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Fly Ash in Soil Stabilization and As An Engineered Construction Material

Fly Ash from bituminous coal:

This fly ash may be a suitable filler for poorly graded sandy soils where it would aid in compaction and would increase density.

For silts, lime is required 24 hours prior to fly ash addition to reduce soil plasticity and improve its workability. Lime is required to promote the pozzolanic reaction (which produces cementitious products.) Lime reacts with clay minerals and the fly ash.

Engineering and Physical Properties

Self-cementing Fly Ash from sub-bituminous or lignite coal:

This fly ash is used to control the swell potential of expansive soils and is used to stabilize coarse-grained soils.

This fly ash contains 15-35% total CaO. Most of this CaO, as measured by elemental analysis, exists in the glassy phase. Some of the CaO is present as free lime ( 0-7 %) which might provide some cation exchange and ion crowding to fine-grained soils (if calcium exists in crystalline phase, it can react with clay minerals) when used in amounts exceeding 20 % by dry weight of soil. This fly ash may contain calcium aluminosilicate glass, tricalcium aluminate, free CaO, MgO, CaSO4, alkali sulfates, carbon and magnetite/hematite.

However many fly ashes are poor sources of free lime (since most of the CaO is combined in the glassy phase), and thus would do little to reduce soil plasticity. If cementation is a desired attribute, especially for coarse-grained soils, this fly ash may be appropriate since it is self-cementing. To bind the soil particles, it is important not to delay compaction to take full advantage of its self-cementing properties (otherwise one will observe a decrease in density.) Retarders such as sodium borate and sodium citrate have been used to retard the set in order to allow for a delay in compaction. Treatment rates of 10-16 % by oven-dry weight of soil have been used.

See ASTM D 5239, Standard Practice for Characterizing Fly Ash for Use in Soil Stabilization. Class C fly ash should be carefully evaluated for its potential to form expansive compounds within the subgrade resulting in heaving of the pavement. Free CaO that is retained on a 45 um sieve may react after soil compaction. Also expansion might result from delayed formation of ettringite due to the presence of aluminates and sulfates in the soil (fly ash also contributes CaO and sulfates.) Evaluate the soil for its sulfate content and evaluate the fly ash for its free CaO and sulfate content. Evaluate the combination. The SO3 in fly ash should not exceed 10 %. Most of the SO3 in the fly ash is present as calcium sulfate or alkali sulfate. The fly ash supplier must be able to provide a uniform product and should have at least six months of testing history.

Determine the moisture density relationship and immediate bearing value for the untreated soil and for increasing additions of fly ash. The Engineer will determine the optimal and economical fly ash content. The swell potential of the treated soil is often of great importance for modified subgrades.

ENGINEERED CONSTRUCTION MATERIAL:

See Iowa State University.

See Lafarge North America for soil-stabilzation projects. See Beach case studies in the United Kingdom. See SON -Haul's experience in soil stabilzation.

Fly ash particles, carried out of the boiler by the exhaust gases, are extremely variable but have some characteristics of interest. The particles are generally less than 250 micrometres in size, spheroidal, have a high mechanical strength, a range of densities from about 3 to less than 0.6, a melting point above 1000°C, low thermal conductivity and are mostly chemically inert.

Density of a soil is important because it determinesthe load which a structural fill will apply to itself and to its foundation and because it influences the permeability, stiffness and strength of the fill, thus affecting the settlement and ultimate stability. Increased density means increased strength and stiffness and decreased permeability. At a specified density, the soil should have the strength values used in the design of the fill. Each soil is represented by a bell-shaped curve of dry density vs. moisture. Peak of the curve represents the maximun dry density. Particle size of soils vary from gravel size to sand size to silts to clays. Fly ash is in the silt category. Fly ash is rated as non-plastic as defined by liquid and plastic limits.

The strength of fly ash when used as a fill material will determine the steepness of fill slopes which can safely be constructed and the magnitude of foundation loads it will support. For retaining walls, the shear strength of the backfill will determine the load the wall must support. Shear strength is the primary strength parameter used in the design of fly ash embankments and backfills. Shear strength is related to cohesion and the angle of internal friction.Cohesion of non self-cementing fly ash is only apparent as cohesion is easily destroyed by complete drying or saturation. Angle of internal friction is reported to be 25-40 degrees. For fly ash, the compression index is rated as 0.1-0.25 and the coefficient of permeability is reported to be 4 x 10-4 to 5 x 10-7 cm/sec.

Engineered Ash Fill Projects:

Title: Ash Recovery Project Manager Company: Duke Energy Corporation BusStr: 400 S. Tryon St. BusCt: CharlotteBusSt BusZip: NC 28201 BusPhone: (704) 382-8991Email: jwnewell@dpcmail.dukepower.comURL: http://www.duke-energy.comInterest: Duke Energy's most successful engineered ash fill project to date was completed in Nov 97.

Lime-fly ash base course(LFA)

. LFA was originally patented in the 1950's as POZ-O-PAC and used in Illinois. An example was the 3.5 mile service road from state route 195 to the Coffeen Power Station in Coffeen, IL. Here 3% hydrated lime + 32.5% class F fly ash and 64.5% boiler slag was used. Nonplastic mixtures of lime, fly ash, and aggregate are mixed and sufficient water is added to achieve maximum dry density ( see AASHTO T-180)upon compaction. Construction is usually limited from April to October for areas of the country experiencing freeze-thaw cycles. Subgrade shouldnot be frozen or muddy. Minimum average compressive strengths of 600 psi(at 14 days at 72F) should be obtained on laboratory specimens with no individual test below 500 psi. Required unconfined compressive strengths must exceed 600 psi, depending on the time of year and freeze-thaw cycles. Outside temperature should exceed 40F.

Typical mix design: (in % by weight of oven-dry aggregate)

LFA is constructed in layers not less than 4 compacted inches. Final thickness and actual proportions are set by Engineer. After LFA has been constructed, the surface must be kept continuously moist until the bituminous curing cover is applied. Surface course paving then may proceed after at least 14 hrs. after curing cover has been applied.

Cement-fly ash base course (CFA)

. CFA was first used in West Virginia in 1971-1972 in the construction of West Virginia Route 2. It had 54% boiler slag + 46% bottom ash and 5% portland cement. NonPlastic mixtures of cement, fly ash, and aggregate are mixed and sufficient water is added to achieve maximum dry density ( see AASHTO T-180)upon compaction. See above.

Typical mix design: (in % by weight of oven-dry aggregate)

See individual state DOT for specifications on materials, equipment, general construction conditions, composition of base course mixture, mixing, placing, compacting and finishing of base course mixture.

Pozzolanic Contracting and Supply Co.-in Tennessee

See Ash Development Association of Australia-Literature on Road Stabilisation