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Publication Number: FHWA-RD-97-148
User Guidelines for Waste and Byproduct Materials in Pavement Construction
Quarry by-products (screenings, pond fines, or baghouse fines) can be used as a filler aggregate or as a partial, or possibly a complete, replacement for the pozzolan component in flowable fill mixes. Screenings can be substituted for sand (or other granular filler materials) and may comprise up to as high as 85 percent by dry weight of such mixes. Pond fines or baghouse fines may be used for some, or possibly even all, of the aggregate content and may comprise up to as high as 95 percent by dry weight of flowable fill mixes.(1)
There are no known commercial applications to date that have made use of quarry by-products in flowable fill applications. However, a report prepared for the National Stone Association identifies flowable fill as a promising application for quarry by-products.(2)
Most flowable fill applications are designed for comparatively low in-place compressive strength development (from 340 to 1400 kPa (50 to 200 lb/in2)). The use of quarry by-products (screenings, pond fines, or baghouse fines) to replace some or all of the sand filler or fines is not considered detrimental to the strength development of the flowable fill.
MATERIAL PROCESSING REQUIREMENTS
It is unlikely that the use of crushed stone screenings as a filler in flowable fill mixes would require any processing since the screenings are produced dry or slightly moist and are generally stockpiled in the same manner as crushed stone or sand and gravel. Minimal sizing on a vibratory screen may be needed if there are any oversize (plus 19 mm (3/4 in)) particles in the screenings. If screenings are stockpiled during a rainy period, some drying may also be necessary.
The use of pond fines as a filler in flowable fill will require some type of dewatering once the fines have been reclaimed from a settling pond. The pond fines may be stockpiled for natural dewatering or passed through a mechanical dewatering device (such as a hydroclone or belt press) to reduce the moisture content.
Some of the engineering properties of quarry by-products that are of particular importance when quarry fines are used in flowable fill mixtures are gradation, moisture content, and unit weight.
Gradation: Gradation is important from the perspective of comparing the gradation of commonly used flowable fill materials (such as fly ash and sand) with potential replacement products.
Screenings are nearly all finer than 3.2 mm (1/8 in) and have low percentages (6 to 12 percent) below a 0.075 mm (No. 200 sieve) sieve size. This indicates that screenings are basically well-graded, medium to fine sand-sized materials with a small amount of silt-sized particles.
Recovered pond fines can be expected to have a size range from 100 percent passing 0.59 mm (No. 30 sieve) down to between an average of 75 to 90 percent passing 0.075 mm (No. 200 sieve). This is essentially the same size range as coal fly ash (a common additive in flowable fill mixes), which is a predominantly silt-sized material. Baghouse fines are all finer than 0.075 mm (No. 200 sieve) and nearly all particles are also finer than 0.045 mm (No. 325 sieve). Baghouse fines are nonplastic and similar in sizing and consistency to a finely sized fly ash.
Moisture Content: Screenings, because they are stockpiled in the open, can have a fairly wide range of moisture content within the same stockpile or from one quarry location to another. Pond fines, when reclaimed, may have moisture contents as high as 70 to 80 percent, and must be dewatered, either by stockpiling or by processing. Baghouse fines are available as a dry powder.
Unit Weight: The unit weight of quarry by-products can vary according to the type of rock processed, but is expected to be within the same range as the rock source from which it was derived. Screenings will normally have a higher unit weight than pond fines, with baghouse fines probably being slightly lower in unit weight than pond fines.
Some of the flowable fill mix characteristics of interest include mix strength, flowability, time of hardening, and bleeding and shrinkage.
Mix Strength: Strength development in flowable fill mixes should not be affected to any great extent by the use of quarry by-products. In mixes with high fly ash content, ultimate strengths in excess of 1035 kPa (150 lb/in2) may require jack hammers if removal is necessary.(3) Use of pond fines or baghouse fines in lieu of some of the fly ash should help to maintain the strength development below this value because the pond fines or baghouse fines are not pozzolanic.
On the other hand, if additional strength is required, screenings used in place of all, or a portion of, natural sand should not be detrimental to strength development since quarry screenings are a manufactured sand from the crushing of rock. The particles are mostly angular with good interlocking characteristics and a relatively high angle of internal friction, similar to or possibly even higher than that of natural sand.
