Concrete Mix design review 4

[vinu180]

I have been asked to do a concrete mix design review. I have received subimittals from the manufacturer and admixture data sheets. As a minimum, what should be specified in a mix design review?

 

[civilperson]

A set of thirty samples and the corresponding breaks, (of that particular mix design)..

 

[henri2]

IBC Table 1704.4 item 4 is a good starting point. Under the sub-heading Referenced Standard, you are referred to ACI 318 Chapter 4 and Sections 5.2 - 5.4.

Chapter 4 deals with durability considerations and Sections 5.2 - 5.4 deals with requirements for the mix (f'cr, workability mix design methodology etc).

If you have more stringent specifications, this must also be taken into account.

In a nut shell I'd do the following:

1. Make sure the mix design has a designation

2. Job address should be shown

3. Name of concrete supplier and source of materials should be shown as a change in these variables can affect the quality of the mix.

4. The 1 cu.yd mix based on an SSD (saturated surface dry) basis must show weights of cement, aggregates, and water, including additive/admixture dosage rate or amount. SSD specific gravities of ingredients must also be shown. If you are doing this on behalf of a contractor also request evidence that the relative yield is greater than 1.

5. Method of mix design must be indicated and back up data requested. If the statistical approach based on field test results is not possible because enough data us unavailable, then request three point curve W/C ratio versus strength data based on trial mixtures as outlined in ACI 318 Sec 5.3.3.2. For the 3 point curve approach, the average compressive strength at 28 days or age specified must equal or exceed the f'cr value shown in Table 5.3.2.2. If neither the statistical method nor method based on trial mixtures is possible because of lack of data, then see if ACI 318 Sec 5.4 will work.

6. The slump must be shown and expressed in a manner consistent with the language used in the job specifications as this may affect the slump tolerances permitted by the inspector in the field. Refer to ASTM C94 Section 6

7. The mix must show method of placement (pump and or tailgate)

8. The structural element/location the mix is intended for must be shown.

We had a similar discussion a while back.

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

If you are not comfortable performing the review, have a qualified individual from a testing lab who has experience with the materials the mix is based on perform the review. Such an individual should be bale to perform this type of review in no time.

 

[BigH]

civilperson - what do you do if you don't have 30 breaks?

I am presuming (maybe incorrectly) that the asker is not from North America or developed country but that the mix design is being done on a project basis - not from a running batch plant operation.

Normally, you will design a mix, say 30 MPa, to have a mix design break at 30 MPa + X MPa - usually 8 MPa or so at this strength level. This is the target strength. When in production, the mix can be adjusted to reflect the statistics of breaks. In remote areas, we don't have batch plants and many times the aggregates are first time use for aggregate.

I would say as a minimum:

1. Make sure you have the mill test certificates for the cement used in the trial mix. Also, if possible, carry out confirmation tests of the cement.
2. Make sure that the aggregate properties have been included in the mix design documentation. Gradations of each part, Los Angeles abrasion (or Micro-Deval), and other tests presribed for coarse and fine aggregates by your particular specifications (ASTM, IS, JIS, etc.).
3. Make sure that you have with the documentation the required water tests to confirm that the water is suitable.
4. Make sure that the mix design computations (as per ACI or BS or IS) are correct.
5. Review the concrete break strengths. Again, you will have a target strength greater than the specified strength. If the aggregates have been used successfully in the past for concrete, the target strength will be lower than if they have not been. The target strength will also depend on the "level of control" that will be used in the concrete production.
6. Get a good book on mix designs (Neville and Brooks or Shetty, say) - they have good chapters on the concrete mix design and how to do it.
7. Make sure you are satisfied that the results can be reproduced and seem reasonable (and not cooked).

Most specs I have been involved with indicate that the contractor is responsible for the mix design he will use. You are only looking to say you have no objections or adverse comments to the proposed mix design. Your "approval" does not relieve the contractor to produce.
If you are not comfortable with making the "decision" - then do your best review and then ask help of a more expereienced individual. But, it will look good on you if you can show him what you have, where you are unclear, and what help you want. Not just pass the ball to him.

 

[dochardnose]

ACI requires a mix that does not have 30 breaks and a standard deviation calculated, be overdesigned by at least 1200 psi.

 

[henri2]

Actually, in the ACI approach, the standard deviation (Ss) can be applied if the concrete production facility has fewer than 30 test results. ACI 318 Sec 5.3.1.2 has modification factors when results are between 15 through 30. The modification factor increases the Ss and consequently the required average compressive strength f'cr.

When the facility has less than 15 test results, the next option is use of a field test record consisting of no less than 10 consecutive test results or trial mixtures. This is where Table 5.3.2.2 kicks in.

All of this is explained in the flow chart shown in Figure R5.3 of ACI 318-05 and Sections 5.3 and 5.4

The overdesign to achieve the target strength/required average compressive strength f'cr based on trial mixtures or field test record depends on the f'c. According to Table 5.3.2.2, the overdesign can be as low as 1000 psi if f'c is less than 3000 psi. For higher strength ranges the over design increases.

As Big H has pointed out, the requirements for mix design depend on which codes and standards apply.

Question for Big H. What do Indian Standards (IS)for concrete mix design methodology typically follow, EN, ACI, DIN, BS....?

 

[BigH]

Mmmmm - Indians seem to develop their own. I would say in many ways it is "home grown" - may reflect the BS method but I think that BS is basically a historical method (?). Shetty, probably the Indian Guru on Concrete (similar to Neville for us) presents the ACI method and the IS method separately. First they determine the target strength which is characteristic plus 1.65 x stdev. There are assumed Stdev for mixes ranging from 3.5 MPa at 15MPa strength to 5MPa at 40MPa. This is assuming good control. If not so good control, you increase the stdev by 1 MPa. You then determine the w/c ration for a characteristic strength. They have different curves for different strengths of cement. (India produces OPC 33, OPC 43 and OPC 53 (which are the strengths of the cement at 28 days). Shetty points out that the method takes into account the strength of the concrete. Then they determine the air content depending on the max agg size. They then select the water content and fine to total aggregate ratio. Calcualte the cement content and calculate the agg content. Then do trial batches. That is it basically.

 

[henri2]

Thank you Big H for the explanation.

I know Adam Neville. He was my dept chair. A true guru in the field of concrete...both in materials and design.

 

[BigH]

henri2 - wow - he as at Alberta for a while, eh? I had Floyd Slate as our concrete man. Interesting chap - did a lot of work on confinement strength - you'll see him referenced a bit - especially in the older books (F.O. Slate).

 

[henri2]

Big H,

I was in high school when AMN was at Alberta. He was my dept chair in the second half of the 70s. He then moved on to Dundee where he became principal and VC.