CONCRETE

Concrete Admixtures

Plasticizers, accelerators, retarders per IS 9103

Also calledadmixtureadmixturesplasticizersuperplasticizeraccelerator
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CODES
IS 9103
Definition

Concrete admixtures are chemical materials added to a concrete mix in small quantities (typically 0.5-2% by mass of cement) to modify fresh or hardened properties. The Indian standard IS 9103:1999 governs admixture specifications and IS 2645:2003 covers integral waterproofing compounds. Modern Indian construction relies on admixtures so heavily that mix designs are unworkable without them — virtually every M25+ pour at an RMC plant uses a polycarboxylate-ether (PCE) superplasticiser to achieve target slump at low w/c ratio.

Functional categories: (1) Plasticisers / Superplasticisers — reduce mix water by 5-15% (plasticisers) or 20-40% (PCE-based superplasticisers) at the same workability, enabling lower w/c without losing flow. (2) Retarders — delay setting, used in hot-weather concreting and long-haul RMC. (3) Accelerators — speed up setting and early strength gain, used in cold-weather and rapid-strip applications; calcium chloride accelerators are banned in RCC due to chloride attack on rebar. (4) Air-entrainers — produce 4-6% entrained air for freeze-thaw resistance (rare in India). (5) Waterproofing — integral chemicals reducing capillary porosity, often used with crystalline systems for underground works.

The most common Indian usage error is over-dosing PCE superplasticisers — beyond manufacturer-recommended dosage (typically 0.6-1.2% by cement mass), they cause excessive bleeding, segregation, and retardation. Site engineers must verify the dosage on the RMC delivery slip against the mix design report. Compatibility between admixture and cement is critical — switching cement brand within a project may require re-trial of the mix design because some cements interact poorly with specific PCE chemistries. Reputed Indian admixture brands: Sika, MC-Bauchemie, Fosroc, BASF Master Builders, Ultratech XPI, Krishna Conchem.

Where used
  • RMC concrete production — superplasticiser dosing for target slump at design w/c
  • High-strength concrete (M50+) — PCE essential to achieve w/c ≤ 0.35
  • Hot-weather concreting — retarder to extend workability time
  • Pumped concrete — superplasticiser to maintain slump at point of placement
  • Underground / waterproof structures — integral waterproofing admixture (IS 2645)
Acceptance / threshold
Per IS 9103: admixture must be tested for compatibility with the project's specific cement; dosage as per manufacturer recommendation and verified on delivery slip; chloride content < 0.1% by mass for use in RCC.
Site example
Site reality: an Ahmedabad project switched cement brands mid-project from OPC 53 (Ultratech) to OPC 53 (ACC) without re-trialing the mix. The new cement had slightly different C3A content; the same PCE dosage caused 90-minute setting delay, and one pour failed acceptance because of 8 MPa low 7-day strength. The supplier eventually credited 80% of the rejected concrete cost. Always re-trial mixes when any input (cement, sand, admixture) changes — admixture compatibility is non-trivial.
Frequently asked
What is the difference between plasticiser and superplasticiser?
Plasticisers (lignosulphonate-based) reduce water demand by 5-15%; superplasticisers (sulphonated naphthalene SNF, sulphonated melamine SMF, polycarboxylate ether PCE) reduce water by 20-40%. PCE is the most common modern Indian superplasticiser — better water reduction at lower dosage, longer slump retention, fewer compatibility issues with high-cement mixes. SNF/SMF are older chemistries still used for bulk applications.
Can calcium chloride be used as an accelerator in RCC?
No — calcium chloride is banned in RCC per IS 9103 because chloride ions attack rebar and accelerate corrosion. Acceptable alternatives: calcium nitrate (Sika), calcium nitrite (specialty), thiocyanate-based, or sodium-aluminate-based accelerators with chloride content < 0.1%. Always verify the chloride content from the manufacturer datasheet before approving any accelerator for RCC use.
What is the right dosage of PCE superplasticiser?
Typical dosage is 0.6-1.2% by mass of cement, but the exact value depends on the specific PCE chemistry, cement type, and target slump. Always do trial mixes — the saturation point (where additional dosage gives no further water reduction) is the optimal. Above saturation, retardation, bleeding, and segregation occur. Site engineers should never over-dose 'to be safe' — under-dose with extra trial is the right approach.
Related concrete terms
Concrete Mix Design
Proportioning of cement, water, aggregates per IS 10262
Concrete Grades (M15-M80)
M20=20 N/mm² characteristic strength at 28 days
Water-Cement Ratio (w/c)
Mass of water to mass of cement. 0.40-0.55 typical.
Concrete Cover
Min concrete from rebar to surface (15-50mm per exposure)
Concrete Curing
Min 7 days for OPC, 10 days for PPC, 14 days for sulphate-resistant
Slump Test / Workability
25-150mm slump per application (IS 1199)
Cube Test (Compressive Strength)
150mm cube compressive strength test at 7/28 days (IS 516)
RCC (Reinforced Cement Concrete)
Concrete with steel reinforcement. Per IS 456:2000.