Fosroc Conplast PA21
Chloride-free air-entraining plasticiser that builds a stable micro-air-void system in concrete for freeze-thaw and de-icing salt durability.
What is it?
A chloride-free, air-entraining admixture based on selected (sugar-reduced) lignosulphonates and compatible surface-active agents, supplied as a brown liquid that disperses readily in the mixing water. It doubles as a mild plasticiser/water-reducer.
Where is it used?
Pavement-quality and exposed concrete: highways, runways, bridge decks, parking decks, kerbs and any concrete subjected to freeze-thaw cycling, de-icing salts or aggressive marine/cold-climate exposure. Also used to improve cohesion in lean or poorly graded mixes.
Why use it?
The entrained micro air-void system relieves internal pressure from freezing pore water and de-icing salt scaling, sharply improving frost durability while also reducing bleeding, segregation and permeability and improving workability and cohesion.
How is it applied?
Dosed into the concrete with the mixing water (never onto dry cement or aggregate) at a typical 0.25-0.60 litres per 100 kg of binder, established by trial mixes against a target plastic air content with calibrated dispensing and an air meter on site.
About Fosroc Conplast PA21
Fosroc Conplast PA21 is a purpose-built air-entraining admixture used to give concrete long-term resistance to freeze-thaw action and de-icing salt scaling. It is a chloride-free liquid based on selected sugar-reduced lignosulphonates blended with compatible surface-active agents, and it disperses instantly when added to the mixing water. The same surfactant chemistry that creates the air-void system also gives the product a useful secondary plasticising effect, so a single admixture both entrains air and improves workability.
The mechanism is physical rather than chemical. The active agents work at the interface between mixing water and the cement and aggregate particles, generating a population of microscopic, closely spaced air bubbles that stay uniformly distributed through the plastic concrete. Once the concrete hardens, those bubbles remain as discrete, non-connected voids. When pore water freezes and expands, the relieving voids absorb the hydraulic pressure that would otherwise crack the cement paste. This is fundamentally different from accidental entrapped air: entrained air is fine, stable and evenly spaced, which is what delivers durability rather than just reducing strength.
For Indian projects, Space Arc Engineering supplies Conplast PA21 (and the closely related Conplast PA21(S) variant marketed for the local market) as an authorised Fosroc distributor and applicator. While air entrainment is most associated with cold-climate frost protection, the admixture is equally valued in India for the cohesion, reduced bleeding and improved finishability it brings to lean, harsh or poorly graded mixes, on hill-station and high-altitude works in the Himalayas and the North-East, and on pavement-quality concrete where a controlled air content is specified. It is compatible with ordinary Portland, Portland-pozzolana and Portland-slag cements and with supplementary materials such as fly ash, GGBS and microsilica.
The technical insight
Air entrainment is a tightly controlled property, not a “more is better” additive. Durability depends on achieving the right total air content and the right air-void spacing factor, not just any air. Because Conplast PA21 is a liquid surfactant, the realised air content in any given mix is sensitive to variables most teams underestimate: cement fineness and alkali content, fly ash carbon (loss-on-ignition), sand grading and fines, mix temperature, slump, mixing time and even the dispensing point. Two batches at the same dosage can land at different air contents if any of these shift, which is exactly why the 0.25-0.60 L/100 kg range is a starting band for trial mixing rather than a fixed recipe. Every production mix should be calibrated against a measured plastic air content using a pressure air meter, and air should be re-checked at the point of placement because pumping, long haul times and over-vibration can all strip entrained air. A second discipline that distinguishes good air-entrained concrete is recognising the strength trade-off: roughly every 1% of entrained air costs in the order of 5% compressive strength, so the mix must be proportioned to deliver both the specified air and the specified grade simultaneously. Finally, because the product is chloride-free, it is safe for reinforced and prestressed concrete where calcium-chloride-bearing accelerators or older admixtures would be prohibited.
