Concrete for a self-build house or a block yard, load by load

A house goes down in a handful of separate pours, and the gap between them is set by how long the formwork has to stay where it is. IS 456:2000 makes strength the governing criterion for striking, then gives periods that satisfy it in the normal case the code describes: ordinary Portland cement, adequate curing, and ambient temperature not falling below 15°C. In that case vertical formwork to columns, walls and beams comes off after 16 to 24 hours; soffit formwork to a slab stays three days and to a beam seven; props under a slab spanning up to 4.5 m stay seven days, and fourteen where it spans further. A self-builder who owns one set of forms therefore pours the footings, waits, strips, resets and pours again. The same question comes back at every pour in that sequence.

The jobsite photographs on this site show that sequence rather than a single event: tied reinforcement standing in timber formwork with an excavator and a tipper behind it, strip footings and formwork set out across a cleared site to a boundary wall, poker vibrators running across a freshly poured slab. None of them carries a country, because we cannot verify where any of them was filmed. What they do establish is the shape of the work. The pours are separate, and far enough apart in time that concrete ordered for one is of no use to the next. Nothing in the striking periods lets a builder bring them together either: the props under one slab are still carrying it while the next lift is being formed. That is the constraint a self-builder is actually working against: not the difficulty of the concrete, which is ordinary, but its arrival in usable quantities on a date he controls.

What puts these pours outside a ready-mix supplier's reach is their size, not their difficulty. A ready-mix plant dispatches and prices by the truckload: a part load still takes a mixer truck off the road, a driver's shift and a return trip, so the economics push a supplier towards a minimum load or a minimum charge. We have not verified the minimum used by any supplier in any market, so no figure for one appears on this site. What can be stated is the other side of the comparison. The smallest rated discharge in our product record is 1.0 cubic metre, then 1.3 and 1.5. Fourteen of the sixteen self-loading mixers in that record carry a rated discharge figure at all, and those fourteen run from 1.0 to 4.5 cubic metres. Whether a quantity of that order sits above or below a given supplier's minimum is a question for that supplier, and worth asking before either answer is assumed.

The code draws the same line by project size rather than by concrete grade. IS 456:2000 clause 10.2 states that ready-mixed concrete shall be preferred, and that for large and medium project sites the concrete shall be sourced from ready-mixed concrete plants or from on-site or off-site batching and mixing plants. It names large and medium sites; it does not extend that instruction to small ones. The same clause is unambiguous about how a batch is to be controlled wherever it is made: the quantity of both cement and aggregate shall be determined by mass, and water shall be weighed or measured by volume in a calibrated tank. Volume batching is permitted only where weigh-batching is not practical and the bulk densities of the actual materials have been established beforehand.

There is no single number, and the two answers most often quoted come from two different questions. The first is durability. IS 456:2000 Table 5 sets a minimum cement content per cubic metre for concrete made with 20 mm nominal maximum aggregate, and that minimum rises with exposure: for reinforced concrete, 300 kg/m³ at mild and at moderate exposure, 320 at severe, 340 at very severe and 360 at extreme. For plain concrete at mild exposure it is 220 kg/m³. Those are floors rather than recipes — the table exists to stop a mix being starved of cement in an aggressive environment. The standard proportions its nominal mixes by the 50 kg bag, so the 300 kg floor is at least six bags to the cubic metre, the 360 kg floor at least 7.2 bags, and the 220 kg floor at least 4.4. That division is performed here; the table itself is written in kilograms.

The second question is proportioning, and IS 456 answers it per bag rather than per cubic metre. Table 9 fixes, for each nominal grade, the total dry aggregate by mass per 50 kg of cement and the maximum water: an M15 mix takes 330 kg of aggregate and at most 32 litres of water to the bag, an M20 mix 250 kg and at most 30 litres, and nominal mixes are permitted up to M20 and no higher. Nothing in that table yields a volume, because the volume a bag makes depends on the density and grading of the aggregate and on how much surface water the sand is already carrying — the standard gives a separate table for that last correction alone. This is why published bags-per-cubic-metre answers disagree with one another, and why the figure a builder uses should come from the code in force where he is building.

