The success of that project and several other demonstration projects in Indiana led to the acceptance and use of boiler slag in Indiana and several other states, including Ohio, Michigan, Missouri, and West Virginia. Boiler slag has also been used as an aggregate in HMA paving in a number of cities such as Cincinnati and Columbus, Ohio, as well as in Tampa, Florida.
Boiler slag provides better coverage per mile than limestone chips and retains a rich black color that is an excellent contrast to road strip colors. The attributes of boiler slag often lead to specifying boiler slag for rehabilitation of airport runways and taxiways. Commingling the rejected pyrite with bottom ash is a practice at some power plants. Material handling operations should be modified to keep pyrite and bottom ash-boiler slag separate.
If pyrites are present in the bottom ash or boiler slag, they should be removed by electromagnets, media separation, or other means. Technologies typically used for bottom ash processing can provide a cost-effective method to remove impurities i.
Blending Boiler slag will almost always require blending with other aggregate sources to meet gradation specifications. Although more well-graded than boiler slag, bottom ash may require blending with aggregates. Drying Aggregates used to produce HMA are dried before blending with asphalt cement; therefore, moisture that may be present in bottom ash or boiler slag should be removed.
Excessive moisture in the aggregates will reduce the production rate of paving material due to the additional drying time required. 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, should be stockpiled and allowed to drain to a surface dry condition. When used in a cold mix application, 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.
Because boiler slag is typically poorly-graded, blending with other aggregates may be needed to meet gradation requirements. Bottom ash is typically a well-graded sand-sized material. Specific Gravity: Specific gravity is a good indicator of the quality of a material. The specific gravity of bottom ash and boiler slag depend on the mineralogical composition of the material as well as the porosity of the particles. A dry bottom ash with a high iron content may have a specific gravity as high as 3.
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. Due to the porous nature of bottom ash particles, the absorption of asphalt binder is higher than conventional fine aggregate. Hence, from a purely economic standpoint, bottom ash may not be a cost effective choice for asphalt aggregate. Sulfur-modified bottom ash mixtures in which bottom ash represents 50 to percent of the aggregate fraction compare favorably with typical surface course mixtures in terms of strength, durability, and asphalt demand.
Most bottom ash samples have some friable particles, while boiler slag normally does not. Los Angeles Abrasion test results have shown that bottom ash samples are not as sound or durable as natural aggregate. However, the test results fall within the specifications of a maximum 50 percent loss by abrasion.
Most of the paving experience with bottom ash has been in cold mixes. Because of the presence of friable popcorn particles in dry bottom ash that can break down under compaction, bottom ash is more appropriate as a base course rather than surface mixtures. Although the asphalt contents of mixes containing bottom ash will be greater than the asphalt contents of conventional asphalt paving mixes, the total weight of asphalt cement used should not be significantly greater because of the low unit weight of the bottom ash.
Bottom ash mixes are also likely to have relatively high air void contents. The high air voids are attributable to the rough surface texture of bottom ash particles, which also produces a high angle of internal friction. For this reason, no more than 30 percent of the aggregate in a asphalt pavement mix should be replaced with bottom ash. Research has shown that sulfur modified bottom ash mixes containing 50 to percent bottom ash aggregate replacement can be achieved with 7.
Stripping occurs when moisture causes a loss of bond between aggregate and asphalt binder.
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Lime additives minimize the moisture susceptibility of pavement mixes. Mixes containing large amounts of fine aggregate will require additional lime because of the increased surface area of the aggregate. Coating dry aggregate surfaces with dry hydrated lime may be difficult. Therefore, dry hydrated lime can be added to damp aggregate that contains 3 to 5 percent water content by weight of lime. The blend proportions of boiler slag and conventional aggregates are designed to meet gradation specifications. Percentages of aggregate replaced by boiler slag range from 40 to 50 percent by weight of the total mix.
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 boiler slags lack microtexture that increases the aggregate-asphalt bond and to provide skid resistance.
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. Kneading compaction improves the stability and flow characteristics compared to Marshall drop hammer compaction.
Obtaining adequate compaction is essential with boiler slag mixtures. Optimum compaction is produced by blending boiler slag with well-graded, angular, rough-textured aggregate and limiting the percentage of boiler slag to 50 percent. Porous boiler slag can be used in greater percentages, but excessively porous slag are weak and can crush. However, pyrites should be removed prior to using bottom ash in asphalt paving. Boiler slag mixtures with acceptable skid resistance that employ boiler slag as the top size aggregate can be designed by limiting the percentage of boiler slag in the mix.
Boiler slag asphalt pavement mixtures should avoid open-graded mixtures with low filler content. Mixing, Placing, and Compacting The same methods and equipment used for mixing, placing, and compacting conventional pavements are applicable for asphalt pavements containing bottom ash or boiler slag.
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In hot mix applications, bottom ash or boiler slag are typically blended with other aggregates using conventional equipment. Dry bottom ash used in cold mix applications may not require blending and can be prepared by mixing with emulsified asphalt at a central pugmill mixing plant. Cold mix asphalt containing bottom ash or boiler slag can be prepared in advanced and stockpiled for 10 or more days. Laydown characteristics of dry bottom ash cold mixes placed with either a spreader box or a conventional paving machine are the same as conventional mixes.
Spreader box lifts of up to mm 8 in uncompacted mix can be placed. Lifts greater than mm 8 in in loose thickness may be difficult to compact. Bottom ash and boiler slag consist of the same chemical components as fly ash; therefore there exists the potential to leach trace elements. Because bottom ash and boiler slag have larger particles and less surface area per unit volume, the potential to leach trace elements is reduced. In addition, coal combustion products mixed in asphalt pavement is considered an encapsulated application that further reduces the potential to leach elements.
A recent leachate study was conducted on test strips of asphalt concrete with bottom ash. Although trace elements were observed in the leachate, there was no evidence that the use of coal ash in asphalt pavements was the source. The performance of wearing surface mixes with precrushed bottom ash aggregate should be evaluated in comparison with more conventional asphalt paving mixes.
Additional research into modifying bottom ash, i. Bottom ash and boiler slag possess unique physical and engineering properties that are different from conventional pavement materials; therefore, standard test methods may reject bottom ash or boiler slags that would provide acceptable performance. New or modified test methods are needed to characterize bottom ash and boiler slag properties that influence pavement performance.
Improved characterization is needed for both abrasion loss and particle size degradation that may occur during compaction. Ksaibati K, Conner GL. Laboratory evaluation of bottom ash asphalt mixes. Ksaibati K, Sayiri, S. Utilization of Wyoming bottom ash in asphalt mixes. Clean coal technology: Coal utilization by-products.
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