Artículo 90. Compete al Comité de Transparencia:
1. Asistir puntualmente a las Sesiones del Comité a las que sean convocados
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.
Figure 9.1 The process of compaction
Figure 9.2 Loss of strength through incomplete compaction
9.3
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 over- vibrate 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.
9.4 EFFECT ON HARDENED
CONCRETE
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 Figure
9.2, 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 (see Chapter 19 Properties of 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 flatwork a careful balance is therefore required to expel entrapped air without bringing excessive amounts of mortar (fines) to the surface of the concrete.
Chapter 9 Compaction
9.5
METHODS AND EQUIPMENT
9.5.1 General
Two types of vibrators are common on building sites (immersion vibrators and surface vibrators. Each has its sphere of application, although on floors and other flatwork it is not uncommon for one to augment the other. A third type, form vibrators, is commonly used in factories for precast work, and sometimes on building sites.
9.5.2 Immersion Vibrators
Frequently referred to as 'poker' or 'spud' vibrators, immersion vibrators consist essentially of a tubular housing which contains a rotating eccentric weight. The out-of-balance rotating weight causes the casing to vibrate and, when immersed in concrete, the concrete itself. Depending on the diameter of the casing, and on the frequency and the amplitude of the vibration, an immersion vibrator may have a radius of action of between 100 and 600 mm Table
9.1.
Immersion vibrators may be driven by:
a flexible shaft connected to a petrol, diesel, or electric motor;
an electric motor situated within the tubular casing;
compressed air.Flexible-shaft vibrators may have either a conical pendulum, which runs around the inside of the casing like an epicyclic gear, or a straight rotating weight. Those with the former have the advantage that they generally have thinner heads (useful in reinforced members). They also have higher amplitudes at the tip than further up the casing. This helps compact the concrete near the top of the pour as the vibrator is withdrawn from the concrete. Electrically powered vibrators, with the motor in the head driving an eccentric weight, are relatively light in weight and, with a switch located at the vibrator, are easy to handle.
Vibrators powered by compressed air normally have the motor driving an eccentric weight located within the casing. They are most common in the larger diameters used in compacting mass concrete, e.g. in dams.
Table 9.1 Characteristics and applications of internal vibrators
Diameter of head (mm) Recommended frequency (Hz)1 Average amplitude (mm)2 Radius of action (mm)3,5 Rate of concrete placement (m3/h per vibrator)4,5 Application
20–40 150–250 0.4–0.8 80–150 0.8–4 High slump concrete in very thin
members and confined places. May be used to supplement larger vibrators where reinforcement or ducts cause congestion in forms.
30–60 140–210 0.5–1.0 130–250 2.3–8 Concrete 100–150 mm slump in thin
walls, columns, beams, precast piles, thin slabs, and along construction joints. May be used to supplement larger vibrators in confined areas.
50–90 130–200 0.6–1.3 180–360 4.6–15 Concrete (less than 80 mm slump) in
normal construction, e.g. walls, floors, beams and columns in residential, commercial and industrial buildings.
80–150 120–180 0.8–1.5 300–500 1–31 Mass and structural concrete of 0 to 50
mm slump deposited in quantities up to 3
m3 in relatively open forms of heavy
construction.
Adapted from Table 5.15 ACI Committee Report: Consolidation of Concrete ACI Manual of Concrete Practice 1993 Part 2.
1
While vibrator is operating in concrete.
2
Computed or measured. This is peak amplitude (half the peak-to-peak value), operating in air. Reduced by 15–20%
when operating in concrete.
3
Distance over which concrete is fully consolidated.
4
Assumes insertion spacing 1½ times the radius of action, and that vibrator operates two-thirds of time concrete is
being placed.
5
Reflects not only the capability of the vibrator but also differences in workability of the mix, degree of de-aeration
Chapter 9 Compaction
Guide to Concrete Construction 9.5
Figure 9.3 Use of an immersion vibrator
Figure 9.4 Alternative patterns for use of immersion vibrators
The effectiveness of an immersion vibrator is dependent on its frequency and amplitude, the latter being dependent on the size of the head, the eccentric moment and the head weight—the larger the head, the larger the amplitude.
Table 9.1 summarises the characteristics and
applications of internal vibrators. As a general rule, the radius of action of a given vibrator not only
increases with the workability of the concrete, but also with the diameter of the head. A good general rule is to use as large a diameter head as practicable, bearing in mind that vibrators with diameters in excess of 100 mm will probably require two men to handle them. Below this diameter, the appropriate head size will be dependent on the width of the formwork, the spacing of the reinforcement and the cover to it.
The frequency of a vibrator is the number of vibrations per second (Hz). In general, high- frequency vibrators are most suited to high-slump concrete and small maximum-sized aggregates, and low frequencies to low slumps and large maximum-sized aggregates. The amplitude is the maximum displacement of the head from its point of rest, measured in mm. It will be larger in air than in concrete which has a damping effect. As a general rule, high-amplitude vibrators are most suited to low-slump/large maximum-sized aggregate concrete and low amplitudes to high slumps and small maximum-sized aggregates.
Immersion vibrators should be inserted vertically into concrete, as quickly as possible, and then held stationary until air bubbles cease to rise to the surface, usually in 15–20 seconds Figure 9.3. The vibrator should then be slowly withdrawn and reinserted in a fresh position adjacent to the first. These movements should be repeated in a regular pattern until all the concrete has been compacted
Figure 9.4. Random insertions are likely to leave
areas of the concrete uncompacted. The vibrator should not be used to cause concrete to flow horizontally in the forms, as this can lead to segregation.
In deep sections such as walls, foundations and larger columns, the concrete should be placed in layers about 300 mm thick. The vibrator should penetrate about 150 mm into the previous layer of fresh concrete to meld the two layers together and avoid 'cold-pour' lines on the finished surface. In small columns where concreting is continuous, the vibrator may be slowly raised as the concrete is placed. However, care should be taken to ensure that the rate of placement is slow enough to allow the concrete to be fully compacted and the entrapped air able to reach the surface. Care should also be taken to avoid trapping air on the form face and a means of lighting the interior of the form while the concrete is being placed and vibrated should be provided.
The vibrator should not be allowed to touch the forms as this can cause 'burn' marks that will be reflected on the finished surface. Generally, the vibrator should be kept about 50 mm clear of the form face. Similarly, the vibrator should not be held against reinforcement as this may cause its displacement.
Chapter 9 Compaction
Figure 9.5 Compaction at stop ends and inclined forms
Figure 9.6 Compacting around void formers and encased beams
Figure 9.7 Typical screed vibrator
Stop-ends, joints and, especially, inclined forms are prone to trapping air. To minimise this tendency, the best technique is to place the concrete close to, but away from, the form and insert the immersion vibrator close to the leading edge of the concrete
forcing it to properly fill the corner Figure 9.5. Void-formers are prone to trapping air on their undersides if concrete is placed from both sides and then compacted. Concrete should be placed at one side and, maintaining a head, vibrated until it appears at the other side. (Note that the void- former will need to be fixed so as to resist the pressure of the concrete – sideways and vertical.) When the top of the concrete is fully visible from above, then placing can continue normally Figure