Industry News

An important naturally occurring process in wastewater
systems is the production of hydrogen sulfide (H₂S) from decaying organic matter. H₂S exists in a dissolved state within the wastewater, but may be released into a gaseous state resulting in the unpleasant “rotten egg” odors that cause so many complaints from neighbors. But the real impact occurs within the system itself, when atmospheric H₂S comes in contact with the moist surfaces of a pipeline or other structure.

Thiobacilli bacteria quickly convert the H₂S to sulfuric acid (H₂SO₄), a colorless, oily liquid that reacts exothermically with water. As hydrogen sulfide levels rise, bacteria colonies proliferate, forming an extremely corrosive “slime layer” that can rapidly cause weakening and decomposition of even the most massive concrete and steel structures.

Many components of a sanitary sewer collection system, including trunk and interceptor lines, manholes, wet wells, and other structures, experience severe degradation of concrete and metallic surfaces due to H₂S. Interior concrete surfaces are particularly susceptible, and can become soft and chalky and suffer a major loss of surface area. The pasty white mass observed on the concrete surfaces above the water line is calcium sulfate, aka gypsum, a primary product of concrete decomposition by sulfuric acid. The calcium sulfate superficially resembles concrete, but it provides very little structural support, especially when wet.

Concrete corrosion is perpetuated when hydrated calcium sulfate hydroxide, known as ettringite, is formed in the contact zone between the soft calcium sulfate layer and the sound, uncorroded concrete surface. The ettringite that forms in wastewater systems is a soft but expansive white substance that occupies more space than its constituent elements. As it forms, the ettringite forces the corroded concrete away from the sound concrete and greatly accelerates the corrosion process by continually exposing fresh surfaces to attack by sulfuric acid.

In most cases, the worst deterioration of the concrete can be seen just downstream from transitions and other turbulent areas (e.g. manholes, drop inlets, etc.) due to H₂S off-gassing. Usually, the extent of the damage is not known until a detailed inspection and analysis is carried out. Only then is it possible to design an appropriate and cost-effective plan to arrest and correct the degradation.

Concrete Conditions: The V&A Inspection and Rating System
V&A has developed a systematic approach to the field investigation of H₂S damage, as well as a convenient rating system to objectively denote various degrees of corrosion damage.

Due to the prevalent high concentrations of atmospheric hydrogen sulfide, entry into corrosion-impacted structures requires a self-contained breathing apparatus (SCBA) and a confined-space permit. During the inspection, the structures are evaluated for structural integrity and samples of the degraded concrete are removed and analyzed. The pH of the concrete samples is measured and often the samples of corroded concrete have pH values of 2 or less; for comparison, new concrete typically has a pH of around 12.

Over a period of a dozen years, V&A has gradually developed a visual standard system for characterizing the type and amount of degradation observed on the H₂S-exposed interior surfaces of concrete tanks, pipes etc., based on the overall appearance of the concrete; loss of hardness; smoothness; cracking; spalling; and condition of reinforcing steel. These standards and the associated rating system are shown in Figure 1.

Damage Level	Description	Photo
1	Overall: little or no damage to concrete Hardness: no loss of hardness of mortar Smoothness: no loss of smoothness Cracking: none Spalling: none Reinforcing steel: not exposed or damaged
2	Overall: observable damage to concrete mortar Hardness: some loss of mortar hardness Smoothness: small-diameter aggregate exposed Cracking: thumbnail-sized cracks of minimal frequency Spalling: shallow spalling of minimal frequency; no related rebar damage Reinforcing steel: may be exposed, but not damaged or corroded
3	Overall: significant loss of concrete mortar; damage to reinforcing steel Hardness: complete loss of mortar hardness Smoothness: larger-diameter aggregate exposed Cracking: ¼" to ½" cracks, moderate frequency Spalling: deep spalling of moderate frequency; related rebar damage Reinforcing steel: exposed, damaged, and corroded, but reparable
4	Overall: rebar severely corroded; significant damage to structure Hardness: complete loss of mortar hardness Smoothness: large-diameter aggregate exposed Cracking: high frequency of cracks, ½" and greater Spalling: deep spalling at high frequency; related rebar damage Reinforcing steel: corroded or consumed; structural integrity diminished

By factoring in these many conditions, the consultant can prepare a quantitative evaluation of the structure. This in turn guides the consultant and owner in choosing the best strategy for corrective action.

Having an objective, quantitative evaluation of the structures has another value for the facility owner as well. It helps the utility or municipality prepare to comply with the new GASB 34 (Government Accounting Standards Board, Statement No. 34) and CMOM (“Capacity, Management, Operations, and Maintenance”) regulations, which will soon require condition ratings and computer-based management systems for virtually all municipal assets, including civil infrastructure.

Choosing a Corrective Strategy
Once the condition of the structure is quantified, there are basically three approaches available in regard to taking the necessary action: rehab, replace, or (most recently) treat.

Rehabilitation. If the structural integrity has not been compromised, most corrosion-damaged facilities can be successfully rehabilitated. The structure will have to be cleaned, using either an abrasive blast or a high-pressure water blast. Lost mortar cover must be replaced. An especially prevalent option to control the effects of H₂S corrosion is by the selective use of materials less susceptible to H₂S corrosion. Corrosion-resistant materials include protective linings (such as PVC), nitrile rubber gaskets, protective coatings (such as epoxies, enamels, polyurethanes, and bitumastics), and a wide range of fiberglass and ceramic products. All of these materials can be used to rehabilitate the structures in question and prolong their remaining life. Usually, it costs less to rehabilitate a structure than to replace it.

Replacement. Replacement of a structure may be the only option available, especially if structural integrity has diminished to the “point of no return.” Replacement costs are generally higher than rehabilitation costs. However, replacement may also create an opportunity to gain a longer-lived facility that will be easier and less costly to maintain.

Chemical Treatment. Numerous chemicals are now available that can be added to the sanitary sewer flows to mitigate and/or control the effects of hydrogen sulfide corrosion. Addition of chemicals such as sodium hydroxide, sodium hypochlorite, hydrogen peroxide, potassium permanganate, and air can be used to reduce H₂S levels. Less H₂S will mean less slime layer and thus less corrosion and less degradation of concrete and metallic structures. The downside is that chemical treatment does require a continual effort, it poses a health risk to the maintenance staff, and it may increase recurring maintenance costs significantly.

Bottom Line
Our experience consistently indicates that the useful life of many concrete structures could be dramatically extended, with proper care, to well beyond their original design life, with potential savings of literally hundreds of millions of dollars to municipalities and taxpayers. Two essential steps in achieving this are (1) obtaining accurate, quantitative condition assessments for these important public infrastructure assets, and (2) securing sound technical and planning advice regarding H₂S mitigation and structure rehabilitation methods.

For further information concerning the V&A Concrete Condition Rating System, call or write
Jose L. Villalobos, P.E., at 510-903-6600 or info@vaengr.com.

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