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Graham Moorfield and Rob Hossell of Grace Construction Products look at the role that corrosion-inhibiting admixtures play in protecting concrete structures.
For over 60 years now, Grace Construction Products has been actively improving the quality of concrete, by developing value-added admixtures that are recognised for their strength and durability enhancement, and ultimately the value that they impart to this unique universal material.
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The raft slab is cast for the Gulf Tower in West Bay, Doha, Qatar. |
Grace Construction Products actively promotes its range of chemical admixtures in the Middle East through its affiliate company Emirates Chemicals of the UAE, which has been a leader in the area for over 20 years.
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Al Habal Bridge in Abu Dhabi, UAE. |
The issue
What are the consequences of corrosion damage to reinforced concrete structures? Cracking, surface spalling, concrete repairs, possible structural failure, and potential liability claims for negligent durability design!
The problem
Many structures require extensive and expensive repairs due in no small part to chloride ions accessing the concrete from various sources such as seawater, groundwater, soil and the atmosphere. The chloride ions diffuse through concrete and eventually reach the embedded steel reinforcement. In the presence of both oxygen and water, these very mobile chloride ions interact with the oxides of iron in the passivating oxide layer surrounding the rebars causing the formation of high volume, "chemical complexes" commonly known as rust.
This expansive formation of rust leads to the tensile strength of the covering concrete being exceeded and ultimately cracking and spalling will occur, thus providing a much more direct route for further salts to reach embedded steel and accelerate the decay process. This goes some way to explaining why the process causes engineers and building owners so many headaches.
What are the answers? Most corrosion protection techniques are now well understood by engineers. These include:
One method that is gaining widespread acceptance is the inhibition of the "rusting" chemical reaction itself.
Integral inhibitors
There is a group of chemical admixtures commonly known as "integral" corrosion inhibitors which, when added to concrete during the batching process, will inhibit the chemistry of the corrosion process after the concrete has been placed. The most common type of corrosion inhibitor is based on calcium nitrite, as these inhibitors have by far the longest successful track record (25 years) in concrete construction throughout the world, and probably account for over 90 per cent of the corrosion inhibitor market.
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Vopak Tank Terminal Jetty in Fujairah, UAE. |
In the Gulf, calcium nitrite-based corrosion inhibitors are rapidly becoming accepted as an effective means of combating chloride-induced rebar corrosion. By providing an integral protection system whereby the workmanship element is not an issue, no additional precautions are necessary once the admixture has been added to concrete. Many other protection systems are reliant on further precautions at the site level (such as additional curing), or constant supervision to ensure no defects in the material application (a difficult task in severe summer temperatures).
Grace Construction Products utilises calcium nitrite technology in its DCI range of products, and has promoted this type of material to the construction industry, such that over 10 million cu m of concrete now contain the admixture (600,000 cu m in the Middle East alone).
It was the first company to develop, promote and market the benefits of calcium nitrite to the construction industry. It has now gained extensive experience throughout the Middle East, understanding and advising clients, specifiers, and concrete producers on the benefits of this type of material.
The fact that the British Board of AgrŽment (BBA), an independent assessor, has certified the performance of Grace DCI and DCI S corrosion inhibitors, also demonstrates confidence in this technology.
Calculating the dosage
It is possible to calculate the level of protection (dosage) that the admixture can provide to the embedded steel. Only calcium nitrite-based corrosion inhibitors have extensive long-term testing that confirms these chloride protection levels are now well established. The rate at which chloride ions diffuse through concrete can be estimated using Fick's second law of diffusion. Diffusion equations are used to predict the chloride concentration at the level of the reinforcement after a given period (the expected life of the structure). The dosage level of the calcium nitrite is then determined to provide the engineer with an "informed estimate" as to the necessary level of protection.
Computer models, such as the 'DuraModel', developed by Grace are used to make predictions. This software package takes account of all of the factors that affect chloride ingress through concrete such as:
Service life and all costings are included to provide a full life-cycle cost analysis, which can quantify the long-term benefit of providing additional protection during construction. Durability modelling described above was used in all the projects illustrated in the article.
Summary
Throughout the world, the use of calcium nitrite-based corrosion inhibitors in structures has been shown to successfully and economically combat rebar corrosion caused by chloride ingress.
The fact still remains, however, that some engineers and specifiers still believe that this type of corrosion protection technology has yet to be proved.
Considering several thousand structures have been built over the past 25 years utilising a calcium nitrite-based corrosion inhibitor, none of which has experienced any corrosion problems, this is a surprising situation.
One of the main advantages of a "one-off" treatment such as an integral calcium nitrite-based corrosion inhibitor is that it is required, and costed, only once - at construction stage - to achieve the desired service life.
As the chloride ions will inevitably reach the reinforcement, the owner of the structure should take comfort in the knowledge that they won't cause a problem because of the elevated corrosion threshold. The structure can then be considered to be ''protected from within''.
