Analysis of alternative sustainable approach to concrete mixture design

Abstract

Keywords: Concrete; Admixture; Sustainability; Fly-Ash; Volcanic Ash

1. Introduction

The Connecticut Department of Transportation currently adheres to the Federal Highway Administration’s catchy slogan of “get in, get out, stay out”, leaving them reluctant to experiment and possibly gamble with new concrete design mixtures that may not have the same longevity as traditional concrete mix designs or those that may require longer time to set risking unnecessary traf-

fic delays. Current concrete mixture designs are up to the supplier who has the freedom to experiment as they see fit as long as they meet the strength specifications set by the Connecticut DOT [1]. Nevertheless, alternative concrete mixtures have been slow to make gains in heavy highway construction. Currently, the Connecticut DOT’s most used sustainable concrete is fly ash concrete since it helps with concrete workability. However, while this is more eco-conscious than traditional concrete, it is not an in-

*Corresponding author. *E-mail address: talats@ccsu.edu Published by Yıldız Technical University Press, İstanbul, Türkiye This is an open access article under the CC BY-NC license (http://creativecommons.org/licenses/by-nc/4.0/).

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definite solution. As climate change is a growing concern, the burning of coal, which produces fly-ash has declined, as this process also causes significant harm to the environment [2]. Further, even if countries were to continue to burn coal, the current rate of consumption will likely deplete coal by 2090 [3]. In turn, coal slag will become scarcer, and the cost will inevitably increase as well. Therefore, with this future date in mind, it is now more critical than ever to explore alternative concrete mixture designs that will prove to have similar material properties as traditional concrete but will be more sustainable in terms of the environment and economy. Two alternatives to be explored are volcanic ash concrete and ground glass pozzotive concrete. With strengths able to meet DOT specifications, it is surprising that Departments of Transportations have not attempted to use the material in their heavy highway builds [4]. This prompts questions involving practicality and supply to determine how these materials, or any other alternative materials, may become a solution for Connecticut highway infrastructure. By surveying the Departments of Transportation nationwide as well as investigating concrete suppliers, it may be possible to see if this reluctance is limited to Connecticut, or if it is nationwide. In all, this project attempts to gather data on alternative concrete mixtures used by the Departments of Transportation nationwide in an effort to offer more sustainable long-term solutions to the Connecticut DOT.

2. Synopsis Of Existing Literature

The plan for this project initially began as a survey to the Departments of Transportation nationwide to gain insight on concrete usage, specifically pertaining to cost and composition in the heavy highway construction industry. Following the surveys, this project’s focus has morphed in response to the survey replies and targeted two alternatives for fly ash concrete. Of the two options discussed, ground glass pozzolan concrete is much more likely to take off in the New England and Tri-State regions compared to the volcanic ash concrete. To assess these mixture designs more clearly in comparison to fly ash concrete, a literature review was performed. Fly ash, which is the resultant of burning coal, is one of the most frequently used additives in concrete composition to offset the amount of Portland cement, which not only makes concrete stronger, but also benefits the environment. Nevertheless, this material will not be around forever, as coal powered plants have experienced a severe decline. In the past 60 years alone, the coal production in the United States dropped 24%, with only 535 million short tons produced [5]. The labor force has shrunk as well, with only 37,000 coal miners working today, as compared to 178,000 miners in 1985 [6]. Figure 1 shows the drastic decline in coal plants from just 2007 to 2016 as wind, solar, natural gas, and hydrothermal plants have become more

