What is new in ACI 301-20, Specifications for Concrete Construction

by sadia_badhon | January 12, 2021 6:20 am

Photos courtesy American Concrete Institute[1]
Photos courtesy American Concrete Institute

by Michelle L. Wilson, FACI

The American Concrete Institute (ACI) published ACI 301-20, Specifications for Concrete Construction, in October 2020. This standard specification establishes criteria for concrete construction that can be incorporated by reference into contract documents. ACI 301 is written to the contractor.

When an architect or engineer (A/E) cites ACI 301, the document is intended to be adopted in its entirety into the contract documents. Requirements set forth in ACI 301-20 meet the minimum requirements in ACI 318-19, Building Code Requirements for Structural Concrete. Approximately every five years, ACI 301 is updated to complement the ACI 318 code cycle.

A/E can apply the reference specifications of ACI 301 to many forms of concrete construction. A notable change was made in ACI 301-20 to the specification’s title (previously the document was titled ACI 301-16, Specifications for Structural Concrete). The new specification title recognizes the breadth of the document. For example, ACI 301 covers industrial floor slabs and architectural concrete.

ACI 301-20 is organized into two parts, distinguishing basic items that are always required for concrete construction from those that may be needed on a particular project. The first five sections cover the core requirements for most cast-in-place concrete. Sections 6 to 14 cover specialty applications that may be designated on a project.

ACI 301-20 contains the following sections:

ACI 301-20 is not a complete specification until the specifier reviews and completes the instructions to the A/E, listed at the end of the document titled “Notes to Specifier.” It includes two checklists for the A/E to use in incorporating ACI 301 into the concrete specification. The “Mandatory Requirements Checklist,” indicates specific qualities, procedures, and performance criteria the specifier must include in a project specification that are not defined in ACI 301-20. The “Optional Requirements Checklist,” identifies choices and alternatives the specifier can include as requirements in a specification. These checklists provide information allowing specifiers and owners to have options to change or customize their specifications where appropriate. The specifier should review each of the items in the checklists and make adjustments to the needs of a particular project by including those selected alternatives or additions as mandatory requirements in the project specification.

What is new in 2020

This revision expanded the scope of ACI 301 and revised many of the requirements that have been in previous versions. The scope of the document was expanded to include shotcrete, internal curing, mineral fillers, and recycled concrete aggregates. Evaluation requirements for concrete made with self-consolidating concrete and structures designed for modulus of elasticity were also included in the update.

Compliance details throughout ACI 301-20 have been updated to align with ACI 318-19. For example, ACI 318-19 saw a major reorganization of Chapter 26, “Construction Documents and Inspection.” Inspection requirements were unified in the chapter. It was also recognized many projects have roles for multiple design engineers; the updated Chapter 26 provided a framework for their coordination of work. Many of the changes in 301-20 were included to reflect the requirements in Chapter 26 on information the engineer must provide in the construction documents to achieve compliance with ACI 318-19.

The new American Concrete Institute (ACI) 301-20, Specifications for Concrete Construction, has been expanded to include shotcrete, internal curing, mineral fillers, and recycled concrete aggregates.[2]
The new American Concrete Institute (ACI) 301-20, Specifications for Concrete Construction, has been expanded to include shotcrete, internal curing, mineral fillers, and recycled concrete aggregates.

An overview and notable changes to ACI 301-20 are provided in the following sections.

Section 1—General Requirements

Section 1 covers general requirements pertaining to the specification; outlines work not specified; defines terms unique to the specification; and references four additional ACI standards, 138 ASTM standards, and 24 other industry documents. The referenced standards provide requirements for the tolerances, materials, and test methods cited in ACI 301-20. As part of the optional requirements in Section 1, specifiers must indicate any additional testing and inspection services, or if the sampling is desired at locations other than point of delivery, if they are applicable.

Notable changes to Section 1 include new definitions for clarity. Definitions of specialty concrete applications covered in separate sections were removed. Specifiers need to refer to specialty sections and designate portions of the work meeting specific types of specialty concrete.

Optional requirements in Section 1 now include a preconstruction conference to review the project requirements, acceptance criteria, and responsibilities of each party, when specified.

Shotcrete is now included in the scope of 301-20. Specifiers must designate the portion of work to be constructed with shotcrete and specify the requirements based on ACI 506.2, Specification for Shotcrete, and ACI 318-19.

Section 2—Formwork and Formwork Accessories

Section 2 covers requirements for design and construction of formwork. Specifiers must indicate locations of required movement joints and waterstops, along with areas where walls require form ties with a positive water barrier, and where coated or corrosion-resistant ties are required.

