Introducing ACI 318-19: Building code requirements for structural concrete
New versions of the American Concrete Institute (ACI) 318 — Building Code Requirements for Structural Concrete — are typically released every three years. Due to changes in the International Code Council’s schedule for reviewing and publishing the International Building Code (IBC), this year’s July release of the document occurred five years after the previous version. It is anticipated that ACI 318–19 will be referenced in the 2021 IBC.
ACI 318 includes the requirements for design and construction of structural concrete that are necessary to ensure public health and safety. The latest round of changes concentrates heavily on responding to developments in materials, structural systems and seismic design. Not only have structural design provisions changed but also new requirements address materials advancements and placement techniques, which will result in procedural changes for manufacturers and contractors.
Many new metrics for building performance, including seismic resistance, are being implemented worldwide. Therefore, performance-based design is becoming more common. Performance-based requirements set measurable objectives but allow freedom in design and construction for how the objectives are met.
Performance-based seismic design verification is commonly done using nonlinear dynamic analysis. Appendix A in ACI 318–19 sets parameters for design verification of earthquake-resistant concrete structures using nonlinear response history analysis. Appendix A is intended to be used in conjunction with Chapter 16, “Nonlinear Response History Analysis,” of the American Society of Civil Engineers/Structural Engineering Institute (ASCE/SEI) 7, Minimum Design Loads for Buildings and Other Structures, which includes general requirements, ground motions and load combinations. Appendix A is also compatible with Guidelines for Performance-based Seismic Design of Tall Buildings, a document published by Pacific Earthquake Engineering Research in conjunction with their partners in the Tall Buildings Initiative. With the release of ACI 318–19, ACI becomes the primary resource for nonlinear dynamic analysis as it pertains to tall concrete buildings.
For seismic design of structural walls, ACI 318–19 introduces several new design requirements. Whereas previous designs permitted the use of crossties with 90-degree hooks at one end, all crossties for special boundary elements must now have 135-degree hooks at both ends. New provisions also restrict the locations of vertical reinforcement lap splices near intended plastic hinge zones. Another new design provision provides a check that detailing is adequate for the calculated earthquake displacement demands. Perhaps most significantly, new provisions will amplify wall design shears based on considerations of wall flexural overstrength and the effects of higher dynamic response modes, which may result in substantial increases in design shears for some walls.
ACI 318–19 adopts the precast concrete diaphragm design procedure of ACI 550.5, Code Requirements for the Design of Precast Concrete Diaphragms for Earthquake Motions. The design method in ACI 550.5 gives designers connection options for selecting the target performance of a precast concrete diaphragm when subject to seismic forces. ACI 550.5 requires connections to be qualified in accordance with ACI 550.4, Qualification of Precast Concrete Diaphragm Connections and Reinforcement at Joints for Earthquake Loading.
The code now clarifies the application and effect of the vertical ground component on earthquake load. There are numerous clarifications and additions to the requirements for column tie spacing in special moment frames — this includes clarifications of tie spacings for columns not considered part of the earthquake-resisting system. Intermediate moment frame requirements for tie spacing are also reduced.
ACI 318–19 includes revisions and additions aimed at eliminating conflicting provisions in ACI 318, ASCE 7 and the IBC regarding design of deep foundations for earthquake-resistant structures. These differences have been a source of confusion for both engineers and code officials. The purpose of the code change is to have all the pertinent concrete-related design and detailing provisions for the seismic design of deep foundations contained in ACI 318–19.
The document also contains numerous miscellaneous clarifications and simplifications. For example, the column-to-beam flexural strength ratio is adjusted for roof-level connections where the column axial load is low. The shear area of concrete walls, Acv, is clarified so it is clear it does not include the area of wall openings.
Code integration and ACI 318–19 reorganization
In addition to its compatibility with the ASCE 7 requirements for design verification using nonlinear dynamic analysis, ACI 318–19 achieves code integration by incorporating information from IBC and modernizing those provisions. ACI 318–19 also identifies areas where personnel are needed to be certified and references appropriate requirements. By providing certification requirements directly in the code and commentary, the information becomes more easily accessible to engineers.
Two chapters of ACI 318–19 have been reorganized. Chapter 17, “Anchoring to Concrete,” which covers anchor design, is reformatted to match other chapters initially adopted for ACI 318–14. Post-installed concrete screw anchors are increasingly used and this anchor type is now recognized, as are shear lugs, which comprise a steel element welded to a base plate. Shear lugs are usually used at the base of columns to transfer large shear forces through bearing to a foundation element. ACI 318.2–14, Building Code Requirements for Concrete Thin Shells and Commentary, which replaced ACI 318–11 Chapter 19, has also been reorganized to be consistent with the rest of ACI 318–19.
