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Federal Highway Administration Research and Technology
Coordinating, Developing, and Delivering Highway Transportation Innovations
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Publication Number: FHWA-RD-97-148
User Guidelines for Waste and Byproduct Materials in Pavement Construction
Fixated or stabilized flue gas desulfurization (FGD) scrubber material can be used as a stabilized base and/or subbase material. The fixated FGD scrubber material is produced in a compactable condition and can be used in essentially the same manner as other lime-fly ash or cement-stabilized base materials. The fixated FGD scrubber material may be used in its "as produced" condition, provided it is capable of developing the required compressive strength and satisfying durability requirements. If this is not the case, then additional fixation reagent (Portland cement, lime, fly ash, etc.) must be blended with the fixated FGD scrubber material to ensure that it does meet strength and durability criteria. Besides additional fixation reagent, an aggregate material (usually coal bottom ash) can also be blended with the fixated FGD scrubber material. Properly designed fixated FGD scrubber material has comparable strength development and durability characteristics to that of conventional stabilized base materials.
The earliest known application in which FGD scrubber material was used in stabilized base compositions occurred during the construction of the TRANSPO'72 International Transportation Exposition held in 1972 at Dulles Airport near Washington, D.C. A portion of the new 40-hectare (100-acre) parking area was used by the FHWA to demonstrate the potential for use of base course mixtures containing a number of calcium sulfate type wastes, including FGD scrubber material.
The demonstration parking area at the exposition was paved with a 127-mm- (5-in-) thick compacted stabilized base mixture containing 3 percent lime, 59 percent fly ash, 17 percent bottom ash, 13 percent crushed limestone, and 8 percent dewatered FGD scrubber material(1) The overall results from the demonstration indicated that FGD scrubber material was potentially useful for highway base course construction. However a combination of poor weather and saturated subgrade conditions at the time of construction contributed to a softening and loss of density and eventually resulted in the removal and replacement of the experimental base material.(2)
Besides its use in TRANSPO'72, fixated FGD scrubber material has been used in stabilized base applications in at least four other states. These applications are listed in Table 6-5.
Table 6-5. Summary of fixated FGD scrubber material use in stabilized base and subbase applications in the United States.
In Florida, crushed lime rock is widely available and commonly used as a roadbase construction material. A typical county road section consists of from 152 mm (6 in) to 305 mm (12 in) of lime rock base placed on a prepared subgrade and overlaid with 50 mm (2 in) to 76 mm (3 in) of asphalt wearing surface. Laboratory test data from the University of Florida indicated that base course compositions of lime-fly ash-dewatered FGD scrubber sludge (referred to as Poz-o-Tec) had better bearing strength characteristics than lime rock base.(3)
Between 1977 and 1989, 12 different projects were constructed in central Florida in which FGD material was used as the base course. These projects consisted of one truck haul road, three parking lots, one landfill access road, one private access road, one heavy steel storage yard, one utility plant access road, and four city or county roads. The base course thicknesses for these projects varied from 152 mm (6 in) to 305 mm (12 in). At least 90,000 metric tons (100,000 tons) of fixated FGD base have been placed in Florida. Performance has been reported as highly satisfactory.(3)
In 1977, a 244-m- (800ft-) long section of pozzolanic road base material composed of lime, fly ash, bottom ash, and dewatered FGD scrubber material was placed as a demonstration on a state secondary road in southwestern Pennsylvania. The entire base was placed in one day in a single lift at a compacted thickness of 25-mm (10-in). The base was covered by a 50-mm (2-in) bituminous concrete binder course and a 25-mm (1-in) bituminous concrete wearing surface.
Test data and visual observations over a 7-year period following installation have demonstrated fully successful performance. Despite severe freeze-thaw cycles, the road exhibited no potholes or broken areas and the base remained intact as verified by periodic coring. The average unconfined compressive strength of road base cores was 4,960 kPa (720 lb/in2) after 1 year, 5,800 kPa (840 lb/in2) after 3 years, and 6,990 kPa (1014 lb/in2) after 7 years.(4)
Two demonstration test sections, each 5.6 m (18 ft) wide, 92 m (300 ft) long, and 305 mm (12 in) thick, were constructed during December 1993 at the Riverside Campus of Texas A&M University. The first test section consisted of fixated FGD sludge (sometimes referred to in Texas as scrubber base), which consisted of a blend of self-cementing fly ash with dewatered FGD sludge, to which was added coal bottom ash and additional self-cementing fly ash. The second test section consisted of the fixated FGD material blended with type II Portland cement.
