Sprayed concrete, or why a free form lasts

You do not pour a free form: no formwork follows an irregular curve. The concrete is therefore sprayed under pressure onto the reinforcement. This technical detour has a consequence the client never sees — and which determines how the waterproofing holds up over time.

The principle

Instead of being poured into a mould, the concrete is sprayed under pressure against a steel frame that has exactly the shape wanted. The formwork is the ground itself: you excavate the shape, build the reinforcement inside it, and spray.

Reinforcement: this is where the shape is decided

Before any concrete, a steel skeleton is built — reinforcing bars and welded mesh — following the shape of the pool, curves included. It is this stage that makes bespoke work possible: the shape is limited only by what steel can follow — including the slope of a submerged shallow area.

One technical rule matters more than the others: the reinforcement must stay buried under a sufficient thickness of concrete. This cover protects the steel from water and air. If it is insufficient, the steel eventually corrodes, swells and bursts the concrete from the inside — a defect that appears years later and is very expensive to put right.

Spraying: in passes, never in one go

The concrete is applied in successive passes, each pass having to begin setting before the next is applied. This produces a monolithic wall.

For a medium-sized pool, the spraying itself takes a few hours. What takes time is everything around it: the earthworks before, the curing times and the waterproofing after.

Dry process, wet process

Two processes coexist. In the dry process, the mix arrives dry at the nozzle and water is added only at the moment of spraying: the concrete contains very little water, which limits shrinkage and therefore cracking as it dries. In the wet process, the concrete arrives already mixed: the work is more even and less dependent on the nozzleman’s hand, but the water content is higher.

Neither is bad. What matters more than the process is the evenness of the passes, the cover over the reinforcement and the quality of the curing — that is, keeping the shell damp while it sets, a point that is particularly critical when spraying in high heat.

The advantage you do not see: the absence of joints

A pool built from hollow blocks or poured in several stages contains cold joints. These are the weak points of the structure: it is almost always there that a pool starts to leak. Sprayed concrete forms a monolithic shell, with no cold joint. The structural risk is not removed, it is moved — to the waterproofing and the singular points.

What still has to be watched

Sprayed concrete makes the structure, not the waterproofing. That comes from an applied covering: water-repellent render, reinforced membrane or mineral finish. The weak points are always the same and deserve most of the attention:

• Inside corners, where the covering moves the most.
• Wall penetrations: skimmers, return inlets, lights, main drain.
• The waterline, permanently exposed to the alternation of air and water.

A pool that ages badly is almost never the structure. It is one of these three points.

How to check it on site

Is the reinforcement held clear of the ground by spacers, rather than laid straight on the earth? Are the passes applied gradually rather than in a single thick layer? Is excess concrete removed instead of being reused in the structure? A site that is clean on these three points is a good sign. Our working method is set out step by step.

One last pointer, specific to hot climates: in high heat, spraying is done early in the morning and the shell must be kept damp while it sets. Concrete sprayed in the full July sun and left to dry on its own will never give the same strengths.