Speaking in Code – Developing Concrete Mix Proportions

Getting Down in the Weeds with the New Concrete Code

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ACI 318-14 “Building Code Requirements for Structural Concrete

ACI 301-10 “Specifications for Structural Concrete

ACI 301-16 “Specifications for Structural Concrete

Are you one of those people who would read a lot more if only books were written in numbers? This article is for you! Fair warning to the rest of you: An abundance of code and section and chapter numbers are contained herein.

All of us that have had to deal with developing, reviewing and/or approving concrete mix proportions in the past have memorized where to go in the ACI 318 concrete code book in order to find the rules and stipulations set forth for concrete mix proportioning and code compliance. In the past concrete building code (ACI 318) editions for the past fifty or so years, that building code section has been ACI section 5.2 “Selection of Concrete Proportions” or ACI section 4.3 in earlier editions of the ACI 318 code.

The new Concrete Building Code (ACI 318-14) has deleted all of that section 5.2 verbiage that addresses the selection of concrete mixture proportions regardless of whether you want to establish concrete mix proportions according to field experience OR trial mixtures. ACI 318-14 no longer contains the old section 5.2 verbiage about the proportioning of concrete mixtures except for the fact that Chapter 26, section stipulates that concrete mixture proportions shall be established in accordance with article 4.2.3 of ACI 301-10**. It seems that the reason for the removal of the section 5.2 verbiage from ACI 318 is that the prescriptive requirements on concrete mixture proportioning was not proper for the code (ACI 318). The prescriptive requirements on concrete mixture proportions were directed to the contractor (it was thought). The thinking was that ACI 301 “Specifications for Structural Concrete” was for them (contractors) and ACI 318 should only provide the acceptance criteria for the concrete which is now provided in ACI 318-14, Chapter 26, and section 26.4.2.

Let’s be clear about one very important item: ACI 318-14 (the CODE) only incorporates ACI 301, article 4.2.3 into the CODE. This is important because ACI 318 does NOT incorporate ALL of ACI 301 nor any other portions of ACI 301; ACI 318 ONLY incorporates article 4.2.3 of ACI 301 (concrete proportioning) into the CODE structure. The good news is that the verbiage comprised by article 4.2.3 is very much like (and almost the same) as the ACI 5.2 verbiage it replaces from previous ACI 318 code cycles. This basically means that the way we go about developing mix proportions according to article 4.2.3 of ACI 301 is not going to be much different from what we have been doing for the past 50 years as we went about our business of making our concrete mixture proportions comply with the “Structural Concrete Building Code” (ACI 318, section 5.2).

According to ACI 301, the concrete mix must be proportioned on the basis of 1) field experience or 2) trial mixtures. The field experience method utilizes field strength test records of consecutive compressive-strength tests with the same mixture proportions; this method relies heavily on standard deviation data and resultant average compressive strength determinations. The material proportions of the concrete mixtures developed must comply with ACI 301, article 4.2.2, “performance and design requirements”, regardless of whether method 1 or method 2 is used. The field experience methodology is most often used by concrete producers. The trial mixture method is based on the data compiled on the production of, at least, three trial mixtures for each class of concrete (required on the project) which will produce a range of compressive strengths that will encompass the required design strength required by the project. The trial mixes will be produced with the same materials to be used on the project and will have the same basic slump, air content, etc. of the project concrete. The trial mixtures must be proportioned to produce a slump as specified for the proposed Work, and for air-entrained concrete, an air content within the tolerances specified for the proposed Work (See Note 1).

The new concrete code (ACI 318-14) actually offers a third method option for the development of concrete mixture proportions in addition to the field experience or the trial mixture methods. ACI 318-14, section offers an alternative acceptable to the licensed design professional. This alternative method shall have a probability of satisfying the strength requirements for acceptance tests of standard-cured specimens that meets or exceeds the probability associated with the method in Article 4.2.3 of ACI 301.

When Article 4.2.3 of ACI 301 is used, the strength test records used for establishing and documenting concrete mixture proportions shall not be more than 24 months old. It should be noted that ACI 318-14, section and ACI 301-16, article are consistent and coordinated in mandating that the strength test records used to establish and document the mix proportions shall be no more than 24 months old.

But wait! We should not end this discussion about developing the proportions of concrete mixtures and compliance with the concrete code (ACI 318.14) without addressing ACI 318, Chapter 19, “Concrete Design and Durability Requirements”. The licensed design professional is charged with the responsibility of assigning exposure classes in accordance with the severity of the anticipated exposure of members for each exposure category as listed in Table of ACI 318-14 and as stipulated in section of ACI 318-14. Based on the exposure classes assigned by the licensed design professional, the concrete mixtures shall conform to the most restrictive requirements set forth in Table of ACI 318. ACI 301-16, article addresses concrete durability and offers concrete exposure tables and information that is similar (but different) to Table of ACI 318-14.

For those of you still with us, since this discussion about “Developing Concrete Mixture Proportions” and the code (ACI 318) has involved a lot of information about ACI 301, perhaps we should elaborate further on the ACI 301 standard:

ACI 301-16 “Specifications for Structural Concrete”

ACI 301-10 “Specifications for Structural Concrete”

ACI 318-14 “Building Code Requirements for Structural Concrete”

ACI 318 is often simply referred to as the “Code” and most folks realize that ACI 318 is incorporated in its entirety (almost) by the International Building Code (IBC). ACI 301 covers specification requirements for general construction and is written in “mandatory” language so that it can be adopted directly into a project specification by the specification writers; however, ACI 301 is not recognized by the IBC building code. When ACI 301 is adopted, it is intended that the specification be adopted in its entirety, rather than parts since “taking them out of context may change their meaning”. Throughout the ACI 301 specification, consistent language differentiates between required and optional provisions. The term “as specified in the contract documents” indicates a mandatory checklist item; “unless specified otherwise” means something is optional. The last ten (10) pages of ACI 301 is comprised of a list of “mandatory requirements” as well as a list of “optional requirements”; both lists are presented in a checklist format.

The first five sections of ACI 301 cover general construction requirements for cast-in-place structural concrete and slabs-on-ground. These sections cover materials and proportioning of concrete; testing/inspection, reinforcement steel, production, placing, finishing and curing of concrete as well as formwork, construction, embedded items, surface defect repair, finishing, etc. The remaining sections are devoted to architectural concrete, lightweight concrete, mass concrete, post-tensioned concrete, industrial floor slabs, tilt-up construction, precast concrete, etc.

ACI 318 (the Code) is incorporated into the general building code; therefore, compliance is required according to the Code. ACI 301 in almost always incorporated into the contract documents; therefore, compliance is required in order to comply with the contract documents. Project special inspectors should review and observe construction methods and materials for compliance to both ACI 318 (because it is Code) and ACI 301 (because it is part of the contract documents).

** ACI 301-16, article 4.2.3 will not be incorporated into the International Building Code (IBC) until the IBC 2018 version of IBC is adopted. IBC 2015 adopts ACI 301-10, article 4.2.3


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