Meeting Expectations When Selecting Concrete Rehabilitation Systems
By Mike Dadik, P.E. S.E., Carollo Engineers, Inc.
By itself, concrete has enormous compressive strength but little tensile capacity. Thus, an unreinforced concrete column can support a large load, but a beam has minimal strength without reinforcing steel. Reinforcing steel, also known as rebar, first came into common use near the end of the nineteenth century. Since then, reinforced concrete has become a preferred building material for water storage and conveyance structures, providing great strength and many possible configurations. These advantages, however, come with the challenge of protecting rebar, which is essential for long-term durability.
Fig. 1: This photo shows the site before the project was bid. The best access available was by removing the covers from the full grit basins. Photos courtesy of the author
Engineers have learned that maintaining concrete’s alkalinity is the key to this protection. With a pH between 11 and 12, concrete’s pH is very high, similar to that of lime or lye. This high pH environment surrounds reinforcing steel, forming a passivating layer that prevents corrosion. Concrete longevity depends heavily on maintaining the high pH environment. Exposure to acid breaks down this protective mechanism.
Coating concrete can minimize, or even stop damage from aggressive environmental exposures that harm the concrete and lower its pH. Properly applied, a coating can stop concrete deterioration before it progresses to the reinforcing steel and prevent costly structural repairs. This practice is commonly implemented in wastewater treatment plants, collection systems and industrial plants where concrete coatings are commonly used to reduce acid exposure. An acid/base reaction occurs when high-pH concrete is exposed to low-pH acids. There are many sources of low-pH acids. Atmospheric carbon dioxide in water forms a weak acid resulting in concrete carbonation. Extensive, widespread concrete deterioration is more commonly seen as the result of acid byproducts produced by bacteria-consuming hydrogen sulfide from wastewater. Referred to as biogenic sulfide corrosion, this may be the most common form of concrete deterioration remediated with protective coatings. Wastewater processes can also become upset, resulting in unintentional acid exposure.1
Applied when concrete is new, coatings can protect it from acid exposure. For a number of reasons, concrete may not be coated until after damage occurs. Coating deteriorated concrete is considerably more difficult than coating new concrete. Understanding the deterioration process is important when specifying and applying remedial coatings.
Reinforced Concrete Deterioration Process
Damage from acid exposure begins at the concrete surface and progresses in toward the reinforcing steel. When exposed to a low pH environment, acid/base reactions break down the concrete paste, reducing the thickness of the high-pH concrete layer that protects the reinforcing steel. The deterioration advances deeper into the concrete, eventually lowering the pH near the reinforcing steel, allowing corrosion to begin. When rebar corrosion initiates, expanding rust forms larger cracks accelerating the corrosion rate and reinforcing steel wasting, ultimately weakening the structure. Coatings can reduce deterioration by eliminating exposure and restoring the corrosion protection by replacing the lost concrete with an impermeable membrane.
Chemicals such as sulfates and chlorides also lead to concrete deterioration. Chlorides from chemicals used for wastewater disinfection will penetrate the concrete. The chlorides break down the passivating layer around the rebar. Without this protective layer, rebar can corrode. Coatings can be used after chloride contamination begins to reduce moisture in the concrete and the rate of corrosion. It is best to remove all of the chloride-contaminated concrete before a coating is applied.
Fig. 2: The contractor begins work removing the lining system to uncover extensive H2S damage.
Sulfates present in groundwater or in concrete can react within the cement paste. Expansive byproducts in the cement paste form microcracks. Chemical changes within the concrete matrix then weaken the cement paste, leaving it susceptible to water intrusion and accelerated corrosion. Sulfates are also a byproduct of biogenic sulfide deterioration, where concrete is exposed to sulfuric acid from H2S reducing bacteria. This is why biogenic sulfide corrosion is so damaging.
Sulfate exposure is best mitigated with a properly designed concrete mix before the concrete is placed. As with chloride contamination, coating to reduce moisture in the concrete after the sulfate contamination has taken place can help. This approach can be used in specific circumstances, but because most external sources of sulfate are in the ground and below-grade, coating is usually not a cost-effective approach.
Evaluating concrete deterioration
Selection of cost-effective coating and concrete repair systems should consider the extent of deterioration and substrate condition. Assessment tools range from easy-to-perform visual and nominally destructive techniques to removing core-drilled samples for laboratory analysis. The test findings provide the specifier and estimator with an idea of how much concrete should be removed during surface preparation and if structural repairs are required.
