Minimum Aggregate Size for Concrete Flatwork

[canman]

I have numerous school site projects in Southern California, that require new concrete flatwork. The specificatons, which I wrote, specify a mix design of compressive strength minimum 4000psi, W/C ratio 0.50 maximum, with 1 inch aggregate passing 95-100%. If it helps in this question, the specs are written per Green Book requirements for a mix design of 618-CLE-4000P.

The contractor has come back asking for a revised mix design that can be used in tight areas, inaccessible for a truck. His mix meets the compressive strength and W/C ratio requirements but his aggregate gradation passes a 3/8" size at 100%, or grade E per the Green Book. How much should the difference in aggregate size affect my decision on accepting this alternate mix design?

Thanks for the help - Canman

 

[dicksewerrat]

He wants to pump this stuff. But he should be able to pump the larger stone also. The smaller rock will give him a
mix with a larger slump. you don't have to accept his recommendation either. IT IS YOUR SPEC.

 

[canman]

I'm trying to work with the guy and was hoping to base my rejection or acception of his mix design on some facts. His mix design shows only a 4" slump and he wants to use a 2" line pump. I am leaning towards rejection since the size of the aggregate for flatwork purposes seems too small.

 

[DRC1]

We have frequently subsituted aggergate size for pumping or finishing. The larger aggergate is cheaper, therefore using the larger aggergate is more economical. The smaller stone is easier to pump and finish, and you will probably like the finish better. Fighting the 1" stones is tough. the 3/8 stones should finish much better with less work, which will bring less water to the surface giving you a better finish. Before you reject it talk to the Q/C engineer at the concrete plant. He does not want any trouble with the concrete either as it will eventually become his problem. So he should give you a good answer on the applicablity of the mix and other projects on which it has been used.

 

[canman]

Guidelines - Expansion joints every 6', #4 bars every 18" on center, both ways. The slump is 5" and W/C ratio is 0.61 - mix is stamped by a P.E. I think I will approve the mix design but I just hope it doesn't come back to haunt me in the future.

 

[cvg]

You have allowed an increase in the W/C ratio - why is this necessary? The slump could have been increased with a superplasticizer.

 

[canman]

The pea gravel mix design is not intended for the school with the max. w/c ratio of 0.50, which I found out recently. I had specified a max. w/c ratio of 0.60 for the schools this mix is to be applied. This mix is using WRDA 64 as an admixture.

 

[DRC1]

I perfer adding water to super P for flat work. With water the concrete stiffens fairly gradually making finishing predictable. With Super P, when it is over, it is over. Usually strenth does not control in flat work (except possibly in suspended slabs)so w/c vs strength is not a big issue. I would dicuss curing with the contractor, as the high w/c can lead to cracking. This is a bigger concern if it is hot out on the pour day.