Vacuum Concreting Process

• Vacuum concrete is the one from which water is removed by vacuum
  pressure after placement of concrete structural member.

• Vacuum concrete has high strength and durability than normal concrete.

• Vacuum concrete is the effective technique used to overcome this contradiction of opposite requirements of workability and high strength. With this technique both these are possible at the same time.

• In this technique, the excess water after placement and compaction of concrete is sucked out with the help of vacuum pumps.

• This technique is effectively used in industrial floors, parking lots and deck slabs of bridges etc.

• The magnitude of applied vacuum is usually about 0.08 MPa and the water content is reduced by upto 20-25%.

• The reduction is effective upto a depth of about 100 to 150 mm only.

The main aim of the technique is to extract extra water from concrete surface using vacuum dewatering. As a result of dewatering, there is a marked reduction in effective water-cement ratio and the performance of concrete improves drastically. The improvement is more on the surface where it is required the most.

Mainly, four components are required in vacuum dewatering of concrete, which are given below:

• Vacuum pump
• Water separator
• Filtering pad

• Screed board vibrator

• Vacuum pump is a small but strong pump of 5 to 10 HP. Water is extracted by vacuum and stored in the water separator.

• The mats are placed over fine filter pads, which prevent the removal of cement with water.
• Proper control on the magnitude of the water removed is equal to the contraction in total volume of concrete.

• About 3% reduction in concrete layer depth takes place.

• Filtering pad consists of rigid backing sheet, expanded metal, wire gauze or muslin cloth sheet.

• A rubber seal is also fitted around the filtering pad as shown in fig.1. Filtering pad should have minimum dimension of 90cm x 60cm.

• Below is the diagram:

Mass Concreting:

• In mass concrete, larger size aggregates (up to 150 mm maximum) and low slump (very stiff consistency) are used to reduce the amount of cement in the concrete mix (normally 5 bags per m3 of mass concrete).

• As the concrete is relatively dry and harsh, it needs immersion type of powder vibrators for full compaction.

• The concrete is normally placed in open forms. Due to the greater mass of the concrete, the heat of hydration (reaction between cement and water) may increase the temperature considerably.

• These can be avoided by placing the concrete in shorter lifts and taking gaps of several days before the next lift.

• During concreting, cold water should be circulated through the pipes buried in the concrete mass may also be useful. If possible, concreting can be done in the winter season to lower the peak temperature in concrete. Alternatively, the aggregates may be cooled before using in the mix.

• The high temperature due to the heat of hydration may result in an extensive and serious shrinkage in the mass concrete. The shrinkage cracks can be prevented by using low heat cement and by rapid curing of the concrete.

• The early age strength is very high compared to later strength concrete cured at normal temperatures. During setting and hardening the volume change of mass concrete is very small but it can produce larger creep at a later stage.