-through consists of a water treatment tanks), is primarily an visual survey and investigates deteriora- visual assessment of the overall conditions industrial structure, or both, and then tion further by collecting samples and of substrates and coatings on plant struc- visually assessing and documenting the conducting tests. tures. Surveyors record the types, extent, exposure and age of the concrete/coating The principals conducting surveys and distribution of defects and failures • Visually surveying the entire layout of must have expertise in coatings and using standard terms and a standard rating the structure. If knowledgeable facility must carefully follow standard test system. Surveyors pay attention to special representatives are available, ask them to method procedures. patterns, such as whether deterioration
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