Conc. Mix Designs based on ACI "trial mixtures" procedure 2

[pelotoner]

We have an ongoing debate in our company:

ACI 5.3.3.2; many concrete mix designs have the 3-point curves comparing compressive strength and w/c ratio. ACI requires:
1. combination of materials shall be those for proposed work.
2. trial mixtures have proportions and consistencies require for proposed work.
3. three different w/c ratios that encompass the required f'c

Q: Does this mean that only the cementitious materials and water are allowed to change? Do the aggregate, HRWR, SuperP, etc. have to be the same?

We have many submittals have completely different mixes (changing aggregate %, HRWR quantity, etc.), basically three mixes off the shelf with the same type (not %) of aggregate; this does not seem to meet the intent of ACI. COMMENTS??

 

[Ron]

The materials used in the trial batches should reflect the materials used in the production. That includes all materials. Different admixtures interact within the mix in different ways. You are trying to predict the behaviour of the concrete as well as its strenght.

 

[pelotoner]

What about the percentage of aggregate (small and large)? Should it be maintained by weight, if the water and or cemetitious materials change?

 

[Ron]

If the cement or water content changes, adjustments must be made in the fine and coarse aggregate as well.

Concrete is batched by weight, for convenience. It starts though, as a volumetric exercise (i.e., 1 cubic yard). All of the constituent weights are converted to absolute volumes to achieve the desired proportions.

Increasing the fine aggregate volume will necessitate an increase in cement and water, because you have to have enough cement and water to coat the increased surface area. Conversely, an increase in aggregate size or coarse aggregate proportion will decrease the amount of cement and water for all other properties maintained. Increasing the amount of coarse aggregate or increasing the coarse aggregate size will decrease the amount of shrinkage in the concrete.

 

[dik]

There are a few web sites with a batch calculator that can be used for a prelim design. I captured one that was a Java script and have it on my laptop as a program.

I use this as a starting point or for checking other batches. Actual mix designs are generally 'tuned' based on a prelim batch design. Aggregate moisture content, generally the fines/sand is often critical.

Dik

 

[pelotoner]

I may have been vague.

We are the Structural Engineer of Record. My question is related to what ACI requirements must a three-point water:cement ratio curve must have.

Specifically, since ACI requires that the water:cement change, is anything else allowed to change? What does it mean when ACI requires that "mixtures have proportions and consistencies require for proposed work"?

 

[Ron]

As I noted earlier, when you change the water-cement ratio, you will need to change all other constituents to maintain the volume relationships. Sometimes those changes are minor, sometimes major.

What ACI means by that statement is that the mix proportions (sizes of aggregate, aggregate ratio, water-cement ratio, etc.) and consistency (slump) should be selected to be compatible with the application.

For instance, in reinforced masonry, the cells are filled with grout. The grout is one form of structural concrete. In this case, it must be very flowable and the aggregate must generally be small. Selecting a small coarse aggregate requires a higher amount of cement and a higher amount of water since the surface area of the aggegate increases with a decrease in size, yet the smaller coarse aggregate is needed to get into the masonry cells and consolidate without large air voids or segregation. In order to do this the mix has to be "soupy" or have a high slump.

 

[henri2]

I’d like to chime in with some additional comments to the valuable ones made by Ron and dik.

We have discussed the concrete mix design review process on several occasions in the past. Here is a link to a discussion I participated in.

http://www.eng-tips.com/viewthread.cfm?qid=146578

For more relevant threads use the search feature at the top of the page.

Regarding the aggregates in the mix, make sure that the MSA (max agg size) is the same and aggregate size combinations are consistent.

To get a good feel for what happens to the amounts of coarse aggregate and fine aggregate as the W/C ratio varies check out Ashraf Ghaly’s concrete mix design procedure (based on ACI 211) on his website.

http://concrete.union.edu/general.htm

dik posted this link a while back.

Let us assume that SG of cement is 3.15, SG for fine and coarse aggregates is 2.65, FM for FA is 2.60, free water on fine aggregate (FA) is 5.0%, absorption of fine aggregate is 1.5%, free water on coarse aggregate (CA) is 1.5% and absorption of coarse aggregate is 1.0%, compute mix proportions for 1 in. MSA 3000 psi, 4000 psi and 5000 psi based on the following additional requirements/steps:

1. Absolute volume method
2. US units
3. Non-air entrained concrete
4. Slump: 4 in. max --- 3 in. min (Step 1)
5. Max aggregate size = 1 in. (Step 2
6. Mixing water for 1 in. MSA, non-air-entrained and slump of 3-4 in. = 325 lb (Step 3)
7. Select W/C ratio for f’c (Step4)....do not use fly ash…ignore exposure condition requirement
8. CA (Step 5) use FM of 2.60 for fine aggregate ,100 pcf for unit weight for CA and SG of 2.65 for CA
9. FA (Step 6) use SG of 2.65 (as stated above)
10. Adjustment for moisture (Step 7). Use free water of 1.0% for CA and 5.0% for FA….absorption of 1.0% for CA and 1.5% for FA. Do not use admixture to keep it simple
11. Summary of Mix Design (Step 8)

After running the numbers, it will be obvious that as the W/C ratio decreases when f’c increases, the total paste (W+C) increases, C increases and the total aggregate (FA +CA) decreases. For a given slump range, W is essentially constant. Furthermore, the FA expressed as a percentage of FA+CA decreases with the corresponding decrease in W/C ratio. Does the amount of CA change much?

It must be pointed out, that ACI 211 is merely a guide to be used when dealing with an unknown aggregate source. Typically what happens afterwards is fine-tuning of the mix, so that it can perform with respect to workability, pumpability, finishability etc in addition to satisfying strength and durability requirements.