Precast Concrete Pavers in condo driveway 2

[ajk1]

This question is directed to anyone who has experience in precast concrete pavers in northern freeze-thaw climates, such as most of Canada and the northeast U.S. The project is actually in southern Ontario, a high-end luxury condo entrance area subject to both pedestrian and vehicle traffic (but not truck traffic).

The following is a description, followed by my questions.

The existing 65 mm thick x 300 mm square precast concrete pavers were set by an experienced contractor specializing in this sort of work, and in two widely separated locations they have moved vertically sufficient to cause a tripping hazard (1/2" ±). This has happened a second time, after the heaved area was reset.

The pavers are set above an un-insulated structural slab which forms the roof level of the underground unheated garage, so there is no source of heat from below the pavers.

The existing construction is, from top down is as follows:

a) 65 mm ± thick x 300 mm square precast concrete pavers
b) 50 mm granular "A" setting bed, 50 mm thick
c) 12 mm stone, 50 to 75 mm thick
d) protection board
e) 2 layer reinforced hot-applied rubberized asphalt membrane
f) reinforced concrete structural slab, sloping mainly in one direction about 3% to a single bi-level drain (a second below paver drain was installed a number of years ago, which is single level, draining only at the top of waterproofing membrane surface. The drainage run is relatively long (about 60 feet perhaps...I will check the drawings later this morning when I get into the office, and see what the drainage run is, and perhaps be able to post the drawing).

There is no leakage below the slab, so the membrane is still working well, although at least 20 years old, and perhaps as much as 30 years old.

The drains are running freely. Removal of a portion of the horizontal pipe runs at each drain shows there is sediment in the pipe (about 25% of the pipe diameter) but not enough to block the pipe.

There is no filter fabric above the stone drainage course (i.e under the granular "A" setting bed).

Water was found at 2 locations in the stone layer when we removed a couple of pavers two weeks ago.

Questions

1. Would installing the proper filter fabric directly under the the granular "A", as recommended by our geotechnical engineer, solve the pever differential vertical movement?

2. Would it be better to remove the stone layer and the granular "A" layer and replace it with 100 mm ± of a proprietary cementitious mortar designed for setting stone, with salt scaling and freeze-thaw resistance, which we have used in the past on a large plaza to set 900 mm square granite pavers (we have not used it for setting the smaller pavers used on the condo project).

My concern about the current installation, even if a filter fabric is added, is that there are voids in the stone layer where water can freeze and heave the pavers, and of course they never settle down in the original location because dirt gets under them.

What would be any negative consequences of eliminating the drainage layer? The only thing I can think of is that the pavers can not dry from the bottom surface, so they may appear wet for a longer period after a rain.

 

[dik]

Some questions:
Do you have a proper drainage system to remove any water from the stone layer?
Do you know what the soil profile is within the active frost layer?
Is the structured slab a slab on grade? and not a 'real' structured slab?
If a real structured slab, is there any void form beneath the system?
Is there any insulation beneath the slab?
What sort of perimeter drainage is provided?
Is there any indication the slab is moving?

Water trapped in the drainage layer could be freezing and causing the pavers to move without the slab being affected.

Dik

 

[ajk1]

To dik:

I believe that just about all the questions that you ask, while very good, are answered in my original post. Perhaps you inadvertently did not read the preamble to my questions...happens to the best of us, including me.
To repeat:
"The pavers are set above an un-insulated structural slab which forms the roof level of the underground unheated garage, so there is no source of heat from below the pavers".

