Waterproof Ground-Bearing Slab with Bentonite
Below the water table there is no such thing as maintenance: whatever is laid before the pour stays there forever. That is why bentonite is not a membrane like the others — it is not glued down, it is allowed to swell, and it works only if something stops it swelling freely.
Overview
TAV. 00A waterproof ground-bearing slab is the point on site where mistakes cost most: the waterproofing ends up under thousands of tonnes of concrete and will never be reachable again. Bentonite panels solve the problem on a different principle from membranes: they contain granular sodium clay that expands ten to fifteen times on contact with water. If that expansion is free, the clay disperses and does nothing; if it is restrained by the weight of the slab, it becomes a dense continuous gel that seals even accidental punctures. Everything follows from this: dry laying, an early pour, a minimum confining thickness, and obsessive care at penetrations, which are the only place where water finds a preferential path.
The site score
TAV. HThe detail of the craftthe edge nobody allows for
The laying surface
Casting the blinding on compacted ground, floating it flat, removing projecting bars and debris, and checking levels and upstand edges.
Don't walk the blinding carrying rebar. Every scratch you make now will be under eight hundred tonnes in three weeks.site notebook — editorial synthesis
click a beat · arrow keys ← → walk the site · dashed pauses are the chemistry at work
The principle, in two drawings
Why bentonite only works when it is confined
Sodium clay swells ten to fifteen times on contact with water. If it can expand freely it disperses into a suspension that the water carries away: exactly what happens to panels left out in the rain. If instead the expansion is restrained by the weight of the raft, the same clay compacts into a dense continuous gel that works into every irregularity of the concrete and seals even nail holes. Confinement is not a precaution: it is the mechanism.
Schematic. It is also why bentonite panels are not used on surfaces that will never carry a permanent load: with no confinement there is no system, only clay.
Three ways of saying no to water
Bentonite, membrane, watertight concrete
Below the water table you choose between three strategies, and each shifts the risk somewhere different. A membrane is a continuous barrier, excellent while it is intact, but once punctured the water travels between membrane and concrete and emerges far away. Watertight concrete moves the problem onto pour quality and construction joints. Bentonite bonds to the concrete and heals itself, but demands confinement and dry laying. The choice is not between good and bad: it is between risks you can manage and risks you cannot.
A qualitative comparison of strategies, not products. In the most demanding cases — water under pressure, spaces that tolerate no seepage at all — the three are combined, with a monitoring and injection system built in at the time of the pour.
How it ages (and what betrays it)
TAV. PQuestions from the site
TAV. QIs groundwater analysis really necessary?
Yes, and it is one of the few tests that costs little and changes the product choice. Sodium bentonite swells beautifully in fresh water, far less in saline or highly mineralised water, and almost not at all in the presence of certain contaminants. In those cases modified bentonites are used, or the strategy changes. Finding out after the pour means finding a problem with no affordable answer.
How much concrete is needed above for confinement to work?
The usual order of magnitude is around thirty centimetres of raft, but the useful figure is not the thickness: it is the permanent pressure bearing on the panel. That is why the critical point is never the middle of the raft, where load is plentiful, but the edges, the upstands and any thin areas. That is where the figure must be checked against the system's data sheet, not at midspan.
If a panel is punctured while the reinforcement is being fixed, what do you do?
Small nail holes close by themselves when the system activates, and that is bentonite's main advantage over a membrane. A real tear, however, is repaired at once: bentonite mastic is applied and a patch of panel lapped well beyond the edge of the damage. The rule is not to use self-healing as an excuse for carelessness: it works on holes, not on tears.
Can bentonite be laid in winter?
Yes, and it is in fact one of its advantages over bituminous membranes, which turn stiff and hard to weld in the cold. The constraint is not temperature but water: melting snow, an excavation that floods overnight, freeze-thaw cycles. A panel that freezes after getting wet is a panel to be replaced. So in winter you lay in small bays and pour straight away.