The Art of Concrete Mix Design Review

Reference Codes and Standards cited in this article: International Building Code (IBC); Building Code Requirements for Structural Concrete (ACI 318); Specifications for Structural Concrete (ACI 301); Standard Practice for Selecting Proportions for Concrete (ACI 211.1)

Performing a concrete mix design review for compliance with the building code and the requirements of the approved construction documents is a substantial task. The building code (ACI 318) comprises many compliance criteria with which the concrete mixture must comply, and the project contract documents set forth considerable acceptance criteria for the concrete mix as well. There are often conflicts between the code requirements and the requirements of the contract documents, and when this happens, the most stringent requirement is generally enforced. However, we must never forget that the building code represents the MINIMUM requirements regarding acceptance criteria for any project. These minimum code requirements must be complied with when the concrete mix design is developed. Only the building official can approve discrepancies that do not meet minimum building code requirements.

Reviewing a concrete mix design for compliance with the building code and contract documents involves examining it fastidiously to ensure it meets the specified strength, potential workability characteristics, durability, exposure requirements, and other performance criteria outlined in the building code and contract documents. The proportions of cement, water, aggregates, and admixtures used – including their “absolute volumes” – must be carefully observed to ensure they align with building code criteria relative to cement factor, water-cement ratio, air content, theoretical yield values, and more.

The “art” of concrete mix design review depends on tapping numerous sources of concrete mixture acceptance criteria. Of course, the mix design submittal must be scrutinized for compliance with code- acceptance criteria specified by ACI 318 and ACI 301. Additional knowledge can be drawn from ACI 211.1 and various ASTM standards, such as ASTM C 33, “Standard Specification for Concrete Aggregates,” which is just one of many ASTM standards listed in ACI 301, Article 4.1, that should be addressed in the concrete mix design submittal package.

ACI 211.1 is a stand-alone one-of-a-kind ACI standard that is chock full of sound concrete mix design technical data. Suppose you possess numerous types of fine and coarse aggregates, cement, fly ash, and concrete admixtures but have little knowledge about how any of them perform in a concrete mixture. In that case, ACI 211.1 is the only ACI standard or publication that provides a series of steps to develop a workable concrete mixture with the desired concrete qualities and workability required of the individual concrete project.

You do not have to know how to develop a concrete mixture in accordance with ACI 211.1 to review a concrete mix design, but some knowledge of ACI 211.1 is helpful and makes the task a bit easier. ACI 211.1, Table A1.6.3.6, listed below, is just one tool that assists in reviewing a concrete mix design and the proportioning process.

Once you know the fineness modulus (FM) of your concrete sand, your dry-rodded coarse aggregate weight, and the coarse aggregate’s nominal maximum size, you can calculate the desired batch weight for the coarse aggregate for one cubic yard of concrete. All this aggregate information should be included with your concrete mix design. 211.1 also introduces the reader to the term “absolute volume,” which is the methodology required to ensure that one cubic yard of concrete comprises 27 cubic feet. A plastic concrete yield of 27 cubic-feet-per-cubic-yard of concrete is important on many levels.

Absolute volume, specifically, means the actual space occupied. With granular solids like cement and aggregate, the apparent volume is larger than the absolute volume because the apparent volume includes spaces between particles. With absolute volume, there is no space between particles. In designing concrete mixes, the absolute volumes of the amounts of all ingredients are calculated from the weights and their specific gravities. Then, the absolute volumes are added together to obtain the absolute volume of the concrete made with those amounts of ingredients. The absolute volume of liquids in the concrete mixture is simply the actual volume.

The code requires a concrete mix design for a project. It directs that all design mix materials and proportions included in the statistical analysis for compliance with ACI 301, Article 4.2.3, be the same materials and brands from the same sources used for the concrete furnished for work on the project.

If the proposed mix design is based on trial mixtures, evaluate compliance with ACI 301, Article 4.2.3.4(c), and verify that trial mixes reflect the same general proportions, slump, and air contents that are proposed for the project. Trial mixes should be developed less than 24 months prior to the submittal date, and 28-day compressive strengths should comply with ACI Table 4.2.3.1.

The mix design submittal should include aggregate gradation test data and exhibit compliance with ASTM C33. It should also include aggregate-specific gravities and dry-rodded weights of coarse aggregate. The mix design submittal should include information confirming that the materials used in mixtures comply with all applicable ASTM requirements cited in ACI 301, Section 4.1.