Chapter 26, “Construction Documents and Inspection,” has seen significant updates since ACI 318–14. Inspection requirements are unified in this chapter, including the relocation of anchor inspection requirements from Chapter 17. The chapter now recognizes projects may have roles for multiple design engineers and provides a framework for their coordination of work.
Structural concrete materials, quality control measures, and construction methods are continually evolving, requiring changes to ACI 318 provisions. Therefore, some changes to ACI 318–19 have been made to keep pace with changes to material characteristics. For example, lightweight concrete’s mechanical properties and unit density are different from other types of concrete. ACI 318–19 adds a new approach for assigning λ, a modification factor used in calculations to account for the reduced mechanical properties of lightweight concrete and is based on the unit weight of the material. This allows it to be defined as early as the project design stage. The method for determining λ based on testing to measure splitting tensile strength has been deleted from the code. However, the method to determine λ based on the composition of the fine and coarse aggregate has been retained. Lightweight concrete provisions throughout the code have seen numerous changes and clarifications based on the new method for determining λ.
As higher strength concretes have been developed over time, using the standard definition of modulus of elasticity may be inadequate for certain projects (such as tall buildings). Therefore, the definition for modulus of elasticity has been updated using data from external documents and best practices.
With the release of 318–19, the use of shotcrete is explicitly addressed in the document for the first time. The new content was developed by updating relevant provisions from IBC, with input from the American Shotcrete Association and ACI Committee 506. In the future, the IBC will reference ACI standards to govern the use of shotcrete.
High strength rebar is another material advancement addressed in 318–19. Current U.S. building codes limit rebar strength based on decades-old research, with most reinforcement used in concrete construction in the United States being Grade 60. Progress in metallurgy, however, has resulted in production of rebar that is almost twice as strong as it was several decades ago. This stronger rebar is able to transfer greater stresses. However, it also may lack benchmark properties of weaker steels, such as required strain-hardening and elongation. Recognizing this, ACI 318–19 includes new provisions for material properties of higher-strength steels. Accompanying these are myriad changes related to strength reduction factors, minimum reinforcement, effective stiffness, and requirements for development and splice lengths of straight high-strength rebar as well as hooks and headed bars. The many updates addressing high-strength rebar are expected to support adoption of these bars, which will, in turn, reduce congestion in heavily reinforced members, improve concrete placement, and save time and labor.
ACI 318–19 raises limits on the specified strength of reinforcement in earthquake-resistant shear wall and moment frame systems. The new standard allows Grade 80 reinforcement for some seismic systems and no longer allows Grade 40 rebar to be used in seismic applications. Shear walls can employ rebar in Grades 60, 80 or 100. Special moment frames can use Grades 60 or 80. Hoops and stirrups in special seismic systems used to support vertical reinforcing steel have a tighter specified spacing to prevent the vertical bars from buckling.
Alternative cements and aggregates
The industry has been exploring alternative cements and aggregates in recent years to improve concrete’s sustainability. Consequently, a number of new materials are on the market, including geopolymers, activated glassy, fly ash, slag cements, calcium aluminate cements, calcium sulfoaluminate cements, magnesia cements, carbon dioxide-cured cement and more. ACI 318–19 now permits the use of alternative cements as well as crushed hydraulic-cement concrete or recycled aggregate. However, it does not cover specifications for design criteria and performance for either alternative cements or aggregates, because not enough industry testing has been done. The use of alternative materials must be approved by the licensed design professional and the building official. This means materials suppliers and concrete producers will be responsible for performing testing and providing data on the expected performance of products. Although this increases the short-term burden on suppliers, once data for a given product or process has been generated, it should have applicability for many other circumstances.
ACI 318–19 Chapter 19, “Concrete: Design and Durability Requirements,” and Chapter 26, which cover concrete materials and mixtures, are the primary sections addressing alternative cements and alternative aggregates. The performance and durability requirements of Chapter 19 remain unchanged from previous versions of the code, although both durability and performance may be achieved differently in concrete mixtures using alternative cements than they are in mixtures with traditional cement.
ACI 318 assesses durability based on anticipated exposure categories such as exposure to freeze/thaw and/or sulfates and contact with water and corrosion protection of reinforcement. Existing mitigation strategies for these categories have been developed for Portland cement concrete based on testing using ASTM standard methods. Durability testing on alternative cements for exposure categories is a long-term undertaking and will be difficult to do on a project-specific basis. Gathering data in advance — for example, by conducting parallel tests of resistant Portland cement concrete and alternative-cement concrete — will be beneficial for producers and suppliers.
