Fosroc Renderoc GP
A single-component, shrinkage-compensated cementitious reinstatement mortar for patch repairs, rendering and reprofiling of damaged concrete on vertical, overhead and horizontal surfaces.
What is it?
A pre-bagged, single-component, medium-weight cementitious repair mortar based on Portland cement, graded aggregates, special fillers and chemical additives. You add only clean water on site to produce a consistent, shrinkage-compensated mortar that bonds to and reinstates damaged concrete.
Where is it used?
Used across India for general-purpose concrete repair and rehabilitation: spalled and carbonated RCC in buildings, bridges, water-retaining structures, basements, columns, beams, soffits and facades, as both a patch-repair mortar and a render.
Why use it?
It offers excellent bond to the substrate, is alkaline so it re-passivates and protects embedded steel, has low permeability to water, carbon dioxide and chlorides, is shrinkage-compensated for dimensional stability, and removes site-batching variability because it is factory pre-blended.
How is it applied?
The substrate is cut back, cleaned and primed (Nitobond AR; reinforcement primed with Nitoprime Zincrich). A 25 kg bag is mixed with about 4.0-5.0 litres of clean water in a forced-action mixer, applied by gloved hand or trowel in 10-30 mm lifts (up to 100 mm in pockets or with formwork), finished, then cured immediately.
About Fosroc Renderoc GP
Fosroc Renderoc GP is one of the longest-serving members of the Fosroc Renderoc concrete-repair family and is a workhorse product for routine reinstatement of deteriorated reinforced concrete. It is supplied as a ready-to-use blend of dry powders that requires only the site addition of clean water, producing a highly consistent, medium-weight mortar suitable for both patch repairs and rendering. The formulation is built on Portland cement, graded aggregates, special fillers and chemical additives, engineered to give good handling characteristics while keeping water demand low. That low water requirement is the key to its performance: it drives fast strength gain, lower permeability and better long-term durability.\n\nBecause the mortar is alkaline in nature, it restores a protective high-pH environment around embedded steel reinforcement, helping to arrest the corrosion that usually drives concrete spalling in the first place. It is also shrinkage-compensated, which limits cracking and de-bonding as the repair cures, and it shows good thermal compatibility with the parent concrete so that repair and substrate expand and contract together. In Indian conditions, where carbonation, chloride ingress in coastal and marine zones, and water-logging in basements and tanks are common causes of distress, this combination of re-alkalisation, low permeability and dimensional stability makes Renderoc GP a dependable default choice for non-structural and light structural reinstatement. It is sold in India through the Fosroc distribution network and is commonly specified by kilogram in 25 kg bags.\n\nIt is important to be precise about positioning: Renderoc GP is a general-purpose repair mortar, not a high-strength structural micro-concrete. Where large-volume, high-strength or high-flow reinstatement is needed, Fosroc’s Renderoc HB, Renderoc RG (free-flow micro-concrete) and similar grades take over. Renderoc GP earns its place by being economical, easy to handle and forgiving across a wide range of everyday repair situations.
The technical insight
The defining engineering feature of Renderoc GP is its very low water:powder ratio. Published properties are quoted at a water:powder ratio of 0.18, which on a 25 kg bag corresponds to roughly 4.0-5.0 litres of water. Keeping water this low is what delivers a compressive strength of around 9 N/mm2 at 1 day rising to about 28 N/mm2 at 28 days, a flexural strength of about 7.0 N/mm2 at 28 days, and the low-permeability matrix that gives the mortar its resistance to chlorides and carbon dioxide. This is also why the TDS is emphatic that the mortar must be machine-mixed in a forced-action mixer for a controlled time and never water-adjusted beyond the maximum dose: a one-pack mortar can feel perfectly workable while being under-mixed or over-watered, yet quietly lose a large part of its designed performance. The other practical lever is build thickness. In bulk areas Renderoc GP is applied at 10-30 mm per lift (up to 20 mm overhead, up to 30 mm vertically), but in small pockets or against sealed formwork it can be built to 100 mm, and greater builds are achieved in layers with a comb-scratched, re-primed intermediate surface. Because the product re-passivates steel chemically rather than acting as a barrier coat, surface preparation and reinforcement cleaning to a bright condition are not optional finishing steps but core to the repair’s durability.
