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Publication Number: FHWA-RD-97-148
User Guidelines for Waste and Byproduct Materials in Pavement Construction
Bottom ash and/or boiler slag can be used as either the fine aggregate fraction or, in some cases, as the entire aggregate in either Portland cement or pozzolan-stabilized base and subbase mixtures. A blend of bottom ash and boiler slag may comprise the entire aggregate portion of the mix if both materials are available. If only bottom ash is available, it may be used as the entire source of aggregate, or it may be blended with a coarse aggregate to meet a specified range of gradation. If only boiler slag is available, it must be blended with sand or other well-graded fine aggregate to produce an aggregate with a suitable particle size distribution. If a broader range of particle sizes is specified, further blending with a coarse aggregate may also be necessary.
Bottom ash and, in particular, boiler slag have been used as aggregate sources in stabilized base or subbase applications since as far back as 40 years ago. Most of these installations have not been well documented, but their service record is believed to have been from fair to very good.
A recent survey reported that in 1996, 0.6 million metric tons (0.7 million tons) of bottom ash and/or boiler slag (predominantly bottom ash) were used as road base or subbase materials. The category for road base or subbase includes stabilized base or subbase, as well as granular or unbound base or subbase installations. The exact percentage used in stabilized base applications was not reported.
According to a 1992 survey of all state highway and transportation agencies, at least five states indicated that they were currently making use of bottom ash or boiler slag in some type of stabilized base or subbase applications. These states include Arkansas, Kentucky, Mississippi, Texas, and Utah. A sixth state, Wyoming, indicated some use of bottom ash as a granular base, but cited instability of the material as the reason for discontinuing the use of bottom ash. Although the nature of the instability was not explained, it is believed to be due to a lack of cohesion in the base, possibly because of the material becoming too dry.
Bottom ash and boiler slag have been used in the past as an aggregate for stabilized base and subbase mixtures in other states, although not necessarily on state highway projects. These states include, but are not limited to, Georgia, Illinois, Michigan, North Dakota, Ohio, and West Virginia. There are currently no state specifications for the use of bottom ash or boiler slag as an aggregate in stabilized base or subbase mixtures. Table 4-8 presents a listing of pertinent data on some selected applications.
Table 4-8. Pozzolanic stabilized base general design and construction data.
Pozzolan-stabilized base compositions consisting of lime, fly ash, and aggregates (LFA) were originally patented in the early 1950's under the trade name Poz-O-Pac. Some of the first LFA compositions in the Chicago area were mixed in place and used boiler slag as the aggregate. These early mixtures contained an average of 5 percent by weight hydrated lime, 35 percent Class F fly ash, and 60 percent boiler slag. Pavements using such mixtures provided many years of satisfactory service, and cores taken from these pavements have developed compressive strengths well in excess of 6900 kPa (1,000 lb/in2)
Typical of these early boiler slag mixes was a 5.6 km (3.5 mile) service road built from State Route 195 to the Coffeen power station near Coffeen, Illinois, during the mid-1970’s. General design and construction data are presented in Table 4-8. The pavement reportedly performed without distress, even though the roadway was constantly subjected to heavy truck traffic.
The first known large-scale use of a cement stabilized bottom ash base course in the United States was in the relocation of West Virginia Route 2 during the 1971-72 construction season. The aggregate used was a blend of bottom ash and boiler slag from American Electric Power Company’s Mitchell and Kammer plants, respectively. General design and construction data are presented in Table 4-8. The blend was necessary in order to meet the West Virginia Department of Highway gradation specifications for cement-treated base course. The roadway provided excellent service for over 10 years at a substantial reduction in cost compared with the use of conventional aggregates.
Since 1984 several hundred miles of low-volume secondary roads in West Virginia have been reconstructed using cement-stabilized bottom ash. Most of these roads were primarily gravel subbase with traffic ranging from 150 to 1,500 vehicles per day. A typical section, presented in Table 4-8, is Route 34 in Putnam County, near Charleston, where a 150 mm (6 in) thick bottom ash subbase was placed and compacted. Successive 150 mm (6 in) thick lifts of cement-treated bottom ash were placed on top of the subbase.
