MPC
Research Projects (2009-10)

Identifying Number

MPC-301

Project Title

Sustainable Concretes for Transportation Infrastructure, Year 2

University

Colorado State University

Project Investigator

Rebecca A. Atadero

Description of Project Abstract

Large quantities of concrete are used in the construction of transportation infrastructure elements such as roads, bridges, barriers and foundations. Most concrete formulations rely on Portland cement as the primary binding agent, a material whose manufacture is associated with significant environmental impacts. This proposal describes the continuation of a project investigating the development of sustainable concretes based on the use of high-lime fly ash as a primary binding agent. The current MPC project at CSU has prepared and tested concrete mixes with fly ash contents ranging from 0-50%. Testing at ages of 3, 7, 14, 28 and 56 days has indicated that mixes with higher fly ash contents do gain strength more slowly than mixes with relatively less fly ash, but by 14 days the mixes with higher fly ash quantities have nearly caught up and there is little difference at and beyond 28 days. Peak strengths were obtained from mixes with fly ash contents in the range of 30-35%, but the mixes with 50% fly ash were found to be comparable to mixes with no fly ash added, and indicate that a significant savings in Portland cement usage is possible. The focus of the research has turned toward durability in the spring semester, with a focus on resistance to freeze-thaw cycling. Numerous sample batches have been made in order to determine the required amount of air entraining admixture needed to achieve recommended air contents. Relatively greater dosages are required as the fly ash content is increased, due to the fine particle size of the fly ash. Currently preparations are almost complete for the beginning of freeze/thaw testing of rectangular prism specimens, and the specimens should be placed in the freezer by mid April. A small series of tests considering the interaction of the fly ash concrete and steel reinforcement will be initiated over the summer.

A continuation is requested for this project in order to allow for further improvement and testing of the concrete mix. Materials research is an iterative process where results from one stage of research are used to make decisions about the next stage. However, the nature of concrete, specifically the need to provide for curing times of at least seven days although ideally 28 days, makes it difficult to try many combinations of materials in short time frames. Through parametric studies, such as the work conducted in the fall with varying fly ash percentages, we can start to get an idea of which mixes might be most viable for further development, but in a single year project there simply is not much time for further refinement before moving on to other types of testing. In the continuation project we will focus on improving the concrete formulation by experimenting with additional admixtures, considering mixes with even higher fly ash contents, and continuing testing of durability and steel/concrete interaction with the new mixes developed.

Project Objectives

The overall objective of this continuation proposal remains the same as the proposed research from last year: to use high-lime fly ash 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 were identified last year, work in the continuation project will build on these sub-objectives.

1. Characterize and enhance the workability of the mix - Low water/cement ratios are known to enhance the strength of Portland cement based concretes and the same is true of concretes with large proportions of fly ash. Last year's proposal indicated that we would consider the effect of water reducing admixtures on the properties of our fly ash concretes. However, as research progressed, we were able to manufacture mixes with reasonable strengths (over 5 ksi) and degrees of workability without the use of these admixtures. It also became apparent that any improvement that might result from the use of these admixtures would likely be most significant to the durability of the mix. For these reasons we decided to postpone the consideration of water-reducing admixtures until after obtaining at least preliminary durability results. Thus this objective is still significant to the continuation project.
2. Utilize Large Aggregates - This sub-objective was based on the fact that early fly ash research at CSU had considered only pastes and mortars without the use of larger aggregates. All of the concrete mixes manufactured for the current MPC project have used 3/4" crushed stone as large aggregate. These mixes have performed well, and this aggregate will be used for all research in the continuation project.
3. Optimize Portland cement content to meet strength requirements - In the first year of this project we considered fly ash contents up to 50%. This decision was made based on the desire to develop a structural grade concrete product in a limited time frame. In the continuation phase we will consider mixes with higher fly ash contents, which with the proper admixtures may be appropriate for structural uses or would likely be suitable for less demanding applications.
4. Investigate potential durability - The acceptance of any concrete developed by this project will depend on the demonstrated durability. A significant effort during the current project has gone into preparing equipment and procedures for freeze-thaw testing of the concrete mixes. While these efforts were necessary, they have occupied a good portion of project time, and it is likely that only one or perhaps two complete rounds of durability tests (consisting of approximately 300 freeze-thaw cycles) will be completed by the end of this project. This will give us information on the potential strengths and weaknesses of our mix, and in the continuation phase we will be able to formulate and test new mixes that provide for enhanced durability.
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. Preliminary testing for this sub-objective is currently planned for the summer when students have a greater availability for work at the ERC. This testing will continue during the continuation project as new mixes are developed and their properties must be determined.

