concrete flash set vs "hot spots" during SOG placement ?? 1

[boffintech]

I understand the PCA definition of "flash set". However, I've got a normal weight 4000psi mix (low shrinkage well graded aggregate flex strength of 700psi at 56-days). The last slab-on-grade (7-inch thick and 25,650ft2) placement suffered from a couple of small areas of what appeared to be flash set. But these areas were only about a truck load in size. The concrete plant is on-site so it's a short haul but the min. 70 revolutions for mixing is obtained at the rack with 20-21 revs./minute and 4-5 minutes at the rack. I don't see how this can be a true "flash set" which is a cement milling QC problem when the +500yds3 didn't appear to have any problems. Using a laser temp doohickey I read a 4-5F degree difference in the areas flashing (84-85F) and the adjacent plastic areas(80F). All of the trucks were chipped out about 3-weeks ago and were subsequently inspected. The slumps on the trucks appear to be consitant. It was a night pour with placement lasting 6 hours from 1am to 7am with air temps around 65-75F and humidity around 80%. Flashing areas approximately 6th and 14th truck with 7-trucks round tripping. FF/FL of 50-35 specified and a couple of "hot spots" super complicate achieving the numbers.

What else causes these hot-spots?

 

[Ron]

Wind. Inconsistent mixing because of too few revolutions or worn blades in the transit mixer. Contaminated aggregate. Improper admixture dosing.

Recommend a batch plant audit to check compliance with NRMCA requirements, including getting in the drums and checking the blades.

 

[concretemasonry]

I would take a look at the truck drum/fins and the driver controls to make sure the mix was uniformly mixed.

Since there are 7 tucks making the runs, it sounds like the same truck could have easily delivered the 6th and 14th loads if there was a minor shift in the dumping or batching order. It was a little surprising you could track it down to the load numbers.

Aggregate differences are not too likely, since all the aggregate usually comes out of the same stockpile. If you had 2 different cements on the site in silos, it is possible (but not likely) some hot cement (very fresh or overground)was in one of the silos.

Dick

Engineer and international traveler interested in construction techniques, problems and proper design.