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Flowable Fill

Flowable Fill is the Smarter Way to Backfill: It's Quick and Cost Effective!

It’s delivered on a mixer truck and has similar ingredients, but flowable fill isn’t concrete. Instead, the cement, sand, water and fly ash combine to create a sturdy substitute for traditional fill that is also sometimes called controlled low-strength material (CLSM). Because it flows into place, flowable fill requires no compaction. Its viscosity and strength can be altered to meet the needs of any backfill projects, and its fluidity allows it to flow long distances to even the hardest places to reach. But that’s not all it can do…

Use Flowable Fill as backfill for:


  • Utility Trenches
  • Building Excavation
  • Bridge Abutments
  • Road Base
  • Sub Footing
  • Floor Slab Base
  • Pipe Bedding
  • Surface Prep for Concrete overlays

Flowable Fill can:

Cut Costs

Because it flows into place and settles on its own, flowable fill reduces the cost of manpower and equipment. While traditional compacted fill requires two laborers for placement and two laborers for compaction, flowable fill requires only one laborer for placement and no compaction.  Using flowable fill also eliminates the need for conventional backfilling equipment such as compactors and backhoes.

Simplify Construction

In addition to cutting costs, flowable fill will help you avoid the headaches caused by traditional backfill materials while speeding up construction:

  • Because it flows into place and settles on its own, flowable fill eliminates the difficulty of properly compacting material under and around pipes.
  • Flowable fill has controllable set times and allows the job to be completed faster.
  • Flowable fill can be placed underwater, so in some cases trenches that contain water or moisture may not have to be pumped before flowable fill is installed.
  • Flowable fill eliminates the site crowding that comes with storing backfill materials on-site.
  • Flowable fill can be color-coded for utility identification.
  • When the time comes to remove entrenched pipes or utility lines, some flowable fill mixes can be excavated with a shovel or pick.
  • It’s easily conveyed by pump, chutes or buckets.

Color Coding for Safety

Using integral color, Reli-a-Fill can be color coded to match the American Public Works Association’s Uniform Color Code.  This clearly alerts those digging in the future that they are near utilities and what type of utilities they are.

Guarantee Backfill Densities

The most common complaint from public works directors is improper backfilling.  Frequently contractors are called back to repair settlement problems and ensure that backfilling will only need to be done once. Even lower strength flowable fill mixes offer twice as much strength as traditional backfill, and some flowable fill mixes can have as much as 10 times the strength of traditional backfill.

Prepare Surfaces for Concrete Overlays

Before a concrete overlay (called whitetopping) is poured, cracks in the existing asphalt should be filled with flowable fill that easily flows into cracks and crevices. This is an inexpensive way to avoid mirror cracking, which occurs when cracks form in the whitetopping in the same places where there are cracks in the asphalt.



How is flowable fill slump measured?
Because flowable fill is such a fluid substance, it is measured by spread instead of slump: a cylinder is filled with flowable fill, then lifted vertically, allowing the flowable fill to spread across a level surface. A mix with good flowability should spread more than 8 inches. Be sure to follow ASTM standards when testing flowable fill.

Is flowable fill more expensive than traditional backfill?
While flowable fill is more expensive than traditional backfill, it more than makes up for it with labor and equipment cost savings: labor and equipment costs for granular fill are 600% more expensive than that of flowable fill.

Sample Labor Cost Comparison:

  Granular Backfill Flowable Fill
Placement (2 laborers @ 35.09*)  $70.18 $35.09
Compaction (2 laborers @ 35.09*) $70.18 n/a
Heavy Equipment Operator $45.82* n/a
 Hand Compactor $15.00* n/a
Backhoe $25.00* n/a
 Total labor/hour  $226.18 $35.09

84% labor cost savings!!!

* National Industry Average including overhead costs

How long does it take for flowable fill to cure?
Flowable fill does not need to be cured like concrete because the mixture does not harden through hydration, but instead the mix settles and excess water bleeds off. The higher the water content of flowable fill the better, because too little water will restrict proper settlement and low water content will not likely speed up the hardening process. Regardless of temperatures and environmental conditions, flowable fill can be designed to accommodate construction needs and can even fast track some projects.


Additional Resources:

NRMCA Flowable Fill Site

MSDS Sheets

Flowable Fill Overview