ACI 318 Water / Cementitious Ratio - Free Water? 2

[dkite007]

Hello,

This question has been bugging me recently: Does the max w/c ratio in ACI 318, Table 4.2.2, reference "free water" or "total water"?

Any sources for your opinion is appreciated.

("free water" being defined as "total water in the mixture minus the water absorbed by the aggregates in reaching a saturated surface-dry condition" according to the Caltrans standard specifications.)

Background:
I work with concrete a lot. I test it, break it, and check mix designs. These max w/c ratios make no sense to me. if the w/c ratio were that high, in practice, the slump would be 9+, seperation of aggregates would occur, and strength would never be achieved. It'd be rejected in the field straight away. Regardless, i see mix designs with these high w/c ratios all the time. In the field, however, the cement plants ALWAYS trims out much of the water (holds back much of the water) to get a w/c ratio of approximately .3. . .They seem to know something the engineers don't.

If you have any opinion on this, please enlighten me!

Thanks

 

[frv]

I'm fairly certain this refers to total water. I recall having to estimate the quantity of water in the aggregates to obtain our w/c ratio for our design mix.

 

[Ron]

I'm not being a smart a$$, but if you work with concrete a lot, you need to stop calling it a "cement" plant. It is a "concrete" plant or a batch plant.

Mix designs are done with the consideration that aggregates are in a saturated, surface dry (SSD) condition. The water-cement ratio is computed based on the free, or added water.

Slump is not a good indicator of water-cement ratio and water-cement ratio is not necessarily a good indicator of slump. There are lots of factors that determine the slump with the water-cement ratio being only one of those factors.

Most ready-mix suppliers do not hold back a lot of water. Let's consider a water-cement ratio of 0.50 for instance. With a w/c of 0.50, and a typical "5-sack" mix (470 lbs of cement), then there will be 235 lbs of water per cubic yard. For a 10 cubic yard transit mixer, that means there will be 2350 lbs of free water to react with the cement. That is 282 gallons of water. If they hold out 20 gallons of water, that only amounts to 7 percent of the water needed to get the design water-cement ratio. So the water cement ratio would now be 0.46, not 0.30 as you noted...and that's assuming that the 20 gallons is not added back in at the jobsite. If you add the 20 gallons back in, the w/c is now...0.50.

You probably need to go through a few more mix design calculations, paying particular attention to absolute volumes of materials and how they change with different mixes.

 

[frv]

I'll defer to Ron on this; I'm quite certain he knows what he's talking about.

When I had to do these calcs it was for my materials class several years ago.

 

[dkite007]

Ron,

Thanks for the response. No need to appologize for the smart a@@ comment. . .you wouldn't be a very good engineer if you didn't care about details. Besides, you're right, and i've gotten a bit sloppy with my jargon.

Anywho,why would slump not be a good indicator of water-cement ratio? (Assuming you have accounted for agg. and sand moisture and are not using additives.)

Thanks

 

[dik]

YOu can add a 'bunch' of water and cement combined to the mix, maintain the same W:C ratio and end up with 'soupy' concrete...

Dik

 

[htfptm]

Regarding slump as a measure of w/c ratio: An overly high ratio (too much water) can be "corrected" with sand.

Ask any kid with a bucket at the beach...

 

[cvg]

why would you assume no admixtures in the mix? If you spec low w/c ratio you are almost guaranteed to get a high range water reducing admixture in your mix. And if you want to put that concrete in wall forms, then you will probably also get a superplasticizer added.

 

[dkite007]

the question is if slump is a good indicator of w/c if there are no additives.

cvg, this question is not related to a specific w/c ratio. It is specific to a mix with no addititves.

htfptm, nothing personal, but i don't think your advice is good practice. Maybe if you're building a shed in your backyard, but not if you're providing a professional service. You wan't both agg and fines in you mix to make a good batch. Merely adding sand is poor quality control.

dik, that sounds plausible. thanks.

 

[cvg]

interesting question, but mostly academic. Even without admixtures, slump is still not a particularly good indicator of w/c ratio. As indicated, there are other factors including amount of sand, aggregates, cement, fly ash, type of aggregates, amount of time in transit, temperature that all affect slump.

 

[hokie66]

dkite007,
htfptm wasn't offering advice, he was just giving an answer to the question of whether slump is a good indicator of w/c ratio. His answer to that question is just as good as dik's.

 

[TXStructural]

Slump is only a relative indicator of water content from batch to batch of the same mix design. You cannot really draw any conclusion about strength, water content, W/C ratio, etc from a slump test - it only tells you if the current batch is like the previous ones or the same design. It is a simple quality control measure to verify that something didn't go terribly wrong in the proportioning.

