Minimum Aggregate Size for 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

 

[SacreBleu]

Sounds like a pea gravel mix. I believe it will aggravate shrinkage cracking, so closer joint spacing and more reinforcing should be the contractor's penalty for the decision.

 

[dicksewerrat]

No reason for you to accept his mix. does he have a PE to back up his request? Tell him to get some wheel barrows.

 

[Rjeffery]

Does he have a PE stamp on the mix design to back up his design? Tell him to use a pump to place the mix (at no additional cost to the owner).

 

[STR04]

What are some guidelines for horizontal pours when the contractor wants to use pea gravel, ie. CJ spacing, pour length, temp. reinf. requirements?

 

[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.

 

[STR04]

What about elevated slabs and beams?

 

[Ron]

canman...you are asking for trouble with this mix and SacreBleu and dicksewerat told you why. Just because a P.E. has sealed the mix only means it is a viable mix design to achieve certain physical properties...that doesn't make it right for the application.

With a w/c ratio of 0.61 and a 3/8" coarse aggregate top size, this will be a high shrinkage mix. Further, because it has more cement in it, you will likely get faster setting times or at the least will get a very fast "end" set.

Your original mix design is much better for the application. There is no reason you can't pump a large aggregate mix. It is done every day. (but you can't do it with a 2" pump pulled behind a pick-up).

Stick to your guns and make them do it right.

 

[SacreBleu]

Ron:
If the slab is only 4" thick, the 1" max aggregate seems large. Why not 3/4"?

 

[jheidt2543]

In my experiance, 1" stone mixes can be successfully pumped in a 6" diameter line with a wet mix (either higher bag mix or plasticizers). Most of the pumping I've seen done is with 3/8" stone for precast toppings and 3/4" for slabs. Talk to the concrete pumper (most are rented these days) or the pump manufacturer. They can give good, tested advice on pumpable mixes. You need tried and true mix designs, not theoretical ones.

 

[dik]

Slump is extremely high for a slab! also, the larger the aggregate the smaller the shrinkage generally... Sawcutting also critical where applicable

Dik

 

[STR04]

http://www.eng-tips.com/viewthread.cfm?qid=126656&page=1

check this post...