Coarse aggregate batch weights should be somewhat similar to the recommendations contained in ACI 211, Table A1.6.3.6, but do not have to be exactly as ACI 211.1, Table A1.6.3.6, proportion methodology indicates. ACI 211.1 proportion methodology yields material approximations and may be adjusted for workability and other desired concrete mixture characteristics.

The data chart listed below displays the type of test data and information required by the code to accompany mix design proportions proposed for project work using the standard deviation method. Article 4.2.3 establishes rules and guidelines governing this data so that it is acceptable for use in the project concrete mix designs.

These rules and provisos include the following: the statistical test data must be from consecutive compressive strength tests and represent concrete produced from the same materials, mix proportions, and quality control procedures (slump, air content, unit weight) expected in the proposed project work. While the standard deviation procedure requires that the field test data comprise 30 consecutive strength tests, fewer than 30 are allowed under certain conditions. Each 28-day compressive strength test shall consist of two test specimens.

If concrete mix proportions are based on trial mixtures, the following rules govern:

??????? Materials and material combinations proposed for the work must be used

??????? Design strength is determined per ACI 301, Table 4.2.3.1:

??????? At least three trial mixtures that will produce a range of compressive strengths that encompass all compressive strengths required by the project must be made

??????? Trial mixtures must produce the same slump and air content as proposed for the work

??????? The proposed concrete mixtures shall meet applicable exposure requirements set forth in ACI 301, Article 4.2.2.7, and ACI 318, Table 19.3.2.1

??????? The trial mixture must be developed less than 24 months from submittal date

??????? For each trial mixture, three compressive strength cylinders must be made and cured for each test date

The concrete code addresses concrete durability based on exposure categories and classes as defined in Table 19.3.2.1 of ACI 318, Chapter 19. Strength values, w/c ratios, air contents, and other Table 19.3.2.1 values are code, and compliance is mandatory for concrete mix designs.

General Checklist for Concrete Mix Design Review:

When reviewing a concrete mix design, the most important aspects to focus on are the water-to-cement ratios, as they significantly impact strength and durability, the quality and grading of aggregates, the type and amount of admixtures used, the target compressive strength, and the intended application of the concrete. You must consider factors like exposure conditions and workability requirements for the proposed project.

??????? Check the absolute volumes of material proportions that comprise concrete mix design for one cubic yard for proper yield

??????? Check the w/c ratio compliance, which directly impacts the strength and durability of concrete (Table 19.3.2.1)

??????? Check that target air content of the mix is in compliance with ACI 318, Chapter 19, Table 19.3.3.1 (see image below)

??????? Review the cement type and quantity – high early, low sulphates, cement factor versus job application

??????? Evaluate the mix design utilizing statistical analysis for

compliance with ACI 301, Article 4.2.3.2, regarding age of test data and be certain that information used for analysis are the same materials that will be used on the project

??????? If the mix design is based on trial mixtures, evaluate compliance with ACI 301, Article 4.2.3.4(c), and verify that trial mixes reflect the same general proportions, slump, and air contents that are proposed for the project – trial mixes should have been developed less than 24 months prior to submittal date and 28-day compressive strengths should comply with ACI Table 4.2.3.1

??????? Check that the mix design submittal includes aggregate gradation test data showing compliance with ASTM C33, aggregate specific gravities, and dry-rodded weights of coarse aggregate – the mix design submittal should include information confirming that the materials used in mixtures comply with all applicable ASTM requirements

??????? Check that the coarse aggregate batch weights are somewhat similar to the recommendations of ACI 211.1,

Table A1.6.3.6 – ACI 211.1 calculated proportions are approximations and may be modified to some degree

??????? Verify that strength, w/c ratio, air content, cement factor, etc., comply with the requirements of ACI 318,

Table 19.3.2.1, for applicable exposure class of concrete

??????? Review available data in the concrete mix design submittal, such as w/c ratio, air content, and slump for workability characteristics required by the project contract documents and the code

But wait! We have a lot more to say!

For a complete picture of the Code and how it relates to Special Inspections, F&R would love to provide a virtual AIA-accredited Lunch & Learn presentation to the professionals at your firm.

Trouble Deciphering the Code? Call the Experts at F&R! Alan S. Tuck, Director of Code Compliance & Training: T 540.344.7939 | M 540.798.4440 | [email protected]