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Vacuum Dewatered Flooring

Vacuum Dewatered Flooring, also known as VDF, is a special type of Flooring Technique to achieve High Strength, Durability, Longer Life, Better Finish and Faster Work. This type of floor is suitable for high abrasion & heavy traffic movement. The Vacuum Dewatered Flooring or VDF Flooring is a system for laying high quality concrete floors where the key is Dewatering of Concrete by Vacuum Process wherein surplus water from the concrete is removed immediately after placing and vibration, thereby reducing the water: cement ratio to the optimum level. Reduced water: cement ratio automatically leads to a noticeable improvement in almost each of the concrete properties.

VDF offers a solution to the problem of combining high workability with a minimum Water / Cement Ratio in case of freshly placed concrete. This is a process where excess water that is available for the workability is extracted from a certain depth of concrete by some mechanical means. The final water / Cement ratio before the concrete sets is thus reduced and as a result, controls the strength. It also imparts higher density, lower permeability, and greater durability with higher resistance to abrasion to the concrete placed with proper care and discipline.

The Process

In this system of VDF Flooring or Vacuum Dewatered Flooring, concrete is poured in place & vibrated with a poker vibrator especially to the sides of the panel for floor thickness more than 100mm. Then surface vibration is done using double beam screed vibrator running over the surface, supported on channel shuttering spaced 4.0 meters apart. The screed vibrator is run twice to achieve optimum compaction & levelling. The vibrated surface is then levelled using a straight edge.

After this a system of lower mats & top mat is laid on the green concrete & this is attached to a vacuum pump. This draws out surplus water if any. The concrete is left to stiffen. When the base concrete has stiffened to the point when light foot traffic leaves an imprint of about 3-6 mm, Floor Hardener, if specified, is applied at this stage evenly at rate of between 3-5 Kg per sqm. ? Any bleed water should now have evaporated, but the concrete should have a wet sheen.? ?The concrete is then further compacted and levelled using Power Floater followed by finishing as per the requirement using Power Trowel.

Vacuum processing improves compressive strength and how vacuum affect the top and bottom part of the slab respectively. Note that after just seven days the compressive strength value of vacuum dewatered concrete will be the same as that of normal concrete after 28 days.

Proper Care

Production of a quality concrete slab requires proper techniques and adequate planning. The following key areas have been identified as the most critical to achieving a satisfactory result.

Subgrade- The subgrade must be properly compacted and drained in order to give the bearing support assumed in design. Without support, the slab has little chance of supporting design loads. Maximum deflection under a fully loaded ready mix truck should be 13 mm.

Vapour Barriers- are normally provided to minimise moisture transmission. The selection of vapour barrier is based on the moisture sensitivity of the floor covering to be installed. Vapour barriers can be placed directly beneath and in contact with the concrete slab, or a layer of granular fill may be installed between the concrete and the barrier. Direct placement of concrete on top of a vapour barrier can contribute to problems such as plastic and drying shrinkage cracking and curling. 

Admixtures- use of suitable admixture for enhancing specific properties depending on the functionality should be carefully chosen and decided. Testing should be performed to determine how the admixture(s) will affect the properties of the concrete given the specified job materials, as well as the anticipated conditions and construction procedures. Often, admixtures are used in combination with one another. A second admixture can significantly affect the dosage requirement of both admixtures. Preliminary tests are recommended to ensure the optimum dosage of each admixture. Successful placements onsite will verify proper workability, finishability and setting time. It is important to consult the manufacturer for specific material handling, dosage and application instructions.

Synthetic Fibres- Synthetic fibres for use in concrete floors increase the cohesiveness of concrete and should meet the requirements specified. The most widely used synthetic fibres are polypropylene and nylon, although other types are available. Fibres are generally available in both fibrillated and monofilament forms. Structural macro-fibres (larger coarse fibres) can be used to provide equivalent post-crack strength to conventional reinforcement depending on the specific fibre dosage (see page 8 of this guide for further information). Synthetic macro fibres can also be used to reduce plastic shrinkage cracks as well as minimizing drying shrinkage cracks when used in a low shrinkage mix (0.04% @ 28 days or less). Synthetic micro-fibres, polypropylene and nylon, are most widely used to reduce the formation of plastic shrinkage cracks and to hold cracks tight.

