PROTECTION OF CONCRETE SUBSTRATES
TECHNICAL considerations for selecting coatings for use over concrete substrates are discussed with a review of some of the generic coatings currently being used.
When applying coatings for corrosion protection; concrete substrates present a unique set of circumstances for material selection, surface preparation and application procedures. There are many different aspects to consider as compared to coating steel, although the basic parameters for any successful coating system are the same.
- Good Specifications
- Proper Coating Selection
- Proper Surface Preparation
- Correct Application Techniques
- Good Inspection (Quality Control)
- Good Records Keeping
Over the years, the technology regarding coatings which are applied over metallic substrates, particularly ferrous metals, has been developed to the nth degree. Only in recent years have the problems associated with coatings being used over concrete substrates received serious attention. The spectrum of applications is very wide indeed. Even though metallic substrates also abound in most of these same environments, the nature of the substrate (which is one of the primary factors that dictate the product selection) is significantly different. The function of the coatings may also be different. Steel is not waterproofed, where waterproofing may be a major function of a coating used over concrete.
In addition to what we normally think of as surface type protective coatings, penetrants and special sealers may be used to protect concrete. These products may serve other functions in addition to corrosion protection, such as:
- Providing protection from thermal cycling, such as freeze/thaw weathering.
- Providing a pre-treatment for subsequent coatings.
The penetrant/sealants work in several ways. Some are merely surface sealers and recoating from time to time is a necessity. Others actually penetrate the porosity of concrete and form a crystalline structure filling the voids in the concrete. Others combine with free lime, which has not been completely reacted in the hydration process, forming an aerosilica gel and filling the porosity and minute cracks in the concrete matrix. Case histories show that some of these products have stopped water seepage when applied to the open face or negative side of a structure as opposed to being applied to the buried or positive side, as is waterproofing.
This discussion leads us to a different set of considerations than is used when selecting a coating, preparing the surface or applying a coating to a steel substrate. Steel has an established set of criteria for surface preparation and for checking film integrity and continuity. (Though being worked on jointly by the National Association of Corrosion Engineers and the Steel Structures Painting Council under a joint task group designation, TG-F, to date there are no consensus industry standards for the surface preparation of concrete).
DIFFERENCES BETWEEN CONCRETE AND METALLIC SUBSTRATES
The three most obvious differences between metallic and concrete substrates, as regards coatings, are density, permeability and flexibility. The inherent porosity of concrete allows for the transmission through, and retention within the concrete matrix, of liquids. Concrete's inflexibility allows cracking of its mass from external influences. Hairline cracks may also develop in the curing process. Such cracking along with concrete's permeability poses the dual problems of water or other liquid influence from outside a structure (particularly below grade) and the effluence of dangerous or hazardous fluids from containment structures in the environment. Moisture retention within the concrete, if excessive, will inhibit the bond of most coatings, although there are moisture insensitive coatings on the market.
Construction and expansion joints must receive special attention. In an area where ground water could create a problem, a vapor barrier should be used under concrete at ground level. Compounding these considerations is the fact that it is extremely difficult (if not impossible) to get consistent or substantially identical pours of concrete with any degree of repeatability. Nearly every pour will differ to some degree from the next, even though they may come from the same supplier on the same day.
The concrete may vary in the amount of air entrainment, the degree of laitance on the surface, honeycombing, finishing methods (if any), curing/hardening/release agents, etc. From a structural perspective, such variances may fall well within acceptable parameters. But protective coatings can sometimes be unexpectedly sensitive to some naturally occurring material characteristics or construction variables. This can prove downright frustrating to the specifier, supplier, applicator and inspector.
Finally, some coatings are sensitive to the chemistry of concrete and will not bond to it without some special handling or treatment. In the past, acid washes, followed by a thorough rinsing with clean water, have been used to neutralize the high pH of the concrete surface, which is caustic in its original state. Current practice is to avoid acid etching where possible, due to environmental and handling problems.
