What are the reasons - Patio slab pointing crumbling.

[utterlydiy]

Roughly this time last year, I filled the gaps between my 30 x 24 slabs.

And now some is crumbling or disappearing. Decided to dig out the pointing to re-point.

Found the pointing so easy to get out, where a screwdriver can easily be pushed through where its so sandy although the pointing was filled to the thickness of the slabs 2" thick.

The mix was 4:1 Portland cement - building sand, mixed just alittle drier than bricking cement. When filling and tamping down to fill the gaps, moisture always came to the surface suggesting to me it was not too dry.

The joints are at the rear of the house north facing and can remain damp in the winter, but no freezing as we had a mild spell.

The cement was just outside its 'use by date', would this make any difference as it was still dusty not binding itself as with damp.

Any clues of why/reasons or what can be done to help prevent the sandy crumble.

thanks

 

[utterlydiy]

According to the 'Paving Expert', the mix was weak and application wrong.

Should have used a Class II Mortar and the joints given a soaking before pointing to stop either the ground or slabs absorbing the moisture out of the mix weakening it to sand/dust.

http://www.pavingexpert.com/mortars.htm#mortar

Comments please ??