ACI 301-20 provides an optional requirement that the contractor must use a licensed design engineer (LDE) for formwork design if specified in contract documents or mandated by jurisdiction where work will be performed. Tolerances for formwork are provided in ACI 117, Specification for Tolerances for Concrete Construction and Materials and Commentary.

No major changes were made to Section 2 in 301-20.

Section 3—Reinforcement and Reinforcement Supports

Section 3 covers materials, fabrication, placement, and tolerances for steel reinforcement and reinforcement supports. Specifiers must indicate locations where field bending or straightening of reinforcing bars is permitted.

Tolerances for reinforcement are provided in ACI 117-10. Specifiers must include the location and the tolerance for placement of welded wire reinforcement. Some projects have experienced problems during placement of the reinforcing bars because of congestion associated with embedments, openings, and other items. ACI 301-20 requires that if reinforcement must be located outside of placing tolerances, the A/E must be contacted for approval.

Notable changes to Section 3 include zinc-coated (galvanized) reinforcing bars for structures designed in accordance with ACI 318-19 must conform to ASTM A767/A767M, Standard Specification for Zinc-coated (Galvanized) Steel Bars for Concrete Reinforcement. However, zinc-coated (galvanized) reinforcement conforming to ASTM A1094/A1094M, Standard Specification for Continuous Hot-dip Galvanized Steel Bars for Concrete Reinforcement, may now be specified for applications where a lesser zinc coating thickness is permitted.

Section 4—Concrete Mixtures

Section 4 covers requirements for materials, mixture proportioning, production, and delivery of concrete. The A/E must indicate the specified compressive strength of concrete, f’c, and must designate exposure classes or specify requirements for portions of the structure to ensure durability. Specifiers should also include any proposed ranges of mixture proportions and changes in required or prohibited concrete materials.

Notable changes to Section 4 include separating requirements for slump flow of self-consolidating concrete (SCC) from slump requirements for other concrete mixtures. Performance and design requirements for slump stipulate a target slump not exceeding 9 in. (unless the specifier selects an alternative limit). Determination of slump is to be in accordance with ASTM C143, Standard Test Method for Slump of Hydraulic-cement Concrete, with slump tolerances in accordance with ACI 117. Target slump flows for SCC, determined in accordance with ASTM C1611, Standard Test Method for Slump Flow of Self-consolidating Concrete, at the point of delivery are not to exceed 30 in. (unless the specifier selects an alternative limit) and shall be used as a basis for acceptance. Slump flow tolerances are provided in ASTM C94, Standard Specification for Ready-mixed Concrete. Also, as a new optional requirement, if specified, passing ability (the relative filling ability around obstructions and through constrictions, such as congested reinforcement or narrow formwork) and static segregation (the separation of cement paste and coarse aggregate when the concrete is at rest) should also be evaluated in accordance with ASTM C1621, Standard Test Method for Passing Ability of Self-consolidating Concrete by J-Ring, and ASTM C1610, Standard Test Method for Static Segregation of Self-consolidating Concrete Using Column Technique, respectively.

A prescriptive table listing minimum cementitious material requirements for floors, which appeared in previous versions of ACI 301, was removed in ACI 301-20, providing more flexibility in designing concrete mixtures.

ACI’s MNL-15(20), Field Reference Manual, which includes the most recent specifications for concrete construction, is recommended for any project where ACI 301 is specified.[3]
ACI’s MNL-15(20), Field Reference Manual, which includes the most recent specifications for concrete construction, is recommended for any project where ACI 301 is specified.

New in Section 4, timelines for the prequalification of aggregates were extended. Data on types, sizes, pit or quarry locations, producers’ names, aggregate supplier statement of compliance with ASTM C33, Standard Specification for Concrete Aggregates, and ASTM C1293, Standard Test Method for Determination of Length Change of Concrete Due to Alkali-silica Reaction, expansion data must be no more than 24 months old.

ACI 301-20 (and ACI 318-19) now permits the use of crushed hydraulic-cement concrete or recycled aggregate if documentation is provided as required by the A/E and its use is determined suitable for a particular project. Mineral fillers, conforming to ASTM C1797, Standard Specification for Ground Calcium Carbonate and Aggregate Mineral Fillers for use in Hydraulic Cement Concrete, are now permitted in 301-20, provided they are obtained from the same sources and consist of the same types as those used in concrete represented by submitted field test records or used in trial mixtures. Also, internal curing by using lightweight aggregate (limited to prewetted fine aggregate conforming to ASTM C1761, Standard Specification for Lightweight Aggregate for Internal Curing of Concrete) is now included in ACI 301.

