URGENT, PLEASE COMMENT

[AdM22]

i've just poured a waffle roof slab(flat) of a single family home. The beams are 6"x6" spaced 24" in both directions with 4-#3rebars and its slab is 3".
Now here comes my problem. The structural engineer specified the concrete resistance at 200kg/cm2 and they used 150kg/cm2. The structural engineer is not in town so I came here....

How critical is it?

*Attached is a JPEG
The area in red is my concern since it spans +/-14'
Everything is in centimeters.

 

[dik]

It was just a headzup... I've seen this done many times and unless people are being unreasonable, it can come to a relatively happy ending... One that I didn't do, but was involved in the litigation that followed ended up very badly for the Owner/Consultant.

With many contracts, the only way to 'technically' deal with it is to reject it outright... they often don't have anything between 'accept' and 'reject'. My response was only cautionary.

Dik

 

[BigH]

. . . in the meantime, might I suggest that you contact the ready mix company and ask them what their "historical" records are for the mix that you requested by mistake? In addition, ask for the records over the last 30 or so samples to see what they have recently attained for 28-day strengths. While this will not obviate the need for the breaks on your job, it might give some comfort (or not) regarding the likelihood that the concrete may be considered acceptable. As SRE and others have said, ensure that you cure properly and don't remove the support. Do you have extra cylinders that you might break for a 3-day strength and a 14-day strength - to get a handle on the potential for reaching an acceptable 28-day value?

 

[BoydZander]

200kg/cm2 (2840 psi) is pretty low for concrete in my area (Midwest USA). Most concrete, even if specified to be less, is going to break higher then that.

I'm not sure what commonly occurs where you're at but I'd hope you could expect high enough breaks.

To eas you mind somewhat, ask the mix provider to provide you with some of their historic test breaks(past jobs) for that mix. It can perhaps alieviate some concern if they typically are breaking higher than 200 kg/cm2

Boyd

 

[BoydZander]

Yep. BigH has you covered on the advice. Good Job

 

[AdM22]

Yes, it could be a relief...or not. I will post the results. Thanks!

 

[AdM22]

Ok, I've just met with the concrete representative at the site and he tested the concrete strenght with a test hammer at 14 days and it already reached 200kg/cm2 and by 28 days he says it should reach 225kg/cm2. He will give me a formal report by the end of the day.

He explained the Kilogram difference of cement in the mixture between the 200kg/cm2 and the 150kg/cm2 is only 5kg.

Thanks all of you that took the time to post.

-Andre

 

[BAretired]

That is indeed good news.

BA

 

[Ron]

AdM22...before you get such a warm fuzzy feeling, keep in mind that the rebound hammer test is a poor indicator of compressive strength of the concrete and almost always overestimates the strength.

Sorry, but I would not rely on that type of test. Get appropriate testing done or if samples were taken, wait for the 28 day results.