The two experimental sections were constructed by spreading the component materials with a motor grader and mixing in place with a pulvi-mixer. A control section, also 93 m (300 ft) long, was also placed between the two experimental sections. The control section consisted of 305 mm (12 in) of crushed iron ore gravel. After 2 years of monitoring, both of the FGD scrubber sludge base sections demonstrated significantly higher strength and stiffness than the control section.(5)
In June 1995, a 77-m- (250-ft-) long test section of cement-bottom ash-fixated FGD scrubber sludge base material was placed as part of a 232-m- (750-ft-) long demonstration haul road at the Gavin power plant in Cheshire, Ohio. The fixated FGD scrubber sludge material consisted of 3 percent quick lime and 70 percent pozzolanic fly ash added to one part by dry weight of dewatered FGD scrubber sludge. The base course mix design involved a blend of 40 percent bottom ash and 60 percent fixated FGD scrubber sludge, to which was added 7 percent Portland cement. The base course mix was produced in a pugmill mixing plant.
The cement-bottom ash-fixated FGD scrubber sludge base was placed at a compacted thickness of 203 mm (8 in) and overlaid with 76 mm (3 in) of asphalt paving. The haul road is used extensively by heavy trucks on a daily basis and has survived one winter and more than one full year of use with no noticeable signs of distress.(6)
MATERIAL PROCESSING REQUIREMENTS
The calcium sulfite scrubber material from FGD systems is generally of toothpastelike consistency (from 25 to 50 percent solids) after the sludge has been dewatered. Dewatering is usually accomplished by belt filter presses or centrifuges. Centrifuges are normally able to dewater the sludge to a higher solids content than filter presses.
Dewatered FGD material must be stabilized or fixated by the addition of dry reagents (quicklime and Class F fly ash, Class C fly ash, or Portland cement) before being converted to a compactable consistency (at least 65 to 70 percent solids) for use in stabilized base compositions. The fixated FGD scrubber material can then be placed and compacted as a base material or, if necessary to meet strength and durability requirements, blended with additional reagent and, in some cases, aggregate or bottom ash prior to being used as a stabilized base or subbase.
Some of the properties of FGD scrubber sludge that are of particular interest when FGD scrubber sludge is incorporated into stabilized bases and subbases include particle size distribution, moisture content, wet and dry density, permeability, moisture-density characteristics, compressive strength, and durability.
Particle Size Distribution: Unoxidized calcium sulfite is composed of approximately 90 percent silt, 8.5 percent clay, and 1.5 percent sand sizes. Oxidized calcium sulfate is composed of approximately 81 percent silt, 16.5 percent sand, and 2.5 percent silt sizes.(7)
Moisture Content: Dewatered calcium sulfite is the FGD scrubber material that is usually stabilized or fixated and used for road base. Prior to dewatering, the calcium sulfite slurry has a moisture content of approximately 60 percent. After dewatering, the moisture content is reduced to roughly 25 percent. Following stabilization or fixation, the FGD scrubber material has a moisture content in the 15 to 20 percent range.(7)
Wet and Dry Density: In slurry form, FGD scrubber sludge has a wet density between 1,500 kg/m3 (95 lb/ft3) and 1,600 kg/m3 (100 lb/ft3) and a dry density between 950 kg/m3 (60 lb/ft3) and 1,050 kg/m3 (65 lb/ft3). In dewatered form, the FGD filter cake has a wet density between 1,600 kg/m3 (100 lb/ft3) and 1,800 kg/m3 (110 lb/ft3) and a dry density between 1,200 kg/m3 (75 lb/ft3) and 1,400 kg/m3 (85 lb/ft3). In stabilized or fixated form, the FGD scrubber material has a wet density between 1,700 kg/m3 (105 lb/ft3) and 1,900 kg/m3 (115 lb/ft3) and a dry density between 1,450 kg/m3 (90 lb/ft3) and 1,600 kg/m3 (100 lb/ft3).(7)