Durability provisions in ACI 301-20 outline new requirements for sulfate exposure categories, and new guidelines for calculating the total chloride ion content of the concrete mixture (or water-soluble chloride ion content of a hardened sample), along with different limitations imposed on concrete mixtures to be qualified to reduce the potential of alkali-aggregate reactivity.

ACI 301-20 prohibits the use of aggregates determined to be susceptible to alkali-carbonate reactions in accordance with ASTM C1778, Standard Guide for Reducing the Risk of Deleterious Alkali-aggregate Reaction in Concrete. ACI 301-20 also revised the three options for qualifying concrete mixtures for resistance to alkali-silica reaction:

Alkali content in concrete, excluding that from supplementary cementitious materials (SCMs), is limited to 4 lb/cy for moderately reactive aggregates and 3 lb/cy for highly reactive aggregates. Option (c) restricts use with natural pozzolan or fly ash that has a CaO content greater than 18 percent, or for aggregate with expansion at one year greater than or equal to 0.24 percent when tested in accordance with ASTM C1293. Fly ash with alkali content greater than four percent must not be used in options (b) or (c). Specifiers are referred to ASTM C1778 for additional optional requirements for mitigating alkali-silica reactions.

Also new to Section 4 is a requirement for submitting test data for modulus of elasticity of proposed concrete mixtures. This parallels updates to ACI 318-19, in which the definition for modulus of elasticity was updated (using data from external documents and best practices) in order to address higher-strength concretes.

Section 5—Handling, Placing, and Constructing

Section 5 covers requirements for the construction of cast-in-place concrete. Included are procedures for handling, placing, finishing, curing, and repair of surface defects. The A/E is responsible for specifying the as-cast finish or type of surface finish, curing method, jointing requirements, and when required, repair materials.

ACI 301-20 updated the temperature requirements for placement in cold weather for massive metallic embedded items or bundled metallic embedded items in concrete and formwork contact surfaces to be above 10 F.

Wording was changed in reference to the placement of beams, girders, or slabs monolithically with columns or walls to clarify timing of placement. 301-20 says horizontal elements should be placed after the underlying concrete is consolidated and there is no bleed water on the surface of the vertical supporting member.

Specifiers are now required to indicate where higher-strength concrete for columns and walls is required to be integrated with floor systems. In this case, the high-strength concrete must be extended through the full floor system depth for at least 2 ft past each face of columns and walls to achieve a monolithic mass.

Surface finish SF-2 is now established as the default surface in ACI 301-20, except for metal lap pan deck formed surfaces and those surfaces that are to be buried or covered with subsequent installed surfaces, which require a surface finish of SF-1. The architect or engineer must designate portions of the structure that are to receive a special finish and include requirements such as color, texture, limited surface voids, or other features along with acceptance criteria. Field mockups may also be specified as an optional requirement. Specifiers should also review Section 6—Architectural Concrete, and determine if the selected members should be designated as architectural concrete.

Compliance details throughout ACI 301-20 have been updated to align with ACI 318-19, Building Code Requirements for Structural Concrete.[4]
Compliance details throughout ACI 301-20 have been updated to align with ACI 318-19, Building Code Requirements for Structural Concrete.

Prescriptive means and methods for bonding underlayment in two-course slabs, which appeared in previous versions of ACI 301, have been removed and now the A/E must specify base slab preparation, bonding and topping materials, type of surface finish, curing method, and jointing requirements.

Also new to Section 5, when curing by ponding, ACI 301-20 requires the temperature of ponding water be at least 50 F and also not more than 35 F cooler than the surface temperature of the concrete at the time the water and concrete come in contact.

Another significant change in Section 5 pertains to new requirements for the installation of adhesive anchors. For anchors in holes that are horizontally or upwardly inclined, and are designated in contract documents to support sustained tension; anchors must be installed by manufacturer’s instructions, in at least 21-day old concrete, and installed by personnel who are certified in accordance with ACI Adhesive Anchor Installer Certification Program or other accepted equivalent program.

Section 6—Architectural Concrete

Section 6 covers construction requirements for concrete designated by the A/E as architectural concrete in contract documents. Many of the requirements for architectural concrete also apply to other finishes and are included in Sections 1 through 5 of ACI 301-20. However, Section 6 contains more strict requirements. A preconstruction conference is mandatory for architectural concrete. Mockups are also required for architectural concrete and may include optional reference samples to match surface quality and appearance of mockup with portions of existing structure as designated by the A/E. As an optional requirement, manufacturer’s technical specialists may be required by A/E to inspect and direct installation of supplied systems and products.

