Are concrete mix details required in drawing set? 12

[StructureMan44]

Sometimes I have seen concrete note drawings specifically list the required concrete mix details. Is this necessary or is specifying ACI 318 (mainly chapter 3-Materials) sufficient?

 

[JedClampett]

I'm not sure what details you're talking about. w/c, strength, I'd say yes. Amount of fine and coarse aggregate, amount of cement, other admixtures, no. If you over specified, the batch plant might have to make a special mix for you. That would require new tests which takes time.
You don't want them to have to reinvent the wheel if they have a mix that already meets your basic requirements. Plus, if they're busy, unless your project is very large, they might not want to be bothered.

 

[Ron]

Agree with JC....specify slump range, specify w-c ratio, specify minimum compressive strength at 28 days, limit some admixtures (like calcium chloride), limit total chlorides if near salt exposure, set coarse aggregate size for the application, allow pumping or not, and require that mix design be submitted for review before placement, which should include aggregate sources, aggregate testing for durability and reactivity.

 

[TLHS]

I find that overspecifying is a big problem in concrete. People specify concrete requirements without really understanding what they're doing. I'm a big fan of the way the Canadian codes have moved, with a descriptive style coding of exposure and surface types that have code defined requirements for the manufacturer.

They can then have standard mixes for different exposures and the engineer can just specify the application using the correct coding system.

 

[wannabeSE]

Typically, I see the concrete strength(s) in the general notes on the drawings. The rest of the mix design requirements are in the specifications.

 

[thaidavid40]

I agree with those who say to simply call out the performance characteristics (strength-either compressive or flexural, slump, entrained air percentage, possibly maximum aggregate size). Then let the concrete supply pros do their job, and specify the mix for your review and approval. Over-specifying concrete will hamstring the concrete supplier, drive up the costs, and maybe end up with a concrete mix that doesn't perform like you would want it to.
Dave

Thaidavid

 

[DamsInc]

If you have access to CSA 23.1, there is a good list of things to specify depending on whether you are going for a performance vs. prescriptive method.

Being a young (Canadian) engineer, I have never understood why many consultant engineers on this board are so keen on specifying all aspects of the concrete. I recently took a class from a leading concrete expert who said that "Engineers have no business specifying the slump. The required slump is a function of the construction method, which (in most cases), the engineer does not specify". There are many ways to vary the slump other than just adding water, so why would you limit the options of a contractor?

Reputable concrete suppliers are the most knowledgeable about admixtures, aggregate properties, SCM's, and their interactions. In most cases they should be the ones to create the mix which satisfies your performance requirements.

If you specify the details of the concrete mix, then you assume the responsibility for the final product. If you specify the performance requirements, then it is on the contractor/supplier.

 

[AnimusVox]

On our drawings we typically specify:
[ul]
[li]concrete 28 day compressive strength[/li]
[li]concrete air content (due to exposure)[/li]
[li]water-cement ratio.[/li]
[/ul]

If there's additional things to address such as inclusion/exclusion of admixtures, fly ash content, max. aggregate size, etc. We'll just put that in the specs.

 

[Ron]

@DamsInc....I respectfully disagree with your concrete "expert". Concrete is an important structural and serviceability material in building construction. The more we only rely on "performance" specifications, the farther the engineer gets away from having to know anything about concrete. In my opinion this is wrong. A structural engineer MUST know the materials upon which his/her design is based. To specify a 3000 psi concrete and give no further direction or limitation is irresponsible, does not meet the standard of care and leaves the engineer open to a variety of claims.

An engineer needs to know the capabilities and the limitations of the materials he/she specifies. They need to know that the smaller the coarse aggregate, the more cement the mix will require and thus more water. They need to know that this will increase shrinkage of the mix and lead to unsightly and uncontrolled cracking unless some placement constraints are placed on the concrete activities. They need to understand that rebar spacing affects the maximum size of coarse aggregate that should be used in the mix. They need to understand that certain admixtures can be deleterious to the rebar and to the performance of the concrete during placement, curing and service.

Performance specifications for concrete are often too little, too late. What you do in the mix design, transport, placement and initial curing all affect the performance of the concrete, so each of those parameters must be carefully considered, and yes, even controlled by the specifications.

Whether you put the requirements on your drawings or in the specifications or a combination of the two is somewhat irrelevant. Don't buy into the crap that we have no say as to means and methods when it comes to concrete. Relying on the contractor (who knows little or nothing of concrete technology other than an 8 inch slump takes less effort to place than a 4 inch slump) to dictate the long-term performance of concrete is not a wise move.

 

[TLHS]

Ron,

Nobody's saying that an engineer should just specify the strength. The Canadian code just separates mix design from performance in the preferred method of specification. So, as the engineer, I specify the stuff that matters to me. I haven't read this whole thing, but it's the first source I had of the code performance specification tables. Refer to appendix J for the tables:

http://betonabq.org/pdf/aBest_Practices_Guide_.pdf

Basically, a concrete plant ends up with a whole bunch of standard mixes that can be used for 95% of projects if people specify using the performance method. You know you're getting a reliable mix, because you're going to be using a standard mix that's been used repeatedly.

There's less communication issues, because they aren't trying to interpret each engineer's requirements and the average engineer is more likely to actually understand the consequences of their specification. I know that I've seen all sorts of engineers copy concrete specs across projects without actually knowing why they're requiring anything in that spec.

And yeah, it means that when I get a mix design to review, I'm only checking the inputs, any admixtures, and that there's nothing weird going on. The guys who spend their lives making concrete know their job better than I do, and we're going to test it, so I do have verification. I can make a concrete mix, but I will happily admit that a guy doing it seven days a week is likely going to make a better mix than I would.

