Belive it or not, concrete has no strength at all. In order to reinforce concrete to the point of having strength or for supporting something heavy, you must use rebar or wire mesh. If you notice on over passes, the concrete road is supported by metal decking, which in turn is supported by steel beams. You will never see an over pass poured entirely out of concrete without some type of rebar, steel, or wire mesh embedded into it first. If you ever took notice of how curbing always cracks, and seems to fall apart so easily, it's because there is no reinforcement, like rebar, used in holding the concrete together. Concrete by itself, will not support a thing, and will break or crack quite easily. And yes, to answer your original question, moisture will deteriorate concrete when it's not properly sealed. Does your concrete sidewalk deteriorate? Of course it does. After being poured, the contractor doesn't seal it, due to added cost. But, if it were sealed, your sidewalk will last a lot longer than your neighbors sidewalk. Ever try breaking up your sidewalk? It will be hard to break due to the wire mesh embedded in the concrete at the time of the pour. Now if you poured your sidewalk without the reinforcement, then you would have chunks of sidewalk breaking apart everywhere, without swinging a sledge hammer. If the concrete is cracked, and water should happen to lay in the cracks and freeze, it will crack the concrete beyond repair. The slab, floor, sidewalk, etc. will have to be replaced. This is why expansion joint is used. The expansion joint lets the concrete expand and contract during weather changes. Sometimes you might see white plastic strips used in concrete sidewalks or big floor slabs. These white strips are called control joints. If the concrete slab or floor needs to crack due to pressure, it will crack at these joints, and is designed to help keep the cracks controlled, and not ruining a complete floor or slab.
Ok, I'm done. That was your free tip of the day. I used to work in commercial construction as a union carpenter for 17 years, and we poured concrete everyday. They used to tell me that I had a gift, because I could almost smell the concrete trucks, when they were still a mile away. They were good learning years, but believe me, I don't miss a bit of it!
Edited by johndeereelfman, August 15, 2012 - 08:50 PM.