Date: July 8, 2015

How Much Do You Know About Liquid Soil?

Here’s a riddle:

What’s batched at a concrete plant, mixed and hauled in a ready mix concrete truck, and delivered to the jobsite by a professional ready mix truck driver, but is not concrete? The answer is Flowable Fill.  The technical name of this handy product is Controlled Low Strength Material or CLSM. The American Concrete Institute defines CLSM as “a Self-Consolidating cementitious material used primarily as a backfill as an alternative to compacted fill.” The simplest way to describe Flowable Fill is its liquid soil that flows freely to fill a hole, trench or pipe and achieves excellent density without any effort.  We’ve all driven over that annoying dip in the road a month after a utility road cut was repaved. That’s because it’s very difficult and not very safe to compact a narrow trench in six inch lifts of granular base material and achieve maximum Proctor density. The uses of Flowable Fill are not confined to filling roadway trenches.  Imagine abandoning an old box culvert or storage tank in place by simply filling it with Flowable Fill.  With typical cut and cover construction, you would need traffic control, excavation, special trench shoring, trucks to haul away debris, purchased clean fill or base material, and tedious mechanical compaction at the bottom of a large excavation. We can make Flowable Fill with very low density for special fill applications or with specific thermal properties for high voltage cable backfilling. Any awkward hole from a sinkhole to a national monument is a good candidate for Flowable Fill.

As mentioned in the riddle, this is not concrete, so we must shift our mindset to soil mechanics instead of concrete materials.  In concrete, we look for high compressive strength as a measure of quality. With Flowable Fill, we only need about 30 psi to equal very high quality soil fill. In fact, we often recommend a maximum unconfined compressive strength of only 300-500 psi to ensure we can excavate the Flowable Fill later if needed.  In concrete we carefully monitor every pound of water that goes in the mix.  Flowable Fill is not adversely affected by additional water, and we often like a consistency of loose cake batter to make it flow freely over long distances. Unlike concrete that bleeds water and shrinks, Flowable Fill subsides.  Subsidence is the natural result of the lighter water floating to the top while the heavier particles settle and consolidate into well compacted fill.  Typical Flowable Fill subsidence is approximately 3/8 inch per foot of depth. In concrete, 400 to 600 pounds per cubic yard of Portland cement combines with water to create the primary “glue”. Flowable Fill requires only 25 to 50 pounds per cubic yard of Portland cement.  Flowable Fill is also a very sustainable material because it can use large amounts of industrial byproducts like fly ash, off-spec sand, crushed returned concrete, and even recycled asphalt pavement (RAP).

Please contact a Chaney Enterprises sales representative for more information on how this versatile ready mix product can speed up your project while protecting your employees.


Hank Keiper, P.E.

The SEFA Group