The amount of water mixed in concrete while making is not a necessity for the full hydration process. In Vacuum Concrete excess water is removed from fresh concrete to increases its strength before hardening takes place.
Vacuum Concrete is an equipment-oriented technique to extract the surplus water after placing a highly workable concrete mix with the help of the vacuum.
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What is Vacuum Concrete?
Vacuum concrete is a type of concrete in which excess water is removed to increase its strength parameter before a hardening start. This water is generally extracted by the use of vacuum mats connected to the vacuum pump.
Necessity of Vacuum Concrete:
The high water-cement ratio is detrimental to the quality of concrete.
The hydration of cement is sufficed by the water-cement ratio of 0.38. Water in excess of this amount adversely affects the quality of concrete in terms of strength. However, a low water-cement ratio reduces the workability of concrete and renders its handling and compaction difficult.
As a result, high strength with higher workability is generally difficult to achieve simultaneously. The vacuum concrete fulfils this contradicting demand of attaining higher strength for workable concrete.
History of Vacuum Concreting:
Vacuum concreting was first invented by Tremix AB, Sweden. Later it was patented in the United States and has been popular throughout Europe since then.
Almost half the concrete works in Sweden utilises this technique at present.
In India, this technique was first introduced by Aquarius Technologies in 1987.
- Increase in strength to about 25%
- Increase in density
- Decrease in permeability
- Increase in durability
- Increase in abrasion resistance
- Increase in impact resistance by 10%
Advantages of Vacuum Concrete:
- Higher workability can be achieved along with a higher strength of concrete.
- The final strength of concrete is escalated to about 25 % (Both compressive and tensile strength of concrete are elevated.)
- Vacuum concrete improves the bonding strength of concrete to about 20% which facilitates easy bonding with underlying concrete
- Formwork can be removed early since concrete stiffens rapidly. This enables its usage in other repetitive works. Additionally, this allows the structure to be put into use early.
- The curing period is reduced. 1 week curing period is reduced to 3 days and 4 weeks curing period can be brought down to 1 week.
- A Reduced water-cement ratio not only increases strength but also reduces the permeability of the concrete.
- The durability and density of vacuum concrete are both quite good, with a reduction in voids
- Final finishing of the floor of concrete can be achieved earlier
- Compaction is made easier and more effective
- The surface of vacuum concrete is pitting-free.
- The top layer has a very high resistance to abrasion, which makes it a suitable choice for the construction of structures in contact with water flowing at large velocities.
Challenges in Using Vacuum Concrete:
- Power is consumed for dewatering which adds on to the cost.
- Special equipment is required.
- The initial cost is much higher.
- Skilled laborers are required for dealing with the dewatering process.
- It is not much suitable for concrete with a water-cement ratio of less than 0.4.
Equipment Required for Producing Vacuum Concrete:
The following equipment’s required to produce vacuum concrete,
- Vacuum pump with hose
- Filtering pad
- Suction mat
- Double beam screed vibrator
- Power float
Mostly MS channels are used for formworks. They perform a twofold function as in stopper & providing support to screed vibrator.
2. Vacuum Pump:
It is small yet powerful. Normally, it is hosts electric motor of 5-10 horsepower.
3. Filtering Pad:
A filtering pad constitutes a backing piece and a rubber seal. The seal has wrapped all-round the periphery. Over this backing piece, an expanded metal sheet is placed, which a sheet of wire gauge covers.
The mat of minimum size 90 * 60 cm should be taken. Smaller mats are not effective in generating vacuum.
4. Suction Mat:
It consists of a large single PVC sheet. Nylon/muslin cloth is laid beneath to work as a fine filter. The top surface of mat houses a junction box and hose.
5. Power Trowel:
During the suction of water, capillaries are formed in concrete when water is forced out of it. These capillaries are detrimental to the concrete and hence should be corrected. Power trowel does exactly that. It fills up the cracks through vibrations.
6. Power Float:
It is used for grinding the surface. As a result, the surface attains wear resistance. It also helps in removing surface modulations so that the surface finishing can be achieved with ease.
7. Double Beam Screed Vibrator:
It is a lightweight electrically driven vibrator weighing 128.5 kg.
