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 is accompany you on your preliminary One tool that can be used when begin- concentrated on the sunny side of a struc- investigation and record their known infor- ning a survey is the ACI 201.1R, ture, on the ocean exposure, near the mation as well as your own initial obser- Guide ground, or in hard-to-reach places. The vations. (A tape recorder may be useful.) for Making a Condition(American Survey of Concrete minimum walk-through is intended to pro- • Drawing a diagram of manageable sec- Institute,Concrete www.aci.org). in Service It presents a sys- vide a benchmark for comparison with later tioned-off zones tematic approach for surveying. It defines surveys, including a detailed survey. Key • Photographing all pertinent physical 10 types of cracking, 31 types of deterio- components of a minimum walk-through conditions and recording them, per zone ration, and 17 surface defects, and it include, but are not limited to, the following. A visual walk-through of a wastewater contains 49 photographs of concrete dis- • Obtaining all relevant information on the treatment tank will allow the surveyors to tress. Summarized from ACI 201.R, the structure, coating and past/present expo- see, for example, if there areas of spalling following guidelines for a minimum walk- sure conditions, such as physical expo- and pop-outs, indicating chloride-induced
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www.paintsquare.com JPCL September 2009 49 Click our Reader e-Card at paintsquare.com/ric rebar corrosion and freeze-thaw deterio- ifying tank may, for example, indicate is a summary of general steps for surface ration. The headspace of the tank might that moisture is present (to be expected), preparation of the substrate and substrate also show signs of degradation. that degradation from an acidic environ- repair (which includes installing repair ment due to an acidic pH has taken materials and protective coatings). The place in the headspace area, and that sidebar describes measures for repairing AfterStep condition2—Diagnose assessment, the Problem a detailed the existing coating on the structure is an deterioration caused by rebar corrosion. survey is needed to fully diagnose the epoxy (could be determined by laborato- • Surface Preparation Basics: SSPC-SP problem. In a detailed survey, zones of ry analysis of a paint chip sample from 13/NACE No. 6, “Surface Preparation of coated areas (diagrammed in the visual the site). Core samples from each deteri- Concrete” (www.sspc.org), provides a survey) are further subdivided, and phys- orated area may give further diagnostic detailed description of surface preparation ical measurements are made of the coat- information, such as the quality of the and should be consulted. SSPC-SP ing to assess its condition. Detailing the adhesion of the existing coating and the 13/NACE No. 6 is relevant to preparing all coating condition includes, but is not lim- porosity of the concrete matrix. concrete for lining but is especially critical ited to, the following actions. for concrete in severe service such as 1. Determine and record the condition wastewater treatment. The standard is of the substrate/surface, e.g., cracks or Step 3—How to Repair summarized below. delamination per zone. Tests of bond Oncea Deteriorated the cause Structure? of corrosion is known, a The mechanical bond between the strength, vapor emissions, pH, and other repair method can be developed. Below substrate and the coating system is factors may be needed. 2. Determine and record the depth of Repairing Chloride-Induced Rebar Corrosion contamination by taking a concrete core sample. Chloride-induced corrosion of rebar is a common cause of concrete degradation and must 3. Determine and record the condition be addressed before further repair measures can be taken. This form of corrosion is usually of the protective coating system per zone displayed as spalling on the concrete surface. Spalling is the chipping or fragmenting of a (if applicable). Coating condition assess- surface or surface coating caused by differential thermal expansion or contraction. Rebar ment also requires various tests to be corrosion is often found in wastewater clarifying tanks. To repair concrete and rebar after performed including a concrete/coating chloride-induced corrosion, follow ICRI Guideline No. 310.1 (International Concrete Repair bond/cohesive test. Obtain dry film thick- Institute, www.icri.org), summarized below. ness per SSPC-PA 9 (www.sspc.org) on 1. Remove all concrete that is loose or delaminating. both failed and intact areas of existing 2. Undercut exposed corroded rebar by creating a minimum ¾-inch clearance between the coating. exposed rebar and surrounding concrete. 