Damage from acid exposure begins with softening of the concrete surface followed by loss of the weakened paste. At the start of the process, when the deterioration is a fraction of an inch deep, the concrete may look nearly new. However, striking the surface with a sharp tool will expose the softened paste. While undamaged concrete is hard to chip, deteriorated concrete can be chipped back without much difficulty. A widespread area must be surveyed to assess the variation on the walls below, near and above the waterline, as well as the underside of overhead slabs. Floor surfaces should also be assessed though floor coatings are not typically used in water containment structures.
In wastewater and petrochemical facilities, biogenic sulfide corrosion starts with acid attack then continues with the formation of sulfates which weaken the cement paste in a two-phase process — a soluble salt byproduct forms during the second phase. The resulting loosely adhered, powdery white deposits are easy to spot and are typically the first visible damage observed after significant deterioration has occurred. The loosely adhered powder can be easily removed exposing aggregate.
As the deterioration continues, lowering the pH of concrete surrounding the reinforcing steel, rusting and spalling occurs, exposing the rebar. Exposed reinforcing steel, rust staining and spalled or cracked concrete usually indicates loss of structural strength. Hammer sounding can identify rebar corrosion below the surface. Concrete that sounds hollow when struck indicates a void possibly caused by rebar corrosion.
Removing damaged concrete and rust to expose the remaining reinforcing steel helps assess the structural repairs required. At a minimum, all reinforcing steel corrosion must be cleaned to near-white condition before repair mortar is applied prior to coating. Depending on the location, adding new reinforcing steel may be required.
Visual assessment benefits from a standardized scale to rate deterioration (Table 1). Though non-proprietary standard scales are not currently available, ICRI 310.2R concrete surface preparation replicas provide a standard to compare surface roughness.2 The replica surfaces are produced by abrasive blasting or more aggressive surface preparation techniques and may not be directly applicable to corrosion-damaged concrete. ACI 201.1 is another resource providing guidance for visual assessment.3
Table 1: Visual Concrete Deterioration Assessment Scale Example.
Visual observation and hammer-sounding can help assess the surface physical appearance and condition, indirectly providing insight into concrete pH. Measuring concrete pH at and below the surface affords a better estimation for the rate of deterioration and the work required to restore reinforcing steel protection.
Indicator pencils, litmus paper, phenolphthalein and pH meters all measure in-situ concrete alkalinity. Measurements must be taken as soon as possible after the concrete is exposed. Concrete in contact with the atmosphere will begin to carbonate lowering the pH measurement. When concrete pH is below 10 to 11, the passivating benefits are reduced and corrosion can begin.4
To measure concrete pH, first chip or grind to expose a fresh concrete surface, then wet the concrete with neutral pH (distilled or deionized) water and wait approximately a minute to solubilize the concrete compounds. Expose indicator pencils or litmus paper to the wetted surface and compare the color change against a reference standard. When using PH meters, place the electronic probe on the wetted concrete to measure the surface pH. ASTM F710, though written for concrete evaluation prior to installation of flooring, provides additional guidance.5
If removing a core sample is an option, petrographic evaluation of concrete core samples provides the best assessment of concrete condition. Phenolphthalein solution is sprayed onto a fresh core for a rapid assessment of the carbonation depth. Microscopic, petrographic examination of the concrete structure will identify other types of deterioration in addition to acid attack.
These test findings will inform the specifier and estimator about the approximate amount of concrete that must be removed during surface preparation, which is useful information when evaluating potential coating and concrete repair systems.
After assessing the concrete condition, coating selection is the next step in developing a repair approach. Rehabilitation has two components: restoring the concrete surface and enhancing the corrosion protection provided by the remaining concrete. Typically, cementitious or epoxy/polymer mortars rebuild the concrete surface. Calcium aluminate cement has inherent resistance to biogenic sulfide corrosion and does not need a coating. Factors influencing the selection of cementitious or epoxy mortars include depth of repair, cost, curing time and surface preparation for coating.
High-build coating systems are commonly used in wastewater exposures. These coatings are solvent-free, epoxy or elastomeric polyurethanes/polyureas. Thin-film coatings are generally not considered suitable for these environments; however, for less aggressive environments, an epoxy mastic or coal tar may be appropriate.
Factors influencing the selection of the coating include concrete moisture content, potential for cracking after coating installation and exposure. Table 2 presents repair mortars and coating and lining systems sorted in a commonly perceived order of increasing chemical resistance. The advantages and disadvantages of the each technology are summarized in Tables 3 and 4 and discussed in the following paragraphs.
Table 2: Repair Mortar and Lining Systems.
