Four years ago, you had to look long and hard to find any coal combustion products (CCPs) being used for roadbed stabilization projects in the Texas Panhandle. There were plenty of opportunities, since much of the road-base material in the area requires subgrade treatment to overcome relatively high plasticity of the soil. But at the time, engineers in the area specified lime for nearly all of these projects.
It was a bit of a mystery. Contractors in most parts of the country have been using CCPs successfully for years for soil stabilization. Every year in the U.S., more than 1.5 million tons of CCPs—primarily fly ash—are used in road-base projects, according to the American Coal Ash Association. In addition, CCPs deliver significant environmental and economic benefits. The U.S. Environmental Protection Agency promotes their use to reduce greenhouse gases, and they are inexpensive and widely available.
Xcel Energy’s Harrington and Tolk coal-fired power plants, located in the Amarillo, Texas, area, produce abundant supplies of Class C fly ash. Lafarge North America and Xcel market the material in the Amarillo area, and road-base stabilization projects appeared to be an ideal application. In this region, lime has to be trucked in from hundreds of miles away. CCPs offered a high-quality, locally sourced alternative, while reducing the considerable environmental impact of mining, processing and transporting lime. Why, then, were highway engineers and contractors in the area so reluctant to use CCPs?
Detective work
As part of efforts to expand utilization of Xcel’s CCPs, the supplier took a closer look at why fly ash had earned such a bad reputation in the area. The problem did not appear to be with the material itself. Quality control was excellent and the material met all applicable specifications for road-stabilization work, including the ASTM C-618 Standard.
Yet four of the Texas Department of Transportation engineers in the area said that when they had specified fly ash in the past, they had experienced problems during construction, specifically regarding compaction and prime oil penetration.
These types of problems are usually related to improper construction methods. In February 2002, a geotechnical forum was conducted for project specifiers and paving contractors. Engineers with experience using fly ash in other regions demonstrated the excellent durability and performance they had achieved, and they reviewed the proper construction methods required with fly ash.
At the forum, contractors were asked how they prepared the subgrade and base material. They sent out separate crews for each part of the process. One crew prepared the road base, often working on a stretch of several miles at a time. Later, another arrived and compacted the base. After that, the paving crew arrived to place the asphalt pavement.
This approach works well for lime, but is inappropriate for CCPs. Fly ash is a cementitious material. It reacts with water, triggering a chemical reaction that results in chemical bonding within the soil-ash mixture. This cement-like property provides strength and stability. But within a few hours the soil-ash mixture begins to stiffen, making it harder to compact and breaking down the bonding of the ash.
With CCPs, the mix should be compacted within two hours after it is placed. But because road crews were not coordinating their schedules, the roadbeds sat for as long as several days before the second crew came out to finish them.
It was learned that a similar situation was occurring with the prime oil. The prime oil should be applied within 24 hours after the roadbed is placed. Delayed application can interfere with bonding between the treated base layer and the pavement, resulting in later delamination, shoving and rutting. However, several days or weeks often went by before the priming crew arrived at the site.
For fly ash, the preferred method is to run a continuous paving operation, with crews working together as a “train.” After a trimmer removes the existing asphalt, a mixer cuts into the old base, mixing the material with fly ash and water. A compactor follows immediately after, ensuring that the material is fully compacted before the fly ash has begun to set. Following compaction, the base material is cut to final grade with the use of a trimmer and finish blading. The prime oil is applied next before the compacted material becomes too dry. Essentially, the road base is built in a single continuous operation.
1,500 ft of convincing
It was clear that many of the problems encountered in past projects had been caused by delays in compaction and application of the prime oil. It was believed that CCPs would perform well if contractors were willing to change their construction practices. They resisted the idea at first. Technical issues aside, many were concerned that a continuous processing operation would create scheduling problems and costly downtime for crews. They believed that it was more efficient, from a manpower standpoint, to have each crew working at its own pace.
A 1,500-ft test pavement on a local highway reconstruction project offered an opportunity to conduct a direct comparison between fly ash and lime. It also offered a chance for contractors to try the new approach and judge for themselves whether it would create excessive downtime.
In spring 2002, Xcel and Lafarge demonstrated fly ash alongside other materials (including lime and oil emulsions) on the trial project on Highway FM 1062 in Randall County, Texas. Lafarge personnel worked closely with the area engineering office to conduct the test and help train engineers, inspectors and contractors.
The paving contractor used approximately 200 tons of CCP material to stabilize the road base. The specification called for compaction no more than two hours after water had been introduced to the mixture.
The contractor was, frankly, skeptical that the specification could be met without significantly slowing up the work. In fact, the opposite occurred. With the old construction method, crews were typically processing six to eight loads of material a day. The contractor found that the increased attention to scheduling allowed crews to work much more efficiently. With the new approach, productivity more than doubled, with crews handling more than 20 loads a day.
The method turned out to be highly efficient for a number of reasons. When lime is used, the subgrade requires a “mellowing period” of 48-72 hours after it is compacted. Then it has to be reprocessed. So crews have to make more passes and have to wait longer between passes. As a result, they spend more time shuttling between worksites or waiting for equipment to arrive.
With the “train” method, all of the equipment arrives on the site and is put into operation almost immediately. Like an assembly line, there is little wasted motion and downtime. The result is that a crew can build the entire road—from subgrade to prime layer—in nearly the same time that it once took to process the subgrade.
Follow-up tests demonstrated the strength and durability of the fly ash roadbed, with results equaling or exceeding those seen with lime-stabilized soil.
These results, along with the improved efficiency and lower construction costs, made a compelling case for CCPs. Engineers and contractors were quick to embrace the material and the new work methods. In three years, local market share for CCPs in geotechnical applications increased from 20% to 90%.
This growth yielded significant environmental benefits. By substituting for processed lime, the CCPs reduced annual greenhouse emissions by at least 1,350 metric ton carbon equivalents in 2003 and diverted approximately 50,000 tons of material from landfills.
