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Projected Concrete

 

When to use projected concrete

Projected concrete may substitute a number of techniques where low (<1m) to more substantial thicknesses (>0,50m) are required. These types of veils are frequently used in underground works, nailed walls or berlinese walls, retaining walls, embankment protection, external reinforcement structures… 

The mechanics of projected concrete

Projected concrete works display roughly the same features as any other concrete works.

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The advantages of projected concrete :

When compared with concrete casings, projected concrete displays several advantages:  
– Optimal adaptation to ground support,  
– producing a thin veil over a large surface is easier to do,  
– Possibility to secure newly-shaped embankments in a record time (it is not necessary to resort to casing),  
– The esthetic aspect of projected concrete facings may be perceived as more acceptable.

Resistance

Projected concretes are generally nearly as resistant as concrete casings. 30 Mpa values are easily attained.

Projected Concrete

Different ways of projecting concrete 

There are two types of projection :
Dry-mix method :The cement and the aggregate are initially mixed without water, water being pumped directly into the projecting hose. The dry-mix method allows for freedom of movement and makes it possible to carry out the operations from a much greater distance than applying the wet-mix method
Wet-mix method : A concrete mixer is used to produce the wet-concrete mix to be projected. 

One or several coats of projected concrete may be applied, depending on the required thickness and iron framework.

Iron frameworks

Iron reinforcement elements are used to compensate the traction strains affecting the concrete section. They may be necessary to compensate shear strains when the concrete section does not suffice to do so.

 When producing a concrete veil, the iron framework is usually made of several layers of wire mesh welded together. Round iron bars may be used to further strenghthen the iron structures. Localized reinforcements are applied as ponctual strains warrant, when the latter exceed the resistance of the concrete section.

Lost casings

Projected concrete may be applied to lost casings, which makes it possible to create evenly-shaped retaining works.

Projected fiber-reinforced concrete

For iron frameworks, armatures are replaced by metal fibers of the appropriate type, which are added directly to the cement-aggregate mix prior to projection. This technique is particularly useful when applied to works with an extremely uneven layout where installing armatures and wire meshes appears difficult and of dubious efficacy. 

thierry-MTPSThierry's advice about projected concrete :

"Applying projected concrete to newly shaped embankments in a record time makes it possible to ensure temporary skin stability and weather protection, which are indispensable requirements when nailing operations are to be carried out."

Conventional typical values

Projected concrete works generally have low thicknesses. In the case of nailed walls, veils are generally 15 to 30 cm in thickness and have two layers of welded wire mesh.

Important note

Any construction site must have a drainage system and meet the necessary requirements in terms of foundations and stability in order to ensure its durability.

projection-beton-par-voie-seche

Keep in mindthat a study of the types of concrete needed is indispensable to ensure the durability of the projected works.

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