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?
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.
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.
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.
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.
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.
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.
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.”
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.
…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.
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.
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.
