When it is impossible or inefficient to use traditional concreting technology, special methods are used, which include vacuuming and gunning of concrete, underwater concreting, concreting with the methods of vertically movable pipe (VPT), ascending solution (BP) and a number of others.
Vacuuming refers to the removal of free water from freshly laid concrete mix using discharged air. Vacuum concrete gains strength much faster, has a higher water resistance, is less prone to cracking and abrasion.
As you know, mixing concrete requires about 20% of water by weight of cement, but for better workability, the water-cement ratio usually ranges between 0.35 ... 0.55, sometimes up to 0.8. Excessive water slows down the process of setting and does not allow to achieve full compaction of concrete. Excess water, evaporating from concrete, promotes the formation of cracks, reduces its strength, insulating properties, etc. Vibrating helps to transfer part of the excess water to the surface of the concrete, vacuuming allows sucking off the excess water more fully. The essence of the method of vacuuming is the compaction of the concrete mix with the simultaneous extraction of excess mixing water and excess air present in the voids of concrete by creating a vacuum in the concrete cavity directed to the surface of the vacuum.
Vacuuming is a technological method that allows to extract about 10 ... 25% of the mixing water from the laid concrete mix with concomitant or additional compaction. The method makes it possible to use concrete mixtures with mobility up to 10 cm, which simplifies and reduces the cost of their distribution and compaction, while achieving a significant improvement in the physicomechanical characteristics of hardened concrete, corresponding to a reduced residual water-cement ratio.
Pumping out is usually used when concreting floors, floors, roofing-shells and other structures with a developed horizontal surface. Due to vacuuming in concrete, not only the water-cement ratio decreases, but also the density and strength increase, the compaction of concrete turns out to be so high that you can walk on freshly laid concrete.
Depending on the type of construction, vacuuming is carried out either from above or from the side surfaces of the structure being erected.
Horizontal and spatial structures, such as interfloor overlappings, roof arches, floors, are evacuated from above using portable rigid vacuum shields or vacuum mats, and walls, columns, and other structures developed in height are from the side surfaces using vacuum formwork.
On a flat surface of fresh concrete, lay a vacuum shield. Structurally, the vacuum shield is a duct (usually in plan size 100x125 cm) with a sealing lock along the contour. The sealed box of the upper shield covering is made of steel, waterproof plywood or fiberglass. From the bottom, the shield is equipped with a vacuum cavity in direct contact with the concrete. The lower surface of the shield, bordering on concrete, is a filter cloth (coarse calico, linen), followed by a frequent and rare metal mesh (the second is a power one) and a cover made of waterproof plywood. Due to the curvature of the wires, the grid in its cross section forms interconnected small (thin) air channels, which together form a thin air gap (vacuum cavity).
Between the cover and the filtering fabric, a cavity of about 4 mm thick, framed with plywood strips, is formed by two metal grids. In the middle of the cap there is a hole with a stopcock and a rubber hose to the vacuum pump.
A vacuum shield has a rubber apron around the perimeter for sealing, which not only fringes it, but also prevents air from being sucked from the outside into the cavity formed when the shield is laid on the surface of freshly laid concrete. When you turn on the vacuum pump inside the shield, a vacuum is formed, and water and air from the concrete mix rushes into it. Filter fabric retains particles of sand and cement, but freely passes water and air.
To create a vacuum in the vacuum cavity, and, consequently, to remove part of the mixing water and air in the center of the vacuum shield, a nozzle is installed, which is connected via a three-way valve to a vacuum source. A crane on the body of the vacuum shield, in one of its positions, opens the access of air into the internal cavity of the shield, equalizing the pressure there, which allows the shield to be freely rearranged to the next section. Usually, upon completion of the evacuation, a vibrator is placed on the shield and additional compaction of the concrete mix is carried out, as a result of which it eliminates the directional porosity that occurs during the evacuation process.
Currently, instead of metal, they are switching to the use of non-corrosive, lightweight, extruded plastic meshes. In order to prevent the cement particles from fresh concrete being carried away from the cement, the entire surface of the mesh facing the concrete is covered with nylon or nylon filter cloth.
Vacuum mat consists of two independent elements: the lower and upper. The bottom element, laid on the concrete, is a filter fabric, stitched with a lavsan distribution grid. The top element is sealing. It is made of dense gas-permeable synthetic fabric and rolled over the filter element. Along the longitudinal axis of the upper element is a suction perforated hose that is connected through a nozzle to a vacuum source.
Vacuum formwork is made on the basis of conventional collapsible formwork. For this, formwork boards from the side of the deck shall be equipped with horizontal heights, isolated from each other, with vacuum cavities, which, as the concrete mixture is laid, are connected to a vacuum source. Vacuum formwork can also be assembled from vacuum shields, while ensuring the immutability of their position by stiffeners and fasteners.
Depending on the conditions for evacuating concrete — using vacuum shields (vacuum mats) or vacuum formwork — the physical processes take place differently.
When concrete is vacuumed with vacuum shields (vacuum mats) that have the ability to move towards concrete, an additional static compaction occurs due to the difference in atmospheric pressure and pressure in the vacuum cavity simultaneously with the suction of water and air. In this case, the current force reaches 70 ... 75 kPa. With the distance from the vacuum surface, the pressure transferred to the concrete decreases as part of the load is spent on overcoming the forces of internal friction and the development of contact stresses in the solid phase.
Vacuuming accelerates the decalcification, increases the final strength of concrete by 20 ... 25%, improves frost resistance, water resistance, reduces the need for cement by 12 ... 20%, accelerates decallation by 1.5 ... 2 times.
The vacuum in the vacuum cavity is at least 350 mm Hg. Art. for large shields and at least 500 mm Hg. Art. for small shields. The duration of the vacuum depends on the thickness of the concrete layer.
The thickness of the concrete, see ........................................... 10 20 30
Duration of vacuuming, min ... 10 25 55
A vacuum unit with a vacuum pump and 40 shields per work shift processes up to 2,000 m2 of surface.
Vacuuming begins no later than 15 minutes after the end of concreting; after the end of vacuuming and vibrocompaction of concrete, it is necessary to immediately treat the surface with troweling machines.
It is desirable to carry out vacuuming at the modes of the highest possible discharge. Vacuuming time depends on the degree of suction, the thickness of the evacuated structure, cement consumption, the mobility of the concrete mix, the ambient temperature and other factors.
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