16 minute read
The future of sustainability in the ACI 318 Concrete Building Code Exploring the ways in which structural engineering regulations can be modified to reduce the environmental impacts of concrete buildings
THE FUTURE OF SUSTAINABILITY IN THE ACI 318 CONCRETE BUILDING CODE
Traditionally, structural building codes have been viewed as pertaining only to the regulation of structural engineering practice, without particular regard for the impacts of building design and construction on the environment. With the recent increase in attention to the sustainability of manufactured building materials, building construction, and building demolition, the writers of building codes and standards have started to examine all aspects of the building design process, looking for ways to reduce environmental impacts, particularly the emission of carbon dioxide. One may naturally ask how structural building codes, which deal primarily with regulations for the proper application of engineering principles to achieve adequate strength, stiffness, and serviceability of buildings, can be expected to address environmental concerns. Upon closer examination of structural building codes, however, it can be understood that structural codes can be written in such a way that they support and promote sustainable building practices. This article explores the ways in which structural engineering regulations can be modified to reduce the environmental impacts of concrete buildings. The focus of this article is the American Concrete Institute’s ‘ACI 318: Building Code Requirements for Structural Concrete’, but the ideas explored here can be applied to any building code that regulates the structural design of concrete buildings.
INTRODUCTION
Before the last decade, structural engineers were seldom involved in consideration of the environmental impacts of the structures they designed. In recent years, however, this has changed. Many of the reasons that the environment is adversely affected by the creation of a new building are related to the production, transportation, and assembly of structural materials and components. It stands to reason, then, that the structural engineer should be an active participant in the movement towards creating building projects that are less detrimental to the environment. Until very recently, structural building codes - the regulations that ensure the strength, stiffness, and serviceability of a structure - have had little or nothing to say about environmental concerns. Structural codes have been almost exclusively concerned with public safety, while the environmental impacts of building construction have been the subject of other codes, guidelines and sustainability rating systems. Major structural design codes have, however, started to address environmental aspects of construction. An important example is the fib Model Code for Concrete Structures 2010 [1]. In section 3.4 ‘Performance Requirements for Sustainability’ of the fib model code, the general principles of design for sustainability are outlined, and performance categories related to environmental impact and to impact on society are listed. Section 3.4 presents a brief, innovative framework for consideration of sustainability in structural design. This effort will be expanded in the next edition of the 2020 fib model code (MC2020). The current proposed outline of MC2020 includes Section 3 ‘Sustainability Perspective’, Section 28 ‘Evaluation of Environmental Performance’, and Section 30 ‘Sustainability Evaluation’. MC2020 will adopt a holistic plan for building life cycle performance, including initial design for sustainability, through-life management and care, assessment of environmental performance, and eventual dismantlement and reuse. Sustainability considerations were first introduced in the 2014 edition of the American Concrete Institute building code, ACI 318-14 [2]. Although these provisions essentially consist of a statement expressing the desirability of considering sustainability in structural design, this represented a modest start on the path towards creating more comprehensive sustainability provisions. The same provisions on sustainability appear in ACI 318-19 [3], but these were supplemented by new rules permitting the use of alternative cementitious materials and recycled aggregates. Looking forward, plans are underway to create an ACI 318 subcommittee charged with writing expanded sustainability provisions for the 2025 edition of the ACI 318 code. In this article, we describe why sustainability should be considered in the design of reinforced concrete structures, we outline the initiatives that the American Concrete Institute (ACI) has undertaken to address the challenges of sustainable structural design, and we provide information on recent plans to expand sustainability provisions in future editions of the ACI 318 code.
ENVIRONMENTAL IMPACTS OF CONCRETE CONSTRUCTION
The effects of concrete construction on the environment have been well documented, particularly the production of carbon dioxide (CO2) during the manufacture of hydraulic Portland cement. Other sources of CO2 associated with concrete construction include the mining and transportation of coarse and fine aggregates, and the production, transportation, and placement of the concrete itself. The creation of CO2 is particularly concerning because anthropogenic CO2 is a major contributor to the accumulation of greenhouse gases, which lead to global warming. Figure 1 illustrates that over the last six decades there has been a 28% increase in atmospheric CO2. Because this trend shows no signs of abating, it is imperative to identify methods to reduce the production of atmospheric CO2. Estimates of greenhouse gas emissions from industrial sources in the United States rank cement manufacturing among the top producers of industrial greenhouse gases. Figure 2 shows nine industries with the highest output of greenhouse gases in the United States. Oil and gas production, followed by chemical manufacturing are the two highest producers, while cement manufacturing is the eighth. Looking at these same statistics for US greenhouse gas emissions in a different way, the total US greenhouse gas production in 2002 was 7,065 million metric tons of CO2 equivalent (MMTCO2E), as shown in Figure 3. Of this total, industrial sources were responsible for 29%, or 2,047 MMTCO2E. The portion of these industrial sources that was the result of cement production was 4%, or 82 MMTCO2E. Thus, about 1.2% (82/7065) of the greenhouse gases produced by the US in 2002 was caused by the manufacturing of cement.
