When to Reject Concrete

[tclat]

Hi All,

Is there a recommended limit for rejecting plastic concrete based on temperature? I've heard many times that concrete begins to set at 90 degrees and obviously any placement which occurs above that temperature will affect the 28 day strength. The average air temperature during the day where I am located is generally between 80 and 90 degrees.

Thanks.

 

[racookpe1978]

Why do you think these slightly warmer temperatures will affect the curing concrete? Is it wetted properly?

Did it have the proper slump when poured?

 

[TehMightyEngineer]

95 degrees is when we employ warm weather curing methods but, overall, as long as the final concrete hits it's strength, slump, and air entertainment, and you have no issues with the final product when you strip the forms, there really isn't any reason to reject concrete poured at a higher temperature. Obviously this assumes that you properly wetted the concrete and shielded it from direct sunlight.


Maine Professional and Structural Engineer
American Concrete Industries

http://www.americanconcrete.com

 

[tclat]

I'm looking for some established guideline for rejecting concrete that sits on the site too long. I'm aware of the 90min (batch to placement) guideline set by ACI but for concrete batched with retarders, the 90min rule may not be a practical guideline for rejection. Rejecting concrete based on slump alone also has its challenges. Any other thoughts?

 

[Ron]

Get the mix design and the mill certificate on the cement. If the time exceeds initial set on the cement, reject it. 90 minutes is a reasonable arbitrary point of rejection (ASTM C94). There are many variables that can decrease or increase this time. Initial set doesn't lie though. It is a tested parameter of the cement and if you place concrete beyond this point, you risk a variety of issues.