Expanded Shale, Clay and Slate (ESCS) compacted geotechnical fills are approximately half the weight of ordinary aggregate fills. This advantage, coupled with the high angle of internal friction of ESCS, can also reduce lateral forces by more than one-half. ESCS has been effectively used to solve numerous geotechnical engineering problems and to convert unstable soil into usable land. ESCS is a reliable, economical geotechnical solution.
T.A.
Holm
and
A.J.
Valsangkar
For more than [80] years, shales,clays, and slates have been expanded in rotary kilns to produce structural grade LWA for use in concrete and masonry units. Millions of tons of structural grade LWA produced annually are used in structural concrete applications. Its availability is currently widespread throughout most of the industrially developed world. Consideration of structural grade LWA as a remedy to geotechnical problems stems primarily from the improved physical properties of reduced dead weight, high internal stability, high permeability, and high thermal resistance. These significant advantages arise from the reduction in particle specific gravity, stability that results from the inherent high angle of internal friction, the controlled open-textured gradation available from a manufactured aggregate which assures high permeability, and the high thermal resistance developed because of the high particle porosity.
Retaining wallbackfill, Providence, Rhode Island
PHYSICAL PROPERTIES OF STRUCTURAL LIGHTWEIGHT AGGREGATES
range of particle sizes ensures a high interstitial void content that approaches 50% in the loose state. North American rotary kiln plants producing expanded shales, clays, and slates currently supply coarse [and fine] aggregatesto readymix and precast concrete manufacturers with 20 to 5 mm (3/4 -#4), 13 to 5 mm (1/2 -#4), or 10 to 2 mm (3/8 -#8) gradations [and various fine aggregate gradings]. With [coarse] gradations there is a minimum percentage of fines smaller than 2 mm (#8 mesh) and insignificant amounts passing the 100 mesh screen.
Particle Shape and Gradation
Particle Porosity and Bulk Density
As with naturally occurring granular materials, manufactured LWA's have particle shapes that vary from round to angular with a characteristically high interstitial void content that results from a narrow range of particle sizes. Applications of LWA to geotechnical situations require recognition of two primary attributes: (a) the high interstitial void content typical of closely controlled manufactured granular coarse aggregate that closely resembles a clean, crushed stone. and (b) the high volume of pores enclosed within the cellular particle.
When suitable shales, clays, and slates are heated in rotary kilns to temperatures in excess of 1100째 C (2012째F), a cellular structure is formed of essentially noninterconnected spherical pores surrounded by a strong, durable ceramic matrix that has characteristics similar to those of vitrified clay brick. Oven-dry specific gravities of LWA vary but commonly range from 1.25 to 1.40. Combination of this low specific gravity with high interparticle void content results in LWA bulk dry densities commonly in the range of 720 kg/m3 (45 pcf). Compaction of expanded aggregates in a manner similar to that used with crushed stone provides a highly stable interlocking network that will develop in-place moist densities of less than [960 kg/m3 (60 pcf)]. Differences in porosity and bulk density between LWA's
Structural grade LWA gradations commonly used in highrise concrete buildings and long-span concrete bridge decks conform to the requirements of ASTM C330. The narrow -I
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LWA's stored in exposed stockpiles in a manner similar to crushed stone will have some internal pores partially filled and may also carry an adsorbed moisture film on the surface of the particles. The moisture content that is defined in ASTM procedures as "absorption" based on a 24-hour immersion and routinely associated in concrete technology with "saturated" surface-dry specific gravity is, in fact, a condition in which considerably less than 50% of the particle pore volume is filled. The issue is further clarified by a schematic volumetric depiction (see Figure 4) of the degree of pore volume saturation of a LWA particle that shows that the sample had a measured damp loose bulk unit weight of 775 kg/m3 (48.4 pcf) with an 8.5% absorbed moisture and would, in fact, represent
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WATERWITHIN POROUSAGGREGATE
SURFACE FILM
1-,-
.48 --1-.28-1...,24-+1
DRYLOOSEBULK SAMPLEDENSITY 714 kg/m3 (44.6pol) ABSORBEDMOISTURE @8.5'!0BYWEIGHT 61 kg/m3 (3.8pcf)
1.83
@ 00
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FIGURE 5 Moisture absorption (by weight) and partially saturated, surface dry specific gravity of lightabsorption characteristics are shown in Figure 5 for a LWA sample with a measured I-day immersion moisture content of 8.5% associated with a partially saturated surface-dry specific gravity of 1.5. When moisture absorption-versus-time relationships are extrapolated or theoretical calculations used to estimate the total filling of all the LWA pores, it can be shown that for this particular LWA the absorbed moisture content at infinity will approach 34% by weight with a totally saturated specific gravity of 1.83. Complete filling of all pores in a structural grade LWA is unlikely because the noninterconnected pores are enveloped by a very dense ceramic matrix. However, these calculations do reveal a conservative upper limit for submerged design considerations.
