What blocks should I use for a concrete pad?

[Paul Maggs]

Hi,

Apologies if this is a daft question, I'm rather new to this stuff.

I need to put in half a dozen concrete pads (450mm x 450mm) for a self build conservatory base. The chap that measured up suggested that I put a few concrete blocks in the holes and concrete over these. Assuming that this approach is okay, do I need medium or high density blocks?

Any help greatly appreciated.

Thanks

Paul

 

[geraldthehamster]

I'd have thought 7N dense blocks, but there'll be a Proper Builder along in a minute I expect.

 

[Paul Maggs]

Thanks. Wickes have both medium and high density 7.3N blocks. Would prefer to go for the medium ones simply because they are easier to transport.

 

[John D v2.0]

It's not a block of flats so anything will do. You could probably lay it on celotex (0.15N) and still not have a trouble. Not that I'm suggesting it though!

 

[John D v2.0]

Just make sure they are suitable for below ground/frost resistance/permanently damp. That's the most important.

 

[Paul Maggs]

Thanks. I'll file this thread for when the front of the conservatory is six foot under!

 

[Paul Maggs]

The medium don't actually say they are suitable for below ground whereas the high do. Will this make any difference though if they are encased in concrete?

 

[John D v2.0]

Don't trust me on it, another way is 7N x 450 x 450 x 6 / 10nkg-¹ = 850 metric tonnes
Then ask if that's enough to hold your conservatory up.
The real risk is the ground bearing capacity (could well be similar to the celotex) and any movement of the ground due to seasonal water changes.

 

[John D v2.0]

The medium don't actually say they are suitable for below ground whereas the high do. Will this make any difference though if they are encased in concrete?

You can get "trench blocks" which are lighter, made of done kind of foam. Have a Google, maybe you can get them delivered too. Not sure on price.

 

[Paul Maggs]

Thanks all, much appreciated.

 

[^woody^]

Two blocks side by side won't spread the load over a 450mm2 area, so if that is important then you need to lay and bond several courses.

 

[cjard]

There are 103 concrete blocks in a cubic metre. Wickes charge 1.80 a block or 1.30 if you buy a pack. This means a cubic metre of blocks from Wickes is £135, then as woody says you need to bugger about mixing up mortar and laying them

Trench blocks are more expensive(faster to lay, come out the ground quicker, you pay more for the speed because time is money)

A cube of concrete from my local ready mix (mix on site) wagon was £120 last time I bought it (6 months ago) in a small quantity

Ditch the blocks idea and just fill the holes with concrete. Cheaper, faster, easier.

 

[tony1851]

Don't trust me on it, another way is 7N x 450 x 450 x 6 / 10nkg-¹ = 850 metric tonnes

Err....Hmmmm............!!!

 

[John D v2.0]

Err....Hmmmm............!!!

Isn't it? I think my working is right. Those things are bloody strong, you can rest a steel on that's holding up half your house!

 

[tony1851]

First of all, the bearing capacity is slightly less than 7N - it's something like 6.4N when you take into account the weakness of the mortar joints.
This is also assuming the blocks are laid upright; most builders think that laying the blocks flat is just as strong but it isn't - bearing strength is down to about 3.4N when laid flat.
Then there are certain reduction factors to allow for. For small-area piers (450 x 450), the strength-reduction factor is around 0.75.
Then there is the safety factor stipulated by the masonry code, which is 3.1 in this case.
Ultimately, if the blocks were to be laid flat and cross-bonded, the allowable stress would be (1.25 x 3.4 x 0.75)/3.1 = 1.02 N/mm², which is a lot less than the 7N we started with!
But I do agree, that in the OP's cse, this won't really matter and his 6 piers will be fine for a conservatory, as long as it isn't build on an old tip or made ground.