4. Identify and record the environmen- 3. Extend concrete removal along the rebar until an area is reached that is free of bond- tal conditions, per zone, under which the inhibiting corrosion and is well bonded to surrounding concrete. coating system will be required to protect 4. Take care to not disrupt non-corroded rebar exposed during undercutting. the substrate. 5. Secure loose reinforcement to secured bars. 5. Measure and record the areas of 6. Remove corrosion from rebar by abrasive blasting. concrete failure per zone for repair. 7. Repair concrete using an appropriate material that also contains an inhibitor for chloride- 6. Measure and record areas of coat- induced corrosion. ing failure per zone. One class of inhibitors—penetrating corrosion inhibitors—can be used in several ways 7. Measure and record total surface to slow chloride-induced rebar corrosion. For example, a liquid amino alcohol-based pene- area to be coated. trating corrosion inhibitor additive can be incorporated into the concrete aggregate, sprayed 8. Prioritize repair/coating time lines directly onto the finished concrete surface, or drilled into an existing structure to effectively per zone. decrease the rebar corrosion. 9. Take samples of the substrate, coat- For the drilling method, a hole is drilled into the cured concrete structure and the additive ing, corrosion by-products, and other is inserted into the hole. The hole is then repaired using a cementitious mortar. When the contaminants per zone if required for lab- penetrating corrosion inhibitor reaches the rebar, it forms a protective layer around the oratory analysis. steel. 10. Write a comprehensive analysis of It should also be noted that the effectiveness of chloride-induced corrosion inhibitors is findings based on all acquired accurate dependent upon the permeability of the concrete and the amount of inhibitor reaching the information rebar. This corrosion prevention method is fairly new; therefore, only a small amount of A detailed survey of a wastewater clar- published literature on its effectiveness is available. www.paintsquare.com JPCL September 2009 51 essential to secure functionality. To and water jetting are also among the water clarifying tank, filler compounds are achieve proper adhesion and prevention methods that can be used to remove con- used to fill voids, irregularities, and air of corrosion of a substrate, several sur- taminants, laitance, and weak concrete, pockets in the concrete. A filler compound face cleaning and preparation methods to expose subsurface voids, and to pro- is a viscous material that has the consis- should be chosen, based on the condition duce a sound concrete surface with ade- tency of a “putty” and is applied using a of the concrete and the requirements of quate profile and surface porosity. smooth plasterer’s rubber float trowel. the coating system to be applied. (All pre- The acceptance criteria for concrete Once the material is applied, any excess pared concrete surfaces need to be surfaces after surface preparation should material is removed using the edge of the repaired to the level required by the coat- be specified, but SSPC-SP 13/NACE No. rubber float trowel. ing system in the intended service condi- 6 provides some guidance. Once the voids, irregularities and air tion.) Where concrete deterioration is severe, pockets are filled, creating a (relatively) One of the first steps when preparing a structural integrity may need to be smooth surface, it is necessary to fill in any concrete surface is removal of protrusions restored prior to application of protective cracks that would allow water infiltration. such as burrs, sharp edges, fins, and con- lining barriers. This is accomplished be use of rapid-set- crete spatter. All concrete that is not Repair Basics: To improve a porous ting mortar or hydroactive grouts. The sound must be removed. All contamina- concrete• matrix in a deteriorating waste- material is mixed and immediately applied tion, form-release agents, efflorescence, curing compounds, and existing coatings Table 1: Repair Products determined to be incompatible with the coating to be applied must be removed. Material Fast Set High High Application Purpose Strength Elasticity Method Detergent water cleaning and steam Portland Cement Gunite, trowel, Resurfacer cleaning are used to remove oils and and Modified cast-in-place grease from concrete. Power tool meth- Calcium Aluminate ods, including circular grinding, sanding, Portland-based Pumpable for Resurfacer and wire brushing, can remove existing Cementitious wet-spray coatings, laitance, other weak concrete, application and protrusions in concrete. Impact-tool Hydraulic Water Plug Hand Waterproofing methods are also used to remove existing Catalyzed Injection Waterproofing coatings, laitance, and weak concrete. Hydrophobic These methods include scarifying, plan- Injection Liquid ing, scabbling, and rotary peening. Elastomeric By hand Waterproofing Dry or wet abrasive blasting, vacuum- Asphalt Modified using a glove assisted abrasive blasting, shot blasting, Urethane Lining Click our Reader e-Card at paintsquare.com/ric www.paintsquare.com 52 JPCL September 2009 Click our Reader e-Card at paintsquare.com/ric Click our Reader e-Card at paintsquare.com/ric 53 JPCL September 2009 ] One approach to repairing and resur- Once a proper substrate barrier exists, For wastewater clarifying treatment Other coating technologies such as Table 1 lists some repair products used Editorʼs note: For a detailed study of sur- www.paintsquare.com facing a wastewater clarifying tank is to use a Portland cement and modified calci- um aluminate material. This fast-set mate- rial might be chosen if a quickservice return is to needed. The material can be applied by trowel and can cure withinhours. 