About Fosroc Cebex NK

Fosroc Cebex NK is a single-component powder admixture engineered specifically for the cementitious grouts that fill the ducts of post-tensioned and prestressed concrete. In a stressed structure the high-tensile steel strand is the heart of the system, and the surrounding grout is its only line of defence against corrosion. A grout that bleeds, settles or segregates leaves water pockets and voids at the top of the duct, exactly where chloride and moisture can attack the tendon. Cebex NK is formulated to stop that from happening. Added to ordinary Portland cement grout, it allows the mix to be made at a deliberately low water/cement ratio so the cured grout is dense and low in permeability, yet keeps the grout fluid enough to flow through congested anchorages and long, often vertical, ducts without choking the pump. As the grout sets, a built-in expansion system produces a small positive volume change that compensates for the natural plastic settlement and shrinkage of the paste, holding the grout tight against the strand and the duct wall. Crucially for prestressed work, Cebex NK contains no added chlorides and no metallic iron, so it introduces nothing that could initiate corrosion or cause rust staining, and it develops useful early strength without aggressive accelerators. For Indian contractors and PT specialists, it is the grouting-admixture member of the Fosroc Cebex family, the counterpart to the better-known Cebex 100 general-purpose grout admixture, but tuned for the stricter demands of tendon protection.

The technical insight

The technical value of Cebex NK lies in reconciling two normally opposing requirements: fluidity and a low water content. A neat cement grout pumpable enough for a long vertical duct would ordinarily need so much water that it bleeds heavily, and the rising bleed water collects at duct high points and crests, leaving a void over the tendon once it is reabsorbed. Cebex NK works on both fronts. Its plasticising fraction disperses the cement so the grout stays mobile at a reduced water/cement ratio, which directly lowers permeability and raises long-term durability and strength. Its expansion fraction generates a controlled, restrained expansion in the plastic state that offsets settlement and plastic shrinkage, so the grout maintains intimate contact with the strand rather than slumping away from it. Because the system is iron-free and chloride-free, none of the classic corrosion-initiation pathways are introduced into the duct. The product is verified against the European grouting standards: it complies with EN 934-4 (Table 1) as an admixture for grout for prestressing tendons, and grouts made with it are shown to meet the test-method requirements of EN 445 (fluidity, bleed, volume change and strength). It is compatible with cements meeting the RC-08 specification and can be combined with other compatible Fosroc admixtures. Net effect: a single powder addition converts a plain OPC grout into a fluid, low-bleed, expanding, corrosion-protective injection medium suitable for critical structural tendons.

Where it is used

• Grouting of post-tensioned floor slabs and transfer beams in commercial and residential towers
• Injection of bridge and flyover prestressing tendons and cable ducts
• Vertical and near-vertical prestressed cable ducts where bleed and settlement are most damaging
• Grouting of ground anchors, rock anchors and long wire/rod anchors requiring even stress transfer
• Encapsulation of high-tensile strand in segmental and balanced-cantilever bridge construction
• Corrosion-protective duct filling in water-retaining and marine prestressed structures

Key advantages

• Permits a low water/cement ratio while keeping the grout fluid and easily pumpable through long or congested ducts
• Controlled expansion compensates plastic settlement and shrinkage, eliminating voids over the tendon
• Effectively removes bleed, segregation and exudation, giving full and uniform duct filling
• Contains no added chlorides and no metallic iron, so it does not initiate corrosion or cause rust staining of the tendon
• Develops high early strength without aggressive chloride accelerators
• Low-permeability, dense hardened grout gives long-term corrosion protection and durability
• Verified against EN 934-4 and EN 445 grouting requirements for prestressing tendons
• Compatible with RC-08 cements and with other compatible Fosroc admixtures, simplifying site mixing

Limitations & considerations

• It is a grout admixture only, not a grout in itself; performance depends on correct cement, water and mixing on site
• Final grout properties are sensitive to water/cement ratio, so dosage and water content must be controlled by weight, not by eye
• High ambient temperature and humidity shorten shelf life and can accelerate stiffening of the mixed grout
• Requires proper grouting equipment (colloidal/high-shear mixer and grout pump) and trained operatives for full benefit
• Published dosage and packaging can vary by region, so the current local Fosroc TDS should govern the actual mix
• As a fine powder it requires dust precautions; avoid skin/eye contact and inhalation during handling
• Not a substitute for sound duct detailing, venting and grouting procedure; voids from poor practice cannot be cured by the admixture alone

Selection & best practice

Specifications

Industry use cases

How we position Fosroc Cebex NK

Within the Fosroc grouting line, Cebex NK sits in the Cebex admixture family alongside the general-purpose Cebex 100 (plasticising/expanding grout admixture) and the dedicated prestressing-cable grout admixtures Cebex CP and Cebex EN, all of which target the EN 447/EN 445 grouting requirements; on Indian PT projects it is commonly specified together with Fosroc’s PT and Conbextra grouting systems. For procurement teams comparing brands, the closest credible alternatives are Sika’s prestressing/cable-duct grouting admixtures and ready-mixed PT grouts (the SikaGrout PT / Intraplast range), Master Builders Solutions (formerly BASF, MBS) cable-grout admixtures and MasterFlow PT grouts, and, for non-critical general grouting only, locally available expanding grout aids from Dr. Fixit (Pidilite) and STP. Where the application is genuinely a stressed tendon duct, the decisive factors are chloride-free and iron-free formulation, low bleed, controlled expansion and demonstrated conformity to EN 447/EN 445, and Cebex NK is positioned squarely for that corrosion-protection role rather than as a generic grout filler. Space Arc Engineering, as an authorised Fosroc distributor and applicator in Delhi NCR and Pan-India, can advise on whether Cebex NK or a sibling Cebex product best fits a given duct geometry, cement and specification.

Related Concrete Admixtures products

View all Concrete Admixtures products →