Concrete Layers in Wollongong

The ground here varies far more than people tend to think. Large parts of the coastal plain and foothills are reactive clay that expands and contracts with the weather, escarpment suburbs such as Mount Keira and Mount Ousley have shale and sandstone sitting close to the top, the low land by the creeks, beaches and Lake Illawarra can be sandy with water not far down, and a single steep block might have rock just below the surface at the top and deep fill at the bottom. Concrete laid over badly prepared ground tracks every bit of that movement and eventually cracks. Removing the poor material, building a solidly compacted base and controlling drainage gives the slab a stable, uniform platform, so it moves as one unit rather than bending over hard and soft patches.

That's governed by the ground conditions and the loads involved. A path on firm natural ground may only want a thin, well-compacted layer, while a driveway over soft clay or silt needs considerably more, and on really poor soil we might put geotextile fabric down first to stop the rock punching into the ground. Whatever the final depth, the base goes in and is compacted in thin lifts, not dumped in one go, since a thick loose layer will keep settling under traffic for months. On house slabs, the depth is set by the engineer's design for the site classification.

Only in some cases. A slab that will become an internal floor generally needs one, because otherwise ground moisture can rise through the concrete, ruin timber, vinyl and tile adhesive, and add to dampness inside, and our humid coastal summers already supply plenty of that. External slabs such as driveways, paths and patios usually go without, as moisture passing through them causes no harm. Where a membrane is used, it only works if it's continuous, with generous taped laps, sealed penetrations around pipes and edges turned up at the perimeter. A ripped or poorly lapped membrane achieves very little, so we inspect it before the concrete goes on.

It should be within the concrete at the height shown in the design, and never resting on the base. Reinforcement works by taking tension, and for most ground slabs that means mesh supported on chairs toward the middle or upper part of the slab. Mesh left lying on the ground achieves next to nothing. It also needs adequate concrete cover so moisture can't reach it easily, which is even more important outside and along the coast where salt is in the air. Getting it right takes very little extra time, yet plenty of crews skip it, so chair heights and cover are checked before every pour we do.

Careful foot traffic is usually fine after a day or two. Cars should stay off for about a week, and heavy vehicles, structural loads or machinery should wait until the concrete is near its design strength, which takes roughly 28 days. Warm weather speeds up the set, but those waiting periods stay the same. Stopping the surface from drying too fast in the first few days is just as important, because rapid moisture loss leads to surface cracking and weaker concrete. On days with harsh summer sun or a dry westerly, we use curing compounds, wet coverings or evaporation retarders to slow the drying.

Concrete always shrinks as it dries and hardens, and unless that shrinkage is given somewhere better to go, it'll crack the slab wherever it likes. Control joints are deliberate weak lines, sawn or tooled to roughly a quarter of the slab's depth, so the crack forms tidily inside the joint instead of meandering across the surface. Timing is critical: if they're cut too late, random cracks may already be forming, and if they're too shallow they won't do their job. Spacing is important too, and long narrow shapes and internal corners need particular attention. We work out the joint layout before pouring to match the shape of your area.

It can, with closer planning and some flexibility. Wollongong gets rain in every season, and a heavy storm or East Coast Low can fill an open excavation, push a pour back at short notice, pit a fresh finish or leave the ground too soft for a loaded truck, particularly on a steep block. Hot, muggy summer days also make concrete set quickly, so we might use a retarder, start early while it's cooler and keep the slab covered and moist as it cures, whereas cold winter mornings slow everything down and call for a bit more patience. We work year-round, allow some buffer in our schedules and will flag likely weather delays early.

In that case we pump the concrete or barrow it in. A line pump or boom pump can move concrete a long distance, including over the house, fences and garden beds, which suits backyards, under-house spaces and the narrow side access common in many of Wollongong's older suburbs. Pumping adds to the cost, and the pump needs a flat place to set up on the street or driveway. Barrowing suits small volumes over short runs but is more labour-intensive. For larger areas, splitting the work into separate pours with a proper construction joint can also help. We decide on the best approach at the site visit and include it in the quote.

That depends on what it's for. A straightforward driveway, path or patio on your own land can often be exempt development if it ticks the NSW Codes SEPP boxes, although connecting a new driveway to the road normally needs council approval for the crossover. Slabs for houses, extensions, garages and larger sheds are usually building work, which may mean a development application or complying development certificate, sign-off from a registered certifier and engineering to standards such as AS 2870. On steep or landslip-mapped land below the escarpment, a geotechnical report might be needed as well. Each situation differs, so confirm with your council or a certifier before work begins. Before any digging, we get service plans through Before You Dig Australia so we can avoid underground pipes and cables.