Figure 1: Atmospheric CO2 Concentration 1959 to 2016 (Le Quéré et al, 2016 [4]).
Figure 2: United States Industrial Sectors With Highest Greenhouse Gas Emissions (ACI 130R-19, 2019 [5] based on data from U.S. EPA, 2008 [6]).
Overall, the global production of greenhouse gases from the manufacturing of cement is even higher than in the US. In a recent report, ACI Committee 130 (Sustainability in Concrete Construction) stated “A 2016 study of the global carbon budget estimated that cement production accounts for 5.6 percent of anthropogenic CO2 released globally” [5]. Thus, about one-twentieth of all greenhouse gases released annually into the atmosphere are caused by the production of cement alone. The proportion would be even higher if the greenhouse gases associated with mining and transportation of materials were included in the total.
In recent years, ACI has sought to address the challenge of reducing greenhouse gas emissions through a range of initiatives and publications. These include more than 40 standards and guides containing information on sustainability in concrete construction; over 60 journal articles related to sustainability; 200 practice-oriented papers and articles on sustainable design and construction methods; more than 100 online learning tools; and sponsorship or co-sponsorship of over 25 events that have supported the advancement of sustainability in concrete design and construction. ACI formed Innovation Task Group 10 (ITG-10) which, in 2019, published two important reports that provide guidance on the characteristics and implementation of alternative cements (i.e. alternatives to Portland cement) - Report on Alternative Cements [7], and Practitioner’s Guide for Alternative Cements [8]. These reports present practical approaches to incorporating alternative cements in concrete mixtures, as a means of reducing greenhouse gas emissions caused by production of Portland cement. It should be noted that a recent addition to the ACI 318-19 building code [3], Section 26.4.1.1.1, provides increased flexibility to engineers and contractors for incorporation of alternative cements in concrete construction. Technical committee AC! 130 - Sustainability of Concrete is very active in pursuing solutions for reducing the environmental impact of concrete. The committee has over 100 voting and associate members, and maintains the following eight subcommittees: • Materials • Production/Transportation/Construction • Structures in Service • Rating Systems/Sustainability Tools • Design/Specifications/Codes/Regulations • Education • Climate Change Impacts on the Sustainability of Concrete • Liaison Subcommittee This year, Committee 130 published ‘Report on the Role of Materials in Sustainable Concrete Construction’ [5] which provides comprehensive background on sustainability topics, and describes available practices for design, production, and construction that reduce the environmental impacts of concrete.
WHAT DIFFERENCE CAN STRUCTURAL ENGINEERS MAKE?
With the recent increase in attention to the sustainability of manufactured building materials, building construction, and building demolition, the writers of building codes and standards have started to examine all aspects of the building design process, looking for ways to reduce environmental impacts, particularly the emission of CO2. One may naturally ask how structural building codes, which deal primarily with regulations for the proper application of engineering principles to achieve adequate strength, stiffness, and serviceability of buildings, can be expected to address environmental concerns. Upon closer examination of structural building codes, however, it can be understood that structural codes can be written in such a way that they support and promote sustainable building practices. Among the actions that structural engineers and structural building code writers can take to improve the sustainability of concrete construction are the following.
Alternative cements
Structural engineers can specify concrete mixtures that incorporate cementitious materials other than Portland cement. The recent report by ACI Committee 130 provides an excellent overview of the science and applications of alternative cementitious materials [5].
Recycled aggregate
Engineers can make use of recycled coarse aggregates whenever feasible. Caution must be exercised in correctly matching the type of recycled aggregate to the performance requirements of the concrete, but recycled aggregates are found increasingly in both structural and non-structural applications. The ACI 318-19 building code [3] contains new provisions that encourage the use of recycled aggregates.
Time period to achieve specified strength
Engineers can consider specifying a time period longer than 28 days (say 42 or 56 days) to achieve the specified concrete strength. This can enable the use of reduced amounts of Portland cement, or alternative cementitious materials that gain strength at a slower rate than Portland cement. In some applications a 28 day period, or even a period as short as 7 days, is specified for the concrete to reach a target strength. Such requirements are often necessary to achieve construction efficiencies, but they may result in the use of more cementitious material than is required for structural strength. In other applications, allowing a longer period of time to reach the target strength would have little or no impact on the construction schedule, and would permit a reduction in the quantity of cement used.