prevalent [7]. Further, recent environmental movements have pushed against coal production in support of greener energy sources [3, 8, 9]. Additionally, one study explains that the American coal supply has been largely overstated, and as coal production declines this will become more apparent, with the U.S. only recovering roughly 20% of the government’s reported coal reserves [10]. This information is startling when considering the significant reliance on fly ash for so many concrete suppliers, and the apparent lack of acknowledgment of this information is just as worrisome. New studies are needed to gain industry insight on this decline and investigate alternatives that suppliers may be looking into to combat future shortcomings. To underscore the significance of the need for an alternative concrete mix design that is more sustainable, one must recognize the damage of traditional concrete design, chiefly, the incorporation of Portland cement. The damage and devastation the Portland cement industry causes on the environment is well known. Research shows concrete production, specifically the Portland cement industry, to be one of the leading contributors of Carbon-Dioxide (CO2) emissions, with estimates ranging roughly around 5% to 7% of global CO2 emissions stemming from Portland cement production [11]. Author Jonathan Watts, of The Guardian, paints the picture of the crisis we face, stating: “If the cement industry was a country, it would be the third largest carbon dioxide emitter in the world with up to 2.8 billion tons, surpassed only by China and the US” [11]. Considering this fact, and how the construction industry is just one small facet of life, it is imperative we change. Further, the making of cement is not the only factor in the concrete process that causes harm to the environment. The harvesting of natural materials causes significant irreparable damage to the Earth [12]. In one study, titled ‘Green Concrete Mix Using Solid Waste and Nanoparticles as Alternatives- a Review,’ the researchers argued for the implementation of natural waste materials to form Green Concrete [13]. Examining the currently used avenues of solid waste management, they focused on the harmful effects of landfills and recycling costs, as well as the harmful damage from cement manufacturing [12]. Breaking down the significant harmful effects of the concrete industry on the environment, it is clear the construction industry needs to strive for a more eco-conscious alternative concrete that will not wreak havoc on natural resources or pollute the Earth [13]. With these facts in mind, there is no turning back to traditional Portland cement concrete. While States have their own specifications for concrete mixtures, the Federal Highway Administration (FHWA) sets general parameters that must be followed. The FHWA also advises on the general benefits of the incorporation of fly ash concrete, touching on improved workability, decreased water demand, reduced permeability, and improved durability [14]. While these points are all valid, it is important to address a difference between Connecticut State and

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Figure 1. Map of Decline of Coal Power in the United States 2007 (top) and 2016 (bottom) [7]. Federal guidelines. Per the FHWA, “Fly ash is used to lower the cost and to improve the performance of PCC. Typically, 15% to 30% of the Portland cement is replaced with fly ash, with even higher percentages used for mass concrete place-

ments. An equivalent or greater weight of fly ash is substituted for the cement removed. The substitution ratio for fly ash to Portland cement is typically 1:1 to 1.5:1” [14]. This is higher than the guidelines provided Connecticut.

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Reviewing the DOT’s specifications listed under section M.03.01 titled ‘General Composition of Concrete Mixes,’ the State of Connecticut allows any qualified composition of concrete if it meets the specifications listed per concrete grade. The design details are as follows: “Portland cement concrete shall consist of an intimate mixture of Portland cement, other approved cementitious material (when used), fine aggregate, coarse aggregate, water, and admixtures, if ordered or permitted by the Engineer, proportioned in accordance with the following requirements” [15]. Essentially, discussing strength alone, Class A and C concrete must meet 3,000 psi (20.7 MPa) compressive strength at 28 days, and Class F must meet 4,000 psi (27.6 MPa) at 28 days. Regarding fly ash replacement in the mixture, Connecticut allows for up to 15% replacement of Portland cement, pound for pound. Therefore, as long as the materials have been sourced and verified by the Division of Materials Testing, and the concrete meets the specified requirements, the engineer may choose any mixture design they deem suitable. Additionally, it is important to point out the need for curing in Connecticut, especially in the colder months. While most concrete work is performed in the warmer months, CT DOT section 4.01 does specify supplemental needs including the laying of straw or hay for protection on days or nights where the temperatures may drop below 35 degrees Fahrenheit (2 degrees Celsius) [16]. Also, if any concrete freezes before it is fully cured, it must be removed and replaced at the contractor’s expense [16]. Therefore, for any new concrete mixture designs to take off in New England, they must be suitable for work in colder temperatures, and not delay projects with longer cure times, which can lead to concrete damages if the material freezes before it is fully cured. Fly ash concrete, either Class C or Class F types, is a commonly accepted alternative concrete mixture in which Portland cement is replaced partially with fly ash. The main difference between these two types of concrete is the chemical makeup of the ash itself. Boral Resources, a leading American supplier of coal combustion products, like fly ash, concisely summarizes these differences, explaining: “Class F fly ash is highly pozzolanic, meaning that it reacts with excess lime generated in the hydration of Portland cement, Class C fly ash is pozzolanic and also can be self-cementing” [17]. This manufacturer further explains specifications and requirements set forth by the American Society for Testing Materials [18]. This society per section ASTM C618 “requires that Class F fly ash contain at least 70% pozzolanic compounds (silica oxide, alumina oxide, and iron oxide), while Class C fly ashes have between 50% and 70% of these compounds. Typically, Class C fly ash also contains significant amounts of calcium oxide – over 20%. Most Class F fly ash contains little calcium oxide; however, some Class F fly ash sources may contain intermediate levels (8% to 16%) of calcium oxide” [17].

