About Fosroc Renderoc Microconcrete

Renderoc Microconcrete is the flowable member of Fosroc’s Renderoc concrete-repair range, catalogued by the manufacturer as Renderoc RG. It arrives on site as a pre-weighed dry powder that needs only clean water, which removes the batching guesswork that plagues site-mixed repair concrete. The blend combines Portland cements, graded aggregates and fillers with additives that generate a controlled gaseous expansion in the plastic state, so the placed material stays in intimate contact with the substrate and the surrounding steel instead of shrinking away from it as ordinary concrete would.

What sets it apart on a repair job is fluidity. Reinstating a corroded column or a deep spalled section usually means working around a dense reinforcement cage inside tight formwork, exactly where a vibrator cannot reach. Renderoc RG is designed to flow and self-compact under its own weight, filling the section cleanly without honeycombing. It can be poured directly or pumped into restricted pockets, cures to low permeability, and carries no chloride admixture, so it protects rather than threatens the reinforcement it encases. For Delhi NCR and pan-India contractors, Space Arc supplies it as an authorised Fosroc distributor with the specification support that structural repairs demand.

The technical insight

The behaviour that matters here is shrinkage compensation combined with a very low water demand. Ordinary repair concrete loses volume as it sets and as bleed water leaves, which opens a micro-gap at the bond line and around embedded steel, precisely where a repair must stay tight. Renderoc RG counters this with a gaseous expansion system that produces a slight, controlled unrestrained expansion of 1 to 4 percent (ASTM C827) while the mix is still plastic, holding it against the old concrete and the rebar. Because the additive package also keeps the water:powder ratio down to 0.16, the mix develops high early strength and a dense, low-permeability matrix rather than the weak, porous structure a high water content would leave behind. That is why the datasheet is emphatic that the water:powder ratio must never be increased; adding water to improve flow trades away strength, durability and the shrinkage compensation the system depends on. The graded aggregate is fine enough to travel through congested steel yet coarse enough to build a genuine structural cross-section, and for sections deeper than 100 mm the mix is extended on site with 5 to 12 mm saturated-surface-dry aggregate to limit the heat of hydration in a thick pour.

Where it is used

• Reinstating corroded and spalled reinforced concrete columns and beams to full cross-section
• Jacketing and encasement build-ups for structural strengthening of existing members
• Bridge pier, abutment and crosshead concrete reinstatement
• Repair of deep, congested sections where a poker vibrator cannot be used
• Pile-top and pile-cap encapsulation as part of a waterproofing system
• Reconstruction of retaining walls and other members with significant section loss
• Pump-placed repairs into restricted or inaccessible pockets behind formwork
• Foundation and plinth repairs where flowable, non-shrink concrete is specified

Key advantages

• Self-compacting flow fills congested reinforcement without vibration, eliminating honeycombing risk
• Gaseous expansion system compensates for plastic shrinkage and settlement, keeping the repair bonded
• Reaches roughly 50 N/mm2 at 28 days with strong early gain for faster reinstatement
• Low water demand gives a dense, low-permeability matrix and long-term durability
• Contains no chloride admixture, so it will not initiate corrosion of the reinforcement it encases
• Factory pre-bagged, removing site batching variation and simplifying quality control
• Can be poured or pumped into restricted locations that hand-placed mortars cannot reach
• Extendable on site with graded aggregate for deep sections, controlling cost and heat rise

Limitations & considerations

• A flowable material needs leak-tight formwork with proper pour and air-vent points; it is not a trowel-applied patch mortar
• Standard placement suits sections up to 100 mm deep; deeper pours require site-added 5-12 mm aggregate to control temperature rise
• The water:powder ratio of 0.16 is fixed and must never be increased to gain flow, which limits how far it can be thinned
• Requires a fully prepared substrate, exposed and cleaned steel, and priming (Nitozinc Primer on steel, Nitobond EP where used) for a sound repair
• Being cement-based, it releases alkali when wet and needs gloves, goggles and dust protection during mixing
• Shelf life is only about 6 months in unopened bags, and less in high heat or humidity, so stock must be rotated

Selection & best practice

Specifications

Industry use cases

How we position Fosroc Renderoc Microconcrete

Renderoc Microconcrete (Renderoc RG) is the flowable, formwork-and-pour choice within Fosroc’s repair range, best suited to deep, congested sections that cannot be vibrated. Where a repair is vertical or overhead and shallow, a hand-applied high-build mortar such as Fosroc Renderoc HB is the more practical companion, and both share the same Fosroc priming system (Nitozinc Primer on steel, Nitobond EP as bond aid). For fast-track patch repairs where early strength drives the programme, Fosroc Renderoc RG (rapid-gain) is a related option. Space Arc also carries equivalent micro-concretes and repair mortars from Sika, Dr. Fixit, MC-Bauchemie and STP, so a specifier can match the exact strength, depth and access constraints of the job; our team can advise on the closest cross-brand alternative when required.

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