Frequently Asked Questions

For a reinforced concrete bridge in Delhi NCR that has been repaired using MC-RocTec 222 structural repair mortar, why is the subsequent application of MC-Color EC anti-carbonation coating an essential final stage rather than an optional cosmetic finish, and how should it be specified?

The application of a protective surface coating such as MC-Color EC over repaired concrete is one of the most persistently omitted stages in Indian bridge and infrastructure repair programmes, and its omission is the single most common reason why structural concrete repairs on Delhi NCR bridges and flyovers fail within 3 to 7 years instead of lasting 15 to 25 years. Understanding why the coating is a structural requirement — not a cosmetic option — requires understanding the mechanism of concrete carbonation and its relationship to reinforcement corrosion. Carbonation of concrete is the slow, progressive reaction of atmospheric carbon dioxide with the calcium hydroxide and calcium silicate hydrate phases in the cement paste, which reduces the pH of the concrete pore water from approximately 12.5 to 13 (highly alkaline, passivating the steel) to below 9 (neutral to weakly alkaline, above which the passive oxide film on the steel breaks down). Once the carbonation front has reached the depth of the reinforcement (the concrete cover depth), the steel reinforcement can corrode in the presence of oxygen and moisture even without chloride contamination. The repaired structure is particularly vulnerable to carbonation re-attack for two reasons. First, repair mortars — even high-quality EN 1504-3 Class R3 products — are not carbonation-free materials: they are cement-based and will themselves carbonate over time at a rate dependent on their water-cement ratio, density, and the ambient CO2 concentration. A new repair mortar exposed without surface protection in urban Delhi (where CO2 concentration in traffic-dense areas can be 500 to 700 ppm, significantly above the rural background level of 420 ppm) will begin to carbonate from the surface inward immediately after curing. Second, the repaired concrete will have cracks — both the repair interface perimeter (the saw-cut boundary between the repair mortar and the existing concrete) and thermal shrinkage cracks in the repair mortar itself — through which CO2 and water can penetrate at a rate far higher than through the intact repair mortar surface. MC-Color EC addresses both vulnerabilities simultaneously. As an elastomeric coating applied over the entire surface of the repaired structure (not just over the repair patches), it provides a uniform, crack-bridging barrier against CO2 penetration over both the repair mortar and the surrounding parent concrete. The high Sd value (greater than 200 m equivalent air layer) means that the annual carbonation depth increment in the concrete under the coating is reduced to negligible levels — effectively freezing the carbonation front at its current depth and preventing it from advancing further toward the reinforcement. This extends the service life of the repaired structure by the number of years it would otherwise take for the carbonation front to advance from its current depth to the reinforcement depth — potentially 15 to 30 additional years of protection in Delhi NCR air quality conditions. For the specification, MC-Color EC should be applied in a system comprising: first, one coat of MC-Bauchemie primer (water-based acrylic primer) at approximately 100 to 150 ml per square metre to seal the porous concrete surface and provide a uniform absorption base; then two to three topcoats of MC-Color EC at 150 to 200 ml per square metre per coat at 4 to 6 hours minimum interval between coats, building a total dry film thickness of approximately 200 to 300 microns, which provides the elastomeric thickness necessary for crack-bridging and the CO2 barrier Sd value exceeding 200 m. The total system should be applied over the entire repaired structure surface — not just the repair patches — as a uniform, continuous protective coating. Selective application over repair patches only leaves the surrounding parent concrete unprotected and creates a patchwork appearance that also looks aesthetically unacceptable. Space Arc Engineering recommends specifying MC-Color EC as the mandatory final stage of every structural concrete repair programme for bridges and car parks in Delhi NCR, where carbonation rates are elevated by high urban CO2 and the consequences of re-corrosion within a few years of costly repair are both structural and financial.