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Vibration is one of the methods used to consolidate poured concrete to achieve required density and, subsequently, the designated concrete strength. Vibrators (internal and external or shutter vibrator) need to be used properly and skillfully; otherwise, the concrete may get over vibrated or under vibrated; compromising the concrete quality in both cases.
That is why it is necessary to consider the practical tips obtained from construction standards such as ACI 309R-5 and field experiences for the correct utilization of vibration equipment and achieving vibration goals.
Practical Tips for Using Concrete Vibrators in the Field
- Poker, immersion, or needle vibrator are the most common names for internal vibrators. The head of the internal vibrator is submerged in concrete to compact it with a vibratory pulse.
- Use a small head to compact concrete in shallow forms or closely spaced bars and, larger head vibrator to consolidate concrete poured in wide forms and steel bars with large spacing.
- The influencing radius of the vibrator head is equal to four times its head diameter. So, a smaller head vibrator needs to be inserted into concrete at smaller intervals than that for a large vibrator head.
- Determine influence radius in the field by noting how far the bubbles from the concrete escape from the vibrator head. Alternatively, use an approximate distance of 1-1.5 times the radius of influence.
- Use either square or offset pattern for vibrator immersion into concrete; see Figure-1 and Figure-2. However, use one vibration pattern throughout the work.
- In the former case, Insert the vibrator instrument with a long square grid pattern to ensure 1/3 of the influencing radius overlapping.
- For offset pattern, employ gride pattern but locate vibrator head in a zig-zag manner.
- Do not operate the vibrator in the air; make sure to turn it on when its tip is in concrete to prevent overheating of the instrument and subsequent damage.
- Insert vibrator head into concrete vertically or nearly vertical, and do not incline vibrator excessively; otherwise, it will get damaged.
- Vertical vibrators facilitate air bubble escape and reduce voids.
- Do not enforce vibrator into concrete since it may be obstructed by reinforcement bars. Instead, let the vibrator penetrate the concrete under its self-weight.
- Avoid hitting reinforcement with the vibrator head since it can break the bond between steel bars and previous layers of stiffened concrete.
- Maintain vibrator head in concrete for 15-20 seconds each time. However, workers with enough experience with vibrators, concrete mix, and formwork can consolidate concrete properly regardless of the durationof vibration.
- Withdraw vibrator slowly at around 2.5-7.5 cm/s; minimum range frequently provides the best result.
- The concrete should fill the hole created after vibrator withdrawal. However, the gap is not filled by concrete in the dry concrete mix. Reinsert vibrator into concrete within a one-half radius of influence to solve the problem. If the issue persists, change either the concrete mixture or the vibrator.
- Maintain a 7-10 cm distance between the formwork edge and a vibrator head to avoid formwork damage.
- Do not use vibrators to move concrete.
- Avoid over vibration to prevent segregation, and inspect forms throughout the work for any leakage.
- Place concrete in evenly dispersed layers with a thickness equal to the length of the vibrator head plus 15 cm.
- The thickness of concrete should not exceed 45-50 cm, such as in the case of large mats and pedestals; otherwise, the weight of concrete prevents the release of entrapped air to the surface.
- When concrete is placed in layers, extend the vibrator 10-15 cm into the previous layer, and move the vibrator up and down for 5-15 seconds to improve interlayer bonding.
- Use adequate vibrator number to cope with concrete pouring rate.
- Continue vibration until concrete levels in the form, coarse aggregate particles become embedded, a thin film of mortar forms on the top surface and along with the formwork faces, and the emergence of air bubbles from concrete stops.
- The vibrator operator should see the concrete surface. So, use lighting if necessary.
- The frequency of the vibrator decreases when it is initially submerged in concrete, then increases, and finally becomes constant when air bubbles escape.
- Have a spare vibrator at the site. Use it when one of the vibrators fails.
- Workers should be instructed to avoid vibration for concrete that has already been vibrated.
- Clean all vibrator parts after each application.
- External vibrators are suitable for constructing precast concrete products and thin walls. They are effective for a maximum depth of 75 cm (18 in).
- Shutter or form vibrators are desired for zero slump concrete and supplement internal vibrators.
- Ensure good support for formworks to avoid failures due to external vibrator application.
- The formwork should withstand pressure from liquified concrete and vibrators. In addition, it should be able to transfer vibration force to a considerable distance.
- Low frequency, high amplitude vibrators affect formwork greatly compared with high frequency, low amplitude vibrators. So, make sure to have extremely strong formwork, such as steel, when low frequency, high amplitude vibrators are used.
- Distribute shutter vibrator to ensure even distribution of vibration force. Table-1 can be used as a guideline to specify the spacing between form vibrators.
- Use your hand or vibrator over the formwork to determine the limit of vibrator effect and determine suitable spacing to distribute vibration force evenly. This avoids areas of over vibration or areas of under vibration.
- Do not attach vibrators directly to the formwork because it can lead to formwork failure.
- Do not turn on the external vibrator until concrete depth in the formwork reaches 15 cm.
- Typically, external vibrators are operated for two minutes. After that, the duration can be increased or decreased based on requirements.
- Stop vibration when concrete levels in the form, coarse aggregate particles become embedded, a thin film of mortar forms on the top surface and along with the formwork faces, and the emergence of air bubbles from concrete stops.
Table-1: External Vibrator Spacing for Consolidation of Concrete
|Concrete consistency||Concrete slump, mm (inch)||Distance between external vibrators, m (foot)|
|Very stiff or stiff concrete||< 25 (1)||1.524 (5)|
|Stiff plastic concrete||25 – 50 (1 - 2)||1.828 (6)|
|Plastic concrete||50 – 127 (2 – 5)||2.134 (7)|
|Flowing concrete||> 127 mm (5)||2.438 (8)|
Concrete consolidation is the process of reducing the volume of entrapped air, voids, and pockets of air in a freshly placed cementitious mixture, typically using manual or mechanical compaction methods.
Determine influence radius in the field by noting how far from vibrator head bubbles escape from concrete. Alternatively, use an approximate distance of 1-1.5 times the radius of influence.
Operating poker or an internal vibrator in the air would cause overheating and subsequent instrument failure.
Maintain a 7-10 cm distance between the formwork edge and a vibrator head to avoid formwork damage.
Continue vibration until concrete levels in the form, coarse aggregate particles become embedded, a thin film of mortar forms on the top surface and along with the formwork faces, and the emergence of air bubbles from concrete stops.
If the concrete is over vibrated, it loses its consistency and suffers from segregation. Aggregates would settle to the bottom of the formwork, and cement paste would raise to the top of the element. As a result, the concrete strength reduces to honeycomb.