Concrete breaks much higher than design strength 1

[magee913]

Hello all, below is my mix design

cement type I/II = 548 lb /cyd
Class F fly ash = 137 lb/cyd
fine aggr = 1269 lb/cyd
coars agg = 1680 lb/cyd
water = 242 lb/cyd
air = 5-7%
slump 7"

W/c = 0.35

The batch plant had to design the concrete for 5200 psi based on ACI 214 table 4.2. Insuffucient break test were unavaible so the plant did a test batch and therefore shot for 5200. Well they made it and more. I am getting breaks at 28 days close to 6500 psi. Should i be worried?
The specs call for a maximum w/c of 0.35.

 

[dik]

With a w/c ratio of 0.35, I'd expect 6500 psi concrete, or thereabouts... unless shrinkage or curling is an issue, I wouldn't be too concerned.

Dik

 

[scone38]

@magee913

Just curious what the application is for this mix design

 

[Ron]

agree with dik....no reason for concern. When the w/c ratio gets that low, it's hard to fine tune the mix because of needed workability concerns.

 

[concretemasonry]

It may be that the concrete has had experience with higher strength concrete, and more importantly is familiar with the aggregates available or commonly used AND how to use them. Often, text book procedures are cook-book methods and do not reflect the actual gradation (within range), the particle shape (angular, flat or rounded) and the local fly ash properties that can all affect workability and placement.

Dick

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

 

[Ron]

I assume you are using a high range water reducing admixture to get the higher slump. Based on your proportions, I would consider this to be a "harsh" mix, having a high FA ratio. Any complaints so far about workability?

Also, why not just use an "off the shelf" 5000 psi mix design...surely the ready mix supplier had one of those with a record of service.

Also, if this mix is being used for structural concrete under ACI 318, one trial batch isn't enough. See Chapter 5 of ACI 318.