There’s a newly released version of the American Concrete Institute’s ACI 318: “Building Code Requirements for Structural Concrete,” the document that governs design and construction of structural concrete. While the document is primarily used by engineers and building officials, it incorporates provisions that address materials advancements and applications that have been making inroads in the marketplace. Therefore, it will have an impact on concrete producers.
|Dr. Jack P. Moehle is the Chair of ACI 318 Building Code Committee and is the Ed and Diane Wilson Professor of Structural in the Department of Civil and Environmental Engineering at the University of California, Berkeley. He has played a leading role in the development of building codes and professional engineering guidelines on subjects related to reinforced concrete and earthquake engineering. He is a Fellow of the American Concrete Institute, Structural Engineering Institute of the American Society of Civil Engineers, and the Structural Engineers Association of California, and is an elected member of the U.S. National Academy of Engineering.
The industry has been exploring alternative cements and aggregates in recent years in an effort to improve concrete’s sustainability. Consequently, a number of new materials are available, including geopolymers, activated glassy cements, activated fly ash cements, activated slag cements, calcium aluminate cements, calcium sulfoaluminate cements, magnesia cements, and carbon dioxide-cured cement. ACI 318-19 now permits 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. Any alternative materials used 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.
When it is decided during the design phase to use an alternative cement or aggregate, suppliers and producers can expect general information on the required characteristics of that alternative to be included in the contract documents, along with identification of appropriate submittals to validate the assumptions of material characteristics. Any project-specific issues would be identified and resolved early in the design process. An equally likely scenario is that the proposal of an alternative material could occur during the bidding or construction phase of a project. In this circumstance, the supplier’s ability to have data on-hand to confirm the material’s fundamental structural characteristics becomes critical to avoid delays. It is hoped that once data for a given product or process have been generated, they should have applicability for many other circumstances.
TESTING ALTERNATIVE CEMENTS AND ALTERNATIVE AGGREGATES
ACI 318-19 Chapters 19 and 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 using traditional cement.
ACI 318 assesses durability based on anticipated exposure categories: exposure to freeze/thaw, exposure to sulfates, contact with water and corrosion protection of reinforcement. Existing mitigation strategies for these categories have been developed for portland cement concrete based upon 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.
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; strength; volume stability; elastic properties; creep; permeability; corrosion of metals; and, reactions with aggregates. Structural design and performance should be adequately tested, as should fire-resistance.
Basic material properties 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
- 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 cement 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. Another consideration is the response of the material to water content. For nonhydraulic materials, the w/cm of mixtures containing alternative cements may not have the same relationship to strength and durability as would portland-cement-based concrete mixtures.
Concrete producers must provide evidence that alternative cements 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; mixture proportioning; compatibility with admixtures; mixing time; and, restrictions on time in the mixer drum.
|ACI 318-19 includes new and updated code provisions on one-way shear, two-way shear, shear wall drift capacity, seismic design, shotcrete, deep foundations, post-tensioning, precast, durability, and lightweight concrete.
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 weather placement
- Cold and hot weather concrete placement
- Finishing techniques
- Bonding for multicourse slabs
- Initial and final curing
- Compatibility with curing compounds
- Contraction joint spacing
More information on the applicability of existing test methods and new testing needs can be found in “ITG-10.1R-18: Report on Alternative Cements. ITG-10R: Practitioner’s Guide for Alternative Cements” provides guidance for practitioners seeking to implement alternative cements and discusses currently available and emerging alternative cements.
Printed and digital formats of ACI 318-19 are available at concrete.org. Versions are available in inch-pound units and in 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.