Where it is used
- Pavement-quality concrete (PQC) for highways, expressways and service roads exposed to frost and de-icing salts
- Airport runways, taxiways and aprons requiring specified air content and scaling resistance
- Bridge decks, approach slabs, kerbs and crash barriers in cold and high-altitude regions
- Concrete in Himalayan, North-East and hill-station projects subject to repeated freeze-thaw cycling
- Improving cohesion, reducing bleeding and segregation in lean or poorly graded / harsh aggregate mixes
- Reducing permeability of lean concrete and improving finish in extruded and slip-formed elements
- Marine and water-retaining structures where reduced bleeding and improved durability are required
Key advantages
- Builds a stable, finely divided and closely spaced micro air-void system for strong freeze-thaw and de-icing salt resistance
- Chloride-free, so suitable for reinforced and prestressed concrete with no corrosion risk from the admixture
- Acts as a secondary plasticiser, improving workability and allowing some reduction of free water
- Markedly reduces bleeding, segregation and surface sand runs, giving better, more uniform finishes
- Lowers permeability and improves overall durability and resistance to aggressive exposure
- Improves cohesion and pumpability of lean, harsh or gap-graded mixes
- Disperses instantly in mixing water and is compatible with OPC, PPC, PSC and fly ash / GGBS / microsilica
- Conforms to recognised standards (EN 934-2 air-entraining; India variant to IS:9103:1999)
Limitations & considerations
- Entrained air reduces compressive strength (broadly about 5% loss per 1% air), so mixes must be re-proportioned to hold the target grade
- Realised air content is sensitive to materials and conditions – cement, fly ash LOI, sand fines, temperature, slump and mixing all shift it, mandating trial mixes
- Over-dosing produces excessive or coarse air, severely cutting strength and finish quality
- Air can be lost during pumping, long transit or over-vibration, so air must be verified at the point of placement
- Requires on-site air-content testing (pressure air meter) and calibrated dispensing for consistent results
- Not a high-range water reducer – where large water reduction or high slump retention is needed, pair with or substitute a Structuro / Auracast superplasticiser
- Compatibility with other admixtures and the specific cement source should be confirmed by trial before bulk use
Selection & best practice
Always establish dosage by laboratory and site trial mixes targeting the specified plastic air content rather than fixing a dose by volume. Add Conplast PA21 with the mixing water, never onto dry materials, and use calibrated dispensing equipment. Verify air content with a pressure air meter on the first loads and re-check at the point of discharge after pumping or long hauls, since entrained air degrades with handling. Re-proportion the mix so that the target compressive grade and the target air content are achieved together, allowing for the strength reduction that air entrainment brings. Keep cement source, fly ash quality and sand grading consistent during production, and re-validate the dose whenever any constituent changes. Avoid over-vibration. Store drums in shade between 5C and 40C, keep them sealed, and use within the stated shelf life; if the product has been exposed to frost, reconstitute by gentle agitation per the data sheet before use.
Specifications
| Product type | Chloride-free air-entraining admixture (with secondary plasticising / water-reducing action) |
| Chemical base | Selected sugar-reduced lignosulphonates with compatible surface-active agents |
| Appearance | Brown to dark-brown liquid; disperses readily in water |
| Specific gravity | Approx. 1.14 – 1.16 at 25C |
| Chloride content | Chloride-free (water-soluble chloride typically less than or equal to 0.10%) |
| Typical dosage | 0.25 – 0.60 litres per 100 kg of binder (starting range; confirm by trial) |
| Function in mix | Entrains a controlled, finely divided air-void system; reduces bleeding and permeability |
| Standards | EN 934-2 (air-entraining admixture); India variant conforms to IS:9103:1999 |
| Compatibility | OPC, PPC, PSC; compatible with fly ash, GGBS and microsilica |
| Packaging | Supplied in drums (200 litre in India); other pack sizes on request |
| Shelf life | Approx. 12 months in sealed original packaging under recommended storage |
| Storage | Store in shade, sealed, ideally 5C – 40C; protect from frost |
Industry use cases
Roads & Highways
Pavement-quality concrete for NHAI and state expressway works in frost-prone corridors, where a specified air content is required to resist freeze-thaw and de-icing salt scaling while keeping the mix cohesive and finishable.
Airports & Aviation
Runway, taxiway and apron slabs at high-altitude and cold-region airfields, where air-entrained PQC is specified for scaling resistance and long pavement life under thermal cycling.
Bridges & Infrastructure
Bridge decks, approach slabs, kerbs and parapets in Himalayan and North-East projects, combining frost durability with reduced bleeding for sound, dense surfaces.
Hydraulic & Marine
Water-retaining and marine structures where reduced bleeding, lower permeability and improved durability are needed alongside resistance to aggressive exposure.
General Building / Precast
Lean, harsh or poorly graded mixes and extruded/slip-formed elements where the admixture’s cohesion and anti-segregation benefits improve pumpability and finish quality.
Frequently asked questions
Is Fosroc Conplast PA21 a plasticiser or an air-entraining agent?
What is the recommended dosage of Conplast PA21?
Does Conplast PA21 contain chlorides? Is it safe for reinforced concrete?
How does air entrainment affect concrete strength?
Which standards does Conplast PA21 comply with?
What pack sizes are available and what is the shelf life in India?
How we position Fosroc Conplast PA21
Within the Fosroc Conplast admixture family, PA21 occupies the air-entraining niche – where Conplast P211 and the WL/SP water-reducing and waterproofing grades target workability or integral water resistance, PA21 is specifically about building a durable air-void system for frost and de-icing salt exposure. On Indian projects it is most often used alongside, not instead of, a high-range water reducer: pair it with a Structuro or Auracast superplasticiser when you need both a controlled air content and significant water reduction or slump retention, and with Conplast WL or an integral waterproofer where low permeability is also specified. Against competing products, credible equivalents include Sika AER / Sika air-entraining admixtures, BASF/Master Builders Solutions MasterAir (formerly Micro-Air), and air-entraining grades from STP and from CICO/Dr. Fixit in the Indian market. Conplast PA21’s strengths in a like-for-like comparison are its chloride-free lignosulphonate base, the proven stability and spacing of the air-void system, the bonus plasticising effect, and – for buyers in India – assured genuine supply, standard-conformant documentation and on-site air-content support from Space Arc Engineering as an authorised Fosroc distributor and applicator. As with any air-entraining admixture, the right choice is ultimately the one that hits your specified air content and grade in trial mixes with your actual materials.
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