Concrete strength requirement! 1

[engr567]

Per ACI 318-08, table 4.2.1 and table 4.3.1, for any exposure condition above F1, the minimum required f'c is 4500 psi. Does it make sense? If we specify 4500 psi concrete for smaller projects where we use concrete for footings and retaininng walls only, we may not get another project next time from this client. What is your experience on this? Am I missing anything here?

 

[Ron]

ACI 318 is a building code mandated document. It is written in imperative language. Read the paragraph before the table and you will see that it uses the term "shall".
That's what it means.

You don't make engineering decisions based on whether you might or might not get a job again. You make engineering decisions based on sound technical judgment. If you need to educate your client then do that, but don't let your client dictate your engineering decisions. It's your license, not his.

 

[engr567]

Ron - thanks for advice.

I think you did not understand my question. Your answer is academic than practical. My question was, does code allow to use lower compressive strength under this condition? Especially when you are dealing with less amount of concrete for small residential project for an example.

 

[Ron]

No. To follow the code, if you have the exposure condition noted, there is no provision for going to a lower strength.

 

[JAE]

Is the sulfate exposure due to soils or some other source?

If it is soils, then Ron is correct as your small amount of footing concrete would be exposed. If the concrete is exposed to the attacking substances elsewhere, there may be some concrete that wouldn't be exposed and thus, per the code, not need the higher f'c value.

I saw nothing in Ron's answer that was "academic".

 

[IDS]

I don't understand why having a small amount of concrete is seen as a reason to reduce the required concrete grade. Surely small volume = small additional cost for reduced risk of corrosion/deterioration!

Doug Jenkins
Interactive Design Services

http://newtonexcelbach.wordpress.com/

 

[BigH]

With such a small quantity of concrete - surely the cost differential is less than the time it has taken the experts here to comment.

 

[engr567]

Thanks all.

 

[concretemasonry]

BigH -

Amen!!!

The incremental minor extra cost for strength is meaningless.

Here (Minneapolis/St. Paul) if you want to pour a driveway, you may not even get delivery of concrete from one of the better producers unless it is minimum 4000 psi with 5% air and 3" slump unless the purchaser signs off on the suppliers liability. The lost business to the supplier caused by poor concrete specifying is less than the liability and negotiating costs later if there is a problem.

Dick

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

 

[rscassar]

In my part of the world the concrete grade is a really small contributor to the overall project cost. If the concrete is being placed in an aggresive environment it is best to specify a more durable concrete grade. Otherwise you might have engineers looking at this concrete in 20 years time after it has shown a considerable degree of deterioration, trying to rectify the situation.