Vacuum Concrete is the type of concrete where excess water is removed by vacuum pressure after the concrete pressure in the structure. Water cement ratio is always crucial for concrete, therefore, we always enforce the proportional value of water and cement to achieve higher strength.
The chemical process of cement with water requires a wc ratio of less than 0.38, whereas the adopted wc ratio is much higher because of the workability requirements.
Vacuum Concrete Procedure.
After the placement of concrete, the excess water will eventually evaporate, leaving capillary pores in the concrete. These pores result in high porosity and less durability of concrete. It proves that workability and high strength are opposite to each other. By using vacuuming technique we can achieve it at the same time. Vacuuming method is used in industrial floors, parking lots and deck slabs of bridges.
The scale of applied vacuum is generally about 0.08 MPA and the water content is reduced by 20-25%. The absorption is effective at the depth of 100-150 mm only.
Equipment Used in concrete vacuuming procedure
The purpose of concrete vacuuming is to remove extra water from the concrete surface by vacuum dewatering. This significant reduction in water cement ratio improves the concrete strength and durability. Generally, four equipments are required in the vacuum dewatering process of concrete are listed below:
- Screed Board Vibrator
- Vacuum Pump
- Filtering Pad
- Water Separator
Vacuum pump is a relatively small, but strong pump of 5 to 10 HP. Water is removed by vacuum and stored in a water separator. The mats are placed over the fine filter pads, this minimizes the loss of cement with water. During this process, around 3% of contraction in concrete depth takes place.
Filtering pad comprises rigid backing sheet, expanded metal, wire gauze or muslin cloth sheet, a rubber seal is also fitted around the filtering pad as shown in the picture below. Filter pad has dimensions of 90cm x 60 cm.
Advantages of Vacuum Concrete
Vacuum Concrete has a number of advantages over normal concrete. Some of the advantages are given below.
- The final strength (28 days) of concrete is increased by about 25%.
- Sufficient decrease in the porosity of concrete is sufficiently decreased.
- Vacuum concrete hardens very rapidly so that the formworks can be removed within 30 minutes of casting even on columns of 20 ft. high.
- This is of significant economic value, particularly in a precast factory as the forms can be reused at frequent intervals.
- The bond strength of vacuum concrete is about 20% higher.
- The density of vacuum concrete is higher as compare to the normal concrete.
- The surface of vacuum concrete is entirely free from pitting and the uppermost 1/16 inch is highly resistant to abrasion. These Properties are of special importance in the construction of concrete structures which are to be in contact with flowing water at a high velocity.
- It bonds well to old concrete and can, therefore, be used for resurfacing road slabs and other repair works.