The Brew Cabin

Posted: **Tue Oct 14, 2008 5:58 pm**

hi - please help!,

We had a terrace laid with random yorkstone flags (deeplane thin)for pedestrian traffic only about 1 month ago. Within a couple of weeks the corners of 5 or 6 flags moved very fractionally (1 or 2mm) when walked on, now there are 15. I don't think it is right, I have looked through this site to see what good practice should be and it was not done with the much maligned 5 spot method. The site was levelled then a layer of ballast was put down, not sure if it was consolidated with a wacker plate but I was told it was going to be, then a full bed of mortar (I think 3 or 4:1) was laid under each slab and it was tamped down. They then used wet mortar, not a very nice grey to point and there is quite a lot on the face of the slabs (architect says some is acceptable!). The mortar bed has gone off rock hard and I feel the flags should be stuck to it not moving on top in places, it is not a flexible bed of sand with flexible joints so it should be 100% set in place. I think Tony will agree but my query is - why has this happened, a)was the mix too dry? or is ballast as a base no good? b) is the rest of the 50 sq meters going to come over the next month or year? I have paid a lot of money for the work and have a meeting with the foreman and architect about it on Friday, they will lift the ones which I can show are loose now and relay, but I don't want to be left with a long term problem, I don't want the flags which I provided at great expense damaged when lifted and I want to be informed if I need to challenge them over the method they have used . One further complication is that you can only really see and hear the flags moving if it has been dry for a while and it has been so wet, last time the architect came I couldn't get one to move, although you can see a crack between the flag and the mortar around many of the flags which have moved. Would be grateful for your thoughts. Lorraine.

???

Posted: **Wed Oct 15, 2008 11:39 am**

It's impossible to comment without actually seeing the problem at first hand, but I'm surprised to hear that there has been a problem with Deep Lane Thins, as they normally bond exceptionally well to a standard mortar without the need for additives such as PVA or SBR. Traditionally, when they were laid in and around Bradford, they'd be laid on cinders but mortar has become the norm over the past couple of decades.

Two possibilities: failure of the flag to bond with the bed, which can be determined by lifting a few of the 'rockers', or partial compaction of the sub-base, which has since moved and settled resulting in cracking of the jointing.

You really need to lift a few loose units to determine the precise cause of the problem.

Posted: **Wed Oct 15, 2008 1:30 pm**

Thanks Tony will get a few lifted.

Posted: **Wed Oct 15, 2008 2:44 pm**

Very hot day, not wetting/brushing the slabs before placing them, the underlay drying too quickly, or the pavers been walked on within the first 3-4 days, are all posssible causes. Even tho the pavor laid on a full bed of concrete, he should have checked for rocking before moving on to the next slab.

Posted: **Sun Oct 19, 2008 3:21 pm**

We lifted one, after the pointing was chipped away it came up easily. Tony can you define your comment about what 'bonded to the mortar' means. The architect tells me the flags are not supposed to be 'stuck' down to the 4 in 1 mix - if they are not stuck down and these flags are only thin - won't they all start to come up over the years? - How stuck should they be, if they are not what keeps them in place - the pointing? fine cracks have appeared between the edge of some of the slabs and the pointing, the builder says this is usual because of the union of 2 different materials. They say they have fixed some of the moving flags by chipping out some of the mortar and pouring some wet cement mix into the gap and re pointing - its not something I witnessed or can see where they have done it but for the moment none of the flags are moving, some of them just sound hollow or like they are vibrating against the bedding mix when walked on. Is the architect correct that they should not be stuck down?

Posted: **Sun Oct 19, 2008 8:21 pm**

I would wait for one of the pros to confirm but your original post reads a "then a layer of ballast was put down" taking this as read, ballast (i.e. grit sand and gravel mixed, at approximately 40% gravel) is no good as a sub base and may explain the problems you are seeing? a lean mix concrete made with ballast and cement would be acceptable?

Posted: **Sun Oct 19, 2008 11:53 pm**

your architect is talking architect, when Tony says bonded he means you would not have a cat in hells chance of lifting the slabs without the aid of a large hammer and bolster. The builder is talking builder, if the slabs have movement this is a very short term solution. Any loose slabs need to be lifted and relayed.
Jim

Posted: **Mon Oct 20, 2008 11:51 am**

The architect is a typical architect. Probably does lovely drawings but knows buggerall about the construction industry. Pass on a message from me: if I promise not to design any buildings, will they promise not to pretend they know what they're talking about when it comes to paving?

When flags are laid onto mortar, it is done to prevent ANY movement, and so the flags must bond (stick, adhere, become attached to) the bed. There are situations where it is not necessary for the flags to bond with the bed, and for these projects we'd use sand, rather than mortar. This clueless architect has specified mortar bedding but then doesn't want the flags to bond: what a pillock!

The remedial work undertaken by the "builder" might do the trick, but given the problems to date, I'd want to leave it 3-6 months before declaring it acceptable.

Posted: **Mon Oct 20, 2008 12:03 pm**

Couldn't have said it better myself.

Posted: **Mon Oct 20, 2008 6:32 pm**

This paving has been part of redeveloping part of the house - you are spot on about the pretty drawings, I could bore you all to death with the problems of trying to work to them, but you all know. Thanks for all of your input, much appreciated. I will compose an appropriate email to them, pursue relaying flags where necessary as some are still moving, hold remaining payment back for 6 months and worry about the ballast (no cement added) sub-base.

The Brew Cabin

Moving flags - Correct base preparation?