About Fosroc Conplast CNI

Fosroc Conplast CNI is a calcium nitrite based corrosion-inhibiting admixture engineered to protect the steel reinforcement inside concrete from chloride-induced corrosion. It is supplied as a pale straw coloured liquid that disperses instantly in the gauging water, carrying a minimum 30% calcium nitrite content, and is added at the concrete batching plant rather than applied to a finished surface. Unlike membranes or coatings that try to keep chlorides out, Conplast CNI works from within the matrix: as aggressive chloride ions migrate toward the rebar, the nitrite ions interact with the steel and reinforce its protective passive oxide layer, raising the chloride concentration that the steel can tolerate before corrosion begins.

For Indian conditions, this matters enormously. India’s long coastline, high humidity, splash-zone marine works and increasingly aggressive ground conditions mean that chloride attack is one of the most common causes of premature reinforced-concrete (RCC) failure. Conplast CNI is positioned for structures in severe and very severe exposure categories, where the designer wants additional durability assurance over and above good cover, low water/cement ratio and dense, well-cured concrete. Fosroc specifies it for use in reinforced and prestressed concrete with a free water/cement ratio of 0.45 or lower, so it complements rather than replaces sound mix design.

Conplast CNI sits within Fosroc’s wider Conplast admixture family and is closely related to Conplast CN, with both products historically covered by British Board of Agrement (BBA) Certificate 04/4171 issued to Al Gurg Fosroc. Space Arc Engineering supplies Conplast CNI as an authorised Fosroc distributor and applicator across Delhi NCR and pan-India, supporting consultants and contractors who need a documented, internationally recognised corrosion-inhibiting admixture for durability-critical projects.

The technical insight

The active chemistry is calcium nitrite, supplied at a minimum 30% concentration. Calcium nitrite is an anodic (passivating) corrosion inhibitor: nitrite ions help maintain and repair the passive ferric-oxide film on the rebar surface so that corrosion does not initiate until the chloride-to-nitrite balance at the steel is overwhelmed. Because protection depends on the nitrite-to-chloride ratio, the dosage is selected against the anticipated chloride loading rather than the cement content alone, which is why Fosroc offers a wide working range of 7.5 to 22.5 litres per cubic metre of concrete. Higher anticipated chloride exposure calls for higher dosage within that band. A practical consequence engineers should plan for is that calcium nitrite can accelerate set and act as a mild accelerator; Conplast CNI is formulated to be compatible with other Fosroc admixtures, but a separately dosed water-reducer or retarder (for example a Conplast superplasticiser, or a retarder where placing in hot weather) is normally combined with it to control workability, slump retention and setting time. The admixture must be added to the mix separately from other admixtures and should not be pre-blended in the same dispenser line. As a liquid added at up to 22.5 L/m3, the water it contributes is part of the free water and must be counted within the design water/cement ratio of 0.45 or lower. Calcium nitrite corrosion-inhibiting admixtures of this type are assessed against ASTM C1582 / C1582M, the standard specification for admixtures to inhibit chloride-induced corrosion of reinforcing steel in concrete.

Where it is used

• Marine and coastal RCC structures: jetties, sea walls, breakwaters and splash-zone elements exposed to seawater chlorides
• Docks, harbours and port infrastructure where tidal and spray chloride loading is continuous
• Bridges, flyovers and elevated decks, including those subject to de-icing salts in cold regions
• Multi-storey and basement car parks where vehicles carry in chlorides and de-icing salt
• Power plants, desalination plants, chemical works and industrial structures in aggressive environments
• Prestressed concrete members in severe exposure where corrosion of high-tensile steel must be avoided
• Coastal precast and ready-mix concrete where the producer wants an internationally documented durability additive

Key advantages

• Protects embedded steel by raising the chloride threshold needed to initiate rebar corrosion, extending RCC service life
• Acts from within the concrete matrix, so protection is not lost if a surface coating or membrane is damaged
• Liquid form disperses instantly in the gauging water for uniform distribution through the mix
• Min. 30% calcium nitrite gives a defined, specifiable active content for durability design
• Wide dosage range (7.5 to 22.5 L/m3) lets dosage be tuned to the anticipated chloride exposure
• No known detrimental effect on the hardened properties of correctly designed concrete
• Compatible with other Fosroc admixtures when dosed separately, allowing combination with water-reducers and retarders
• Backed by recognised third-party assessment (BBA Certificate 04/4171 for the Conplast CN/CNI corrosion inhibitors)

Limitations & considerations

• Not a standalone fix: it supplements, and does not replace, adequate cover, low water/cement ratio, good compaction and proper curing
• Specified for concrete with a free water/cement ratio of 0.45 or lower; ineffective design or porous concrete undermines its benefit
• Calcium nitrite can accelerate setting, so a separately dosed retarder/water-reducer is often needed, especially in hot Indian weather
• Dosage must be matched to anticipated chloride loading; under-dosing relative to chloride ingress reduces effectiveness
• Must be added separately from other admixtures and not pre-blended in the same line
• Liquid added at up to 22.5 L/m3 contributes to the free water and must be accounted for in the mix water/cement ratio
• Being a nitrite chemical, it requires standard PPE and safe handling; consult the current SDS before use
• Cannot retrofit protection to already-corroding reinforcement in existing concrete; it is an additive for new pours

Selection & best practice

Specifications

Industry use cases

How we position Fosroc Conplast CNI

Conplast CNI is Fosroc’s calcium nitrite based corrosion-inhibiting admixture, sitting alongside its near-identical sister product Conplast CN within Fosroc’s broader Conplast concrete-admixture range. In practice it is specified together with Fosroc water-reducers and superplasticisers such as the Conplast SP430 family to balance the slight set acceleration nitrite can cause, and it complements barrier-side durability products like Fosroc’s integral waterproofing admixtures (for example Conplast WL and Conplast Crystalline) and rebar primers such as Nitoprime Zincrich. Among credible alternatives, the most directly comparable nitrite-based product is Sika CNI; other recognised options include GCP/MBS-type calcium nitrite inhibitors such as DCI and MasterLife CI 30, while waterproofing-led approaches from Dr. Fixit and STP (Pidilite/STP) target the same chloride-durability problem by different mechanisms. Conplast CNI’s strengths are its defined minimum 30% nitrite content, wide dosage range for tuning to chloride exposure, and recognised third-party assessment.

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