Flowability: The use of quarry fines may have an effect on the flowability of a flowable fill mix, depending on the type and amount of quarry fines that are used. The use of pond fines, or baghouse fines, in place of a portion of the mix may cause the mix to be somewhat less flowable at the same moisture content, because of the angularity of the particles of quarry by-products compared with the spherical shape of fly ash particles. Somewhat lower flowability may also occur when using screenings instead of natural sand. To maintain or improve flowability, additional water and/or other additives (such as coal fly ash, which enhances the flowability of the mix) may be needed. Although chemical admixtures (such as water reducing agents) are not ordinarily added to flowable fill mixtures, the use of an appropriate admixture may also help to improve flowability.
Time of Set: Setting time for flowable fill mixes is determined mainly by the cement and water content of the mix, as well as atmospheric conditions. The use of quarry by-products in mixes is not expected to adversely affect the normal setting time of flowable fill mixes in which they are used, provided the cement content of the mix is not altered. Similarly, if pond fines or baghouse fines are used, the time of set should also not be adversely affected as long as the amount of cement in the mix and the water-cement ratio remains essentially the same.
Bleeding and Shrinkage: The use of screenings to replace sand in flowable fill mixes should have minimal effects on bleeding and shrinkage for these mixes. Likewise, the use of pond fines or baghouse fines in lieu of some fly ash in high fly ash content mixes should also not adversely impact bleeding or shrinkage. Evaporation of bleed water in some mixes (e.g., mixes containing high fly ash content) often results in a shrinkage of approximately 10.4 mm per m (1/8 in per ft) of flowable fill depth.(4)
In most cases, flowable fill mixes are designed on the basis of a given strength criterion, not on prescribed amounts of ingredients. If quarry by-products are incorporated in these mixes, then specific mix designs will need to be prepared to establish design criteria. The increased fines content in a flowable fill mix when pond fines or baghouse fines are used may require an increase in the water content and/or cement content in order to ensure a flowable mix.(5) With little or no fly ash in the mix, its flowability is likely to be noticeably reduced.
Flowable fill mixtures are usually designed on the basis of compressive strength development, generally after 28 days of ambient temperature curing, but sometimes on the basis of longer term (90 days or more) strength. They are usually designed to have high fluidity (typical slump of 150 mm (6 in) to 200 mm (8 in). In many applications, especially pipe trench backfills where future access for repairs may become necessary, a maximum unconfined compressive strength (usually in the 690 kPa (100 lb/in2) to 1035 kPa (150 lb/in2) range) is the design criteria. For structural support applications, such as abutment backfills supporting bridge approach slabs, the design criteria may range from 4140 kPa (600 lb/in2) to 8280 kPa (1,200 lb/in2).
When using screenings to replace all or part of the natural sand in flowable fill mixes, the amount of Portland cement and fly ash used should remain essentially the same. The overall density of the flowable fill mix should not be too different when using screenings, since the screenings have a density that is in the same range as natural sand. Some additional water may be needed, compared with the sand mixes, to achieve a desired range of flowability.
Pond Fines or Baghouse Fines
If pond fines or baghouse fines are to replace part or all of the pozzolan, such as fly ash (in high fly ash content mixes), it is possible that a small increase in the amount of Portland cement in the mix may be needed in order to achieve a desired level of compressive strength. The pond fines or baghouse fines may be somewhat denser than fly ash, resulting in a somewhat increased mix density. Some additional water may also be needed to maintain the desired range of flowability.
Structural design procedures for flowable fill materials are no different than geotechnical design procedures for conventional earth backfill materials. The procedures are based on using the unit weight and shear strength of the flowable fill to calculate the bearing capacity and lateral pressure of the material under given site conditions.
Material Handling and Storage
The same general methods and equipment used to handle, store, and stockpile conventional aggregates are applicable for quarry by-products.
Mixing, Placing, and Compacting
Flowable fill can be produced at a central concrete mixing plant in accordance with ASTM C94(6) and delivered by concrete truck mixers or using a mobile, volumetric mixer for small jobs. It is important that high fluidity (slump greater than 150 mm (6 in)) be maintained to ensure that the flowable fill material entirely fills all voids beneath pavements and around structures and utilities.
Various standard field and laboratory tests for flowable fill mixes are given by AASHTO T27,(7) as well as ASTM Provisional Standards 28(8) and 29.(9)
Quarry by-products are basically untried in field flowable fill applications. Experience is needed in the design, placement, and performance of flowable fill mixes containing different sources of quarry by-products, including some mixes with coal fly ash. Data are also needed on long-term strength development, flowability, and other engineering properties of such mixes.