Fosroc Renderoc HB40
Medium-weight, polymer-modified high-build repair mortar for overhead and vertical concrete reinstatement up to 40 mm in a single pass, without formwork.
What is it?
A single-component, polymer-modified cementitious patch-repair mortar from Fosroc, engineered with lightweight fillers so it builds thick on vertical faces and soffits without slumping or formwork.
Where is it used?
Reinstating spalled or corrosion-damaged reinforced concrete on beams, columns, slab soffits, bridge decks, car parks and tunnel linings.
Why use it?
It lets a repair crew place up to 40 mm vertically (30 mm overhead) in one lift while still reaching 35 MPa at 28 days, cutting cold joints and site time.
How is it applied?
Prime the reinforcement and substrate, mix one 20 kg bag with 3.0-3.2 L water in a forced-action mixer, apply by gloved hand or trowel onto the tacky primer, finish and cure.
About Fosroc Renderoc HB40
Renderoc HB40 sits in the middle of Fosroc’s hand-applied Renderoc repair range, and that positioning is the whole point of the product. It is a ready-blended dry powder of Portland cement, graded aggregates, lightweight fillers and chemical additives that needs only clean site water. The lightweight fillers pull the fresh wet density down to around 1780 kg/m3, which is what allows an applicator to trowel a genuinely thick section overhead or on a wall and have it stay put rather than sag off. At the same time the polymer modification keeps the cured mortar dense and low in permeability, so you are not trading durability for handling. It is designed to be compatible with parent concrete in the 30-45 MPa band, making it the sensible default for the majority of RCC repair work on Indian infrastructure and commercial buildings.
For engineers and consultants writing a repair specification, HB40 is attractive because it removes the formwork step from a large slice of patch-repair scope. Corrosion-driven spalling on a beam soffit or a car-park slab underside is exactly the situation where shuttering is slow, expensive and awkward, and where a high-build thixotropic mortar earns its keep. Because it is pre-bagged as a single component, site batching variation is taken out of the equation, and the material contains no added chlorides, so it is safe against the very reinforcement it is protecting. Where the concrete is stronger or the loading is more severe, Fosroc offers higher-strength options in the same family, but for standard-grade structural repair HB40 is the workhorse most contractors reach for first.
The technical insight
The mortar works because it balances two properties that normally fight each other: build thickness and strength. Lightweight fillers lower the density enough that surface tension and the thixotropic gel structure can hold a 40 mm vertical section against gravity, yet the low water demand (a water:powder ratio around 0.15) keeps the cement matrix tight, so it still develops roughly 10 MPa at 1 day, 25 MPa at 7 days and 35-38 MPa at 28 days. Shrinkage compensation is built in, giving positive expansion characteristics rather than the drying-shrinkage cracking that plagues site-batched mortars. Durability comes from the polymer, which drops the capillary absorption and severely retards the ingress of water, chloride ions and carbon dioxide that drive rebar corrosion in the first place. The result is a repair that is chemically and dimensionally matched to the parent concrete, not just a cosmetic filler, which is why it qualifies as an EN 1504-3 Class R3 structural repair mortar.
