Size of Coarse Aggregate

[chencfp]

We are going to construct a reinforced concrete continue beam. Since the long span and loads, the beam is heavy reinforced. We concerns that the standard coarse aggregate is difficult to be poured into the beam. We want use small size coarse aggregate. The ready mix company told us that the only avaiable for small size coarse aggregate is max. size 3/8 Coarse Aggregate. My questions are:
1. Is it ok to achieve a 4000 psi strength in 28 days for max. coarse aggregate of 3/8"?
2. Will this size coarse aggregate coccrete will cause more shingkage cracking than standard size coarse aggregate? Does this type cracking affect the beam capacity?
3. Any disadvantage of using 3/8" coarse aggregate?

Thank you very much.

 

[youngstructural]

What do you mean by a continue beam???

Okay, setting that question aside, if I understand your question properly, you're worried primarily about any possible negative effects of smaller max aggregate size. I'll answer you three points while maintaining your numbering, and add a couple of my own...

1) There is no reason this cannot be acheived, techincally, by proper mix design.
2) Yes; It is possible an increased amount of shrinkage cracking will result. It would normally be finely distributed micro cracking, if it occurs at all, and would not affect the beam capacity.
3) Cost, cost, cost. By using a smaller maximum size, you will increase the amount of cement required. The fines cannot be increased too much to compensate without causing other problems, and course aggregate is one of the cheapest (normally, unless it is trucked and others are not) ingredients in concrete. Thus you are increasing your overall concrete cost.

My additional points to consider:

1) It is not possible to meet the minimum spacing (Smin) requirements between bars of your code? If Smin is met, typical concretes may be used.
2) You call this a continue beam; Is it to continue an existing beam? If that is the case there are much more important concerns relating to the concrete, such as interface bond.
3) You may actually incur a heat of hydration issue, and the surface area available to offset the heat build up should be carefully checked with the help of your mix designer.

Good luck!

YS

B.Eng (Carleton)
Working in New Zealand, thinking of my snow covered home...