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Ready-Mix Concrete

Concrete that is not mixed on site, but which is batched at a central plant and delivered to the site by transit mixers.

Concrete Transit Mixer

Usually a road registered truck that brings concrete from a batching plant to the job site. When it is travelling the bowl is turning and the concrete is being mixed.

Concrete Batching Plant.

Usually situated in an industrial area a plant that delivers all the ingredients of a concrete mix into concrete trucks for delivery of fresh concrete to construction sites.

A medium sized batching plant. The silos contain the cement and the various aggregate arrive in the hopper above the truck via conveyor belts. The ingredients are progressively weighed before they are discharged into the waiting truck.

Ready mix became available in the 1930's but it did not take off until the 1960's when the equipment became more sophisticated. The advantages of a large separate plant in an industrial area that has quality control and a large capacity are obvious when compared to the ad-hoc, low quality and wasteful methods of batching concrete on each site that prevailed before the introduction of ready mix.

It has a couple of downsides which mainly concern the distance of travel. The ingredients are added at the plant and if the job site is an hours travel away then the concrete is already one hour old when it arrives on site. Not a good scenario and to be avoided. However retarder chemicals can help with this problem.

One technique that I have used on bush jobs in the past is to take the cement to the job beforehand and with a small scaffold or staging add the cement to the mixer when it arrives on the job. Expensive, yes, paying the extra traveling time but still a good alternative to site mixing in many cases

The rear end of a small 3 cubic metre concrete transit mixer. The trucks in the previous photo with the twin rear axles and twin steer are 8 cubic metre capacity. Trucks are even larger and also smaller than this.

Each truck carries it's own delivery chutes that can be shortened or extended to adjust the reach. The height of the chute can be adjusted by a few pumps of a hydraulic lever to lift it up.

Trucks also carry a supply of water to clean up after discharging the load, and to add a little water to the mix if necessary.

This rather poor shot shows the inside of the mixer bowl. When the bowl is rotating anti-clockwise then the concrete is being mixed. When the direction of the rotation is reversed then the screw like fins draw the wet concrete upwards and into the discharge chute.

The technical term for this device is the Archimedian Screw which has been used in the middle east for raising water since biblical times.

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