Frequently Asked Questions

A civil works contractor in Ghaziabad has just struck formwork from a reinforced concrete bridge pier column in a hot summer pour and found the column face has extensive bug holes (5 to 15 mm diameter, 3 to 8 mm deep) over approximately 40 percent of the surface area plus two areas of honeycombing (50 to 100 mm diameter, 8 to 15 mm deep) and two shutter step lines (2 to 3 mm rise) — the column will receive MC-Color 2K anti-carbonation coating — what is the complete cosmetic repair sequence using MC-Dur 100, and at what point can the MC-Color 2K coating be applied after the skim coat repair?

Cosmetic defect repair of a freshly struck concrete column in summer requires a systematic approach because the hot ambient temperature and direct sun drying of the fresh skim coat mortar are the primary causes of skim coat cracking and delamination — the most common failures of this type of repair on summer bridge projects in Delhi NCR. The complete repair sequence is as follows. Initial assessment and prioritisation: after formwork striking, inspect the full column face in both direct light and oblique light (oblique light reveals form step lines and surface undulations that are invisible in direct lighting). Document the defect types, depths, and areas. Confirm: bug holes less than 10 mm depth — MC-Dur 100 directly; honeycombing 10 to 15 mm depth — confirm no exposed reinforcement steel before skim coat; if reinforcement is visible or corroded within any honeycombed zone, reclassify as structural repair requiring MC-Dur 200 or MC-Dur 600 and rebar treatment before skim coat. Substrate preparation for skim coat: the concrete surface must be clean and free of release agent (mould oil) before MC-Dur 100 application — release agent prevents adhesion. Wash the full column face with clean water and stiff brush to remove release oil; or wash with dilute detergent solution and rinse — check by water drop test on the surface (water should sheet rather than bead; beading indicates oil contamination that requires further cleaning or light grinding). Pre-wet the surface to SSD condition immediately before applying MC-Dur 100 — hot dry concrete in summer will absorb moisture from the fresh mortar and cause it to stiff-prime too rapidly for good trowelling and adhesion. Application for each defect type: Bug holes 1 to 5 mm depth: mix MC-Dur 100 to a stiff paste consistency (minimum water, about 4.5 litres per 25 kg) — a stiffer consistency reduces the risk of the mortar slumping out of the overhead and vertical bug holes. Pack the mixed mortar firmly into each bug hole with a pointing trowel or small palette knife using a pressing and twisting motion to fill completely — do not just smear over the surface; the bug holes must be filled solid. Build up level with the surrounding concrete surface in one pass for depths up to 8 mm; for depths 8 to 15 mm, apply in two passes (first pass to half depth, allow to firm up 1 to 2 hours, second pass flush to surface). Honeycombed zones 10 to 15 mm depth: square-cut the edges of the honeycombed zone with an angle grinder (4 to 5 mm depth saw cut around perimeter) to eliminate feather edges; blow clean with compressed air; dampen; apply MC-Dur 100 in layers, building up from the substrate in 5 to 8 mm layers — allow each layer to firm up before the next. Finish level with or 1 to 2 mm proud of the surrounding surface — allow to dry and then fine sand or grind flush if proud. Form step lines 2 to 3 mm height: apply MC-Dur 100 as a skim feathering from the high side of the step down to zero at the low side — the fine aggregate of MC-Dur 100 allows application as thin as 1 mm at the feather edge. Use a long straight edge or scratch pad to blend the skim coat into the column face surface for a smooth transition. Final skim coat application: after all defects are filled and the patches have been cured for minimum 24 hours, apply a full-face skim coat of MC-Dur 100 at 2 to 3 mm thickness over the entire column face to achieve a uniform, smooth surface for coating — this full-face skim eliminates the patchy colour and texture variation between the repaired areas and the original concrete face that would show through the MC-Color 2K coating. Apply with a wide plastering trowel (600 mm x 150 mm) in a single pass, working from top to bottom, and finish with a damp steel trowel for maximum smoothness. Post-application curing: immediately after the skim coat surface has closed (is no longer wet but is still slightly damp, approximately 30 to 60 minutes in summer heat), cover the full column face with polyethylene sheet to prevent rapid moisture loss — tape the sheet edges securely to prevent wind from lifting it. Alternatively, mist with clean water spray 3 to 4 times over the following 24 hours. In direct sunlight, shade the column face with a tarpaulin for the first 6 hours. Timing for MC-Color 2K coating application: MC-Color 2K must not be applied over freshly-applied wet or green skim coat — the epoxy primer will seal the surface and trap moisture vapour which causes blistering of the coating. Wait minimum 7 days at 25 degrees Celsius ambient before applying MC-Color 2K over MC-Dur 100 skim coat — this allows the skim coat to dry to less than 4 percent moisture content (CM meter check). In summer at 40 to 45 degrees Celsius and low humidity (May-June), 4 to 5 days of drying after the skim coat is typically sufficient — confirm by CM meter rather than by fixed days. After moisture content is confirmed less than 4 percent, apply MC-Floor 2K EP or MC-Color 2K directly onto the skim coat surface — no additional priming is required on a sound, dry MC-Dur 100 skim. Space Arc Engineering supplies MC-Dur 100, MC-Color 2K, and technical support for bridge column cosmetic repair, precast concrete finishing, and structural concrete surface preparation in Ghaziabad, Delhi NCR, Noida, and Uttar Pradesh.