concrete mixing in practice question 2

[blights]

Hello

I would like to ask what is the reason that in practice, when someone is making concrete manually (not from the concrete plant) and wants to make let's say 1 m3 concrete it is required to use more aggregates that when summed up would give 1,2 - 1,4 m3 alone. I'm a student and in the lab we've been doing only the lab method, where everything is weighted and we also check the water content of the materials and subtract the aggregates water from the total water to be added. However, we dimension having 1 m3 in total in the end and not more. What's the reason for this big difference between this methods?

Thanks for the help

 

[Ron]

I'm not sure I understand your question. The volume of the end mix is based on the absolute volume of the constituents, including the aggregates. If you are "over yielding" on the concrete, your calculations are not being done correctly. Please provide more information.

 

[blights]

what I mean is that in practice when engineers don't have scales on the field use more aggregates in order to obtain a cubic meter of concrete.
Maybe this image helps

http://www.ppc.co.za/media/21174/Concrete-p1-06.png

- if you sum up the sand and gravel you will get more than 1 m3.

 

[Ron]

You are referring to volumetric batching of concrete, not weight (mass) batching. Volumetric batching is often done in small quantities based on experience of proportioning the mixes such as 1 part cement, 2 parts sand, and 3 parts coarse aggregate.

The example you show is conservative and will likely provide a strength higher than 30mPa.

The reason you seem to see disparity in the numbers is that the coarse aggregate has large voids between the particles before mixing. When you mix sand in with the coarse aggregate, the sand fills some of the coarse aggregate voids, but does not contribute its whole pre-mixing volume to the total mix volume. The same is true of the cement. The remaining voids created by both the coarse aggregate and the sand are then filled by the cement so that you have a dense mix of solids.

As an example, your mix has 0.71 m³of coarse aggregate. Assuming a specific gravity of the aggregate of 2.65 and a dry rodded unit weight of, say, 1350 kg/m³, you would have an absolute volume of 0.51 m³ of the coarse aggregate if it were a solid mass. But it is not solid so those voids (0.71-0.51=0.20 m³) could be filled with sand, and so on with the cement. The numbers might be off a bit, but you can get the idea of how it works.

 

[SlideRuleEra]

Ron - A really good explanation.

http://www.SlideRuleEra.net

http://www.VacuumTubeEra.net

 

[blights]

Ok it makes sense... I suspected it has something to do with that.
How did you get the specific gravity 2.65? It means you considered a density of aggregates of 2650 kg/m3. Why is that? I checked some older spreadsheet I have from my courses and I saw that we used similar values for that but can't find the reason why (saturated grain density or however you want to call it).
Also based on what you are saying it means that using mass batching the void ratio is taken into consideration when creating the mix?

Thanks a lot

 

[Ron]

Thanks, SRE...I appreciate your compliment!

blights....the specific gravity of most soil and rock materials will fall between 2.60 and 2.70. It is a parameter that can be tested in the laboratory using standard test methods; however, if you are familiar with the geology of an area, you can probably guess the value as accurately as you can test it. I generally use 2.63 to 2.65 for fine and coarse aggregate that comes from a dense rock source such as most metamorphic rocks. When you are dealing with sedimentary rocks, particularly calcareous materials, the specific gravity will go down, sometimes as low as about 2.50. For igneous materials the ranges can be vast depending on parent material, porosity, cooling rate, etc.

In batching concrete by its weight (mass), material proportions are selected based on a first decision of strength required and the corresponding water-cement ratio, then working through to finding the absolute volume of materials. A simplified approach is contained in ACI 211, a publication of the American Concrete Institute. Similar techniques are covered in publications by the Portland Cement Association ("Design and Control of Concrete Mixtures") as well as other sources.

 

[blights]

Yeah the way I learned to do it is based on the ACI "model" but there was nothing involving void ratio or anything in that from what I recall and once you obtain the quantities you could basically create the mix.
As for the specific gravity... isn't that defined as the ratio between the density of a material and the density of the water?

 

[Ron]

blights...yes

 

[concretemasonry]

Here, when I was involved with testing and mix designs, we usually used 2.70 or 2.75 as density of the aggregate because our aggregates were all igneous and more dense than either sedimentary or metamorphic. Our aggregate supplies were mainly glacial, so the aggregate was rounded, very strong and durable (unless some shale got into it).

Ron's explanation of the voids is very good. I always like the simplistic example of taking a crate and adding some clean rock(or cannon balls) and then weighing it. Then add some BBs then fill the crate and weigh it again.

Dick



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

 

[racookpe1978]

But remember to add the weight - NOT volume! - of the water in the total required volume = concrete weight/concrete density calc.

Now, at normal pressures and temperatures (and that is the only region important when concrete is being mixed), water volume and density and weight are each constants. But the water is mixed into the powder + sand + aggregate mix, so the water "volume" is "gone" because it chemically reacts to the powder and across the surfaces of the mixture. The "water weight" remains in the mix.

Since water (at a job site) is available through a hose (time is known, flow only approximated), from a volumetric can or bucket (volume known precisely but slowly), or is pre=mixed from the concrete yard into the truck, people don't think of water the same way and at the same time as they measure the other three ingredients. (Bags of concrete, bags of sand, pounds or tons of aggregate or cement or sand, etc.) In the budget and in the transportation costs, water is "free."

Also, there is little penalty (other than the very cost of the extra cement) for over-ordering the mix: If the form is even slightly "empty" when the final cement truck leaves empty, you have a tremendous problem. It is as likely that the forms or excavation is too deep or too long than it is "too small" Leaks and spills over over-flows do not "add" extra concrete to the job, are losses from a "perfect pour" or a "perfect form job".