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+--- Thread: high mortar bed?? (/showthread.php?tid=59866)
high mortar bed?? - phil on a fazer - 14-08-12
When laying bricks/blocks/ e.t.c i use about a 10mm mortar bed (cement & builders sand). if you make the bed higher it becomes weaker as there's no aggregate in the mix...is this correct? As you've guessed...I'm not a trained builder!
i ask because i need to put some slabs down in my garden. There steps and half of each slab will be seated on concrete. So i was going to lay them how i always have, normal mortar mix with around a 10mm bed.
but i messed up one of the steps when i was at the shuttering stage.(made it too low) So to even it out i need to lay one of the slabs on a bigger bed.....40mm would be perfect, 25mm would do.
If i mix up my mortar as usually but add say 1/2 builders sand and 1/2 sharp sand, will this allow me to use a larger bed but maintain the strength?
Hope that makes sense :o
cheers
Re: high mortar bed?? - manuel32 - 14-08-12
Any depth of bed will be strong as long as you put cement in,3 to 1 with sand or use concreteing sand,(fine gravel) with cement at about 5 to 1.
On flat areas dont lay slabs on blobs or dabs of cement , use dry mix sand cement and level out on whole area and place slabs on the flat dry mix.
Re: high mortar bed?? - phil on a fazer - 14-08-12
(14-08-12, 06:13 PM)manuel32 link Wrote: Any depth of bed will be strong as long as you put cement in,3 to 1 with sand or use concreteing sand,(fine gravel) with cement at about 5 to 1.
On flat areas dont lay slabs on blobs or dabs of cement , use dry mix sand cement and level out on whole area and place slabs on the flat dry mix.
oh right, i thought the bed would weaken the bigger it got due to the lack of ballast. The last patio i layed was onto wackered down MOT grade 1 stone and i layed them wet (i "spotted" them), then pointed around the edge before laying the next one (they were quite thick slabs)
I will lay these slabs wet as a good portion of the slab is onto concerete so the bond will be better. (rather than using a dry mix). the slabs are the treads on a flight of stairs.
Re: high mortar bed?? - manuel32 - 14-08-12
No the bed thickness has no impact on strength,,the strength is in the mix, obvious the thicker the bed then use something suitable, , suppose it was 4 inch , then use concrete,its just a matter of mix consistency , to keep it in the joint and not running out,you might have to shutter the joint if you use a wetter mix to stop it running out.
Or cut the joint ,bed, back and point it after it has set. to get a good finish.
Re: high mortar bed?? - chris.biker - 14-08-12
The higher the bed of mortar the less weight it will support, what i would do is go for the 40mm bed but lay 10mm bed with a slate or tile first then then place the another bed of mortar on that to get an over all 40mm, then place the slab on that. Doing this as a dab in each corner.
Re: high mortar bed?? - phil on a fazer - 15-08-12
(14-08-12, 11:08 PM)chris.biker link Wrote: The higher the bed of mortar the less weight it will support, what i would do is go for the 40mm bed but lay 10mm bed with a slate or tile first then then place the another bed of mortar on that to get an over all 40mm, then place the slab on that. Doing this as a dab in each corner.
Thats kind of what i was thinking.....normal bed of mortar...then slate...then another bed on top of that for the slab to sit on???
Im trying to do the best job i can as i need it to last.
Re: high mortar bed?? - pitternator - 17-08-12
Phil
Its no problem using a 40mm bed ( or dab) if its left to dry before you try to use the steps.Dabbing slabs onto a concrete base is an acceptable way to lay slabs.Its also far quicker than a solid bed method.I used to lay York Stone for a living 20 odd yrs ago, and the variance of that stuff means your mortar dabs varied alot too.Just dont skimp on using enuff compo where you need it ! I often build sets of steps,and where possible I like to cast concrete bases to dab slabs etc onto.
In bricklaying a 10mm ( compressed) bed is the standard for various reasons- aetsthetics/ strength/ speed of laying/ standardisation of build etc.But its not essential.Old imperials ( 73 - 80mm bricks) often had up to 15 mm beds.Stone has much thicker joints too..