RMRC • User Guidelines for Waste and Byproduct Materials in Pavement Construction •

Bottom ash or boiler slag can be used as the fine aggregate fraction or as the entire aggregate in Portland cement or pozzolan-stabilized base and subbase mixtures. Bottom ash and, in particular, boiler slag have been used as aggregate in stabilized base or subbase applications since the 1950's. Most installations have not been well documented, but there is no indication of unsatisfactory performance.

PERFORMANCE RECORD

In 2006, the American Coal Ash Association reported that over 740,000 metric tons (815,000 tons) of bottom ash were used as road base or subbase materials.⁽¹⁾ The road base or subbase category used by the American Coal Ash Association includes the use of coal bottom ash as an unbound base, stabilized subbase, or stabilized base material.

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 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 service and cores taken from these pavements have developed compressive strengths over 6900 kPa (1000 lb/in²).

Case studies report that fly ash stabilized base courses may contain 65 percent bottom ash or boiler slag by weight, while Portland cement mixes may contain up to 95 percent bottom ash or boiler slag. The remaining percentage of a pozzolan stabilized base mix is fly ash, lime, or Portland cement. Depending on the blending with natural aggregates, bituminous stabilized bottom ash or boiler slag base courses require between 5 to 7 percent asphalt content.⁽²⁾

MATERIAL PROCESSING REQUIREMENTS

Bottom ash and boiler slag are both well-draining materials that can be dewatered in 1 or 2 days. Ponded ash, which typically contains fly ash, requires a longer time to dewater, up to two weeks. The higher the percentage of fly ash in ponded ash, the longer the dewatering time.

Bottom ash is generally a more well-graded aggregate than boiler slag, which is normally more uniformly graded between the No. 4 (4.75 mm) and No. 40 (0.42 mm) sieves. Pond ash may be a blend of bottom ash and fly ash, and will vary in gradation, depending on the proximity to the discharge pipe in a lagoon. 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 (½ in) or 19 mm (¾ in) screen.

As produced, bottom ash may meet gradation specifications for stabilized base or may require blending with other coal combustion products or natural aggregates to meet specifications. Boiler slag, being poorly-graded, will require blending to meet gradation specifications. Well-graded aggregates normally require less activator or reagent than poorly graded aggregates in order to produce a well-compacted stabilized base.

Deleterious materials in bottom ash or boiler slag, especially coal pyrites, should be removed prior to use as an aggregate. Pyrites oxidize (or weather) over time, causing expansion and popouts of individual particles from the matrix. Soluble sulfates exist in some bottom ashes. Low pH values in bottom ash are often used as an indicator for the presence of sulfates. Technologies exist for processing bottom ash that can provide a cost-effective method to remove impurities (i.e. unburnt coal and pyrite) so that bottom ash meets product quality targets.⁽³⁾

ENGINEERING PROPERTIES

Engineering properties of bottom ash and boiler slag that are important when using this material as aggregate in stabilized base or subbase mixtures are gradation, specific gravity, unit weight, durability, and soundness.

Gradation: The size limits in Table 4 are recommended for cement-treated aggregate base by the Portland Cement Association and are applicable to bottom ash or boiler slag use in cement-treated base course mixes.⁽⁴⁾

Sieve Size	Percent Passing
19 mm (3/4 in)	100
9.5 mm (3/8 in)	70-90
4.75 mm (No. 4)	55-90
3.35 mm (No. 8)	40-70
1.18 mm (No. 16)	30-60
0.075 mm (No. 200)	0-30

Specific Gravity: The specific gravity of bottom ash is in the range of 2.1 to 2.7,⁽⁵⁾ but values as low as 1.9 and as high as 3.4 have been recorded.⁽⁶⁾ Bottom ash with relatively low apparent specific gravity is often indicative of the presence of porous particles trapping gases that effect the test results. Bottom ash with relatively high specific gravity may indicate a high iron content. The specific gravity of boiler slag is in the range of 2.3 to 2.9.

