There are workers outside the building where I work, and they’re pouring cement for new sidewalks. Why do they lay down wire grids before pouring the cement?
that wire mesh is used to add strength to the concrete (or without the stone which would be cement) so when it dries it is less likely to crack or break under pressure. large buildings made mostly of concrete (parking garages e.g.) use a thicker wire called rebar (reinforcing steel) to make the floors and pillars even stronger for holding up it’s own weight, plus several hundred or thousands of tons of vehicles.
OK, so is there a simple explanation as to how/why this works?
steel is stronger than concrete, and it makes more sense to pour concrete than 1200 degree molten steel. it’s just like the bones in your body adding strength and support for your muscles, well, sorta bad example
Or like the glass cloth in fiberglass–it’s normally something you can bend, but with a stiffener, it’s much stronger and more resilient. More so than the resin used.
Concrete has great compressive strength but only so-so strength in tension, so it makes sense to add something that will keep it from pulling apart (i.e., cracking on the bottom if it sags or on top if it lifts.)
Wire or rebar help with this. Wire can also limit how much it cracks during drying, but (in my layman’s opinion) reducing the water or adding modifiers when pouring the stuff is a better way to reduce the shrinkage that causes that.
there is a fiberglass addative that is relatively new, but more expensive. it can be used in the place of rebar, but it has to be mixed in with the water.
Ohhh, those answers are close, but not quite there.
To begin, understand that concrete, while very strong in compression, is incredibly weak in tension. (Think chalk) You can push on it and nothing - because its like stone, but pull it and it will crumble. There is very little internal cohesion. Steel though is incredibly stong in tension, but not as good in compression - it tends to bend.
When you pair them together the concrete takes the compressive forces while the steel takes the tension. The key is that the concrete has to mold to the steel, which is why rebar is bumpy. The wire mesh is used in sidewalks and other slabs that are shallow so the tension forces are not large. The mesh allows the concrete to resist forces in two directions whereas rebar can only do one direction. Ideally the mesh should be set about an inch or two from the bottom of the slab because that is where the tension forces are (think bending - the bottom gets longer)
Just to add a bit to the discussion; the steel wire adds tensile strength, which concrete (in theory) does not have. The wire is placed in the upper portion of slabs to help keep the concrete acting together in one piece as it cracks from normal curing stresses or from ground action. In beams, reinforcing steel is generally placed in the bottom third of the piece, with shear reinforcing loops (stirrups) placed along the length, again to add tensile strength.
45ACP
Hey, near simulpost!
The one clarification I need to make is that exterior slabs-on-grade generally have the wire toward the top (the ground itself acting to hold any bending force) while suspended slabs would have the wire toward the bottom as in epeepunk’s example.
45ACP
(who must remember to do better previews)
Or how about this:
When you’re driving across a concrete bridge, it’s the rebar that’s holding you up. The 'crete just gives you a flat surface.
I used to cut rebar for precast bridges and stuff, and sometimes it’d look like there wasn’t any room left in the forms for the concrete.
JLC article: Can You Substitute Synthetic Fibers for Wire Mesh?
Going back to the OP, the mesh is included to keep the concrete from falling apart as it cures, and as the temperatures rise and fall. In fact, the type of mesh you refer to is often called “temperature steel” by designers of such things. If the concrete you’re seeing is actually for a sidewalk, I’m surprised there’s any steel at all; sidewalks seldom require steel, since they’re basically slabs on-grade, with essentially no loading.
“Temperature steel” is meant to hold the surface of the concrete together, and is therefore usually placed near the surface. Reinforcing steel, or “re-bar” is another concept altogether. In a simple slab or beam application, the longitudinal re-bar is placed as near to the bottom of the slab or beam as possible. The “bottom third” estimate I read in another post is as correct as would be the “bottom half” or the “bottom 99%.”
Any basic text on statics and strengths of materials will explain this stuff better than I can.