Spiderman, MacGyver and concrete
November 5, 2012
I’ve heard it said that, “A Swiss Army knife can do a thousand and one things, but none of them well.” Trying to make one thing serve two purposes usually means that one purpose gets the short end of the stick. Spiderman is a hero, while Peter Parker is looked on as a nerd. Superman is another hero, but Clark Kent never gets Lois Lane. In the construction industry we are constantly told, “Use the right tool for the job.” A screwdriver doesn’t make a good crowbar. In fact, MacGyver is the only person I know who can turn a fountain pen into a nuclear detonator and have every assurance that it will do the job. So why do we make the abbreviation “f’c” try to do two things?
F’c is defined in ACI 318 as the “specified compressive strength of concrete”. When an engineer designs a building, he designates f’c on the drawings or in the specifications. (I hope you will forgive me if I don’t get this exactly right, since I am not an engineer. I don’t even play one on TV.) Usually the strength shown is the strength required to support building loads. After that, restrictions may be placed on the minimum cementitious content or maximum water/cementitious ratio. (Don’t get me started on “cement vs. cementitious”.) In the end, the concrete producer is often faced with something like a requirement for 3000 psi concrete with a maximum w/cm of 0.50. The resulting concrete strength will usually exceed 5000 psi. What happened to the 3000 psi requirement?
The problem lies in the fact that engineers usually use f’c to designate the required structural strength of the concrete, but concrete producers use it as the basis for their mix design and cementitious quantity selection. We are trying to use f’c to do two totally different things.
ACI 318 tries to address this in Para 4.1.1 as follows:
4.1.1 —The value of fc ′ shall be the greatest of the values required by (a) 1.1.1, (b) for durability in Chapter 4, and (c) for structural strength requirements and shall apply for mixture proportioning in 5.3 and for evaluation and acceptance of concrete in 5.6. Concrete mixtures shall be proportioned to comply with the maximum water-cementitious material ratio (w/cm) and other requirements based on the exposure class assigned to the concrete structural member. All cementitious materials specified in 3.2.1 and the combinations of these materials shall be included in calculating the w/cm of the concrete mixture.
ACI 318 says that f’c should be adjusted for the durability requirements in Chapter 4, but Chapter 4 includes both minimum strength requirements as well as maximum w/cm requirements. For example, concrete exposed to sulfates at levels appropriate for Exposure Class S1 requires a minimum f’c of 4000 psi and a maximum w/cm of 0.50. If I produce concrete using a high range water reducer, fly ash and a well graded aggregate, I can probably achieve over 6000 psi at a w/cm of 0.50. Even so, I frequently see specifications requiring a concrete mix design with an f’c of 3000 psi and a maximum w/cm of 0.45. Apparently some designers haven’t gotten the memo.
One of the problems with this overlap of terminology is the process of calculating strength overdesigns. If we have two 3000 psi concrete mixtures, one designed for strength only and the other designed for a maximum 0.50 w/cm, the average strength of the first mix may be only 3500 psi, but the average strength of the second mix may be over 5000 psi. While these two mixtures may nominally have the same design strength, they are designed for two totally separate conditions. If we combine the tests for both mixes the standard deviation will go through the roof. ACI 214 and 318 allow for standard deviations for mixes with similar materials and cast under similar conditions to be combined, but the only specific factor for determining whether they can be combined is the design compressive strength, f’c. In reality these standard deviations should never be combined, because they are two totally different mixes, even if they are made up of the same materials.
I think there should be a new variable based on the design strength the mix designer is using – something like f’cp. This way the specifier could specify the minimum structural requirement, plus the minimum cementitious content and maximum w/cm ratio and then the mix designer could decide the actual design strength (f’cp) of the mixture. Alternately, the mix designer could specify the characteristic strength of the concrete and determine how to combine mix data based on characteristic strength. (FYI, characteristic strength is a concept used in Europe and is typically a value that 95% of the concrete tests will exceed. The characteristic strength is based on the average strength, number of tests and the standard deviation. It is NOT based on the design strength, f’c. In the example of a mix with an f’c of 3000 psi and a maximum w/cm of 0.50, the required average compressive strength (f’cr) of the concrete may be 3600 psi, but the average strength may be 5500 psi and the characteristic strength may be 4800 psi, depending on the standard deviation and number of tests.
Even if you don’t follow the math, the ambiguity of the term f’c creates difficulties for concrete producers and mix designers, and possibly even for the engineers who specify it. I’m not really certain what the next step should be in getting this issue resolved, but I probably need to send this discussion to ACI 318 for their consideration.
What do you think?
Jay Shilstone