Fosroc Renderoc HB25
Lightweight, polymer-modified, low-shrinkage patch repair mortar engineered for high-build vertical and overhead reinstatement of reinforced concrete without formwork.
What is it?
A single-component, pre-bagged, polymer-modified cementitious repair mortar that uses lightweight fillers to allow extra-high-build vertical and overhead concrete reinstatement with minimal drying shrinkage.
Where is it used?
Repairing spalled and carbonated reinforced concrete – facades, columns, beams, soffits, balconies, parking decks, bridges, water-retaining and marine structures – where lightweight, low-permeability patch repair is needed.
Why use it?
Its lightweight formulation builds up to 80 mm vertical and 60 mm overhead in one pass without formwork, while polymer modification gives very low permeability to water, CO2 and chlorides and its shrinkage-compensation system protects long-term durability.
How is it applied?
Mix one 20 kg bag with 3.45-3.70 litres of clean water in a forced-action mixer, prime steel with Nitoprime Zincrich and substrate with Nitobond HAR, then trowel or wet-spray onto the prepared substrate and cure with Nitobond AR or Concure A99.
About Fosroc Renderoc HB25
Fosroc Renderoc HB25 is a high-performance, lightweight, low-shrinkage patch repair mortar designed for the structural reinstatement of damaged reinforced concrete. It belongs to the well-established Renderoc family of hand- and spray-applied repair mortars and is positioned as the lightweight, high-build member of the range. Supplied as a ready-to-use blend of Portland cement, graded aggregates, lightweight fillers and chemical additives, it requires only the on-site addition of clean water to produce a consistent, cohesive, thixotropic mortar that can be worked by gloved hand, trowel or wet-spray equipment.\n\nThe defining feature of HB25 is its lightweight chemistry, which lets it be built up to far greater thicknesses than conventional repair mortars without sagging or the need for shuttering. In a single application it can be placed up to 80 mm thick on vertical faces and up to 60 mm overhead, and by wet spraying these figures rise dramatically to roughly 175-250 mm vertically and 150-200 mm overhead. This makes it especially valuable for deep spalls, large reinstatement zones and overhead soffit repairs where formwork would be slow, costly or impractical. The mortar is engineered for maximum compatibility with parent concrete in the 15-30 MPa compressive-strength band, so it behaves elastically in sympathy with the substrate rather than introducing a stiff, incompatible patch.\n\nBecause it is polymer-modified, the cured mortar has very low permeability to water, carbon dioxide and chloride ions – the three main drivers of reinforcement corrosion – and its alkaline matrix re-establishes a passivating environment around embedded steel. Combined with a dedicated shrinkage-compensation system that keeps drying shrinkage below about 400 microstrain at 7 days, this delivers durable, dimensionally stable repairs. The product is chloride-admixture free, suitable for both internal and external work, and is compatible with impressed-current cathodic protection and Galvashield XP incipient-anode systems, making it a sound choice for long-life concrete rehabilitation across the Indian construction and infrastructure sector.
The technical insight
The number to read carefully on HB25 is its 28-day compressive strength of about 23 MPa – this is deliberately modest, not a shortfall. Renderoc HB25 is formulated for true compatibility with the 15-30 MPa concrete it most often repairs, so its modulus of elasticity (approximately 1.10 x 10^4 MPa, i.e. ~11 GPa) and strength are tuned to flex with the substrate rather than against it. A stiffer, stronger patch mortar concentrates stress at the bond line and tends to debond or crack the surrounding concrete over thermal and load cycles. Where the parent concrete is genuinely high grade, or where higher strength is the design driver, Fosroc itself directs the specifier to step up to Renderoc HB40, HB70 or LA55. The other technically important point is that durability here comes from low permeability and shrinkage control, not from raw strength: chloride diffusion is around 2.29 x 10^-12 m2/sec and drying shrinkage stays under 400 microstrain at 7 days, which is what actually keeps chlorides and CO2 away from the rebar over decades. To realise these properties on site, the bonding system is not optional – exposed steel must be primed with Nitoprime Zincrich and the substrate with Nitobond HAR (or Nitobond EP where the repair will be permanently wet or immersed), and the mortar must be water- or membrane-cured immediately after finishing.
