Darwin Concrete Layers

Darwin's ground is far more varied than most people expect. A lot of the city sits on lateritic gravel and ironstone, some suburbs have clayey soil that drains badly and goes soft in the wet, low ground near creeks and mangroves can be soft and salty, and one block can have ironstone just under the surface at one end and deep fill at the other. A slab poured on poorly prepared ground follows all that movement and sooner or later it cracks. When we dig out the worst material, build a well-compacted base and manage drainage, your slab sits on an even, stable platform and moves as one piece instead of flexing over hard and soft spots.

It depends on your ground and what the slab will carry. A path on firm natural ground might only need a thin, well-compacted layer. A driveway over soft clay or silt needs a lot more, and on very weak soil we may lay geotextile fabric under the rock so it doesn't push down into the ground. However deep it ends up, we place and compact the base in thin layers rather than tipping it all in at once, because a thick, loose layer keeps settling under traffic for months. For house slabs, the depth comes from the engineer's design for your site classification.

Not always. If your slab will be an indoor floor, it generally should have one. Without it, moisture from the ground can rise through the concrete, damage timber, vinyl and tile glue, and add to indoor humidity, which is high enough in the tropics already. Outdoor slabs like driveways, paths and patios usually don't need one, since moisture coming through them doesn't hurt anything. When we do lay a membrane, it has to be unbroken to work: wide taped overlaps, sealed pipe penetrations and edges turned up around the outside. A torn or badly lapped sheet won't do much, so we check it before any concrete goes in.

Inside the concrete, at the height the design calls for, and never lying on the base. Steel does its job by handling tension, and in most ground slabs that means sitting the mesh on chairs towards the middle or upper part of the slab depth. Mesh left on the ground does almost nothing. It also needs enough concrete cover so moisture can't easily get to it, which matters even more outdoors and near the coast where there's salt in the air. Getting this right adds very little time, but plenty of crews cut corners, so we check chair heights and cover before every pour.

You can usually walk on it carefully after one to two days. Keep cars off for about a week, and hold off on heavy vehicles, structural loads or machinery until the concrete is close to its design strength, which takes about 28 days. Hot weather makes concrete set faster, but it doesn't shorten those wait times. Keeping the surface from drying out too quickly in those early days matters just as much, because fast moisture loss causes surface cracks and weaker concrete. In harsh sun or a drying breeze, we use curing compounds, damp covers or evaporation retarders to slow things down.

All concrete shrinks as it dries and hardens, and that shrinkage will crack your slab somewhere unless we give it a better place to go. Control joints are planned weak lines, cut or tooled to about a quarter of the slab depth, so the crack forms neatly in the joint instead of wandering across your surface. Timing is everything: cut too late and random cracks may have started already, cut too shallow and the joint won't work. Spacing matters as well, and long skinny shapes or inside corners need extra care. We plan your joint layout before the pour to suit the shape of the area.

Yes, with tighter planning and a bit of flexibility. A big storm can flood an open excavation, force a pour to be rescheduled at short notice, pockmark a fresh finish or leave the ground too soft for a loaded truck. The heat and humidity of the build-up and wet season make concrete go off fast too, so we may add a retarder, pour in the cooler early morning and keep your slab covered and damp while it cures. We work all year round, but wet-season timelines include more buffer and we'll warn you early about likely weather delays. If your job isn't urgent, the dry season from about May to October is often easier.

Then we pump it or barrow it in. A line pump or boom pump can send concrete a long way, and over your house, fence and garden beds, which is handy for back yards, under-house areas and tight side access in many older Darwin suburbs. Pumping does cost extra, and the pump needs a level spot on the street or driveway to set up. Barrowing works for small amounts over short distances but takes more labour. On bigger areas, splitting the job into separate pours with a proper construction joint can help as well. We work out the best method when we visit and include it in your price.

It depends on what the slab is for. A simple driveway, path or patio on your own property often doesn't need a building permit, but linking a new driveway to the street usually needs your local council's approval. Slabs for houses, extensions, garages and bigger sheds generally count as building work, which may need a building permit from a registered building certifier and engineering to standards such as AS 2870. Every case is a little different, so check with your council or a certifier before you start. Before we dig, we request service plans through Before You Dig Australia so we can steer clear of buried pipes and cables.