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Road Salt Tied to Long-term Corrosion

Friday, November 9, 2012

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With road-salting season around the corner, scientists are shedding new light on just how severely the world's most common de-icing chemical may be damaging roads, bridges and other infrastructure.

The United States alone spreads millions of tons of road salt each year. New research offers a sobering look into the long-range consequences of that strategy, showing that the chemical may be corroding steel-reinforced concrete structures over the long term.

Road salt corroded bridge
Corrosion Doctors

The Swedish scientists say chloride ions in road salt are corroding steel-reinforced concrete structures.

Effects of Exposure

A team of Swedish scientists has looked at the effects of exposure to road salt over a 10-year and a 25- to 30-year service period. Using computer simulations of chloride ingress into concrete structures, the study aims to help assess existing reinforced road and bridge structures.

Their research demonstrates that the chloride ions in the salt are responsible for corroding the steel rebar and mesh reinforcement in concrete structures, said Luping Tang, from the Chalmers University of Technology in Gothenburg, and Dr. Anders Lindvall, from the Central Laboratory at Thomas Concrete Group AB in Gothenburg.

Tennessee DOT

The scientists are studying 10-year and 25- to 30-year effects of road salt.

Concrete can be penetrated by the chloride ions, which then reach the steel reinforcements, leading to corrosion after months or years of use.

"Chloride-induced corrosion of reinforcement in concrete is still one of the main concerns regarding durability and service life of reinforced concrete structures," the team wrote in the International Journal of Structural Engineering.

"Prediction of chloride ingress in concrete is one of the important parts in durability design of reinforced concrete structures exposed to the chloride environments," they reported.

Simulated Models

DuraCrete and ClinConc were the two models tested to simulate heavy traffic over steel-reinforced concrete structures exposed to road salt over several years.

Road salt usage
U.S. Salt Institute / Corrosion Doctors

The use of rock salt for deicing in the U.S. has grown steadily over the decades.

ClinConc tests for the effects of exposure over 25 to 30 years and is valid against field data from real concrete roads and bridges, the team says. DuraCrete predicts the corrosive effects of chloride after a 10-year period.

The scientists say they hope their findings will not only inform those assessing existing reinforced road and bridge structures but also allow design engineers to take into account the needs of such structures during the winter months and ensure that they find ways to reduce or avoid potentially damaging chloride ingress.

   

Comment from chris atkins, (11/9/2012, 3:55 AM)

New research shows chlorides cause corrosion, also in other breaking news, it gets dark at night. The work is good, the headline may be oversimplifying somewhat. All the best Chris


Comment from Curt Harvey, (11/9/2012, 8:41 AM)

Florida DOT has run numerous tests on the subject, as numerous bridges in this state are exposed 24/7 to chlorides. The Journal may have archive copies of the study which was conducted back in the mid '80's. Testing consisted of various methods of barrier systems, induced current into rebar, penetrating sealers and combination. I think current practice for new construction in FL is for concrete bridges to have both a penetrating sealer applied to exterior, and possibly a form of cathodic protection tied to the rebar. At least it might be worth taking a look at the testing protocols and results.


Comment from Roberta Hilko, (11/9/2012, 9:42 AM)

Salt not only corrodes steel , what about the effects on water that is eventually consumed by humans. We need to curtail and cut back on how much and where salt is used and get the motoring public to slow down and not try to drive like the roads are clear and dry.Pomalo is a Slovak word for "take it easy"; we all need to go pomalo.


Comment from Craig Roberts, (11/9/2012, 9:43 AM)

I agree bridges are under “a-salt” . Technology has progressed to allow an exterior prep to the metal decking of a bridge with a water based phosphoric wash which will consume all impinged contaminates including but not limited to all sulfides and chlorides. The result is a toothy iron phosphate substrate that will encapsulate the anode and cathode sites thus stopping the rusting process which is the result of all steel corrosion and coatings systems failures.


