Concrete Mix Design Review

[RFreund]

My main question here is in regards to the water / cement ratio and if you apply a limit on this. However, any input on what you typically check for in regards to a typical concrete mix design review would be helpful.

Below are some typical checks and my question regarding W/C ratios.

For footings/foundation walls:
Concrete strength (typically historical data)
Maximum aggregate size
Air Entrainment
W/C ratio - for foundation walls and footings, this would be Exposure category S per ACI 318. However I'm unsure how to judge which severity applies? If it falls in S0 then there is no limit on W/C. Do you have a limit that you typically apply to this?

For Interior Slabs:
Concrete strength (typically historical data)
Maximum aggregate size
Air Entrainment (typically none or small)
W/C ratio - Here the severity would be F0 (at least for most cases) so again there is no limit on W/C. Do you have a limit that you typically apply to this?

For Exterior Slabs:
Concrete strength (typically historical data)
Maximum aggregate size
Air Entrainment (typically none or small)
Maximum aggregate size
Air Entrainment
W/C ratio - for foundation walls and footings, this would be Exposure category F per ACI 318. Typically where I am at there severity would be F1-F3. However I'm unsure how to judge which severity applies? If it falls in S0 then there is no limit on W/C. Do you have a limit that you typically apply to this?


In a particular case I am looking at a 0.63 W/C with 4" of sump (didn't realize that was possible) for an interior slab. This seems high but the historical cases meet strength requirements. The slab use is office space. Any thoughts on this?

Thanks!!

EIT

http://www.HowToEngineer.com

 

[wannabeSE]

In the project specification, we specify the max w/c ratio for different concrete uses. The mix design submittal should meet the project specifications. A w/c = 0.63 seems high for a slab-on-grade. I would worry about shrinkage cracking and potential for curling.

SEAONC has a fairly comprehensive guide for reviewing concrete submittals:

http://www.seaonc.org/committees/co...rum/guidelines-reviewing-concrete-mix-designs

But this doesn't provide information on specifying concrete (No info on typical w/c)

 

[Ron]

RFreund...if I remember correctly you are in the US. Take a look at ACI 211 for some recommended w-c ratios and slumps for different applications. ACI 211 tables are generally without the benefit of admixtures, other than air entrainment.

As you noted, a w-c ratio of 0.63 and a slump of 4" doesn't compute! It would imply to me that your aggregate gradation is relatively harsh (high FA to CA ratio). This will cause a higher water demand and a lower slump because of the increased aggregate friction.

When I review a mix design, I look for a few things:....

1. Is the w-c reasonable for the application? In general the w-c ratio defines the strength. Strength begets durability. Durability is more important in a slab on grade than strength! If you design for durability, strength will almost always be adequate. For a slab on grade without forklift or other traffic, 0.55 is my max with a reasonable aggregate ratio (about 38 percent FA, 62% CA) of the aggregate fraction. If you have forklift or other vehicle traffic, the max. w-c should be droppedd to 0.50 max.

2. Aggregate durability should be proven. Get certification that the aggregate contains no reactive contaminants and will not expand when used in the concrete. It should have a good abrasion resistance.

3. Look at the supplier to make sure their batch reports and trip tickets comply with ASTM C94.

 

[RFreund]

Thanks for the info!

One question - does Fly Ash typically get included in the W/C ratio? I thought it did. Also it can only be a certain percentage of the w+p total.

EIT

http://www.HowToEngineer.com

 

[Ron]

RF....yes, fly ash is included in the WC ratio.

 

[TLHS]

I love the fact that the Canadian concrete codes have incorporated a serviceability spec where the engineer just has to specify a couple of variables (generally exposure class, cement type, strength, density and air content category), all of which have handy charts in the code. Then the concrete supplier is required to determine that their mix meets those specifications.

It means that the engineers actually understand what they're specifying, and the suppliers can have a library of standard mixes that they know work because there are only so many common combinations of the different requirements that will typically be called out.

 

[Ron]

TLHS....go figure! Specifying concrete for strength only is easy and unless someone really screws up, it will meet the strength requirements. When you throw in serviceability (durability) you actually have to learn what concrete is and how it works. As I've said many times, if you design the mix for the required durability, strength is just a side effect (and it will likely always be met!!).

Yes, as a structural engineer I know that we need to quantify f'_c. However, if that is our only consideration, we are seriously overlooking the need to design concrete for its use, not only for its strength.