During the early 1980’s, Georgia Power Company successfully constructed a lime-stabilized bottom ash base with a 38 mm (1-1/2 in) asphalt wearing surface near Rome, Georgia. In 1985, the Georgia Department of Transportation successfully constructed a 305 m (1,000 ft) section of cement stabilized pond ash base on State Route 22.
In 1987, pond ash from subbituminous coal was used to reconstruct approximately 2.4 km (1.5 miles) of State Route 15 in Stone County, Mississippi. The reconstruction involved five different sections, four with lime-stabilized ash and one with cement-stabilized ash. A 1.36 km (0.85 mile) control section of mechanically stabilized sand-clay subbase was also constructed. All sections were mixed in place and had a double bituminous surface treatment as a wearing surface. Stabilized base design data are presented in Table 4-8.
Deflection measurements were taken each year after construction through 1990. The sections with 6 and 7 percent lime and 7.5 percent cement all had much lower deflection readings than the control section and the section with only 2 percent lime. After 3 years of service, the control section and the section with 2 percent lime had no observed cracking, while the cement stabilized section had the most cracking. The shrinkage cracking of cement-stabilized granular materials is a fairly common occurrence, especially in soil-cement mixtures. The cracking in traditional soil-cement mixtures is attributable to the hydration of Portland cement. None of the cracking that was observed was considered structural in nature.
MATERIAL PROCESSING REQUIREMENTS
Bottom ash and/or boiler slag are both well-drained materials that can be readily dewatered in 1 or 2 days. Ponded ash reclaimed from a lagoon for use as a base course aggregate should be stockpiled and allowed to drain prior to use. Ponded ash will require a longer dewatering period because it usually includes some fly ash. The higher the percentage of fly ash in the ponded ash, the longer will be the time required for dewatering.
Crushing or Screening
Well-graded aggregates normally require less activator or reagent than poorly graded aggregates in order to produce a well-compacted mixture. Bottom ash is generally a more well-graded aggregate than boiler slag, which is normally more uniformly graded between the 4.75 mm (No. 4) and 0.42 mm (No. 40) sieve sizes. Pond ash may be a blend of bottom ash and fly ash, and will vary in gradation, depending on its location in the pond relative to the discharge pipe. Bottom ash may contain some agglomerations or popcorn-like particles. These agglomerations should either be reduced in size by clinker grinders at the power plant or removed by scalping or screening at the 12.7 mm (1/2 in) or 19 mm (3/4 in) screen.
When necessary to achieve a specified gradation, bottom ash or boiler slag may need to be blended with other aggregates. This is normally not necessary with bottom ash, but may be necessary with boiler slag.
Removal of Deleterious Materials
Deleterious materials, especially coal pyrites, should be removed at the power plant prior to use of bottom ash or boiler slag as an aggregate. The pyrites oxidize (or weather) over time, causing expansion and possible popouts of individual particles from the matrix. Soluble sulfates also occur in some bottom ashes. Low pH values are often used as an indicator for the presence of sulfates.
Some of the engineering properties of bottom ash and/or boiler slag that are of particular interest when used as aggregates in stabilized base or subbase mixtures are gradation, specific gravity and unit weight, durability, and soundness.
Gradation: The size limits in Table 4-9 are recommended for cement-treated aggregate base by the Portland Cement Association and are applicable to bottom ash and/or boiler slag use in cement-treated base course mixes.
Table 4-9. Recommended gradation for cement stabilized base.
Specific Gravity and Unit Weight: The specific gravity of bottom ash usually ranges from 2.1 to 2.7, with dry unit weights ranging from 720 to 1600 kg/m3 (45 to 100 lb/ft3). The specific gravity of boiler slag usually ranges from 2.3 to 2.9, with dry unit weights ranging from 960 to 1440 kg/m3 (60 to 90 lb/ft3). With bottom ash, lower specific gravity is usually indicative of the presence of porous, popcorn-like particles, which readily degrade under compaction.
Durability: In ASTM C131 (Los Angeles Abrasion) tests, bottom ash has had loss values between 30 and 50 percent. Boiler slag has had loss values between 24 and 48 percent. Most bottom ashes have loss values less than 45 percent, enabling them to meet ASTM requirements for soil-aggregate base and subbase materials.
