concrete on concrete

[longdogs]

Hi

I have a large concrete area at the end of the garden approx 20ft x 12ft and about 6'' thick. I want to put a shed 10x8 on the area and will need to build up the concrete at one end because the 'fall' is a bit too much.

My question is, can I just make up some form work and lay concrete on top of the old concrete or do I have to treat the surface in any way. The original concrete has been there for about 20 years if that makes any difference.?

Thank you.

 

[PrenticeBoyofDerry]

Providing the addition depth is over 100mm, you shouldn't have a problem.
If the addition depth is less, sometimes it don't bond that well.

 

[Norcon]

If its only foot traffic then I would have said 50mm minimum using a 10mm aggregate.
And the base well power washed.

 

[longdogs]

That's great! Thanks guys.

 

[theoldun]

If its only foot traffic then I would have said 50mm minimum using a 10mm aggregate.
And the base well power washed.

50mm 0f 10mm aggregate with a cement grout slurry, is plenty good enough.

Would suggest, if shed has own floor, bed and level three rows of path edging and sit shed on them. Will be fraction of the price concrete.
oldun

 

[crayonnick]

Yep, you can pour new concrete over the old slab — I’ve done it a few times myself — but don’t just throw it on and hope for the best. Since that pad’s been there for 20 years, the prep is what really makes the difference.

First, give it a proper clean — pressure wash or scrub off all the dirt, moss, and flaky bits.

Then rough up the surface a bit with a grinder or chisel so the new mix has something to bite into.

Brush on a bonding agent right before you pour — that helps the two layers stick together. Before you pour, it’s also worth figuring out exactly how much mix you’ll need — nothing worse than running out halfway . If you’re worried about miscalculating, you can use online calculators, like https://concretecalculator.org or https://www.calculator.net/concrete-calculator.html. They make it quick to get the volume and bag estimates right.

Finally, set up your forms and pour, making sure the new layer’s thick enough (2" minimum is a good rule), especially if you’re building up one end.

 

[stevie888]

Possibly done by now??

 

[MaxBuilds]

Yes, pouring new concrete over the old slab works fine - I've done it a few times - but the prep is what makes or breaks it. Pressure-wash the old slab, rough up the surface so the new mix has something to bite into, and brush on a bonding agent right before you pour. Keep the new layer at least 2" thick where you're building up the fall.

Before you mix anything, work out the volume so you don't run short halfway:

1. Measure the area in feet: length x width.
2. Convert the average thickness to feet (inches / 12).
3. Multiply to get cubic feet, then divide by 27 to get cubic yards.
4. Add about 10% for waste.

Example: topping up a 10 ft x 8 ft area by 2" on average -> 10 x 8 x (2 / 12) = 13.3 cu ft -> / 27 = about 0.5 cu yd. An 80 lb bag yields roughly 0.6 cu ft, so you'd need about 22 bags, plus 10% = roughly 24 bags.

If you want to double-check the numbers before you buy, this free concrete calculator makes it quick: https://concretecalcus.com

One extra tip: cover the new layer with plastic sheeting for a few days while it cures - a thin topping dries out fast and that can cause cracking.