Where it is used
- Patch repair of spalled, cracked or delaminated reinforced concrete in buildings, bridges and industrial structures
- Reinstatement of cover concrete lost to carbonation or chloride-induced reinforcement corrosion
- Rendering and reprofiling of damaged vertical and overhead concrete surfaces
- Repair of beams, columns, soffits, slabs, chajjas, parapets and facade elements
- Concrete rehabilitation in water-retaining and below-grade structures such as tanks, basements and retaining walls
- Repairs to marine and coastal structures exposed to aggressive chlorides and carbon dioxide
- Filling of honeycombing, blow-holes and form-tie pockets up to 100 mm depth
- Use as part of cathodic / re-alkalisation systems, being approved for use with Norcure realkalisation and desalination
Key advantages
- Single-component and pre-bagged – only clean water is added on site, eliminating site-batching variations
- Alkaline matrix re-passivates and protects embedded steel reinforcement
- Low permeability to water, carbon dioxide and chlorides for durable, protective repairs
- Shrinkage-compensated for long-term dimensional stability and reduced cracking
- Excellent bond to a properly prepared concrete substrate
- Good thermal compatibility with parent concrete – repair and substrate move together
- Fast early strength gain (about 9 N/mm2 at 1 day) thanks to low water demand
- Contains no chloride admixtures and is non-flammable (BS 476 Class 0 fire rating)
- Versatile across vertical, overhead and horizontal locations and usable as both a patch mortar and a render
- Can be hand/trowel applied or spray applied for fast, high-build repairs on large areas
- Compatible with the wider Fosroc repair system (Nitoprime Zincrich, Nitobond AR/EP, Renderoc Galvashield XP, Dekguard)
Limitations & considerations
- A general-purpose mortar, not a high-strength structural micro-concrete – use Renderoc RG/HB or micro-concrete grades for high-strength or large-volume structural reinstatement
- Minimum applied thickness is 10 mm; feather-edging is not permitted, so repair edges must be saw-cut square
- Must not be applied when substrate/air temperature is 5 deg C and falling; needs winter-working precautions in cold conditions
- Performance depends critically on correct forced-action machine mixing and strict water control – over-watering or under-mixing sharply reduces strength
- Part-bags must not be mixed, and free-fall mixers must not be used
- Should not be exposed to moving water during application, and heavy rainfall before final set can cause surface scour
- Requires immediate, disciplined curing (e.g. Nitobond AR film or taped polythene) to avoid plastic shrinkage and loss of strength
- Reinforcement priming and substrate priming are mandatory steps, not optional – skipping them compromises bond and corrosion protection
- Cured material can only be removed mechanically; tools must be cleaned with water before set
Selection & best practice
Treat preparation as the repair. Saw-cut the perimeter at least 10 mm deep to a square edge, break out all unsound concrete to a minimum 10 mm depth, and expose corroded steel fully – cleaning it to a bright condition by abrasive blasting, paying attention to the back of the bars. Prime reinforcement with one full coat of Nitoprime Zincrich (a second coat if any doubt about coverage), then bring the substrate to a saturated-surface-dry condition and apply Nitobond AR primer, scrubbing it well in; apply Renderoc GP while the primer is still tacky. Machine-mix one full 25 kg bag with 4.0-5.0 litres of clean water (never exceed 5.0 litres) in a forced-action mixer for 3-5 minutes until homogeneous, always adding powder to water. Compact firmly onto the primed substrate and around steel, working at 10-30 mm lifts in bulk areas and building thicker sections in re-primed, comb-scratched layers. Finish with a steel float without overworking, then cure immediately – large areas as trowelling progresses – using a continuous Nitobond AR film or taped polythene, protecting from rain, fast drying and freezing. Where the structure is at ongoing risk from carbonation or chlorides, overcoat the surrounding concrete with a Fosroc Dekguard anti-carbonation coating to bring it up to the same protective standard as the repair.
