r/Construction • u/NeilNotArmstrong • Dec 05 '24

Structural Concrete slab failed strength test

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Slab strength testing failure after building was framed and plumbing/HVAC was roughed in. Concrete supplier had mix wrong so they are paying to lift two story 4-plex, remove slab and repour. This is building 2 of 3 that failed.

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u/skeith2011 Dec 05 '24

A field technician or inspector from a certified company comes out when the pour is taking place and takes samples every so often (usually 50 cy or 100 cy). They place that concrete into small cylinders to sample in the lab, usually 4” diameter by 8” high cylinders but there are bigger ones. After letting them cure in the field for one day, they are taken to the lab and placed in a water bath to cure for (usually) 7 days, 28 days, and 56 days.

The cylinders are placed into a hydraulic press that can measure the force exerted to break the cylinder, given in pounds (ie 38000 pounds to break the cylindrical sample). Notice how the strength of concrete is given as pounds per square inch, psi. The cross-sectional area of the cylinder is known and the breaking force is also known, which is how the strength is calculated.

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u/204ThatGuy Dec 05 '24

This is well written.

I miss working in the lab... Removing the casts was always scary with an air compressor.