Placing Sequence- In many cases, the most efficient way to place concrete in large areas is in long alternating strips. Strip placements allow superior access to the sections being placed and most suitable in case of VDF.

Bleeding- Excessive bleeding is a major deterrent to achieving quality slab surfaces always. Though in case of VDF this is taken care of by the very process, care should be taken in respect of proper concrete mix proportioning, the use of a well graded aggregate, controlled vibration and slump, concrete temperature control help. Collected bleed water should be removed before the start of finishing operations or the application of dry shake surface hardeners or toppings.

Evaporation-Rapid evaporation and moisture loss can result in plastic shrinkage cracking in the slab surface. This undesirable appearance can be minimized or prevented by dampening the subgrade, cooling water and aggregates, using windbreaks or sunshades, eliminating vapour barriers, using fog sprays, and/or treating the concrete with a monomolecular film just after floating.

Typical Application Areas

• Warehouses, Store houses, Factories
• Roads, Sports Courts
• Cellars, Parking Areas
• Manufacturing Areas,
• As the base floor for special finish Floorings

Benefits & Features

• Increased Compressive strength by up to 60%  
• Reduced Cement consumption by 40% as no cement is required separately for finishing. 
• Increased Abrasion resistance by @ 60%
• Reduced Shrinkage.
• Minimizing dusting, crack formation
• Uniform homogeneous floor  with High flatness accuracy
• Increased wear resistance
• Earlier utilization and Reduced maintenance cost
• Lower water permeability due to increased density.
• Minimizes dry shrinkage and plastic shrinkage
• Controlled working Cycle
• Earlier start of finishing operation
• Fewer forms faster reutilization
• High early strength minimizes damage on newly cast floors

VDF- Step by Step

1. Clean the location free from dust and other unwanted material. If required floor wash to be carried out to ensure proper bonding between the concrete and the surface to receive the concrete.
2. Provide a suitable type of Vapour Barrier if required as per specification.
3. Mark the area longitudinally so as to place the end channels at a spacing not to exceed 4 meters.
4. Provide Reinforcement (Mesh or In-situ fabrication) with provision for dowel bars. These dowel bars are required as alternate panels are cast.
5. Place end channels at about 4 meters apart, well anchored avoiding any movement.
6. Place Concrete as per the design mix preferably through a transit mixer.
7. Spread and Compact properly using needle vibrators.
8. Run the screed vibrator spanned across the two end channels twice (Figure 3). Double Beam Screed Vibrator- used for levelling as well as compaction of green concrete it consists of high quality steel bar (4.2meter) with of 250 spacing MM in between. Our special water protective vibrator motor is mounted in the centre which produces 1830 N centrifugal force which is most ideal for compaction of green concrete. These vibrators are also available in different sizes from 2 meter to 5.5 meter.

9. Spread the Vacuum mat system. The vacuum is applied through porous mats connected to a vacuum pump. The mats are placed on fine filter pads which are provided to prevent the removal of cement together with the water. The mats can be placed on the concrete immediately after screeding, and can also be incorporated in the inside faces of vertical forms. The vacuum applied is normally of the magnitude 0.08 Mpa. This vacuum reduces the water content usually by about 20%.  The reduction is greater near to the mat. Care should be taken proper trained staff operates and applies the correct pressure.It is normally observed that whenever a

   vacuum pressure is applied, it compress the concrete sufficiently so that it is necessary to place extra concrete to compensate. Usually the slab gets compressed about 2 percent. This means that for a 6-inch-thick slab the screeds must be set high by 1/8 inch.

