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.
