Surface Treatments for Concrete Protection and Enhancement

Concrete, designed correctly and constructed with appropriate precautions, has proven to be a maintenance-free and durable material. However, there are times when concrete is subject to severe environmental attack during its service life. The use of surface treatments is beneficial in such cases. These treatments provide additional means of protecting the structure or improving its performance over and above what could otherwise be obtained.

The function of surface treatments is two-fold: they can act as a barrier against aggressive environmental attack and they could also be used to improve the visual appearance of concrete, blending it into an overall aesthetic scheme, masking stains and blemishes or giving the surface better self-cleaning properties. Thus, surface treatments fulfill the functions of protection and aesthetic enhancements at the same time.

Uses of Surface Treatments:

• Enhancing appearance (colour, texture, opacity, cleanability, reflectance, etc);

• Improving chemical resistance to sulphates, acids, brewery/dairy products, etc

• Controlling ingress to chlorides, oxygen, carbon dioxide, water vapour, moisture, etc. 

• Improving mechanical and physical endurance such as resistance to abrasion impact, skidding, etc.

The benefits of treatments could apply to both old and new concrete. For a new structure, the surface treatment is expected to counteract the anticipated problems. For the old concrete structures, it is however essential to determine the exact cause of deterioration by inspection and testing so that the most suitable surface treatment can be chosen.

Surface treatments may be expected to perform satisfactorily under widely variable and extreme service conditions; hot and humid, cold, wet/dry cycles, covered under water. They often have to fulfill several distinct requirements at the same time: for example, chloride/carbonation barrier with acceptable appearance, ease of cleaning with slip resistance, etc. As a result, selection of an appropriate surface treatment is always a difficult task and hence needs to be done carefully.

The surface treatment products can be classified on the basis of the main generic material components, viz, inorganic, organic or organo-metallic / organo-silicon. Alternatively, they could be divided into two main categories; first those materials which impregnate concrete and do not depend on the formation of a significant surface film and second comprising of coatings which depend upon the formation of a continuous film, thus shielding the concrete surface completely.

The former category could be further sub-divided into two types, namely, pore-lining penetrants and pore-blocking sealers. While the penetrants line the pores and prevent the ingress of liquids into concrete, the sealers block the pores and are thus able to resist the ingress of liquids under a pressure gradient. Virtually all pore-lining penetrants are based on hybrid organo-metallic / organo-silicon materials such as silanes, silioxanes, siloconates, stearates, drying oils, etc. The pore-blocking products and materials physically block the pores without reacting with concrete. Solvated systems based on thermoplastic and thermosetting resins (acrylics, polyurethanes, epoxies) could be tailored to match specific requirements, depending upon the porosity of concrete and the number of applications that are acceptable.

The other category, namely, surface coatings could be sub-divided into two types, namely, coatings which have a dry film thickness of upto approximately 2mm with high-build system as the upper limit and thick or heavy linings with a thickness in excess of 2mm. The materials of the coatings could be either inorganic or organic. The former comprises of decorative masonry, paints based on cement and polymer modified coatings. The presence of polymer in the polymer-modified cementations coatings improves a host of properties such as adhesion, flexibility, barrier properties and application characteristics. The polymer dispersions generally used include: acrylics, styrene-butadiene, styrene-acrylics and ethylene-vinyl acetate.

Organic coatings mainly fall under four categories namely, thermoplastics and synthetic rubber (widely used for decorative and protective treatment), alkyl and drying oils (improved versions of traditional oil-based and oleo-resinous paints), bituminous system (for damp proofing and water proofing) and thermosetting polymers (commonly based on epoxy and polyurethane resins).

Ingress control, particularly moisture control, is one of the important properties of a surface treatment. Some treatments provide a barrier to liquid water but are less effective at inhibiting vapour transmission. The ability of many treatments to reduce liquid water transport, while allowing the passage of water vapour can be helpful for both coating and concrete performance. A totally impervious treatment will not allow water trapped behind the treatment to escape as vapour. Vapour pressure can build up behind the coating, leading to failure by blistering.

For selecting an appropriate coating, it is essential to take into consideration the technical information supplied by manufacturer, laboratory data, case histories, etc. It is important that a broad view is maintained and that as many factors as possible are considered in selecting the surface treatments. The concept of whole life-cycle performance should be considered in the final selection.

Before application of surface treatments, it is essential that proper surface preparation techniques are used. Effective techniques include: wet, dry and vacuum-abrasive blasting, high-pressure washing with/without emulsifying detergents, steam cleaning and mechanical wire brushing (for small areas). Application could be done with use of sprayers, roller, brush application, etc. Ambient temperature, humidity and wind speed have a significant effect on application and curing/drying behaviour of surface treatments. All these factors need due consideration before application.

It is imperative that the requirements for protection and constraints of application, maintenance and service life need to be matched to the properties of materials by qualifiable tests. A large number of tests are available for performance evaluation of surface treatments, covering aspects such as properties relating to appearance and integrity, chemical resistance, ingress control and mechanical and physical resistance.

Until recently, the technology of surface treatments for concrete had been generally based on practice, rather than on science. It is essential that attention must be focused on more basic and subtle underlying mechanisms of gas and liquid transport through the concrete/air interface. Problems such as blistering, pin holing, cracking, volume change and detachment – all need to be addressed and understood. Long term migration processes also need to be understood so that materials can be properly formulated.