You walk outside one morning, look at the driveway and there it is.

A crack.

Yesterday you had a perfectly respectable concrete driveway. Today you're staring at a line across it wondering whether the whole thing is about to fall apart.

The good news? Not every crack in concrete means there's a serious problem.

Concrete can develop cracks for several reasons, including normal shrinkage as it cures, ground movement, poor drainage, heavy loads and inadequate preparation. Some cracks are largely cosmetic. Others can be warning signs that something underneath the concrete needs attention.

Here's how to tell the difference.

Is it normal for concrete to crack?

Some cracking in concrete is relatively common.

Concrete contains water when it is placed. As it hardens and dries, it undergoes changes in volume and can shrink. Because concrete is strong in compression but comparatively weak in tension, stresses can result in cracking.

This is one reason control joints are deliberately installed in concrete.

They're designed to encourage shrinkage cracking to occur at planned locations rather than wherever the concrete feels like making its own design decisions.

However, not all cracks should simply be dismissed as "normal concrete cracking".

The width, location, pattern, movement and cause all matter.

Let's look at eight common reasons concrete cracks.

1. Concrete shrinkage

One of the most common causes of cracking is drying shrinkage.

As excess moisture leaves concrete after placement, the concrete can contract. If that movement is restrained, tensile stresses develop and cracks may form.

These are often relatively narrow cracks and don't automatically indicate that the slab has structurally failed.

Correct concrete design, placement, curing and appropriately positioned control joints all help manage shrinkage.

Should you worry?

Fine, stable shrinkage cracks are often more of an appearance issue than a major structural concern.

However, cracks that continue widening, show vertical displacement or allow significant water penetration deserve closer attention.

2. Poor ground or base preparation

What's underneath your concrete matters enormously.

Before a driveway, pathway or concrete slab is poured, the ground needs to provide appropriate support. Depending on the job, this may involve excavation, preparing the subgrade and installing and compacting suitable base material.

If the ground or base underneath the slab settles unevenly, the concrete above may lose support.

Eventually, the slab can crack.

Imagine putting a biscuit across a gap and pressing down on it.

Actually, don't. We've all seen enough biscuits sacrificed in the name of science.

The point is simple: concrete needs proper support underneath it.

Should you worry?

Cracking accompanied by sinking, rocking sections, noticeable height differences or significant movement can indicate an underlying support problem rather than simple surface cracking.

3. Ground movement

Melbourne doesn't have identical soil conditions everywhere.

Ground can expand, contract or move due to changes in moisture, soil characteristics, nearby excavation and other site conditions.

If the ground underneath concrete moves, the slab may move with it.

This is particularly important where there are reactive soils or significant variations in moisture.

Should you worry?

If cracks repeatedly widen, sections of concrete are lifting or sinking, or the crack appears to be associated with substantial ground movement, it's worth having the area assessed.

Simply filling the visible crack may not address what's causing it.

4. Poor drainage and water

Water and concrete need to get along during construction.

Afterwards, we'd prefer water to know where it's going.

Correct levels and falls help direct surface water away from areas where it shouldn't collect.

Water entering around or underneath concrete can contribute to erosion, soil movement and loss of support in some situations. Drainage problems can therefore contribute to cracking or make an existing problem worse.

Look for signs such as:

• water pooling beside the crack

• erosion around slab edges

• constantly wet areas

• water travelling underneath the concrete

• nearby drainage problems

• sections beginning to settle

Should you worry?

A crack plus an obvious drainage issue deserves more attention than the crack alone.

Fixing the crack without addressing the water problem may simply mean meeting the same crack again later.

5. Control joints are missing or poorly positioned

Concrete is going to move.

Control joints — also called contraction joints — are deliberately created to help manage where shrinkage cracking occurs.

You'll recognise them as the straight grooves or saw-cut lines across many concrete driveways, pathways and slabs.

Their spacing and location should be considered as part of the concrete layout.

Without appropriate joints, the concrete may create its own.

Unfortunately, concrete has absolutely no interest in symmetry.

Should you worry?

A crack between control joints isn't automatically a serious structural problem, but widespread uncontrolled cracking can indicate issues with joint layout, shrinkage, curing or other aspects of the installation.

6. Heavy loads

Concrete thickness and construction should suit the loads the area is expected to carry.

A residential driveway regularly used by passenger cars isn't necessarily designed for repeated use by heavy trucks, machinery or other substantially heavier vehicles.

Excessive loading can place additional stress on the slab, particularly around:

• slab edges

• driveway entrances

• unsupported areas

• service trenches

• weakened sections

• areas where the base has settled

Should you worry?

If cracking appeared after heavy vehicles or machinery started using the area, particularly where sections have moved or broken, loading may be contributing to the problem.

If you know heavier vehicles will regularly use a new driveway, tell your concreter before construction — not after the concrete starts complaining.

7. Tree roots and nearby vegetation

That beautiful tree beside the driveway can occasionally become a less charming neighbour.

