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  #1  
Old 04-16-2010, 12:45 AM
endotoxin endotoxin is offline
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Why do concrete sidewalks buckle?

Title pretty much says it all.

I'm sure most people here have seen this. A nice long street with a well-maintained sidewalk, and out of nowhere a gigantic MOUNTAIN of a hill, where two concrete slabs have pushed up against each other.

HOW?

I realize concrete can continue to cure for decades after it has been poured, but I was told that 99% would cure within the first 30 days (My father, a general contractor). So what gives? Micro-plate tectonics? Gophers? Secret conspiracy of masonry pranksters?

A few searches of the Internet have revealed several photography projects, and several companies selling concrete alternatives, but no real hard data other than "Tree Roots." Piffle, I say! I live in the southwestern United States, and I have seen far more and acute buckling and shifting in concrete slabs than can be accounted for by the few measly cacti seen growing nearby. Hell, I've ridden some of them on my bike! (Oh my aching rear!)

My significant other says that it might have to do with the way concrete stratifies, or it might have something to do with the annual winter freezes. I confess I just can't see it. So to any urban engineers out there, can you set my troubled mind at ease? Or at least my posterior?
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  #2  
Old 04-16-2010, 02:07 AM
Mangetout Mangetout is offline
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Could it be thermal expansion? Things get bigger when they heat up in the sun. There are supposed to be expansion gaps in the right places to allow enough slack for thermal expansion to occur without causing damage, but sometimes, people screw up and omit them, and sometimes they end up under-engineered for the amount of expansion that actually takes place.

The other thing that can happen is that repeated cycles of expansion and contraction could make a slab 'walk' in one direction, closing up an expansion gap at one end - so that further cycles of expansion tend to deform the material.
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Old 04-16-2010, 02:15 AM
Patch Patch is offline
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Yup. Thermal expansion will do it.
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  #4  
Old 04-16-2010, 06:37 AM
racer72 racer72 is offline
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Where I live tree roots are the biggest cause of uneven sidewalks. Once the concrete slab has been disturbed this lets water in then the multiple freeze and thaw cycles of winter do the rest.
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  #5  
Old 04-16-2010, 06:43 AM
Harmonious Discord Harmonious Discord is offline
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For you it has to be thermal expansion without sufficient expansion joints. Concrete roads will sometimes explode from thermal expansion pressure. I know you can find video for that on YouTube. Scale that down for sidewalks.
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  #6  
Old 04-16-2010, 07:42 AM
johnpost johnpost is online now
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Quote:
Originally Posted by endotoxin View Post
Micro-plate tectonics?
the earthquake gods have been angry

Quote:
Originally Posted by endotoxin View Post
Piffle, I say! I live in the southwestern United States, and I have seen far more and acute buckling and shifting in concrete slabs than can be accounted for by the few measly cacti seen growing nearby. Hell, I've ridden some of them on my bike! (Oh my aching rear!)
i think the southwest can have large temperature swings with lots of prolonged hot periods. when in cold temperatures and the concrete contracts the gaps can't be too big. the extreme hot temps expand beyond the gap provided.

i think buckling is more of a problem in the southern states.
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  #7  
Old 04-16-2010, 08:44 AM
Cheshire Human Cheshire Human is offline
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Frost heave. Water in the soil expands as it freezes, shifting the concrete slightly. Repeat that for a number of cycles, and it can affect the size of the thermal expansion joints. Hot days in summer then do their bit. Winter comes again and continues the process. Repeat as needed to make big mess. They all do their tiny contributions until it all adds up.
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  #8  
Old 04-16-2010, 10:49 AM
Cat Whisperer Cat Whisperer is offline
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All the things that have been said already (frost heave is a HUGE problem here), plus improper preparation of the underlying site. We have some problem areas in town here (McKnight Blvd. for Calgary Dopers) that every single winter heave up and turn a high-speed road into a slow road with de facto speed bumps in it. They've re-done the asphalt there any number of times, but until they rip the road out and re-do the road bed, it will continue to heave every winter.
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  #9  
Old 04-16-2010, 02:02 PM
endotoxin endotoxin is offline
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Thanks for the answers everyone. It's such a dramatic image to see this sort of heaving in practice, and realize that this is merely caused by the freezing and thawing of water. Definitely one of those, "I understand the science, even though my eyes tell me it can't possibly cause something that dramatic."

