Underground Construction - June 2017 - 14

Job-Site Geo
By Amster Howard

Test Standards For

Flowable Fill

Although agencies and owners are increasingly using flowable fill, they are
often not aware of the existence of applicable construction control test
standards. Flowable fill is a mixture of soil, cementitious matter and water
that hardens into a material stiffer than compacted soil. Typically placed
as a self-leveling slurry, flowable fill may be an economical alternative to
compacted soil.
Flowable fill (also known as CLSM or CDF) may be used as bedding,
embedment and backfill for pipeline installation. As long as strength and
flowability requirements are met, the ingredients and mixing of flowable
fill may vary widely. Pipeline projects have been using concrete test standards not realizing that ASTM has standards specifically for flowable fill.
Because flowable fill has properties unique to its use, there are separate
ASTM standards for strength, flowability, set time, yield and unit weight,
and sampling.
Flowable fill for pipe installation is gaining popularity because of the
reduced amount of inspection and testing, and the elimination of reworking and retesting compacted soil. A consistent mix and placement are
much easier to maintain with flowable fill than with compacted soil. Soil
requires re-compaction when the field density tests performed on the soil
result in unacceptably low densities.
Typical construction can result in 10 to 25 percent of the soil having to
be re-compacted, especially at the beginning of construction when some
experimentation with the compaction procedures is necessary to obtain
the required density. Instead of density tests, construction control of
flowable fill placement typically involves testing for compressive strength,
flowability and set time.
The ASTM standards applicable for flowable fill are:
* ASTM D 4832, Standard Test Method for Preparation and Testing of
Controlled Low Strength Material (CLSM) Test Cylinders
* ASTM D 5791, Standard Practice for Sampling Freshly Mixed Controlled Low Strength Material (CLSM)
* ASTM D 6023, Standard Test Method for Unit Weight, Yield, Cement
Content, and Air Content (Gravimetric) of Controlled Low Strength
Material (CLSM)
* ASTM D 6024, Standard Test Method for Ball Drop on Controlled Low
Strength Material (CLSM) to Determine the Suitability of Load Application
* ASTM D 6103, Standard Test Method for Flow Consistency of Controlled Low Strength Material (CLSM)

Sampling
The ASTM standard specifically for sampling of flowable fill material is
ASTM D 5791. The standard is similar to ASTM C 172 for concrete.
Currently, ASTM D 5791 states that it applies only to central mixed
flowable fill transported to the job site. For volumetric mixers and trench
side mixers, the project specification should state requirements for sampling, or use wording such as "obtain samples using procedures similar to
principles in ASTM D 5791."

Compressive strength
Compressive strength testing involves determining the seven-day and/or
28-day compressive strength of the hardened mixture. The compressive

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strength tests are performed to
check the adequacy and uniformity of the mixes. A minimum
of two 6- by 12-inch compressive
strength cylinders should be prepared to represent each sampled
batch. In the initial stages of a job,
three test cylinders are recommended in order to obtain representative data. Multiple 6 by 12
cylinders are recommended because the strength can easily vary
± 50 percent due to the low strength, inherent variability of the material,
handling and testing machine sensitively.
The preparation and testing of the flowable fill cylinders should be in
accordance with ASTM D 4832. The test procedure is similar to ASTM C
39 for concrete, except that flowable fill cylinders are more fragile, and
require more care in handling and testing.
A minimum compressive strength value of 40 pounds/inch2 is suggested (Howard 2015) so that the flowable fill would be stronger than
the existing soil, and have enough strength to be tested. The compressive
strength need not be higher than 80 psi because more strength is not necessary. The flowable fill should be able to be removed using an excavator,
not jackhammers. Sometimes there is a need to remove the hardened
flowable fill in case of repairs, crossing lines, etc.

Flow test
Using a standard slump cone for flowable
fill is not practical because the material can
spread up to three feet due to its fluidity.
The consistency (flowability) of the flowable
fill mixture can be measured using ASTM D
6103. This procedure was developed specifically to quantify the flow characteristics of
flowable fill. Those who work with concrete
are trained to keep the water content as low as possible. As a result, it is
sometimes difficult to get construction workers, transit mix operators, or
construction inspectors to add enough water to flowable fill so that the
mix is fluid enough to fill all void spaces between the pipe and the trench
walls. The test involves filling a 3- inch-by-6-inch open cylinder with the
slurry, lifting the cylinder, and measuring the diameter of the patty that
results. Two diameter measurements, at 90 degrees apart, are averaged,
and the average should be between 8 and 12 inches for the mix to have
the proper consistency.
For flowable fill mixtures placed on a slope, or when a stiffer mix is
required, the slump cone test (ASTM C 143) becomes a more useful indicator of consistency. A trial may be necessary at the beginning of the
placement to establish the appropriate slump. A balance should be found
between stiffness and filling all the space around the pipe.

Set time
Backfilling over flowable fill
is usually the day after the
flowable fill is placed. However, backfill and/or pavement over the flowable
fill sometimes needs to be
placed as quickly as possiUnderground Construction June 2017

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