How Permeable Concrete Works

7 mm in Victoria, 10 mm everywhere else

That's the stone we mix into Permcon, sized to leave enough gap between each piece once the fines are left out of the mix. The stone binds only where it touches its neighbour, so the cured slab is threaded with connected voids, 20 to 25 per cent of its volume, running from the surface down to the base. Ordinary concrete has none of that: fine sand fills every gap, and rain sheets off it toward the nearest grate. Rain passes straight through the Permcon surface instead, at 8,000 to 10,000 mm/hr, which is the surface side of how permeable concrete works.

Getting the binder right is the actual skill

Too little binder and the slab loses strength. Too much, and it closes the voids you were trying to build. That balance is why we design, supply and place the Permcon mix ourselves rather than leaving it to whoever's pouring the job that week. Ultracon runs the same principle with a finer 2 to 5 mm stone, for a smoother surface on driveways, pool surrounds, tree protection zones and car parks.

Under the slab: a bed of screenings

Picture the slab as a door and the base underneath as the bucket. That base is single-sized screenings with the fines washed out, and the gaps between the stones hold the water once it's through. Every 100 mm of base stores 28 litres per square metre at 85 per cent design capacity: a footpath's shallow profile holds that much; a driveway, built deeper, holds 37 to 42 litres; a car park or right of way holds 42 to 56 litres. The depth is set to the storm the site has to handle, not a standard figure. That's the base side of how permeable concrete works: store first, release second.

Two ways for that water to leave

Sand, gravel or a free-draining loam takes it straight down, topping up the water table. Heavy clay, or a council wanting discharge kept under control, gets an under-drain in the base instead, releasing the water slowly into the stormwater system. Either way, the peak flow off the site drops.

None of this costs strength

Permcon is 25+ MPa. Same mix on a driveway, same mix under a commercial car park; thickness is the only thing that moves between jobs. 125 mm is standard for driveways and footpaths. A car park carrying heavier vehicles is built thicker. The surface carries a P4 slip rating and is rated for a design life in excess of 30 years, backed by a 10-year structural warranty.

What it changes around a tree

Air and water still reach the roots underneath a Permcon path, which matters inside tree protection zones, where a sealed surface would cut the root system off from both. The same voids trap up to 70 per cent of the heavy metals, hydrocarbons and suspended solids that would otherwise wash into a drain, and where a council accepts the area, it counts toward the site's permeable area requirement. Water doesn't sit at the low point of a driveway either. It's gone through the slab before it gets the chance to pool.

Silt is the only real threat

Fill the voids with fine sediment and the surface stops draining. The fix is short: once a month, clear leaves and debris, check the slab still drains after rain, look for sediment sitting on top, and keep bare soil upstream from washing onto it. Sweep it once a year if the monthly checks haven't already covered that, and wash out any moss showing in the voids. A slab that's been let go for a season isn't finished. It comes back with a wash.

What's on paper, and what's on site

Strength, infiltration, base storage: they're all published on our technical page, and that's the first thing a drainage engineer will ask for. What isn't on that page is your soil, your catchment or what your council will accept, and that's the part we look at before we put a number on your job.