Soundness: The durability of an aggregate for possible use in stabilized bases or subbases can be evaluated by the sodium sulfate soundness test. Bottom ash has had sodium sulfate soundness loss values that normally range from 1.5 to 10.5 percent. Boiler slag has had sodium sulfate soundness loss values of between 1 and 9 percent. The lower the specific gravity, the higher the probable percentage of deleterious material in the ash, which will likely be reflected in a higher value for soundness loss.
For pozzolan-stabilized base (PSB) mixtures containing coal fly ash (along with either lime, Portland cement, or kiln dust as an activator), the initial step in determining mix design proportions is to find the optimum fines content. This is done by progressively increasing the percentage of fines and determining the compacted density of each blend. Fly ash alone can be used to represent the total fines. A Proctor mold and standard compaction procedures are used for each blend of bottom ash and/or boiler slag and fines. Fly ash percentages ranging from 25 to 45 percent by dry weight of the total blend are suggested for the initial trial mixes.
At least three different fly ash additions are needed to establish the optimum fines content, which is the percentage of fines that results in the highest compacted dry density. The dry density for each fly ash percentage is then plotted to identify the optimum fines content. An optimum moisture content must then be determined for the selected mix design proportions.
Once the design fly ash percentage and optimum moisture content have been determined, the activator (lime, Portland cement, kiln dust, etc.) percentage must also be established. Trial mixtures using a gradual increase in the activator percentage are recommended. Final mix proportions are selected based on the results of compressive strength and durability testing, using ASTM C593 procedures. The objective is to meet strength and durability criteria with the most economical mix design.
For cement-stabilized bottom ash and/or boiler slag mixtures, the only mix design consideration is a determination of the percentage of Portland cement to be added to the mixture. As with the PSB mixtures, trial mixtures using several increasing percentages of cement will be necessary. Usually between 5 and 12 percent Portland cement will be needed to properly stabilize bottom ash and/or boiler slag for use as a roller-compacted base course. The results of ASTM C593 compressive strength and durability testing should be the basis for selection of final mix proportions.
The compacted unit weight of bottom ash and/or boiler slag mixes is usually considerably lower than the compacted unit weight of stabilized base mixtures containing conventional aggregates. Consequently, a cement content of 10 percent by weight for a base course mix containing bottom ash and/or boiler slag may be the equivalent of a 7 percent by weight cement content for a similar mix containing a normal weight aggregate.
In general, the trial mixture with the lowest percentage of cement (or activator plus fly ash in PSB mixtures) that satisfies both the compressive strength and the durability criteria is considered the most economical mixture. To ensure an adequate factor of safety for field placement, it is recommended that the stabilized base or subbase mixture used in the field have an activator content that is at least 0.5 percent higher (1.0 percent higher if using kiln dust) than that of the most economical mixture.(18)
The thickness design of stabilized base or subbase mixtures containing bottom ash or boiler slag can be undertaken using the standard structural equivalency design method for flexible pavements described in the AASHTO Design Guide.(19) This method uses an empirical structural number (SN) that relates pavement layer thickness to performance.
Table 4-10 lists recommended structural coefficient values based on studies of pozzolanic and crushed stone base materials(19) for stabilized base or subbase mixtures. The values are for stabilized base or subbase materials that attain a given range of compressive strength, regardless of the source of aggregate used or the type of reagent(s) in the design mix. These coefficient values are based on the use of a1 = 0.44 (used for a bituminous wearing surface) and a value of a3 = 0.15 (used for a crushed stone base).
Table 4-10. Recommended structural layer coefficient values for stabilized base and subbase materials.
The main factors influencing the selection of the structural layer coefficient are the compressive strength and modulus of elasticity of the stabilized base material. The value of compressive strength recommended for determination of the structural layer coefficient is the field design compressive strength, which is the compressive strength developed in the laboratory after 56 days of moist curing at 73° F (23° C).(18) However, other time and temperature curing conditions may be required by various specifying agencies.
When a Portland cement concrete (PCC) roadway surface is to be designed with a stabilized base or subbase, the AASHTO structural design method for rigid pavements can be used.(19)
Material Handling and Storage
Both bottom ash and boiler slag can be handled and stored using the same methods and equipment that are normally used for handling and storage of conventional aggregates.