By advancing on the results obtained during the current MPC project with respect to these sub-objectives, a viable alternative to existing concrete mixes will be developed.

Project Approach/Methods

The continuation phase of this project will rely heavily on laboratory study. The key tasks to be achieved in the coming year are described below.

Task 1 - Assessment of Existing Results and Development of Testing Plan - A vital task of the continuation project will be to evaluate the results collected during the first year of the project and to determine the most important directions for new research. This planning phase is particularly important to the continuation project because during the first year many of the necessary mixing and testing techniques were established through a time-consuming, trial and error process. As a result we now have a collection of working techniques and through proper planning we can have several different series of tests proceeding simultaneously, a huge advantage when considering the required curing times for concrete materials.

Because the final project plan for the continuation phase will not be developed until the first year of the project is concluded, the remaining tasks will be subject to modification. However, based on the current state of research, several areas of study can be identified.

Task 2 - Evaluation of Potential Admixtures - Many modern concrete mixes rely on the addition of admixtures. The only admixture considered in the first year of the study has been air entraining agents. As results from the initial durability tests come back at the end of the current project, we will be better able to evaluate the limitations of our current mixes, and during the continuation phase we will seek to improve the mix with the addition of admixtures. Water reducing agents are the most likely for consideration, but if other admixtures are identified that offer potential improvements these products will also be tested. Testing will consider different dosages of selected admixtures, and will be conducted over time providing results on the rate of strength gain as well as the "final" strengths.

Task 3 - Testing of Mixes with Fly Ash Contents over 50% - The work conducted during the first year of this project indicates that mixes with up to 50% fly ash can achieve strengths that would make them suitable for many structural purposes. In order to further reduce the amount of Portland cement we will consider mixes with even higher fly ash contents, most likely in conjunction with admixtures. Our goal initially will be to create structural grade mixes, but we will also consider these mixes for applications with lower strength requirements if necessary.

Task 4 - Continuing Assessment of Durability - The number of cycles required for a complete freeze-thaw test makes these tests time intensive, and will likely prevent us from testing various means of improving the freeze/thaw performance during the first year of the project. During the continuation phase new mix designs will be developed in order to enhance the freeze-thaw performance and I anticipate nearly continuous freeze-thaw testing of various mixes during the course of the continuation project. The continuation project will also give us the opportunity to consider, at least in a preliminary fashion, other factors that can affect the long term performance of concretes.

Task 5 - Interaction with Steel Reinforcement - Testing of the behavior of our fly ash concrete mixes in conjunction with steel reinforcement during the first year of this project will necessarily occur with preliminary mix designs. While these tests may indicate areas for improvement they will primarily be useful in developing the required test procedures. As adjustments to the mix are made during the continuation project continued testing of steel/concrete interaction will be necessary. Bond testing, and a limited study of corrosion potential within the various concrete mixes are studies of particular interest.

Task 6 - Reporting - The final task involved in this project will be producing the final technical report following the required MPC format.

Scheduling
Because techniques for testing many of these characteristics were developed during the first year, I anticipate that testing during this continuation phase will not occur in a linear fashion. Task 1 will occur during the first month of the project. The remaining research tasks (2-5) will be addressed simultaneously with a research plan that prevents long down times due to curing.

MPC Critical Issues Addressed by the Research

USDOT Strategic Goal (3) Infrastructure that Minimizes Environmental Impacts. This project has the potential to reduce the environmental impact of new construction by using a material that uses an existing by product as a key ingredient and which has a reduced carbon footprint when compared to traditional materials.

Contributions/Potential Applications of Research

Clearly, 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. The results presently available from the current project indicate that initial strengths of this material are adequate for many purposes, and are likely to be improved with the addition of admixtures. By providing enhanced durability results, this continuation project will help to verify the suitability of this material, and following this project the material would be appropriate for use on a demonstration basis.

Technology Transfer Activities

The transfer of the technology developed in this project to implementation 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, 2009 through June 30, 2010

Total Project Cost

$40,666.00

MPC Funds Requested

$18,475.00

TRB Keywords

Fly ash, pavements, sustainable development

References

• 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.