question about concrete 2

[JOHNINFLA]

I'm a field technican here in Jacksonville, Fl. my question is this. i work on a rather large FDOT project, and was informed about a problem with placing concrete at a diaphram. The inspector said they were having a hard time placing a 2-3 inch slump in such a small tight area. it just so happen on this day concrete was delivered at about a 3 1/2 - 4 inch slump. Ambient temp was around 89-90 degrees,( target slump is a 3 ) needless to say placement went fairly smooth. Untill the inspector told me to tell the mixer driver to speed up the rotation on his drum. in between filling bucket drum was mixing at a slow speed. which i see as a good thing. mixing at a faster speed will only cause concrete to lose slump and air at a quicker rate...maybe i'm wrong?..please help if you can.

 

[Ron]

JohninFla...you are correct. Running the drum speed up will typically dry the concrete a bit in your area, particularly because the coarse aggregates are rarely SSD when batched and slump loss is common, assuming you are using the typical crushed limerock aggregate, common in Florida.

Check the rotation counter on the drum. Total revolutions should not be more than about 300 revolutions and running at mixing speed should only be done for about 75 to 100 revolutions, although the FDOT will allow you to adjust the consistency of the mix using 30 revolution "spurts", up to a maximum of 160 revolutions at mixing speed. Don't allow them to exceed either.

I assume you are working for a commercial testing lab in that area. I do not believe it is your job to tell the concrete supplier to do anything to modify his mix, though I understand your position of being directed by the inspector. That puts your firm in a liability position that perhaps your principals would not want. I am a firm believer that only the concrete supplier should make modifications to the mix and then only if it is allowed by the specifications. In your case, the contractor has to have a Quality Control Plan in place, which outlines any anticipated modification that can be made to the mix in the field. If the QCP is not followed, the FDOT will not be particularly pleased! See Section 346-6 of the FDOT Standard Specifications for Road and Bridge Construction.