Specific answers to your questions, are given in italics below:

Do you have a proper drainage system to remove any water from the stone layer? Drainage layer under the pavers is described in detail in the original post.
Do you know what the soil profile is within the active frost layer? This is not on grade, so question seems not relevant
Is the structured slab a slab on grade? and not a 'real' structured slab? As noted in originalmpost, the slab is roof of garage, so it is not a slab-on-grade
If a real structured slab, is there any void form beneath the system? There is no void form anywhere
Is there any insulation beneath the slab? As noted in the original post, there is no insulation in the system
What sort of perimeter drainage is provided? I do not know, but since the perimeter b aseemnt wall below the suspended slab that supports the pavers is on the lot line, I expect that the vertical drainage board placed againstwood lagging shoring wall
Is there any indication the slab is moving? No. Is not on grade

All responses are appreciated but I am particularly interested in responses from those who have been involved in specifying or constructing such driveway pavers & bases over unheated garage roofs in northern climates.

(I have > 50 years structural engineering experience, including some paver experience, so I am not so interested in hearing the basics, but all responses are appreciated).

 

[dik]

Need more information than contained in the original post... why no drainage if water is trapped? Missed that it was a roof subject to pedestrian and vehicle traffic. If heaving is occurring and it is a roof, then you simply have to get rid of the water. You may have to add an intermediate topping to provide proper drainage with a cross slope of approx 1-1/2%. Construction should be concrete, protection board, membrane, clean drainage material to a 'real' drain, non-woven geotextile, limestone screenings, and pavers.

Dik

 

[oldestguy]

The fact that the drain collection pipe has a lot of silt in it means some or most of the fill granular materials are dirty which cause substantial frost heave. I'd replace all those granular materials with CLEAN MATERIALS. You also may have some displacement of "marble" sized materials, with low internal shear strength. Well graded, clean (zero P-200) gravel and sand mix, such as concrete aggregate, likely would perform much better. Much better shear strength, meaning traffic loads less likely to cause slab rocking.

 

[ajk1]

To dik:

ok, thank you. Owner does not want exposed concrete. Wants the appearance of separate precast pavers. Could do patterned concrete, but that is expensive, subject to shrinkage cracking (which precast pavers are not so much), and would required to be thicker than the 65 mm that the pavers are, and there is not space for a 100 mm slab. CSA A23.1 requires 100 mm thickness for separate toppings. There are many other precast pavers on other nearby buildings that do not seem to be having the paver heave/tripping hazard. There is not space to change the drainage slope. It is 1.6% which although not great, is adequate. However the drainage basin length is 45 m which is extraordinarily long. We could add more drains, but the slopes would not be able go be altered to direct water directly to the added drains. But may still be worthwhile to add drains.

To oldestguy:

Very interesting. How do we tell if the materials are dirty? Our geotechnical engineer looked at the sample of top 2" setting bed we brought him in a ziplock bag and he said that it is granular "A" which he says is the right material to use under the pavers. Is a simple visual look all that is required, without lab analysis? Is lab analysis required to tell if material is CLEAN?

There is no filter fabric separating the 2" of granular "A" from the 2" of 1/2"stone. Do you think that if we added a filter fabric, that would solve our problem? Is quite expensive to remove all the pavers and granular "A" to install the filter fabric and then put the material back, so we have to be very sure that will fix the problem. The material was just installed within the last couple of years (we had nothing to do with that work, thank goodness).

The area is about 230 m2. Do you think if we just added the filter fabric in the 2 areas where the paver heaving occurred that would fix the problem? Or would a similar problem eventually occur in other areas?

 

[OldBldgGuy]

Sorry ajk1, I haven't been on the site in a bit & maybe am too late to comment, but I'm in your neck of the woods & I am constantly dealing with materials reacting to frost & owners/clients not believing that they will. (Just as an aside, I was in Yellowknife last week & I would not want to have their frost issues). I think you have too much stone above the slab, but if you're going to have it, gran A is the wrong material for you. Clear stone is free-draining and doesn't require/change with compaction, and doesn't have fines that will get between the pavers and prevent settling back down after heave. My choices (I have done many similar situations) would be 6mm or less clear stone bedding for the full 100mm under the pavers (I like 3mm clear), or set them in mortar as you described. I think the clear stone is preferable to anything.