Compaction of concrete is one of the important site operations that simultaneously enable fresh concrete to reach its potential design strength, density and low permeability. Done properly this ensures that the concrete surrounds completely and protects the reinforcement, tendons and cast-in inserts. It also has a direct effect on the specified surface finish.

Compaction is the process that expels trapped air from freshly laid concrete and packs aggregate particles together to increase the density of the concrete.

Compaction and finishing of concrete are usually two separate operations but sometimes, especially with flat horizontal surfaces, they become part of a single operation. In such circumstances, it should be noted that a smooth surface finish does not necessarily imply good compaction beneath it. Care should always be taken to ensure that the concrete is sufficiently compacted.

1. purpose of concrete compaction

Compaction significantly increases the final strength of the concrete and enhances the bond with the reinforcement. It also increases the abrasion resistance and general durability of concrete, reduces permeability and helps reduce its shrinkage and creep characteristics.

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

During laying the concrete should be compacted so that:

  • A monolithic mass is created between the ends of the member, the planned joints or both;
  • the formwork is completely filled to the desired level;
  • Trapped air is expelled;
  • All reinforcements, tendons, tubules, anchors and embeddings are fully enclosed;
  • The specified finish is provided to the formed surfaces of the member;
  • The required properties of concrete can be achieved.

2. concrete compaction process

When first put into form, normal concrete (ie except those with very low or very high workability) will have between 5% and 20% by volume of trapped air. The aggregate particles, although coated with mortar, also tend to lean against each other and are prevented from collapsing or being consolidated by internal friction.

Figure-1 Process of Concrete Compaction
Figure-1 Process of Concrete Compaction

Therefore compaction of concrete is a two step process. Figure 1, First, the aggregate particles are set in motion and the concrete is consolidated to form fill and give a level top surface (liquefaction). In the second stage, the trapped air is expelled. This description of the process holds true whether compaction is done by manual methods like roding, tamping, and the like, or when vibration is applied to concrete. The latter, by temporarily ‘liquefying’ a large amount of concrete, is generally far more efficient than hand tamping or rodding, and is therefore almost universally used.

It is important to understand that compaction is a two-stage process and to recognize each step, because with vibration, the initial consolidation of concrete (liquefaction) can often be achieved relatively quickly. The concrete liquefies and the surface level, giving the impression that the concrete is compacted. It takes a little longer for the trapped air to rise to the surface. Therefore compaction should be prolonged until it is complete, that is, until air bubbles appear on the surface.

3.1 effect on fresh concrete

The effect of vibration on the properties of fresh concrete must be understood to ensure that the type and amount of vibration applied to the concrete is appropriate. Otherwise, defects such as excessive damage to the mortar and other forms of separation may occur.

The concrete mix supplied for the project needs to be properly proportioned. Concrete lacking fineness can be difficult to compact and, even when fully compacted, can have high porosity.

On the other hand, those with a lot of fine material, especially if they also have a high meltdown, can be prone to separation and excessive bleeding. Nevertheless, it should be noted that properly proportioned concrete is difficult to overvibrate and that the caution notes in the specification regarding over-vibration can potentially result in the concrete being low on the project with the resulting loss of strength and durability.

Concrete with low workability, i.e. stiffer mix, will require more energy input to compact completely. This can be achieved by using a high-energy vibrator or by vibrating the concrete over a long period of time. In the latter case, the vibrator must have at least sufficient capacity to liquefy the concrete. Conversely, more practical blends would require less energy input.

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

3.2 impact on hard concrete

Figure-2 Power loss due to incomplete compaction
Figure-2 Power loss due to incomplete compaction

Since concrete compaction is designed to expel trapped air and optimize the density of the concrete, it benefits most of the properties of hardened concrete. as can be seen from line drawing number 2, its effect on compressive strength is dramatic.

For example, 10% of the trapped air strength of concrete can be as low as 50% of concrete when fully compressed.

In addition to expelling trapped air, it promotes a more uniform distribution of pores within the concrete, closing them. This results in an improvement in the durability of concrete, perhaps, except in freeze-thaw situations, where excessive vibration may expel a volume of intentionally-entered air that is designed to increase the freeze-thaw resistance of hardened concrete.

The abrasion resistance of concrete surfaces is normally improved by substantial compaction. However, excessive vibration, or overworking of the surface, can cause excessive amounts of mortar (and moisture) to accumulate on the surface, reducing its potential abrasion resistance.

Flat work requires careful balancing to expel trapped air without bringing an excessive amount of mortar (fine) to the surface of the concrete.

Er. Mukesh Kumar

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Er. Mukesh Kumar is Editor in Chief and Co-Funder at Civil Engineering Website. Mukesh Kumar is a Bachelor in Civil Engineering From MIT. He has work experience in Highway Construction, Bridge Construction, Railway Steel Girder work, Under box culvert construction, Retaining wall construction. He was a lecturer in a Engineering college for more than 6 years.