Comment from trevor neale, (11/9/2012, 10:12 AM)

I suspect that report should be dated 1980 not November 2012.As Curt pointed out a search of back issues of the JPCL is almost certain to contain a wealth of information on this subject.


Comment from Glenn Summers, (11/9/2012, 10:21 AM)

At the risk of spilling the beans--- This Is NOT NEWS !!! What planet have these folks been on that they just came to this startling revalation? Ridiculous ! What's going to be next? will they discover the wheel or maybe Fire?? Should we let the general membership of NACE in on this secret??


Comment from Stephen Pinney, (11/9/2012, 12:47 PM)

Glenn You hit that one on the head. There is loads of research done on this subject. At this point, the only question in the most economical method of evaluating the presence salts and removing them. Go to goolge for a start. There are volumes available as far back as the 80s.


Comment from bryan marsh, (11/9/2012, 2:05 PM)

I fear this is simply poor reporting. We all know road salt attacks reinforcement in concrete but up to now there hasn't been a particularly good model for prediction of the rate of chloride ingress through the concrete towards the reinforcement - probably because of the uncertainty of the chloride concentration at the concrete surface and the intermittent nature of the exposure unlike, say, exposure to sea water. So maybe we could ease up on the sarcasm.


Comment from Mark Schilling, (11/11/2012, 10:22 AM)

I have to agree with Glenn Summers. This is NOT news. What's next? Will these braniacs invent the wheel or discover fire? And Chris Atkins made a comment that was right on target. Breaking News!! It gets dark at night! I started work in the corrosion and materials engineering field in 1978 and I knew more about salts THEN than most "paint experts" know today. For example, why is it that when the paint industry talks about salt it is almost exclusively about chlorides (plural)? Unless we want to start talking about radioactive isotopes, all chloride ions are absolutely identical, and isotopes don't make a difference with respect to the corrosion of rebar in concrete. So why is it "chlorides" (plural)? It's because most people don't have a clue what they are talking about and it's just a bunch of words. Salts are composed of anions AND cations. As far as deicing salts go, calcium chloride is significantly more corrosive than sodium chloride (and although not deicing salts - ferrous and ferric chloride are really big time trouble with respect to corrosion). So if you want to talk about salts (plural) you need to consider the cations, not just chloride as an anion. If you want to talk about chloride ion, keep it singular. Why all the fuss about this supposedly most evil thing in the cosmos, chloride anion? It has little if anything to do with corrosion mechanisms and everything to do with the fact that field testing for chloride ion is simple, quick, and inexpensive. Measuring chloride ion is an excellent yardstick. But that's all it is. It is a metric. There is so much that we need to know. Chloride ion is not "magic." NaCl does not "corrode" carbon steel. NaCl is a corrosive; it is not a corrodent. There is a difference. The corrodent is most often dissolved oxygen (oxygen in water), and the corrosion product is a mix of iron oxides. By definition with electrochemical corrosion, the corrodent is the chemical species that steals electrons away from the steel. If the steel cannot find a way to dump electrons electrochemical corrosion grinds to a halt. Make no mistake. Chloride ion can be bad news from a corrosion perspective. It's a potent depassivating agent. It promotes pitting. Chloride ion is an anion and it is drawn to active anodic areas on the steel. It will concentrate in the bottom of corrosion pits and in so doing it promotes a low pH in the pit (because it helps to keep the iron ions soluble, NOT because of some dumb hair-brained notion that chloride ion is a perpetual cyclic acid regenerator (HCl) or that it reacts with and attaches to stell and blah, blah, blah). Salt improves the conductivity of the electrolyte. Our problem over the last 20 or 30 years is that the paint industry makes sh*t up. It's all about the $elling. It's hype. It's not education - it's indoctrination. And my colleague Steve Pinney hit the nail on the head. Tons of research have been done and published on this topic. I have stuff in my personal files that dates back to the 50s and 60s, if not the 40s. However, I would caution you all. A lot of pure BS has been published, especially in the last 20 or so years by JPCL and NACE MP. You have to be smart enough to recognize the difference between right and wrong. You have to have a good bullsh*t detector up and running. You have a responsibility to yourself. You cannot just believe whatever you read - because some of what you can find in print is pure crap. I live on the Jersey Shore and we got hammered. The power is back on. If you want info on salts you can e-mail me at welbytoast@aol.com and I will try to get back to you. Two weeks with no power or only limited, intermittent power - I have a lot of catching up to do.


