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.
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.
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.
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.
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.
Heat buckled a concrete highway in Wisconsin in 2012.