The Brew Cabin

Posted: **Fri Feb 17, 2006 10:48 pm**

Can anyone advise me please.
I have just laid a large rear patio 70m2 using cast in mold riven slabs with chamferred edges. Laid on sharp sand using the individual method. There was a large variation in slab thickness 25mm to 45mm thus various bedding sand thickness used. slabs were butted resulting in 15mm tapered joint. A 3:1 sharp sand cement pointing mortar was used. These slabs were provided by the client!!

A lot of the pointing has cracked and lifted out of joints.

Is there a problem with adhesion of the jointing mix within the V shape formed within the smooth face chamfers from these mould cast slabs?

Is there a suitable material to re-point slab to ensure it does adhere and not crack?

Posted: **Fri Feb 17, 2006 11:50 pm**

Did you use a plasticiser with the mortar?

Sharp sand often makes a mortar with reduced adhesion, compared to a mrtar made with building sand, so a plasticiser and/or a bonding agent is usually a good idea.

Also, you say the flags were laid so the base of each flag was tight against its neighbour - this gives a v-shaped plug of mortar that's just begging to be flicked out. I have discussed this with several manufacturers in the past but the long and the short of it is that they don't give a Donald Duck - it makes mould removal that much easier for them, and, after all, they're not in business to make life better for the contractors or customers, are they? :;):

Final point: if the flags are bone dry when pointed, they suck all the water out of the pointing mortar, reducing the water/cement ratio and thereby creating a weaker mortar with poor adhesion. It's always a good idea to soak the flags an hour or two before pointing.

You could re-point using an "improved" Class II mortar based on a soft building sand with a plasticiser, or you might prefer to consider using a quality polymeric product such as Rompox.

Posted: **Sat Feb 18, 2006 12:09 am**

Thanks for the info Tony
Should I have laid the slabs apart? wouldn't this make the joint size unacceptable?

Posted: **Mon Feb 20, 2006 9:04 pm**

Whether the flags should have been separated further depends on the joint profile. Ideally, you don't want more than a 15mm gap maximum at the surface, but some numpty manufacturers now create wet-cast flags that are wider at the base than they are at the surface, so unless you lay them 'tight' you end up with a 25mm crevice to fill with mortar.

And while they whine on about "lovingly re-creating hand selected stone originals", they fail to mention that most riven stone flags have edges that taper inwards - ie: the joint expands underneath the flag, whereas their "re-creations" do the exact opposite. How authentic is that? :p

The Brew Cabin

Mortar cracking on mould cast riven slabs - Info requested