Comment from Glenn Summers, (11/12/2012, 6:49 AM)

There are hundreds of new construction efforts that are incorporating reference cells inside a structure and anodes tied to the reinforcing steel before the concrete is poured. It then becomes a mater of monitoring the building to detect corrosion cell formation.. At that point it then becomes the adjustment of the mili-volt of current to be induced to the structure to control the corrosion. Don't give up, Mark, you'll get your power back. Wonder if the author of this article is ready to hear about solar powered Impressed systems that you don't have to plug in. Impressed Cathodic IS the answer to corrosion. A Smart design team will incorporate a quality penetrating sealer to Stop/Slow ingress of contaminates and increase the resistance of the matrix, That will delay when the Impressed Cathodic system is energized. Indoctrination is a good word for it !! Lets use this breathable coating because of ??????. Keep your B/S Meter turned on Mark, and you'll be fine !!


Comment from Frank Goodwin, (11/12/2012, 7:05 AM)

ACI Committee 20's, "Guide to Durable Concrete", issued in 1994, states the limit for chloride in prestressed concrete as 0.06 percent of cement weight, and in conventional reinforcing concrete in moist environment and exposed to external chlorides as 0.1 percent. This grew out of many years of work. Good early work on the effect of chlorides on corrosion in concrete are the Cornet & Bresler paper "Corrosion of Steel and Galvanized Steel in Concrete" presented at the NACE Western Regional Conference in October 1964 and before that the Baumel and Engell paper "Corrosion of Steel in Concrete" Arch Eisenhuettenw, 1959. Tons of work since then, too as others have said.


Comment from Tom Schwerdt, (11/12/2012, 8:12 AM)

If I properly parsed it under the "gee, chlorides!" stuff, it sounds like this article really should have been describing the development of computer modeling of chloride-accelerated corrosion. Being able to accurately model the likely corrosion environment inside a concrete structure over time without cutting a ton of cores would be quite useful


Comment from Mary Chollet, (11/12/2012, 8:17 AM)

All: The goal of the research was not to revisit the very well-established connection between road salt and corrosion. The work developed a model for predicting the extent of that damage over the long term (up to 30 years), to help maintenance engineers with their road and bridge programs. You may want to see http://bit.ly/Rx3426. Thanks.


Comment from Glenn Summers, (11/13/2012, 8:05 AM)

Such models already exist and this article did nothing to further promote their use.


Comment from Mark Schilling, (11/13/2012, 8:27 AM)

To Glenn Summers - I say Bingo! Computer models are a dime a dozen. And most computer models turn out to be wrong. Most people don't have a good understanding and appreciation of the basic chemistry and physics with respect to salts or to electrochemical corrosion. They don't even have good command of the English language (e.g., chlorides - plural). So, let me get this straight. We don't know what we are talking about but a computer model will give us THE answers. Really?! I mean no personal offense to anyone but I have to ask - collectively, how ignorant if not just plain stupid are we? It's people who don't know pretending to know and that can be dangerous. There is an old saying that too many of us have apparently forgotten. Garbage in; garbage out. Why do we believe this computer modelling nonsense? It's because we are generally lazy. We lack intectual rigor. We don't want to actually know and understand the facts. We have watched too many episodes of Star Trek. Take the easy path, the path of least persistence. Ask the computer and then do whatever the computer tells you to do. We need better detection, correction and prevention - and fewer computer models giving us the "magic" answers that only a machine could possibly provide.


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