COATINGS USED ON CONCRETE
We cannot cover all of the coatings applied over concrete in this short space. We will look at some of the more commonly used coatings.
ORGANIC COATINGS - THIN FILM
For the purposes of this paper, thin film coatings are defined as those coatings usually applied by brush, roller or spray; are not reinforced with a scrim or other filler; are applied at less than 30.0 mils in a single coat. It should be noted that thin film coating systems, consisting of more than one coat, may exceed 30.0 mils.
Epoxies - Probably the most commonly used coatings in this category are the high solids/high build epoxies. Generally, a mist or wash coat is first applied to fill and seal the natural porosity of the concrete. This helps prevent bubbling and blistering often seen as the result of air entrapment when applying the full strength materials over porous surfaces.
Phenolic modified epoxies are used in flooring applications where they have some better chemical resistance and wearability under high traffic conditions.
Coal tar epoxies and their performance are well documented. They were the standard for many years in the wastewater and pipeline industries. In recent years, they have been challenged by newer technologies.
100% solids epoxy coal tars were originally designed to go over green concrete, that is, concrete which has cured just long enough to support and accept the coating. They have proven to be very effective in certain containment and lining applications, where quick turn around is a requirement. Their application characteristics are somewhat unique and require a degree of application expertise. Some are applied by plural component spray.
Oil/moisture tolerant epoxies have been under development for years. Many are 100% solids materials. Recent developments have provided thin film epoxies that bond directly to oil saturated concrete (and oil contaminated steel). The activated resin system absorbs and cross links chemically with certain hydrocarbons. Patch tests should be conducted to ensure good performance, especially where the oils and/or greases may be animal or vegetable in origin. These coatings can significantly reduce the amount of surface preparation necessary to achieve a good bond.
Furans that are acid cured will not bond properly to unprimed concrete substrates, and therefore are not applied directly over concrete. They are used in a thick film state as grouts or as the resin for polymer cements.
Conversely, non-acid cured furans have excellent broad spectrum chemical resistance and do bond tenaciously to abrasive blasted concrete.
Chlorinated rubber coatings, once widely used in industry, are now limited for the most part to swimming pools. Because of V. O. C. requirement and newer technologies, these products are not often specified today.
New 100% acrylic resins are finding more favour in industry. The waterborne epoxies also are becoming more popular V. O. C. and other environmental concerns, as well as ease in handling, are having a positive impact on the widening use of these coatings. Waterborne coatings are not generally specified for immersion service. They are not yet up to the performance levels of their solvent based sister coatings.
There are a limited number of vinyl esters on the market for thin film applications. They are relatively expensive but do offer some advantages over the epoxies in certain chemical environments.
PV A Latex, epoxy esters, waterborne elastomeric acrylics and others may be used as architectural coatings over concrete substrates. There are other high performance coatings, such as polysulfones, used in special applications.
ORGANIC COATINGS - THICK FILM
Thick film coatings, as defined for this paper, are those coatings and coating sytems, exceeding 30.0 mils in film thickness or system thickness. They may be applied by spray, roller, squeegee or trowel and may contain additives or fillers such as cloth scrims, sand, talc, cab-o-sil, etc. for additional strength, thickness or decorative purposes.
These coatings are often used as linings for storage facilities and secondary containment liners, as well as flooring systems.
The family of elastomeric polyurethane coatings, after some bumpy starts, has begun to show significant growth. They offer the advantages of 100% solids coatings, such as V. O. C. compliance, good film build at sharp edges and corners, and the ability to span smaller holes and cracks in a seamless continuous film. The elastomeric quality allows it to expand and contract (within limits) should the substrate move. They are applied in thicknesses from 60.0-120.0 mils or greater, depending on the condition of the substrate. Polyurethanes are not effective in some of the higher concentrations of acids and caustics. They are not usually recommended for organic solvent service.
