Frequently Asked Questions

A site engineer managing structural repair of a reinforced concrete building in Ghaziabad asks: after breaking out the spalled concrete and exposing the corroded rebar, what is the correct sequence of operations for rebar preparation and MC-Dur 900 application before applying the repair mortar, and what are the common application errors that cause repair failures in this critical step?

The rebar preparation and MC-Dur 900 corrosion protection application step is the single most quality-critical operation in the entire structural concrete repair sequence — failures at this step lead to repair failures that are expensive to diagnose and remediate. Here is the complete correct procedure and the common errors. Phase 1 — Concrete removal to sound substrate: remove all carbonated, contaminated, or delaminated concrete to expose fully sound concrete — the correct test for sound concrete is hammer sounding (a hollow or drum sound indicates delamination) and the Schmidt hammer rebound (less than 25 on the Schmidt hammer indicates weak concrete that must be removed); do not stop removal at the first sight of the rebar — continue removal until you have a minimum 10 mm clearance all around the rebar circumference to allow the repair mortar to fully encapsulate the bar and develop adequate cover. Phase 2 — Saw-cutting repair perimeter: before breaking out concrete, always saw-cut the perimeter of the repair to a minimum 10 mm depth — this prevents edge feathering (repair mortar applied as a thin wedge at the repair boundary) which is a primary cause of edge debonding; the saw cut creates a square repair edge that supports minimum mortar thickness at the perimeter. Phase 3 — Rebar cleaning — the most critical step: clean all exposed rebar to a minimum of St 3 standard (vigorous wire brushing with a power wire brush removing all loose rust, scale, and contamination — the steel surface should appear bright metallic grey with a slight metallic lustre on the high points; residual tight rust with no visible loose scale is acceptable at St 2 — the minimum for chloride-contaminated conditions is St 3 or Sa 2 grit blast; the reason cleaning standard matters: loose rust and scale are not bonded to the steel and will continue to corrode under the MC-Dur 900 coating — the corrosion products will expand, generating stress and debonding the coating and repair mortar. Common error 1 — under-cleaning: wire brushing only the front face of the rebar that is visible from the concrete surface, leaving the back face (against the remaining concrete) with loose rust — the back face is where the original chloride or carbonation attack was most active and where the most extensive corrosion product accumulation exists; use angle grinders with wire cup brushes to clean the full 360 degrees of the rebar perimeter. Phase 4 — Chloride contamination assessment: if the cause of corrosion is chloride (coastal structure, de-icing salt environment, seawater contamination), use a rapid chloride test kit (Rapid Chloride Test or QuanTab) to measure the soluble chloride content of the concrete dust sampled at the rebar depth — if soluble chloride exceeds 0.4 percent by mass of cement, the chloride-contaminated concrete has not been fully removed or the rebar surface is contaminated; additional concrete removal or rebar cleaning is required before MC-Dur 900 application. Phase 5 — Mix MC-Dur 900 per the data sheet ratio: add the polymer Component B to Component A powder and mix with a slow-speed drill and paddle mixer for 3 minutes to form a lump-free creamy consistency — do not add water; do not over-mix (air entrapment); mix only the quantity you can use within the pot life (typically 30 to 60 minutes at 25 degrees Celsius). Phase 6 — First coat application: brush apply MC-Dur 900 generously with a stiff-bristle brush (not a soft paint brush — you need the stiffness to work the material fully around the rebar perimeter); apply in a scrubbing motion to ensure full coverage of the front, sides, and back of the rebar; apply to both the exposed rebar and to the concrete substrate immediately adjacent to the rebar; the first coat should be 1 mm thick and fully cover the metallic grey steel — no bare metal visible. Common error 2 — painting the rebar and not the concrete: the corrosion protection coating should extend onto the concrete substrate at least 20 mm around the rebar perimeter — the bond between rebar and concrete at the edge of the exposed repair is where corrosion most commonly restarts; leaving bare concrete at the rebar-concrete interface immediately adjacent to the repair allows carbonation and moisture to re-attack the steel at the repair perimeter within a few years. Phase 7 — Second coat application: apply the second coat after the first coat has set sufficiently to be touch-firm (typically 2 to 4 hours at 25 degrees Celsius) but before it has fully dried — the second coat must bond to the first coat through mechanical interlocking while the first coat still has some surface moisture; the second coat brings the total coating thickness to 2 mm. Common error 3 — applying repair mortar over fully dried MC-Dur 900: if the MC-Dur 900 is allowed to dry completely (dusty, chalky surface) before repair mortar application, the bond between MC-Dur 900 and the repair mortar is significantly reduced — the repair mortar should be applied while MC-Dur 900 surface is still touch-damp; if fully dried, pre-wet with clean water before repair mortar application. Phase 8 — Bonding coat (if specified): some specifications require a separate cementitious bonding slurry (MC-Contact) between MC-Dur 900 and the repair mortar; apply MC-Contact to the cured MC-Dur 900 surface and to the concrete substrate immediately before repair mortar application, working the repair mortar into the wet bonding coat — do not allow bonding coat to dry before repair mortar application (the bonding coat is a wet-on-wet application). Common error 4 — applying repair mortar in ambient temperatures above 35 degrees Celsius without shading: in Indian summer conditions (April to June, 38 to 45 degrees Celsius in Ghaziabad), the MC-Dur 900 and the concrete substrate can exceed 50 degrees Celsius in direct sun — at these temperatures, the pot life of MC-Dur 900 is halved, the material sets before it can be worked into the rebar perimeter, and the repair mortar applied subsequently may exhibit accelerated setting and plastic shrinkage cracking; schedule rebar coating and repair mortar work in early morning (before 9 am) or shade the work area throughout. Space Arc Engineering provides full technical support for MC-Dur 900 application, including site supervision for large or complex structural repair projects across Ghaziabad, Delhi NCR, Noida, and Uttar Pradesh — contact +91 9999155255 for pre-project consultation.