{QUESTION} We are a producer in the Midwest. Last spring we received several calls about scaling on concrete driveways and parking lots. We had been very diligent in monitoring our mix designs and delivery procedures, so this spike in complaints surprised us. More interesting, several calls came from owners whose concrete surface had been in use for several years, and had performed well before last winter.

Something seemed out of the ordinary. It all came together when we received a call from an engineer in our local public works department. They also had been experiencing an unusual amount of scaling on much of their concrete pavement. He provided us some interesting information. Its transportation department recently bought a different type of de-icing material to replace the normal rock salt used in the past.

Does the type of de-icing material affect concrete's durability? And if there is a difference between de-icing materials, how can we advise public works departments, contractors, and clients to help better protect concrete surfaces?

{ANSWER} The type of de-icing material does affect the chemistry of a hardened concrete surface. In a recently published paper, University of Kansas researchers provided some interesting insights. In “Effects of Deicers on Concrete Deterioration,” David Darwin, JoAnn Browning, Lein Gong, and Sean Hughes compared how concrete samples reacted when exposed to four of the most widely used de-icing chemicals.
The research team, supported by the Structural Engineering and Materials Laboratory at the University of Kansas, subjected the specimens to weekly southern exposure-type cycles of wetting and drying. In addition to a control sample placed in distilled water, samples were immersed in solutions of sodium chloride (NaCl), calcium chloride (CaCl2), magnesium chloride (MgCl2) and calcium magnesium acetate (CMA).

Samples were tested for periods up to 95 weeks. Researchers evaluated the samples based on changes in the dynamic modulus of elasticity and appearance.

The researchers concluded that applying significant quantities of CaCl2, MgCl2, and CMA over the pavement's life will negatively impact the concrete's long-term durability. But there was a difference:

  • at lower concentrations, NaCl and CaCl2 have a relatively small negative impact;

  • at high concentrations, NaCl has a greater but still relatively small negative effect;
  • at low concentrations, MgCl2 and CMA can cause measurable damage to concrete;
  • at high concentrations, CaCl2, MgCl2, and CMA can cause significant changes in concrete that result in loss of material and reduced stiffness and strength.

Perhaps your best source for help in mitigating the problems from de-icing material is provided in the ACI 201.2 “Guide to Durable Concrete,” which describes specific types of concrete deterioration. It includes chapters that discuss the mechanisms involved in lessening a concrete's service life and provides recommendations for individual components of concrete, quality considerations for concrete mixtures, construction procedures, and influences of the exposure environment.

These are all important considerations to ensure concrete durability. Unfortunately, the current ACI document does not address the effects of MgCl2 or CMA on hardened concrete.

To review the report by the University of Kansas researchers, visit www.iri.ku.edu.