If I have a strange need, then sure I can specify the specific properties or mix that I need, but that should be the exception rather than the rule.

Also, on the matter of slump specifically, I would agree that it should be the purview of the contractor. In the world of superplasticizers, I don't see a need to tell the contractor how to do actual construction. If I have a pressing need for a slump I'll specify it, but most of the time I think it would be meddling in the contractor's means of construction.

 

[BUGGAR]

Not unlike welding.

 

[JedClampett]

Sorry, north of the border guys (or gals), in my area, Water and Wastewater Plants, I've got a 33 page concrete specification that I'm not going to give up. There is no way I'm going to let the batch plant steer that ship. Per code, I'm required to limit w/c, cementatious constituents (low alkali, Type V, Fly Ash, etc.) and air content. All the batch plants in the area have a standard mix that meets these requirements (MAG Type AA for you Arizona engineers) that I expect to get. On occasion I need to tweak the specification for other areas of the country, but it never deviates from the basic requirements.
Think of it this way; Who's going to get the call 20 years from now about a concrete issue? Me or the batch plant? Will the concrete submittals be available? All we're going to have to go on is the General Notes Drawing (Specifications are usually long gone) and my memory.
I'll totally agree that as far as concrete knowledge, the concrete technicians and scientists could run circles around me. But as long as the lowest price rules, I can't afford to have a .53 w/c ratio mix in one of my clarifier walls. I need to set tight ground rules.
The original question had to do with specifying concrete on a drawing. I'm not a fan. Give enough information for future reference and provide a tight specification.

 

[Ron]

JC....Yes!!

 

[BUGGAR]

A lot of this is from long, long ago. Roebling couldn't rely on ASTM Specs at the time.

 

[MacGruber22]

...or is specifying ACI 318 (mainly chapter 3-Materials) sufficient?

It is not. Simply invoking ACI 318 does nothing special for the contractor's understanding of the project, nor to hold them responsibly for important aspect of concrete performance.

I refuse to wait till concrete submittal time to decide whether the concrete contractor and his/her "expert" is proposing a mix design that meets my project's performance objectives. More importantly, not every job in the world has a concrete supplier that is an "expert" - actually, many don't, and I am not talking about in the middle of a third-world country. I am talking about little suppliers in the middle of nowhere USA. Also, the supplier works for the contractor, which means the contractor will push the supplier to provide concrete that is as cheap as possible for the given project requirements (which may not be too much in your proposed delegation of mix design". If in the next 20-years your specs don't prescribe in sufficient detail, more than once you will have a major headache on your hands. I think your seminar expert has convinced you that all concrete suppliers are experts and thoroughly understand the nuances of all projects that come their way. That is mighty trustworthy of you.

Suggestion, StructureMan44: learn, learn, learn about how to fashion a mix design to meet the performance requirements in as many situations as possible. Some important performance variables overlooked are total cement content and aggregate/cement ratio. Also, ACI does not cover all situations, and your engineering judgement needs to be razor sharp to know when the situation is problematic and in need of the EOR to set things straight. Many times you will need to invoke your judgement, because you know that it is the right thing to do, not because you will receive credit for making the right decision. As others have mentioned - on the other hand, when the wrong decisions are made (by your contractor and their "expert"), and the results are ugly, YOU will become the primary complaint department.



"It is imperative Cunth doesn't get his hands on those codes."

 

[Ron]

MG22...well said!

 

[MacGruber22]

Thanks, Ron.

"It is imperative Cunth doesn't get his hands on those codes."

 

[cloa]

Surely an alternative is to specify the performance requirements as much as you need and design the mix but allow for professional operators to submit a counter mix to meet the performance requirements if they can sell you that it is cheaper/more practical in their particular local conditions and achieve the same results.

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[MacGruber22]

Cloa, absolutely. It isn't always a black or white decision - "meet my spec or pound sand".

At least in my neck of the woods, contractors read construction documents as if they were transparent, and they propose dozens of products or methods that clearly contradict the spec as written. I have no problem entertaining alternative products or methods, but when I do say no, my spec needs to clear and detailed on what I want (mix design, etc.). It is nearly impossible to demand something from the contractor when the contract documents under-specify. And when you under-specify, that is when contractors all of a sudden understand every word in the CD's - they love know what they were not told to do.

"It is imperative Cunth doesn't get his hands on those codes."

 

[BigH]

Almost all the projects I have worked on over the last 20 odd years are in outlying areas in Asia and the concrete mix is developed on site without history. Sometimes I see specs say that the slump, for instance, is to be maximum of, say 75 mm - but when one uses plasticizers, it may go up to 150 or more. In these cases the specifications say that the non-plasticized slump (at the time of the mix design) shall be 75 mm - but the mix design would develop, with plasticizers a slump of 120 or . . . and that is the one that is utilized. If a specification is to identify the slump - then one needs to be specific on a non-plasticized slump or a slump with plasticizer.

Also, many of the mixes in mass concrete have 75 mm aggregate (sometimes larger) - the slump, in my view, in these mixes is of little use - taking out the 40 mm plus aggregate leaves you with what? with respect to the real mix. We have also used fly ash up to 50% substitution. The specs say what the w/c ratio must be (less than 0.7 for 15 MPa concrete (cylinders)) to 0.45. What gripes me, though, is when a spec says to use a w/c ratio of such and such and a requirement on strength but also gives a minimum cement content.

The above - for what it is worth. We have had some very good concrete made on these jobs - especially the one in which we substituted 50% fly ash for the type 1 OPC.