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Super Salts: A Powerful Solution

Super Salts: A Powerful Solution

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    Super Salts can magnify stresses on concrete pavements in winter months, especially around sealed joints.

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    A troubling trend: Test cores show damage that is not visible on the surface.

Any producer involved with infrastructure projects knows that transportation agencies are encountering the same economic issues businesses face. They now do more with less. The challenge is to do so without creating more expensive repairs in the long run.

For example, snow removal and deicing are expensive tasks involving many hours of employee overtime and tons of deicing materials. Highway officials must adopt new technologies to meet budget pressures while keeping roads safe and clear.

The traditional method involved applying rock salt (sodium chloride) to roads after snowstorms. As salt prices drastically increased, agencies learned that dissolving the salt into a liquid and applying it to the road as an anti-icing agent before a storm was a much more effective use of their salt supplies. The brines prevent ice from forming on roadways down to – 6 ° F. This innovation led to a new generation of anti-icing technology: treating sodium chloride brine with other chemicals to further lower the freezing point as low as –76 ° F.

Agencies were thrilled with these products. They reduced their salt use, lowered overtime plowing costs, and created a safer environment for drivers. All was good.

But what chemicals are being added to the sodium chloride brines? And what impact do the chemicals have on concrete pavements?

Negative side effects

Super Salts can potentially reduce service life of concrete pavements. These materials include calcium chloride, magnesium chloride, and other proprietary products.

As these products lower the freezing point of water in the concrete, the pavement may experience more freeze/ thaw cycles throughout the year. Applying brine to pavements can increase the amount of chloride migration into the pavement and even encourage greater penetration. And once in the concrete pavement, these salts attract moisture and increase concrete's saturation levels. In several Snow Belt states, the corrosive effect on concrete roads ranges from minor to severe.

Interestingly, the damage appears to be largely limited to control and construction joints and not in the body of the pavement. Of particular concern are pavements that have no visible damage, but reveal joint damage below the surface when test cores are taken.

There are several theories to explain how Super Salts cause this damage. Larry Sutter, PhD, from Michigan Technological University in Houghton, Mich., has led research on the effects of deicing chemicals on concrete. Sutter's work describes how these salts, when in solution, can chemically react with portions of the paste matrix to create damaging expansion. The damaged area later fills with water and severe freeze/thaw damage can quickly occur.

At Purdue University in West Lafayette, Ind., Jan Olek, PhD, P.E. has also done extensive work on these deicing agents. Olek and his team have taken cores of damaged pavements from several locations in Indiana. Originally, the cause of the damage appeared to be straightforward: a substandard air void system.

But how could the air void system at the damaged joints be that much different than the air void system in the middle of the panel that was not damaged? The Purdue team further concluded that the salts chemically react with the paste structure to create either ettringite or Friedel's salts that, when in solution, fill the entrained air bubbles. The infilling of the air void system leaves the pavement quickly susceptible to freeze/thaw damage.

Combating the elements

Concrete producers must understand their customers' needs and provide guidance on how to design and construct durable pavements that will be exposed to Super Salts. The first step is delivering a quality product. If a concrete pavement has substandard quality (higher water/cement ratio, poor air void parameters), damage will be unavoidable and extensive.

Unfortunately, high-quality pavements have also been damaged. Producers must insist that pavement designs minimize the chances of saturation. The damage in quality pavements appears to be more extensive at longitudinal saw cuts rather than at transverse saw cuts or construction joints. Longitudinal saw cuts are often tied with rebar, whereas transverse saw cuts typically receive round, smooth dowels designed to move. One school of thought is that moisture is breaching the joint sealant and getting into the saw-cut joints. The tie bar under the longitudinal joint minimizes or prevents drainage from under the joints, while the sealant prevents drying of the joint from above.

Some states have eliminated joint filling, finding that it has not provided an adequate return on investment, compared to initial and long-term costs. Even a narrow, 1/8 -inch unsealed saw-cut joint encourages drying from above. Using properly sized tie bars (when necessary) will promote drainage from underneath the joint.

Christopher Tull is the founder and owner of CRT Concrete Consulting. He consults for contractors, ready-mix producers, owners, and the PCA. Visitwww.crtconcreteconsulting.comor e-mailchris@crtconcreteconsulting.com.


Read more in the [WEB EXTRA] How to Fight Super Salts
Two reports may assist designers in creating an environment where pavement saturation can be minimized.