Some trivia picked up at work, if anyone's interested. We had to have labs crush test cylinders of concrete for work we have done. At the construction site, the plastic test cylinders are poured. They are roughly 6" in diameter and maybe a little over a foot in height. The lab does a minimum of two breaks, and I forget when the first one is done - think it's 7 days - then a second break at 28 days. I never saw it done, but was told they cut slices from the cylinders and crush them by placing the slices in a machine that records the PSI applied at failure. The "weight" or compressive strength of concrete is the amount of pressure applied to crush it between two smooth surfaces, and does not indicate a sidewalk, floor, wall or bridge deck will hold that weight. What it does is verify that the material is up to specification for the engineer's design of the assembly, which may include reinforcing steel, compacted earth, etc. An example would be a floor that is designed for a 250#/SF load. The design may use 4,000# concrete, that doesn't mean the floor will hold 4,000#/SF. It means the test machine will need to apply at least 4,000PSI to crush a sample of the concrete. If everything else used to build the floor is also up to spec and installed properly, it will hold the 250#/SF plus whatever safety factor the engineer has built into it.
I live where there are freeze/thaw cycles, and the concrete plants need to know if the concrete they are sending to the job is for interior or exterior use because the concrete used for sidewalks and such are to be "air-entrained" to help avoid damage from Mom nature......................chim