A block yard is the mirror image of a house: one address, a small batch, every working day, indefinitely. The house is a handful of pours spread over months and then it stops. That difference does not change the concrete, but it changes what matters about the machine. On a house, access and mobility dominate; in a yard, what matters is whether the same batch can be repeated to the same proportions every working day, and whether the machine brings its own water and its own weighing to that job. The configuration lists in our record speak to the water side plainly. Of the sixteen self-loading mixers, fifteen carry a water-drawing system, under two different wordings: seven list a self-filling water controller together with an external suction pump, and eight list a high-capacity water tank with its own filling system. On those machines the water comes off the site rather than off a bowser.

Batching to a written mix is the part a yard has to get right every time, and here the record is thinner than sales material elsewhere would suggest. Electronic hydraulic weighing appears as standard equipment on exactly one of the sixteen mixers, and as a listed option on six more; on the remaining nine the record says nothing either way. Since IS 456 requires cement and aggregate to be determined by mass, a buyer batching to a specified grade should name the weighing system in the order rather than assume it. Monitoring splits the same way: seven machines list a camera covering reversing and drum monitoring, and eight list a high-definition in-cab driving display which the record does not connect to a drum camera. The standard also requires the mixer to be fitted with a water-metering device, and sets a mixing time of at least two minutes.

The small end of the range, and it is identified here by width rather than by model name. Ten of the sixteen self-loading mixers in our record have measured overall dimensions; the other six have no dimensioned drawing in our hands and appear in no dimension statement on this page. Across those ten, width runs from 1.60 m to 2.30 m and length from 4.35 m to 7.06 m. Pairing rated discharge against measured width, the four narrowest give 1.0 cubic metre at 1.60 m, 1.3 at 1.65 m, 1.5 at 1.80 m and 2.0 at 1.85 m. The narrowest of those four is also the only machine in the range measured to pass through a 40 ft high-cube door and load two to a container; the container guide on this site sets out the door-frame arithmetic behind that verdict.

The uncomfortable part is that the machines this description points at are the ones we can say least about. All four of the narrowest carry an engine model in our record, but not one of them has an engine brand, a rated power or an operating weight. Rated power is on file for seven of the sixteen mixers and runs from 55 to 91 kW; operating weight for three, from 5,800 to 8,500 kg. None of those seven and none of those three is among the four narrowest. The most completely documented machine we publish sits inside that same band by volume and outside it by width: the SLM-18, 1.6 to 1.8 cubic metres discharged, 78 kW, 5,800 kg, and 2.21 m wide — 610 mm more than the narrowest machine on the list. The trade between documentation and access is a real one, and it is better made with the gaps visible than with them filled in.

Access is a width question, and it is the one thing a buyer can settle for himself before anything is ordered. Measure the narrowest point the machine has to pass — the gate, the corner between two finished walls, the gap left between the house and the boundary — and set it against the measured widths: 1.60 m at the narrowest of the ten mixers we hold measurements for, 2.30 m at the widest. Height is where our record is weaker, and it is worth saying why. Only one of those ten, the SLM-18, comes from a drawing that separates the height of the machine standing on its tyres, 2.58 m, from the height with the bucket at full lift, 3.45 m. For the other nine the source does not state which posture was measured, so we publish no travelling height for them, and a low gateway or a canopy has to be checked against the machine itself.

Two further things are worth knowing before the site visit. Articulated steering is named on six of the sixteen configuration lists, but none of those six is among the four narrowest machines, and only one of the six has a measured width at all — the SLM-18, at 2.21 m. In our record a narrow body and articulated steering do not yet arrive together, so a buyer who needs both has to ask for the pair rather than assume it. Then weight: the three mixers with an operating weight on file run from 5,800 to 8,500 kg, against a median declared unit weight of 13,617 kg across 308 country-years of trade under HS 8705.40. They sit well below the middle of what moves under that heading, which matters on soft ground and on a plot already half built on — with the caveat that whether a customs net weight and an operating weight are the same measure is itself unverified.