Fly ash concrete has long been studied and is widely accepted in the heavy highway industry [19–22]. However, none of these studies discuss the impending loss of the material or possible solutions. Notably though, one study in 2014 in Wisconsin did address the lack of the fly ash material (Class C) and suggested an expansion of their current specifications to allow for the incorporation of Class F fly ash concrete [23]. It is remarkable that this study was completed seven years ago, and the problem remains the same. Furthermore, it will not be feasible in the long run to adopt a different fly ash mixture since this would be a temporary fix; a permanent solution is needed for the heavy highway construction industry. To expand further on the limitations and finite timeline of fly ash concrete, fly ash is the byproduct of burning coal for energy production, while energy production is becoming less reliant on coal. As the world moves greener and environmental crisis shapes the world, new initiatives are implemented to become more environmentally conscious in all areas of life, especially in significant areas such as electricity. In recent years the production of coal has declined due to shortages, as well as its harmful effects and toxicity. Even disregarding the environmental and health risks of coal consumption, research has shown coal supply is shrinking and may become extinct in the 70 years [8, 9, 24]. Therefore, as power plants move from burning coal to more environmentally friendly options like natural gas or hydroelectricity, the loss of fly ash might be sooner than predicted. The future of sustainable alternative concrete must take into consideration materials that are in abundance and not comprised of finite resources, like fossil fuels. Natural pozzolans, like volcanic ash, have a great potential as a substitute for fly ash. The volcanic ash used for concrete mixtures can be obtained in several ways: explosive volcanic eruptions, phreatomagmatic eruptions, and the transports in pyroclastic density currents. However, it is important to note that these variations can result in different mechanical properties [25]. Nevertheless, harvesting this material is significantly less damaging to the environment in comparison to the mining of lime to create Portland cement. Collecting this material also eliminates the negative effects of volcanic eruption to human life [25]. In the United States alone, there are 169 volcanoes. Most are in Alaska, but they are across the U.S. as well. Harvesting this material is not damaging to the Earth in the same way that lime mining is, and the process for refining this ash to be useable in concrete does not release the same amount of carbon as Portland cement or burning coal [26, 27]. The chemical properties of this material make this a strong cementitious material with a comparable compressive strength to traditional concrete mixture designs with ranges of 6% to 10% replacement of Portland cement [28]. Interestingly, cure time for this material is roughly comparable to traditional concrete [29]. It is also important to note that one study identified volcanic ash as beneficial in improving the ability of concrete to resist freeze–thaw cycles with limited mass loss ratios

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(2.1–2.3%) [30], which is extremely important in regions with fluctuating temperatures, like New England. Another promising alternative to fly ash concrete is the supplementation of ground glass pozzolan as a partial replacement for Portland cement in concrete. Several studies have assessed this mix design and found it to be comparable, if not stronger than traditional Portland cement concrete [31–35]. For this mixture, 20% to 25% may be most optimal for heavy highway construction [31–33]. The chemical properties of this material make this a strong cementitious material with increased compressive strengths of 16% with only a 15% replacement of Portland cement [32], and increased compressive strength with 10% glass replacement [33]. Moreover, Khudair et al. [34] reason compressive strength could be the result of “pozzolanic activity”, in which glass particles react with the cement hydrates forming new gel bonds, which can block the pores of the concrete, making pores smaller and not connected, which leads to greater density. Additionally, studies have also concluded this material to be longer lasting, with estimated service life five times greater than traditional concrete [36]. Cure time may be slightly longer than traditional concrete, however if protected properly from the elements and properly planned for in scheduling, this should not be a significant issue. Silica Fume is another alternative mix concrete that has proven to increase strength and durability, while also reducing the carbon impact by replacing varying quantities of the Portland cement. Several studies have focused on this material, all finding it suitable for both lightweight and heavyweight concrete construction [37–40]. While this material has great benefit, there are drawbacks including potential cost increases or fluctuations in various regions where this material may not be readily available. The extra cost of the material is examined in one study in which quarry dust was incorporated as a filler in addition to silica fume, to strengthen the concrete mix while lessening the amount of silica fume or fly ash for regions that may not have as much access to these materials [41]. This information is useful as it shows that research is expressing concern over availability and cost of materials, however, there is not a similar study like it for just fly ash which is especially needed now as coal production declines. It is also important to examine what other surveys and studies have assessed the current market of concrete production. Overall, there are relatively few studies in this regard. Most closely related is a survey completed in 2015. In this study, the scholars surveyed concrete suppliers and manufacturers to gain insight on the intersection of academic knowledge in comparison to industry production [42]. This study surveyed numerous suppliers, one trade organization, as well as the Ohio Department of Transportation. This study concluded that the most used concrete mixtures were fly ash (Class C and F), ground granulated blast-furnace slag (GGBFS), Silica fume, and shale. This study provided insight on manufacturers understanding and usage of alternative concrete mixture designs. Nevertheless, given how stringent state public jobs can

