W/c ratio for 3000 psi mix design 1

[dianium500]

Hi All

I recently started reviewing concrete mix designs and I have noticed the w/c ratio is higher than the typical 0.45 recommended amount. The main complaint I am getting is that 0.45 is typical of 4000 psi and not 3000 psi. I am concerned about durability and cracking. Also, I noticed the air content is really low. I don't want to change the design specs but if 0.45 is not appropriate for lower strength concrete then I would like to know.

Thanks

 

[DaveAtkins]

I will express my opinion here, and see if everyone else comes down on me

I have always disagreed with putting too many prescriptive parameters in specifications. If you want 3,000 psi concrete, ask for it, but don't tell the concrete supplier what water/cement ratio to use--let him do his job.

If you want a 0.45 water:cement ratio, ask for it, but don't tell the concrete supplier the concrete will be 3,000 psi.

Now, I will admit all of the cementitious additives we use these days (fly ash, slag, silica fume) make it difficult to know what w/c ratio goes with what strength.

DaveAtkins

 

[jayrod12]

In Canada, we require sealed mix designs confirming that it meets our strength, air and exposure requirements. What they actually put in the concrete I could really care less about.

 

[IFRs]

I usually ask for test cylinders which confirms the mix. We break 1/3 after 7 days, etc...

 

[BUGGAR]

Many specifiers will call for a low W.C. ratio for "durability" when they are really after strength. By doing this they can specify "uninspected" concrete and save some dollars while still getting the strength. I frequently see concrete with a specified 28 day strength of 2500 psi with a W.C. ratio good for 4000 psi. But this is on the coast.

 

[dik]

jayrod:
CSA has a couple of alternatives for concrete supply. Alternative A is the most common that I've encountered. It does not require a design, it is prescriptive. The mix design can be treated as s proprietary mix. The supplier has to only provide a history of use. I usually stipulate that the concrete must be supplied in accordance with A23.1, except as noted herein (Project Notes).

I spec a minimum concrete strength and a max W/C ratio, usually. For a W/C ratio less than approx. 0.5, you will likely get a strength of 3000 psi min. With all the admixtures, W/C seems to be less significant, but I spec it anyway, for durability, or whatever.

Dik

 

[Once20036]

Maybe this is getting away from the original post - but for you folks that specify the strength and let the supply figure out how to do it, how do you handle shrinkage/curling/cracking?
I`m under the impression that too much water can exacerbate each of these problems, even if the concrete meets the strength that you specify.

Does the concrete supplier accept that responsibility?
Couldn't they just as easily blame the control joint spacing, reinforcing, etc that you specified - thus making the cracking from the wet concrete your problem?
The client doesn't come after you for issues associated with these cracks?

 

[Ingenuity]

I frequently see concrete with a specified 28 day strength of 2500 psi with a W.C. ratio good for 4000 psi. But this is on the coast.

BUGGAR,

Really, 2500 psi - what application? Round here, never see anything less than 4,000 psi specified, and more often 5,000 psi min.

 

[Ron]

It would be a good idea for all structural engineers to know a bit more about concrete technology, since we specify it routinely.

If you want a specific w/c ratio, then understand why you want that w/c ratio. Most often, a limitation on w/c ratio has to do with durability, although the compressive strength of concrete is clearly and directly related to the w/c ratio. The lower the w/c ratio, the higher the strength (Abrams Law).

The lower the w/c ratio, generally, the higher the durability of the concrete for most applications.

For 3000 psi concrete, the w/c ratio can be as high as about 0.60. For 4000 psi concrete, the w/c ratio can be in the range of 0.50 to 0.52, depending on other aspects of the mix design.

Review ACI and PCA requirements for mix designs.

 

[msquared48]

Durability affects the ability of the concrete to maintain it's strength by keeping the steel reinforcing intact. It's a tradeoff...

Mike McCann, PE, SE (WA)

 

[CELinOttawa]

Mike: Not sure I see your point. Increased durability mainatains long term strength by protecting the steel. But that's the only affect I am aware of.... The specific parts which contribute to the durability (permeability, autogeonous healing, free paste, etc) can be discussed but I am confused my your trade-off comment. I also respect you enough to want to understand your thinking...

That's a bald-faced and blatant bit of flattery, but you deserve it. Merry Christmas.

 

[Ron]

CEL...durability affects almost all parameters and properties of concrete. Its most pronounced affect is the resistance to deterioration of the concrete when the concrete is exposed to weathering and other detrimental environments; however, it does have an effect on critical structural parameters such as bond to the rebar and corrosion protection of the rebar, both from the enhanced compressive strenght and lower permeability.

 

[darthsoilsguy2]

off top of head, i've usually seen the 0.45 w/c ratio on standard specs tied to 4 ksi or higher from specs. 0.55 and 0.50 sound more familiar for under 4 so i think i agree somewhat with the producers on this one. i haven't paid too much attention to it though. Some egrs specs take it a prescriptive step further and spec w/c and the # of bags of cement in the mix. w/c ratios seem like they come from older and simpler times then now and mixes seem to have more admixtures and pozzolans in the works now. We don't have any approved way to field verify w/c ratio (because the RMC producers are on the ASTM committees and they don't want the microwave test checked against the batch weights) so it just seems like extraneous info. but, if the thing you really want is durable concrete with low volume change and w/c cement ratio is your way of getting it... then sure... a 4 ksi mix is suitable for 3 ksi work. i'm not really that much of a fan of 3ksi air mixes. i think there is something to having that extra ksi to have a reliable air mix.