Qualified applicators, once scarce, are now available throughout all areas.
Synthetic Rubber (Elastomers)
Cloth inserted rubber (elastomer) sheet, such as hypalon and EPDM, are in fairly common use as liners for containment areas and waste collection ponds. They have excellent chemical resistance as well as UV resistance. The installation of these materials require the proper sealing of the seams and in some instances special arrangements for holding them in place or fastening them to the concrete substrate.
These systems are often used in flooring and lining applications which are multi layer and may consist of a thin film primer, a resin-rich layer rolled or squeegeed out, a reinforcing or filler layer (cloth, sand, mineral) and may have a resin-rich top coat or seal coat. The glass cloth reinforced systems are commonly called FRP or GRP (fiberglass reinforced plastic; glass reinforced plastic).
There are three popular generic resin systems used for most applications - polyester, epoxy and vinyl ester. There are numerous formulations and modifications to these basic resins and resin-rich systems depending on the in service conditions.
They are used extensively in architectural applications for decorative and functional flooring systems, such as kitchen areas. The more chemically harsh environments, like wastewater petroleum, chemical and pulp and paper facilities, call for more chemical resistant resins than are used for the architectural applications.
A potential major drawback to these coatings and systems is that they bond so tightly to concrete that, even when reinforced, they may crack if the substrate moves or cracks. Under the current E. P. A. mandates for secondary containment, such cracks would call for repair when discovered. Relatively stable and dense substrates, such as microsilica concrete and the extra reinforcing of standard concrete, can help reduce the problem of cracking.
Polymer concretes have also been researched and worked on for some years with varying degrees of success. Early attempts were frustrating because of the lack of batch to batch consistency. The handling of some of them, such as the furan concrete, was very difficult as they were ultra-sensitive to variations or changes in environmental and climatic conditions. Manufacturing consistency has improved and there are both polyurethane and furan polymer concretes on the market.
These are not coatings in the strict sense, in that they do not bond to concrete. They are generally anchored to the substrate in some mechanical fashion. This can be a significant drawback should the retaining mechanism fail.
Some systems have anchoring mechanisms molded into the plastic material. The material is shaped or cut to fit a form, the seams are sealed and the concrete is poured around the corrosion resistant liner. When the concrete cures, the plastic is locked in place.
Although there are some polypropylene and polyethylene plastic used, PVC is the usual material for these liners.
Brick or Tile and Mortar Systems
Since when did brick and tile become organic? They, of course, are not. But the setting beds and mortars used today for corrosion resistant applications are. Since acid cured furans will not bond to concrete because of chemical incompatibility, most setting beds are epoxy. Vinyl ester, epoxy, and furan or carbon-filled furan may be formulated as grouts. These systems are used in dairies, food processing plants, paper mills and other processing plants. Furans are chemically resistant over the full pH range.
Machinery Setting Grouts
Where process equipment used in corrosive environments must be either leveled or stabilized, corrosion resistant organic grouts are used. These are very similar to the polymer concretes in that they are designed for strength as well as corrosion protection. Most are epoxy resin based.
The inorganic corrosion resistant coatings for concrete that are widely used are ...concrete. That is they are cementitious.
Some standard concrete types are considered more corrosion resistant than others. But practically, they are not really very corrosion resistant in H2S gas environments, such as found in wastewater applications where MIC (microbiological induced corrosion) is prevalent. All one needs to do is to put a drop of 10-15% H2SO4 on them and observe the results.
But there are corrosion resistant cementitious products (potassium silicates, sodium silicates and some calcium aluminates, for example, which perform extremely well in concentrated acidic environments at elevated temperatures. These materials are manufactured in castable and gunite grades, as well as mortars. On vertical and overheads surfaces they are nearly always applied over an anchoring system. Thickness may range from a uniform 1.5 inches to as much as 6-10 inches to rehabilitate badly deteriorated concrete. The products are not promoted as structural replacements for standard concretes, but their physical properties may exceed those of the concrete over which they are being applied. Although these materials will perform very well in very low pH (acidic) conditions, they may dissolve in a few days is the pH exceed 8-9.0. They are not recommended for highly caustic in service applications.