Table 1. Department of transportation concrete mixture survey What is the traditional makeup of your concrete? What is your concrete budget? Do you use flyash?

How much does your flyash concrete cost? Do you use ground glass/ Pozzotive concrete?

be, more investigation is needed to see the concrete materials States would allow in their heavy highway jobs. Additionally, it would be useful to compare the results of the study that was done seven years ago with the results presented in this paper, specifically to evaluate if new concrete admixtures are being used by the various Departments of Transportations.

3. Study Methodology

The primary goal of this study was to identify what concrete mixtures that are allowed by the Departments of Transportation across the nation. The secondary goal of this study was to evaluate the availability these materials, and hopefully highlight the practicality of these alternative compositions. To complete these tasks, a comprehensive literature review was completed that identified possible alternative mixture designs, as well as an investigation was performed that identified major suppliers, by state, of sustainable mixes currently produced in America. Then, with the aim of producing findings for the Connecticut DOT, a survey was sent out to the Departments of Transportation nationwide. To increase the likelihood of gaining information on concrete suppliers and materials currently in use while not being a customer, a thorough review of leading suppliers was completed to assess the materials they advertise and offer for more sustainable options. Table 1, inserted below, shows the survey sent to the Departments of Transportation. This survey addressed questions regarding annual budget, concrete mixtures used, and concrete cost. To increase the likelihood of responses, the survey was kept short. Respondents did elaborate on their answers and provided further insight and thoughts on sustainable concrete mix designs. The survey was sent to the Materials Testing Divisions, and other relevant departments, listed publicly on the Departments of Transportation websites. If specific emails were not found, the survey was sent to the general forum in hopes of it being passed to the appropriate department. The next step of this study was to interpret the various data these surveys provided. The results revealed information on various materials, costs, sale percentages, concrete usage and more. This information was illustrated by graphical representations. Additionally, the analysis of the data from the survey responses will lead to further

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investigation of alternative materials and refine plausible solutions that is expected to be helpful to the Connecticut Department of Transportation to provide a more permanent sustainable alternative of concrete mixture for heavy highway construction.

4. Results

This survey was sent out nationwide to multiple divisions of each the State’s Departments of Transportation excluding Connecticut, as the information was obtained directly from the Connecticut DOT interviews. The authors received responses from 12 States. The data pooled revealed several important trends and identified more areas for future surveys to research. The results of the interview and the surveys are as follows.

5. Review Of Concrete Manufacturers

It was important to look to the leading concrete suppliers and evaluate the feasibility of different alternative mixture designs. For this study, four leading suppliers were researched: CalPortland and Cemex, Inc., each in a different region of the country, and two Connecticut suppliers, Tilcon and Urban Mining. Urban Mining was of particular interest, as they only manufacturer ground glass concrete.

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Fly ash, slag, natural pozzolans, and blended cementitious materials. We see a lot of blended hydraulic cement such as type IL cements with the use of fly ash and slag replacing portland cement

ASTM C595 blended cements and C1240 drydensified silica fume

Table 7. Other types of “green” admixtures in concrete (continue)

Ground granulated We have been doing fly ash No blast furnace slag, and GGBFS for over 40 silica fume years, we have optimized our gradations and reduced our cement contents, we use IL cements, we have approved the use of carbon cure with a 3% cement reduction