We hope to remove the requirement for the engineer to specify slump from ACI 301 in the next cycle.

 

[rscassar]

Why is the structural engineer specifying slump when the person who best knows the concrete is the readymix supplier?

 

[Ron]

dkite007...back to your question about slump and water-cement ratios:

Depending on the other proportions in the mix, you can get the same slump with 0.50, 0.55, and 0.60 water cement ratios. The other proportions tend to increase or decrease the viscosity of the mix as much or more than the cement content or w-c ratio. An example: If you have a w/c of 0.50 and your fine aggregate ratio is low, the slump will increase. If you have a higher w/c and your fine aggregate ratio is high, your slump will go down. The gradation of the aggregates, the angularity of the aggregates, their absorption, and their hardness can all have a bearing on the final slump.

It takes all of the mix constituent proportions to define the slump, not just the water and cement. Yes, changing those without changing other parameters will affect the slump, but then so will changing the aggregate proportions.

With or without admixtures, the slump test should be used primarily to assess batch-to-batch consistency for a given mix design, and to assess in general terms, the workability of the concrete.

@kikflip...The structural engineer should be familiar enough with materials and their properties to properly specify them, including the slump of the concrete. I would not leave the slump to the ready mix supplier, since he/she does not know the design conditions of confinement, rebar sizes and spacing, durability requirements and needed strength gain until YOU, the structural engineer, tell him.

 

[TXStructural]

According to ACI:

http://www.concrete.org/Technical/CCT/FlashHelp/W.htm

water-cementitious material ratio — the ratio of the mass of water, excluding that absorbed by the aggregate, to the mass of cementitious material in a mixture, stated as a decimal.

--
ACI 301 4.2.2.2 requires 4 inch slump unless otherwise specified. Obviously, that is unworkable for most building applications these days, since the norm is to pump concrete when it has to move any significant distance.
ACI 301 4.2.2.9 requires the contract documents to specify minimum compressive strength, and "where required" W/C or W/CM ratio (usually for durability requirements of ACI 318 Ch 4.)

Many engineering firms routinely specify all three in all applications out of habit.

 

[cvg]

most standard specifications also require it and the guys behind the permit counter require these standard specs. That's why engineering firms specify it. It's hard to fight city hall and just not worth it. Easier for the contractor to submit an RFI asking for permission to increase the slump after the permit is issued.

 

[HeavyCivil]

I do see batch plants holding back water quite often with higher strength mixes. On a 5000 psi mix with a specified max w/c of .40 we usually get a mix with an actual w/c of .31 to .34 shipped.

That brings me to another point - both w/c's I'm talking about - that which we spec'd and that listed on the batch slip - take moisture content of coarse and fine aggregate into account. When stockpiles at the batch plants are almost always exposed to the elements, how could w/c's based only on free water be a good indicator of anything? Proportioning mixes per the PCA book 'Design and Control of Concrete Mixtures requires, as far as I know, the actual w/c ratio. Maybe I'm missing what Ron and TXStructural are saying - but neglecting agg moisture content would result in a very varying mix. It's bad enough most quarries around here only bake/probe samples when you require it. After a rain storm the ticket will read the same as during a drought unless their yields are off or they get pressure to check it.

I agree that slump is subjective and that its main purpose is to ensure/verify workability and pumpability and consistancy from mix to mix. That - to me - is why water absorbed by aggregate must be reflected in w/c. If its not then we have another subjective measurement.

cvg- I call H.R.W.R. 'superplasticizer'. Maybe I'm wrong to do so, but I consider them one and the same. I do understand that mid-range water reducer is different and has its own place in certain mixes (eg flatwork).

 

[Ron]

VTEIT...you don't neglect the water absorbed in the aggregates, but it is not included in the calculated water-cement ratio. It requires adjustment during batching for the actual moisture content of the aggregate.

As an example, lets say that you have a coarse aggregate with a moisture content in the saturated surface dry (SSD) condition of 3 percent upon which your design is based. The moisture content of the coarse aggregate in the stockpile at the time of batching is measured to be 2 percent. That means it is 1 percent deficient in moisture. That one percent (1% x dry weight of aggregate) is then added to the free water content, but is not included in the water cement ratio calculation...it is just added to bring the aggregate back to its design condition...SSD. If the aggregate moisture content was found to be 4 percent when measured at the time of batching, then the free water content would be reduced by an amount of 1% x dry weight of aggregate.