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Shotcreting

Shotcrete is concrete (or sometimes mortar) conveyed through a hose and pneumatically projected at high velocity onto a surface, as a construction technique. It is reinforced by conventional steel rods, steel mesh, and/or fibers. Fiber reinforcement (steel or synthetic) is also used for stabilization in applications such as slopes or tunneling.

Shotcrete is usually an all-inclusive term for both the wet-mix and dry-mix versions. In pool construction, however, the term "shotcrete" refers to wet-mix and "gunite" to dry-mix. In this context, these terms are not interchangeable (see "Shotcrete vs. gunite" discussion below).

Shotcrete is placed and compacted at the same time, due to the force with which it leaves the nozzle. It can be sprayed onto any type or shape of surface, including vertical or overhead areas.

Advantages of Shotcreting

Shotcrete is used in lieu of conventional con-crete, in most instances, for reasons of cost or convenience Shotcrete is advantageous in situat-ions when form work is cost prohibitive or impractical and where forms can be reduced or eliminated. Access to the work area is difficult, thin layers or variable thicknesses are req-uired, or normal casting techniques cannot be employed. Additional savings are possible because shotcrete requ-ires only a small, portable plant for manu-facture and placement. Shotcreting operations can often be accom-plished in areas of limited access to make repairs to structures.

Guniting

The guniting is the most effective process of repairing concrete work which has been damaged due to inferior work or other reasons. It is also used for providing an impervious layer.

The gunite is a mixture of cement and sand, the usual proportion being 1:3. A cement gun is used to deposit this mixture on the concrete surface under a pressure of about 20 to 30 N/cm2.

The cement is mixed with slightly moist sand and then necessary water is added as the mixture comes out from the cement gun. A regulating valve is provided to regulate the quantity of water.

The surface to be treated is cleaned and washed. The nozzle of gun is generally kept at a distance of about 750 mm to 850 mm from the surface to be treated and the velocity of nozzle varies from 120 to 160 m/sec.

Advantages of Guniting

The high compressive strength is obtained. Strength of about 56 to 70 n/mm2 at 28 days is generally obtained.

The high impermeability is achieved.

The repairs are carried out in any situation in a short time.

Sandblasting

Sandblasting is a general term used to describe the act of propelling very fine bits of material at high-velocity to clean or etch a surface. Sand used to be the most commonly used material, but since the lung disease silicosis is caused by extended inhalation of the dust created by sand, other materials are now used in its place. Any small, relatively uniform particles will work, such as steel grit, copper slag, walnut shells, powdered abrasives, even bits of coconut shell. Due to the dangers of inhaling dust during the process, sandblasting is carefully controlled, using an alternate air supply, protective wear, and proper ventilation.

Advantages of Sandblasting

Sand blasting is used to clean the surface and round-off the corners. Workpieces with a cross-section of 2 mm or more are sand blasted automatically.

A surface quality of up to SA 2 1/2 can be achieved as per ISO standard 8501-1. This process removes scale caused by laser cutting and welding, rust, coatings and contamination, to achieve the best possible adhesion of the coatings.

Full-power, medium-power or no sand blasting can be selected, depending on the material and level of contamination.

Waterproofing in construction

In construction, a building or structure is waterproofed with the use of membranes and coatings to protect contents as well as protecting structural integrity. The waterproofing of the building envelope in construction specifications is listed under "07 - Thermal and Moisture Protection" within MasterFormat 2004, by the Construction Specifications Institute, and includes roofing material as well as waterproofing materials.

In building construction, waterproofing is a fundamental aspect of creating a building envelope, which is a controlled environment. The roof covering materials, siding, foundations, and all of the various penetrations through these surfaces need to be water-resistant and sometimes waterproof. Roofing materials are generally designed to be water-resistant and shed water from a sloping roof, but in some conditions, such as ice damming and on flat roofs, the roofing must be waterproof. Many types of waterproof membrane systems are available, including felt paper or tar paper with asphalt or tar to make a built-up roof, other bituminous waterproofing, EPDM rubber, hypalon, polyvinyl chloride, liquid roofing, and more.

Walls are not subjected to standing water, and the water-resistant membranes used as housewraps are designed to be breathable to let moisture escape. Walls also have vapor barriers or air barriers. Damp proofing is another aspect of waterproofing. Masonry walls are built with a damp-proof course to prevent rising damp, and the concrete in foundations needs to be damp-proofed or waterproofed with a liquid coating, basement waterproofing membrane (even under the concrete slab floor where polyethylenesheeting is commonly used), or an additive to the concrete. A potential problem in earth sheltered houses is too much humidity, so waterproofing is critical in these houses. Water seepage can lead to mold growth causing significant damage and air quality issues. Properly waterproofing foundation walls is required to prevent deterioration and seepage.

The penetrations through a building envelope need to be built in a way such that water does not enter the building, such as using flashing and special fittings for pipes, vents, wires, etc. Some caulkings are durable, but many are not a reliable method of waterproofing.

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