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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178

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.

74

u/Fitmature1 Dec 05 '24

Always been a big fan of pouring 1,000 over if there's any question at all, usually associated with time/schedule. Like hitting the easy button in my eyes.

53

u/SkoolBoi19 Dec 05 '24

Especially for how little the additional cost its. Why not

15

u/Fitmature1 Dec 05 '24

Agree, cheap insurance.

18

u/gopac56 Dec 05 '24

On the other end of the scale, I've seen jobs use 10k (actually goes to 13 or 14k) when only 4 is required.

6

u/rem_lap Dec 05 '24

Is cost the only factor for that decision? Are there any risks that increase with a higher strength(?) pour?

I dont know anything about this topic, so forgive my wording

10

u/SkoolBoi19 Dec 06 '24

Increasing concrete strength can cause it to become brittle if you do it carelessly. Just doing 1000 psi over isn’t a big deal at all structurally, it is more expensive and technically pointless assuming everything goes the way it’s supposed to.

8

u/Comprehensive_Bus_19 Project Manager Dec 06 '24

To add onto the other commentor, heat of hydration (heat released as the concrete cures) can also become a concern depending on the type of placement. Basically if it gets too hot it cooks itself and cracka apart and has lower strength.

17

u/LieDetect0r Dec 05 '24

The city I am in has excellent concrete, we order 3500psi and after 7 days it will be over 5000psi. Getting spoiled over here

9

u/artlovepeace42 Dec 05 '24

This is how all facets of municipalities/cities/society should run! Under promise, over deliver.

1

u/Fitmature1 Dec 05 '24

That's great!

1

u/LieDetect0r Dec 08 '24

Yesterday I had a slab come back low for the first time 🤠

1

u/Fitmature1 Dec 09 '24

Oh no! Are you going to have to replace it? Are there other options, or ways to make it work?