Concrete placing and compaction has to be right first time. The degree of success will not be fully known until the hardened concrete can be inspected. At this stage it will be difficult and expensive to correct any defects. Important aspects of managing the placement process are the selection of suitable equipment and correctly trained staff.
A placement method that transports and places the concrete in a single operation is the most productive approach.
The concreting team needs to know what action to take if the appearance of any concrete is abnormal. While being focused on productivity, they must also know that it cannot be achieved at the cost of quality.
Developments in concrete technology allow concrete to be dropped much further than before. Whereas traditional construction methods for columns involved numerous construction joints, it is now possible - with the involvement of the contractor, specifier and Lafarge Readymix - to design concrete to pour a 15 meter high column in a single lift. A special concrete has to be designed to meet this special demand. Also, should such a pour be undertaken it is important to increase the strength of the formwork to cope with the increased hydrostatic pressure of the concrete.
Dropping inappropriate concrete over a long distance or allowing it to fall through congested reinforcement will produce honeycombing and a poor standard of surface finish.
It is important to discharge the concrete as close as possible to where it is required in order to reduce the risk of segregation, which could lead to honeycombing or unsightly grout lines. The concrete should be evenly spread before compaction. Unless there is careful control of concrete layers, thicker sections may be inadequately compacted. Where it is not possible to place the concrete in its final position (e.g. in the corner of a wall), it should be placed as close as possible and then made to flow into place with a poker vibrator. The concrete must still be compacted afterwards.
Compaction of concrete is the process of removing air bubbles that have become entrapped during mixing and discharge of the concrete from the drum or other handling equipment. If the air is not removed the appearance, strength and durability of the concrete will be inferior. Entrapped voids or pockets of air have a natural tendency to rise, but in general concrete is not sufficiently fluid to allow air to escape.
The role of compaction is to make concrete more fluid so that entrapped air is able to rise to the surface. Properly compacted concrete is dense and impermeable. The structure being built is correspondingly stronger and more durable. Generally, every 1 % of air that is trapped in the concrete results in the loss of 5 % of design strength.
The fundamental principle of compaction is to use the most powerful vibrator that can be conveniently handled. The usual method of imparting vibration is by using poker vibrators. However, even large diameter pokers have a limit to the radius of their compacting effect and need to be inserted in an overlapping pattern so that all the concrete is properly vibrated. This is particularly important with walls where reinforcement makes it difficult to position the poker closer to the form face. In this case, insertions must be made at closer centres to ensure the important cover zone will be adequately compacted.
When the first concrete is placed, the lower vibrators should already be running so that the removal of air is progressive from the bottom upwards. It is a mistake to place a thick layer of concrete before inserting the poker as the compaction of the top layer would prevent air trapped down from escaping. Vibration should continue until no further air bubbles appear on the surface. It is never possible to achieve 100% compaction and some small blow-holes will always be evident on a vertical concrete surface.
It may not be practical to use poker vibrators on slender columns and walls containing congested reinforcement. In such cases, external vibrators clamped to the form work will work best. It is essential that the formwork is sufficiently robust to transmit the compactive force to the external vibrator, in addition to the hydraulic load of the concrete.
Beam vibrators are used to compact thin slabs providing sufficient concrete coverage can be maintained. However, at the edge of the slab, much of the energy will have been dissipated and proper compaction will require the use of poker vibrators aswell.
How compaction affects quality
Lack of proper compaction results in:
- Poor appearance: as a result of air voids and honeycombing.
- Reduced strength: every 1% of air causes drop in strength of between 5 and 6%. In other words, 5% under compaction produces around 25-30% loss of concrete making it vulnerable to deterioration form:
- Freeze/thaw effects.
- Sulpahtes in solution.
- Increased rates of carbonation, particularly where chlorides are present.
- Corrosion of reinforcement steel.
Note: Air voids produced in concrete by an air en-training agent should not be confused with entrapped air. Entrapped air exists in randomly distributed pockets and has relatively large bubble sizes. Entrained air consists of tiny, discreet bubbles dispersed throughout the concrete that have a beneficial on durability by allowing the concrete to expand without any adverse structural effects.
As compaction continues, the air pockets become fewer and the quality of the concrete is enhanced. As a general rule, it is not possible to over-compact concrete. In any event, the possible consequences of over vibration are less serious than the real problems arising from stopping compaction prematurely.
- Very high workability concrete: segregation of the sand and aggregate may occur.
- Inappropriate mix or difficult aggregate: if a mix is prone to segregation, the aggregate may sink leaving a layer at the top of deep lifts that is deficient in coarse aggregate. This is rarely a problem.
- Exposed aggregate architectural finishes: in the case of over-vibration, the aggregate deficient top layer may simply have to be removed and replaced with a quantity of fresh concrete.
Correct specification, mix design and careful placing generally minimize any segregation problems caused by over-vibration.