Dry Unit weight: The dry unit weight of bottom ash is in the range of 7.07 to 15.72 kN/m³ (45 to 100 lb/ft³) while the dry unit weight of boiler slag is in the range of 9.43 to 14.15 kN/m³ (60 to 90 lb/ft³).⁽⁵⁾ The dry unit weight of bottom ash can reach as high as 18.08 kN/m³ (115 lb/ft³) while the dry unit weight of boiler slag may be as high as 17.29 kN/m³ (110 lb/ft³).⁽⁶⁾

Durability: Bottom ash and boiler slag exhibit marginal durability as measured by ASTM C131 (Los Angeles Abrasion) tests, with bottom ash percent loss values between 30 and 50 and boiler slag between 24 and 48 percent.⁽⁵⁾ 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.⁽⁷⁾

Soundness: The durability of an aggregate used 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.⁽⁵⁾ Boiler slag has had sodium sulfate soundness loss values of between 1 and 9 percent.⁽⁵⁾ Lower the specific gravity may indicate a higher percentage of deleterious material, which will be reflected in a higher soundness loss.

DESIGN CONSIDERATIONS

For pozzolan-stabilized base (PSB) mixtures made with bottom ash or boiler slag and containing coal fly ash (along with 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. Each blend of aggregate and fines is compacted into a Proctor mold using standard compaction procedures. Fly ash percentages ranging from 25 to 45 percent by dry weight of the total blend are suggested for the initial trial mixes. The optimum fines content selected by this procedure should be 2 percent higher than the fines content at the maximum dry density. The optimum moisture content must then be determined for this mix design.

Once the design fly ash percentage and optimum moisture content have been determined, the ratio of activator to fly ash must be determined. Using a series of trial mixtures, final mix proportions are selected on the basis of the results of both strength and durability testing according to ASTM C593 procedures.⁽⁸⁾

For cement-stabilized bottom ash and boiler slag mixtures, the only mix design consideration is the percentage of Portland cement. Trial mixes between 5 and 12 percent Portland cement are needed to properly stabilize bottom ash 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 the final mix.

The compacted unit weight of bottom ash or boiler slag mixes is 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 or boiler slag may be equivalent to 7 percent by weight cement content for a similar mix containing a natural aggregate.

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, stabilized base or subbase mixture used in the field should 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.⁽⁹⁾

Designing pavement structures that include stabilized base layers with bottom ash or boiler slag aggregate should follow AASHTO pavement design methods provided in the Guide for Design of Pavement Structures,⁽¹⁰⁾ or the Guide for the Mechanistic-Empirical Design of New and Rehabilitated Pavement Structures.⁽¹¹⁾ The AASHTO methods account for the predicted loading (the number of 80 kN equivalent single axle loads), required reliability (degree of certainty that a design will function properly during the design life), serviceable life (ability to maintain quality during the pavement life), the pavement structure (characterized by the structural number), and subgrade support (related to the resilient modulus of the subgrade).⁽¹⁰⁾

A hierarchical approach in the mechanistic-empirical design method allows for varying levels of material characterization depending on project criteria. Mechanistic material properties such as dynamic modulus, resilient modulus, and Poisson’s ratio are employed to evaluate pavement performance. The levels in the hierarchical system can directly measure strength characteristics (level 1), can use correlations to develop strength characteristics (level 2), or can use typical material property default values (level 3). Both asphalt stabilized base materials and chemically or cement stabilized base materials are included in the mechanistic-empirical design method under different material categories.

Pavement design employing a structural number accounts for the relative strength of the constructed materials. The total structural support from the surface course, base course, and any subbase course equals the required structural number. Layer thicknesses are calculated using layer coefficients that define the structural support. The layer coefficients can be obtained from the relationship provided by AASHTO based on CBR or MR.⁽¹⁰⁾

When a Portland cement concrete roadway surface is to be designed with a stabilized base or subbase, the AASHTO structural design method for rigid pavements can be used.⁽¹⁰⁾

CONSTRUCTION PROCEDURES

Both bottom ash and boiler slag can be handled and stored using methods and equipment that are used for conventional aggregates.

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. Stabilized base material should be spread in loose layers that are approximately 50 mm (2 in) thicker than the 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 used. For fine-grained mixtures, a vibratory sheepsfoot roller, followed by a pneumatic roller, provide quality stabilized base compaction.⁽⁹⁾

To develop the design strength of a stabilized base mixture, the material should be well-compacted at the optimum moisture content. To avoid drying, plant-mixed materials should be delivered to the job site as soon as possible and should be compacted within a reasonable time after placement.