Permeability: As FGD scrubber material is successively reduced in moisture content through dewatering or stabilization, the permeability of the material also decreases. Prior to dewatering, calcium sulfite slurry has a permeability of 8 x 10-5cm/sec. After dewatering, the filter cake has a permeability of 1 x 10-5 cm/sec. Shortly after stabilization, the permeability is about 1 x 10-6 cm/sec. After fixation, the permeability may be as low as 5 x 10-8 cm/sec.(7)
Moisture-Density Characteristics: Depending on the properties of the dewatered FGD scrubber material and the mix design proportions and reagents used for stabilization or fixation, the maximum dry density values of fixated FGD scrubber material may range from 1,300 to 1,650 kg/m3 (80 to 102 lb/ft3), with optimum moisture normally between 11 and 19 percent when tested in accordance with the standard Proctor (ASTM D698)(8) method.(9)
Compressive Strength: The compressive strength of most fixated FGD material base course mixtures continues to increase over time. Strength gain is more gradual with pozzolanic stabilized mixtures than mixtures stabilized with Portland cement or Class C fly ash. After 7 days, compressive strengths can range from 1,400 kPa (200 lb/in2) to 2,800 kPa (400 lb/in2) for pozzolanic stabilized mixtures and from 4,100 kPa (600 lb/in2) to 6,200 kPa (900 lb/in2) for mixtures stabilized with Portland cement or Class C fly ash. These strengths continue to increase over time and can exceed 13.8 MPa (2,000 lb/in2) after 1 or more years.
Durability: Durability testing of fixated FGD scrubber material may involve either freeze-thaw or wet-dry testing, or both. Freeze-thaw testing should be performed in accordance with ASTM D560(10) procedures. Wet-dry testing should be performed in accordance with ASTM D559(11) procedures. There has been limited durability testing of fixated FGD scrubber materials. Generally, stabilized base or subbase mixtures containing fixated FGD scrubber material that achieve between 2,800 kPa (400 lb/in2) and 4,500 kPa (650 lb/in2) compressive strength within 7 days will pass durability tests.
Mix design for FGD scrubber sludge mixes involves blending fixated FGD sludge with one or more stabilization reagents (lime, fly ash, or Portland cement), and possibly also including coal bottom ash or conventional aggregate. As with other stabilized base mixes, mix design proportions must be developed to meet the requirements of ASTM C593.(12)
Unless otherwise specified, a minimum unconfined compressive strength of 2,800 kPa (400 lb/in2) is recommended after ambient curing for at least 14 days, but no longer than 28 days. If Portland cement is used as the stabilization reagent, some states require 4,500 kPa (650 lb/in2) unconfined compressive strength after curing for 7 days.
Mixes containing fixated FGD scrubber material should be tested for moisture-density relationships and molded as close as possible to optimum moisture content and maximum dry density. If bottom ash is used as an aggregate, the ratio of FGD scrubber sludge to bottom ash is often in the 1.5:1 to 1:1 range. The addition of bottom ash usually enhances the strength development of stabilized base mixes.
When using Portland cement as a stabilization reagent, type II (sulfate resistant) cement should be used. When a pozzolanic fly ash and quicklime are used to stabilize the FGD scrubber material, adequate strength can usually be achieved by the addition of up to 7 to 8 percent cement by weight of dry solids, or by adding more quicklime. If a self-cementing fly ash is used as the FGD material fixation reagent, then adding a lower percentage of cement (possibly 3 to 4 percent) or the addition of more fly ash may be needed to achieve the required strength.
The thickness design of stabilized base or subbase mixtures containing fixated FGD scrubber material can be undertaken using the structural equivalency design method described in the AASHTO Design Guide.(13) This method uses an empirical structural number (SN) that relates pavement layer thickness to performance.
