PLACING AND COMPACTING CONCRETE
Ensuring the full and proper compaction of concrete when it is placed is of fundamental importance because any voids or pockets left in the finished concrete will compromise its final strength. As an indication, voids that total 5 per cent of the concrete by volume will reduce the strength of the concrete by as much as 30 per cent.
Concrete with minimal water will be high in strength but will need mechanical compaction, whereas a high water content will give workable concrete that is easily compacted by hand, but the concrete will be weaker. Very wet mixes are very easily compacted since the concrete will practically flow into position but, again, final strength and durability will be further compromised by the high ratio of water to cement.
Freshly mixed concrete goes stiff after 30 minutes. This is referred to as the ‘initial set’ and once concrete has reached
this stage it should not be mixed up again. Beyond initial set the concrete hardens and rapidly gains strength. After about 10 hours the concrete is hard, having reached what is called the ‘final set’. Having achieved its final set, concrete then slowly continues to harden and gain strength over many years and decades.
Accordingly, placing and compacting need to be done quite quickly, before the initial set takes place, especially where the concrete mix has a proportionally high cement content. Such a concrete will set more quickly than a ‘lean mix’ containing less cement.
For small concreting jobs, a short length of 50 x 50mm timber can be forcefully poked or rammed into the freshly placed concrete to remove any air pockets and ensure compaction.
For slabs and/or larger areas constructed within a frame of timber formwork, a heavy, straight timber plank (of up to 125mm x 50mm sectional size), preferably with raised handles or cut-out handles at the ends, can be operated by a man at each side of the formwork. The ‘tamping board’, as it is known, is raised and lowered whilst gradually working all the way along the concrete surface to achieve compaction.
Fig. 15 Use of a tamping board to compact and level concrete within timber framework.
Where necessary, the tamping board can be ‘shuffled’ from side to side as it is drawn forward along the top of the formwork in order to grade off any excess wet concrete and to obtain a flat surface. Generally, the timber straight-edge used as a tamping board must be long enough to extend 100mm beyond both sides of the formwork in order to be effective. Hammering the exposed sides of the timber formwork will also assist compaction of the concrete at its perimeter sides.
The construction of formwork and shuttering is covered in greater detail in Chapter 5.