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Publication Number: FHWA-RD-97-148
User Guidelines for Waste and Byproduct Materials in Pavement Construction
Bottom ash and boiler slag can be used as aggregates in hot mix asphalt base courses or wearing surfaces, emulsified asphalt cold mix bases or surfaces, and asphalt surface treatments or seal coats.
Bottom ash, produced in dry bottom boilers, is usually sufficiently well-graded that it does not need to be blended with other fine aggregates to meet gradation requirements. However, bottom ash particles are less durable than conventional aggregates. Consequently, bottom ash is better suited for use in base course and shoulder mixtures or in cold mix applications, as opposed to wearing surface mixtures. Most of the previous use of bottom ash has been in cold mix projects on low-volume secondary roadways.
Boiler slag is produced in wet bottom boilers, is uniformly sized, and consists of hard, durable, glassy particles. Boiler slag has to be blended with other fine aggregates to meet gradation requirements, but has been used more frequently in asphalt paving than bottom ash. Boiler slag has been used mainly in hot mix wearing surfaces, where it has been found to enhance skid resistance, and, to a lesser extent, in surface treatment or seal coat applications.
Bottom ash and boiler slag have been used with considerable success as fine aggregates in asphalt paving mixtures for at least the past 25 years in different sections of the United States. The American Coal Ash Association recently reported that during 1996 more than 75,000 metric tons (83,000 tons) of boiler slag and nearly 14,400 metric tons (16,000 tons) of bottom ash were used in asphalt paving.(1)
A 1994 survey of all 50 state transportation agencies indicated that five states have made some recent use of bottom ash and/or boiler slag as an aggregate in asphalt paving on state roadways. These five states are Arkansas, Missouri, Texas, West Virginia, and Wyoming.(2) Previous surveys have reported bottom ash and boiler slag usage in up to as many as 11 different states, although such usage includes some projects on nonstate roadways.(3)
Bottom ash is a somewhat variable material that may sometimes contain pyrites or lightweight, porous "popcorn" particles. As a result, it is not frequently used as an aggregate source in hot mix asphalt paving mixes, especially wearing surfaces. Bottom ash has been used more frequently in cold mix emulsified asphalt mixtures, hot mix base course mixtures, or in shoulder construction, where gradation requirements and durability considerations are not as critical as in hot mix wearing surface mixtures.(4)
The most extensive use of bottom ash in bituminous paving has been in West Virginia, where, during the 1970's and 1980's, bottom ash was cold mixed with 6 to 7 percent by weight of emulsified asphalt and used in the paving of secondary roads where traffic volumes are lower and durability concerns are reduced. In some cases, the bottom ash was also blended with boiler slag. Similar applications have also been made in eastern Ohio.(5)
Bottom ash has been used occasionally as an aggregate in hot mix asphalt, but usually only when blended with conventional aggregates. There have been periodic indications of problems with paving mixtures in West Virginia containing bottom ash, in which pyrite contamination in the bottom ash had not been considered. Pyrite particles will weather in service, despite being coated with asphalt cement, causing popouts and deep red stains in the pavement surface.(8) Bottom ash has also been used in hot mix asphalt in some western states.