Table 6-6 lists recommended structural coefficient values for stabilized base or subbase mixtures. 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). The following structural coefficient values are derived from studies of pozzolanic and crushed stone base materials performed at the University of Illinois.(14)
Table 6-6. Recommended structural coefficient values for stabilized base mixtures.
Structural layer coefficient values of 0.30 to 0.35 have been recommended for Portland cement-stabilized bases.(15) Compressive strength development in laboratory cured cement-stabilized test specimens is ordinarily determined under ambient temperature conditions, rather than 38°C (100°F) conditions.
The main factors influencing the selection of the structural layer coefficient for thickness design using the AASHTO method 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. The field design compressive strength is simulated by the compressive strength developed in the laboratory after 56 days of moist curing at 23° C (73°F)(15) However, other curing conditions may be required by various specifying agencies.
Construction procedures for stabilized base and subbase mixtures in which fixated FGD scrubber material is used are essentially the same as those used for more conventional pozzolanic stabilized bases and subbases.
Material Handling and Storage
Fixated FGD scrubber material is usually stockpiled on a concrete pad for a period of as much as 24 hours. The material can then be blended with additional reagent (such as lime or Portland cement), blended with bottom ash, boiler slag or other aggregate, or simply transported and placed as is on the job site.
Mixing, Placing, and Compacting
The blending or mixing of dewatered FGD scrubber material with appropriate reagents in stabilized base mixtures can best be done in a mixing plant. 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. Because of the toothpastelike consistency of dewatered FGD scrubber material, mixing in place with reagents is not recommended.
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. Fixated FGD scrubber materials should be delivered to the job site as soon as possible after mixing and should be compacted as soon as possible after placement. This is particularly the case with mixtures in which Class C fly ash is used as an activator.
Fixated FGD scrubber base materials should not be placed in layers that are less than 100 mm (4 in) or greater than 20 to 22 cm (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. Smooth drum, steelwheeled vibratory rollers are most frequently used for compaction, although satisfactory compaction results have also been obtained using smooth drum, steelwheeled static rollers. The smooth drum roller also seals the surface of the road base to minimize adverse impacts from rainfall.(3)
After placement and compaction, the fixated FGD scrubber base material must be properly cured to protect against drying and 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. For most types of stabilized base materials, the seal coat is applied within 24 hours after placement.
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, vehicles should not be permitted to drive over the material until it has achieved an in-place compressive strength of at least 2,410 kPa (350 lb/in2).(15)
Cold Weather Construction
Stabilized base materials containing fixated FGD scrubber material 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 cut-off 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.(15)
Use of Self-Cementing Fly Ash
When self-cementing fly ashes are used as an activator in stabilized base mixtures, including those containing fixated FGD scrubber material, 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.(16)
Crack Control Techniques
Fixated FGD scrubber base materials, especially those in which Portland cement is used as the reagent, may be subject to crack development. The cracks are almost always shrinkage related and are not the result of any structural weakness or defects in the stabilized base material. 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. Cracking is also likely to occur when FGD scrubber material is blended or fixated with quick lime and pozzolanic fly ash, or with self-cementing fly ash.
Recommended methods for minimizing reflective cracking associated with shrinkage cracks 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 approximately 9 m (30 ft) have been suggested.(15) For parking lots, the joints should be cut in two directions, perpendicular to each other at approximately the same spacing. The joints should all be sealed using a hot poured asphaltic joint sealant.
Although field installations of stabilized base course materials containing fixated FGD scrubber material have almost always been successful, scrubber sludge is perceived as an unusual, difficult to handle material. There is a general lack of knowledge and understanding of how FGD scrubber material can be converted from a sloppy, unstable mess before dewatering into a competent engineering material after being dewatered and fixated, by being blended with other aggregates and reagents to form a quality road base material.
More laboratory mix design work is needed to develop suitable base course mixtures in which dewatered FGD material is blended with Portland cement or self-cementing fly ash and a variety of unconventional aggregate sources, such as coal bottom ash, reclaimed paving materials, nonferrous slags, or mineral by-products. More fixated FGD field installations also need to be placed, monitored for performance over several years, and documented in order to demonstrate to the highway engineering community that properly designed fixated FGD material can produce excellent base courses.