What type of concrete for an outdoor terrace ?

The sun and the mildness returned, nothing is more pleasant than the idea of ​​lounging in the open air. The outdoor concrete terrace has all the qualities to ensure the serenity and comfort of these privileged moments. If the resistance and robustness of the material are no longer to be demonstrated, today it pleasantly loses its greyness, surprises with its new aesthetics and imposes itself as an essential architectural element for outdoor terraces.

Concrete for outdoor terrace, a thoughtful and motivated choice

Concrete is a composite assembly of various materials, held together by a binder. There are therefore as many concretes as there are aggregates and binders. The physical, chemical and appearance properties of each vary widely. The outdoor terrace consists of two superimposed layers: give it, structural element taking up all the forces and mechanical stresses of the structure and the finishing surface which models and personalizes its appearance.

The outdoor terrace slab

To support the loads specific to terraces, the thickness of the slab must be between 7 and 12 cm. Thinner, it is subject to cracking and micro-bursting due to harsh climatic conditions, the thicker its cost becomes prohibitive and its weight is likely to undermine supports or foundations. The slab is cast on a bed of compacted sand, or rests on walls, support beams or even on a slab floor.

The concrete making up the slab

The concrete that composes it is a mixture of sand, gravel and gravel of different sizes. For common works, such as terraces, quarries also prepare a concrete mix ready to use, more practical and faster to implement. Its grain size range is between 0/15 mm and 0/20 mm. The usual binder is a ciment Portland class CPJ-CEM II -32.5 R, dosed at 300 kg/m3 for surfaces with little traffic and 350 kg/m3 for more resistant structures or structures receiving a metal reinforcement. This concrete is compatible with the use of fibers organic, effective in reducing the appearance of shrinkage cracks unsightly.

The slab for large terraces

For large terraces, it is essential to provide joints de dilatation. These are cuts in the mass of concrete, made every 15 to 20 m² of surface area. They allow free dimensional variations due to fluctuations in temperature and humidity. This simple precaution permanently avoids cracks in the slab during retractions or excessive pressures applied to adjacent walls during dilations.

Three main operating modes make it possible to save these spaces:

  • Pour the concrete in successive sections of 15 to 20 m², interposing between them a sheet of polystyrene over the entire thickness of the slab.
  • Before pouring, seal split joints (special PVC profiles) on concrete pads. These joints constitute supports for pulling the surfacing rulers.
  • Cut the already hardened concrete with a diamond disc of large diameter. All business equipment rental companies offer this type of tool.

Concrete mixing

Mixing components is of paramount importance. Up to 5 m², it is possible to envisage a gâchage manuel. It is, however, a physically demanding and technically risky option for the solidity of the structure. Indeed, each zone of the slab must imperatively be poured in one go, otherwise there will be significant variations in appearance and less resistance to reworking. Therefore, favor the use of a cement mixer which ensures faster mixing with less effort and better controlled viscosity. The use of mixer trucks (spinning tops) guarantees optimum quality, but is only of interest for terraces with large surfaces intended to be covered afterwards of their topcoat.

Finishing exterior concrete terraces

The choices for finishing terraces are wide and often a matter of personal taste. There are two main groups : integrated finishes et reported finishes. The robustness and longevity of these finishes are largely conditioned by the care taken in their technical and delicate implementation. Final protection, color fastness, stain resistance and perfect impermeability are provided by varnishes based on synthetic resins with very high wear and UV resistance. The final appearance of the surface can be, at will, matte, satin, glossy or at wet effect.

In this file, only concrete or derivative finishes are treated, deliberately ignoring the use of tiles, stones, wood or composite materials, worthy of interest, but irrelevant.

Integrated finishes

In essence, these facings are installed on the still loose and damp concrete of a freshly poured slab. They are therefore intimately linked to the structural mass of the concrete terrace. There are various types of integrated finishes:

  • Smoothed concrete, polished concreteor waxed concrete are the traditional and inexpensive finishes, applicable to gray concrete or colorful in the mass. The surface is smoothed by the passage of a long manual trowel (flemish) or using a motorized trowel (helicopter). This mechanical action tightens and raises to the surface a mixture of cement laitance and fine grains of sand. The hard, regular and satin surface layer thus created can be impregnated, after a few hours of setting, with a synthetic varnish or a protective paint.
  • self-leveling concrete or self-placing allows to obtain a smooth concrete by saving sweat and time. This new product, compatible with complex shapes and metal reinforcements, spreads and flattens on its own, under the effect of gravity.
  • stamped concrete uses a modeling technique, by pressing silicone molds on the surface of fresh concrete, sprinkled with hard patching powders, such as quartz. The large number of imprints associated with a large color chart allow all fantasies or the reproduction of most noble materials in all layouts known (geometric arrangements): stones, cobblestones, bricks, tiles with regular shapes or not, opus insertum, wooden strips, rustic parquet floors, etc. The result is astonishing visual realism, while retaining the intrinsic qualities of concrete, in all climates.
  • Stamped concrete resumes the printed concrete technique to apply it to traditional raw or colored concrete.
  • deactivated concrete or washed concrete is a classic concrete, with gray or white cement, treated in such a way as to reveal aggregates of chosen colors on the surface. This effect is produced by spraying a product that limits the grip on the surface, followed by pressure washing. The result, very decorative, is particularly non-slip.

The reported finishes

thin layer concreteor millimetric concrete is a recent innovation. It makes it possible to pour over large surfaces, leveling screeds of extreme resistance, from 0.5 to 4 cm thick. Most of these products, compatible with the decorative concrete techniques described above, suffer no shrinkage and can be applied to all kinds of stable and hard pre-existing substrates. They provide ideal solutions, in restoration, to cover old tiles, wood or old concrete. Their fault lies in the price, inversely proportional to the thickness of the layers: count between 90 and 150 €/m².

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