My wife and I moved into a 6 year old ranch home in the north eastern Ohio area last May. The basement is a 13 course block with a walkout onto a cement pad. The pad was cracked when we moved in. The interesting thing about the cracks were that the patio was flat in the middle but each side was broken down at the ends much like an upside down U. I had the slab removed and replaced in the summer. The old slab was 6 inches thick and poured directly on the soil without a gravel base yet it still cracked. The new slab is 4 inches thick and poured on a gravel base. It has expansion Celotex joints at all points near the house and wrapped around the deck posts. Once it got very cold (below 10 degrees) you would hear occasional loud popping (similar to a handgun firing) coming from the deck walkout on the first floor. I discovered that the slab had risen by the walkout door and pushed up the side trim framing the door. The concrete slab rose 3/4 inches and I assume this pushed up the trim. The slab also has a hairline crack running across the width of the slab. I am not sure what caused the popping sound.
My question is: Did the slab rise due to moisture freezing under the slab? If this is the cause, can I reduce the moisture in the ground by putting drainage pipe 16″ underground and at the end of the slab to drain moisture to a lower level of the yard?
Maclf
Replies
I'm sure some folks with more knowledge than I in regards to soil mechanics will chime in here, but......
I try to do slabs like this one over a decent (8" or better) of 1-1/2" stone. I find that in addition to the value it gives in regard to drainage, it has some voids that (I think) give a little expansion room for when the frost heaves the slab, as it looks like yours has.
Drainage around the slab might help, but my opinion is that it will be minimal. Most slabs DO move up and down a little (1/2"+-) seasonally......at least in my neck of the woods.
As a builder, I'd be a lot more concerned with the fact that the slab was poured flush with the bottom of the door...big no-no in my opinion. Primarily for the reason that it's begging for water infiltration at the door, and all along that sill area, but secondarily (is that a word?) for the problem that you're having; if the slab comes up, it wreaks havoc with trim, and can push up on the threshold to the point of the door binding.
Someone will undoubtably be along with an idea to reduce movement....Sorry to say that I'd be inclined to re-do if the job were dropped in my lap.
Bing
From the way the slab looks discolored, I would speculate that the water is draining off the raised grade to the right and has no where to go BUT onto the slab. Putting a drain 16" down will not correct it unless you have a place for it to drain to.
I also agree that the slab being on the same grade as the basement slab will just create problems in the future.
Solution:
Jack this slab up, put down a DEEP gravel base that will act as a sump ror the runoff and repour....or just live with the cracking/shifting and accept the fact that you will never be able to put a pool table there!
The slabs aren't being pushedup because of a little water under them. They are heaving because the entire earth surrounding your house is expanding!
Typically, the heat of the house will keep the immediate surrounding area of the house from freezing but once you get past a foot or two, everything is frozen solid and expanding. The slab would raise up and be cantilevered over that soft zone next to the house.
You should have provided at least 1" clearance for the slab to raise up near any trim. I would have opted for a bit more but 1" will usually be sufficient for most winters. You might consider cutting the concrete back or cutting the trim up to provide relief.
One other option would be to cover the slab inside that cove to keep it from freezing. You wouldn't need much cover to prevent it from heaving. Snow is actually a good insulator but the heat escaping from the house melts it fast on the patio.
You have a lot of water feeding those downspouts that go through the slab, I would wonder where they are leading or if there are leaks in the line. also appears the surrounding landscaping is too high.
In my experience, seems like you can only have frostheave if you have enough moisture to freeze, if it was drained properly you wont retain enough moisture.
I agree, though, the slab is too high, I like it at least several inches below the door.
Best fix in my opinion would be to remove slab, dig up and check downspout drainage, then regrade to provide better slope away from the foundation.
I also have wondered if there are problems with downspout drain leaks under ground which might add more water than the normal drainage. The main house roof above the deck above has two valleys on each side that will direct water over the gutter in a heavy downpour. The water falls through the deck and into each corner area of the slab next to the house. In this last winter there was an ice dam in each valley which during periods of winter warmups would steadily drip water onto the deck which then dropped down to the slab. This makes another source of extra moisture. This is one reason I was considering a deck drainage system to prevent the water from ending up on the concrete slab. I will cut the trim up so that it is not near the slab and I will look at getting the concrete contractor to cut the slab near the house to relieve some of that movement during winter and reevaluate the slab.Maclf
On a new house, you shouldn't have to accept ice dam problems, you should investigate that issue first.
If the downspouts/gutters regularly overflow, they are undersized. That does look like a lot of roof for two downspouts.
Where do the downspouts drain to?
Rumor has it that I'm still an excavation contractor. Back when there was enough left of the economy to actually go to work and get paid, a lot of what I did was fix structural problems as they relate to geo-technical and drainage conditions.
First, there are three things required for a frost heave; moisture, freezing temperatures, and suitable soil (soil that will retain moisture and behave in a plastic manner). Remove any one and the ground will remain stable.
