Demonstration of Ash Utilization 

in Low Volume Roads

What Need Did We See?

When doing construction on a road that is not expected to have a lot of traffic, it can 

be very expensive to haul away poor-quality soil or existing road material and replace it 

with new, more stable base materials. Using poor base materials, however, can result in a 

bumpy road with a shortened pavement life. 

An alternative is to stabilize the existing base materials using fly ash, a by-product of 

coal-fired power plants that is normally landfilled. Fly ash is self-cementing: It chemically 

reacts with soil (or recycled pavement material, or road surface gravel) and water to 

form a strong and stable base for hot-mix asphalt pavement. This can increase pavement 

life, permit use of a thinner HMA layer, and improve ride quality.

Pavement reconstruction using fly ash to stabilize recycled materials costs considerably 

less than traditional total reconstruction; some estimates of this savings run as high as 

66 percent. While fly ash has been used to strengthen soils for more than 20 years, its 

long-term effectiveness in road reconstruction has previously been largely undocumented in Minnesota. There are also environmental concerns about its use, and Mn/DOT has 

not yet established design guidelines for fly ash stabilization. A systematic demonstration 

and implementation effort is needed to achieve widespread acceptance. 

What Was Our Goal?

This study was undertaken to assess the effectiveness and environmental suitability of 

using fly ash to stabilize Minnesota’s local roads. Through field demonstrations, investigators aimed to provide documentation for the effectiveness of fly ash as a stabilizing agent 

for recycled pavement material and for road surface gravel.

What Did We Do?

The project involved two field demonstrations:

• Reconstruction of a city street in Waseca. Crews pulverized 0.5 km (0.3 miles) of 

existing asphalt, gravel and soil to a depth of 300 mm, spread fly ash (10 percent by 

dry weight) evenly over the surface, then mixed the ash with water and the reclaimed 

pavement materials. This mixture was compacted and covered with 75 mm of HMA.

• Construction of a flexible pavement on a gravel country road in Chisago County.

Fly ash was spread in strips over 3.5 km (2.2 miles) of existing gravel, mixed into 

254 mm of the gravel layer with water, compacted, and then covered with two layers 

of HMA.

Researchers took samples of the subgrade both before and after the fly ash was mixed in, 

and created additional mixtures in the laboratory of fly ash and the base materials. They 

used numerous tests to determine the strength and stiffness of the pavement, and to see 

how it reacted to freeze-thaw cycles (which also involved installing weather and other 

sensors at the sites). Laboratory tests included resilient modulus and California bearing 

ratio measurements, and field activities included soil stiffness, soil penetration and falling 

weight deflectometer tests.

Investigators installed a pan lysimeter beneath the pavement at both sites to monitor 

moisture movement through the pavement. They tested the liquid for trace element 

concentrations, and performed additional leaching tests for comparison on the mixtures 

created in the laboratory.

A water truck and road-reclaimer 

blended the ash, water and

reclaimed pavement materials on

7th Avenue in Waseca.

2007-12TS

Published 3-24-08

continued

Project cost:

$212,995

Technical

Summary

Technical Liaison: 

John Siekmeier, Mn/DOT

john.siekmeier@dot.state.mn.us

Administrative Liaison: 

Alan Rindels, Mn/DOT

alan.rindels@dot.state.mn.us

Principal Investigator: 

Tuncer Edil, University of Wisconsin

Research

S e r v i c e s s e c t i o n
What Did We Learn?

These tests showed that adding fly ash to either reclaimed pavement materials or road 

surface gravel significantly improved stiffness and strength. The resilient modulus at the 

end of construction was about 1.7 to 3 times higher for the ash-treated material than for 

the untreated material. This performance did not degrade over a single winter season.

There is concern that repeated freeze/thaw cycles may reduce resilience: laboratory 

tests reduced performance by 50 percent. However, this is still an improvement over the 

results of the same testing on the untreated recycled pavement material and road surface 

gravel.

The chemical analysis of the leached moisture showed that none of the trace elements 

normally associated with health risks exceeded the U.S. Environmental Protection Agency’s maximum contaminant levels or the health risk levels established by the Minnesota 

Department of Health.

This project also served to refine testing procedures; this will streamline subsequent 

studies, which are needed to further develop these techniques.

What’s Next?

This project is a step toward the broader goal of increasing the acceptability of fly ash 

use in Minnesota’s low-volume roads. Engineers will need a framework of design procedures, construction guidelines, and source and availability characterizations to use when 

incorporating fly ash stabilization into various pavement systems.

Investigators are continuing to monitor these demonstration sites for freeze-thaw performance and trace element levels. LRRB has funded a second project to quantify the 

benefits of pavement design using fly ash: Does fly ash allow the HMA layer thickness to 

be reduced while retaining the same pavement life? 

Additional Mn/DOT and national research efforts are aimed at more comprehensively 

establishing the environmental impact of fly ash use. Mn/DOT is currently implementing statewide a screening tool for hazard evaluation prior to the use of new types of 

recycled materials, and a follow-up study on the leached moisture from fly-ash-stabilized 

roadbeds that have been in place for 10 or 20 years has been proposed to establish longterm impacts.

“We demonstrated that 

you can create a satisfactory base for asphalt 

pavement using in situ 

recycled material with 

fly ash stabilization. This 

provides a cost-effective 

option for reconstruction 

projects.”

–Tuncer Edil,

Professor, Civil and Environmental Engineering, 

University of Wisconsin

Produced by CTC & Associates for: 

Minnesota Department 

of Transportation

Research Services Section

MS 330, First Floor

395 John Ireland Blvd.

St. Paul, MN 55155-1899

(651) 366-3780 

www.research.dot.state.mn.us

This Technical Summary pertains to the LRRB-produced Report 2007-12, “Demonstration of Ash Utilization in Low Volume Roads,” published May 2007. The full report can be accessed at 

http://www.lrrb.org/PDF/200712.pdf.

Researchers installed water content and temperature sensors to monitor the subgrade and road surface gravel on the newly paved County 

Road 53 in Chisago County for moisture and trace element levels.

“Fly ash is particularly 

useful for city and county 

roads that have lower 

traffic volumes, since 

agencies need to use 

locally available materials 

and keep costs down.”

–John Siekmeier,

Mn/DOT Senior Research 

Engineer