Guidelines on Repair and Rehabilitation of StructuresSamir Surlaker & Sunny Surlaker, MC-Bauchemie India Private Limited
IntroductionVersatility of Concrete is an established fact on account of its mouldability and durability. This makes it attractive for use in varied exposure conditions. Vulnerability of concrete to deterioration has always been a cause of deep concern. Concrete Cover plays a key role in this phenomenon. Generally, the process of deterioration begins almost immediately after the casting. Basically from material science point of view, the durability of concrete structure is a direct function of achieving specified cover not only in dimensions but also in quality. Considering a mix of complicated construction, quick completion deadlines, speed of construction and economy, it becomes virtually impossible to cast specified cover to reinforcement. The outermost layer of concrete therefore becomes the weakest one. If formworks are not impermeable and slump of concrete is not adequate, the cement paste does not cover the reinforcement optimally to render it passive to corrosion. This facilitates deterioration and the subsequent need for repairs.
Since cover is the first line of defense to inhibit corrosion, repair and rehabilitation is recreation of cover with the highest protection quotient. We therefore need to use the latest technologies like polymers, fibres, etc. to lower the permeability, minimize the cracks and provide adequate bonding, to stop further deterioration. Given actual working conditions, it is difficult to create a cover that will be resistant to carbonation or chloride ingress. To remove this lacuna from the system, it is preferable to resort to protective coating that work on principles to equivalent cover to ensure durability of repairs. Therefore, the last step of Concrete Repair, which is anti- carbonation or chloride ingress resistant coating, ensures the durability of Repairs.
Cause of DeteriorationIt has been established that the increasing environmental stress is one of the key factors causing concrete damage. Figure 1 shows this process, due to carbonation attack. It is recognized that steel embedded in a heavily alkaline medium like concrete with pH-values from 9 upwards will not rust. The concrete's hydration process ensures the concrete's alkalinity, producing a pH-value of more than 12.6, which renders the steel surface passive to corrosion. Due to this, even the occurrence of small cracks (up to 0.1 mm in width) or blemishes in the concrete need not necessarily lead to damage. Environmental influences and carbon dioxide in particular, will reduce the concrete's pH-value (carbonation) and will remove the passivating effect. In conjunction with existing humidity, the result is corrosion of the reinforcement.
Basic Principles for the Protection and Repair of Concrete Structures
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