Vacuum Concrete

Vacuum ConcreteNikhil MathewDixit MakwanaSuhel MansuriA Brief IdeaA high quality concrete floor or pavement having high compressive strength, high wear-resistance, reduced shrinkage and minimum water permeability.facilitates use of concrete with better workability to enable it to be placed in complicated molds or around extensive environment. water extracted through mats placed in contact with the concrete; such that only water, and no cement, or fines (out of the aggregates) can be sucked from the concrete by the vacuum pump.Recent use of steam creates low pressure, pulls air from concrete in a sealed container; stronger concrete due to absence of air voids

Timeline of Vacuum concreteNot a recent phenomenon, concept of removing excess amount of water from concrete i.e. dewatering has been known since ages. The Romans developed a method of dewatering concrete: they stuck brick and lava chips into it; the chips protruded above the surface and acted as wicks to draw water from the concrete. In the USA, dewatering of concrete by vacuum was patented in 1935 by K. P. Billner and all subsequent development has been based on his extensive ideas. His method was used in the 1940s and early 1950s, with the last known practical application in the United States being in 1965.Vacuum dewatering got a new lease on life mainly after two or three Scandinavian firms succeeded in simplifying the equipment enough to make it practical for almost any builder.Why Vacuum Concrete ?Manages both the w/c ratio as well as workability characteristics properlyovercomes the contradiction of opposite requirements ofworkability and high strength.

No honeycombing due to adequate workabilityIn untreated concrete, excess water will eventually evaporate leaving capillary pores in the concrete. These pores result into high permeabilityandlessstrengthintheconcrete.Vacuum concrete is majorly used in slabs-on- grounds as an alternative to waiting for the 28 day period to attain strengthOther than slabs, vacuum concreting can also be used in vertical surfaces and on forms for precast concrete.

Comparison b/wnon treated & vacuum concreteThe effects of vacuum dewatering on both fresh and hardened concrete properties were investigated through tests on plain and processed specimens prepared under outdoor conditions in the experimental study presented.

1. Concrete slab, 200 600 20 cm in size (no vacuum), control concrete (CC);2. Concrete slab, 300 400 20 cm in size (18 min. vacuum applying), half vacuum concrete (HVC);3. Concrete slab, 300 400 20 cm in size (35 min. vacuum applying), vacuum concrete (VC).

Strength gain in the top and bottom parts of a testslab. Data are from tests at Hannover Technical University,West Germany

Preparation of Vacuum Concrete:Billner MethodMainly four components are required in vacuum dewatering of concrete,(1) Vacuum pump (5 to 10 HP)(2) Waterseparator(3) Filteringpad(4) Screedboardvibrator

Part of the concrete surface sealed with a rigid plate or a flexible mat below which vacuum are created in the concrete slab.Atmospheric pressure will then act on the plate or mat with a considerable force, about 1,600 to 2,000 pounds per square foot (8,000 to 10,000 kilograms force per square meter) at the vacuum generally used. The concrete is compressed and part of the surplus water is squeezed out and sucked away from the concrete surface by the vacuum pump

Vacuum mats to be arranged to begin dewatering of the concrete soon after it is placed. (preferably within 30 minutes)Waterremovedequalto the contraction in total volume of concrete. About 3% reduction in concrete layer depthtakesplace.Next to the concrete surface is a filter padconsisting of rigid backing sheet, expanded metal, wire gauge ormuslin cloth sheet, which prevents the finest particles of the concrete from following the extracted water. .A rubber seal is also fitted around the filtering pad of minimum dimension 90 cm x 60 cm.On top of the cloth is a shallow air chamber(some type of net between the filter cloth and the airtight top cover or plate). The mat or plate projects slightly beyond the filter cloth on all sides and thus forms a seal against the wet concrete. The top cover is connected to the vacuum pump.

the water squeezed out of the concrete passes through the filter cloth into the space below the top cover, moves toward the suction inlet and is sucked away to the vacuum pump. Water isextracted by vacuumand storedin thewater separator.vacuum cant remove water needed for hydration because the capillary diameters in the cement paste decrease as the water cement ratio decreases. Thus there is no danger that the water-cement ratio will fall below 0.30, which is well above the minimum of 0.20 required for hydration of cement.vacuum processing to continue for 1 to 3 minutes per 25 mm of slab thickness.effectiveness of water removal decreases with slab depth. For practical purposes the water- cement ratio of the concrete can be reduced to a depth of about 12 inches (30 centimeters) or in some cases 16 inches (40 centimeters).

Removing water by the vacuum process reduces a slabs thickness. a 150 mm slab cast with concrete having a 150 mm slump will be reduced in thickness 3 to 6 mm thickness. If the slump is 90 mm or less, the thickness reduction is usually less than 3 mm. when necessary, the thickness of the slab as placed should be increased to obtain the full design thickness after dewateringWhen a concrete slab is vibrated there is some relocation of its component materials. Coarse aggregate sinks toward the bottom of the slab; the fines, cement and water move toward the surface. Well - designed mixes have much less tendency than others to undergo such segregationTremix MethodPioneered by TREMIX AB, SWEDEN and introduced by Aquarius in India in 1987Used for laying high quality concrete floors with superior cost-effectiveness. 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.

Screeding being done to get a levelled surface

Advantagesboth workability & high strength are achieved simultaneously.enhances the wear resistance of concrete surface. An improvement of the wear resistance by up to 2-3 times can be achieved through floating and repeated power trowellingReduction in w/c ratio may increase the compressive strength by 10-50 % and lowers the permeability. After seven days, the compressive strength value of vacuum dewatered concrete will bethe same as that for normal concrete after 28 days24-hour-strength of the vacuum dewatered concrete slab is much higher, which reduces the risk of damage on a newly cast floor. The final strength is about 50 % higher than that of a conventional concrete floorHighest compressive strength value for a vacuum dewatered slab is on the top surface i.e. a part of the floor that will be exposed toa lot of stress. A vacuum treated concrete slab shows no curling because the slightest shrinkage will be on the top part of the slab where the water/cement ratio is lowest. Therefore, the edges will remain in their correct position

The surface obtained after vacuum dewatering is plain & smooth due to reduced shrinkage. The formwork can be removed early and the surface can be put to use early.

Thank You!!!