Mixing, Placing, and Compacting
The blending or mixing of bottom ash or boiler slag in stabilized base mixtures can be done either in a mixing plant or in place. Plant mixing is recommended because it provides greater control over the quantities of materials batched and also results in the production of a more uniform mixture. Although mixing in place does not usually result in as accurate a proportioning of mix components as plant mixing, it is probably used more frequently with mixes involving bottom ash or boiler slag and will still produce a satisfactory stabilized base material.
Stabilized base materials should not be placed in layers that are less than 100 mm (4 in) or greater than 200 to 225 mm (8 to 9 in) in compacted thickness. These materials should be spread in loose layers that are approximately 50 mm (2 in) greater in thickness prior to compaction than the desired compacted thickness. The top surface of an underlying layer should be scarified prior to placing the next layer. For granular or coarse graded mixtures, steel-wheeled vibratory rollers are most frequently used for compaction. For more fine-grained mixtures, a vibratory sheepsfoot roller, followed by a pneumatic roller, is often employed.(18)
To develop the design strength of a stabilized base mixture, the material must be well-compacted and must be as close as possible to its optimum moisture content when placed. Plant-mixed materials should be delivered to the job site as soon as possible after mixing and should be compacted within a reasonable time after placement.
When self-cementing fly ashes are used as a cementitious material in stabilized base mixtures, compaction should be accomplished as soon as possible after mixing. Otherwise, delays between placement and compaction of such mixtures may be accompanied by a significant decrease in the strength of the compacted stabilized base material, unless a retarder is used. A commercial retarder, such as gypsum or borax, may be added at the mixing plant in low percentages (approximately 1 percent by weight) without adversely affecting the strength development of the stabilized base material.(18)
After placement and compaction, the stabilized base material must be properly cured to protect against drying and to assist in the development of in-place strength. An asphalt emulsion seal coat should be applied to the top surface of the stabilized base or subbase material within 24 hours after placement. The same practice is applicable if a PCC pavement is to be constructed above the stabilized base or subbase material. Placement of asphalt paving over the stabilized base is recommended within 7 days after the base has been installed. Unless an asphalt binder and/or surface course has been placed over the stabilized base material, it is recommended that vehicles should not be permitted to drive over the material until it has achieved an in-place compressive strength of at least 2400 kPa (350 lb/in2).(18)
Cold Weather Construction: Stabilized base materials containing bottom ash and/or boiler slag that are subjected to freezing and thawing conditions must be able to develop a certain level of cementing action and in-place strength prior to the first freeze-thaw cycle in order to withstand the disruptive forces of such cycles. For northern states, many state transportation agencies have established construction cutoff dates for stabilized base materials. These cutoff dates ordinarily range from September 15 to October 15, depending on the state, or the location within a particular state, as well as the ability of the stabilized base mixture to develop a minimum desired compressive strength within a specified time period.(19)
Crack Control Techniques: Stabilized base materials, especially those in which Portland cement is used as the activator, are subject to cracking. The cracks are almost always shrinkage related and are not the result of any structural weakness or defects in the stabilized base material. The cracks also do not appear to be related to the type of aggregate used in the base mix. Unfortunately, shrinkage cracks eventually reflect through the overlying asphalt pavement and must be sealed at the pavement surface to prevent water intrusion and subsequent damage due to freezing and thawing.
One approach to controlling or minimizing reflective cracking associated with shrinkage cracks in stabilized base materials is to saw cut transverse joints in the asphalt surface that extend into the stabilized base material to a depth of 75 mm (3 in) to 100 mm (4 in). Joint spacings of 9 m (30 ft) have been suggested.(18) The joints should all be sealed using a hot poured asphaltic joint sealant.
As noted above, control of shrinkage cracking has been long considered by many state transportation agencies as a prime concern associated with stabilized base mixtures, especially cement-stabilized mixtures. Since most mixtures that include bottom ash and/or boiler slag as the aggregate have been placed on secondary roads, haul roads, and parking lots, as opposed to higher-type highway facilities, the issue of crack control has not been as great a concern to the owners or administrators of these installations. However, additional mix designs with reduced potential for shrinkage cracking need to be developed, especially if these materials are someday to be used on higher-type facilities.
Pyrites must be removed before bottom ash or boiler slag can be used. Soluble sulfates in bottom ash may warrant removal if found in sufficient quatity to be considered detrimental. Improved techniques for timely removal of these detrimental constituents are needed.