"
MOISTURE
GRAVITY
weight aggregate(LWA) versus time of submersion
DEGREE
ABSORBED
SATURATED SPECIFIC
~ ffi 1.8
Absorption Characteristics
Durability Characteristics The durability of LWA's used in structural concrete applications is well known. More than 400 major U .S. bridges built using structural lightweight concrete (LWC) have demonstrated low maintenance and limited deterioration. Long-term durability characteristics of LWA's were demonstrated in 1991 by reclaiming and testing samples of the LWA fill supplied in 1968 to a Hudson River site. Magnesium soundnesstests conducted on the reclaimed aggregate sample exposed to long-term weathering resulted in soundness loss values comparable to those measured and reported in routine quality control testing procedures 23 years earlier, indicating little long-term deterioration due to continuous submersion and freeze-thaw cycling at the waterline. AlthoughASTM standard specifications C330 and C331 for lightweight aggregate make no mention of corrosive chemicals limitations, foreign specifications strictly limit SO3 equivalents to 0.5% (Japanese Industrial Standard J5002) or 1.0% (German Standard DIN 4226). The American Concrete Institute Building Code (ACI 318) mandates chloride limitations in the overall concrete mass because of concern for reinforcing bar corrosion, but no limits are specified for individual constituents. Numerous geotechnical projects specifications calling for lightweight aggregateshave limited watersoluble chloride content in the aggregate to be less than loo ppm when measured by AASHTO T291.
VOIDS
TOTALDAMPLOOSE BULKSAMPLEDENSITY 775 kg/m3 (48.4pcf) WATER
Fractionof totalbulkloosesampleoccupiedby absorbedmoisture= 1~ Degreeof saturationof the lightweight aggregateporevolume by thesamplemoisturecontent
= .06
06 = 24 = .25
'Saturated," surfacedry (SSD),aftera onedayimmersionrepresents approximately one-quarter degreeof saturationof the poresof the particularaggregate
FIGURE 4 Degree of saturation of partially saturated lightweight aggregate (LWA) a condition in which approximately 25% of the pore volume is water filled. Structural grade LWA exposed to moisture in production plants and stored in open stockpiles will contain an equilibrium moisture content. LWA's that are continuously submerged will, however, continue to absorb water over time. In one investigation, the effective specific gravity of a submerged LWA sample was measured throughout a one-year period to demonstrate long-term weight gain. Long-term
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APPLICATIONS During the past decade several hundred diverse geotechnical applications have been successfully supplied with structural grade LWA. The applications primarily fit into the following major categories: .Backfill behind waterfront structures, retaining walls, and bridge abutments; .Load compensation and buried pipe applications on soft soils; .Improved slope stability situations; and .High thermal resistance applications.
fication of an angle of internal friction greater than 40 degrees. No constructability problems were experienced by the contractor while transporting, placing, and compacting the LWA soil fill. Peak shipment were more than 1,000 tons per day without any logistical difficulties. The material was trucked to the point of deposit at the job site and distributed by front-end loaders. This project used approximately 20,000 m3 (27 ,000 yd3) of compacted LWA and resulted in overall savings by reducing sizes of sheet piling and lowering costs associated with the anchor system. On the Charter Oak Bridge project, Hartford, Connecticut, constructed in 1989 to 1990, LWA fill was placed in the east abutment area to avoid placing a berm that would have been necessary to stabilize an earth fill embankment. According to the designer, construction of a berm would have required relocating a tributary river. LWA fill was also used in other areas to avoid increasing stresses and settlements in an old brick sewer (7). When all applications were totaled, this project incorporated more than 100,000 tons of structural grade LWA.
Load Compensation and Buried Pipe Applications on Soft Soils In numerous locations throughout North America, design of pavements resting on soft soils has been facilitated by a "load compensation" replacement of heavy soils with a free-draining structural grade LWA with low density and high stability. Replacing existing heavy soil with LWA permits raising elevations to necessarylevels without providI El. +19 ing any further surcharge loads to the El. +17 ~ Gran~se Steel Pile Curb Cap lower-level soft soils. Rehabilitation of Colonial Parkway near Williamsburg, Steel Sheet Pile Wall Dike Virginia, built alongside the James and Wale El. +3 MLW El. Existing . York rivers, provides a representative Timber example of the procedure. Soft marsh Reinforced Bulkhead Concrete soil sections of this roadway had a low Deadman load-bearing capacity, and had experiDredge EJevation enced continuous settlement. The confor Barge 2 Berth\ I. crete roadway slabs were removed El. -28+ ~ along with the soil beneath to a depth of CROSS SECTION AT SHEET PILE WALL PORT OF ALBANY MARINE TERMINAL more than 3 ft. The normal weight soil ~ , + El. -32 Dredge Elevation ---"' was then replaced with structural-grade For Tanker Berth LWA with a compacted moist density of less than 960 kg/m3 (60 pcf), providing Rehabilitation of Port of Albany, New York [1981] effective distribution of load to the soft FIGURE 6 soil layer, load compensation, and side slope stability. Reconstruction was completed in two stages Backfill Behind Waterfront Structures, by first completely rehabilitating in one direction, followed Retaining Walls and Bridge Abutments by excavation of the opposing lane with delivery, compaction, and slab construction routinely repeated. A classic example of how unusable river front was Construction of pipelines in soft soil areas has frequently reclaimed and large industrial site extended by the use of been facilitated by equalizing the new construction weight sheet piles and lightweight fill is demonstrated in Figure 6 (pipe plus LWAbackfill) to the weight of the excavated nat(6). LWA fill specifications for this project required rotary ural soil. Supporting substrates do not "see" any increased kiln expanded shale to have a controlled coarse aggregate loading and settlement forces are minimized. gradation of 20 to 5 mm (3/4- #4) and laboratory test certi-5-
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