Grout and Mortar Surfacers
Portland Cement Mortar
These general-purpose mortars are trowel- or machine-applied at thicknesses ranging from a 1/4 of an inch to over 4 inches. Sometimes thick placements require multiple lifts. When the substrate is saturated surface-dry and neat grout is scrubbed in, the bonding strength improves.
Usually, cement mortar requires at least a week of cure time, sometimes up to 28 days before the coating can be applied. It is also a recommended practice to use abrasive blasting to remove laitance and loose material prior to coating.
Calcium Aluminate (CaAl) Cement Mortar
This specialty mortar has similar properties to cementitious mortars with the added benefit of being resistant to hydrogen-sulfide-induced corrosion. Because of this, it does not need to be coated before return to service.
These systems are commonly used in collection systems to repair manholes and vaults, since high-quality surface preparation is difficult for these confined, hard to access and isolated locations. Performance history indicates that CaAl cement mortars perform well and can also restore structural capacity, essentially building a new manhole inside the existing manhole.
Fig. 3: The finished project used calcium aluminate cement.
There are a wide variety of epoxy mortars. Some have significant structural capacity to rebuild small vaults and manholes similar to CaAl cement mortars and are also corrosion resistant.
Epoxy grouts, which are used in coating applications to restore a concrete surface, are made by mixing epoxy paste with fine sand or silica fume. Trowel- or spray-applied, these mortars are typically no more than 1-inch thick, since application thickness is limited by the material’s cost and heat generated from the epoxy curing. As with cement mortar, the bond strength is improved by scrubbing into the substrate. A dry surface will improve bond. Epoxy mortar surfacers are easy to apply and usually require a short cure time before coating. Abrasive blasting the repaired mortar surface may not be required.
Table 3: Grout and Mortar Surfacers Comparison.
Coating and Lining Systems
Coating systems can protect concrete in vapor spaces and below the water surface in tanks. Lining systems installed as sheets, are effective only in the vapor space above the water surface.
Epoxy mastic and coal tar epoxies were industry workhorses in the mid-to-late twentieth century. With tighter VOC limits, concern for worker safety and development of new technologies, these coatings are now applied less frequently. Nonetheless, these products are surface tolerant and can be applied at 10-to-20 mils thick, providing a good membrane to protect the concrete.
Even with these benefits, entirely covering surface defects and blowholes is difficult with this type of coating. In immersion service, entrapped solvents can lead to osmotic blistering that compromises the film’s integrity and cracks can reflect through the thin coating film leaving concrete exposed at the crack edges. When biogenic sulfide corrosion begins at these very fine cracks, widespread damage occurs as expansive byproduct peels off the coating, progressively exposing more concrete.
Elastomeric polyurethane and polyurea, amine-cured epoxy and novolac epoxy are often 100-percent/ultra-high-solids formulations that do not contain solvents. Prior to application, these two-component coating systems (resin and catalyst) are mixed to begin the curing process. Depending on the formulation and temperature, the curing reaction can last less than a minute to almost an hour. The rapid cure time reduces working time.
Some coatings can be premixed (hot-potted) and applied with airless spray equipment. All of these coatings can be applied using plural-component systems with metering pumps that mix the components immediately before application. The curing process requires accurate component ratios. The pumps and application equipment must be well maintained to ensure proper mixing. Operating the metering pumps and mixing the coatings requires specialized training; additional inspection and quality-control measures are highly recommended.
The solvent-free coatings allow for thicker-film application from 40-to-125 mils or more. The thick film provides a less permeable and more durable membrane to protect the concrete than thin-film coatings.
These coatings can address reflexive cracking concerns. Elastomeric coatings’ tensile strain capacity allows them to stretch, bridging shrinkage cracks. The epoxy coatings have less tensile capacity but are recognized as having enough capacity to bridge most common concrete cracks.
Concrete age must be considered when evaluating a coating’s ability to bridge cracks. Newly installed concrete is more prone to drying shrinkage cracking than concrete that has been in service. Existing cracks in aged concrete should be evaluated because they will continue to open and close with temperature cycles. To a lesser extent, varying loads will cycle existing cracks. Given this information, crack width, condition and temperature variation are important factors to assess when selecting a coating.
In addition to these three factors, substrate surface texture must also be taken into consideration for coating selection. Plural-component coatings tend to be viscous and require heating to pump and apply. On an irregular surface, air can be trapped behind the uncured film. The trapped air then heats and expands causing pinholes in the coating film. Some products address this problem with fillers or by expanding the film with an inert gas: the fillers or bubbles intercept the pinhole before it can penetrate the film. Elastomeric polyurethane/polyurea coatings and epoxy coatings use this approach.
