Concrete Design 4

[windies]

I am a materials engineer but have been away from concrete design for some while so would appreciate some assistance.
My client wants a 4000psi concrete with a slump of 3 to 4 inches and a water cement ratio of 0.45 the coarse aggregate is granite with a specific gravity of 2.68 the fine aggregate is a manufactured sand with a fineness modulus of 2.2 and a specific gravity of 2.57 whilst the f.m is a bit low i have to work with it. On my initial paper designs the cement and water quantities for a cubic yard both seem high.

Thanks in anticipation for the assistance.

 

[ishvaaag]

Look if

FirstMix version 2.0

a downloadable mix program -by Ravi Sikka- can be useful to you. Make a google search for it.

Look also ACI 211

and

Graphical Approach to Mixture Proportioning by ACI 211.1-91
Ken Hover
Concrete International, Sep 95
ACI

 

[Briansch]

Windies, if you haven't already, reference ACI 211.1, "Standard Practice for Selecting Proportions for Normal, Heavyweight, and Mass Concrete" and PCA's "Design and Control of Concrete Mixtures".

You didn't indicate what the mix was to be used for (paving, structural, etc), the size of coarse aggregate, admixtures to use (air entrainment, water reducers). The fineness modulus you indicated is kind of "off the chart" on the low end. The PCA reference above says ASTM C33 requires the fineness moduduls to be not less than 2.3 or greater than 3.1 nor vary by more than 0.2 from the value assumed in selecting proportions. It would be good if you could get the FM up a bit - the smaller the aggregate, the more cement you'll need to cover the surface of the aggregates and "glue" them together.

You will need to work with the local ready mix supplier(s) to "tweak" the mix design which may require some trial batches if you have time. If not, see what he has in his "recipe book" of mix designs (with test data) that would be close to what you need.

The ACI reference above lays out a procedure for proportioning concrete including approximate mixing water. For 3" to 4" slump, air entrained concrete, the mix water is around 295 lbs per cy for 1" aggregate. Keep in mind this water includes the water content of the aggregate (saturated surface dry). If your w/c ratio is 0.45, the cement content is 656 lbs per cy. This seems a bit high. I would look at reducing the cement content to say 605 to 610 lbs but maintain the same w/c ratio. This may reduce workability (which the construction crew doesn't like if the work requires some serious finishing). Seriously consider the use of a low range water reducer (ASTM C494 Type A) to keep water content down (5% to 12%) and workability manageable. Air content should be 4% to 6% for moderate exposure.

It is also advisable to have frequent if not full time inspection of the work. Look at retaining a professional testing firm at least for the initial placements so you establish somewhat of a baseline. Then, have someone around who knows a little bit about concrete to look at the slump (or at least see it come out of the chute-someone who's been around ready mix can tell what the slump is by how it rolls off the fins in the drum), take cylinders and to visit the batch plant once in a while to look at the aggregate and watch them batch some trucks. This individual should be retained by the owner. The key is to work out a mix design with the proportions you need and sticking to it. Don't let your only measure of quality be compressive strength. You need to look more at durability (low w/c) and exposure, among other things. Focus more on what and how much you're putting into the mix. The strength will follow if the proportions are right.

What can blow all your hard work to heck is a contractor adding water at the site because it's not flowing around his boots and no one is there to tell him he can't do that!

Briansch