Concrete inFocus - Spring 2013 - (Page OC3)

www.nrmca.org/news/connections infocus online connections Impact of Specifications on Concrete Quality Part 13 of the NRMCA Concrete Quality Series Karthik Obla, Ph.D., P.E. C oncrete specifications can play an important role in ensuring good quality concrete. This article makes suggestions on concrete specifications with the aim of improving concrete quality. Allow Use of Standard Deviations Almost all concrete project speciﬁcations have a speciﬁed compressive strength (ƒ´c) requirement for concrete mixtures. Mixtures are designed to meet the required average compressive strength (ƒ´cr) to ensure that the strength tests meet the strength acceptance criteria. ACI 3181 and 3011 provide two options to establish this required average strength ƒ´cr for ƒ´c equal to or below 5,000 psi: Option A: If past test records are available, the test standard deviation, S, is calculated and the mixture should be designed to achieve an average strength ƒ´cr greater than or equal to the larger of the following two equations: f´cr = f´c + 1.34 x S f´cr = f´c + 2.33 x S – 500 Option B: If no past test records are available, ƒ´cr is established as 1,000 to 1,200 psi greater than ƒ´c. Many speciﬁ cations require concrete mixtures to be overdesigned by 1,200 psi above the speciﬁed strength (i.e., option B). Forcing the use of option B when a test record is available is not desirable. The standard deviation, S, from a strength test record is a measure of the ability of the concrete supplier to control the quality of the product. It should be noted here that testing is a component of this standard deviation. There should be a beneﬁt and incentive to use this information. Table 1 compares the required average strength for a speciﬁed strength of 4,000 psi by option A and B for diﬀerent levels of concrete quality as measured by S.1 Option B requires ƒ´cr of 5,200 psi regardless of the producer’s standard deviation. On the other hand, if option A (based on past test data) is used, a producer with S = 350 psi has to design the mixture for an average strength of 4,470 psi, whereas a producer with S = 1,250 psi has to target at least 6,410 psi. An average materials cost savings of \$1/yd 3 can be assumed for every 200 psi decrease in average strength. By proportioning the concrete to target a lower average strength the producer who has a lower variability (S) will lower the materials cost of concrete. Figure 1 shows the potential materials cost savings. For this example, with a speciﬁed strength of 4,000 psi, when the producer’s standard deviation is greater than 730 psi, option B will allow a lower average strength than option A. A speciﬁcation that requires option B does not beneﬁt a quality-conscious producer but does beneﬁt the one with higher variability. Option B is surely not a conservative option when S > 730 psi and can lead to a higher number of strength problems during the project. Statistically it can be shown that for ƒ´c = 4,000 psi for a concrete with average strength of 5,200 psi with S = 1,250 psi, the probability of failing the strength acceptance criteria is about 10 times higher compared to one with S = 730 psi and the same average strength! In summary, speciﬁcations that require option B merely tell the producer not to pay attention to quality. Prescriptive versus Performance-Based Specifications Concrete mixtures designed to prescriptive speciﬁcations are typically substantially overdesigned. It is likely that for a given set of materials a knowledgeable engineer of record can optimize the mixture proportions Table 1. Target average strengths for ´c = 4000 psi QC Standards (ACI 214) S, psi ´cr , psi (Option A) ´cr , psi (Option B) Excellent Very Good Good Fair Poor 350 450 550 650 750 850 950 1050 1150 1250 4470 4600 4780 5020 5250 5480 5710 5950 6180 6410 5200 concrete INFOCUS ı OC3 http://www.nrmca.org/news/connections http://www.naylornetwork.com/nrc-nxt/

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Taking It to the Streets
Impact of Specifications on Concrete Quality

Concrete inFocus - Spring 2013

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