Making the grade - The required compressive strength in concrete design

A few years back, I discussed the merits of the NPCA Plant Certification program with the owner of a small precast plant.  He did not feel the need to become certified.  His exact statement was something like: “I’ve made 5,000 psi concrete for 30 years.”  That is a strong statement, and one that hopefully was supported by fact.  In a non-threatening manner, I responded with a question: “do you have one piece of paper that documents the evidence of this claim?”  He was silent.

A quality system will not magically transform the average precaster into super precaster.  And, the strength of concrete is only one part of making a quality product.  As I think about the comments made by this precaster, I wonder just how many folks understand the compressive strength requirements.  Jay Shilstone (2012), in his blog “Missed it by that much” – Concrete tests and f’c, got me to thinking about the importance of this quality principle. 

The product design calculations, or possibly the customer, will state a minimum compressive strength requirement.  The desire is that no concrete compressive test result will be lower than the specified strength.  All processes have variation, and concrete is no different.  A population set of data will be distributed in such a manner that when plotted by a curved line, the data points will form a bell shaped curve.  In a normal distribution curve, the apex of the curve will represent the mean or average.  Theoretically, 50% of the data will be to the left, and 50% will be to the right of the mean.

ACI 318 defines the standard method for determining the target compressive strength required to assure that 99% of the time the compressive strength will be greater than the specified strength.  According to ACI 318 section 5.1.1, the average strength is called the required strength or f’_cr.  The specified strength is noted as f’_c.  Section 5.3.2.1 provides the formulas for establishing the required strength when the specified strength and sample standard deviation are known.  In the formula below, s_s is the sample standard deviation.

Specified compressive strength, psi	Required average compressive strength, psi
f’c ≤ 5000	Use the larger value computed from Eq. (5-1) and (5-2) f’cr = f’c + 1.34ss                  (5-1) f’cr = f’c+2.33ss – 500             (5-2)
f’c ≥ 5000	Use the larger value computed from Eq. (5-1) and (5-2) f’cr = f’c + 1.34ss                  (5-1) f’cr = 0.90f’c+2.33ss              (5-2)

                              Table 5.3.2.1 from ACI 318

The standard sample deviation is calculated from “30 consecutive tests or two groups of consecutive tests totaling 30 tests” (ACI, 2005).  A modification factor is allowed when the number of tests is less than 30 but greater than or equal to 15.  When the number of consecutive tests is less than 15, the required average strength will be [f’_cr = f’_c + 1200] when the specified strength is ≥3,000 psi and ≤5,000psi.  For a specified concrete compressive strength of >5,000 psi, the required average strength will be [f’_cr = 1.10f’_c + 700].

For the precast producer, making consistent concrete with less variability will reduce the sample standard deviation.  This will allow the precaster to produce a concrete design with a required average strength that is lower, while still meeting the over design requirements of ACI 318.  This might result in a more economical concrete mix, and it will also provide a better batch-to-batch consistency for concrete products.  So, the next time you tell someone that “you make 5,000 psi concrete”, consider the statistical variation.  Maybe you do, and maybe you don’t.

References:

ACI. 2005. Building Code Requirements for Structural Concrete (ACI 318-05). American Concrete Institute: Farmington Hills, MI.

Shilstone, Jay. 2012. “Missed it by that much – Concrete Tests and f’c. Accessed on September 27, 2012 from http://www.commandalkonconnect.com/2012/09/26/missed-it-by-that-much-concrete-tests-and-fc/.