5 Once the resurfacer cures, the tank should be checked for areas ofinfiltration. If water water infiltration is found, the affected areas can be corrected by inject- ing a catalyzed hydrophobic liquid. For the headspace of the tank, an elastomeric asphalt-modified urethane lining can be applied by hand using a glove. Because the headspace is prone to deterioration from hydrogen sulfide gas, the urethane system could be chosen for its strong resistance to this gas. it is time to apply atem. protective coating Depending sys- on the coating system chosen, application can be accomplished by trowel, airless spray, or plural-compo- nent spray. The coating system should be applied per the manufacturer’s product data sheet (PDS) to ensure optimum physical properties and a uniform surface without pinholes or holidays. tanks, the chemical resistance of the coating is extremely important. Epoxy coatings offer a high degree of chemical resistance and ease of application if the surface is prepared and the coating is applied For appropriately. example, two to three coats of a polyamide epoxy might be suitable for lining a wastewater tank. polyureas, urethanes, and vinyl esters also offer a high degree of chemicaltance, resis- but often are not environmentally friendly or easily applied. You can consult by injection to the area displaying leakage. for the interior of a wastewater tank. [ face preparation of repair materials before lining application in wastewater environ- ments, see the article by V. OʼDeaissue in on this pp. 32–45. coating manufacturers and other experts a quality inspection and maintenance Pre-Surface Preparation for all commonly recommended protec- program is the • Post-Surface Preparation SSPC Guide for Planning tive coatings for concrete substrates and (www.sspc.org). It is • Coating Conditions for Application Coatings Inspection wastewater treatment facilities, in particu- intended to assist coating and lining • Coating Application lar. Be sure to ask about the advantages inspection companies, contractor quality • Post-Application of Coating and disadvantages of each system. control (QC) personnel, and owners in • Post-Curing developing a key tool to ensure that coat- • ing and lining inspection is the best it can Summary Step 4—Develop an Inspection be. This tool will provide the contractor Wastewater treatment creates severely Toand reduce Maintenance costly and Program timely repairs, it is with guidance on how to plan to do com- corrosive environments that call for pro- important to develop an inspection and prehensive QC, which will provide a tection of all structures at wastewater maintenance program for the structure. record of objective evidence that the plants. This article gave an overview of Again, such a program is relevant to any work has met contract requirements. concrete deterioration, diagnosis of its structure but is particularly important for The quality inspection and mainte- causes, repair, and prevention, using a structures in severe service such as nance program should consist of inspec- concrete clarifying tank to illustrate key wastewater treatment tanks. tion hold points, which are critical periods points. If corrosion is detected on a struc- The inspection and maintenance pro- during the project when work is stopped ture, it is important to determine what the gram should include a procedure for con- until the work-to-date has been inspected cause of corrosion is and create a repair ducting surveys of structures in a waste- and the contractor has been authorized method to stop corrosion from continuing water facility periodically to enable early to proceed. Hold points will vary, but the or recurring. There are steps that can be detection of concrete or coating deterio- ones listed here should be considered as taken to prevent corrosion or detect it in ration. basic for most work, including waste- its early stages to reduce structural prob- A tool that can be used to help develop water treatment tank jobs. lems and the costs of their repair. JPCL
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www.paintsquare.com 54 JPCL September 2009 Is wastewater eating your tank? Protect it with CIM.
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