Concrete Columns in Kent

A column is a small volume of concrete doing a large structural job. It is poured vertically into tall, narrow formwork, usually around a dense cage of reinforcement, and it is often the element everything else waits on.

Three consequences follow. The concrete has to travel down a confined space without separating. It has to pass between reinforcement bars that may be closely spaced. And it has to be compacted properly in a space too narrow to get much into. None of these is a strength problem. All of them are placing problems, and they are solved when the concrete is ordered rather than when it arrives.

Consistence and aggregate size

Two properties on the order matter as much as the strength class.

Consistence is how readily the concrete flows. Column concrete is normally supplied at a higher consistence class than a slab, so that it fills the formwork and surrounds the reinforcement without needing to be forced.

Maximum aggregate size decides whether it can get between the bars at all. Where reinforcement is congested, or the cover to the bars is small, a 10mm aggregate is specified instead of the usual 20mm. A 20mm mix poured into a tightly reinforced column arches over the bars and leaves voids behind them, which are only discovered when the formwork comes off.

Both are on your engineer’s specification. Both need to be on the order.

Formwork pressure and the rate of pour

Fresh concrete behaves like a heavy liquid, and the pressure it exerts on formwork rises with the height poured, the speed of pouring and the temperature. The reference document for calculating that pressure in the UK is CIRIA Report 108.

The practical point for the concrete order is this: the rate of pour is part of the formwork design, not a matter of convenience. Pouring a column faster than the formwork was designed for is how formwork fails. Agree the placing rate before the concrete is booked, and order a delivery pattern that supports it rather than one that forces the pace.

Placing and compacting

Concrete dropped from height into a tall column separates: the aggregate falls faster than the paste and lands in a heap at the bottom. Use a tremie, a trunk or a pump line so the concrete is delivered near the level it is being placed at.

Compact in layers with a poker vibrator, each layer worked into the one below so the two combine. Over-vibration is as damaging as under-vibration and drives the mix to segregate. The finished evidence of getting this wrong is honeycombing on the face of the column when it is struck, which is a repair at best and a structural question at worst.

Working out column volume

A rectangular column is width times depth times height. A column 300mm by 300mm and 3 metres tall is 0.3 x 0.3 x 3, so 0.27 cubic metres.

A circular column is pi times the radius squared, times the height. A 400mm diameter column 3 metres tall is 3.14 x 0.2 x 0.2 x 3, so roughly 0.38 cubic metres.

Columns come in sets, so multiply by the number being poured, then add for the starter kicker at the base of each one and for whatever stays in the pump line or the chute. Individual columns are small volumes and the losses are proportionally large.

Getting concrete to the column

Columns are usually poured at height, above the reach of any chute, which makes a concrete pump the normal answer. A boom pump places the concrete directly into the top of the formwork. A line pump with hose works where a boom cannot be set up.

The alternative on smaller jobs is a skip lifted by a crane or telehandler and discharged into the formwork. Whichever is used, it needs booking alongside the concrete so that both arrive together and neither waits for the other. Kent Readymix arranges the delivery around the placing method rather than the other way round.

Talk to us about a column pour

Send the designation, consistence class and maximum aggregate size from the specification, the column dimensions, how many you are pouring, the height and the placing method. Your quotation, delivery and payment are agreed directly with the approved supplier appointed to your pour.

Where any of those values is missing from the drawings, ask your engineer rather than us. We supply what the specification names and we will not substitute for it.