ACI 318 has long relied on ASTM standards to define the requirements of cementitious materials. Standards currently recognized are:
- ASTM C150/C150M, Standard Specification for Portland Cement;
- ASTM C595/C595M, Standard Specification for Blended Hydraulic Cements; and
- ASTM C1157/ and C1157M, Standard Performance Specification for Hydraulic Cement.
Materials specifications in previous versions of ACI 318 have applied to the cementitious material alone or in a mortar, with no testing having been done on a mixture that might be considered structural concrete. Therefore, factors that should be tested for an alternative cement’s influence on concrete include (but are not limited to):
- thermal cracking;
- volume stability;
- elastic properties;
- corrosion of metals, and
- reactions with aggregates.
Structural design and performance should be adequately tested, as should fire-resistance. Basic material properties (according to ASTM C150/C150M) must be assessed, including:
- chemical composition;
- loss on ignition;
- air content of mortar;
- fineness (or other measure of particle size);
- autoclave expansion;
- compressive strength;
- heat of hydration; and
- sulfate resistance.
A critical consideration is how a concrete mixture using alternative cements develops hardened properties. Materials specifications in previous versions of ACI 318 have applied to hydraulic cement (a material that sets and hardens by chemical reaction with water and is capable of doing so under water). Many alternative cements, however, do not rely on a chemical reaction with water. If the material is hydraulic, it may fall under ASTM C1157/C1157M. Another consideration is the response of the material to water content. For nonhydraulic materials, the water/cementitious materials ratio (w/cm) of mixtures containing alternative cements may not have the same relationship to strength and durability as Portland cement-based concrete mixtures.
Suppliers of alternative cements must provide evidence those materials will behave consistently during batching, transportation, and placing. Concrete mixtures made with the alternative cement will also require testing to determine how production should be modified (if at all). For example, considerations should include storage of material, mix proportioning, compatibility with admixtures, mixing time, and restrictions on time in the mixer drum.
Tests for constructability can be performed by conducting test placements and looking for issues with:
- pumping and other concrete conveying;
- workability and finishing characteristics;
- control of internal concrete temperature;
- segregation avoidance and consolidation techniques;
- slump loss and setting characteristics;
- wet, cold, and hot weather concrete placement;
- finishing techniques;
- bonding for multicourse slabs;
- initial and final curing;
- compatibility with curing compounds; and
- contraction joint spacing.
Additional changes to ACI 318–19
ACI 318–19 addresses concerns in the industry that previous shear provisions were inadequate for design of thick slabs or deep beams. As more large structures are designed to include thick slabs to support upper floors, these updates are timely. ACI 318–19 sections on one-way shear and two-way punching shear consolidate what were previously a wide range of equations. They also provide a method to include size effect in shear design to avoid issues wherein increasing a member’s size can reduce the design unit shear strength of a section. The new shear equations also allow the design engineer to take the effect of reinforcement ratio into consideration.
A variety of other industry needs are now addressed in ACI 318–19. For instance, post-tensioning updates include clarifications of the construction requirements regarding loss of prestress, use of a new reference document for determining prestress losses, deformed and bonded reinforcement spacing limitations, and several clarifications to requirements for anchorage zone reinforcement.
Numerous changes have been made to the durability of concrete sections, including additional requirements for sulfate exposure classes and concrete exposed to water.
Updates to strut-and-tie methodology (STM) include the removal of the terminology “bottle-shaped struts” from the code and the inclusion of minimum reinforcing requirements in STM. Other STM improvements include curved-bar nodes, knee joints, and seismic-design provisions.
Shrinkage and temperature reinforcement requirements have been simplified. Load testing provisions have been modified to become more consistent with other ACI standards.
Commentary language allows ACI 318–19 to be used for analysis, repair, and rehabilitation of existing structures and to recognize ACI 562, Code Requirements for Assessment, Repair and Rehabilitation of Existing Concrete Structures.
Several analysis clarifications and additions have been made. Commentary language regarding vibration analysis helps the user find guidance for designing a structure when vibrations are design criteria. Calculations of the effective moment of inertia, Ie, have been adjusted for nonprestressed reinforced concrete based on more accurate estimated deflection calculation results in both the laboratory and the field.
Find out more on ACI 318-19
Printed and digital formats of ACI 318–19 are available at concrete.org. Versions are available in inch-pound and SI units. ACI 318–19 is also available to subscribers of the online ACI Collection of Concrete Codes, Specifications, and Practices. Additionally, the institute is hosting public and in-house seminars to introduce users to ACI 318–19. Visit concrete.org for locations and to learn more.