The Brew Cabin

Posted: **Tue May 26, 2009 4:48 pm**

I've got an engineer telling me I'm a fool, that you can use the sand:cement (1:3) laying course to introduce the pavement falls...
In the standards they all say to employ falls at sub-base/sub-grade level. But his argument is that, thats the case for flexible (sand) bedding only not for rigid (mortar) bedding. That the mortar bedding can be any thickness, in my case he wants to use up to max. 70mm and min. 25mm
(your main part of the website say falls to be in the sub-base)
I'm saying the mortar bedding should me 25mm (after compaction) with a +5mm or -10mm tolerance levels.

The pavement is 600x400x80mm concrete flags to take up to 5ton cherry pickers once or twice a year for maintenance. The flag supplier say the tensile bending strenght of the flags is 16.9kN so these should take the cherry picker.

If I'm wrong I'll have to eat my hat!!!!
can please someone let me know if Im right or wrong.....
Thanks

Posted: **Tue May 26, 2009 5:17 pm**

My opinion is 25mm is to thin mortar bed for the slab size and possible loading of 5ton, I would suggest 50mm bed on average
fall's need to be in the sub-base because over a long distance you will have to thicken the sand-cement mix to created the falls that will cost you more than grading the sub-base to fall's.

if you went by your tolerance +5 and -10 on 25mm thick screed you could be only 15mm thick which is worthless,

80mm slabs on mortar bed seams like alot of hard work ? why not compact,screed 50mm sand and lay? MSH

Posted: **Tue May 26, 2009 5:18 pm**

I'd say your engineer is wrong, sub base is used to create your falls,if you run a 9 ton cherry picker over a 75mm bed, there's more chance of the weight splitting the slab, rather than a 25mm ,as this would transfer the weight to the sub base.

IMHO

Posted: **Tue May 26, 2009 10:51 pm**

Ah you see, my problem is only in one particular area. where I've got a concrete roof (over a basement carpark) which is flat and only have 150mm free space between that and the door treshold.
I had specificed to use pedestal and mechanically fix those to the concrete roof and lay flags over those.... and when it came to drive the 5ton cherry picker for maintenance in that area, to take the flags up, drive the cherry picker on the concrete deck and when finished relay the flags!!!! I was quickly shot down with that suggestion!!!
I feel uncomfortable saying that a 70mm thick 1:3 sand:cement bed is fine and to introduce the falls in it, coz that is what they want me to say or am I being too fussy?
(Oh I told the engineers that they have to be satisfied that the sand:cement bed will be able to withstand the loads imposed on it)
I know a flexible bed is better where vehicular overrun in expected, but there are other areas where limestone slabs are being used on a 50mm sand:cement bed (with no vehicular overrun) so by going with the one construction throughout makes life alot easier

Your opinions please coz my head is wrecked!!!!

Posted: **Tue May 26, 2009 11:17 pm**

if he is prepared to sign it off ,do it
but do not do it off your own back
LLL

Posted: **Wed May 27, 2009 12:18 pm**

every job we have ever done or been on falls have come from the subbase up, then everything is uniform and the calculations work out otherwise you have a massive variation in bed thickness which throws out the calcs.

The Brew Cabin

Falls introduced into sand:cement laying course - Can this be done?