Concrete, fibreglass or panel-and-liner: which suits you

The question is usually asked backwards. It is not about which is "better", because all three can be built well and all three can be built badly. It is about which one fits your plot, your access, your ground and the use you will give it over the next twenty years.

And two Colombian constraints settle the answer before any comparison of finishes: how the shell gets in and what lies under the ground. Let us start with what they are.

The three families

• —Concrete, cast in place or sprayed. It is built in the hole: steel, concrete, waterproofing and lining. Free shape, free depth, free edge — including an infinity edge and any kind of overflow pool. It is the slowest of the three and the only one that can be repaired indefinitely.
• —Fibreglass, one-piece shell. A reinforced polyester shell moulded at a factory and transported whole to site. Installation is measured in days, not weeks. Shape, size and depth are those of the mould: there is no "nearly".
• —Panel-and-liner. A structure of panels — galvanised steel or polymer — assembled in the hole and lined inside with a membrane. It is the quickest to assemble and the cheapest to buy, and its membrane is a consumable that gets replaced.

What the rules require of all three alike

None of this changes with the material, and it is worth being clear about before comparing prices: two quotations, one compliant and one not, are not comparable.

• —Two or more submerged drains at the deepest point, hydraulically balanced, at least 0.90 m apart, each with its anti-entrapment cover — the rule spells it "antientrampamiento" — of an area at least four times that of the discharge pipe, plus a vacuum release system and an emergency stop button.
• —One skimmer — the surface outlet, which the Colombian rule calls a desnatador — for every 46.5 m² of water surface, on opposite sides of the shell. Above 312 m² of surface it gives way to perimeter overflow, and it does not apply to infinity-edge pools.
• —A turnover period of 4 to 6 hours, two to four turnovers a day and 8 to 24 hours of daily circulation in a restricted-use pool. That is what sizes the pump and the filter.
• —A maximum velocity of 2.4 m/s on the pressure line and 1.8 m/s on the suction line, and a maximum diameter of eight inches.
• —A watertight lining, easy to clean and disinfect and stable against the treatment chemicals — and the rule extends the requirement to perimeter channels and balance tanks. It applies to fibreglass gelcoat and to a liner membrane exactly as it applies to concrete tiling.

The fact that settles a fibreglass pool: the road

A one-piece shell is not assembled on site: it arrives finished, on a bed, slung from a crane. And every dimension of that shell has to clear every bend on the way to your plot.

The national rule is specific. Resolución 4959 de 2006, from the transport ministry, classes anything wider than 2.60 m as an oversized load, and beyond that width haulage stops being a delivery and becomes a permit application.

Hydrostatic uplift: why the shell is a structure

This is the second constraint, and the one that produces the most expensive damage and the hardest conversation with a client afterwards.

An empty pool is a hollow box buried in the ground. If the water table rises above its floor, ground water pushes upward with a force equal to the weight of water displaced. In our reference pool — 32 m² of floor — with ground water 1.40 m above that floor, the push is around 45 tonnes. A concrete shell does not weigh that. A fibreglass shell does not weigh a twentieth of it.

The trade has a name for the result: the pool "comes out of the hole". It lifts, it tilts, it snaps the pipework and it wrecks the perimeter walkway. And it nearly always happens on the day somebody drains it to clean it, in the wet season.

That is why the shell is a structure and not a finish: the soil investigation and the structural design are signed by a civil engineer, as the earthquake-resistant construction regime requires. On sloping ground the lateral push of the earth is added to all this, which is another conversation: see building on a sloping plot.

The comparison, unvarnished

What you will be told, and what sits behind it

1. 1"Fibreglass does not crack." A one-piece shell does not crack the way concrete does, true. But it deforms if the sand bed is poorly compacted and it blisters if the gelcoat misbehaves, and both are harder to put right than a crack in concrete.
2. 2"Concrete lasts forever." The shell, nearly. The waterproofing and the tiling, no: they have a service life and they have to be renewed. A budget that does not say so is pushing that bill down the road.
3. 3"You just change the liner and it looks new." True, and that is exactly the point: it has to be changed. Ask in writing what service life the manufacturer states for the membrane, and what the warranty covers.

When each one suits

• —Concrete when the shape is not negotiable, when there is a slope, when you want an infinity edge or any overflow variant, when the ground is demanding, or when the pool is in collective use and will work every day of its life.
• —Fibreglass when access is generous and verified, when a catalogue size suits you as it stands, when the programme rules, and when the ground is firm and dry. It is the cleanest of the three if those four hold.
• —Panel-and-liner when the initial outlay is the binding constraint and you accept, with open eyes, that the membrane is a consumable with a date on it.

The person who leads this line of work at the practice brings fifteen years of designing hydraulic systems and bodies of water, and in that time the argument about materials almost never settled a project. What settled it was always the same three things: how it gets in, what lies underneath, and who signs the calculation. Choose those first; the material then chooses itself.