Permeable Concrete.

Permeable concrete, from the bottom up

Start underneath. A layer of clean screenings goes down first, and that's where the water is stored. From there it soaks into the ground, or, on slow soil or a site where soakage isn't allowed, it leaves through an under-drain at a controlled rate. That base is the part we size for your site.

On top goes the slab, which is the fast part. It's concrete that lets water through. With the fines left out of the mix, nothing fills the gaps between the stones; they lock together where they touch and leave 20 to 25 per cent of the slab as voids, joined from the top face to the bottom. Rain lands and drops straight through. It never gets the chance to run anywhere. From the kerb it's a textured concrete, and on our jobs it's Permcon, poured on site as one continuous slab by our own crews.

What councils are asking for

On smaller Victorian sites the stormwater check is BlueFactor, which replaced STORM. Bigger jobs get modelled in MUSIC, and so do sites under the Queensland councils that ask for a MUSIC model with the development application. In NSW, each council writes its own detention and WSUD controls. Behind all of them sits one problem. Every roof, driveway and car park turns rain into runoff that carries oil and grit and reaches the pit at the same moment as the rest of the street. Water sensitive urban design is how councils answer it, with caps on impervious area, on-site detention and targets for how clean the outflow is.

Permeable concrete meets those requirements on ground you were paving anyway. The slab is your pavement; underneath, it takes care of how much stormwater leaves and how clean it is. Designed as a passive system, it counts as pervious, much as grass would. That drops your impervious total. The base is storage an engineer can count, so the detention tank shrinks or goes, taking some of the pits and pipes with it. Whichever tool your site goes through, the pavement fits it.

Assessed to ISO 14067:2018, Permcon's cementitious materials carry 62 per cent less embodied carbon than conventional concrete, and our environmental page has more on that.

What it carries

A 71 m² bus stop we poured at Mornington went down at 175 mm, inside the tree protection zone of an established canopy tree. Most jobs don't need that. The standard slab is 125 mm, and it covers nearly every driveway and path; our loading chart estimates a 5.8 t axle at that depth. Heavier vehicles get more depth, because the mix never changes and the thickness does. The technical page sets an estimated axle load against every thickness and explains the method.

Each slab is poured as one continuous surface with no units or joints, so weeds have no joint to root in. It carries a P4 slip rating. Structurally it's warranted for 10 years (terms and conditions apply), and it's designed to last in excess of 30 years on a monthly look and a yearly sweep.

Permcon and Ultracon

Standard Permcon has an open, stony texture, from 7 mm stone in Victoria or 10 mm in other states. Colours run plain grey, charcoal, soapstone and sandstone. For something smoother underfoot there's Ultracon, the finer one, with small stone held in a permeable resin.

Other names you'll hear

Pervious concrete is the engineers' word. On a homeowner's quote request it's usually porous concrete, and water permeable concrete says it plainest.

Two products get mixed up with it. No-fines concrete is lower in strength and isn't a substitute for Permcon. Pavers and grids are separate units draining through the gaps between them; permeable concrete drains through its whole face and has no pieces to move.

Jobs we do

• permeable concrete driveways, the job we pour most. Rain drains through before it can pool at the garage door.
• Council footpaths and shared paths: no ponding at the kerb, and the water goes to the street trees.
• Car parks, often the largest hard area a site has, draining where each car stands.
• Tree protection zones, so roots under the pavement still get water and air.
• Sports courts that are back in play soon after rain, with sure footing on the baseline.
• Commercial properties whose permit carries an impervious cap or a detention condition.
• Airport tarmac: aprons, taxiways and hardstands with no standing water.
• Around the house, pool surround and courtyard included.