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case study on vacuum dewatering concrete in pdf

A high quality concrete floor or pavement requires not only to be level but it should also have high wear resistance, high compressive strength, reduced shrinkage and minimum water permeability.
Vacuum dewatered concrete, popularly know tremix in India, is developed by Tremix, Sweden. It is a system for laying high quality concrete floors with superior cost-effectiveness. The key to the use of this method is the dewatering of concrete by vacuum process. Immediately after placing and compaction, excess water is removed by vacuum dewatering process. Thus reducing water content, optimum water to cement ratio can be achieved. A lowered water : cement ratio automatically leads to a noticeable improvement in almost each of the concrete properties.

THE PROCESS OF VACUUM DEWATERED CONCRETE :

It is a vital part to execute all activities in right sequence to obtain good quality of concrete pavement.Initially, poker vibration is essential, especially at the panel edges. Use of needle vibrator ensures elimination of entrapped air and voids. One can not achieve levelled surface while doing compaction with needle vibrator.It is therefore essential to combine this vibration with surface vibration (screeding), in order to obtain a vibrated concrete with a levelled surface. Two passes with surface vibrator are normally recommended. The Surface Vibrator is guided by two men, standing on either side of the panel. Vacuum dewatering process is achieved by filter pad, suction mat and pump.. The process starts immediately after surface vibration.
After compaction by surface vibrator, filter pad are placed leaving all sides exposed about 4 “. The Top Cover is then placed on the filter pads and rolled out till it covers the strips of exposed concrete on all sides. The Top Cover is then connected to the vacuum pump through a suction hose and the pump is started. Vacuum is immediately created between the filter pads and the top cover. Atmospheric pressure compresses the concrete and the surplus water is squeezed out.Water content in the concrete is reduced by 15-20% during vacuum dewatering process.Usually it takes about 1.5 – 2 mins for de watering process for one centimetre thickness of slab.. This is the indication of concrete being properly de watered and ready for finishing.
The finishing operations –
Floating & Trowelling take place right after de watering. Floating operation is done with Floating disc. This ensures after mixing of sand & cement particles, further compaction and closing the pores on the surface. Floating operation generates skid-free finish. Trowelling is done with Trowelling blades in order to further improve the wear resistance, minimize dusting and obtain smoother finish.

ADVANTAGES OF USING VACUUM DE WATERING SYSTEM :

1.compressive strength increases about 55 %
2. Tensile strength increases by @ 70%
3. Cement consumption is reduced to the extent of 40%, no cement is required separately for finishing the surface.
4. Abrasion resistance of the floor increases by @ 60% resulting in less wear and rear of the floor surface
5. Shrinkage of concrete is reduced and floor wraps less.

VACUUM CONCRETE: TECHNIQUES, EQUIPMENTS & ADVANTAGES
Water-cement ratio is detrimental for concrete. We always try to restrict the water-cement ratio in order to achieve higher strength. The chemical reaction of cement with water requires a water-cement ratio of less than 0.38, whereas the adopted water-cement ratio is much more than that mainly because of the requirement of workability. Workability is also important for concrete, so it can be placed in the formwork easily without honeycombing.

After the requirement of workability is over, this excess water will eventually evaporate leaving capillary pores in the concrete. These pores result into high permeability and less strength in the concrete. Therefore, workability and high strength don’t go together as their requirements are contradictory to each other.

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.

Technique and Equipments for Vacuum Concrete:

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 gauge or muslin cloth sheet. A rubber seal is also fitted around the filtering pad

What are the disadvantages of vacuum dewatering concreting?