IS 4925 : 2004Concrete Batching and Mixing Plants - Specification

IS 4925 specifies the equipment, layout, and performance requirements for concrete batching and mixing plants — central-mix, dry-batch (transit-mix), and on-site mobile plants. It's the producer-side companion to IS 4926:2003, which governs the *concrete*; IS 4925 governs the *plant that makes the concrete*.

A designer engaging a Ready-Mixed Concrete supplier should confirm IS 4925 compliance during source qualification, especially when: - High-volume pours (raft / mat foundations) where batching consistency over 8-12 hours matters - High-grade concrete (M40+) where weighing accuracy directly governs target strength - Pumped concrete where slump retention demands tight admixture metering - Onsite mobile plants for remote infrastructure where the project sets up its own RMC operation

IS 4925 is also the type-certificate basis for new plant procurement — equipment manufacturers (Schwing-Stetter, Macons, Apollo, Ammann) certify each model line against IS 4925 Type 1 (manual), Type 2 (semi-automatic), or Type 3 (fully automatic).

IS 4925 classifies plants by mixing technology and automation.

By mixing system: - Tilting drum mixers — small site mixers, capacity ≤ 0.4 m³/batch. Not used in modern RMC. - Pan mixers — fixed pan with rotating blades. Common in 1-2 m³/batch precast plants. Excellent for SCC and harsh mixes. - Twin-shaft (planetary) mixers — two horizontal shafts with intermeshing arms. Industry standard for high-capacity (60-180 m³/h) RMC central-mix. - Reversible drum — drums tilted on horizontal axis, mix in one direction, discharge in reverse. Common in older plants.

By batching automation: - Type 1 (manual) — operator weighs each material on a single scale, manually opens charge gates. Strength variability higher. - Type 2 (semi-automatic) — operator triggers preset weigh sequences; PLC controls charge gates. Most small RMC plants. - Type 3 (fully automatic) — recipe-based, dispatch-system integrated, moisture probes auto-correct water dosing. All large urban RMC plants.

Capacity classes (typical): - 30 m³/h — small urban plant, single twin-shaft - 60 m³/h — mid-size, two cement silos - 90-120 m³/h — large infrastructure plant - 180+ m³/h — mega-pour or central infrastructure (highway/airport runways)

Weighing accuracy (Clause 6.5): - Cement and water: ±1 % of batch quantity, ±0.5 % of full-scale deflection (FSD) - Aggregates: ±2 % of batch quantity, ±0.5 % FSD - Admixture: ±3 % of batch quantity, ±0.5 % FSD

Mixing time (Clause 7.4): - Pan / twin-shaft mixer: 30 s minimum after all materials are charged (M20-M40 conventional concrete) - Tilting drum: 60 s minimum - For SCC and high-strength (>M50): 45-60 s; verify by uniformity test (IS 4634)

Mixer uniformity test (IS 4634 — verify on commissioning and monthly): - Two samples taken from front-and-back of same batch - Difference in: density (≤ 16 kg/m³), slump (≤ 25 mm), coarse-aggregate content (≤ 6 %), water content (≤ 1 %), 7-day strength (≤ 7.5 %)

Storage: - Cement silo capacity: ≥ 50 t per silo (typical urban RMC has 2-4 silos) - Aggregate bins: ≥ 4 separate compartments (10-20 mm coarse, 4.75-10 mm coarse, fine sand, manufactured sand or 2nd fine source) - Admixture tanks: jacketed for chilled storage in hot climates; FIFO discharge

Plant calibration cadence: - Weighbridge / load cells: monthly with certified test weights - Moisture probes: weekly correlation with oven-dry test - Discharge timing: per shift change - Mixer uniformity: at commissioning, after major repair, monthly otherwise

• IS 4926:2003 — what comes out of the plant: the concrete supply specification.
• IS 4634:1991 — mixer uniformity test method.
• IS 1791 — batch-type concrete mixers (older code, the predecessor to IS 4925 for small mixers).
• IS 12119 — general purpose concrete mixers.
• IS 8112 / IS 12269 / IS 1489 — cement standards (silo storage compatibility).
• IS 383:2016 — aggregate standards (bin separation requirements).
• IS 9103:1999 — admixture standards (dosing system accuracy).
• IS 456:2000 — RCC design code (the upstream reason all this matters).

1. Reading 'IS 4925 Type 3 plant' on a brochure as a guarantee. Type certification is at *manufacture*. Calibration drift over 5 years of operation is real. Audit the plant's calibration records; demand monthly weighing-system verification logs. 2. Manual moisture correction on automatic plants. Even Type 3 plants need monthly probe correlation with oven-dry; otherwise the plant adds the same water regardless of actual aggregate moisture, and slump drifts. 3. Insufficient mixing time on twin-shaft plants under schedule pressure. Operators may cut from 45 s to 25 s to push more loads/hour. Result: poor uniformity, batch-to-batch slump scatter. Insist mixing time matches IS 4925 minimum. 4. Mixed cement silos. If a silo holds residual OPC 43 and is then topped-up with PPC, the next 50 t of concrete has unknown cement composition. Insist on dedicated silos per cement type, with logged changeover purges. 5. Aggregate cross-contamination at the bin walls. If the 20 mm bin spillover bridges into the 10 mm bin, your gradation is shifting. Insist on tall vertical separation walls or covered bins. 6. Discharge-timing drift between plant clock and operator instruction. The PLC may say 30 s mixing but the discharge gate opens at 25 s if the timer is mis-set. Spot-check during plant audit. 7. Calibration-weight provenance. The 1 t test weights used for monthly calibration must themselves be NABL-traceable. Many plants use uncertified weights — meaningless calibration.

Sample BOQ language for an RMC supply contract:

> *Concrete shall be supplied from a batching plant compliant with IS 4925:2004 Type 3 (fully automatic), capacity not less than 60 m³/h, equipped with twin-shaft mixer, ≥ 4 separated aggregate bins, dedicated cement silos per cement source, automatic moisture compensation on fine aggregate, and admixture dosing accurate to ±3 %. Plant shall hold a current NABL-equivalent calibration certificate (weighing system, moisture probes, mixer uniformity per IS 4634), reissued not less than monthly. Buyer reserves the right to audit the plant calibration records and witness a uniformity test before contract confirmation and at any time during supply.*

That single paragraph gates ~80 % of RMC quality issues. The rest is on-site verification per IS 4926.