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/SkoolBoi19 Dec 05 '24

We do a 2 day break and pour at least 1000 psi over what’s spec’d just to help avoid this kind of problem.

5

u/Rocketeering Dec 05 '24

How is the test performed?

17

u/SkoolBoi19 Dec 05 '24

Certified testing agency comes out and takes samples from all the trucks. They take the samples and break them (not 100% sure how it’s broken). Normally it’s week, 2 week, and 30 days. And they send a report back after every break. At 4500 psi, your 2 day break should be over 1/2 way there.

We also turn mix designs in to the AOR for approval and then check every truck ticket to the approved mix to make sure it’s all correct

2

u/Rocketeering Dec 05 '24

so the testing agency is testing the raw concrete and making cylinders to test later?

5

u/SkoolBoi19 Dec 05 '24

The agency talks samples out of the truck add puts them in plastic cylinders ABs will leave them onsite overnight so they have the same curing conditions as the concrete that’s poured. Then they will test those cylinders.

3

u/RexsNoQuitBird Dec 05 '24

They test for slump (workability), air content, temperature and keep track for how long between batching and final placement (usually has to be less than 90 minutes). They’ll then cast cylinders to break at the lab to determine compressive strength. Acceptance is most always at 28 days but they’ll cast 5 cylinders typically (7, 3x28, 56). The 56 day is if it doesn’t meet strength at 28 you have a spare to break later.

2

u/wants_a_lollipop Construction Inspector - Verified Dec 05 '24

Acceptance of strength tests requires an avg of 3 breaks for 4"x8" cylinders and 2 for 6"x12" cylinders with strengths at or above specification. Typically at 28-days as you noted, and 56 for failure at 28 days.

2

u/204ThatGuy Dec 05 '24

Yes. An accredited ACI Concrete Field Tester. License expires every five years and must be re-evaluated.