Fill. ..Easier, Faster, and Economicalby Stefanie R. Fishman, Project Assistant I

Controlled low-strength material (CLSM),commonly called "flowable fill," has many uses:to backfill utility, sewer , or conduit trenches; fillbuilding excavations; repair behind bridge abut­ments; create sub-bases for foundations; and fillunderground voids. Flowable fill is often the easi­est, fastest, and most economical way to accom­plish these activities .

Flowable Fill Properties

CLSM is a mixture of sand, cement, andwater. Fly ash is sometimes included to increaseits ability to flow. CLSM is designed to have thestrength of a strong compacted soil. When placedagainst a permeable soil, the water drains quicklyfrom flowable fill. After placement, it will settleslightly, about 1/8 inch per foot of depth. The topseveral inches are often like loose sand andcement. After the water drains out, the remainingend product is like a firmly compacted soil. It willnot settle after it hardens.

When delivered, flowable fill has the consis­tency of pancake batter. This self-flowing propertyleads to its many advantages.

Advantages and Disadvantages

CLSM's fluid-like property allows it to beplaced without compaction. Although it costsabout as much as concrete, it can be more eco­nomical than conventional excavation and voidfilling operations. It fills a cavity as it is poured,requiring a minimum of labor. Therefore it savesthe labor and equipment expense of compaction asfewer people are required to place the materialthan in traditional backfill methods.

Worker safety is also an advantage. Workersneed not work in trenches compacting soil, withvibratory equipment. They spend less time in workzones with vehicles driving by the site.

The use of CLSM reduces the time a trench orexcavation is open. Once placed, traffic canresume after a steel plate or soil has been placed

Road Busin ess, Spring 2000, Vol. 15, No.1

over it. Prior to paving or other surface treatment,crews can remove the soil-like CLSM to finishgrade.

Flowable fill is designed to be excavated so itmay be easily removed if necessary. It acts likecompacted sand and can be excavated withequipment or hand tools.

Other advantages are all weather constructionand efficient use of materials. Road crews canplace flowable fill in rain, snow, or freezingweather. Ready-mix trucks deliver the exactamount of material needed for the job. On sitematerial storage, and hauling away excess soil, isnot necessary.

CLSM should be used in situations appropri­ate for compacted sand. That is, flowable fillapplications should be underground, and in con­fined areas. Large pours require a longer time forCLSM to reach an effective strength. It resistsfreezing and thawing providing it is not directlyexposed to surface freezing conditions, abrasiveerosion action, or aggressive chemicals. It should 'not be used as a substitute for concrete.

Placing Flowable Fill

When applied in roads, the CLSM finishgrade should be at or lower than the bottom of theroad base. Note the level at which crews are plac­ing the flowable fill in the picture on the nextpage.

CLSM is inadequate for road base or otherhigh strength use. It behaves like compacted sandand is unsuitable for road base or other highstrength uses. Until crews have experience withflowable fill, they should place it several inchesabove the finish grade, and scrape off the topmaterial before placing the base course .

Crews should use caution when placing flow­able fill around or under tanks, pipes, or largecontainers. It is a fluid, and may cause objects tofloat or shift. When containers are abandoned, useCLSM to fill them.

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Workers backfill a utility cut with flo wable fill.

Success Stories

The city of Concord first tested flowable fillaround storm culvert s. Concord has been pleasedwith its performance and is now considering usingit to hold curb stones in place.

In Goffstown, the department has used flow­able fill to backfill abandoned pipes. They havefound it to be convenient since it does not requirethe department to excavate the road. The depart­ment is proposes to use flowable fill to backfill avault near a bridge abutment. In cases like theseflowable fill becomes very advantageous sincedigging around the vault area would disturb thebridge leaving the town with an extremely costlyrepair.

Getting More Information

Highway Departments can purchase flowablefill from many New Hampshire Ready Mix Plants.The March 5, 1999 NHDOT regulations for flow­able fill can be found in the state "SupplementalSpecifications" Amendments for sections 203,209,508, and 603. Contact Northern New EnglandConcrete Promotion Association for more infor­mation on flowable fill and locations in the NewHampshire area, 1-888-875-3232.

Road Business, Spring 2000, Vol. 15, No.1

Source s

"Controlled Low Strength Materials". American Concrete Institute Com­mittee 229. 1994.

"Ready Mixed Flowable Fill". National Ready Mix Concrete Association.1980.

"What. Why. & How? Flowable Fill Materials". Concrete in Practice 17.National Ready Mixed Concrete Associa tion. 1989.

Bergstralh-Shaw-Newman, Inc. Utility Cuts in Paved Roads Field Guide.U.S. Department of Transportation & Federal HighwayAdministration. September 1996.

Gress. David. "The Effect of Freeze-Thaw and Frost Heave on FlowableFill". New Hampshire Department of Transportation. U. S. Depart­ment of Transportation. Federal Highway Administration. & Univer­sity ofNew Hampshire. October 1996.

Spe cia l T hanks

Douglas Bernard. City Engineer. Concord. NHPeter Hodgen. City Concrete. Portsmouth. NHRobert Myers. Executive Director Northern New England Concrete Promo­

tion AssociationCarl L Quiram. PE. Director ofPublic Works. Goffs town. NH

T2 ChallengeCome to Mountain of Demos and take the T2

Challenge. Earn Road Scholar credit while at theRoad Agents Association Show. This is the secondT2 Challenge to be held at the Mountain. Thechallenge was held in 1997 at Waterville Estates.It was such a success that the UNH T2 Center hasdecided to have it again this year in Gilford at theGunstock Ski Area. So, mark your calendars forthe Mountain of Demos on May 25th and stop bythe T2 Challenge booth , beside the registrationtable. Dave, Kathy or Marisa will be happy to signyou up so you can earn Road Scholar hours!

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