During the winter of 2002 and 2003, western New York experienced a significant increase in scaling and spalling concrete. As a large materials supplier located in Rochester, N.Y., the Dolomite Group knew it was producing products that met expected quality standards, but these products weren't meeting functional expectations. Fortunately, the severity of the damage has not been equaled since, but there is more damage and replacement of concrete than anyone wants or expects. As Dolomite conducted its investigation, it noticed the problem occurred sporadically across the country with no apparent rhyme or reason.
Searching for an answer
Some of the concrete problems in western New York were related to inadequate air entrainment, low quality aggregates, and poor workmanship - no product is perfect. Changes in materials and design philosophies brought about through customers' insistence also were looked into, as well as revisions to building codes, the advent of new technology, and various other economic and societal forces. No significant trends were found.
The next step was to focus on the use of specific products, the environment of that use, and evaluate damage by application.
Unfortunately, these comparisons produced not only inconclusive results but even worse, conflicting results. There were no consistent patterns of failure due to application, mix design, raw material use, concrete production factors, contractor involvement, or inspection level. None of this made sense from a routine concrete failure perspective. However, despite the inconclusive results, a pattern emerged. The damage plotted well geographically. Some localitieshad severe problems; others had negligible ones.
The photo above shows a section of agutter, located in a town where Dolomite was responsible for almost 40,000 lineal feet of concrete gutter and the accompanying asphalt overlays. It also should be noted the town did not install plows on their trucks between 2002 and 2003, choosing to reduce truck maintenance and expenses by "burning the snow off" the roads. When local municipalities were surveyed to find out which deicing chemicals were used, a variety of productsand combinations were discovered, including:
- Sodium chloride or rock salt (NaCL)
- Magnesium chloride (MgCl2) andorganic-based performance enhancer (OBPE)
- NaCL with MgCl2 and OBPE
- NaCL with Magic-O - a proprietaryblend of OBPE and 30% MgCl2
- Calcium chloride (CaCl2) and OBPE
- NaCL until 15° F degrees, then NaCLwith CaCl2 until 5° F degrees, thenNaCL with sand
Deicing agent technologies are protected as proprietary by their manufacturers and approach the complexityof concrete technologies. The deicing industry conducts significant research in order to design products for optimum efficiency. Deicing agents sometimes contain organic-based performance enhancers (OBPE), which are primarily agriculture by products. They are added to reduce the corrosiveness of the salts. Ironically, these by products also are used to make products to dissolve concrete residue on ready-mix trucks.
The properties of deicers
Every material is subject to the laws of nature and the natural processes they dictate. There was much debate about deicer materials chemically attacking concrete. In 2002, the federal government funded a study that was conducted by credible agencies including FHWA, USDOT, 10 state DOTs, and several universities. The results were published in April 2008 in the report SD2002-01 "The Deleterious Effects of Concentrated Deicing Solutions on Portland Cement Concrete."
The study determined there is significant evidence magnesium and calcium chloride compounds chemically interact with cementitious paste resulting inexpansive cracking, increased permeability,and significant loss in strength. SD2002-01 found that portland cementis susceptible to both CaCl2 and MgCl2 attack, slag is susceptible to MgCl2 but not CaCl2, and fly ash is susceptible to CaCl2 but not MgCl2. Conversely, NaCl appears to have little to no chemical interaction with the cement but its corrosive nature on reinforcement steel is well documented.
The results for calcium magnesium acetate were inconclusive. The deicer reacts with the glue created by hydration and converts it into expansive, watersoluble compounds. SD2002-01 alsofound that CaCl2 potentially can affect aggregates. This finding makes asphalt susceptible to spalling as well as concrete.This is not surprising because portland cement, fly ash, slag, and aggregate soften contain the same compounds. This may not be exclusive to CaCl2. It's beennoticed over the years that sometimes an aggregate will fail ASTM C88 significantly when using MgSO4 yet pass easily using NaSO4. Is this a lack of physical or chemical durability?
The report reinforced claims that some deicing compounds chemically attack concrete and aggregates but fell short of addressing why this issue has occurred sporadically throughout the country inconsistently. If it was assimple as a chemical attack, the issue would arise almost every winter and would be easily correlated to chemicals.
The perfect storm
The effect of temperature and dilution on deicing chemicals has interesting ramifications. The charts on the next page shows phase change diagrams for NaCL, CaCl2, and MgCl2, provided by the Salt Institute, Alexandria, Va. InFigure B, areas above the lines are where salts function. The area to the left shows the results of a solution with too little salt-refreezing will occur unless more deicing salt is applied. The area to the right shows the results of a solution with too much salt causing surfaces to quickly refreeze. Notice the angle of "refreezing due to too much salt" in relation to "too little salt." Too much or too little salt affects the freeze point, diminishing or enhancing the effectiveness. Because deicers become diluted as they melt ice and snow, they remain at their ideal ice melting concentration very briefly. It is possible to freeze a surface by applying too much deicer and to thaw a surface by snowfall. What does this mean whenit comes to the number of freeze/thaw cycles possible by repeated salt runs combined with snowfall?
Another factor is the thermal behavior of the reactions by which different deicers function. NaCl deicing action is theresult of an endothermic reaction (which requires heat) andMgCl2 and CaCl2 work as a result of exothermic reactions(which liberate heat). Despite the fact that they work off ofopposing reactions, they are routinely combined.
During the 2002-2003 winter season, upstate New York experienced more snowfall in moderate amounts and less major snow events - not more snow, but more frequent snowfall. Significant snowfall is plowed while light snowfall often is salted away. Salt phase charts show limited windows of effective concentrations. More frequent snowfall caused deicers to cycle from concentrated to diluted. There also were more frequent applications of multiple deicing products. Temperature swings caused deicers to go from effective to ineffective to effective repeatedly. How often were the freeze/thaw windows broken?
When each additional variable is considered, the total number of freeze/thaw cycles during a winter season can be exponential. Cycles occurring when the air entrainment system is blocked by expansive by products of the chemical attack, as well as by precipitated salt recrystallization, can be destructive.
What to do and how to advise owners
Definitive answers that everyone wants aren't available at the present time. The mitigation strategies recommended by SD2002-01 are not a revelation to the industry. Fundamentally, they recommend following the practices the industry has recommended for years. Further research conducted by FHWA, ACI, and ASTM could help to supply answers. Owners should be advised that deicers are intended to disrupt the bond between ice and a surface in order to facilitate physical removal. Excessive application of deicers known to chemically attack surfaces to burn the ice away will cause damage.
Thomas A. Leasure (firstname.lastname@example.org) is concrete technical services manager for the Dolomite Group, Rochester, N.Y.
Editor's Note: A 30-minute PowerPoint presentation, a copyof SD2002-01, and additional information are available at http://usftp1.oldcastlematerials.com/thinclient/login.aspx. To access, login with username "email@example.com" and password "toml." These are free for industry use and dissemination, as long as they are not altered.