Building code requirements for structural concrete

By Jack P. Moehle, PhD, PE

All images courtesy American Concrete Institute
All images courtesy American Concrete Institute

The American Concrete Institute (ACI) published ACI 318-19, Building Code Requirements for Structural Concrete, in July. ACI 318 presents requirements for design and construction of structural concrete that are necessary to ensure public health and safety. It is addressed to the engineer or the building official who is responsible for the contract documents.

It has been five years since the reorganized ACI 318-14 edition was published and the latest round of changes concentrates heavily on responding to developments in materials, structural systems, and seismic design. The Canadian Standards Association (CSA) A23.3, Design of Concrete Structures, gives structural engineers discretion in using new materials and methods for building construction, and historically the profession has been proactive in doing so. It is anticipated the new provisions within ACI 318-19 will give engineers further background and tools to support such adoption.

Engineers in Canada must comply with CSA A23.3, as adopted by the most recent building code, in the province in which they work. It is anticipated, some, or all of the new requirements in ACI 318, will be adopted in an upcoming version of A23.3, if they have not been adopted already.

Alternative cements

The viaduct over Almonte River located in Garrovillas de Alconétar, Cáceres, Extremadura, Spain, is the 2018 American Concrete Institute (ACI) Excellence in Concrete Construction Awards Overall Winner.
The viaduct over Almonte River located in Garrovillas de Alconétar, Cáceres, Extremadura, Spain, is the 2018 American Concrete Institute (ACI) Excellence in Concrete Construction Awards Overall Winner.

Portland cement, long used as a binder in concrete, has been under scrutiny because of the energy required for its production, as well as associated carbon dioxide (CO2) emissions. ACI 318-19 has added provisions allowing the use of alternative cements, defined by ACI Innovation Task Group (ITG) 10 as inorganic cements that can be used as complete replacements for Portland or blended hydraulic cements, and are not covered by applicable specifications for Portland or blended hydraulic cements. A few commonly used examples include geopolymers, activated glassy, fly-ash, and slag cements, calcium-aluminate cements, calcium-sulfoaluminate cements, magnesia cements, and CO2-cured cement.

Materials specifications in previous versions of ACI 318 have applied to hydraulic cement (cement that sets and hardens by chemical reaction with water and is capable of doing so under water). Many alternative types of cement, however, do not rely on a chemical reaction with water. Further, the materials specifications have applied to the cementitious material alone or in a mortar, without any testing done on a mixture that might be considered structural concrete.

Existing standards do not address these conditions, and until data are available industry-wide at some point in the future, materials suppliers will be responsible for conducting laboratory and field testing and distributing the findings. Design professionals and building officials will be responsible for assessing the influence a given alternative cement will have on concrete, including but not limited to:

  • thermal cracking;
  • placeability;
  • strength;
  • volume stability;
  • elastic properties;
  • creep;
  • permeability;
  • corrosion of metals;
  • reactions with aggregates; and
  • resistance to freezing and thawing, chemicals, or high temperatures.

Basic material properties, similar to those required by existing standards, must be established to ensure uniformity of supply of a given material. 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 the storage of materials, mixture proportioning, and compatibility with admixtures. Structural design and performance should be adequately tested, as should fire-resistance. In addition to warning that data must be available to demonstrate compliance with all project requirements, Section 26.4, “Concrete materials and mixture requirements,” of ACI 318-19 further warns designers 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 would.

Alternative aggregates

Another strategy for making concrete more sustainable is to incorporate more recycled materials into the mixture. As with its treatment of alternative cements, ACI 318-19 permits crushed hydraulic-cement concrete or recycled aggregate if it is approved by the licensed design professional and the building official. Here, too, the burden is on the project team to show concrete using the aggregates meet all project requirements. The material supplier is responsible for developing data to show the alternative aggregate will perform as intended, since the nature of a given recycled aggregate is likely to change from project to project. The designer must determine whether the data presented are adequate for the application.

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