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Publication Number: FHWA-RD-97-148
User Guidelines for Waste and Byproduct Materials in Pavement Construction
Coal bottom ash and, to a lesser extent, boiler slag have been used as a granular base material for roads, streets, and parking areas on state, local, and private highway projects. To be successfully used in this application, the bottom ash (or boiler slag) must be able to satisfy conventional material specifications for gradation, soundness, and abrasion loss.
Recent statistics for coal ash utilization indicate that, in 1996, approximately 0.6 million metric tons (0.7 million tons) of bottom ash were used as road base or subbase materials.(1) The road base and subbase category includes use as a granular or unbound base course, as well as stabilized base or subbase. A 1992 survey of all 50 state highway and transportation agencies indicated that at least four states have recently made use of bottom ash or boiler slag as a granular base material. These states are Georgia, Texas, Utah and Wyoming.(2) Wyoming has reportedly discontinued its use of bottom ash for this purpose because of reportedly poor performance. The other three states have indicated satisfactory performance.
Bottom ash and/or boiler slag have also been used in the past as an aggregate in unbound base course applications in other states, although not necessarily on state highway projects. Many such non-state installations are not well documented. Table 4-6 presents a list identifying a number of locations where the use of bottom ash and boiler slag has been documented.
In general, the performance of pavements using bottom ash or boiler slag as a granular base have reportedly performed in a satisfactory manner.
MATERIAL PROCESSING REQUIREMENTS
Dewatering (Moisture Control)
Bottom ash may require stockpiling for a short period of time (at least 1 or 2 days) to allow excess water to drain. Ponded ash reclaimed from a lagoon should be stockpiled and allowed to drain for a somewhat longer time period, perhaps up to 1 or 2 weeks, depending on the amount of rainfall. The ash should not be allowed to become too dry before use. If the ash becomes dusty while in a stockpile, or if dust is generated when the ash is being handled or loaded, it has become too dry and should be wetted enough to suppress further dusting before being used.
Table 4-6. General design and construction data for selected bottom ash/boiler slag granular base applications.
After size reduction, bottom ash can be screened to produce different size ranges, if desired. Boiler slag is more uniformly graded and usually does not require any screening prior to use. Blending with conventional aggregate may be required to meet specifications.
Deleterious materials, such as soluble sulfates or coal pyrites, should be removed from the bottom ash, boiler slag, or pond ash before attempting to use these materials in a base course or subbase application. Pyrites can be removed from the coal before it is burned using sink-float techniques, or from the bottom ash or boiler slag using magnetic separation.
Some of the engineering properties of bottom ash and/or boiler slag that are of particular interest when bottom ash and/or boiler slag are used as an aggregate in granular base applications are gradation, specific gravity and unit weight, moisture-density relationship, degradation under compaction, shear strength, bearing strength, and corrosivity.
Gradation: Bottom ash and boiler slag are considered to be fine aggregates.(7,8) Sometimes, in order to improve the sizing characteristics of the bottom ash or boiler slag, a conventional aggregate or a slag aggregate may be blended with the ash prior to its use in a base or subbase. Table 4-7 lists recommended size limits for ash when used as a granular base material.(9)
Table 4-7. Recommended gradation for granular base material.
Specific Gravity: The specific gravity of bottom ash usually ranges from 2.1 to 2.7,(10) but values as low as 1.9 and as high as 3.4 have been recorded(11). Bottom ash with relatively low specific gravity (below 2.2) is often indicative of the presence of porous popcorn particles. Bottom ash with relatively high specific gravity (above 3.0) may indicate a high iron content.
The specific gravity of boiler slag usually ranges from 2.3 to 2.9. The dry unit weight of boiler slag usually ranges from 960 to 1440 kg/m3 (60 to 90 lbs/ft3) whereas the dry unit weight of bottom ash may range from 720 to 1600 kg/m3 (45 to 100 lb/ft3).(10) Occasionally, the dry unit weight of bottom ash may reach 1840 kg/m3 (115 lb/ft3) and the dry unit weight of boiler slag may reach 1760 kg/m3 (110 lb/ft3).(11)
Moisture-Density Relationship: The laboratory moisture-density curves for dry bottom ash are similar in shape to those of typical cohesionless materials. These curves are characterized by a fairly high dry density for the air-dried condition, a lower dry density at intermediate moisture, and a high dry density at or near saturation. Generally, field compaction curves also exhibit maximum dry density at either an air-dried condition or a "flushed" or wet condition. It is recommended that bottom ash be maintained in a "flushed" condition in order to attain the highest degree of densification.(11) When compared with conventional granular materials, bottom ashes have lower maximum dry densities.(12)
Degradation Under Compaction: To quantify the extent of degradation under compaction, an index of crushing has been developed by calculating the mean size of a material before and after compaction and expressing the index of crushing as the percent reduction between the two mean sizes the initial mean size. The higher the index, the easier it is to crush the material. The index of crushing for pulverized coal bottom ash has been found to be roughly twice that of conventional aggregates, whereas the index of crushing for boiler slag is essentially the same as that of conventional aggregates. The index of crushing for bottom ash from a stoker-fired boiler was found to be about three times greater than the index of crushing for bottom ash from a pulverized coal boiler.(11)
Shear Strength: Bottom ash, because of its rough surface texture and angularity, has a slightly higher friction angle than conventional granular soils when it is compacted to a high relative density. Direct shear tests conducted on dry bottom ash samples under loose and dense relative density conditions indicated that the angle of internal friction for most bottom ashes ranged from 35° to 50° , depending on the extent of densification, with some dense bottom ash samples exhibiting friction angles as high as 55° . The angle of internal friction for boiler slag was found to fall within the same range as that of most natural granular soils (between 36° and 46° ).
Bearing Strength: California Bearing Ratio (CBR) testing of bottom ashes has indicated that soaking does not negatively affect the CBR of bottom ash. The CBR values at high moisture contents were found to be higher (between 40 and 70 percent) than corresponding CBR values on the dry side (between 35 and 60 percent). These findings further suggest that it is advantageous to compact bottom ash or ponded ash in a "flushed" or wet condition.(11) Typical CBK values for conventional granular base materials are in the range of 40 to 80 percent.
Corrosivity: When using bottom ash and/or boiler slag as a base or subbase material, there is the potential that metal structures that come into contact with the base or subbase material may eventually corrode.(13) The parameters of a base or subbase material that are most closely related to corrosivity are pH, electrical resistivity, soluble chlorides, and soluble sulfates. A study of 11 bottom ash or boiler slag samples from Indiana indicated that, using the above criteria, seven of the samples were considered corrosive, principally because of low electrical resistivity.(13) All bottom ash and boiler slag materials (including reclaimed ponded ash) being considered for prospective use as an unbound or granular base or subbase material should first be investigated for corrosivity prior to their use.
When using bottom ash or boiler slag (including reclaimed ponded ash) as an unbound or granular base/subbase material, the use of the AASHTO structural equivalency design method(14) is recommended as a granular base design procedure for either flexible or rigid pavements, whichever is applicable.
A layer coefficient value of 0.10 is recommended for the design of flexible pavement systems in which bottom ash, boiler slag, or reclaimed ponded ash are used to construct an unbound or granular base or subbase. A coefficient of 0.10 for bottom ash and/or boiler slag recognizes that bottom ash and/or boiler slag are not structurally equivalent to crushed stone, which is typically given a larger coefficient of 0.15.
Material Handling and Storage
Both bottom ash and boiler slag can be handled and stored or stockpiled using the same methods and equipment that are normally used for handling and storage of conventional aggregates.
Placing and Compacting
Bottom ash and/or boiler slag can be dumped and spread with a motor grader or bulldozer or, preferably, placed into a spreader box or paving machine for more accurate grade control. The material can be spread and compacted very well when placed at, or slightly above, the optimum moisture content, as determined by standard Proctor compaction procedures.(15)However, bottom ash loses stability when it dries out, making it necessary to keep the material wet (or "flushed") so that construction equipment can operate satisfactorily on its surface.(10)
Compaction of bottom ash and boiler slag bases and subbases can be accomplished by static steel-wheel or pneumatic rollers, as well as vibratory compaction equipment. The material, regardless of whether it is bottom ash, boiler slag, or reclaimed pond ash, must be kept moist during and after compaction.(15) However, no matter how well the material has been compacted, it is still possible that some material may become unstable upon drying. The addition of up to 30 percent fines in the form of fly ash may remedy the loss of stability upon drying.(10)
Once a bottom ash granular base layer has been properly compacted, it must be protected. A prime coat of asphalt emulsion can be applied to the top surface of the base material to prevent rapid moisture evaporation, stabilize the surface, and provide a bond between the base layer and an asphalt or Portland cement concrete wearing surface.(5) An asphalt binder and/or wearing surface or a concrete pavement should be installed within a reasonable time after sealing the granular base layer in order to minimize traffic on this layer.
Bottom ash and/or boiler slag aggregates possess somewhat unique engineering properties and characteristics when compared more conventional sources of aggregate materials. For example, bottom ash may contain some particles that may crush or degrade easily, and boiler slag is very uniformly graded. Standard test methods and specifications have been developed to evaluate conventional aggregate materials. Some sources of bottom ash and/or boiler slag, although they may provide satisfactory performance as an aggregate in granular base material applications, are not always able to satisfy all test criteria and/or specification requirements for such aggregates. This is particularly the case as far as particle size distribution specifications and abrasion loss requirements for graded base courses are concerned.