Two pads, same street in Glen Eden, poured eleven months apart by different crews. One is a shed floor that has not shifted a millimetre. The other is a patio slab with a crack running corner to corner and a 6 mm step across it, and the owner has been told three times that “clay does that”. Clay does do that, but not on its own. What separates those two slabs is not the concrete. It is the 150 mm underneath them and what happened to the ground in the fortnight before the truck arrived.
What Auckland clay actually does
Most of the isthmus and the western suburbs sit on stiff, plastic clay that changes volume with moisture. Wet it and it swells; dry it out over a long February and it shrinks and cracks. The vertical movement across a season on a reactive site can be 20 to 40 mm at the surface, and it is never uniform — the shaded side of the house holds moisture while the north-facing edge dries out, so the slab is being lifted at one end and dropped at the other. A pad is not fighting gravity. It is fighting a ground surface that is quietly changing shape underneath it.
NZS 3604 works off the idea of “good ground” — broadly, ground with an ultimate bearing capacity of at least 300 kPa that is not subject to material movement. Plenty of Auckland clay meets the bearing number and fails the movement test. That is the trap: the site feels solid underfoot and still moves seasonally.
The base is where slabs are won or lost
A pad on clay wants a compacted hardfill layer — commonly 100 to 150 mm of GAP40 or similar, placed in layers and actually compacted rather than tracked over with the digger twice. Under that, the topsoil and any soft or organic material has to come out entirely. Over it goes a polythene damp-proof membrane, taped at the laps, so the clay cannot wick moisture up into the slab and so the slab cannot bleed its water down into the base. Those three steps cost very little and they are the first things skipped when a price is being sharpened.
Reinforcement matters, but not the way people think. Mesh does not stop a slab cracking. It holds the two sides of a crack together so the crack stays tight and stays level. SE62 mesh on chairs, in the upper third of the slab, does that job. The same mesh dropped on the ground and walked on until it sits in the bottom of the pour does nothing at all, and you cannot tell from the finished surface which of those two happened.
Joints, thickness and the curing fortnight
Domestic pads are usually 100 mm at 20 to 25 MPa, going to 125 or 150 mm where a vehicle or a heavy shed load is involved. Control joints get saw-cut to about a quarter of the slab depth, within roughly 24 hours of the pour, at spacings in the order of 3 to 4 metres and arranged so no panel is a long thin rectangle. Concrete will crack. Joints decide whether it cracks where you chose or where it felt like it.
Curing has the worst ratio of effort to consequence. The first seven days set the surface. Poured onto dry clay on a nor’-westerly afternoon with no curing compound and no cover, a slab loses water out of the top faster than it can bleed, and you get plastic shrinkage cracking and a dusty surface that wears badly. The questions worth asking before a pour are about the base, the joint plan and the curing — which is what a contractor like Auckland Concrete Contractors will be talking about while everyone else talks about the finish.
Reading a pad that has already moved
Hairline cracks that follow the saw cuts are the system working. A crack that wanders diagonally across a panel with a step across it is movement, and the step says the two halves sit on differently behaving ground. A crack that opens in summer and closes in winter is a shrink-swell signature and points at drainage or a tree. Before anyone talks about replacement, watch for a season whether the crack breathes — relaying a slab on the same untouched base just buys the same crack again.
Frequently asked questions
Do I need council consent for a concrete pad? A ground-level pad that is not part of a building and does not support a structure generally does not, but a shed floor, a garage slab or anything supporting a wall can change that answer. Confirm the specifics with your council before you book the pour.
Is a thicker slab the fix for reactive clay? Not by itself. A 150 mm slab on an uncompacted base still follows the ground down. Thickness buys stiffness across a short span; base preparation and drainage buy stability.
How long before I can park on a new driveway pad? Foot traffic after two to three days is normal, light vehicles at around seven days, and full loading at 28 days. Cold weather stretches all three.