Flag-on-edge questions - Need some advice on laying flag on edge

[oldreptonian]

Hi All

I intend to build a short (c.200mm) retaining wall using flag on edge as described on the site. I have dug the trench ready for the concrete but now have a question about the practicalities of laying this:

I intend to set the flags on 100mm of concrete and then support the flags with about 300mm of concrete on the inside. My question is: do I put down 100mm of concrete and then set the flags on top of that? if so I need the height of the flags to be the same so how do I manage this on wet concrete? is it substantial enough to place flags exactly like this or is there a danger of them sinking?

I'm planning on using a piece of timber laid parallel along the length of the trench as a guide to rest the flags against as they set. Is this advisable? Can I then back fill this with concrete easily while the base layer is still wet?

Lastly what mix of concrete is recommended for this type of thing?

I hope these questions make sense? I'm a novice at this sort of thing so would appreciate any advice.l
Cheers

Michael

[seanandruby]

Hi Michael. The trick is don't have the concrete to wet for the bedding as they will sink/float and be hard to line up. Add enough water to send the concrete off and make it workable, not to dry either. I'd use a c20 mix. You can use a length of timber as a straight edge to keep them in line at the top. Lean them back slightly 5 to 15 degrees into the bank.

[oldreptonian]

Hi Sean

Many thanks for the reply. when you say 'send the concrete off' what exactly do you mean by that?

in another area of this great resource referring to Windrow bedding, which looks similar to what I'll be doing, it suggests: "The mixed concrete should be damp enough to be moulded into a ball without falling apart." See here: http://www.pavingexpert.com/edging3.htm#mixos

Would that be a good test for what I require?

Cheers

Michael

[seanandruby]

Sorry about the terminology i used, we tend to forget most DIYers inderstanding. It means the concrete setting/ curing. Dry concrete would never go off it needs added water which makes it a lot easier to work with. Normally for concrete a slump test is carried out but for what your doing no need. To wet and your flags will sink and push over, to dry and it'll be useless and won't go off. You can use C7.5 to bed them in and C20 for haunching. Windrow bedding is nothing to do with how wet concrete is but to do with the shape of it, the concrete laid out in a heap is Windrow. But i guess you knew that
:;):

[oldreptonian]

I do now

I guess I'll just have to be careful with the water and make a judgement on how wet/dry it is. I'm going to get a mixer so is there a good rule of thumb to identify a good consistency in absence of a slump test?

If it is too wet and the flags sink a little can I wait until it has started to harden before putting the flags in place and haunching?

Thanks

Michael

[lutonlagerlout]

i would try and have at least 150mm of concrete under the slabs
we do this detail for the local authority and that is their spec,also 1/3 of the slab should be underground

the mix for this is basically dry leanmix c15

the haunching is wetter and stronger but care needs to be taken
cheers LLL