   Suction Mat & Top Mat, which is the most important part of the vacuum de watering system. Suction mat is also known as the Filter Mat and comprises of a filter cloth on a wire mesh. The filter mat ensures that the cement fines are not sucked at the time of the vacuum process. These are available in size of 35 sq     meters total. The top mat is only placed on top of the filter and work for sealing the vacuum.

10. Extract excess water from concrete using vacuum pump-Vacuum pump, Filter mat and Top mat are important components in the VDF process. They help to extract the excess water as described above from the concrete matrix. The excess water thus sucked is diverted ensuring not to get remixed with the concrete that is already placed.
11. Allow concrete to initially set. Start finishing operation using mechanical floaters. (Figure 6)- Power Trowel that is surface finishing equipment that polishes the surface after floating operation and 3 to 4 passes are required for giving fine finish to the floor. Power Floater, which is surface grinding equipment powered by 3 H.P. Electric motor through gear box  with floating RPM of 130. The Power Floater offered by us grinds the surface to make it wear resistance.
12. Edges to be finished manually or using smaller discs.
13. Avoid damage to the finished floor.
14. Cure the concrete
15. Cut the groves as specified and fill the sealant.
16. Protect the concrete till the adjacent panel is cast.

The Operation

In order to obtain a high quality concrete floor using this method, it is essential to follow the various operations in the correct sequence. Initially, poker vibration is essential, especially at the panel edges. This results in proper compaction of the concrete and hence elimination of voids and entrapped air. Poker vibration never really gives a levelled surface. 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 removes surplus water always present in the concrete. This is done using the Vacuum Equipment comprising of Suction Mat Top Cover, Filter pads and Vacuum Pump. The process starts immediately after surface vibration. Filter pads are placed on the fresh concrete leaving about 4 inches of fresh concrete exposed on all sides. 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. This process lowers the water content in the concrete by 15-25%. The dewatering operation takes approximately 1.5 - 2 minutes per centimetre thickness of the floor. The dewatered concrete is compacted and dried to such an extent that it is possible to walk on it without leaving any foot prints. This is the indication of concrete being properly dewatered and ready for finishing. The finishing operations - Floating & Trowelling take place right after dewatering. 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. Repeated passes with disc and blades improve the wear resistance substantially.

Factors affecting VDF

Fresh concrete contains a system of water filled channels and the application of a vacuum to the fresh concrete surface results in water being extracted from a certain depth of the concrete, especially near to the top surface. This water is referred to as ‘water of workability’. The final water / cement ratio at the surface is thus reduced and as this ratio largely controls the strength of the concrete and a higher strength will be obtained. However it must be noted that some of the water extracted leaves voids and as such the theoretical advantage of removing the water may not be fully achieved in practice. It is also observed in practice that if Vacuum Dewatering process is applied over a 25 minute period, it can reduce the water content by 20% but is only really effective at depths of 100 – 150mm. 

Although there are several advantages using Vacuum Dewatering process on concrete floors resulting in increased strengths, increased density and assists increase abrasion resistance, there are some factors that affect the the entire process. However, while designing an effective VDF, following factors are to be considered.

• The withdrawal of water produces settlement of the concrete, possibly up to 3%. This can be topped up with the application of a dry shake but with little bleed water at the surface there is a risk of subsequent delamination.
• In practice the VDF process produces voids in the concrete and it has been found that with the same water / cement ratio, ordinary concrete has been found to have a somewhat higher strength than VDF concrete.
• VDF concrete stiffens very quickly. This is acceptable in cold climatic conditions but leaves the window of workability very short in hotter climates.
• Some of the finer materials are removed with the VDF process and fine sands and cement contents of greater than 350 Kg / m should be avoided.

Groove Cutting

Concrete expands and contracts constantly with changes in the temperature, the moisture content of the air and due to drying of cement which results in shrinkage.? The movements result in stress that can cause cracks in the concrete and destabilization of the base.

