Vacuum Concrete: Techniques, Equipment, Advantages and Disadvantages

One solution to the problem of converting a sufficiently high workability to a mix with a minimum desired water-cement ratio is attained by Vaccum- Dewatering of Freshly Placed Concrete.

Concreting thin sections of slabs and walls will demand a mix of water cement ratio of 0.50 to 0.60. This is to facilitate proper placing and compaction. This mix will have less strength and abrasion resistance.
A treatment method that involves the removal of excess water and air by using suction is called as the vacuum treatment of concrete.

Fig.1. Vacuum Concrete Procedure

Vacuum Dewatering Concept

For the past 10 years, the method of vacuum dewatering is fairly used in the construction factory floors. The process is equipment oriented. It will require the following:

1. Formwork in the form of channels
2. Internal Vibrators
3. Screed Board
4. Vacuum Pump
5. Filter Pads
6. Disc Floater
7. Power Trowel

Fig.2: Vacuum Dewatering Process Schematic Representation

Vacuum Dewatered Concreting Procedure

A mix of medium workability is placed in the form, in the usual manner. As the fresh concrete consists of a continuous system of water-filled channels, the application of a vacuum to the surface of the concrete results in a large amount of water being extracted from a certain depth of the concrete.
Air bubbles are removed from the surface as they do not form a continuous system.
The final water-cement ratio before the concrete sets, thus reduces and as this ratio largely controls the strength, vacuum -dewatered concrete has the following properties:

•  Higher strength
•  Higher density 
•  Lower permeability
• Greater Durability
• High Abrasion Resistance

Fig.3: Vacuum Process In Detail

Fig.4: Step by step procedure in Vacuum Dewatering

The increase in strength due to the vacuum dewatering is directly proportional to the amount of water that is removed up to a critical value beyond which there is no significant increase. Hence it is advised not to have prolonged dewatering with an expectation o an increase in strength. This critical value mainly depends on the thickness of the concrete layer and the mix proportion.

Consideration in Concrete Dewatering

The vacuum is applied through the porous mats connected to the vacuum pump. The mats are placed on the fine filter pads.This will prevent the removal of cement together with the removal of water. The mats can be placed on the top of the concrete immediately after screeding and can also be incorporated in the inside faces of vertical forms.

Fig.5: Vacuum Concrete Accessories

Vacuum is created by the vacuum pump. Its capacity is governed by the perimeter of the mat and not on its area. The magnitude of the applied vacuum pump is usually about 0.08MPa. This vacuum reduces the water content by 20%.

The withdrawal of water will produce the settlement of the concrete by an amount of 3% of the depth over which the suction is applied. The rate of withdrawal of waterfalls off with time and it has been found that processing during 15 to 25 minutes is usually most economical.Beyond 30 minutes, there is a very little reduction of water content.
 

The vacuum treatment is not effective for water cement ratio less than 0.4. The suction pressure on the concrete is about 1/3rd the atmospheric pressure.

Advantages of Vacuum Dewatering

• It reduces the time for finishing the floor and stripping of the wall forms
• Increases the strength of the concrete. Compressive strength is increased by 10 to 50%
• Lowers the permeability
• Smooth and clean finish surface
• Early removal of formwork
• Increase the abrasion resistance
• Decrease the total shrinkage
• Provides good bond with the underlying concrete

Disadvantages of  Vacuum Dewatered Concrete

• The inherent porosity of the concrete allows water, oil, and grease to seep through, consequently weakening the concrete. 
• Joints are a necessity for concrete floors (to accommodate shrinkage, thermal movements etc) which can lead to joint breakage as well as seepage of the above contaminants. 
• Concrete floors generate dust due to abrasive movement of vehicles commonly found in industrial plants which can cause tangible and intangible damage to plant and machinery, sub-assemblies etc. 
•  The best-laid floor (Trimix) can have undulations of above 5mm (not normally visible to naked eye). While it is typically accepted in various engineering industries, it must be kept in mind that heavy movement of vehicles can wear out the surface faster

Also Read:

1.     Step by Step Inspection of Building Structures

2.     How to Repair Interior Plaster of Masonry Walls?

3.       How to Correct Hairline Cracks on a Concrete Surface?