The Brew Cabin

Glad I found this site - it looks like I might get a good response to my questions here, judging by the quality of the other threads I have read!

I have got an old (dry) cess pit in the garden. It has a 250mm thick 3M diameter concrete cap with an inspection manhole in it. The cess pit is either unused or no longer in use. i would like to lay a circular patio over it, rather that try to break it up. I need to raise the level by a total of 120mm to be the same as the grass. I have bought some pre-cast slabs to form a circle which are 35mm thick. So, the problem is I need to raise these slabs by up to 85mm where the existing concrete cap is lowest. The methods open to me are, as I see it:

1 The '5-dab' method. I think 85mm would be too high, and I'm not keen on this method anyway - voids holding water and rodents.
2 Lay in an 85mm bed of 10:1 dry mortar
3 Cast a new slab on top (thickness?) and then lay the slabs in a bed of 10:1 dry mortar or sand. I would prefer to point to suppress weeds, so sand may not be an option.

Which of these options would be best, or are there any more I've missed?

Thanks for a great site

The "5 dab" method will get you banned from this website, or, as this is a first offence, a minimum of 100 lines of "I have been very silly and I will never again think of using spot bedding for a paving project".

There is never a valid reason to use spot bedding for laying flags. There's a full explanation of why it is a method used only by fools and incompetent buffoons on the main website, on the Laying Flags page.

Laying on 85mm of bed is possible, but fraught with difficulty, mostly difficulty in achieving proper compaction.

The correct way to do this is to lay a screed of cement bound material over the existing slab. Make up a lean mix concrete using 6 parts of 10mm or smaller gravel with 4 parts of grit sand and 1 part of cement. No need to add extra water - use it semi-dry. Spread this lean-mix concrete (it's actually referred to as CBM - cement bound material) over the existing slab to a thickness of around 80mm and run the plate compactor over it a few times, up and down, side-to-side, to make sure you get a well-compacted base.

This base will have now compacted down to around 50-60mm thickness, so you can top it up with more of the semi-dry mix, and use this as your laying course. As this laying course is only 3-40mm thick, it will behave just as a 'normal' bedding layer and give no problems.

The advantage of this methodology is that it's not messy, you're not mixing all sorts of different concretes and bedding mixes, and there's no need to rush, as the semi-dry mix gives you at least 4 hours working time, even on a day like this!

How's that? :)

Thanks for the speedy repsonse Tony,

A few more questions as a result of your response:

1 Wickes are my nearest supplier who can deliver. They sell a couple of products by the 'Jumbo Bag'. One is MOT Type 1, the other is Ballast. I presume I need the latter for this project?
2 I don't have access to a plate compactor - could this be achieved realistically using any other non-mechanised method?
3 Will the concrete bond to the slabs when using a dry mix like this? Would the slabs need to be soaked first?

Thanks for your advice

Laurence

1 - Ballast is fine for making a general purpose concrete. MoT, as you call it, is a sub-base material not really suited to making concrete.

2 - If you;ve no plate, then you can rely on trampling the concrete with your boots to get a fair degree of compaction. This is not recommended for depths greater than 75mm, but for what you are doing, it should be ok.

3 - Why do the flags need to bond to the bedding? This is a common misconception, I'm afraid. There is no need for them to bond to the laying course material: in fact, it's better if they don't! :)

Thanks again Tony for the advice and correcting some misconceptions!

Some final Q's before doing the job this weekend:

1 Wickes don't know what preportion of sand to grit is in their ballast! Would it be safe to assume it would be OK for this job, or should I go elsewhere? I am guessing it is 2:1 gravel to sharp sand

2 Hire of a vibrating plate is about £30 for a day. Would this be money well spent for a circular patio of 3M diameter (7 sq m) or should I use my trusty boots and a tamping bar?

3 Should I let the first screed go off before putting down the laying course of semi-dry mix, or can it be laid straight away?

4 I presume that a cement dye can be used in a semi-dry mix for brushing into the joints for pointing?

Thanks for all your advice

1 - As a general rule, Wickes don't know which is their left and which is their right! The all-in-ballast is, nine times out of tenn, fine for a general pupose concrete.


2 - Forget the plate compactor - your boots will be fine for such a relatively small job using a cement bound base and bedding layer


3 - You can lay the bedding material directly over the base layer, no need to wait.


4 - Not really. Cement dyes need moisture to bring out the colour, so they don't work with dry brush-in grouts. :(

Well, the slab laying went well using Tony's advice. The only issue now is finding somwhere that stocks white Portland cement to use for the pointing. I mixed up some grout using ordinary Portland and various amounts of cement dye and left it for a few days, but the colours looked awful. I asked in Wickes if they has any white Portland and they looked at me as though I was from Mars! No surprise really I guess. I wanted to point up straight away, but this has put the preverbial spanner in. Off out tomorrow to some merchants to see if I can get any of the stuff.

Any decent BM will be able to supply white cement, or order it in for you.

However, the difference in shade afforded by using whirte cement is minimal for darker colours of mortar. It's the colour of the sand that has a dominant effect. If you're using a red building sand, as is common in much of western Britain, then browns and shades thereof are easy to achieve, but with the yellow sands of Eastern and south-eastern England, then browns are more tricky and blacks/greys are almost impossible.

Have you thought about using a silver sand instead of, or even as well as, a white cement?