What type of concrete for an outdoor patio?


The sun and the sweetness 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 stands out as an essential architectural element trend for outdoor terraces.

Concrete for outdoor decks, a thoughtful and motivated choice

Concrete is a composite assembly of various materials, joined together by a binder. There are therefore as many concretes as there are aggregates and binder. The physical, chemical and appearance properties of each vary considerably. The outdoor terrace has two superimposed layers: the slab, structural element taking up all the mechanical forces and constraints of the structure and the finishing surface which models and personalizes the appearance.

The outdoor patio slab

To withstand the specific loads on terraces, the thickness of the slab must be between 7 and 12 cm. Thinner, it is subject to cracks and to micro-bursts due to climatic rigors, the thicker its cost becomes prohibitive and its weight is liable to undermine the supports or foundations. The slab is poured 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 which composes it is a mixture of sand, chippings and gravel of different calibers. For routine work, such as terraces, quarries also prepare a concrete mix ready to use, more practical and faster to implement. Its particle size range is between 0/15 mm and 0/20 mm. The usual binder is a portland cement class CPJ-CEM II -32.5 R, dosed at 300 kg / m3 for surfaces with little stress and 350 kg / m3 for more resistant structures or those receiving a metal frame. 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 expansion joints. These are cuts in the mass of concrete, made every 15 to 20 m² of surface. They allow free dimensional variations due to fluctuations in temperature and humidity. This simple precaution durably prevents cracks in the slab during retractions or excessive pressures applied to adjacent walls during dilations.

Three main operating modes allow these spaces to be saved:

  • Pour the concrete in successive slices of 15 to 20 m², interposing between them, a sheet of polystyrene over the entire thickness of the slab.
  • Before pouring, seal fractionation joints (special PVC profiles) on concrete pads. These joints constitute supports for drawing the surfacing rules.
  • Cut the already hardened concrete using a diamond disc 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 consider a manual mixing. This is, however, a physically painful and technically risky option for the strength of the structure. Indeed, each area of ​​the slab must imperatively be poured in one go, otherwise there will be significant variations in appearance and less resistance to rework. Therefore, prefer the use of a cement mixer which ensures faster mixing with less effort and better controlled viscosity. The use of concrete mixer trucks (spinning tops) guarantees optimum quality, but is only of interest for large-surface terraces 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 and reported finishes. The robustness and longevity of these finishes are largely conditioned by the care taken in their implementation, technical and delicate. Final protection, color retention, stain resistance and perfect impermeability are provided by varnishes based on synthetic resins with very high resistance to wear and UV. The final appearance of the surface can be, at will, matt, satin, glossy or to 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 off topic.

Integrated finishes

In essence, these facings are implemented on the still loose and wet 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:

  • Smooth concrete, polished concrete, or 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 float (Flemish) or by means of a motorized trowel (helicopter). This mechanical action tightens and brings up to the surface, a mixture of cement laitance and fine grains of sand. The hard, regular and satiny surface layer thus created is impregnable, 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 frames, spreads out and hovers on its own, under the effect of gravity.
  • Printed concrete uses a modeling technique, by pressing silicone molds on the surface of fresh concrete, dusted with hard leveling powders, such as quartz. The large number of prints associated with a wide range of colors allow all the fantasies or the reproduction of most noble materials in all layout known (geometric arrangements): stones, paving stones, bricks, tiles with regular or irregular shapes, opus insertum, wooden slats, rustic parquet floors, etc. The result is an astonishing visual realism, while maintaining the qualities intrinsic to concrete, in all climates.
  • Forged concrete takes over the technique of printed concrete to apply it to traditional raw or colored concrete.
  • Concrete deactivated or washed concrete is a classic concrete, with gray or white cement, treated so as to reveal on the surface, aggregates of chosen colors. This effect is produced by spraying a product that limits the surface setting, followed by washing under pressure. The very decorative result is particularly non-slip.

The reported finishes

Thin-bed concrete, or millimeter concrete is a recent innovation. It allows pouring over large surfaces, screeds of extreme resistance, 0.5 to 4 cm thick. Most of these products, compatible with the decorative concrete techniques described above, do not suffer from any shrinkage and are applicable on all kinds of stable and hard pre-existing supports. 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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