Coatings World - July 2017

they are typically not high gloss finishes – usually satin or semigloss finishes. Since the films are relatively thin and largely on the surface of the concrete, they usually need to be reapplied every other year to maintain protection.

Solvent-based acrylics are also commonly used as sealers and are usually applied after the concrete has aged for a month. These sealers are very durable because of the acrylic backbone, but still low enough molecular weight to penetrate into the concrete pores and provide a darkening of the surface (called a wet look) which can be desirable. They are applied at higher film thicknesses than the acrylic emulsions and can give a high gloss when dried. These acrylics can be formulated in exempt solvents like acetone, however the rapid dry speed and odor can present a significant issue during application.

Two part epoxies are used to seal concrete in environments where a high level of chemical resistance is needed. These can be cured with polyamines, polyamides or polysulfides, although the poly-amide-cured epoxies typically have lower chemical resistance. The epoxies have good adhesion to the concrete and can be formulated to be relatively oil tolerant for use on garage floors, or designed to cure at low temperatures. Epoxies can chalk on exterior exposure, however, and they are not usually used for outdoor applications.

In the case of chloride attack on the re-
inforcing steel, it is also possible to reduce
the effect by applying a thin overlayment
of polymer modified cement to reduce the
surface porosity of concrete. The poly-
meric admixes function by reducing the
amount of water necessary for a given
concrete fluidity resulting in a lower wa-
ter to cement ratio. In addition, as the
concrete dries the polymer forms films in
the pores and blocks the capillaries. The
slower cure of the concrete and the dried
polymer in the matrix results in reduced
porosity. 6 While the reduced porosity of
the overlayment helps reduce the penetra-
tion of water carrying chloride ions, the
polymer can also improve adhesion to the
concrete substrate and can improve du-
rability by increasing the tensile strength,
resistance to abrasion and dusting.

Summary

In order for the complex chemical reactions in concrete to occur, water is required. To build optimum strength and avoid cracking at the surface, it is also necessary to keep concrete moist for several days after being poured. Polymeric coatings and cure and seal coatings can help develop the strength of concrete by reducing the porosity at the surface and keeping the water in the concrete longer. Cured concrete is also subject to chemical attack or physical degradation by freeze thaw cycles. Sealers or polymer-modified overlayments applied after cure can help protect concrete from these forms of degradation. CW

References

1. Thomas, J.; Jennings, H. (n.d.). Science of Concrete. Retrieved June 28, 2016, from http://iti.northwestern.edu/cement/ index.html

2. Thomas, M. (2007). Optimizing the use of Fly Ash in Concrete. Retrieved July 15, 2016, from http://www.cement.org/ docs/default-source/fc_concrete_technol- ogy/is548-optimizing-the-use-of-fly-ash- concrete.pdf

3. Farny, J.; Kerkhoff, B. (2007). Diagnosis and Control of Alkali-Aggregate Reactions in Concrete. Retrieved June 14, 2016, from http:// www.cement.org/docs/default-source/ fc_concrete_technology/is413-02---di- agnosis-and-control-of-alkali-aggregate- reactions-in-concrete.pdf?sfvrsn=2

4. Kerkhoff, B. (2007). Effects of Substances on Concrete and Guide to Protective Treatments [Pamphlet]. Skokie, IL: Portland Cement Association.

5. Neville, A. (1995). Chloride attack of reinforced concrete: An overview. Materials and Structures, 28, 63-70. 6.ATS 11 - Polymer Dispersion Admixtures. (2016, April 21). Retrieved June 14, 2016, from http:// www.admixtures.org.uk/download/ ats-11-polymer-dispersion-admixtures/

7. Duvallet, T. (2014). Influence Of Ferrite Phase In Alite Calcium Sulfoaluminate Cements (Published doctoral dissertation, 2014). University of Kentucky. Retrieved August 8, 2016, from http://uknowledge. uky.edu/cme_etds/27/