Thanks once again!
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  #10  
Old 04-16-2010, 03:05 PM
Cat Whisperer Cat Whisperer is offline
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Come north and see just what the freezing and thawing of water can do - our roads are beat to shit every spring.
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  #11  
Old 04-16-2010, 10:18 PM
msmith537 msmith537 is offline
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Quote:
Originally Posted by Cheshire Human View Post
Frost heave. Water in the soil expands as it freezes, shifting the concrete slightly. Repeat that for a number of cycles, and it can affect the size of the thermal expansion joints. Hot days in summer then do their bit. Winter comes again and continues the process. Repeat as needed to make big mess. They all do their tiny contributions until it all adds up.
I believe this is the correct answer. The ground under the concrete swells and contracts due to frost, water, dryness and so on.

IIRC, concrete doesn't expand or contract that much. It's usually the steel rebars or girders in the case of a bridge that expand.
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  #12  
Old 04-16-2010, 10:55 PM
Snarky_Kong Snarky_Kong is offline
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Originally Posted by msmith537 View Post
I believe this is the correct answer. The ground under the concrete swells and contracts due to frost, water, dryness and so on.

IIRC, concrete doesn't expand or contract that much. It's usually the steel rebars or girders in the case of a bridge that expand.
Concrete and steel have extremely similar thermal expansion constants. If this wasn't the case then rebar would debond from the concrete any time there was a temperature change.
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  #13  
Old 04-16-2010, 11:26 PM
Civil Guy Civil Guy is offline
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...And just because there isn't a tree there now, doesn't mean there never was a tree. Look for uplift in both the sidewalk and the nearby curb - that would be evidence of tree roots. There doesn't have to be much actual uplift in flat areas to cause a noticeable bump or gutter ponding - just look for the cracks. The tree (or removed tree) doesn't have to be near the curb; could be on (or missing from) the adjacent property.

If the street is pretty flat with trees up and down both sides, fixing one address won't take care of the problem, really.

There could be a storm drain or sewer pipe nearby with a crack in it. If so, water and soil can work itself through that crack and down the pipe, causing a hidden sinkhole, eventually causing a different kind of ponding. Or maybe it's some utility company's access vault, rather than a pipe.

That, or thermal expansion problems, maybe soil problems. Lots of things.
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  #14  
Old 04-16-2010, 11:35 PM
beowulff beowulff is offline
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Quote:
Originally Posted by Cheshire Human View Post
Frost heave. Water in the soil expands as it freezes, shifting the concrete slightly. Repeat that for a number of cycles, and it can affect the size of the thermal expansion joints. Hot days in summer then do their bit. Winter comes again and continues the process. Repeat as needed to make big mess. They all do their tiny contributions until it all adds up.
I see buckled sidewalks all the time out here, and I can guarantee that there is no frost heaving going on.
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  #15  
Old 04-16-2010, 11:53 PM
Snarky_Kong Snarky_Kong is offline
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Originally Posted by beowulff View Post
I see buckled sidewalks all the time out here, and I can guarantee that there is no frost heaving going on.
Just because it's not frost heave by you doesn't mean it isn't frost heave near the OP.

Buckling is generally a term I'd associate with thermal expansion though, true.
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  #16  
Old 04-16-2010, 11:59 PM
Johnny L.A. Johnny L.A. is offline
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Quote:
Originally Posted by Harmonious Discord View Post
Concrete roads will sometimes explode from thermal expansion pressure. I know you can find video for that on YouTube.
Link?
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  #17  
Old 04-17-2010, 12:28 AM
aceplace57 aceplace57 is offline
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They are supposed to put a bed of pea gravel or sand under the concrete. The gravel allows drainage. The sand gives some flexibility for the concrete to expand/contract.

A lot of contractors don't give a crap. They put up the forms and pour the concrete right over the grass. They figure there's not much weight on a sidewalk anyhow.
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  #18  
Old 04-17-2010, 07:47 AM
johnpost johnpost is online now
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sidewalks can have frost damage because of drainage away from buildings on its way to the lower street level.
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  #19  
Old 04-17-2010, 08:15 AM
Cicero Cicero is offline
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The Master wrote a column on something similar.

Here.
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  #20  
Old 07-07-2011, 08:05 PM
tim-n-va tim-n-va is offline
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I realize this thread is over a year old but it came up in my search...

Regarding cases when sidewalks buckle due to heat how does if happen? I have two options but there could be more.

Option one: As the sidewalks heat up they expand and exceed the capabilities of the expansion joints. The pressure causes the adjoining sections to gradually rise.

