Concrete Mix Design Proportioned by Trial Mixes

[wannabeSE]

What documentation do you expect to see in a concrete mix submittal that is designed with trial mixes per ACI 318 rather than field experience? I am accustomed to submittals with field experience where a summary of the test result are provided and the statistical calculations justifying the concrete strength. For trial mixes, do you expect the submittal to include the all of the trial mix designs or just the final design with satisfactory f'cr. Should all compression test results be included for all of the cylinders tested, or just the average for each trial mix?

 

[Ron]

You should get their final mix design with appropriate backup test data. This would include aggregate source reports, aggregate reactivity tests or a statement of historical record of service, cement mill certificates, admixture data and water data. You should get compressive strength reports for 28-day results. Keep in mind that the target strength must be considerably higher if you do not have sufficient historical data on the mix design. Trial batch designs must show compressive strength results that are higher than the intended f'c as required in ACI 214 and ACI 318

 

[wannabeSE]

Ron,

Thank you for the information. When a mix design has no historical records, would you expect to see the actual compression test report from a testing agency, or will a simple statement from the mix design engineer stating the average compression test result be sufficient (the required f'cr is 1000+psi over f'c per ACI 318 table 5.3.2.2 or similar table in ACI 301)? At a minimum, I want to know the date of the test(s) since aggregates can change over time.

By the way, we have started to see trial mixes rather than historical data for typical concrete mix submittals.

 

[Ron]

wannabeSE...statements from the "mix design engineer" are worthless without backup data. For trial batches, all information should be contemporaneous. There is no reason that a trial batch done last year should be used for a submittal, since there should be adequate moving average test data since the trial batch to show compliance with ACI 318 and ACI 214. If that can't be done, then have them do new trial batches or have a local testing laboratory do a mix design verification.

Stick to your guns on what you want to see. I don't do mix design review routinely anymore, but when I did the producers in our area could spot my requirements for mix design submittals. They knew they would have to comply strictly with ACI requirements and provide me with all the aggregate info I typically requested. Those submittals became known by my name, though the comments were not necessarily endearing! More like....Oh $hit...another "Ron" mix design submittal....but they complied and my clients were better protected both functionally and legally. One of the 3 or 4 local producers actually took pride in the fact that his submittals could pass my scrutiny.

 

[wannabeSE]

Thanks again Ron,

I appreciate the encouragement. Since I can be excessively detail oriented, I really make an effort to approve submittals as noted rather than have them revised and resubmitted. I am amazed at some of the simple things that are missing or wrong in submittals. I'd better stop typing before I get up on a soap box and start whining about poor quality submittals.

 

[BOluyemi]

Someone please tell me in which British standard i can get the charts and tables used in designing a concrete mix, for example: CCS 4: Relationship between compressive strength and free-water/ cement ratio.

CCS 1: Approximate compressive strength (N/mm2) of concrete mixes made with a free-water/cement ratio of 0.5

etc.

Thanks in advance.

 

[Ron]

BOluyemi....please start a new thread with a new question. You might consider placing your questing in one of the concrete forums rather than the structural forum.

 

[BigH]

Almost all our concrete work in which I have been involved with over the last 15+ years is based totally on trial mix designs. The mix designs are worked up with ACI or BS (now EN) standards. Usually a standard deviation is used to determine the target strength above the characteristic (design) strength. Thus the target strength would be the characteristic strength plus 1.64xStdev (usually taken as 5 MPa) . . . For normal concrete strength ranges this is typically 8 MPa plus or minus a bit. There are some contracts, though, that actually specify the difference in the trial basis mix design's target strength and the characteristic strength. We must have the strength of the trial mix cylinders/cubes at the requisite date (28 days but now getting towards using 90 days for mass concrete and much reinforced concrete as fly ash is now being typically used). Only then would we accept for approval a mix design for placement into the works.

On the projects in which I have been associated, we have not experienced any problems with this method for strength (we are normally out in the boonies and not in a city where there are ready mix plants at hand). Also, as aggregate (or cement, fly ash) sources may vary, any change of a source would require a new trial mix design. For instance on one project the contractor had a back-up cement source in case his nominated source went down or was unable to produce sufficient cement. This can be particularly important when fly ash is used as its properties are dependent on the source of coal used in the burning. As statistics become available, the stdev of 5 MPa can be modified based on the history - I have found contractors, though, reluctant to do so especially if their unit rate is based on m3 of concrete placed - likely as they feel the owner will want a reduction in price if he can reduce his cement content . . . or that "if it ain't broke, don't fix it."