Concrete is fairly durable as a material,material; it is typically the reinforcement that corrodes.
As the steel corrodes, it expands up to eight times its original volume. Expansive forces cause extreme pressures within the concrete, which eventually are relieved when the concrete cracks. In turn,these cracks admit more water and chloride salts, accelerating the corrosion process, which leads to more cracking, then more corrosion, in a progressive cycle of damage. Over time, the cumulative process reduces the cross-sectional area of the reinforcing steel, compromising the parking structure’s structural integrity.
Elastic deformation of concrete is an instantaneous and reversible deformation of concrete caused by loading, while the phenomena known as creep is an irreversible plastic deformation of concrete caused by sustained loading.
Thermal movement of concrete comprises expansion on a rise in temperature or contraction on a fall in temperature, in the same way as almost any other material.
Once corrosion has progressed to concrete delamination and exposed rebar, the cost of rehabilitation can be high.
Carbonation is the reaction of carbon dioxide in the environment with the calcium hydroxide in the cement paste. This reaction produces calcium carbonate and lowers the pH to around 9. At this value the protective oxide layer surrounding the reinforcing steel breaks down and corrosion becomes possible. The reaction of carbon dioxide and calcium hydroxide only occurs in solution and so in very dry concrete carbonation will be slow. In saturated concrete the moisture presents a barrier to the penetration of carbon dioxide and again carbonation will be slow. The most favourable condition for the carbonation reaction is when there is sufficient moisture for the reaction but not enough to act as a barrier. In most structures made with good quality concrete, carbonation will take several (or many) years to reach the level of the reinforcement.
For corrosion to occur water and oxygen needs to be present. Oxygen is present in the air. Therefore, if there are any damp or wet areas of concrete corrosion will take place. Once corrosion begins the rust produce expands and cracks the concrete reducing the strength of the concrete.
Carbonation is the process by which this protective environment degrades, subjecting the reinforcement to corrosive forces. When rainwater combines with carbon dioxide in the air, it forms carbonic acid, which combines with the concrete’s calcium hydroxide to form calcium carbonate.
Concrete is tested for carbonation by drilling into the concrete. Freshly exposed concrete is sprayed using a solution of phenolphthalein indicator that appears pink in contact with alkaline concrete with pH values in excess of9 and colourless at lower levels of pH
Migrating corrosioninhibitor (MCI) compoundssuch assuchas Corrosion inhibitingimpregnation Sika Ferroguard. These are used to soak into the concrete and provide a protective film around the steelwork. The intention is to reverse the PH value of the concrete.
When the carbonation depth is shallow its rate of ingress can be limited by preventing the environment in getting to the concrete. Elastomeric Hydrophobic Impregnating Hydrophobic Impregnating compounds Surface treatments place a barrier on the concrete.
When the inclusion of chlorides in concrete together with oxygen, reach the embedded steeland set up electrochemical corrosion in the rebar.
De-icing chemical damage is a de-icing material is rock salt (i.e.sodium chloride), which is extremely corrosive to steel and destructive to concrete.
The concrete is drilled in and tested for Chlorides. The concrete is drilled in sets of 3 to confirm the depth that chlorides have penetrated.
The electrical resistant of areas can be checked with half cell potential tests. These are taken over a 1m or500mm square area at strategic locations to obtain the chloride content.
Treatments look to alter the Electron Flow within the steelwork. Active protection is often successful in existing structures. Croft will need to appoint an engineer from either the National Association of Corrosion Engineers (NACE) or The Institute of Corrosion level 4 to provide a suitable design. The active measure slook to place a small electrical charge into charge into the Concrete
Surface treatments are the primary means of protection for existing parking decks, as they are non-destructive and relatively straightforward to apply. Preventative coating use and prompt addressing of cracks and surface defect scan avert corrosion and significantly prolong the structure’s life.
Silane-siloxane treatments improve the water repellent properties of the concrete, and concrete and achieve this by hydrophobic impregnation of the concrete to prevent water ingress
Elastomeric coatings that form a barrier, locking out moisture and chlorides to protect the underlying concrete and reinforcement. Composed primarily of epoxy, methylmethacrylate, or urethane
A migrating corrosion inhibitor (MCI) can be applied to the surface of an existing concrete parking structure, as well as incorporated as an admixture into new concrete construction. In theory, a surface-applied MCI is drawn into the pores of the concrete through capillary action, penetrating down to the reinforcing steel level. Through ionic attraction, the MCI adsorbs into the steel, forming a protective coating that displaces chlorides and other corrosive compounds. In practice, however, some studies have shown the MCI may not successfully reach the steel reinforcement in some applications.