Uncontrolled cracking can cause an uneven surface, which is subject to increased wear over time and water seepage, which can damage the substrate. ?Though cracking is almost impossible to prevent, it can be controlled.? The use of specific types of control joints helps accommodate the movement of the concrete and avoid long-term damage.

Groove Cutting is cutting the laid concrete providing grooves of required size within 48 hours of placing of the concrete to form bays of 4Mtrs X 4Mtrs using heavy duty cutting machine with diamond cutting wheel and filling of the grooves with appropriate sealant.?

Making Grooves is one such method to insert contraction joints into slabs to guide cracks along a predetermined line. The purpose of the Groove Cutting is to weaken the slab along the approved line so that the slab cracks there instead of somewhere else. In order to be effective, the contraction joint must be at least one quarter as deep as the slab is thick.

??In case of heavy load traffic, a Load Transfer mechanism is used. Load transfer devices prevent the cement slabs from shifting under heavy loads. Shifting can cause uneven slabs and breakdown in the joints. Dowels, load plates, or slots can be embedded into the concrete to act as load transfer devices. These devices are laid perpendicular to the construction joint, extending into both slabs across the joint. Their purpose is to distribute the load evenly between slabs thereby protecting the concrete along the joints.

Joint Sealants

Joint Fillers and Sealants are hard, semi-rigid materials typically used to fill the construction joints in concrete floors. The joint filler transfers the load of heavy wheeled traffic across the joint, protecting the joint edges from damage.

Joint sealants are flexible materials that expand and contract along with the concrete. Their main purpose is to prevent water, other liquids, and debris from entering the joint. They can also improve the appearance of a floor. Most sealants are available in colours and can make the joints “disappear”. Sealants are one or two-component polyurethanes and should not be used on floors that will receive heavy traffic or loads because sealants will not support and protect the joint edges.

Dry shake floor hardeners come in mineral aggregate and metallic varieties. The selection of a dry shake hardener is dependent on the specific solution intended for the particular application. Floor hardeners provide a dense, tough surface capable of withstanding the abrasion and impact loading seen by floor slabs in a wide range of commercial, industrial and manufacturing facilities. Dry shake hardeners provide 2 to 8 times the abrasion resistance of plain, cured concrete. Most manufacturers offer hardeners in a range of colours.

Special Finishes

Metallic floor hardeners are formulated with graded, non-oxidizing or oxidizing metallic aggregate in a high strength cementitious binder. Mineral hardeners contain a mixture of well graded, non-metallic aggregates, plasticizer and cement binder. Both are recommended for use in either interior or exterior applications where a hard, long wearing, heavy duty floor is required. Metallic hardeners should not be applied to concrete with intentionally added chloride.

Non-oxidizing metallic and mineral aggregates come in a light reflective version designed to increase reflectivity to improve lighting levels. In combination with providing increased abrasion resistance, this type of dry shake floor hardener can boost reflectivity in excess of 60%. Benefits can include lower electrical requirements and fewer light fixtures resulting in significant cost savings.

The use of embedded mineral or metallic hardeners is usually intended for industrial floors exposed to moderate or heavy traffic. In some cases, floor hardeners are applied where impact resistance is required. The manufacturer’s recommendations should be followed along with the procedures.

Accordingly, dry shake floor hardeners should be embedded near the top surface of the slab to obtain the required surface hardness, toughness and impact resistance. It is recommended that the total air content of normal weight concrete should not exceed 3% except when service conditions expose the concrete to freeze/thaw cycling and the slab is not hard-trowel finished. Consult the manufacturer and ACI for specific installation guidelines since these vary slightly between metallic and mineral hardeners due to differences in their properties. In general, it is advised that good construction practices be followed. This includes obtaining flat and level surfaces and joints, using proposed mix proportions when installing test panels or placements and making any required adjustments at that time. A concrete mix design review should be conducted prior to installation of a dry shake floor hardener with all appropriate parties present. Issues such as air content, use of blended cements, concrete admixtures, and placing and finishing techniques should be discussed and agreed upon before the project begins.