The type of formwork allowed for architectural concrete has been updated in ACI 301-20, requiring a structurally rated plywood with a nonporous finished surface, bonded to a sanded hardwood veneer substrate (unless the A/E selects alternative form facing requirements).

Section 7—Lightweight Concrete

Section 7 covers construction requirements for concrete designated by the A/E as lightweight concrete in contract documents. Lightweight concrete must be proportioned to meet the equilibrium density (the density determined after the concrete has dried to a constant weight) specified by the A/E. The equilibrium density is to be correlated with fresh density of concrete in accordance with ASTM C567, Standard Test Method for Determining Density of Structural Lightweight Concrete. ASTM C567 includes an analytical method for determining the equilibrium density. Fresh density is used as the basis for acceptance. Concrete for which fresh density varies by more than 4 lb/cf from the required fresh density should not be used. As an optional requirement, A/E’s may specify alternative methods of determining equilibrium density and other density tolerances (It is important to note some Underwriters Laboratories [UL] assembly ratings only permit ±3 lb/cf of the equilibrium density).

No major changes were made to Section 7 in 301-20.

Section 8—Mass Concrete

Section 8 covers construction requirements for concrete designated by the A/E as mass concrete in contract documents. A thermal control plan is required for mass concrete applications to meet temperature limits specified (unless the specifier selects to remove the requirement). The maximum temperature in concrete after placement must not exceed 160 F, and the maximum temperature difference between center and surface of placement must not exceed 35 F. Maximum temperature is limited to minimize future durability concerns due to delayed ettringite formation (DEF), restraint cracking, and potential reductions in ultimate strength. The A/E may specify alternative temperature limits or other methods (given in the Optional Checklist) to reduce the potential for DEF.

Improvements to Section 8 were made to provisions in thermal control plan submittals, including removing material restrictions for specific types of cementitious materials. These are now to be specified by the designer or default to cementitious materials specified in Section 4 within ACI 301-20. Section 8 also now allows permissible changes to materials that do not require updates to the thermal control plan as specified by the A/E.

Section 9—Post-tensioned Concrete

Section 9 covers construction requirements for structural members designated by the A/E as post-tensioned concrete in contract documents. If the A/E delegates structural design of post-tensioned concrete members to the contractor, drawings and design criteria used for designing post-tensioning must be signed and sealed by a licensed design engineer.

For grouted tendons, grout constituents must conform to Post-Tensioning Institute (PTI) M55.1, Specification for Grouting of Post-tensioned Structures, with the following exception to meet ACI 318-19 requirements: water-soluble chloride ion content of grout must not exceed 0.06 percent by mass of cement when tested in accordance with ASTM C1218/C1218M, Standard Test Method for Water-soluble Chloride in Mortar and Concrete.

Installation of post-tensioning shall be performed by personnel certified in accordance with PTI training programs. The term “bonded post-tensioning” has been revised to “grouted post-tensioning” to correspond to PTI Multistrand and Grouted Post-Tensioning certification programs in ACI 301-20.

Additional language was included in Section 9 for clarity when designing post-tensioned systems for aggressive environments. Non-encapsulated tendons are prohibited in slabs-on-ground that are exposed to external sources of chlorides from deicing chemicals, salt, brackish water, seawater, or spray from these sources. They are also prohibited where stressing pockets are subject to wetting or direct contact with soil during service.

Also new to Section 9, for tendons comprising encapsulated anchorages, excess lengths of tendons beyond anchorages must be removed to ensure proper fit of encapsulation cap. Cutting of tendons should not damage wedges or compromise encapsulation system.

Evaluation requirements for concrete made with self-consolidating concrete and structures that are designed for modulus of elasticity are included in the updated ACI 301-20.[5]
Evaluation requirements for concrete made with self-consolidating concrete and structures that are designed for modulus of elasticity are included in the updated ACI 301-20.

Section 10—Shrinkage-compensating Concrete for Interior Slabs

Section 10 covers requirements for concrete made with cementitious material conforming to ASTM C845, Standard Specification for Expansive Hydraulic Cement, for interior slabs designated by A/E in contract documents to be constructed of shrinkage-compensating concrete. Specifiers must indicate the reinforcement requirements and location of isolation joints and any alternative materials permitted other than those conforming to ASTM C845. The contractor must submit expansion test results determined in accordance with ASTM C878, Standard Test Method for Restrained Expansion of Shrinkage-compensating Concrete.

No major changes were made to Section 10 in 301-20.