Compaction of fly ash stabilized bottom ash or boiler slag mixtures should be completed as quickly as possible after placement. The stabilized material can lose strength capacity if the fly ash hydrates in an uncompacted state. The pozzolanic reaction between Class F fly ash and lime is a relatively slow reaction, and a maximum delay of 4-hours should be followed whereas a maximum delay of 2-hours is recommended for Class C fly ash.^(12;13) To slow the reaction, a commercial retarder, such as gypsum or borax, can 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.⁽⁹⁾

After placement and compaction, the stabilized base material should 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 within 24 hours after placement. The same practice is applicable if a Portland cement concrete or asphalt pavement is constructed above the stabilized base or subbase material. 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 material, vehicles should not drive over the stabilized base until an in-place compressive strength of at least 2400 kPa (350 lb/in²) has been achieved.⁽⁹⁾

SPECIAL CONSIDERATIONS

Cold Weather Construction: Stabilized base materials containing bottom ash or boiler slag that are subjected to freezing and thawing conditions should develop a level of in-place strength prior to the first freeze-thaw cycle. For northern states, many state transportation agencies have established construction cutoff dates for stabilized base materials. These cutoff dates range from September 15 to October 15, depending on the state, or the location within a particular state.⁽¹⁰⁾

Crack Control Techniques: Stabilized base materials, especially those in which Portland cement is used as the activator, are subject to cracking. The cracks are typically shrinkage related and are not the result of structural weakness or defects in the stabilized base material. The cracks also not related to the type of aggregate used in the mix. Shrinkage cracks will reflect through the overlying asphalt pavement and should be sealed at the pavement surface to prevent water intrusion and subsequent damage due to freezing and thawing.

An 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 to 100 mm (3 to 4 in). Joint spacing of 9 m (30 ft) have been suggested.⁽⁹⁾ Joints should be sealed with a material such as hot poured asphaltic joint sealant.

ENVIRONMENTAL CONSIDERATIONS

Although stabilized, bottom ash and boiler slag used in stabilized base and subbase layers is considered an unencapsulated use due to the high hydraulic conductivity. Therefore, bottom ash and boiler slag have the potential to leach trace elements. Use of bottom ash and boiler slag in base material requires good management and care to ensure that the material does not result in a negative impact on the environment. In particular, areas with sandy soils possessing high hydraulic conductivities and areas near shallow groundwater or drinking aquifers should be given careful consideration. An evaluation of groundwater conditions, applicable state test procedures, water quality standards, and proper construction are all necessary considerations in ensuring a safe final product.⁽¹⁴⁾

UNRESOLVED ISSUES

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 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 a major concern. However, additional mix designs with reduced potential for shrinkage cracking need to be developed, especially when these materials are used on higher-type facilities.

Pyrites should be removed before bottom ash or boiler slag is used as aggregate in a stabilized base. Soluble sulfates in bottom ash may warrant removal if found in sufficient quantity to be considered detrimental. Improved techniques for economic removal of these constituents are needed.

REFERENCES

A searchable version of the references used in this section is available here.
A searchable bibliography of bottom ash and boiler slag literature is available here.

[ Material Description ] - [ Asphalt Concrete ] - [ Granular Base ] - [ Stabilized Base ] - [ Embankment/Structural Fill ]

COAL BOTTOM ASH/
BOILER SLAGUser Guideline

Stabilized Base

INTRODUCTION

PERFORMANCE RECORD

MATERIAL PROCESSING REQUIREMENTS

ENGINEERING PROPERTIES

DESIGN CONSIDERATIONS

CONSTRUCTION PROCEDURES

SPECIAL CONSIDERATIONS

ENVIRONMENTAL CONSIDERATIONS

UNRESOLVED ISSUES

REFERENCES

[ Material Description ] - [ Asphalt Concrete ] - [ Granular Base ] - [ Stabilized Base ] - [ Embankment/Structural Fill ]

[ Material Description ] - [ Asphalt Concrete ] - [ Granular Base ] - [ Stabilized Base ] - [ Embankment/Structural Fill ]

COAL BOTTOM ASH/BOILER SLAGUser Guideline

Stabilized Base

INTRODUCTION

PERFORMANCE RECORD

MATERIAL PROCESSING REQUIREMENTS

ENGINEERING PROPERTIES

DESIGN CONSIDERATIONS

CONSTRUCTION PROCEDURES

SPECIAL CONSIDERATIONS

ENVIRONMENTAL CONSIDERATIONS

UNRESOLVED ISSUES

REFERENCES

[ Material Description ] - [ Asphalt Concrete ] - [ Granular Base ] - [ Stabilized Base ] - [ Embankment/Structural Fill ]

COAL BOTTOM ASH/
BOILER SLAGUser Guideline