About Fosroc Structuro 305

Fosroc Structuro 305 is a ready-to-use liquid admixture that combines two jobs in one bottle: it plasticises the mix so you need less water, and it retards the set so the concrete stays workable for longer. It belongs to Fosroc’s Structuro family of concrete admixtures and is aimed squarely at the situations where standard concrete starts to fight the crew, long transit times from a ready-mix plant, a pour that runs through the hottest part of an Indian afternoon, a tall column being pumped storey after storey, or a raft so large that the first batch risks stiffening before the last truck arrives. In all of these, the enemy is a mix that loses slump and starts to set too soon, leaving cold joints, honeycombing and rejected loads. Structuro 305 is formulated to push that window open.\n\nFunctionally it sits in the family of water-reducing and retarding admixtures described under ASTM C494 (Type B retarding and Type D water-reducing-and-retarding) and the equivalent BS EN 934-2 and IS 9103 categories. As a water reducer it lets the concrete reach a target slump at a lower water-cement ratio, which raises density, strength and durability; alternatively the same water content can be kept and the surplus plasticising used purely to improve flow and finish. As a retarder it delays the onset of stiffening in a controlled, dose-dependent way, so a batching team can dial in roughly how much extra placing time the job needs. Crucially, retardation here is about timing, not weakening, the product is designed so that once the concrete does set it continues to gain both early and ultimate strength normally.\n\nFor Space Arc Engineering’s customers in Delhi NCR and across India, Structuro 305 is most relevant to ready-mix producers, infrastructure contractors and any site batching large volumes in warm weather. It is supplied through Fosroc’s authorised distribution network, and because admixture performance is sensitive to cement chemistry, ambient temperature and the rest of the mix, the right dosage is always confirmed by trial mix rather than copied blindly from another job. The exact numeric properties, specific gravity, chloride limit, dosage band and pack sizes, are stated on the country-specific Fosroc technical data sheet for your region, and we recommend ordering against that current TDS.

The technical insight

The behaviour of Structuro 305 comes from how the admixture acts at the cement-water interface. Plasticising admixtures adsorb onto the cement grains and disperse them, breaking up the flocs that normally trap mixing water. That freed water becomes available for workability, which is why a smaller total water charge can still hit the target slump, the heart of the water-reduction benefit. The retarding action works alongside this by slowing the early hydration of the aluminate and silicate phases, so the dormant period before stiffening is stretched out. Because both effects are dose-driven, a batching plant can tune the same product from a mild plasticiser at low addition to a meaningfully delayed set at higher addition, within the manufacturer’s recommended range. The practical engineering point is that water reduction and retardation must be balanced together: over-dose and you can get excessive retardation and slow early-strength gain in cool conditions; under-dose in 40-plus-degree heat and the mix may still flash-set during a long haul. This is exactly why trial mixes with the actual job cement, aggregates and expected site temperature are non-negotiable, and why the verified specific gravity from the TDS matters, dispensers are calibrated by volume against that density to deliver the correct mass of admixture per cubic metre.

Where it is used

• Ready-mixed concrete delivered over long distances or in heavy city traffic, where transit time would otherwise cost slump
• Hot-weather concreting in Indian summer conditions, to hold workability and avoid premature stiffening
• Pumped concrete for high-rise columns, walls and slabs, reducing the risk of line blockages from a stiffening mix
• Large mass pours such as rafts, pile caps and foundations, to prevent cold joints between successive batches
• Slip-form construction of cores, silos and chimneys, where a controlled, extended placing window is essential
• Site-batched structural concrete that needs extended placing time due to access, reinforcement congestion or sequencing

Key advantages

• Extends the workable, placeable life of the concrete so long hauls and big pours can be completed without cold joints
• Reduces water demand at a given slump, raising density, compressive strength and long-term durability
• Controlled, dose-dependent set retardation that crews can tune to the job and the weather
• Protects early and ultimate strength gain, retardation delays setting without weakening the hardened concrete
• Improves cohesion and pumpability, lowering the chance of blockages and segregation in pumped mixes
• Can allow modest cement economy by getting more workability and strength from the same or less water
• Supplied as a single ready-to-use liquid, simple to meter through standard plant dispensing equipment

Limitations & considerations

• It is a retarder, over-dosing can cause excessive set delay and sluggish early strength, especially in cool weather, so dosage must be controlled
• Performance varies with cement type, aggregate, temperature and mix design; results from one job do not transfer directly to another without a trial
• It is an admixture, not a waterproofer, sealer or strength miracle, it cannot fix a fundamentally poor mix design or bad placing practice
• Must be added to the gauging water or wet mix, never to dry cement, and is best dispensed through calibrated equipment for accuracy
• Compatibility with other admixtures (for example a separate superplasticiser or air-entrainer) should be confirmed before combining in the same mix
• Exact specific gravity, chloride limit, dosage band and shelf life must be read from the current country-specific Fosroc TDS, not assumed
• Like most liquid admixtures it should be protected from freezing and used within its stated shelf life

Selection & best practice

Specifications

Industry use cases

How we position Fosroc Structuro 305

Within the Fosroc range, Structuro 305 sits in the concrete admixtures family as a retarding water reducer, a step below the high-range superplasticisers in the same Structuro line. If your priority is workable time and hot-weather set control with moderate water reduction, 305 is the right pick; if you need high flow, big water reduction or self-compacting concrete, look at Structuro 311, Structuro 351 or Conplast SP430, which can also be paired with a retarder for long-haul flowing mixes. For pure set-retardation with water reduction in the Conplast family, Conplast RP264 is a comparable Fosroc option. Against the wider market, it competes with retarding/water-reducing admixtures such as Sika Plastiment and Sika ViscoCrete (set-retarding grades), BASF/Master Builders Solutions (MBS) MasterPozzolith and MasterSet retarders, and the admixture lines of Dr. Fixit (Pidilite) and STP Limited. The honest distinction is rarely the chemistry headline, it is consistency, technical support and correct dosing for Indian site conditions. Space Arc Engineering positions Structuro 305 for RMC producers and contractors who value reliable hot-weather performance backed by trial-mix support and authorised Fosroc supply, rather than chasing the cheapest unverified admixture. Always specify against the current Fosroc TDS for verified properties and dosage.

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