Minimise concrete volume
An obvious way to reduce the usage of Portland cement on a project is to reduce the total volume of concrete required. Designing structural members with the minimum
possible volume of concrete will result in the minimum weight of cement used, with corresponding reductions in greenhouse gas emissions. In some construction markets, where the cost of labour is high relative to the cost of materials, it may be most economical to design oversized structural members that take advantage of labour efficiencies while sacrificing materials efficiency. When environmental considerations take precedence, however, it may become necessary to sacrifice certain labour efficiencies as a way to achieve minimal consumption of materials.
Participate in codes and standards development
The structural engineer can play an active role in the drafting of structural design standards that promote sustainable design and construction with concrete. Engineers should play a leading role in the development of codes and standards because engineers have the practical experience that enables them to identify design processes and materials that will lead to more sustainable applications of concrete construction. Engineers can, in fact, develop not only new design approaches and materials specifications, but also suggest new areas of research that can reduce the environmental impacts of concrete.
EXISTING SUSTAINABILITY PROVISIONS IN ACI 318-14
As was mentioned previously, the first provisions related to sustainability appeared in the 2014 edition of the ACI 318 building code [2]. Section 4.9 of ACI 318-14 simply states that engineers may take sustainability considerations into account when designing concrete structures. Section 4.9 goes on to stipulate that considerations related to safety must take precedence over considerations of sustainability. While this provision represents a modest start to the inclusion of sustainability provisions in the ACI 318 code, it created a placeholder that would lead to further development of sustainability provisions in future editions of the code.
NEW SUSTAINABILITY PROVISIONS IN ACI 318-19
In addition to the general principles of Section 4.9, the 2019 edition of the ACI Code [3] (published in June 2019) contains additional sections that provide engineers with tools for sustainable design.
Cementitious materials defined by ASTM
Section 26.4.1.1.1(a) lists specific cementitious materials that are permitted in concrete construction. These are defined by American Society for Testing and Materials (ASTM) standards. Table 26.4.1.1.1(a) lists the names of permitted cementitious materials, and the corresponding ASTM standard that defines each material.
Alternative cements
Section 26.4.1.1.1(b) allows, within limits, the use of alternative cementitious materials, i.e. materials not listed
Table 26.4.1.1.1(a) – Specifications for cementitious materials
Cementitious material Portland cement Blended hydraulic cements
Expansive hydraulic cement Hydraulic cement Fly ash and natural pozzolan Slag cement Silica fume
Specification
ASTM C150 ASTM C595, excluding Type IS (≥70)and Type IT (S≥70) ASTM C845 ASTM C1157 ASTM C618 ASTM C989 ASTM C1240
Figure 4: Table 26.4.1.1.1(a) reproduced from ACI 318-19.
in Table 26.4.1.1.1(a). These alternative materials are permitted only if approved by both the licensed engineer and the building official. Furthermore, “Approval shall be based on test data documenting that the proposed concrete mixture made with the alternative cement meets the performance requirements for the application including structural, fire, and durability”. In other words, an alternative cementitious material may be used if sufficient test data is available to demonstrate adequate structural and non-structural performance, and if this performance meets with the satisfaction of both the structural engineer and the building official.
Recycled aggregates
In addition, in Chapter 2, the commentary language related to the definition of the term ‘aggregate’ has been revised to include a discussion of recycled aggregate - “The use of recycled aggregate is addressed in the Code in 2019. The definition of recycled materials in ASTM C33 [the US material standard for concrete aggregates] is very broad and is likely to include materials that would not be expected to meet the intent of the provisions of this Code for use in structural concrete. Use of recycled aggregates including crushed hydraulic-cement concrete in structural concrete requires additional precautions. See 26.4.1.2.1(c)”.
Section 26.4.1.2.1(c) states the following:
“(c) Crushed hydraulic-cement concrete or recycled aggregates shall be permitted if approved by the licensed design professional and the building official based on documentation that demonstrates compliance with (1) and (2). (1) Concrete incorporating the specific aggregate proposed for the Work [the project] has been demonstrated to provide the mechanical properties and durability required in structural design. (2) A testing program to verify aggregate consistency and a quality control program to achieve consistency of properties of the concrete are conducted throughout the duration of the project”. These provisions - Sections 26.4.1.1.1(a), 26.4.1.1.1(b), and 26.4.1.2.1(c) - create a framework within which the
engineer can assess and incorporate alternative cements and recycled aggregates into the design of concrete structures.