Section 11—Industrial Floor Slabs

Section 11 covers construction requirements for concrete slabs-on-ground that are designated by the A/E as industrial floor slabs in contract documents. Drying shrinkage test results, for the proposed concrete mixture are to be determined in accordance with ASTM C157/C157M, Standard Test Method for Length Change of Hardened Hydraulic-cement Mortar and Concrete, with a modified storage requirement of seven days of moist curing followed by at least 21 days of air drying (unless the specifier selects alternative criteria).

New provisions for joint filler materials are given in ACI 301-20. Joint fillers must have 100 percent solids content and Shore A hardness requirements of at least 85 when measured in accordance with ASTM D2240, Standard Test Method for Rubber Property—Durometer Hardness, and an elongation below 90 percent when measured in accordance with ASTM D638, Standard Test Method for Tensile Properties of Plastics. Under optional requirements, the A/E is to specify if they require an alternate joint filler material. For smaller, harder wheels, the minimum Shore A hardness may need to be increased to 90 or greater.

Section 12—Tilt-up Concrete

Section 12 covers construction requirements for concrete panels designated by A/E as tilt-up construction in contract documents. Unless the A/E specifies alternative requirements, field mockups are required for tilt-up concrete; with a minimum of two panels, and each panel shall be at least 4 x 8 ft. The A/E also needs to specify when lifting and bracing design drawings need to be signed and sealed and by a licensed design engineer.

Smooth panel finishes (SPF), resulting from casting on a hard-troweled surface, require a SPF-2 finish. The A/E may specify an alternative type of smooth panel finish other than SPF-2 based on normal intended service. Requirements for SPF have been broadened in ACI 301-20 in the Optional Checklist and wording has been revised to clarify each recommended classification based on visibility of panels. For any SPF classification, the A/E must specify if panel cracking needs to be repaired, along with the scope and method of crack repair.

Section 13—Precast Structural Concrete and Section 14—Precast Architectural Concrete

Sections 13 and 14 cover the construction requirements for members designated by the A/E as precast structural concrete (Section 13), and precast architectural concrete (Section 14) in contract documents. These sections are aligned with documents available through the Precast/Prestressed Concrete Institute (PCI). For example, erector quality control, plant quality control, and the fabricator qualifications shall be certified in accordance with PCI MNL-116, Manual for Quality Control for Plants and Production of Structural Precast Concrete Products, and MNL-117, Manual for Quality Control for Plants and Production of Architectural Precast Concrete Products, as appropriate.

Updates to Section 13 and 14 in ACI 301-20 include changes to fabricator qualifications that now include optional requirements for an alternative certification program from the National Precast Concrete Association (NPCA) applicable to some structural precast concrete products, except those that are prestressed.


The new ACI 301 provides the latest construction practices and concrete-related material information that is necessary for today’s contract documents. If industry personnel are not well-versed in the changes that have been incorporated in ACI 301-20—which is the primary specification document in use throughout the industry for concrete construction—it could have a negative impact on a project’s budget and schedule. Producing a concise specification that takes full advantage of the content within ACI 301-20 will help avoid conflicting or confusing requirements.1

The most direct way to reference ACI 301 is for specifiers to use general note G3, provided in the “Notes to the Specifier” section of ACI 301-20:

Work on (Project Title) shall conform to all requirements of ACI 301-20, Specifications for Concrete Construction, published by the American Concrete Institute, Farmington Hills, Michigan, except as modified by these Contract Documents.

In pursuit of its mission to develop consensus-based standards and technical resources covering the entire industry, ACI releases updates annually. With so many resources available, users need to understand how to structure their approach to applying ACI guides and specifications. MNL-15(20), Field Reference Manual, which includes the most recent structural concrete specifications, is recommended for any project where ACI 301 is specified. In addition to containing ACI 301-20, the new Field Reference Manual includes more than 20 selected reference documents that are needed to complete specifications, including references on formwork; measuring, mixing, transporting, and placing concrete; concrete pumping methods; consolidation; and hot and cold weather concreting.

[6]Michelle L. Wilson, FACI, is director of concrete knowledge at the Portland Cement Association (PCA), Skokie, Illinois. She has over 20 years of experience relating to concrete materials, specifications, performance, troubleshooting, and repair and is co-author of PCA’s Design and Control of Concrete Mixtures. Wilson is a member of the American Concrete Institute’s (ACI’s) Board of Direction and ASTM International Committee C09–Concrete and Concrete Aggregates.

  1. [Image]: https://www.constructionspecifier.com/wp-content/uploads/2021/01/iStock-867395460.jpg
  2. [Image]: https://www.constructionspecifier.com/wp-content/uploads/2021/01/301-20_highres.jpg
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