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RCC Slab

RCC Slab

RCC Slab

We are required to cast a RCC Slab of 600 mm depth and size 21M x 11M. in 2 parts of one day duration each ( Non-continuous) at 3.8 m lvl above G,.L. The constraints are: 1) Mixing of M20 grade of concrete ( Cement quantity 401 Kg.per Cu.M. of concrete) by concrete mixers at site and 2) Manual placing of concrete Questions are: What should be the sequence of concreting to avoid formation of a cold joint? Will there be any creep if at all such joints are formed? How the construction joints shall be treated to avoid such problems? Will use of chemicals for reducing setting time affect the strength of concrete? Any advise on the matter will be of great help Thanks

RE: RCC Slab

Your pour is only 139 cubic metres in volume.  Unless your site batcher is incredibly slow or you are placing concrete via a skip, then you should have few problems in completing this pour without a cold joint.

You can place the concrete either by direct tipping into a chute made from (say) a steel tube suspended from a crane for ease of handling, or you could use a concrete pump.

As for pouring procedure, start in one corner and keep two faces live with concrete as you move away from the corner.  If you are still concerned about cold joints then I suggest you use temporary stop ends from materials such as Expamet.  These are sheets of woven stainless or galvanised steel through which your rebar can be punched.  Once the concrete reache the expamet it tries to pass through but can't.  This leaves a very rough key for the next stage of your pour to bond onto with no scabbling or pre-treatment required.  Traditional timber temporary stop ends could be used but need a good concrete gang working with them.

Regarding admixtures, you need a retarder to extend the fluid life of the concrete in order to prevent cold joints.  This will inhibit the rate of strength gain initially but should not affect the design strength of the concrete.


Andy Machon

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