Boiler slag has been used more frequently in hot mix asphalt than bottom ash because of its hard, durable particles and resistance to surface wear. Boiler slag has also been used in hot mix asphalt wearing surface mixtures because of its affinity for asphalt and its dust-free surface, which aids in asphalt adhesion and resistance to stripping.(6) Another of the properties of boiler slag that enhances its value as an aggregate in bituminous paving is its permanent black color, which is not affected by sun or weather. The black color also aids in the melting of snow from the road surface during the winter.(7)
Since boiler slag has a uniform particle sizing, it is commonly blended with other aggregates for use in asphalt mixtures. As a rule of thumb, paving mixture stability is likely to suffer if the percentage of boiler slag exceeds 50 percent. (8)
Boiler slag was first used in asphalt paving many years ago in Hammond, Indiana, where, on an experimental basis, it was blended with conventional aggregate to help solve the problem of aggregate polishing. The success of that and several other demonstration projects in Indiana led to its acceptance and use in that state and several others, including Ohio, Michigan, Missouri, and West Virginia. Boiler slag has also been used as an aggregate in hot mix asphalt paving in a number of cities such as Cincinnati and Columbus, Ohio, as well as in Tampa, Florida.(9)
In West Virginia, boiler slag has been blended with graded river sand for resurfacing applications, where thin overlays are used. Typical sections range in thickness from 12.7 to 50.8 mm (1/2 to 2 in) and are composed of 50 percent by weight boiler slag, 39 percent river sand, 3 percent fly ash, and 8 percent asphalt cement. Some of these sections were able to provide more than 10 years of service with little change in texture, despite being subjected to heavy truck traffic.(6)
Some 10,000 tons of boiler slag were used to construct the wearing surface and shoulders of a portion of Interstate Route 94 near the Detroit Airport. This section of roadway was placed during the late 1970's and reportedly performed well into the mid 1980's.(10)
Boiler slag from lignite coal was successfully used for street resurfacing work in parts of southern Texas for many years. The paving mixes consisted of a blend of 75 percent by weight of lignite boiler slag and 25 percent limestone screenings, with an asphalt content of 6 to 7 percent by weight of aggregate. These pavements have reportedly held up well with no signs of shoving or raveling, despite heavy truck traffic, while maintaining a dark, black texture, nonskid properties and smooth riding quality.(11)
The City of Sioux City, Iowa, has incorporated optimum amounts of boiler slag and fly ash mineral filler in an asphalt hot mix surface course called "Carpet Coat." The mix consists of 56 percent by weight boiler slag, 17 percent quartzite, 14 percent sand, 7 percent fly ash, and 6 percent asphalt cement.(12)
Boiler slag has also been used very successfully as a seal coat aggregate for bituminous surface treatments in a number of states, especially in municipal road construction projects. The boiler slag provided better coverage per mile than limestone chips and retained its rich black color, while the surface of the stone chip gradually acquired a faded, gray appearance. Significant cost savings using boiler slag in this application during the mid 1980's have been documented.(13)
The use of power plant aggregates in surface treatment or seal coat applications is believed to be confined almost exclusively to boiler slag. There are no known uses of bottom ash as a seal coat aggregate in asphalt surface treatments.
MATERIAL PROCESSING REQUIREMENTS
Pyrites present in the coal before burning should be removed during the pulverizing step and should not be allowed to be collected together with the bottom ash or boiler slag. If pyrites are present in the bottom ash or boiler slag, they should be removed by electromagnets, media separation, or other means.
Oversize or agglomerated popcorn particles may be present in some bottom ash sources and should be removed by screening or scalping the material over a 19 mm (3/4 in) or 12.7 mm (1/2 in) screen.
Boiler slag will almost always require blending with other aggregate sources to meet applicable gradation specifications. Bottom ash may require blending with other aggregates, although it is considerably more well-graded than boiler slag.
Since aggregates used to produce hot mix asphalt are dried before blending with asphalt cement, moisture that may be present in bottom ash can be removed. Excessive moisture in the aggregates, however, will reduce the production rate of the paving material. Both bottom ash and boiler slag are relatively easy to dewater, particularly boiler slag, which consists of glassy particles. Ponded ash, which is usually a mixture of fly ash and bottom ash or boiler slag, must be stockpiled and allowed to drain sufficiently prior to use, preferably to a surface dry condition.
When used in a cold mix application, the bottom ash should be at least surface dry so that moisture does not interfere with the coating of the ash particles by the emulsified asphalt. Boiler slag should also be in a surface dry condition when used as a seal coat aggregate.
Both bottom ash and boiler slag are used as fine aggregates in asphalt paving applications. Properties of bottom ash and boiler slag that are of particular interest when they are used in asphalt concrete include gradation, specific gravity, absorption, and durability. Table 1 shows some of the engineering properties of bottom ash and boiler slag from power plants in West Virginia.(15)
Table 4-5. Engineering properties of selected bottom ashes and boiler slags
Gradation: ASTM D1073 defines a fine aggregate in asphalt paving mixtures as an aggregate that passes the 9.5 mm (3/8 inch) sieve.(14) Boiler slag, with few exceptions, meets this size requirement and most bottom ash sources, with minimal screening of oversize material, will satisfy this definition of a fine aggregate. Bottom ash is predominantly sand-sized, usually with 50 to 90 percent passing a 4.75 mm (No. 4) sieve.
Specific Gravity and Absorption: Bottom ash generally has lower specific gravity and higher absorption values than limestone sand, while boiler slag is comparable in specific gravity with lower absorption than limestone sand.(15)
Durability: Bottom ash and boiler slag exhibit marginal durability as measured by the Los Angeles abrasion test, with bottom ash percent loss values between 30 and 50 and boiler slag somewhat lower. Most bottom ash samples have some friable particles, while boiler slag normally does not.
Soundness: Soundness values are generally found to be within ASTM D1073 weight loss specifications of not more than 15 percent after five cycles when sodium sulfate is used, or not more than 20 percent after five cycles when magnesium sulfate is used. Values for both boiler slag and bottom ash are generally found to be less than 10 percent.(15)
Dry bottom ash has received much less usage than boiler slag in asphalt paving and most of the experience in using bottom ash for this purpose has been in cold mixes. A serious consideration with some sources of dry bottom ash is the presence of friable popcorn particles, which can break down under compaction. For this reason, bottom ash is more appropriate for use in base course rather than surface mixtures.(16)
Because many bottom ashes contain friable popcorn particles that are also absorptive, the asphalt contents of hot mixes or cold mixes containing some percentage of bottom ash will be higher than those of mixes with conventional aggregates. Although the asphalt contents of mixes containing bottom ash will be greater than the asphalt contents of conventional asphalt paving mixes, the total amount of asphalt cement used should not be significantly greater because of the reduced unit weight of the mixes containing bottom ash. Bottom ash mixes are also likely to have relatively high air void contents. The high air voids are attributable to the high angle of internal friction and the rough surface texture of the bottom ash particles.(16)
Because of comparatively high optimum asphalt contents of mixtures using bottom ash as the only aggregate, combining bottom ash with conventional aggregates is recommended. Increased percentages of conventional aggregate result in a reduction in the optimum asphalt content. The primary benefit to be realized from blending with conventional aggregates is an improvement in the durability of the paving mix.
The addition of bottom ash can alter Marshall stability values, and stability must be examined in each mix design. Immersion-compression testing(17) indicates that moisture damage potential does not appear to be critical in paving mixtures containing bottom ash.(18) However, this test is too short in duration to detect particle degradation due to pyrites in the bottom ash.(16) It has been reported that kneading compaction(19) more closely approximates field compaction conditions than Marshall drop hammer compaction(18)
The uniform gradation and smooth surface texture commonly associated with most boiler slags require that these materials be blended with other aggregates for use in asphalt paving mixtures. The blend proportions of boiler slag and conventional aggregate(s) will be dictated mainly by the size distribution of the materials and the requirements of the gradation specifications. Percentages of boiler slag ranging from 40 to 50 percent by weight of the total mix have been successfully used on a number of past projects. The best use of boiler slag is as a partial replacement for the sand fraction of hot mix base and surface course mixtures. The type of aggregate used and the relative proportions of the boiler slag and aggregate have a significant influence on the properties of the paving mixture.(8)
Marshall stability and flow values have been found to decrease as the percentage of boiler slag is increased for a given compactive effort. Mixes blended with rounded siliceous aggregates, such as uncrushed river sand, result in lower quality mixtures than blends containing crushed stone, which possess more desirable angularity and surface texture. Blending crushed stone aggregates with boiler slag is recommended because most boiler slags lack microtexture, which increases the ability of aggregate to retain its asphalt coating and to provide skid resistance.(15)
Optimum skid resistance using boiler slag is best found in open graded sand mixes using boiler slag as the top-sized aggregate. However, such mixes should limit the percentage of boiler slag in the mix and avoid low filler content. Rounded river sands should also be avoided. Boiler slag does not appear to be as helpful in terms of skid resistance in coarse graded mixtures, especially if the coarse aggregate is polish susceptible.(8)
The effect of compaction method on mixture properties is quite pronounced with blends of boiler slag and sand. Kneading compaction improves the stability and flow of such mixes, compared with Marshall drop hammer compaction. Obtaining adequate compaction is essential with boiler slag mixtures. The best mixtures are produced by blending boiler slag with well-graded, angular, rough-textured aggregate and limiting the percentage of boiler slag to 50 percent or less.(17)
It is possible that some of the more vesicular (porous) boiler slag sources could be used in greater percentages, but excessively vesicular slags tend to be weak and lack crushing resistance.(16) These types of boiler slag may also be more absorptive than typical boiler slag sources and require a higher percentage of asphalt cement.
Boiler slag asphalt mixtures have performed well with respect to their retention of stability in the presence of water. When evaluated using the Marshall immersion-compression test,(17) boiler slag mixtures yielded acceptable stability retention values.(8)
Conventional AASHTO pavement structural design methods are appropriate for asphalt pavements incorporating bottom ash/boiler slag in the mix.(20
Similarly, pavement thickness design procedures for cold mix overlays containing bottom ash or boiler slag, or a blend of the two, should not be any different from the thickness design procedures normally used for cold mix overlays using conventional aggregates. Modified structural numbers (SN) for cold mix overlays containing bottom ash and/or boiler slag should be the same as those normally used by the local jurisdiction for conventional cold mix overlays.
Material Handling and Storage
Bottom ash can be handled and stored or stockpiled using the same methods and equipment that are normally used for handling and storage of conventional aggregates. However, as noted previously, prospective users of bottom ash must be aware of the possible presence of pyrites in the bottom ash and, if such pyrites were not removed prior to burning the coal, they must be removed from the bottom ash prior to its use in asphalt paving. Mixing, Placing, and Compacting
The same methods and equipment used for mixing, placing, and compacting conventional pavements are applicable to asphalt pavements containing bottom ash.
When bottom ash is used in hot mix applications, it usually must be blended with other aggregates. However, dry bottom ash used in cold mix applications may not have to be blended with other aggregate. Such mixes can be prepared cold by mixing with emulsified asphalt at a central pugmill mixing plant and can usually be stockpiled for 10 days or more.
Cold mixes containing bottom ash can be placed with a paver or a spreader box, or, in some cases, can even be dumped and leveled with a grader. Adequate compaction is usually achieved from three to four passes with a pneumatic roller, followed by one or two passes from a steel-wheeled roller.(15)
Laydown characteristics of dry bottom ash cold mixes have been found to be excellent with the use of either a spreader box or a conventional paving machine. Lifts of up to 200 mm (8 in) loose were attempted in a spreader box with good results. It is believed that lifts greater than 200 mm (8 in) in loose thickness would probably be difficult to compact.(15)
Material Handling and Storage
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.
Mixtures with acceptable skid resistance that use boiler slag as the top size aggregate can be designed by limiting the percentage of boiler slag in the mix and by avoiding open-graded mixtures with low filler content.(18)
Mixing, Placing and Compacting
The same methods and equipment used for mixing, placing and compacting conventional pavements are applicable to asphalt pavements containing boiler slag.
Some, but not all, bottom ash sources may contain pyrite particles and/or soluble iron sulfate particles. These particles, if not separated and removed prior to mixing with asphalt, will eventually weather in the pavement, producing popouts and causing unsightly staining. These particles are usually associated with low pH values, which are indicative of the presence of excessive sulfate. A more direct test method is needed to identify these undesirable particles, particularly the pyrites, so they can be removed from the bottom ash before being incorporated into a paving mix.
Bottom ash may also contain friable, porous "popcorn" particles. If so, such bottom ashes should not be used in asphalt surface mixes unless precrushed before being mixed with asphalt. The performance of wearing surface mixes with precrushed bottom ash aggregate should be evaluated in comparison with more conventional asphalt paving mixes.
Some standard test methods are not appropriate for evaluating bottom ash and boiler slag and can result in the rejection of otherwise acceptable materials. Bottom ash and boiler slag possess unique physical and engineering properties that are different from conventional construction materials, for which the standard test methods have been developed. Some new or modified test methods are needed to provide a more complete evaluation of bottom ash and/or boiler slag properties. This is especially the case with respect to abrasion loss characteristics and particle size degradation during compaction for bottom ash.