Short answer
Concrete cracks because it shrinks as it dries and because the ground under it moves. Fine, straight hairline cracks that stay the same width are normal and cosmetic. Cracks that are wide, growing, stepped so one side sits higher, or that let water pool, point to a problem under the slab, usually a base that was never compacted on reactive clay or a slab poured too thin, and those are fixed by replacement, not patching.
Why concrete cracks at all
Concrete is a liquid that becomes a rock, and it loses water as it does. That loss makes it shrink by a small amount, and because the slab is held in place by the ground, the friction and the edges, something has to give. What gives is the concrete, in a crack. This happens to every slab ever poured, including the ones at the Sydney Opera House, which is why concreters cut joints: a straight groove, tooled in while the concrete is wet or sawn once it has hardened, that is deliberately the weakest line in the slab so that the inevitable shrinkage crack opens there, out of sight in the bottom of the groove.
Both local councils write this into their crossing standards. City of Newcastle’s drawing A1300 requires tooled joints no more than 3 metres apart on a driveway crossing, and Lake Macquarie asks for a full depth saw cut at the property boundary and an expansion joint where the driveway meets the layback. The same logic applies inside your boundary. A driveway with no joints will still have joints; it will just have chosen them itself.
The second reason concrete cracks has nothing to do with the concrete. It is the ground underneath moving. A slab is strong in compression and weak in bending, so if the base under one corner settles, or swells, the slab is left spanning a gap it was never designed to bridge, and it breaks. The specification on this site describes the base, steel and thickness that keep that from happening; this post is about recognising when it already has.
Which cracks are harmless and which are not
| What you see | Likely cause | What to do |
|---|---|---|
| Fine, straight, hairline, same width all along, both sides level | Drying shrinkage | Cosmetic. Fill or seal if you want, otherwise leave it |
| Fine cracks in a web or map pattern on the surface only | Surface dried too fast in heat or wind | Cosmetic, but a sign of rushed finishing |
| Crack running from a corner, a pit or a change in shape | No joint or steel at a stress point | Usually stable; watch it for a season |
| One side higher than the other, you can feel the step | Base settled or ground heaved | Structural. The slab is bridging a void |
| Crack that keeps widening, or a slab that rocks underfoot | Base failure, soft ground, water under the slab | Structural. Replace the section, rebuild the base |
| Crossing split at the kerb or layback | Movement at the join with council’s kerb, thin slab, no dowels | Replacement, with a new Section 138 application |
The test that separates the two groups is movement. Put a straight edge across the crack: if both sides are level and a coin dropped in the gap does not disappear, the crack is almost certainly shrinkage. If there is a step, or the gap has grown since you first noticed it, the ground is moving and the slab has already failed, however small the crack looks today.
What Newcastle’s ground does to a slab
The region changes underfoot within a short drive, and the ground explains most of the cracked concrete in it.
Reactive clay to the west. From Wallsend out through Cameron Park, Edgeworth and Fletcher, and across much of Lake Macquarie’s inland suburbs, the soil is clay that swells when it is wet and shrinks when it is dry. A slab laid straight onto it rides up in a wet winter and drops in a dry summer, and after a few of those cycles it cracks at whichever point the movement was greatest. The defence is a base: the soft and reactive material cut out, a granular base brought in and compacted in layers where the ground calls for it, so the slab sits on something that does not change shape with the weather. The Cameron Park and Wallsend pages describe what those blocks are like.
Sand near the coast and the lake. Merewether, Belmont and the lakeside suburbs sit on sand that drains well and does not swell, which is good news. Its weakness is at the edges: sand migrates from under an unconfined slab edge, especially where water runs along it, and the corner is left unsupported. Thickened edges and a confined base are the answer.
Fill and old workings in the older suburbs. Council’s own history records that parts of Merewether were built on brick clay pits filled with colliery waste, that Wallsend’s town centre stands on raised swamp, and that Lambton had eleven collieries. Nobody can see any of that from the footpath. A slab in an older suburb can be sitting on a century of fill that was never compacted to a building standard, and the honest answer at a measure is to dig until the ground is firm and build up from there.
Trees. Street trees and large garden trees do two things to concrete: roots lift it from below, and in a dry spell the tree draws moisture out of clay and shrinks the ground under the slab. Lake Macquarie’s driveway standard goes as far as requiring small trees within 2 metres of a new crossing to be removed and replanted further away, and City of Newcastle’s drawing keeps a crossing 3 metres from a trunk. Both councils are telling you something about how often trees crack driveways.
The two faults a concreter builds in
Ground is the cause you cannot control. These two are the ones you can, because they are decided on pour day.
