Concrete sub base for flags - Will shrinkage cause issies

[Clawhammer]

I need to lay a patio, approx. 3m x10m on poor ground in my garden. I am planning on 25-30mm sandstone flags on a strong wet grit sand/cement bedding mix, on a concrete sub base. I know a concrete sub base might be over the top but want to avoid any future movement given the soil conditions. If I don’t use expansion joints are any shrinkage cracks likely to move the flags. Given the area I will probably need to do the sub base over 3 days and use simple undoweled construction joints. Will this make things any worse ? Would 142 mesh, probably totally over the top, make any difference With staggered slab laying.

I suspect you might tell me to forget about concrete altogether and use alternative sub base. However light soil and large trees in neighbours garden give me some cause for concern.

Thanks

[Tony McC]

You need some form of movement joint in a slab that size. At 100mm concrete thickness, the maximum recommended span would be 3m, so I'd look at splitting that 10m length into three bays at 3.33m width.

For a simple patio, it's not essential to carry these through the laying course and paving layers, as the jointing between flags will act as a line of 'failure' . We would bring them right up to surface level on a commercial pavement, but the loads on a patio are much less, so you can get away with it.

No great need for dowels, either. You *could* use them, if you wished, but they're not critical for a Class IV pavement (patio). I'd opt for poly-fibres rather than steel mesh, too - that bit cheaper and so much easier to work with.

[Clawhammer]

Tony thanks for your help. I've used this site for years but never planned anything quite as big before.

Just so I've got this right I can get away with simple butt joints between the bay's with no need for anything else between them and when I lay the flags I need to avoid joins along the bay joints ?

If I did it when warmer I was thinking of using a DPM. Would it best best to avoid that between the bay joints and just stick with concrete to concrete to prevent any bigger gaps than necessary ?

Thanks Stuart

[Tony McC]

You *could* rely on Day Joints between alternate bays of the concrete base, but for what it costs, I'd slip in a bit of fibreboard or similar just to make sure there's room for movement.

Then don't fret about the paving joints: let them land where they will. They act as the weakest point in the pavement surface, so any movement will reflect through them, zig-zagging as necessary in most cases, althoughh, if you are using units larger than say 600x600, there may be the occasional cracked flag if the bond between bed and flag is particularly strong. The alternative is to mirror the Movement Joints by creating full width joints in the paving directly above the MJ, but that can sometimes mar the look of the finished project.

I'd *always* use a DPM with concrete, regardless of the season. It maintains the critical w/c ratio while the concrete cures and then it offers protection against corrosion from aggressive groundwater, which should always be considered in indiustrial/urban areas or anywhere mining, tanning or steel-making took place.

[Clawhammer]

Many thanks. Will do as advised. Have just finished smaller patio using your recommendations, mixes calculations etc. and it went like a dream. My other one from 17 years ago has also stood the test of time.

Stuart

[Tony McC]

Glad to be of service!