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What is Compaction of Concrete?

Compaction of concrete is one of the important site operations that together enable the fresh concrete to reach its potential design strength, density and low permeability. Properly carried out it ensures that concrete fully surrounds and protects the reinforcement, tendons and cast-in inserts. It also has a direct impact on achieving the specified surface finish.

Compaction is the process that expels entrapped air from freshly placed concrete and packs the aggregate particles together so as to increase the density of the concrete.

The compacting and finishing of concrete are generally two separate operations but sometimes, particularly with flat horizontal surfaces, they become parts of the one operation. In such circumstances, it should be noted that a smooth surface finish is not necessarily evidence of good compaction underneath it. Care should always be taken to ensure that concrete is adequately compacted.

1. Purpose of Concrete Compaction

Compaction significantly increases the ultimate strength of concrete and enhances the bond with reinforcement. It also increases the abrasion resistance and general durability of the concrete, decreases the permeability and helps to minimise its shrinkage and creep characteristics.

Proper compaction also ensures that the reinforcement, tendons, inserts and fixings are completely surrounded by dense concrete, the formwork is completely filled – i.e. there are no pockets of honey-combed material – and that the required surface finish is obtained on vertical surfaces.

Concrete shall be compacted during placing so that:

  • A monolithic mass is created between the ends of the member, planned joints or both;
  • The formwork is completely filled to the intended level;
  • The entrapped air is expelled;
  • All reinforcement, tendons, ducts, anchorages and embedments are completely surrounded;
  • The specified finish to the formed surfaces of the member is provided;
  • The required properties of the concrete can be achieved.

2. The Process of Concrete Compaction

When first placed in the form, normal concretes (i.e. excluding those with very low or very high workability) will contain between 5% and 20% by volume of entrapped air. The aggregate particles, although coated with mortar, will also tend to arch against one another and are prevented from slumping or consolidating by internal friction.

Fig-1 The Process of Concrete Compaction
Fig-1 The Process of Concrete Compaction

Compaction of concrete is, therefore, a two-stage process Fig-1. First, the aggregate particles are set in motion and the concrete consolidated to fill the form and give a level top surface (liquefaction). In the second stage, entrapped air is expelled. This description of the process is true whether compaction is carried out by rodding, tamping and similar manual methods, or when vibration is applied to the concrete. The latter, by temporarily ‘liquefying’ a much larger volume of the concrete, is generally much more efficient than tamping or rodding by hand, and hence is almost universally used.

It is important to understand that compaction is a two-stage process and to recognise each stage because, with vibration, initial consolidation of the concrete (liquefaction) can often be achieved relatively quickly. The concrete liquefies and the surface levels, giving the impression that the concrete is compacted. Entrapped air takes a little longer to rise to the surface. Compaction should therefore be prolonged until this is accomplished, i.e. until air bubbles no longer appear on the surface.

3.1 Effect on Fresh Concrete

The effect of vibration on the properties of fresh concrete needs to be understood to ensure that the type and amount of vibration applied to the concrete are appropriate. Otherwise, defects such as excessive mortar loss and other forms of segregation can be caused.

The concrete mixture as supplied to the project needs to be properly proportioned. Concretes lacking fines can be difficult to compact and, even when fully compacted, can have a high porosity.

On the other hand, those with too high a fines content, particularly if they also have a high slump, may be prone to segregation and excessive bleeding. Nevertheless, it should be noted that properly proportioned concretes are difficult to overvibrate and cautionary notes in specification regarding over-vibration may result in concrete on the project being under-vibrated with resulting loss of potential strength and durability.

Concretes with lower workability, i.e. stiffer mixes, will require a greater energy input to compact them fully. This may be achieved by using a high-energy vibrator or by vibrating the concrete for a longer time. In the latter case, the vibrator must have at least sufficient capacity to liquefy the concrete. Conversely, more workable mixes will require less energy input.

The size and angularity of the coarse aggregate will also affect the effort required to fully compact concrete. The larger the aggregate, the greater the effort required, while angular aggregates will require greater effort than smooth or rounded aggregates.

3.2 Effect on Hardened Concrete

Fig-2 Loss of strength through incomplete compaction
Fig-2 Loss of strength through incomplete compaction

Since compaction of concrete is designed to expel entrapped air and optimise the density of the concrete, it benefits most of the properties of hardened concrete. As may be seen from Fig-2, its effect on compressive strength is dramatic.

For example, the strength of concrete containing 10% of entrapped air may be as little as 50% that of the concrete when fully compacted.

In addition to expelling entrapped air, promotes a more even distribution of pores within the concrete, causing them to become discontinuous. The durability of the concrete is consequently improved except, perhaps, in freeze-thaw conditions, where excessive vibration can expel amounts of purposely-entrained air which is designed to increase the freeze-thaw resistance of hardened concrete.

The abrasion resistance of concrete surfaces is normally improved by adequate compaction. However, excessive vibration, or excessive working of the surface, can cause an excessive amount of mortar (and moisture) to collect on the surface, thereby reducing its potential abrasion resistance.

In flat work a careful balance is therefore required to expel entrapped air without bringing excessive amounts of mortar (fines) to the surface of the concrete.

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