VACUUM CONCRETE:
Vacuum concrete is that from which the water is removed by vacuum pressure after the placement of the concrete structural elements.
In plastering thin sections such as slabs and walls, it is convenient to place and compress a liquid mixture with a water-cement ratio of 0.50 to 0.65. Such a blend leads to relatively low strength and poor corrosion resistance.
In such cases, vacuum treatment of concrete may include removing excess water and air by suction. The concrete vacuum system using suction through the surface layer connected to the vacuum pump is shown in Figure 14.2.
The duration of treatment depends on the water-cement ratio and the amount of water to be removed. For slabs that vary in thickness from 25mm to 125mm, it usually lasts 1 to 15 minutes. The treatment effect is initially more pronounced and falls faster.
Therefore, there is no benefit to extending treatment periods beyond these values. A sequence of operations for vacuum dehydration.
The concrete is illustrated in Figure 14.3.
Vacuum treatment for water-cement ratios less than 0.4 may not be effective. Absorption pressure on concrete is one-third of atmospheric pressure. The vibration of the concrete before the vacuum treatment helps the process. The application of vibration simultaneously with vacuum treatment after initial vibration is very effective.
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Persistent vibration beyond the 90s can damage the concrete structure and therefore the vibrations must be stopped beyond this period and only apply the vacuum for the rest of the treatment period.
Vacuum treatment has been found to significantly reduce the time required for the final finishing of the floor and removal of all forms. The strength of the concrete and its resistance to wear and corrosion increases and the total shrinkage decreases. Vacuum-treated concrete provides good bonding with the underlying concrete.
It achieves its compressive strength of 28 days in 10 days and has a 25 percent higher crush strength. The details of the improvement of the structural properties are:
1. Compressive strength of the floor increases by up to 60 percent.
2. Tensile strength increases approximately 70 percent.
3. Cement consumption is reduced by up to 40 percent; no cement is required separately for surface finish.
4. The abrasion resistance of the floor increases by almost 60 percent.
5. Shrinkage of concrete and floor wrap is reduced.
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Vacuum processed concrete is widely used for factory production of simple and reinforced concrete precast units.
The other important application is in the construction of horizontal and inclined concrete slabs, such as floor slabs, road and airport pavements, thin load-bearing walls, and partitions.
Vacuum treatment can also be used effectively in road pavement resurfacing and repair.