Frequently Asked Questions

A state PWD in UP is planning preventive maintenance of 120 highway flyover structures across Ghaziabad and Meerut where condition surveys show that the concrete on pier columns and abutments has carbonated to 10 to 18 mm depth on many structures — the rebar has not yet begun to corrode — the engineer is deciding between surface protection by anti-carbonation paint alone versus a thin MC-Nafufill SB spray mortar render plus anti-carbonation paint — what is the technical basis for choosing between these two approaches and what are the cost and durability implications?

The decision between anti-carbonation paint alone versus a thin mortar render plus paint as the surface protection intervention for carbonated but not-yet-corroding flyover concrete in the Ghaziabad and Meerut highway network is one of the most important and cost-impactful choices in a preventive infrastructure maintenance programme. Here is the technical analysis. Current condition and risk: the survey shows carbonation depth of 10 to 18 mm on pier and abutment surfaces. The structures are highway flyovers in the Delhi NCR urban industrial environment where CO2 concentrations are elevated (3 to 5 times higher than background) from vehicle exhaust, and the concrete was probably designed in the 1990s to 2000s when M25 or M30 was standard for flyover substructure — these grades typically achieve carbonation depths of 10 to 20 mm within 10 to 15 years in the Delhi NCR environment. If rebar cover is 40 mm (nominal per IS 456 for moderate exposure), and carbonation has reached 10 to 18 mm, the residual uncarbonated cover protecting the rebar is only 22 to 30 mm. Without intervention, carbonation will continue to advance at approximately 1 to 2 mm per year in this environment (following the square root of time diffusion law), and rebar depassivation (onset of corrosion) is likely within 10 to 15 years for the most-carbonated areas. The maintenance objective is to prevent the carbonation front from reaching the rebar depth before the next planned major rehabilitation cycle (target 20 to 25 years). Anti-carbonation paint alone: a high-quality anti-carbonation paint system (e.g., an elastomeric anti-carbonation coating at 150 to 200 microns DFT, or a silicone-acrylic anti-carbonation system) applied to the existing concrete surface will provide an equivalent air layer thickness (sd value) of 50 to 200 metres — equivalent to adding a significant effective cover depth against CO2 penetration. A paint system with sd value of 100 metres is equivalent to approximately 50 to 70 mm of additional effective carbonation-free cover — more than sufficient to prevent carbonation from reaching the rebar over the 20-year target life if the paint system remains intact. However, the surface condition of carbonated concrete is often problematic for direct paint application: the surface layer has a higher porosity (carbonated cement paste has a looser microstructure than intact high-pH cement paste), may have fine surface cracks or surface irregularities from previous weathering, and may have patches of surface scaling or friable laitance that would cause paint delamination. If the paint is applied directly over such a surface, it may bubble, peel, or crack at the surface irregularities within 3 to 5 years, compromising the CO2 barrier well before the design life is achieved. MC-Nafufill SB render plus paint: applying a 10 to 15 mm MC-Nafufill SB spray render over the prepared concrete surface achieves three simultaneous objectives. First, it removes and replaces the degraded carbonated surface layer — the spray render is a fresh, low-permeability, polymer-modified cement mortar with very low CO2 permeability that itself provides anti-carbonation protection. Second, it provides a new, sound, smooth surface for the subsequent anti-carbonation paint system to adhere to, eliminating the surface irregularity adhesion problem. Third, it adds 10 to 15 mm of effective concrete section over the remaining sound substrate, restoring the total cover depth to the designed level. The render-plus-paint system provides a combined barrier to CO2 that is substantially more durable than paint alone on a degraded carbonated surface — expected performance life of the combined system on properly prepared and applied piers is 20 to 30 years versus 10 to 15 years for paint alone on a borderline surface. Cost comparison: for 120 flyover structures with an average pier and abutment surface area of 300 square metres per structure, total surface area is 36,000 square metres. Anti-carbonation paint alone: estimated material cost at INR 200 to 300 per square metre for a quality two-coat anti-carbonation paint system = INR 72 to 108 lakh. MC-Nafufill SB render at 12 mm average thickness plus anti-carbonation paint: estimated combined cost at INR 500 to 700 per square metre = INR 180 to 252 lakh. The render-plus-paint approach costs approximately 2 to 2.5 times more than paint alone in this scenario. However, on a life-cycle basis, if the paint-alone system requires reapplication after 10 to 15 years (INR 72 to 108 lakh plus mobilisation cost) versus the render-plus-paint system lasting 25 years without reapplication, the net present value of the two options over a 25-year horizon is much closer — and the render approach delivers much higher confidence of achieving the preventive maintenance objective without emergency structural repair intervention within the target period. Recommendation: for flyover structures where the carbonation depth is below 10 mm and the surface is in good condition (no scaling, no surface cracking), anti-carbonation paint alone is appropriate and cost-effective. For structures where carbonation depth is 10 to 18 mm and the surface shows visible deterioration (scaling, surface cracks, irregular texture), the MC-Nafufill SB render plus anti-carbonation paint system is the technically superior intervention and the PWD should specify this for the highest-priority structures in the programme. Space Arc Engineering supplies MC-Nafufill SB and the complete MC-Bauchemie concrete repair and protection system for PWD and infrastructure maintenance programmes in Ghaziabad, Meerut, Noida, and Uttar Pradesh.