Where it is used
- Repairing spalled and delaminated concrete on beam and column faces
- Overhead reinstatement of slab and bridge-deck soffits without shuttering
- Corrosion-damage repair on car-park decks, service reservoirs and water-retaining structures
- Patch repair of columns, walls and cantilevers where formwork is impractical
- Tunnel lining and infrastructure concrete rehabilitation
- Reinstating cover to reinforcement after chloride or carbonation-driven distress
- Large-area repairs applied by wet-spray for higher build and faster placement
- Localised patch repairs to plinths, ledges and structural projections
Key advantages
- Builds up to 40 mm vertically and 30 mm overhead in a single hand-applied layer
- Medium-weight formulation resists sag on soffits and vertical faces without formwork
- Reaches around 35 MPa at 28 days, compatible with 30-45 MPa parent concrete
- Low permeability to water, chloride ions and carbon dioxide protects embedded steel
- Shrinkage-compensated for long-term dimensional stability and fewer cold joints
- Single-component and pre-bagged, eliminating site-batching variability
- Contains no chloride admixtures, so it will not accelerate rebar corrosion
- Can also be wet-sprayed for much higher build on large-area repairs
Limitations & considerations
- Not designed as a broad-scale building render; large surface areas need specialist advice
- Being medium-weight, it should not be used in areas subjected to vehicular or foot traffic
- Requires a primed substrate and reinforcement; it is not a stand-alone bagged patch
- Do not mix part bags or use a free-fall (drum) cement mixer, a forced-action mixer is essential
- Must not be applied below 5 C and falling, or exposed to moving water or heavy rain before set
- Minimum applied thickness is 10 mm, so it is unsuitable for very thin skim repairs
Selection & best practice
Success with HB40 is mostly about preparation and priming. Saw-cut the repair perimeter at least 10 mm deep to avoid feather edges, break out to a minimum 10 mm (20 mm behind exposed steel), and clean the concrete back to a sound, roughened surface. Expose and abrasive-blast corroded reinforcement to bright metal, then apply Fosroc’s zinc-rich rebar primer. Soak the substrate, remove standing water, and apply the Fosroc bonding primer; place the mortar while the primer is still tacky, scrubbing a thin first layer well in for adhesion. Mix one 20 kg bag into 3.0-3.2 L of clean water in a forced-action mixer for 3-5 minutes, always adding powder to water. Compact firmly around the steel, strike off, and close with a steel trowel once the material has stiffened. Cure immediately with a Fosroc curing membrane or taped polythene, and in hot weather store bags in shade and mix with cool water. For thicknesses beyond a single lift, build up in scratch-keyed layers or move to wet-spray application.
Specifications
| Product type | Single-component, polymer-modified cementitious high-build repair mortar |
| Standard | EN 1504-3 Class R3 structural repair mortar |
| Compressive strength | ~10 MPa (1 day), 25 MPa (7 days), 35-38 MPa (28 days) |
| Flexural / tensile strength | 5.8 MPa flexural, 3.1 MPa tensile at 28 days |
| Single-layer build | Up to 40 mm vertical, up to 30 mm overhead (hand/trowel); min 10 mm |
| Fresh wet density | Approx. 1780 kg/m3 (medium-weight) |
| Yield | Approx. 12.9 litres per 20 kg bag |
| Water demand | 3.0-3.2 litres per 20 kg bag |
| Chloride ion content | ~0.004% (no added chlorides) |
Industry use cases
Bridges & Infrastructure
Reinstating spalled bridge-deck soffits and pier concrete where hanging formwork over live traffic or water is impractical.
Commercial & Car Parks
Repairing chloride-driven delamination on multi-storey car-park slab undersides and beam soffits with minimal downtime.
Water & Sewage
Rehabilitating reservoir walls, tanks and treatment structures where a dense, low-permeability repair is essential.
High-Rise & Facades
Restoring cover to corroded reinforcement on columns, cantilevers and external RCC members on ageing buildings.
Tunnels & Utilities
Large-area lining repairs applied by wet-spray for high build and rapid placement in confined works.
Frequently asked questions
How thick can Renderoc HB40 be applied in one coat?
What compressive strength does it reach?
Do I need a primer, or can I apply it straight onto the concrete?
What is the coverage or yield per bag?
Can it be used on floors or trafficked areas?
How is HB40 different from other Renderoc repair mortars?
How we position Fosroc Renderoc HB40
Renderoc HB40 is the go-to hand-applied, high-build grade within Fosroc’s Renderoc concrete-repair family, and it rarely works alone. It is specified alongside a Fosroc zinc-rich rebar primer (Nitoprime/Nitozinc range), a Nitobond bonding primer, and a Dekguard or Emer-Clad anti-carbonation coating to bring the surrounding structure up to the same protective standard; Galvashield sacrificial anodes can be embedded where galvanic corrosion control is required. For repairs demanding higher compressive strength or heavier structural performance, step up to Fosroc Renderoc HS Xtra. Where clients specify an alternative brand, comparable structural repair mortars in our portfolio include Sika MonoTop and Dr. Fixit / MC-Bauchemie repair grades, and we can advise on the closest equivalent for a given design intent.
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