Help! Water under newly laid flagstones

[s219]

I've spent the last couple months putting in a patio with the help of a family friend. We had to build a block wall, backfill, then poured a 4" slab on top. Slab cured for about 40 days. The last major step was done 2 weeks ago, when we laid bluestone squares (2x2) over the slab with mortar. It seemed to go well. Here's a shot after we laid out all the stone and got it sponged off 2 weeks ago:



I went out there today to prepare for grouting/tucking the joints, and noticed water squishing out of the edges of about half the flagstones when I walked on them (primarily the ones in the center, but also some edges). Thinking that was not good, I was able to pry a couple flagstones loose with my bare hands. The mortar bed underneath looked very continuous and was a mirror image of the bottom of the stone, so clearly we had good contact. But there was a lot of moisture under there. Here's a shot of the underside of one stone (flipped upward), and the bed it was on:



Not sure if it's due to all the rain we've had, or maybe it was a mistake to try and cover the stones with a tarp when it rained, possibly trapping moisture? I would have thought any moisture would have wicked into the slab.

Anyhow, now I am wondering how to salvage this mess. Lots of time/labor, and of course the flagstones weren't cheap. Anybody have ideas?

Since the mortar bed looks decent, I am really tempted to re-glue the stones down with some sort of adhesive. But I thought I'd reach out for suggestions from the brains here. I am no mason, but decent with mortar/concrete. However, this experience has me feeling outgunned.

thanks,
219

 

[oosik]

For whatever reason the flagstone did not bond well with the mortar. I think it may have been the rain you mentioned. You should not have been able to lift the flagstone. I would lift the flagstone and first see if the mortar bonded well with the concrete slab. The bond there may not be good and perhaps all the mortar should be removed.

If the mortar/concrete bond is good - remove all the flagstone and let the mortar completely dry. After it is completely dry lay down another thick base of waterproof mortar, set the flagstone, then grout.

In any case all you have lost so far is some time and perhaps some mortar. BTW - nice looking patio.

 

[Snobdds]

What kind of mortar did you use. There is no physical bond between the stone and mortar.

It could also be the tarp. I know I tarped some fresh concrete in a prolonged rain, and the top dried funky. So who knows.

 

[s219]

You know, the strange thing is that the stones were held on really snug initially. In fact, when we were laying the stone, during lunch break we noticed one stone that could have been leveled a little better. We decided if there was enough mortar left at the end of the day, we'd pop that stone loose and re-do it. Well, it gave a pretty good fight getting that popped loose, just 7-8 hours after it was laid. A couple days later I went out there and walked on the tiles when I laid the tarp, and they seemed solid then too.

I think the bottom line is that the mortar didn't stick to the stone properly once it cured. Perhaps it was rain water, or moisture trapped by the tarp, or perhaps it was some other effect (heat?).

If I can establish that the mortar is stuck to the concrete slab strongly, and it does seem that way, then I was thinking I could switch to using thin set to reattach the stones. At this point, the mortar bed under each stone has the exact shape for the stone. I could lay thin set over the mortar with a square-notch trowel, then drop the stone back into place were it matches up with the mortar bed. I found some thin set rated for stone and exterior use. Any thoughts on that?

At this point I have nothing to lose, and can experiment a bit.

 

[/pine]

Regardless of the remedy...I would use a quality bonding agent on all surfaces and in any new mortar/thinset etc...

 

[CurlyDave]

When I attached natural stone to concrete I used an "acrylic admix" to promote bonding between the stone and the mortar. 3 years and all is holding.

The other thing we did was to wash the stone thoroughly before placing it. You would be amazed at the amount of dust on seemingly clean stone and how that prevented good adhesion.

I do not think it was a water problem. Water helps curing of portland cement, the active ingredient in mortar, and usually promotes bonding.

 

[oosik]

I used an acrylic mix of some type when I set the quarry tile in the house - about 450 sq ft of tile. It runs thru the kitchen and down the hallway to the bedrooms. The one thing that was stressed was that I had to soak the quarry tile before I set them. That was 34 years ago and they are still attached as though they were welded in place.

We put them in 5 gallon buckets full of water and let them set overnight.

 

[EddieWalker]

I found that when using mortar to attach stone, it needs to be on the wet side. From what you describe, I think your mortar was too dry. I'm guessing that you spread it all out at the same time, then laid the stone on top of it. The first few stones are probably fine, but the last few are the ones that are coming off easily.

The moisture isn't important, but there was no reason to put plastic over it.

In the last couple of years I've gone from dealing with the short working time of mortar to the much better and stronger bond of using regular thinset mortar that everyone uses for tile. I also back butter each tile, or stone. This controls the mess, and there is never a dry spot in the mortar. If the surface is porous, like tiling over brick or block, I skim coat that too.

Do you have a SDS rotary hammer with a flat chisel bit? I would use that to clean off the existing mortar and start over fresh.

 

[Streetcar]

Stone set with mortar is a failing proposal in a freezing climate
As other have said use clean damp stone prior to setting , use acrylic bonding agent will make the stone last longer before failing. The stone and mortar will fail
Lately I have set stone in sand bed and reset in five years. With sand bed resetting stone is not complicated

 

[SPYDERLK]

I've spent the last couple months putting in a patio with the help of a family friend. We had to build a block wall, backfill, then poured a 4" slab on top. Slab cured for about 40 days. The last major step was done 2 weeks ago, when we laid bluestone squares (2x2) over the slab with mortar. It seemed to go well. Here's a shot after we laid out all the stone and got it sponged off 2 weeks ago:

View attachment 474085

I went out there today to prepare for grouting/tucking the joints, and noticed water squishing out of the edges of about half the flagstones when I walked on them (primarily the ones in the center, but also some edges). Thinking that was not good, I was able to pry a couple flagstones loose with my bare hands. The mortar bed underneath looked very continuous and was a mirror image of the bottom of the stone, so clearly we had good contact. But there was a lot of moisture under there. Here's a shot of the underside of one stone (flipped upward), and the bed it was on:

View attachment 474086

Not sure if it's due to all the rain we've had, or maybe it was a mistake to try and cover the stones with a tarp when it rained, possibly trapping moisture? I would have thought any moisture would have wicked into the slab.

Anyhow, now I am wondering how to salvage this mess. Lots of time/labor, and of course the flagstones weren't cheap. Anybody have ideas?

Since the mortar bed looks decent, I am really tempted to re-glue the stones down with some sort of adhesive. But I thought I'd reach out for suggestions from the brains here. I am no mason, but decent with mortar/concrete. However, this experience has me feeling outgunned.

thanks,
219

Falling water. Porosity. Hydraulics.