Driveway cracks and sealer

Hi All

We had a new concrete driveway laid about 5 months ago. Coloured concrete 32mpa, 120mm thick, reo throughout with saw cuts every 3m approx. After it had cured for around 6 weeks (with no use) we sealed it with Crommelin Diamond Coat. Looked fantastic.

Now it still looks fantastic when dry, but when it rains the surface appears to have a 'spiderweb' cracks over the whole surface. There is also one legitimate crack across the width, which from the edges, appears to go through the full thickness.

After a day of rain the crack lines disappear - but when you compare to other new driveways in the area, the others seem to soak in water, while ours seems to repel water more.

Question is - are these cracks (either major one or spiderweb cracks) a concern and is there anything I can do? e.g. different sealing product, crack filler etc?

See attached photos taken when first wet (showing cracks) and completely wet (cracks not visible) but in both cases water largely sitting on surface, not soaking in. When dry it looks normal.




Cheers

Are they cracks - or just pooling along your paint lines ?

But - I'd be interested in how you do fix a crack in a path - a hair line one.

I've seen and poured a few slabs over the years where to help speed up the drying process so the concreters can finish off the top surface they have sparsely thrown cement dust over the surface then trowelled it. Similarly with some colouring agents/surface finishes etc.
Is your coloured concrete all the way through or just a surface treatment ?
The fine hairline cracking by the way is nothing to worry about - a lot of slabs do it , coloured or otherwise. The other crack however is more of a concern but if the reo is doing its job, should not get any worse.

Stewie

Thanks Stewie and Saint Mike.

I was here when the guys laid the driveway. No cement dust was used on the surface. The concrete mix was coloured all the way through. They used a float and then one of those spinning/fan style finishing machines.

The reo seemed reasonable, was well supported, overlapped and joined and in line with the engineer specs.

Is there anything I can/should do to treat the crack? Would you expect the sealer to behave this way (i.e. prevent water from soaking in) - when I look at other new driveways, they just seem to get wet and don't highlight the hairline cracks.

Hi
I think part of the problem is that 32MPA is an overkill for a driveway and 20-25MPA would have been sufficient. Higher cement content causes more shrinking and more dense concrete hence more impermeable concrete.
Location of your mesh is also very important, it should be 30mm down from top surface and if concrete had excessive slump when poured then more shrinkage would have occurred.

Also if reo is lower that the depth of control cut it would not have been cut, hence your significant crack.

Who specified concrete and reo?

Thanks Building-Expert.

We had engineer specify requirements for an extension we did, including the carport slab. The carport slab and driveway were done in one pour (roughly 3.3 x 15m). The engineer specified 25mpa and (from memory) SL82 mesh. The concretor said he always uses 32mpa for driveways, and I figured it was better so didn't question it.

Here is a pic of the mesh in place.



What's done is done, I guess. Is there anything I can do now to treat this crack or prevent further issues?

Also note the saw cuts are approx 15mm deep.

Your surface cracking is shrinkage cracking and there is a not a lot you can do except resurfacing.
In my similar recent report for VCAT I recommended demolition of paving.



Your control joints have not been cut deep enough to introduce a plane of weakness.

Control joints should have been marked and along the line reo should have been cut every second bar to introduce weakness along which concrete will crack.

My tip? Re cut control joints to 30mm with intermittent deep (every600mm )cuts to 60mm(through reo). That way some reo will still be holding paving together.

Cheers

The 'spiderweb' cracks have the technical name of 'mapping cracks'

The cracks are typically only 2-3mm deep and are as a result of the surface concrete shrinking more, and sometimes faster, than the underlying concrete.

The reasons for this greater/faster shrinkage include:
■ Adding water to improve concrete workability.
■ Starting finishing while 'bleedwater' (standing water from mix after concrete levelled) is still on the surface
■ Add cement to dry up 'bleedwater'.
■ Letting the surface dry before proper curing (see http://www.anewhouse.com.au/2012/05/con ... e-is-laid/)

It is a cosmetic issue, which does spoil the look, although it doesn't significantly affect the strength of the concrete.

Thanks Bashworth and Building Expert - appreciate the comment and explanations.

Re mapping cracks - These currently only are visible when the driveway is just wet. As it ages, would you expect it to get worse? is there anything that can be done (i.e. sealer etc) to prevent them from getting worse? (I've already sealed with Diamond Coat)

Re big crack - I checked the three control cuts, and on all three you can see a crack through both sides of the slab - see picture. Given this, and the driveway being now 5 months old. Is it still worth deepening the control cuts to 30mm?

They become visible because the water penetrates into the cracks. . . . so the more a sealer fills the cracks the less visible they will be.


If the slab is 5 month old the concrete should not shrink any more, and the cracks at the 'control' joints have developed to the full depth, the need to deepen the joints is less critical. (However if there is some tension in the concrete cutting as Building Expert has mentioned will help release it.)

So - if you have one crack - hairline - is there a good way to "hide" it ...

Hi Guys,
I have a similar problem, had my driveway poured about 4 months ago and now i have noticed a crack. My drive is about 5 meters wide and about 9 meters long. My now concern is, the tradie made only one cut running across the width. No cuts running across the length. Now am likely to see more cracks as a result or is there anything i can do to minimise this risk? Thanks guys.