Growing roots can affect soil conditions and, in some circumstances, exert pressure beneath concrete.

Trees can also influence the moisture levels in surrounding soil.

The result may be lifting, cracking or movement of nearby pathways, driveways and other concrete areas.

Should you worry?

A crack associated with obvious lifting near a mature tree is worth investigating.

Don't immediately start chopping roots, though. Removing major roots can damage or destabilise a tree, and tree controls may apply depending on the location and council.

The cause should be assessed before deciding on the repair.

8. Poor concrete placement, finishing or curing

A driveway isn't finished simply because the concrete truck has left.

Placement, finishing and curing all influence the finished concrete.

Concrete needs suitable conditions to develop strength. Rapid moisture loss, inappropriate finishing practices and other installation issues can contribute to surface defects and cracking.

Hot, dry or windy weather can make moisture management particularly important.

Should you worry?

Fine surface cracking may be cosmetic, but extensive cracking, deterioration or other defects should be assessed to determine whether you're dealing with a surface issue or something more significant.

What do different concrete cracks mean?

The appearance of a crack can provide clues, although diagnosing concrete purely from a photograph isn't always reliable.

Hairline cracks

Very narrow cracks can occur due to shrinkage and may remain stable.

If there's no movement or height difference between the sides, they may primarily be cosmetic.

Keep an eye on them.

Wide cracks

A wider crack deserves more investigation, particularly if it has grown over time.

The width alone doesn't tell the entire story, but significant widening can indicate movement.

Cracks with one side higher than the other

This is more concerning.

If one side of the concrete has lifted or dropped relative to the other, there may be movement underneath the slab.

It can also create a trip hazard.

Cracks with sinking concrete

If the slab surrounding a crack has visibly settled, the problem may involve the base or ground underneath.

Simply filling the top of the crack won't necessarily solve that.

Cracks near the edge of a driveway

Edges can be vulnerable, particularly if they're subjected to vehicle loading without adequate support.

Broken corners and edge cracking should be assessed to determine whether a local repair is practical.

Cracks that keep coming back

You repair it.

It returns.

You repair it again.

At some point the crack is trying to tell you something.

Recurring cracks can indicate that movement or another underlying cause hasn't been addressed.

When should you worry about cracks in concrete?

Consider having the concrete inspected if you notice:

• the crack is getting wider

• one side is higher than the other

• the concrete is sinking or lifting

• several significant cracks are developing

• pieces are becoming loose

• water is entering or collecting around the crack

• the crack creates a trip hazard

• cracking appeared following ground movement

• heavy vehicles have damaged the area

• previous repairs keep failing

A crack doesn't automatically mean the entire driveway needs replacing.

But it's much easier to make a sensible repair decision once you understand why it cracked.

Can cracked concrete be repaired?

Often, yes.

The right concrete crack repair depends on what caused the damage, how much movement has occurred and the condition of the surrounding concrete.

Depending on the situation, repair options may include crack treatment, localised concrete repairs, removal and replacement of a damaged section, or more extensive replacement where the existing concrete is no longer performing adequately.

The important part is determining whether you're repairing:

the crack

or

the problem causing the crack.

They're not always the same thing.

Should you repair or replace cracked concrete?

Repair can make sense when damage is localised and the surrounding concrete remains sound.

Replacement may make more sense when there is extensive cracking, substantial movement, widespread deterioration or multiple failed sections.

There isn't much value in repeatedly patching concrete that's waving a white flag.

On the other hand, ripping out an otherwise sound driveway because of one minor stable crack can be unnecessary.

A proper assessment should consider:

• extent of cracking

• slab movement

• condition of surrounding concrete

• drainage

• base condition

• cause of the damage

• safety

• appearance

• expected future use

• cost of repair versus replacement

How can you reduce the risk of concrete cracking?

No reputable concreter should promise that concrete will never crack.

What good concrete construction can do is reduce the risk and manage how the concrete behaves.

That means paying attention to:

• site preparation

• suitable base material and compaction

• appropriate slab thickness

• reinforcement

• control joints

• drainage and falls

• concrete specification

• correct placement

• finishing

• curing

• expected vehicle loads

Most of that happens before you get the satisfying finished-driveway photo.

Unfortunately, the least glamorous parts of concreting are often the parts doing the most work.

Found a Crack in Your Concrete?

Don't panic — and don't automatically reach for a tube of filler either.

Look at the crack.

Has it changed? Is the concrete moving? Is water involved? Is one section sinking? Is it creating a trip hazard?

Understanding what's happening around the crack is the first step towards deciding whether it needs monitoring, repairing or replacing.

DENZ Concrete & Property Services provides concrete repairs across Melbourne for driveways, pathways, slabs and other residential concrete areas.

We can assess the damaged area, look at the surrounding concrete and discuss whether a localised repair or replacement is the more practical option.

Have cracked or damaged concrete? Request a quote from DENZ Concrete & Property Services.