Urban Mining, in Beacon Falls, Connecticut is a local Connecticut concrete plant which has revolutionized the process of making ground glass concrete even more sustainable as it can accommodate all types of glass containers, including ceramic and other “nontraditional” glass bottles [44]. This is significant, as it maximizes the options of resources to make Pozzotive concrete, their brand of pozzolan. Recently, Connecticut has attempted to increase recycling statewide, specifically that of beverage bottles (and cans). To reduce littering and increase recycling, the state’s legislature introduced the Connecticut Bottle Modernization Bill (Section 8 of PA 21-58) [45]. In 2020, Urban Mining commented on this bill, requesting the Department of Energy and Environmental Protection to make the Beacon Falls facility an important element of the in-state processing option for wine and liquor beverage containers sold in this state [44]. Urban Mining argued the addition of Pozzotive is aligned with the new bill’s objectives of “finding ‘the highest and best use’ for glass, and the addition of Pozzotive to concrete mixes creates stronger and longer lasting concrete for our communities while reducing the carbon footprint of concrete on a nearly ton-for-ton basis” [44]. Urban Mining further explains that the use of Pozzotive ground glass concrete is “five times more impactful in reducing global CO2 emissions than repurposing the glass back into bottles or fiberglass” [44]. Overall, this is promising, and this bill should increase the likelihood of recycling. By keeping the recycled glass in Connecticut and making it easier for people to participate, the amount of glass available will likely increase. Tilcon is renowned as a reliable and long-standing concrete supplier in Connecticut. Comprised of 20 facilities, this company has supplied concrete, aggregates, and asphalt for 100 years, and provides a significant amount of the con-

crete for the Connecticut DOT’s various concrete projects. In 2020, Tilcon’s environmental report recorded 46% of their revenue came from sustainable products, 36.5 million tons of alternative materials and alternative fuels recycled, and 1 million tons of CO2 emissions were prevented [46]. This information is significant and shows what great efforts for the environment have begun to take shape in leading plants, while still providing their high-quality products. CalPortland is one of the largest suppliers on the West Coast with facilities in Oregon, Washington, California, Nevada, and Arizona. This supplier is a leading manufacturer of concrete and has worked to provide reliable and strong blended concretes with a variety of different materials including: limestone, slag, fly ash, and silica fume. Additionally, this company has introduced calcined clay as a natural pozzolan into their concrete mixes. Further, in 2020, this company released a new environmentally friendly ASTM C595 blended cement. This material offers greater Green House Gas and is comprised of a blend of SCMs including limestone, natural pozzolan, or other approved materials [47]. This reinforces the responses provided by Washington’s survey as they commented on the frequent usage of ASTM C595. Between the survey responses and the information provided from CalPortland’s website, it can be argued that ASTM C595 blended cement is a viable option for Connecticut as well. However, this material seems to rely on the addition of limestone. While this is a great solution, it stands to reason that Connecticut can progress further towards sustainable construction on all levels. Cemex, Inc. is headquartered in Houston, Texas and readily serves the surrounding Southern region. This company advertises their sustainable progress in several ways, including safety actions, environment efforts, and their agreement with Climate Action policy. Their site details

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their mixtures of fly ash and slag cement, along with their incorporation of chemical admixtures. However, there is no reported data on the use or incorporation of natural pozzolans or ground glass in their mixture designs. While this supplier is a leading supplier in the South and reinforces the sustainable efforts that other top suppliers are attempting, there was little additional data found.

6. Conclusion

In summary, this study provided further data on the current status of the Departments of Transportation concrete usage, materials used, cost, and use of alternative “green” mixtures. In comparison to the two studies completed in 2014 [23] and 2015 [42], it’s clear how little has changed in the means of concrete manufacturing and how reliant states are on fly ash concrete especially. Nevertheless, by carefully analysing the answered surveys, and through careful investigation of these top suppliers, it can be argued that most states may be open to alternative mixture designs, if they meet the required build specifications, and that states are more concerned with product availability and means. Further, the survey responses received do correlate with the advertised materials from the several concrete manufacturers previously reviewed. This is interesting because, if the larger suppliers market this material more and continue to push it into production on projects, it is more likely that it will be fully accepted by government and state agencies overtime as the material will be less “new”. This survey also expressed the significant amount of fly ash currently used, which raises red flags. This reinforces the concern and critical need for an alternative replacement compound that is fully sustainable on all levels to make its way into heavy highway construction. The time to begin the switch to this new mixture design is now, as the depletion of fly ash only continues with each build and with less availability comes increase in cost. This study leaves room for future studies to attempt interviews with suppliers and investigate pipeline mixture designs as well as marketability for future mixture designs. The authors feel it is important to focus on the economics of alternative concrete use as it would have a major impact encouraging the States to use green concrete more frequently. It is thought provoking to look at the cost of alternative concrete and compare it with traditional concrete, comparing not only its cost of production, but the environmental savings from the recycled material and its lower carbon footprint. Sustainability is a growing area of concern, especially surrounding the concrete industry.

Financial Disclosure

The authors declared that this study has received no financial support.

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