Option two: Same as option one except the pressure builds until it exceeds the friction between the adjoining sections and the sections pop up quickly.
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  #21  
Old 07-07-2011, 08:16 PM
mac_bolan00 mac_bolan00 is offline
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bare soil, when trodden upon often enough, gets flattened and firmed. but a concrete pavement takes in the pedestrians' weight and distributes them so the soil is basically free to expand, buckle due to alternate freezing, loosened by tree roots, or get "toothpasted" upward due to the greater weight of vehicles on the road beside it.
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  #22  
Old 08-15-2014, 10:14 AM
boydg boydg is offline
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Quote:
Originally Posted by endotoxin View Post
Title pretty much says it all.

I'm sure most people here have seen this. A nice long street with a well-maintained sidewalk, and out of nowhere a gigantic MOUNTAIN of a hill, where two concrete slabs have pushed up against each other.

HOW?

I realize concrete can continue to cure for decades after it has been poured, but I was told that 99% would cure within the first 30 days (My father, a general contractor). So what gives? Micro-plate tectonics? Gophers? Secret conspiracy of masonry pranksters?

A few searches of the Internet have revealed several photography projects, and several companies selling concrete alternatives, but no real hard data other than "Tree Roots." Piffle, I say! I live in the southwestern United States, and I have seen far more and acute buckling and shifting in concrete slabs than can be accounted for by the few measly cacti seen growing nearby. Hell, I've ridden some of them on my bike! (Oh my aching rear!)

My significant other says that it might have to do with the way concrete stratifies, or it might have something to do with the annual winter freezes. I confess I just can't see it. So to any urban engineers out there, can you set my troubled mind at ease? Or at least my posterior?
I have seen many sidewalks heave. It always happens on a hot day in the summer time. Once I had a call from a person who had heard a loud sound behind him while he was walking on a sidewalk. About 10 feet behind him a section of sidewalk heaved about 1.5 feet. He said he would have been sent in orbit if it had happend when he was stepping on that section of sidewalk. Why did the sidewalk heave? There was no expansion joint on either side of the heaved sidewalk within 75 feet. Why do you need expansion joints every 150 feet or less? Sand and other grit will enter the control joints (sawed to 1/4 the depth of the concrete sidewalk depth usually every 5 feet in order to control cracking in a straight line) when the concrete contracts in the colder months of the year. When the weather gets hot in the summer the concrete on each side of the control joint will expand but will not be able move back into the control joint because of the sand and grit. All the expansion must now be accommodated by the expansion joints. If there are no expansion joints or they are placed too far apart, the sidewalk will heave. It is important that expansion joints be made of the correct material (we use 1/2" bituminous board) and separates the concrete on each side of the joint by 1/2". I have seen gaps in the bituminous board that allows concrete to span the 1/2" which is the same as having no expansion joint.
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  #23  
Old 08-15-2014, 10:23 AM
dougie_monty dougie_monty is offline
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In Redondo Beach, CA, quite a few trees have been removed because their roots buckled the sidewalks or streets--trees 70 years old or older.
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  #24  
Old 08-15-2014, 11:50 AM
Ranger Jeff Ranger Jeff is online now
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In addition to thermal shrinking and expansion of the ground the concrete is poured on, tree roots, failed water mains, and wrongly spaced expansion joints, there's one more possible cause...

Some contractors might not use the specified mix. It might be a little cheaper if they used a bit more sand or gravel in the mix. And they might be able to stretch out the concrete if they lay a 3.5 inch slab instead of the specified 4 incher.
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  #25  
Old 08-15-2014, 12:16 PM
dougie_monty dougie_monty is offline
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Quote:
Originally Posted by Ranger Jeff View Post
In addition to thermal shrinking and expansion of the ground the concrete is poured on, tree roots, failed water mains, and wrongly spaced expansion joints, there's one more possible cause...

Some contractors might not use the specified mix. It might be a little cheaper if they used a bit more sand or gravel in the mix. And they might be able to stretch out the concrete if they lay a 3.5 inch slab instead of the specified 4 incher.
It could also be that older sidewalks were poured before exact specifications were established.
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  #26  
Old 08-15-2014, 12:21 PM
Yllaria Yllaria is online now
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Expansion joints also need to be kept in good repair. If the fiber pads get worn or removed, or if the plastic fill gets cracked, the joint can fill with dirt, sand, and debris, which will reduce or negate the joint's ability to function.
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  #27  
Old 08-15-2014, 01:41 PM
Duckster Duckster is offline
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Heat buckled a concrete highway in Wisconsin in 2012.

An SUV driver did not heed the warning signs.
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