MPC
Research Projects (2008-09)

Identifying Number

MPC-301

Project Title

Sustainable Concretes for Transportation Infrastructure, Year 1

University

Colorado State University

Project Investigator

Rebecca A. Atadero

Description of Project Abstract

Concrete is a material used to construct a large and varied portion of transportation infrastructure. Common applications of concrete in the transportation sector include pavements, bridge structures, vehicle barriers, and foundations. Currently, interest is growing in the application of sustainable construction practices as more and more states, communities, and individual members of society recognize the global impact of human activities. Sustainability is often thought of in the context of merely environmental impact, but social and economic considerations also contribute to the sustainability of a particular construction project. From an economic standpoint, concrete can be a sustainable choice, for example in the case of pavements where well-constructed concrete pavements are much more durable than their asphalt counterparts, lowering life-cycle costs. However, from an environmental and energy standpoint present concrete formulations leave much to be desired in terms of sustainability. The key binding ingredient in most concretes is Portland cement. The process used to manufacture Portland cement requires high temperatures, making it very energy intensive. Furthermore, CO2 is released as part of the chemical process that occurs during manufacture. As a result of the energy demand and the chemical process, the ratio between tons of cement produced and tons of CO2 emitted is close to 1 to 1 for manufacturing in the United States [1]. While the sheer volume of concrete used in the transportation sector represents a tremendous amount of energy usage and CO2 emissions, that same large volume means that even small changes to improve the sustainability characteristics of concrete can have a large impact on reducing energy consumption and carbon emissions.

One means to improve the sustainability of concrete that has been proposed and studied to some extent is to replace large percentages of the Portland cement with fly ash. Fly ash is a byproduct of coal powered energy generation. Different types of coal have different types of impurities, resulting in ashes composed of different primary components. Based on the chemical composition of the ash ASTM specifies two types of fly ash for use in concrete, Class F and Class C [2]. Fly ash in concrete is generally characterized as a pozzolan, a finely divided material that reacts with the products of the cement hydration reaction to form further cementing compounds. However, Class C fly ashes have a high calcium oxide content and display self-cementing properties when mixed with water. An existing research project at CSU has studied the application of high volume fly ash concretes reinforced with small polymer fibers resulting from tire recycling operations to the manufacture of building products (roof tiles). The study initially considered products made purely with Class C fly ash with and without the recycled fibers, and has recently studied the addition of small percentages of Portland cement (10-20%) to enhance the properties of the resulting material through activation of the fly ash. Some promising results have been observed with this material. Mixes with Class C fly ash as the only binding agent and a water to ash ratio of 0.40 had 28 day strengths of 10 MPa w52ith fibers and 8 MPa without fibers, while a mix with no fibers, but 20% Portland cement had a 28 day strength over 35 MPa. The addition of fibers contributes significantly to the toughness of the mixes. Flexural specimens cast with fibers demonstrated a toughness six times greater than specimens with no fibers included.

The large volume fly ash mixes studied at CSU are unique in that the typical approach has been start with a concrete mix based entirely on Portland cement and see how much fly ash can be added without substantially reducing the strength. At CSU we have started with 100% fly ash and are now identifying what combination of fibers, cement and possibly other compounds can be added to achieve necessary strengths. The material under development has the potential to offer a sustainable alternative to traditional concretes based primarily on Portland cement as the binding agent. Environmental benefits include 1) the beneficial use of fly ash, which is often stockpiled or land filled 2) the beneficial use of recycled tire fibers, and 3) the reduction of energy used to manufacture Portland cement. In addition to environmental benefits, the utilization of waste materials also has the potential to lower costs. While initial results with the fly ash based materials have been promising, this material is not yet ready for application, even on a demonstration basis. Significant further development is required.

Project Objective

The overall objective of this proposed research is to use Class C fly ash and building on preliminary results to develop a viable and sustainable alternative to Portland cement based concrete for use in pavements, transportation structures, barriers and foundations. In order to reach this objective several sub objectives have been identified:

1. Characterize and improve the workability of the mix – the current research has identified the clear benefit of lower water to fly ash ratios on the strength of the resulting mix. In order to achieve these low ratios while manufacturing a product that can be reliability applied in the field water-reducing admixtures (superplasticizers) are necessary and the effect of these compounds on fly ash based concretes must be investigated.
2. Utilize large aggregates – current research efforts have focused on the applicability of the potential mix to the manufacture of building products, for example roofing tiles. For this reason only sand has been used as aggregate and testing has been conducted on small cube specimens. For applicability as a structural material the behavior of the mix utilizing larger aggregates and tested using traditional concrete cylinders is necessary.
3. Optimize Portland cement content to meet strength requirements – Depending on the anticipated application it may be necessary to increase the strength of the proposed concrete alternative. The required strength gain will be achieved with the use of Portland cement as an activator.
4. Investigate potential durability – For any concrete mix to perform successfully it must be durable. To date no durability tests have been conducted on the fly ash based material being studied. Although long term testing is not possible during the time frame of this specific proposal, short term measures can be used to assess the potential for durable behavior. Durability promoting admixtures (such as air entrainers) must also be considered.
5. Investigate the compatibility of the mix with steel reinforcement – As concrete is rarely used without steel reinforcement, it is necessary to ensure that the alternative concrete developed by this project works with reinforcement in ways similar to existing Portland cement based concrete mixes.

