Monday, February 18, 2008

December 15th

One week later, there was concrete. Smooth and clean; ready for a house to sit on top of it.

The concrete was very smooth except for any areas outside the home. In the external areas it looks like they sprinkled something onto the surface of the concrete to make it draw away into little recessed pits.

I thought this was for safety reasons until I saw the garage is smooth. I now believe this was an HOA ordinance designed to keep people from laying around “bleeding up” the neighborhood driving property values down.

Speaking of property values, by this time they had raised the prices of the home we bought $2,000 above the price we signed. Cool…

They poured the concrete for the window wells and some little cylinders. These cylinders are related to the cables that are now run through the poured concrete slab.

From the Post Tensioning Institute
“Simply put, Post-Tensioning is a method of reinforcing concrete, masonry, and other structural elements. Post-Tensioning is a method of prestressing. Prestressed concrete or masonry has internal stresses (forces) induced into it during the construction phase for the purpose of counteracting the anticipated external loads that it will encounter during its lifecycle."

Concrete is great at withstanding pressure (crushing) forces but is pretty much crap at handling tension (pulling) forces. Steel has great tension strength. So they tighten the cables at the ends and as they try to contract they press against the concrete with crushing forces. Newton’s laws, remaining remarkably relevant on humanoid biped scales, mean the concrete pushes back against the steel resulting in tension forces on steel. It works out nicely as long as neither gets the advantage. In order to determine when the cables are ready to be tensioned, they take one of these concrete cylinders and put it in a press and crank the pressure up to 3,000 lbs. If it cracks, it’s not ready. If it doesn’t crack, they start tensioning the cables. If I remember correctly they tighten them up to a few thousand pounds.

If you look carefully you’ll see the two surround speaker boxes that we set next to the outlet box in the center of the great room.

This is going to be so cool.

No comments: