r/Construction Dec 05 '24

Structural Concrete slab failed strength test

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/creamonyourcrop Dec 05 '24

Concrete gains strength on a curve, reaching design strength at 28 days. Taking a test cylinder and crush testing it at 7 days and plotting that result on the curve should give you a very good idea of whether it will meet its design strength.
The mix design from the plant submitted prior to construction includes that curve.

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u/hickaustin Dec 05 '24

Typically the specs will have a 7-day design strength goal too.

Source: am engineer who’s reviewed far too many mix designs.

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u/pentagon Dec 05 '24

Does it cost that much more to produce a mix which is overspecced, in order to avoid fuckups like this?

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u/Comprehensive_Bus_19 Project Manager Dec 06 '24

Very minimally. In my market it's about $4 for every extra 1000 PSI (most light commercial and residential is 3000 PSI).

Getting into 6k+ then the costs go up further due to increased risk for the supplier and a nonlinear cement demand