By meeting these five sub-objectives a viable alternative to existing concrete mixes will be developed. The utilization of recycled fibers will also contribute to the properties necessary for some applications (pavements and barriers), in particular by enhancing the deformation capacity of the hardened mixes.

Project Approach/Methods

Development of a sustainable alternative to Portland cement based concretes will primarily be conducted through laboratory study. The key tasks of this project are described below. A tentative calendar of events is provided following the descriptions.

• Task 1 – The first task will consist of a parametric study to consider the effect of water reducing admixtures, larger aggregate sizes and Portland cement content on the properties of fresh and hardened concrete with and without the recycled fibers. A series of different mixes incorporating these different variables will be developed. 15 cylinders of each mix will be cast allowing for compressive testing at increments of 7, 14, 21, 28 and 56 days. Nine flexure specimens will be cast for each mix allowing testing at 14, 28 and 56 days. During mixing and casting properties of the fresh concrete will also be measured. Months 1 and 2 will be dedicated to gathering materials, developing the parametric study including mix design and casting of specimens. Months 3 and 4 will be occupied in testing of the various mixes and analysis of results. At the end of month 4, several of the best performing combinations will be selected for further study in Tasks 2 and 3.
• Task 2 – In the second task durability of the mixes identified as likely candidates in Task 1 will be considered. The first step taken in Task 2 will be to conduct a literature review identifying key factors affecting the durability of concrete. Currently, the effect of sulfur is of particular interest because the fly ash available at the local power plant has slightly elevated sulfur content due to scrubbing processes in use at the plant. Following this review, specimens will be cast for durability testing incorporating different amounts of air entraining admixtures. Testing will be conducted to evaluate the freeze-thaw performance of the fly ash based concrete. The literature review portion will be conducted during months 3 and 4 while testing for task 1 is ongoing. New specimens for durability tests will be constructed during month 5 with exposure and testing occurring during months 6, 7 and 8.
• Task 3 – The third task will study the performance of this concrete alternative with steel reinforcement. Testing will primarily be conducted to determine bond length. Small reinforced members may be constructed to evaluate overall structural performance. Although it will not be possible to consider corrosion in detail, a permeability study may be conducted to evaluate the potential for chloride ion transport. Specimens for testing with reinforcement will be cast during months 8 and 9 with testing conducted during months 10 and 11.
• Task 4 – The final task will consist of writing the final MPC Technical Report. Although this work will be ongoing throughout the project, the report will primarily be written during months 11 and 12.

MPC Critical Issues Addressed by the Research

17 Environmental Impacts of Infrastructure This project has the potential to reduce the environmental impact of new construction by using a material that is primarily composed of byproducts or recycled materials (in the form of recycled aggregates and recycled tire fibers).

Contributions/Potential Applications Research

As discussed in the introduction, large volumes of concrete are used each year for a variety of purposes in the construction and maintenance of transportation infrastructure. Thus a product that can replace traditional Portland cement based concrete will have many potential areas of application. Efforts will be made to reach fairly high strengths while still using Class C fly ash as the primary binding agent, there are also numerous lower strength applications for which this material may be appropriate. Within its described scope this project will consider and seek to address most of the remaining concerns for the use of a Class C fly ash based concrete and following this project this material will be ready for use on a demonstration basis.

Technology Transfer Benefits

The transfer of the technology developed in this project is to be implemented by state DOTs will be facilitated by working to develop a mix that meets existing performance based provisions of DOT concrete specifications. The current Colorado DOT specification includes both prescriptive and performance based requirements, but a material that can meet the performance requirements will at least merit a second look. Furthermore, this material will be applicable as a concrete alternative in projects beyond just transportation. The power plant that will be supplying the fly ash for this project is very interested in potential beneficial uses of their fly ash, and they will be eager to see the results of this project as part of their efforts to sell the ash. These sales efforts will provide for technology transfer beyond just the transportation community.

Time Duration

July 1, 2008 - June 30, 2009

Total Project Cost

$41,613

MPC Funds Requested

$18,697

TRB Keywords

Fly ash, pavements, sustainable development

References

1. Hanle, L.J., Jayaraman, K.R., and J.S. Smith (2004). "CO2 Emissions Profile of the U.S. Cement Industry," 13th International Emission Inventory Conference, Clearwater, FL, June 8 - 10, 2004. http://www.epa.gov/ttn/chief/conference/ei13/ghg/hanle.pdf
2. ASTM International (2008). C 618 Standard Specification for Coal Fly Ash and Raw or Calcined Natural Pozzolan for Use in Concrete. West Conshohocken, PA: ASTM International.