Fill. . .Easier, Faster, and Economical

Controlled low-strength material (CLSM), commonly called "flowable fill," has many uses: to backfill utility, sewer, or conduit trenches; fill building excavations; repair behind bridge abutments; create sub-bases for foundations; and fill underground 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, and water. Fly ash is sometimes included to increase its ability to flow. CLSM is designed to have the strength of a strong compacted soil. When placed against a permeable soil, the water drains quickly from flowable fill. After placement, it will settle slightly, about 1/8 inch per foot of depth. The top several inches are often like loose sand and cement. After the water drains out, the remaining end product is like a firmly compacted soil. It will not settle after it hardens.

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

CLSM’s fluid-like property allows it to be placed without compaction. Although it costs about as much as concrete, it can be more economical than conventional excavation and void filling operations. It fills a cavity as it is poured, requiring a minimum of labor. Therefore it saves the labor and equipment expense of compaction as fewer people are required to place the material than in traditional backfill methods.

Worker safety is also an advantage. Workers need not work in trenches compacting soil, with vibratory equipment. They spend less time in work zones with vehicles driving by the site.

The use of CLSM reduces the time a trench or excavation is open. Once placed, traffic can resume after a steel plate or soil has been placed over it. Prior to paving or other surface treatment, crews can remove the soil-like CLSM to finish grade.

Flowable fill is designed to be excavated so it may be easily removed if necessary. It acts like compacted sand and can be excavated with equipment or hand tools.

Other advantages are all weather construction and efficient use of materials. Road crews can place flowable fill in rain, snow, or freezing weather. Ready-mix trucks deliver the exact amount of material needed for the job. On site material storage, and hauling away excess soil, is not necessary.

CLSM should be used in situations appropriate for compacted sand. That is, flowable fill applications should be underground, and in confined areas. Large pours require a longer time for CLSM to reach an effective strength. It resists freezing and thawing providing it is not directly exposed to surface freezing conditions, abrasive erosion action, or aggressive chemicals. It should not be used as a substitute for concrete.

When applied in roads, the CLSM finish grade should be at or lower than the bottom of the road base. Note the level at which crews are placing the flowable fill in the picture on the next page.

CLSM is inadequate for road base or other high strength use. It behaves like compacted sand and is unsuitable for road base or other high strength uses. Until crews have experience with flowable fill, they should place it several inches above the finish grade, and scrape off the top material before placing the base course.

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

Success Stories

The city of Concord first tested flowable fill around storm culverts. Concord has been pleased with its performance and is now considering using it to hold curb stones in place.

In Goffstown, the department has used flowable fill to backfill abandoned pipes. They have found it to be convenient since it does not require the department to excavate the road. The department is proposes to use flowable fill to backfill a vault near a bridge abutment. In cases like these flowable fill becomes very advantageous since digging around the vault area would disturb the bridge leaving the town with an extremely costly repair.

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

Sources

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Return to UNH T2 Center, 33 College Road, Durham NH 03824 603-862-2826 Fax 603-862-2364