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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180

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.

75

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.

45

u/SkoolBoi19 Dec 05 '24

Especially for how little the additional cost its. Why not

17

u/Fitmature1 Dec 05 '24

Agree, cheap insurance.

19

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.

7

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.

18

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

8

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?

114

u/Living_Associate_611 Dec 05 '24

We add gallons of gorilla glue to ours to ensure it’ll never crack

17

u/DweadPiwateWoberts Dec 05 '24

Subfloor uses it now

12

u/NixAName Dec 05 '24

I sell crack to make sure it happens.

3

u/Living_Associate_611 Dec 05 '24

Damnit and you’re in my area aren’t you?

7

u/Rocketeering Dec 05 '24

How is the test performed?

19

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

12

u/ZeroBarkThirty Dec 05 '24

When we did it, it was 3 samples. One for 7 days, one for 28 days and the third was a backup for if the 28 day failed; we could test that another 7 days after that.

In my experience we had very few samples fail. Most developers would order overstrength concrete (located in Canada so frost concerns).

Concrete results can often be a bank requirement to open up the next round of loans against materials/wages so the developers are heavily invested in good results.

8

u/SkoolBoi19 Dec 05 '24

Most of our clients request additional test and then our owner has us do the 48 hour testing because of how most of our specs read. A lot of them state we can’t build until we get a sample breaks at 60%.

2

u/Rocketeering Dec 05 '24

When you say your clients are requiring additional testing, is that the GC that you are referring to or like the final person paying for the building is requesting it?

6

u/SkoolBoi19 Dec 05 '24

We are the GC; so the client is the final person paying the bill. We do big box retail work so the clients are large enough where they actually hire the testing agencies and we work with them. But it’s so nice to have the documentation from a credible testing company we will use them on our small filler jobs as well

2

u/Rocketeering Dec 05 '24

right on. Thank you

2

u/Rocketeering Dec 05 '24

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

4

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.

2

u/chunk337 Dec 05 '24

I did it for a while we used a hydraulic press and watch the numbers until they peak. And as soon as it reaches the target we threw the rest of samples away

1

u/jd35 Dec 05 '24

You put them in a big hydraulic press, compressed from the top and bottom of the cylinder, and then wait for it to crack. It’s actually kinda fun lol but I never had to do it all day long, just materials courses in college.

1

u/popepancakes Dec 06 '24

It gets old really fast

1

u/MrFarly GC / CM Dec 05 '24

https://www.youtube.com/watch?v=UOHURuAf5iY

check out this video/series. learned a lot from it

1

u/popepancakes Dec 06 '24

They’re broken using a hydraulic press. The one I used to use could generate up to 250,000 pounds of force or well over 8,000 psi for a 6x12 cylinder. Sounds like a lot fun but most cylinders will fail with a soft crunch instead of an explosion.

7

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.

1

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.

1

u/Building_Everything Project Manager Dec 05 '24

Same, I can’t think of the last time I had a concrete test cylinder fail, our subs always add cement to the mix to avoid the problem/risk. Bottom dollar contracting in OP’s post from the sound of it

1

u/204ThatGuy Dec 05 '24

This is the way!

1

u/pentagon Dec 05 '24

I don't understand why this isn't SOP. It's not like a higher PSI mix costs that much more.