The trick for you in this case is that you could put anything you want under the slab including crushed stone, and with the amount of water you appear to have under there you could still have a problem. At that point, we're not talking about frozen soil, we're talking about solid ice.
So as most of the previous posters have pointed out, any water that does make it to the vicinity of that slab needs to be drained away. The least expensive fix is always to divert it before it hits the ground. After that, a sub-grade drain to a run of infiltrators is probably next best.
I can't really tell from the photos, but it looks like the posts for the deck are sitting on that slab. If this is indeed the case, it would be best to address this sooner than later. If that deck starts moving with the slab you could find yourself with a whole new set of problems.
If you can't find a way to divert the water and you need to put in a drain system, post back here and I'll give you some ideas.
In my area there is a lot of clay and I assume that is what was used as backfill around the house. I honestly didn't pay close attention to the soil type when the pad was repoured other than they did use about 4" of stone before pouring. The posts are anchored into the soil and not attached to the pad. The posts are wrapped in celotex and the concrete was poured up to it.
The downspouts are the typical size (4"?) but not anything larger than that. As I mentioned before there can be a waterfall of water down the valleys that jumps over the gutter and onto the deck. There is a sheet of aluminum attached the gutter at the bottom of the valley to block the water but it is not wide enough because the water goes past it and onto the deck. I am thinking of hiring a plumber with a camera who can snake the downspouts and the drainage pipe to see if there are any breaks in it that may be adding to the water problem. We currently are receiving quite a bit of sustained rain and I can see that the area of lawn at the end of the pad does have water standing. It would be helpful to have some drainage pipe put in just to reduce that situation.
Maclf
My question is: Did the slab rise due to moisture freezing under the slab?
This is a problem of slab-jacking due to the column footings and slab being monolithic. I've seen it before.
Typically, the column footings are poured in sonotube (in virgin soil) which creates the perfect piston which, when freeze/thaw cycles begin, simply goes up and down with groundwater freezing.
Yes, you need a drainage plane under the slab, but also this is a perfect case where column footings and surrounding slab need to be decoupled (not connected).
View Image < Column footing (sonotube likely) jacking slab here
Jeff
Edited 3/8/2009 9:26 pm ET by Jeff_Clarke
Jeff, I'm thinking the same thing, see my post #8.Even if the pier is not the culprit, you are still right, the post needs to be decoupled from the surrounding slab.
>> Typically, the column footings are poured in sonotube (in virgin soil) which creates the perfect piston which, when freeze/thaw cycles begin, simply goes up and down with groundwater freezing. <<
Would the post footings be moving up and down assuming they were dug down below the frost line?
>> but also this is a perfect case where column footings and surrounding slab need to be decoupled (not connected). <<
In the OP's first post he said: >> The new slab is 4 inches thick and poured on a gravel base. It has expansion Celotex joints at all points near the house and wrapped around the deck posts. << So they are at least somewhat decoupled. OTOH, you are right that the top of the post footings could be pushing on the bottom of the slab if these post footings are indeed moving.
OP: So, I guess another question is the deck experiencing seasonal movement? (up and down)? And... Were the expansion joint around the posts nailed to the posts? If so, what is the position of the top edge of the expansion joint relative to the top surface of the slab?
It looks like a combination of things. Most of which have been discussed.
The one that hasn't is thermal shrinkage cracking; if the slab is 4-inches of 3500-psi concrete, on 4-inches of gravel. The cracks you are seeing in the slabs are most likely from thermal shrinkage stress. The control joints on a 4-inch slab need to be no more than 5-feet apart. If your temperature differential is high then they need to drop down to about 4-ft.
The gravel needs to be considerably deeper than 4-inches.
The water needs to be kept away, or moved away. If it gets under the slab and freezes it will push it up into the door and trim. I would also recommend at least 3/8-inch to the foot of fall in the slab away from the house.
Nothing to do with your post, but if I was in the circular stair business, I'd give you a call.
Think he isn't going round and round over this already?;)
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My thanks for the suggestions and observations about this slab issue. I will try to address as many as I can within my budget restrictions.
Regarding the stairs on the deck - I hate it! It is ugly and anything would be an improvement.
Larry
You didn't design/build the staircase, so your not GUILTY. Tight budget, a firemans' pole would work for starters and an improvement.
Do you think the wooden deck is raising during the winter freeze? A level layed on the deck boards might tell a lot....
There have been lots of good suggestions with regards to vertical clearances and control of drainage water. I would only add the suggestion that if you end up digging the existing slab out in addtion to drainage improvments you also add a layer of 2" polystyrene installation under the slab and extend it out at least two feet beyond the perimeter of the slab. I would also recomend that at the perimeter you angle the insulation downward to minimize drying out the grass along the edge. Roger
As far as the stairs, I opened a nice cut on the top of my thinly hair-covered head when I went up the stairs with my head down and one of the crankout Anderson windows was open. As you might guess, I caught the point of the aluminum clad window. Bled very well!Larry