The promise and problem of fly ash
Each year, American power plants and other sources produce approximately 70 million tons of fly ash as a byproduct of burning coal. About 40 percent of all fly ash produced is utilized as concrete additives, flowable fill, or stabilizers for road subgrades. The other 60 percent goes into landfills. It would ease the overall solid waste disposal situation if more fly ash could be put to good use. Furthermore, fly ash has properties that make it attractive for road building. Some types are self-cementing and have significantly higher strength compared to natural aggregate materials. But researchers are careful about using fly ash because it contains environmental contaminants.
At the ninth annual Minnesota Pavement Conference, held February 17 at the University of Minnesota, two such researchers discussed how to use fly ash and how to assess its environmental impact.
What is fly ash?
Tuncer Edil, of the Civil and Environmental Engineering Department at the University of Wisconsin-Madison, began by explaining that, as shown in the table below, there are three major classifications of fly ash.
| Fly ash types per ASTM C618 | ||
| Fly ash designation | CaO (lime) content | Self-cementing? |
| Type F | < 10 percent | No; needs an activator |
| Type C | > 20 percent | Yes |
| Off-specification | Undefined | Yes |
Stabilizing subgrade soil with fly ash
Edil then focused on how fly ash can be used to stabilize soft subgrade soils. He said soft subgrade is a major problem in the Midwest; for example, he estimated that about 60 percent of Wisconsin has poor quality subgrade soil. The usual practice is to excavate subgrade soil and replace it with "breaker run" aggregate, but he pointed out that this is expensive, time consuming, and uses natural materials.
Waseca demonstration project
Edil described a pavement reclamation project where fly ash was used to stabilize sub-base material. The project, undertaken in 2004, involved a section of pavement approximately one-third of a mile long at the intersection of 7th Street and 7th Avenue in Waseca, Minnesota. The construction sequence was as follows:
- Mill and mix the top 9 inches of existing material (various layer thicknesses were encountered in different places).
- Remove top 3 inches of the recycled pavement material (RPM).
- Mix Class C fly ash with RPM.
- Compact mixed material to form stabilized RPM base layer.
- Place new hot-mix asphalt wearing course.
Testing
To monitor the Waseca project:
- Samples were collected of the existing subgrade, the milled material, and the mixture of milled material with fly ash.
- Four nondestructive field tests were conducted: soil stiffness gage (SSG), dynamic cone penetrometer (DCP), rolling weight deflectometer (RWD) and falling weight deflectometer (FWD).
- The collected materials were tested in the laboratory to determine California bearing ratio (CBR), resilient modulus (MR), and compressive strength (qu).
- To analyze leachate, water samples were collected by a lysimeter placed in the subgrade soil during construction.
- Instruments were placed to automatically measure soil moisture and temperature, rainfall, air temperature, and humidity at four-hour intervals and send information to a server at UW-Madison.
Early test results
Initial mechanical results are impressive:
- SSG: Stabilized material is three times stiffer on average than previous material.
- DCP: Stabilized material has on average 30 percent lower dynamic penetration index than previous material.
- CBR of stabilized material: 10 to 53 percent
- MR of stabilized material: 50 to 110 Mpa
- qu of stabilized material: 134 to 198 kPa
In fact, these properties led Edil to speculate on whether the thickness of the asphalt wearing course could have been reduced because of the stiffness and strength of the underlying fly-ash-stabilized material.
Initial results of leaching tests were not available from this site, but results from a similar stabilization project were encouraging. Concentrations of contaminants in field-gathered leachate were typically 1.5 to 2.5 times lower than concentrations in fly ash alone. Furthermore, concentrations of contaminants found in field samples were similar or up to four times lower than the concentrations obtained in column leaching tests performed on the same materials in the laboratory.
New fly ash environmental screening tool
In the same session of the pavement conference, Paul Bloom, of the Department of Soil, Water, and Climate at the University of Minnesota, presented a new computer-based tool that screens fly ash for environmental contaminants.
At the outset of his presentation, Bloom posed the essential problem: "Fly ash has some elements at concentrations that are elevated compared to what you might find in normal soil… So how much can we add and still meet environmental guidelines?"
In response to this question, Bloom and his associates have developed a screening tool called STUWMPP (Screening Tool for Using Waste Materials in Paving Projects) (650 KB PDF). STUWMPP testing is based on concentration limits defined by the Minnesota Pollution Control Agency (MPCA) for 21 potentially harmful contaminants.
STUWMPP testing is based on the legal concept of due diligence, which Bloom defined as "the diligence exercised by a person who seeks to satisfy a legal requirement or obligation. What it means is you do your darnedest to meet the guidelines of the state."
Other basic assumptions
Screening using STUWMPP is based on several assumptions:
- Maximum concentration levels are defined for both residential and industrial applications since inhabitants of a residential area—notably children—might be in the area 24 hours a day, while exposure in industrial areas is likely to be limited to 8 to 10 hours per day.
- Harm may come either directly from the soil used on a site (through ingestion) or from water that has leached contaminants from soil. Therefore, STUWMPP contains two screening levels, called the Soil Reference Value (SRV) and the Soil Leaching Value (SLV); both levels must be passed.
- SRV—the first tier of testing—assumes a worst-case scenario: that a road constructed with fly ash in the subgrade has been abandoned at some time in the past, and that the subgrade soil is then recovered and spread on the surface of land where housing is built. Therefore, SRV uses risk analysis to define soil concentrations that would protect human health when soil has been ingested.
- SLV—the second tier of testing—contains standards for drinking water and mitigating factors that take into account rainfall, soil type, and distance to the water table.
Bloom demonstrated how to use STUWMPP. The operator can select contaminant data sets obtained for common fly ash sources in the Midwest, such as power plants and factories, or plug in individual contaminant values from direct testing of site-specific fly ash and soil samples. If the latter is to be done, STUWMPP guidelines suggest:
- Taking fly ash samples from each truckload.
- Taking soil samples from the strata that will be mixed with the fly ash.
- Sampling from every soil-mapping unit, a minimum of every 1,320 feet.
Bloom said STUWMPP is available for use by any public agency and that he and his associates are in the process of obtaining additional standard contaminant values for Midwestern sources.
—Richard Kronick, LTAP freelance writer