Why do they cut lines in concrete?

[shortylickens]

Separate slabs allow the earth to move underneath without actually cracking the concrete. But if you're talking about what I think you're talking about, they do it cuz it looks cool. Which is annoying.

 

[LTC8K6]

http://www.concretenetwork.com/concrete/slabs/controljoints.htm

 

[GrumpyMan]

It's for traction, they don't want you to fall off of the Earth.

 

[Red Squirrel]

Come to think of it, why is it that basement slabs normally don't have these? Is it because they are poured so thick that they're not needed?

 

[mmntech]

The sidewalk outside my building was recently redone by the city. They poured like a 50-foot long slab and then they cut "lines" in it to make it look like individual 5-foot sections. The lines don't go all the way through; they are only about 1/4" deep.

Why do they do that? Just for aesthetic reasons? Sounds like a waste of manpower to me. What's wrong with a 50-foot slab of concrete that looks like a 50-foot slab of concrete? Confusing concrete construction is confusing.

Expansion joints. Gives the slabs room to move as it heats up and cools down through the seasons. Otherwise they'll heave and crack. Most concrete structures have these cut into them. That's what the metal lines on bridges are.

 

[KLin]

Come to think of it, why is it that basement slabs normally don't have these? Is it because they are poured so thick that they're not needed?

Plus the dirt below should already be compacted.

 

[lxskllr]

Come to think of it, why is it that basement slabs normally don't have these? Is it because they are poured so thick that they're not needed?

Probably due to depth. A basement slab should be below the frost line.

 

[wirednuts]

yup. its also because you want a flat floor for indoor use. and if you notice, basement floors crack in crazy ways, and its not nice looking. but again, you want a flat floor so you just let it crack and the patch it. if it even makes a difference. if you have carpet, let it crack and you will never see it.

 

[vi edit]

Come to think of it, why is it that basement slabs normally don't have these? Is it because they are poured so thick that they're not needed?

Part of it is because the basement doesn't have as much temperature fluctuation as a driveway or sidewalk exposed to all sorts of elements. Another reason is because many basements get covered up with carpet and nobody cares. Another reason is because the contractor was lazy and the owner/CG didn't care. And another reason yet is because a basement slab will normally contract away from the foundation walls as it cures and then expand a bit as it goes through temp changes. But nothing to the extent of an unprotected/insulated slab.

 

[PhoenixEnigma]

They also provide kids with something to do by trying not to walk on them.

Wait, was I supposed to outgrow that?

 

[crashtestdummy]

Expansion joints. Gives the slabs room to move as it heats up and cools down through the seasons. Otherwise they'll heave and crack. Most concrete structures have these cut into them. That's what the metal lines on bridges are.

This. The lines more than just separate the slabs (though they do also keep cracks from propagating between segments). They are designed to provide some space so that cracks from thermal expansion and frost heaves don't occur in the first place.

 

[vi edit]

This. The lines more than just separate the slabs (though they do also keep cracks from propagating between segments). They are designed to provide some space so that cracks from thermal expansion and frost heaves don't occur in the first place.

Concrete is going to crack, it's just the nature of the beast. You can mitigate it somewhat with rebar, slow cures, act but it still wants to crack with wild temperature swings. Control joints help you to tell it where to crack.

 

[rayfieldclement]

<--is definitely a bit OCD. I guess it shows?

Maybe it is OCD but you asked a legit question!

 

[Gooberlx2]

But, localizing cracks makes a lot more sense than them just trying to fuck with OCD people.

:hmm: Methinks they should make the cuts in irregular intervals. Two birds, one stone.

 

[zinfamous]

so that you have something to jump over, thus preventing your mother from breaking her back.

duh.

 

[Miramonti]

The joints are created so that when the cement contracts, it will hopefully do so at these places in order to be more effective at tripping up annoying skateboarders.

 

[XZeroII]

The expansion cracks are not used for drainage. In fact, you want to make sure that you don't get water in them by filling them with an expandable material. The reason is because in the winter, when the water freezes it will expand and push the concrete up and cause big bumps. This also increases the stress and can cause it to crack as well. Never underestimate the power of freezing water.

 

[SparkyJJO]

:hmm: Methinks they should make the cuts in irregular intervals. Two birds, one stone.

I've seen sidewalks like that. It bothers me. A lot D:

 

[Sluggo]

Concrete is going to crack, it's just the nature of the beast.

As the concrete guys say "If you don't want concrete to crack, don't take it out of the bag."

 

[Squisher]

http://www.youtube.com/watch?v=JXXe2aVa1Ek

 

[MichaelD]

I must admit that there were a lot of educational replies to my question. Thanks guys! I learned something today.

 

[Pray To Jesus]

CRACK happens

 

[Humpy]

I must admit that there were a lot of educational replies to my question. Thanks guys! I learned something today.

There were a lot of replies but most of them are wrong.

People are confusing/combining the control joint, and its function, with the expansion joint.

 

[OverVolt]

I figure sidewalks used to be individual slabs of concrete and then to cut corners they did the 50 foot slab pretending to be individual slabs deal and it just stuck. I've seen tons of sidewalks cracked right down the middle I don't think it matters lol.