Corrosive Environments
Rebars in Reinforced Concrete
Rebars are a steel rod or structure that are used within concrete. The purpose of using thee rebars is to provide the concrete with extra strength. In reinforced concrete the rebar may have many separate areas at different energy levels. The concrete acts as the electrolyte and the metallic connection is made by the wire ties, chair supports or the rebar itself
Corrosion is an electrochemical process involving the flow of charges. At active sites on the bar, called anodes, iron atoms lose electrons and move to the surrounding concrete as ferrous ions. This process is called half cell oxidation reaction or the anodic reaction and is represented as:
2Fe —–> 2Fe2+ + 4e–
The electrons remain in the bar and flow to cathodic sites where they combine with water and oxgen in the concrete. A common reduction reaction is:
2H2O + O2 + 4e- —–> 4OH-
To maintain electrical neutrality, the ferrous ions migrate through the concrete pore water to these cathodic sites where they combine to form iron hydroxides, or rust:
2Fe2+ +4OH– —–> 2Fe(OH)
Above: Fig 1- A diagram to show the electrochemical corrosion of rebar as discussed above)
(http://www.cement.org/for-concrete-books-learning/concrete-technology/durability/corrosion-of-embedded-materials, last accessed, 27th Dec 2014)
“Chloride induced rebar corrosion is one of the major forms of environmental attack on re-enforced concrete, which may lead to reduction in the strength, serviceability and aesthetics of the structure. The accumulation of corrosion products (oxides/hydroxides) in the concrete pore space near the steel rebar can build up hoop stresses around the rebar and result in cracking or spalling of the concrete, which in turn facilitates the ingress of moisture, oxygen and chlorides to the embedded rebar and accelerates the corrosion of steel.”
(Page 126, Construction and Building Materials-Shi, Xie,Fortune & Gong, 2011)
Reading into the above review it explains how the rebars within the steel can undergo corrosion by the oxides and the hydroxides within the pores around the steel itself. To reduce these hydroxides and oxides would reduce the build up of them, in turn the corrosion that is able to take place within the concrete.
“Increasing the Strength and durability by using admixtures such as fly ash, silica fume, slag and metakaolin is used. This is because it increases the chloride binding properties, decreasing the chloride permeability”
(Page 126, Construction and Building Materials-Shi, Xie,Fortune & Gong, 2011)
The above idea highlights the method of stopping the chlorides passing through the concrete in order to stop it getting to the steel. The longer it takes the chlorides to get through the concrete, the longer it will take for the steel to corrode.
Adding to this fusion bonded expoxy coated steel performs well in chloride contaminated concrete up to about 3.9% chloride ion in content
(9:55, Corrosion, LL Shreir, RA Jarman and GT Burstein, 1994)
By adding coatings such as these epoxy coatings to the steel it further protects the steel from corrosion once the chlorides get through to it and will increase the lifespan of it.
Other materials that could be used but react differently with the concrete when wet. Aluminium reacts vigorously with wet concrete where hydrogen is evolved. Zinc initially reacts with cement based materials with the evolution of hydrogen. It is more protective in carbonated concrete.
Copper is unaffected by the concrete
(9:53, Corrosion, LL Shreir, RA Jarman and GT Burstein, 1994)
Because copper is unnafected by the concrete it would be ideal to use, however it would not provide the same strength as steel would. Zinc would be ideal to use as a coating to the steel to provide further protection along with the aluminium.
References
Construction and Building Materials-Shi, Xie,Fortune & Gong, 2011
Corrosion, LL Shreir, RA Jarman and GT Burstein, 1994
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