THE FUTURE OF SUSTAINABILITY IN ACI 318-25
Looking forward, a new technical subcommittee of the ACI 318-25 building code has been created specifically to draft expanded provisions related to sustainable concrete design and construction - Subcommittee N, Sustainability. This is the first ACI 318 building code technical subcommittee dedicated exclusively to writing provisions to support and promote sustainable design and construction practices. In addition to the sustainability provisions already in ACI 318-19, the 2025 edition of the ACI 318 code may address the following topics:
Alternative cements
Additional provisions related to the use of alternative cements may be written. The objective of these provisions would be to facilitate the use of alternative cements as a way of reducing greenhouse gas emissions from the manufacture of Portland cement.
Minimise concrete volume
Provisions encouraging the design of optimised, compact, concrete elements may be developed. These provisions would promote minimisation of the total volume of concrete used for a project, possibly with the consequence of slightly reduced construction efficiency in some cases. For example, a series of members that might normally be constructed with the same dimensions might be re-designed to consist of a series of members with a range of structurally optimised sizes.
Voluntary reporting of EPD or GWP
Guidelines for voluntary reporting of the Environmental Product Declaration (EPD) or Global Warming Potential (GWP) of concrete mixtures may be included. Knowledge of the EPD and GWP permits designers and contractors to select concrete mixtures with reduced environmental impacts.
Time period to achieve specified strength
Provisions that encourage consideration of a period of time longer than 28 days to establish the acceptability of in situ concrete strength (e.g. 42 or 56 days) may be included. By permitting concrete a longer time to reach the required design strength, it is sometimes possible to reduce the cement content of the concrete mixture.
Recycled aggregates
More detailed provisions may be developed to support the use of recycled and non-traditional aggregates.
The influence of durability requirements on cement content
A review will be conducted of existing requirements for the durability of concrete subject to various adverse exposures. Some exposure categories require a minimum concrete strength, which may raise the cement content of the concrete above what is necessary for structural strength. It may be possible to allow alternative means of protecting concrete from adverse environments without resorting to increasing the cement content.
CONCLUSIONS
In this article, we have presented a brief summary of the impacts of concrete construction, particularly cement production, on greenhouse gas emissions. We have also summarised the current state of sustainability provisions in the American Concrete Institute building codes ACI 318-14 and ACI 318-19. In the 2025 edition of the ACI 318 code, there will be additional provisions related to sustainability, with the objective of making it easier for designers to implement sustainable engineering practices and for building officials to approve projects that incorporate sustainable features. A technical subcommittee of the ACI 318 code committee has been formed specifically to address these issues - Subcommittee N, Sustainability. Over the next six years, this subcommittee will be working with researchers, practitioners, and other ACI 318 technical subcommittees to develop code provisions that will enable sustainable practices in the design and construction of concrete structures.
REFERENCES
[1] Fédération Internationale du Béton (fib), Model Code for Concrete Structures 2010, Lausanne, Switzerland (2013). [2] American Concrete Institute (ACI), Building Code Requirements for Structural Concrete (ACI 318-14) and Commentary (ACI 318R-14), Farmington Hills, Michigan, USA (2014). [3] American Concrete Institute (ACI), Building Code Requirements for Structural Concrete (ACI 318-19) and Commentary (ACI 318R-19), Farmington Hills, Michigan, USA (2019). [4] Le Quéré, Corinne, et al, ‘Global Carbon Budget 2016’, Earth System Science Data, Vol 8, pp 605-649 (2016). [5] ACI Committee 130, Report on the Role of Materials in Sustainable Concrete Construction, ACI 130R-19, American Concrete Institute, Farmington Hills, Michigan, USA (2019). [6] US Environmental Protection Agency, 2008, ‘Greenhouse Report’, Washington DC, USA. https://archive.epa.gov/sectors/ web/pdf/greenhouse-report.pdf (accessed June 30, 2019). [7] ACI Innovation Task Group ITG-10, Report on Alternative Cements, ACI ITG-10.1R-18, American Concrete Institute, Farmington Hills, Michigan, USA (2018). [8] ACI Innovation Task Group ITG-10, Practitioner’s Guide for Alternative Cements, ACI ITG-10R-18, American Concrete Institute, Farmington Hills, Michigan, USA (2018). (This article is based on a Keynote Paper authored by Andrew W Taylor and Shana T Kelley, KPFF Consulting Engineers, USA, and presented at the 44th Conference on ‘Our World in Concrete & Structures’ held in Singapore, from 29 to 30 August 2019. The conference was organised by CI-Premier Pte Ltd).