Where it is used
- Reinstatement of spalled, delaminated or carbonated reinforced concrete on building facades, columns and beams
- High-build vertical and overhead patch repairs to soffits, slabs and balcony undersides without formwork
- Deep-section structural repairs where multiple cold joints are to be avoided
- Large-area repairs applied rapidly by wet-spray (gunite-style) technique
- Repair of concrete in car parks, bridges, flyovers and other RC infrastructure
- Marine and water-retaining structure repairs requiring low chloride and water permeability
- Concrete rehabilitation used in conjunction with impressed-current cathodic protection or Galvashield XP incipient anodes
- Localised patch repairs to honeycombed or damaged concrete in new construction and refurbishment
Key advantages
- Lightweight formulation enables extra-high-build (up to 80 mm vertical / 60 mm overhead in one pass) with fewer cold joints
- Frequently eliminates the need for formwork, saving time and labour on overhead and vertical work
- Polymer modification gives very low permeability to water, carbon dioxide and chlorides for durable corrosion protection
- Shrinkage-compensation system keeps drying shrinkage low (<400 microstrain at 7 days) for dimensional stability
- Modulus and strength tuned for compatibility with 15-30 MPa parent concrete, reducing patch debonding risk
- Single-component, pre-bagged – just add water, removing site-batching variability
- Can be applied quickly by trowel or efficiently over large areas by wet spray
- Contains no chloride admixtures; suitable for internal and external use
- Compatible with cathodic protection and Galvashield XP anode systems
- Alkaline matrix re-establishes passivation around embedded reinforcement
Limitations & considerations
- Modest 28-day compressive strength (~23 MPa) – not suited where high-strength repair is the design requirement; use HB40, HB70 or LA55 instead
- Minimum applied thickness is 10 mm; feather-edging must be avoided by saw-cutting square edges
- Not designed for use as a broad-scale building render
- Must not be applied when temperature is below 5 C and falling; needs warm-water/winter precautions in cold conditions
- Performance depends on correct priming (Nitoprime Zincrich on steel, Nitobond HAR or EP on substrate) – skipping primers compromises bond
- Requires a forced-action mixer; free-fall mixers must not be used
- Requires immediate and proper curing (Nitobond AR / Concure A99 or polythene) to develop strength and durability
- Polymer bonding agents must not be used when paired with impressed-current cathodic protection (use OPC:water 2:1 slurry instead)
- 36-month shelf life that can shorten under high heat or humidity; sensitive to overwatering
Selection & best practice
Saw-cut the repair perimeter to at least 10 mm and break out to a minimum 10 mm depth to give a square edge and avoid feather-edging. Fully expose and clean corroded steel by abrasive or high-pressure water blasting, then apply one (or two) coats of Nitoprime Zincrich and let it dry. Pre-soak the substrate, remove excess water, work in Nitobond HAR primer and apply HB25 while the primer is still tacky (use Nitobond EP where the repair will be permanently wet or immersed). Mix one 20 kg bag into 3.45-3.70 litres of clean water in a forced-action mixer – add the powder to the water, run for at least 3 minutes to activate the polymers, then adjust water up to the 3.70 litre maximum. Build vertical sections up to 80 mm and overhead up to 60 mm per pass (or use wet-spray for greater build), keep above the 10 mm minimum, scratch-key and re-prime intermediate layers, finish with a steel trowel without overworking, and cure immediately with Nitobond AR or Concure A99 – protecting roughly 0.5 m2 at a time as trowelling progresses. In hot weather store in shade and mix with cool water; below 5 C use warm water and winter precautions.
