Forum: Block Paving
Topic: Section of damp blocks
started by: Gappo

Posted by Gappo on 05 Jan. 2019,11:38
We have damp blocks on our recently installed drive which donít seem to be drying out, any help would be appreciated

Link to picture
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Posted by seanandruby on 05 Jan. 2019,13:10
"Recently laid" says it all, it is winter and materials will be damp. Give it a few months untill dryer weather arrives, if still damp then get back to us  :;):   :)
Posted by Tony McC on 05 Jan. 2019,17:47
Some individual blocks can hold on to moisture for longer than their neighbours. I have a classic photoie soemwhere that shows every Large block in a 3-size mix of a poipular tumbled block remaining damp while every Medium and Small is bone dry.

In that particular case, a slightly different concrete mix had been used for the Large blocks, and it turned out to be slightly more permeable/porous. The Large blocks not only held on to moisture for longer after a shower of rain, they were also more adept at drawing moisture from the sub-layers during warm, dry periods.

However, as Sean said, at this time of year, when it's rare we get a truly dry external surface, it really isn't uncommon for newly laid blocks to remain damp well into the Spring.

Your photie link is password protected, so I can't see what you're looking at, but I would hold off panicking for at least a couple of months, I reckon.

Posted by Gappo on 05 Jan. 2019,19:23
Thanks for the advice, it also has not had the sand brushed in yet, would you suggest just brushing in the dry areas first?

New link to picture

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Posted by Tony McC on 09 Jan. 2019,11:25
Here's the photie you sent to me....

...and it shows a classic case of a single pack of blocks that are more porous than other packs, and therefore appear damp, and, more worryingly, there has been practically no randomisation of the blocks from this pack with blocks from other packs.

More or less every CBP manufacturer stipulates that blocks should be drawn from a minimum of three packs and randomised prior to laying to even out any variation in shade, texture and porosity. As soon as I see something like this, I know that the installation team is of the "Couldn't Give An Eff" variety.

However, it's not the end of the world. As the blocks dry out naturally and then get re-wetted with rain and suirface water, the interstices (tiny gaps and voids) within the concrete will become filled with micro-particles from the environment and, in 6-12 months' time, they'll be virtually indistinguishable from their neighbours.

Is there a reason why there has been no sand jointing done? It's critical that CBPs are jointed as soon as possible after installation and compliance checks, even if the surface is damp and minimal sand falls into the joints. Some sand, no matter how little, is better than no sand, and the surface can be topped up with further KDS as soon as practical.

It's a bad, bad idea to traffic a CBP surface that hasn't been jointed. The KDS is what locks the surface into place and minimises/prevents settlement.

Posted by Gappo on 10 Jan. 2019,14:52
Thanks for the reply Tony. The reason no sand has been brushed in is because we have not really had a dry day since the drive has been completed (there has been no traffic over it yet either) Hopefully will get chance to brush some sand in this weekend. Do you recommend doing the dry bits first & waiting until the damp blocks dry out even more or just attempt the whole lot & top up later if required?
Posted by Tony McC on 10 Jan. 2019,17:03
Do the dry bits first, then the damp, but GET SOME SAND IN NOW!

There is a technique used far more extensively in Northern Europe for jointing damp block surfaces, but it needs the sub-base to be super-draining and someone with a bit of experience (and a brush-in machine if they have one). It relies on using water to wash-in the jointing sand, and it really does do a phenomenally good job.

For damp British surfaces, you can brush-in as much as possible, and leave surplus sand on the surface. Re-brush every day, adding more sand if necessary, until the joints seem as full as they are going to be.

Bear in mind that, even during hot weather with a bone dry surface, we expect jointing sand to drop and require topping-up after about 6-8 weeks, so don't expect miracles in January!

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