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  • Writer's pictureArizona Contractor & Community

How to Backfill Fast: Flowable Concrete Fill

By Luke M. Snell, P.E. and Frank Kozeliski, P.E.

Concrete is often considered a building material used to construct our buildings and roads. The next series of articles will examine several concretes designed to meet different uses. These types of concrete may not look or perform like regular concrete; however, by altering the standard mixture or changing the ingredients, we can create concrete that can solve unique problems.

Backfilling a trench can be tedious work. For example, when using a good quality structural fill material such as compacted soil as backfill, it must be placed in about a six-inch layer, then compacted. This time-consuming process also requires that workers are in the trenches, which can result in injuries or death if a cave-in occurs. An alternative, less dangerous method is available.

Flowable fill was first used to backfill a trench and fill an abandoned underground fuel tank in 1964. The flowable fill was self-consolidating with a strength, in most cases, less than 150 psi. It would be placed directly from the concrete truck's chute and require no vibrating equipment to consolidate the mixture. A single person directs the chute of the concrete truck. Flowable fill can be placed quickly with no one in the trenches; thus, this is a much safer and quicker way to backfill a trench. If piping is in the trench, the flowable fill will also provide stable bedding for the pipe.

Contractors initially used flowable fill with a high volume of fly ash in the mixture because it was inexpensive compared to cement. But fly ash has become more expensive and is unavailable in some areas. So now, most batch plants make a flowable fill with 50 - 200 pounds of cement with enough fine aggregates (sand) or 3/8-inch crusher fines and water to make the mixture flowable and easy to place.

The slump will be about 11 inches, and the water/cement ratio will be more than 4. This ratio compares to regular concrete water/cement ratio between 0.4 - 0.50. Thus, flowable fill has about 8- 10 times the water/cement ratio as standard concrete.

The strength of the flowable fill will vary depending on the amount of cement added to the mixture. On most projects, strength is not considered important since the flowable fill only needs to be as strong as compacted soil. Typical flowable fill strengths are 50 - 300 psi at 28 days. An essential element of flowable fill is that it will be set in just a few hours so that a worker can be on the surface and it can even be paved over four hours after placement.

The low strength of flowable fill allows for relatively easy access to conduit or pipes if repair or new construction is needed. Flowable fill with a strength of 50 psi can be removed with hand equipment, while a backhoe is required for a strength of 150 psi. Above 300 psi, flowable fill is difficult to remove. Thus, higher strengths are not recommended.

To read the rest of this article, you are invited to purchase the digital issue here.

This article originally appeared in the bimonthly Arizona Contractor & Community magazine, May/Jun 2023 issue, Vol. 12, No. 3.


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