A floor slab that cracks slightly is still a floor slab. A reservoir that cracks slightly leaks. That is the whole difference between a building structure and a hydraulic one: the second has no margin, and watertightness is not added afterwards — it is built with the structure.
The construction joint is the weak point
Every interruption in pouring creates an interface between hardened and fresh concrete. That interface is, by its nature, weaker and less watertight than the rest of the wall. It is where water gets through.
It is dealt with by preparing it: scabbling or washing to expose the aggregate, removing laitance, damping without leaving standing water, and setting a water-stop within the thickness. The number of joints is also reduced by sequencing: one bay poured in a single operation beats a wall assembled in four.
Water does not pass through concrete. It takes the defects left for it.
Shrinkage, the other source of cracks
Concrete shrinks as it hardens. Prevented from doing so freely — by a joint, a rigid foundation, an awkward geometry — it cracks. On a reservoir, these restrained-shrinkage cracks often go right through.
The levers are well established: limit the heat of hydration through the mix, divide the structure into bays separated by joints, reinforce more heavily so cracking is distributed as fine cracks rather than a single one, and extend curing well beyond what a floor slab would need.
The filling test settles it
- Gradual filling in stages, rather than loading the structure all at once
- A settling period before any measurement, while the concrete absorbs water
- Daily level readings, corrected for evaporation and rainfall
- Inspection of external faces, joints and penetrations
- Checking connections and hydraulic fittings before commissioning
The filling test is not negotiable and cannot be replaced by a visual inspection. It is the only check that loads the structure the way service will, and the only moment when a poorly treated joint reveals itself before the reservoir goes into use.
A hydraulic structure therefore takes longer than a building of comparable volume. That time is either found in the programme, or spent later on remedial waterproofing — considerably more expensive once the structure is buried and connected.
