If you're paying for a concrete path or driveway, this is what a properly built one includes. Use this case study as your checklist before the concrete truck arrives, because once it's poured, the only real fix is a jackhammer.

Background

We've been advising businesses in the NDIS home modifications space on construction quality. The sector has real quality problems: work is often priced on cost alone, scoped by people without construction training, and checked by no one. Occupational therapists do the scoping, but they aren't trained to design construction work, and many of these jobs are priced below the threshold where a contract and licensing requirements kick in.

The new home industry has similar problems, but it's a bit further along in managing quality. The lessons in this job apply to anyone paying for concrete work.

The job

I was asked to review a new concrete path after the client said the work didn't match the scope and they were worried about cracking. I assessed it on 17 April 2024.

Quoted scope: a 5.8 m x 2 m plain concrete path, 100 mm thick, reinforced, broom finished, with spoil removed from site.

What was built

subgrade photo
No adequate sub grade or prep done
Poor concrete pathway work with no bar chairs or subgrade preparation
No bar chairs, no subgrade
no anything that is required
No joints
Concrete pathway with no dowels installed
No dowels installed for lateral restraint
Concrete pathway with no expansion joints
No expansion joints
Below-average concrete workmanship
Below-average workmanship all round
Concrete edge left untidy and unfinished
No care at all for something that takes perhaps 20 mins to do

What was wrong

  1. Thickness: the concrete was not 100 mm thick as quoted.
  2. No subgrade preparation: existing material was barely removed, with no compacted road base, crusher dust or sand bed, and no plastic membrane.
  3. Undersized mesh: it looked like SL52 from the photos, which is light for crack control.
  4. No bar chairs: the mesh sat on the ground instead of at the correct cover inside the concrete, so it does little to control cracking.
  5. No dowels into the existing concrete: the new path can move independently of the old one, creating a lip that will need grinding to stop it being a trip hazard.
  6. No edge finishing: square, off-form edges with no hand finishing. A bullnose edge looks better and gives sealant a proper surface to bond to. It's about a 20-minute job.
  7. No control joints: these should be at about 1.8 to 2 m spacing along the path.
  8. No isolation joints: needed against the house slab and around the downpipe penetration.
  9. No expansion joint where the new path meets the old path.
  10. No polyurethane sealant at the house and downpipe. Sealant keeps water and dirt out of the joints and reduces movement and cracking.
  11. Garden edge not finished: it wasn't chamfered down to the path level, leaving a new trip hazard.
  12. No fall away from the house: see the drainage requirements below. The falls and transitions should have been agreed with the OT and signed off before work started.

Behind all of these is the real problem: there was no detailed scope, no quality management and no hold points, and no understanding of what the NCC and Australian Standards require. There should have been a proper contract setting out scope, time, cost and quality.

Plan of a typical driveway and path showing control joints at 3.0 m max in the driveway and 2.0 m max in the path, isolation joints at the house, garage and pit, and an expansion joint at the boundary.
Figure 2 - Typical Isolation Joint Locations

What a properly built path includes

A clear scope and sketch

Every quote should come with a sketch showing size, shape, location and total area, plus falls, levels, set downs, thickenings, edge finishes and the location of every control, isolation and expansion joint. Describe the area properly, for example: "11.6 m² plain concrete path, broom finished, to front elevation adjacent to front entry door."

Concrete strength and thickness

  • Strength: N20 (20 MPa) minimum. Use N25 or higher where salt or groundwater is present in the soil.
  • Thickness: 75 mm is the minimum for foot traffic and 100 mm for vehicles under 3 tonnes. I'd always do 100 mm on a path. The extra cost is small, and most people expect 100 mm anyway.
Common slab sizes for paths and driveways
UseMin thicknessMax joint spacingMesh, minor crack controlMesh, moderate crack control
Footpath75 mm2.0 mSL52SL92
Driveway100 mm3.0 mSL72SL102
Heavier driveway150 mm5.0 mSL922 layers SL92

Typical values. See our C23 driveway checklist for site-specific checks.