Read More: Self Compacting Concrete Mix Design
Top or Side Vacuum Processing?
Vacuum processing can be done from either the top of the side surface. There is not much difference in terms of efficiency. However, top processing gives slightly higher efficiency.
At least one surface should be left open to the atmosphere so that the pressure difference can be created, which will give rise to a vacuum. The suction pressure developed is nearly 1/3 of the atmospheric pressure.
Steps for Laying Vacuum Concrete:
1. A concrete mix of sufficient workability is prepared and laid in the formwork.
2. The surface of the laid concrete is compacted by a needle vibrator. Then, further compaction is done with the help of a double beam screed vibrator. Here, a smooth surface is obtained.
3. The vacuum dewatering is started immediately after compaction using vacuum equipment which includes
- Suction mat
- Filter pads
- Vacuum pump
4. Filter pads are rested upon the fresh concrete leaving 4” (inch) filter cover exposed on all sides
5. The top cover is placed on the filter pads covering the exposed concrete surface left by filter pads. This top cover is connected to a vacuum pump through the suction hose.
6. The vacuum pump is then started for about 15-25 min decided by the thickness of the concrete floor. As the pump operates, a vacuum is immediately created within the boundary of the mat and top cover.
The time for pumping is usually taken 15-25 min from an economic point of view. Pumping beyond 30 min causes an insignificant reduction in water content, hence should be avoided.
7. The atmospheric pressure compresses the concrete and the surplus water is suctioned out from the concrete through fine wire gauge/muslin cloth giving the following results:
- Water content lowered by 15-25% of the original water
- Concrete layer depth decreased by 3%
8. Dewatering takes 1.5-2 min/cm of floor thickness. This dewatered concrete is compacted & dried until the concrete does not leave footprints if someone walks over it. This dried concrete implies that
- Concrete is dewatered adequately
- Concrete is ready for finishing
9. The concrete surface is hardened and smoothened with the help of a power trowel & power floater after skim floating it. These re-vibrate and assists in removing the capillaries or channels formed during suction of water.
10. Surface hardeners can also be used as the dewatered surface provides ideal conditions to receive powdered hardeners. This adds to the abrasion resistance of the concrete.
All the above steps are listed out in the flow chart below:
Synopsis of Procedure
Importance of Vibration Post-vacuuming:
Generally, the rich mix (higher cement content)can be observed near the processing surface. This may be because, at the time of pumping out the excess water, cement is also displaced and gets deposited near the surface. Higher cement enhances the strength of the top layer near the surface.
Not only this, but the water-cement ratio is also lower than the one taken. Lower water also provides increased strength to the top layer.
Both the above facts state higher strength of the top layer. This means the concrete as a whole, is not possessing uniform strength.
Vibration post-vacuuming ensures that the cement and water are uniformly distributed throughout the concrete.
If vibration is not done, the increased strength will not be attained with respect to the decreased water content through pumping.
How Much Water should be Extracted?
The rate of water to be taken out depends upon the following factors:
Workability of the mix: Workability-free water directs the amount of water to be removed. This reduces the otherwise higher water-cement ratio.
Maximum size of aggregate: If the water-cement ratio is optimized, and cannot be reduced further, then to extract the water, either of the following is followed:
- The maximum size of aggregates increases.
- The amount of fines in the mix is reduced.
Application of Vacuum Concrete:
- Large scale production of precast PCC & RCC units in the factory
- Construction of horizontal & sloping concrete slabs
- Workshops & warehouse construction
- Railway platforms
- Airport pavements & Docks
- Thin load-bearing walls
- Partition walls
- Parking lots & Basements
- Resurfacing & repair of road pavement (as it bonds well with the previously laid concrete)
- Hydropower plants
Vacuum concreting is the technique through which excess water to compensate for workability is extracted out to enhance the strength.
This reduction in the water-cement ratio not only enhances the strength of concrete but also increases durability, density, abrasion, and bonding strength. Lesser water enables to formwork to be removed earlier. Compaction can also be achieved effectively for concrete having a water-cement ratio of more than 0.4.
It has wide applications, especially where higher workability is essential, as in slabs.
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