Specifications
| Product type | Lightweight, polymer-modified, shrinkage-compensated cementitious patch repair mortar |
| Composition base | Portland cement, graded aggregates, lightweight fillers and chemical additives (chloride-free) |
| Form / supply | Single-component pre-bagged powder; 20 kg bag (code FC387080-20KG) |
| Water demand | 3.45 – 3.70 litres of clean water per 20 kg bag |
| Yield / coverage | 16.1 – 16.6 litres per 20 kg bag (approx. 1.6 m2 at 10 mm thick) |
| Minimum applied thickness | 10 mm |
| Max single-pass build (hand/trowel) | Up to 80 mm vertical; up to 60 mm overhead |
| Max single-pass build (wet spray) | 175 – 250 mm vertical; 150 – 200 mm overhead |
| Compatible substrate concrete | Compressive strength 15 – 30 MPa |
| Compressive strength | 5 MPa @ 1 day; 19 MPa @ 7 days; 23 MPa @ 28 days (AS 1478.2) |
| Flexural strength | 4.2 MPa @ 28 days (AS 1012.11) |
| Tensile strength | 1.8 MPa @ 28 days (AS 1012.10) |
| Modulus of elasticity | 1.10 x 10^4 MPa (~11 GPa) (AS 1012.17) |
| Fresh wet density | 1400 kg/m3 |
| Setting time | Initial 3 hours; final 5.5 hours (AS 1012.18) |
| Drying shrinkage | <400 microstrain @ 7 days; ~500 microstrain @ 28 days (AS 1478.2) |
| Chloride diffusion | 2.29 x 10^-12 m2/sec (Nordtest NT Build 443) |
| Application temperature | Not below 5 C and falling; above 35 C use shade and cool water |
| Curing | Nitobond AR or Concure A99 membrane, or polythene sheeting |
| Shelf life | 36 months in original unopened bags (cool, dry storage) |
Industry use cases
Building facade & structural repair
Reinstating spalled and carbonation-damaged columns, beams and facades on ageing RC buildings, where the lightweight high-build allows deep vertical patches without formwork.
Bridges & highway infrastructure
Repairing soffits, piers, crash barriers and deck undersides on bridges and flyovers, combined with cathodic protection or Galvashield anodes for long-life rehabilitation.
Car parks & multi-storey decks
Overhead soffit and beam repairs in parking structures where low permeability resists chloride and carbonation-driven corrosion of reinforcement.
Marine & water-retaining structures
Patch repair of jetties, tanks and treatment works using Nitobond EP priming where sections become permanently wet or immersed, leveraging low chloride diffusion.
Industrial & commercial refurbishment
Large-area concrete reinstatement applied rapidly by wet spray to minimise downtime in factories, warehouses and commercial podiums across Delhi NCR and pan-India projects.
Frequently asked questions
What is Fosroc Renderoc HB25 used for?
How thick can Renderoc HB25 be applied in one pass?
How much water does a 20 kg bag need and what is the coverage?
What compressive strength does Renderoc HB25 reach?
Do I need primers with Renderoc HB25?
How should Renderoc HB25 be cured?
How we position Fosroc Renderoc HB25
Within the Fosroc range, Renderoc HB25 is the lightweight, high-build, durability-focused choice rather than the high-strength one. It sits alongside Renderoc HB40, Renderoc HB70 and Renderoc LA55, which Fosroc itself recommends when higher compressive strength is the priority – so the right way to specify HB25 is for deep, overhead and large-area repairs on medium-grade (15-30 MPa) concrete where low permeability and formwork-free build matter more than raw strength. It is supported by companion Fosroc products on every project: Nitoprime Zincrich for steel priming, Nitobond HAR and Nitobond EP as substrate bonding aids, Nitobond AR and Concure A99 for curing, and the Dekguard range of anti-carbonation coatings to protect the surrounding structure. For corrosion-management schemes it integrates with Galvashield XP anodes and impressed-current cathodic protection. Among credible alternatives available in India, comparable repair mortars include Sika MonoTop and SikaTop ranges, MBS (MYK) and STP repair mortars, and Dr. Fixit Pidicrete/repair-mortar systems; for free-flow, formwork-and-pour micro-concrete situations Fosroc’s own pourable Renderoc grades or equivalents may be more appropriate. As an authorised Fosroc distributor and applicator serving Delhi NCR and pan-India, Space Arc Engineering can advise on selecting between HB25 and the higher-strength Renderoc grades, supply the full bonding-and-curing system, and support applicator method statements for repair and rehabilitation projects.
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