Since these cementitious products, like standard cementitous products, like standard cements, are porous (even though they are normally more dense than the concrete substrates they cover), they are usually applied over a corrosion resistant, organic membrane. The membrane serves two functions. First of all, being a corrosion resistant product in its own right, it is a back up to the corrosion resistant cementitious material which may be damaged or cracked. Also being porous, corrosive contaminates can migrate through the corrosion resistant cement and attack the substrate. Secondly, it may also serve as a physical isolation barrier from the substrate. In this manner, the transfer of cracks from the substrate through the topping may be reduced, since the top coat is not bonded directly to the substrate.
Corrosion resistant cementitious mortars are used in conjunction with acid brick for lining exhaust stacks for industrial applications and powder plants. They are also used in applications for brick liners in process areas and certain containment vessels. Where used as linings for containment, there is usually a back up membrane behind the brick and mortar lining.
These products, as compared to thin film coatings, are relatively expensive. But their performance in certain applications have made them cost effective. Their elevated operating temperatures and resistance to physical abuse cannot be matched by most thin film coatings. The gunite grades' ability to fill badly deteriorated areas of concrete substrate eliminates or reduces the rehabilitation work prior to applying the protective cementitious coating.
NEW VERSUS AGED OR DETERIORATED SUBSTRATES
Most of this discussion has revolved around the assumption that the substrate is new concrete. But when concrete has been in service and has been corroded and eroded, a whole new set of conditions rear their ugly heads. Even though steel may be contaminated after years of service containing certain chemicals, such contamination is not as pervasive a problem as it is with concrete that is saturated with a contaminant. The contamination may penetrate the concrete matrix to the reinforcing bars, even exposing them after the concrete has spalled.
To be most effective, some coatings require a relatively smooth surface. How does one reconstruct or rehabiliate such a surface in order that it may be protected with a corrosion resistant coating? We are back once more to a surface preparation consideration, assuming the deterioration is not beyond repair. For many coatings, the most cost effective way to rehabilitate is to apply a less expensive underlayment prior to applying the corrosion resistant coating. Other coatings, notably the 100% solids materials, can be filled with an additive, such as sand, talc, etc., to a putty consistency and used to repair damaged areas or fill voids in a surface prior to applying full top coat(s) of the coating. The manufacturer of the coating being used will specify his requirement for the most effective performance of his product in these areas.
Assuming that a product has been selected and applied over a properly prepared concrete substrate, what assurance do we have that all of the steps have been correctly followed and the desired results achieved? Standards for inspections, like standards for surface preparation of concrete, are somewhat hit and miss. There are some ASTM test procedures and other practice, which are used as inspection guidelines. But there are no consensus industry standards here either. For example, adhesion testing on concrete has to have a different set of evaluation criteria than for steel because of the basic differences in materials.
In the meantime, we must rely on historical data of coating system on concrete and the manufacturer's best information. Testing procedures and information, if available, should be compiled for inspection standards for different generic coatings over concrete. Hopefully, the coating manufactures will establish the standards where they do not exist.
Because of the inherent nature of concrete, the wide variety of surface conditions resulting from construction process or in service conditions, and the requirements of a specific application, the protection of concrete substrates by using protective coatings pose a unique set of problems to industry. Many generic types of coatings are being tailored specifically to cope with these tasks. Most importantly, industry standards need to be defined for coatings used to protect concrete. This is particularly true in the area to surface preparation and inspection procedures. Environmental concerns and federal regulations resulting from those concerns have created a major expansion of applications, such as secondary containment. There is an increasing challenge to the coatings industry to develop products and standards in the area of corrosion control for concrete substrates.