Subgrade

  • Remove topsoil, vegetation, roots, debris and other foreign matter.
  • Install 100 mm of road base, dampened and compacted.
  • Then a polyethylene film or about 20 mm of fine sand, so the slab can slip across the subgrade as it expands and contracts.
  • Unless you have a soil report, assume the site is reactive and prepare it accordingly.
Cross-section of a 100 mm concrete driveway: mesh on bar chairs with 30 mm top cover, 20 mm sand or plastic, 100 mm compacted road base on natural clay.
Pavement cross section - subgrade

Reinforcement

  • Mesh such as SL82 or SL92, unless the soil conditions are known to be stable enough for a lighter grade.
  • Mesh supported on bar chairs, at least 4 per m² or enough to hold it in place during the pour. Don't rely on lifting the mesh into position during the pour.
  • About 30 mm top cover and 40 mm end cover to the mesh.

Joints and sealant

  • Control joints at regular spacing along the path.
  • Isolation joints: 10 mm minimum compressible filler (such as Abelflex) against walls, the house slab and around penetrations such as downpipes.
  • Expansion joints with dowels where new concrete connects to existing concrete, so the two can't move apart vertically.
  • Polyurethane sealant over the filler: about 12 mm deep and no more than 15 mm wide. Mask the edges, apply the sealant, dust it with fine sand, blow off the excess, remove the tape and clean up.
Section where a concrete path meets a brick veneer house slab: full-depth compressible filler with sealant on top, shown enlarged with the sealant at a width to depth ratio of 2:1 (best for elasticity, lowest modulus) and the filler setting the sealant depth. The path falls away and stays 75 mm below the top of the exposed slab edge for termite inspection, below the flashing, DPC and weep holes. The flashing runs from the stud frame across the cavity to the outside face of the brickwork, with weep holes in the course above.
Isolation joint detail
Section through a dowelled expansion joint in a 100 mm slab: full-depth compressible filler with sealant on top, a 12 mm dowel at mid-depth cast into one slab and sliding in a capped sleeve in the other, and mesh stopping 40 mm short of the joint.
Concrete expansion joint detail

Drainage and levels

Under the NCC 2022 Housing Provisions, paved surfaces next to a house must fall away from it: at least 25 mm over the first 1 m in low rainfall intensity areas, and 50 mm over the first 1 m otherwise. In most of coastal Queensland, plan on the 50 mm (1:20) fall. The path also needs to finish at least 50 mm below the top of the house slab.

Edges and transitions

Finish all edges with a bullnose trowel (about a 12 mm radius) and blend any garden edging down to the path level so there's nothing to trip on.

Repair options

Option 1: an applied coating. Before I saw the pre-pour photos, a coating such as a spray-on overlay looked like a possible fix. After seeing them, I didn't recommend it:

  • The client would likely want adjacent concrete coated to match, which grows the scope.
  • A cover-up is an admission there's a problem without actually fixing it.
  • A coating won't stop the plain concrete underneath cracking, and the cracks will come through.
  • The subgrade is likely to move once the ground is saturated, and the coating will crack with it.
  • Trust had already been lost. Anything short of replacement would look like a half fix.

Option 2 (preferred): remove and replace the path using the specification above.

Repair cost plan
Note: Connolly Expansion joints were deleted from the updated cost plan (not required)

Lessons for anyone paying for concrete work

  • Get a written contract with the scope, cost, time and quality requirements spelled out.
  • Ask for the specification in the quote: concrete strength, thickness, mesh type, subgrade preparation, joint locations and edge finish.
  • Check before the pour. Look for compacted base, membrane or sand, mesh on bar chairs, formwork set to falls and joint locations marked.
  • Don't accept "this is how we always do it". Good trades work to the Australian Standards and use inspection and test plans with hold points.
  • Remember who is responsible. It's always the licensed contractor's responsibility to comply with the NCC, the standards and the Queensland Development Code, even on small jobs.

For NDIS and insurance-funded work, the bar should be higher, not lower. This work is paid for with public or insurance money, the contractor carries full responsibility for it, and one day it may be audited against the standards.

References

  • National Construction Code 2022, ABCB Housing Provisions Standard
  • AS 3600:2018 Concrete structures
  • AS 1379 Specification and supply of concrete
  • AS 2758.1 Aggregates and rock for engineering purposes, Part 1: Concrete aggregates
  • CSIRO, Foundation Maintenance and Footing Performance: Understanding and preventing soil related building movement
  • Cement Concrete & Aggregates Australia, Guide to Residential Slabs and Footings in Saline Environments (T56)

Report compiled by Anthony Painter.