Concrete structures, especially those located in coastal areas or in regions of the country where deicing salts are used, are deteriorating at a rapid rate. The bill for needed repairs is huge and growing fast. Some estimates exceed $100 billion annually for corrosion induced failures in the U.S. alone. We think of concrete as a durable and permanent building material, so how can this be?
What causes concrete to deteriorate?
Exposure to the atmosphere (carbonation, chlorides, salt’s) and other environmental corrosives contribute to the electrochemical reaction on the reinforcing steel. Unless steps are taken to protect the steel from the effects of contamination, a predictable failure will occur. The high alkaline environment (pH) of the cement, as well as the naturally occurring passivating film on the steel, prohibit the formation of anodic and cathodic sites needed to begin the corrosion cycle. As concrete is exposed to moisture and atmospheric carbon dioxide – a condition known as carbonation – the alkaline environment is neutralized, and the corrosion is initiated. Moisture will infiltrate either through cracks resulting from shrinkage of the cement or with movement of the structure.
Analysis of the chemical contaminants within the concrete is essential to determine the appropriate responsive treatment. It should also be noted that the placement of the reinforcement steel within the concrete cover contributes to its exposure to moisture. Obviously the more porous the concrete, the easier it is for these contaminants to penetrate. Chlorides from ocean breezes, acid rain, and deicing salts all contribute to accelerated corrosion of embedded steel reinforcement. We now know that, frequently, chlorides have been introduced into a concrete mix by way of the aggregate or as calcium chloride (a cure-accelerating admixture). Chlorides, like carbonation, will lower pH and attack the steel’s passive oxide coating.
How Can You Tell If Your Concrete Needs To Be Repaired?
Concrete cracking and spalling are the first visual signs of failure. Spalling is caused when the reinforcing steel corrodes. As steel corrodes, the rust expands- up to eight times its original size. The resulting expansive pressure on the surrounding concrete causes it to crack. This eruption can eventually cause a complete separation – known as a delamination. Once this process has started, merely surface patching or coating contaminated concrete will not stop the steel inside from corroding. In fact, this has often been shown to make things worse. Trapping chlorides and moisture together inside deteriorated concrete can be a disaster waiting to happen.
What Can You Do About Deteriorated Concrete?
The first requirement is to have a specialist perform a condition or corrosion survey of the structure. It is important to both quantify the amount of deterioration and measure the corrosion rate of the embedded steel. Several nondestructive chemical tests can be performed that will provide the critical data necessary for restoration activities. Too often, contractors simply mark a delaminated area of concrete, jack hammer it out, clean or replace the steel bars, install a replacement patch, and then apply a waterproof overcoat. While this conventional technique is required in severe cases of corrosion, it will only address the worst problems temporarily. Within a few years, your contractor will be back to do more of the same work on other areas. This is because the underlying corrosion issues have not been addressed.
There are other methods that can be more appropriate and certainly more cost-effective. Many engineers and consultants are now recommending the use of chemistry as an effective method to prevent and arrest the corrosion process. Corrosion inhibitor technology has been around since the 1960’s but has only been available to the construction industry for the past 10 years or so. These chemicals, when properly installed, will stop the corrosion from within. This is accomplished by penetrating the concrete through cracks and pores and then reacting with the cement and reinforcement steel. It is essential to attack corrosion on several fronts to successfully restore the passivating film to the steel and extend the useful life of the concrete structure.
How Is A Corrosion Inhibitor Used To Treat Concrete?
The application of the corrosion inhibitors and post-project testing is equally important. The proper sequence of products, uniform application, time between applications, and post-application cleaning are all critical for lasting results. Successful projects will have engineers and contractors who fully understand the conditions which cause structures to deteriorate in the first place and that the objective is to arrest or prevent them. It is not as simple as just replacing the concrete.
Professionals on the cutting edge of their field understand that the chemical reactions taking place must be addressed for a long-term solution. Handling these problems is highly specialized because the solution will be different if you are dealing with a structure located in Chicago, corroding due to deicing salts, versus a structure in Miami. We find conditions change from floor to floor within the same structure because the concrete properties also change.
How Will You Know If Corrosion Inhibitors Work?
Testing and test methods have greatly improved over the past five years. Many manufactures have contracted with laboratories to perform corrosion testing on their clients to see if they are happy with the results. But recent advances in test equipment technology give lab-like capabilities to the analyst in the field. There are many tests that can be performed, not only to assess the condition of the structure, but also to verify the performance of corrosion inhibitors applied after the fact. Testing should be done before and after every project to verify the performance of chemical treatment and periodically thereafter.
Given what we know today, the use of chemistry and chemical products to extend the durability and useful life of concrete is taking on greater importance. It provides attractive and effective alternatives to the traditional (and expensive) replacement philosophy. Do yourself a favor and investigate the benefits that chemistry offers as a solution to corrosion problems. It could be a most practical option in maintaining your investments that are “concrete” in nature.