Ultra-High-Solids Elastomeric Polyurethane, Polyurea and Hybrids
This subset of thick-film coatings uses polyurethane, polyurea or polyurethane-polyurea-hybrid chemistry to provide good chemical resistance and tensile elongation capacity. The resulting elastomeric film is well-suited to bridging cracks.
These compounds are hydrophobic while curing. Moisture will repel the uncured coating. The primary mechanism for bonding to concrete is wetting into small pores exposed at the concrete surface. Moisture near the surface of the concrete pore structure reduces the coating bond. This can be problematic when the moisture source is groundwater or an adjacent tank. Water on the opposite side of the wall being coated reduces the coating bond, then exerts a hydrostatic pressure through the porous concrete on the cured impermeable film potentially delaminating the coating. The failure can be localized blisters or widespread. Some manufacturers recommend a thin-film epoxy primer to hold back moisture and improve coating adhesion.
Table 4: Coating Systems Comparison.
Ultra-High-Solids Amine-Cured Epoxies
These thick-film coatings have very good chemical resistance. Additives and fillers modify the properties to bridge cracks. They do not have the crack-bridging properties of elastomeric coatings; however, in-service performance demonstrates that they are effective at protecting concrete, especially after early-age cracking has occurred. Epoxy coatings are moisture tolerant, providing a good bond when small amounts of moisture are present. This makes them a good choice when groundwater or adjacent tanks may apply hydrostatic pressure.
Novolac epoxy is similar to amine-cured epoxy but has better chemical resistance. It is the most chemical resistant of the three types of ultra-high solids coatings.
Plastic Sheet Liners
PVC or HDPE sheet lining is commonly used in the U.S. for new concrete construction. The sheet liner is installed on the inside face of concrete forms before new concrete is placed. During retrofit application, a thick layer of epoxy mastic is applied and the sheets are pressed into the surface. After the sheets are installed, the seams are sealed using heat-welded closure strips.
Properly installed plastic sheet lining systems provide long-lasting protection from H2S exposure in the vapor space. This system is not as effective for submerged applications.
Other Sheet Liners
Sulfur cement and stainless-steel plates are similar to plastic sheets except that they are attached to the concrete with anchor rods. These systems are rarely used in the U.S. Stainless steel can also be used with structural repair materials to rebuild heavily damaged concrete that has lost structural capacity.
CONCRETE REPAIR AND COATING SPECIFICATIONS
After a repair and coating system is selected, the next step prior to application is to specify the repair and coating system. For competitively bid or sole-source projects, the specifications must include necessary information for bidders. A well-defined scope of work reduces the risk of misunderstandings and costly changes. Table 5 lists important topics to consider when writing a scope of work or specification.
Table 5: Specification Topics.
Concrete deterioration can be arrested and repaired to extend a structure’s useful life with a properly selected and specified repair and coating system. Perhaps the biggest problem that arises during this work is misaligned expectations: it is the specifier’s responsibility to clearly spell out what the applicator can expect to encounter. During concrete repair and coating projects, the engineer, estimator and applicator all benefit from a good understanding of the damage, appropriate repair methods and conditions or expectations that will require special effort. Applicator input will also be useful when uncertainties arise.
With an accurate assessment of the concrete condition, exposure, and work constraints, along with a clear understanding of the benefits and limitations of coating systems, a well-written, concise specification will be the basis of a successful concrete rehabilitation project — a project in which everyone’s expectations can be met.
About the Author
Mike Dadik is a structural engineer with more than 20 years of experience specializing in wastewater and water construction. He has worked on concrete rehabilitation and coating projects throughout his career, through which he has earned an in-depth understanding of the unique challenges presented by this type of work. Dadik has been a member of the SSPC Northern California-Nevada Chapter steering committee since 2012.
R. Nixon, Corrosion Probe, Inc. and Corrosion Testing Laboratories, Inc., “Design Considerations for Corrosion Protection of Anaerobic Digesters in Wastewater Treatment Plants,” SSPC 2015, Las Vegas, Feb. 3 to 6, 2015.
International Concrete Repair Institute (ICRI): 310.2R: Selecting and Specifying Concrete Surface Preparation for Sealers, Coatings, Polymer Overlays, and Concrete Repair.
ACI 201.1 Guide for Conducting Visual Inspection of Concrete In Service.
Portland Cement Association (PCA): IS536: Types and Causes of Concrete Deterioration.
ASTM International (ASTM): F710: Standard Practice for Preparing Concrete Floors to Receive Resilient Flooring.