Mesh on the dirt. Reinforcing mesh works only when it is inside the concrete, held up on bar chairs so it sits in the middle of the slab’s depth. Laid on the ground and poured over, it ends up at the very bottom or outside the slab entirely, doing nothing. It is the easiest corner to cut because the finished surface looks identical, and it is the most common thing found when a cracked driveway is broken out. Ask any concreter quoting you how the mesh is supported, and expect the answer “on chairs”.
Thin concrete. A driveway for cars is commonly poured around 100 mm thick in 25 MPa concrete, which is also City of Newcastle’s minimum for the crossing. Lake Macquarie asks for 125 mm on its crossings. A slab poured at 75 mm to save a few cubic metres has about half the bending strength, and it is the first thing to go when a delivery truck or a loaded trailer stands on it. If a quote does not state thickness, strength and mesh grade, it is not describing a slab you can compare.
Does mine subsidence crack concrete?
It can, but it is near the bottom of the list. Parts of Newcastle and Lake Macquarie lie in declared mine subsidence districts, including the Newcastle, Lake Macquarie and Killingworth-Wallsend districts, and the region does have a documented case: between 2012 and 2014 about thirty homes in Roberts Circuit and McCubbin Way in Lambton were damaged when old Lambton Colliery workings about 30 metres down partially collapsed, and the state spent over $7 million on buy-backs and grouting. That is the kind of event the districts exist for.
For a driveway or path, though, Subsidence Advisory NSW’s exempt development list includes driveways and hard stand spaces under every guideline except 4 and 5. In plain terms, a flat slab is light, flexible in the ground’s terms and cheap to replace, so the state does not regulate it. If your slab has cracked and you are in a district, the cause is still almost certainly moisture, base or thickness. If your house has cracked as well, that is different, and Subsidence Advisory NSW is the call. Check any address on the NSW Planning Portal Spatial Viewer.
Repair or replace
The deciding question is whether the base under the slab is sound.
- Shrinkage cracks, no movement: leave them, or fill them with a flexible sealant. On exposed aggregate and coloured concrete, sealing the whole surface keeps water out of the cracks and slows staining.
- A single dropped panel in an otherwise sound slab: that panel can be cut out at the joints, the base corrected, and the panel repoured. Expect a colour difference between old and new concrete.
- Stepped cracks, rocking panels, cracks in more than one panel, or a crossing that has split: the base has failed across the job and removal and replacement is the only fix that lasts. Pouring a new slab over the old one copies every crack through within a season and raises the levels at the garage and the footpath.
Replacement is also the moment to get what you would have chosen the first time: a driveway wide enough for two cars, a path to the door in the same pour, or an alfresco slab sized for a table. One mobilisation costs less than three.
What a replacement costs in Newcastle
Supplied and laid, per square metre: plain broom finished concrete $95 to $130, coloured $115 to $160, exposed aggregate $130 to $185, shed and garage slabs $110 to $150, with removal of the old concrete adding about $35. A 40 m² single driveway replaced in plain concrete therefore sits around $5,200 to $6,600 including removal; the same driveway in exposed aggregate around $6,600 to $8,800. The cost guide shows how those figures are built and the calculator will bracket your own job from length and width. Published rates are a guide for budgeting. The price for your job is the one on the written quote after a site measure.
One more cost people forget: if the council crossing is part of the failure, replacing it needs council approval under Section 138 of the Roads Act, and Lake Macquarie states that a replacement needs a fresh application and fee. Both councils hold the owner, not council, responsible for the crossing. The Section 138 guide explains the process for each council, step by step.
Before you accept a quote for a replacement
Ask every bidder for six things in writing: slab thickness, concrete strength, mesh grade and how it is supported, what happens to the base, where the joints go, and who lodges the council application if the crossing is included. Then read the answers against the specification. A quote that is cheaper because it leaves out the base or lays the mesh on the dirt is a quote for the same crack again, a few winters from now.
Sources, checked October 2026
City of Newcastle standard drawing A1300, Driveway Crossing Standard Design Details (joints at 3 metre maximum spacing, 100 mm minimum, 25 MPa, 3 metres to a tree trunk). Lake Macquarie City Council, Driveways page and Minor Public Works page (owner responsibility for the crossing, replacement needs a new application, 125 mm minimum, saw cut at the boundary, expansion joint at the layback, tree removal within 2 metres). Subsidence Advisory NSW, Exempt Development List (April 2023) and Mine Subsidence District maps. Newcastle Herald and ABC News reports on the Lambton subsidence repairs, 2015 to 2017. City of Newcastle heritage walk notes for Merewether, Wallsend and Lambton. Councils change their documents without notice; treat the council’s current page as the authority.