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GeoInsighter Fall/Winter 2004 Newsletter

HARD AND FAST

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Have you ever been stopped in traffic and watched the often slow progress of a utility trench being backfilled by workers? They carefully place layers of soil in the trench and then compact each layer with multiple passes of a plate vibrator to make the new material dense enough to support the weight of your car. This is a common and important procedure to properly densify the material. However, there are other alternatives, including the placement of a weak concrete like material called flowable fill. In many circumstances, flowable fill can take the place of concrete or compacted soils and is particularly useful to fill around pipes or void areas that otherwise require significant manual labor.

As the name implies, flowable fill is a material with a very low slump (soupy consistency) that is basically poured into an excavation or void to harden and provide load-bearing support. Once in its hardened state, the material resembles a crumbly sandstone and can be re-excavated fairly easily. The compressive strength of cured flowable fill is general in the 100 to 500 pound per square inch (psi) range, while the strength of regular concrete starts at about 2,500 psi and goes up.

Flowable fill mixtures are most commonly used as backfill in road construction where the time to return a road to service is critical, to reach voids that would be difficult to fill with soil (i.e., abandoned tanks or irregular, narrow excavations), and where unstable trench sidewalls or sensitive structures discourage the use of compaction equipment. Although it is typically prepared at a concrete batch plant and delivered in a concrete mix truck, the cost of flowable fill is significantly less than concrete. In general, the cost to purchase and place flowable fill is only a little more expensive than placing and compacting structural soil fill. However, the economics quickly favor flowable fill when it saves time because of the examples given above.

Flowable fill mixtures are usually comprised of coal fly ash, water, aggregate (usually sand), and Portland cement. Many states have specifications for flowable fill and several American Society of Testing and Material (ASTM) laboratory procedures exist that are applicable to flowable fill quality control testing. These specifications address the physical characteristics of the flowable fill mixture, such as compressive strength, flowability, time of set, bleeding, and shrinkage. The use of flowable fill is an excellent method of practicing sustainable development because it puts significant quantities of coal fly ash and spent foundry sand, two materials that would otherwise have to be landfilled, back to good use.
An old issue of Concrete Construction magazine listed the following benefits of using flowable fill.
• The material is now commonplace and readily available around the country, with ready mixed concrete producers using locally available materials to produce flowable fill to meet most project specifications.
• The material can be delivered by ready mix trucks in specified quantities to locations wherever concrete could also be placed (i.e., it can be poured directly from the truck, lifted in batches by a crane bucket dump, or pumped).
• Because flowable fill is self-leveling, it needs little or no spreading or compacting, which speeds construction and reduces labor requirements.

• Flowable fill mix designs can be easily adjusted to meet specific fill requirements to address strength, flowability, permeability, and insulating qualities.
• The strength of cured flowable fill is typically higher than for compacted soil or granular fill.
• Flowable fill with compressive strengths in the 50 to 100 psi range can easily be re excavated with conventional construction equipment and, yet, will be strong enough for most backfilling needs.
• During placement, soil backfill must be tested after each lift for sufficient compaction; whereas, flowable fill self-compacts consistently and does not need extensive field-testing.
• Flowable fill does not form voids during placement and will not settle or rut under loading, as soil often will.
• The use of flowable fill minimizes excavation needed because narrower trenches can be used; the trench can be excavated to the dimension of the utility and not wider to accommodate compaction equipment.
• Workers can place flowable fill without having to enter an excavation, thereby reducing their exposure to possible cave-ins.
• Flowable fill will displace most standing water left in a trench from rain or melting snow and can reduce the need for dewatering pumps.
• Placement of flowable fill is possible during freezing conditions by implementing the same mix and curing techniques used for cold weather concrete.
• Unlike soil or granular backfill, flowable fill can be placed without loaders, rollers, or tampers.
Further information regarding flowable fill can be obtained from American Concrete Institute at www.aci-int.org and American Coal Ash Association at www.acaa-usa.org.

Michael C. Penney, P.E., L.S.P.
mcpenney@geoinc.com
 

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