Untreated rust spots will come back through the sealcoating in a relatively short time.
aggregate to the elements of weather. The ageing process of asphalt may take months, so depending upon the geography and climatic conditions, it could be some time for the aggregates to be exposed to the elements of weather. Although the documented physicochemical aspects of pyrite’s oxidation and its conversion into various compounds are complex and beyond the scope of this article, it will suffice to say that once the aggregate is sufficiently exposed, pyrites in the aggregate start reacting and producing iron compounds (Iron Sulfate, Iron Hydroxide and Iron Oxides) through complex chemical and biochemical processes. The brown spots and then brown streaks on the pavement are caused by the resulting iron oxide. This progression of the oxidizing process is manifested as expansion in size, exploding and finally popping thorough the surface, thus creating minor craters in the pavement. The rusted aggregate become powdery and weak. Test it with a pen knife and you will see how easily the aggregate can be removed. If such pavements are sealcoated without a proper surface treatment, rust spots come back in a relatively short time as brown spots and streaks on the sealcoated surface. No matter how many times Rust-inhibiting primers and coatings either block the permeation of moisture or tie up oxygen thus effectively retarding the oxidation of pyrites containing aggregates.
you sealcoat, rust will come through. So, what’s the big deal? It’s a big deal because if the rust problem is not treated properly then the very purpose of sealcoating will be defeated at two major fronts, i.e. the beautification as well as the protection of the pavement. Not only are such spots unsightly but the popped out (rusted) aggregate creates pathways for water to seep in and damage base courses.
The Remedy Rust spots must be treated with rustinhibiting primers and coatings, prior to sealcoating. Such products either block the permeation of moisture or tie up oxygen thus effectively retarding the oxidation of pyrites containing aggregates. These specialty products, formulated with polymers and with specialty chemicals, have been found to be quite effective. They are commonly supplied as a water-dilatable or -miscible compositions, with a coverage rate of
a nominal 150 square feet per gallon. Specialty clear coatings have been found quite effective for concrete surfaces. Please note that it is a preventive measure and not a one-time cure. You may have to repeat the process of priming for the first couple of rounds of sealcoating the property. With the proper understanding of the rust spotting problem, you will be in a much better position to recommend the remedial steps to the property owner.
Conclusion It is imperative that all pavement defects including rust spots and streaks on asphalt and concrete pavements be remedied prior to sealcoating, using specialty coatings and primers. If ignored, rust spots will certainly come through the sealcoating and continue to negate the benefits that are supposed to be achieved thorough sealcoatings.
Girish C. Dubey is president of STAR Inc., Columbus, OH, which has affiliate sealer producing operations throughout the United States. Rust stains texture Anthony Paz/iStock/ Getty Images Plus
Using a rust-inhibiting primer is a preventive measure and not a one-time cure. You may have to repeat the process of priming for the first couple of rounds of sealcoating to adequately protect the pavement. www.ForConstructionPros.com/Pavement • PAVEMENT • March/April 2019 23
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