Specifications
| Product type | Single-component, medium-weight, general-purpose cementitious concrete reinstatement / repair mortar |
| Composition / system | Portland cement, graded aggregates, special fillers and chemical additives (alkaline, shrinkage-compensated) |
| EN classification | Classified to BS EN 1504-3; current UK formulation is a Class R3 repair mortar (suitable for methods 3.1, 3.3, 4.4, 7.1, 7.2) |
| Certification | British Board of Agrement (BBA) Certificate No. 98/3461 |
| Water : powder ratio (test basis) | 0.18 (properties quoted at 20 deg C) |
| Mixing water | Approx. 4.0-5.0 litres of clean water per 25 kg bag (5.0 litres maximum) |
| Compressive strength (BS 1881 Pt 116, dry cure) | ~9 N/mm2 at 1 day; ~28 N/mm2 at 28 days |
| Flexural strength (BS 6319 Pt 3) | ~7.0 N/mm2 at 28 days |
| Fresh wet density | Approx. 1850 kg/m3 (consistency-dependent) |
| Initial / final set (BS 5075) | Initial set ~3 h 30 min; final set ~4 h |
| Coefficient of thermal expansion | 7 to 12 x 10^-6 /deg C |
| Fire rating (BS 476 Pt 6 & 7) | Class 0; non-flammable |
| Application thickness (classic global formulation) | Min 10 mm; up to 20 mm overhead and 30 mm vertically in large areas; up to 100 mm in pockets, horizontally or with formwork; thicker in layers |
| Application thickness (current UK formulation) | Up to 70 mm vertical and 60 mm overhead in a single application without formwork |
| Substrate compatibility | Concrete of compressive strength approx. 20-35 N/mm2 |
| Application temperature | Do not apply when substrate/air is 5 deg C and falling; usable at 5 deg C static or rising |
| Coverage / yield | Approx. 16.0 litres per 25 kg bag (about 1.6 m2 at 10 mm thickness); ~2.2 kg/m2 per mm of thickness |
| Application method | Gloved hand / trowel, or dry/wet spray for large high-build areas |
| Pack size | 25 kg bags |
| Shelf life / storage | 6 months (reducing to ~4 months in high temperature/humidity) in original unopened bags, stored dry under cover |
Industry use cases
Buildings & Real Estate
Reinstating spalled cover concrete on RCC columns, beams, slab soffits, chajjas and parapets in ageing residential, commercial and institutional buildings across Delhi NCR, where carbonation-driven corrosion is widespread.
Bridges & Infrastructure
Patch repair and reprofiling of distressed concrete on bridge piers, abutments, girders and decks, restoring cover and protecting reinforcement as part of structural rehabilitation programmes.
Water & Wastewater
Repair of water tanks, sumps, STP/ETP structures, retaining walls and basements where low permeability and resistance to water and chlorides are needed.
Marine & Coastal
Reinstatement of chloride-attacked concrete in jetties, sea-front structures and coastal buildings, combining re-alkalisation of steel with a low-permeability repair matrix.
Industrial & Manufacturing
General concrete repair on factory floors edges, plinths, equipment foundations and structural members exposed to aggressive gases and moisture.
Heritage & Facade Restoration
Rendering and reprofiling of damaged concrete facades and architectural elements where a versatile, trowel-applied mortar is required for vertical and overhead work.
Frequently asked questions
Is Fosroc Renderoc GP a structural repair mortar?
How much water do I add per 25 kg bag, and why is this so important?
What is the coverage of Renderoc GP?
What thickness can be applied in one pass?
Do I need to prime the steel and the concrete before repair?
Is Renderoc GP available in India and is Space Arc an authorised source?
How we position Fosroc Renderoc GP
Renderoc GP sits at the general-purpose, economical end of Fosroc’s concrete-repair range and is best understood alongside its siblings. Where Renderoc GP handles routine patch repairs and rendering, Fosroc Renderoc HB (high-build) suits thicker overhead and vertical sections, Renderoc RG is a free-flow, high-strength micro-concrete for formed structural reinstatement, and Renderoc FC is a fine fairing coat for blow-holes and surface defects. It is normally specified as a system with Nitoprime Zincrich (reinforcement primer), Nitobond AR/EP (bonding and curing aid), optional Renderoc Galvashield XP galvanic anodes for incipient-anode control, and Dekguard anti-carbonation coatings to protect the surrounding structure. Against competing products in the Indian market, Renderoc GP competes with Sika MonoTop polymer-modified repair mortars (e.g. MonoTop-615/-722), MBS (MB Solutions / formerly MBT) and BASF MasterEmaco repair mortars, Dr. Fixit Pidicrete and Pidcrete repair systems from Pidilite, and STP (Shalimar/STP Ltd) cementitious repair products. Its differentiators are the proven Fosroc system pedigree, the alkaline shrinkage-compensated formulation with low permeability, and BBA certification – but for high-strength structural work or very large pours, an engineer may legitimately prefer a dedicated micro-concrete or a higher-class R4 mortar. As an authorised Fosroc distributor and applicator, Space Arc Engineering can help specifiers select the correct grade and, where relevant, compare it objectively against these alternatives for a given exposure class, strength requirement and budget.
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