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While the broader definition includes materials with compressive strength less
than 1200 psi (8.3 MPa), most applications use mixtures with strength less than 300 psi
(2.1 MPa). The late-age strength of removable CLSM materials should be in the range of 30
to 200 psi (0.2 to 1.4 MPa) as measured by compressive strength of cylinders. It is important
that the expectation of future excavation of flowable fill material be stated when specifying
or ordering the material.
WHY is Flowable Fill Used?
Flowable fill is an economical alternative to compacted granular fill, considering the
savings in labor costs, equipment and time. Since it does not need manual compaction,
trench width or the size of excavation is significantly reduced. Placing flowable fill
does not require people to enter an excavation, a significant safety concern. CLSM is
also an excellent solution for filling inaccessible areas, such as underground tanks,
where compacted fill cannot be placed.
Uses of Flowable Fill include:
Strength: For later excavatability, the ultimate strength of the flowable fill must
be kept below 200 psi (1.4 MPa) to allow excavation by mechanical equipment, like back¬hoes.
For manual excavation the ultimate strength should be less than 50 psi (0.3 MPa). Mixtures
containing large amounts of coarse aggregate are more difficult to excavate. Mixtures with
entrained air in excess of 20% by volume are used to keep the strength low. Higher-strength
structural fills can be designed for a specific required strength. Compressive strength of
50 to 100 psi (0.3 to 0.7 MPa) provides an allowable bearing capacity similar to well-compacted soil.
Setting and Early Strength may be important where equipment, traffic, or construction loads
must be carried or subsequent construction needs to be scheduled. Judge the setting characteristics
by scraping off loose accumulations of water and fines on top and see how much force is necessary to
cause an indentation in the material. ASTM C 403 or ASTM D 6024 may be used to estimate the load-carrying
ability of the flowable fill. Penetration values by C 403 between 500 and 1500 psi are adequate for
loading flowable fill.
Density in place is usually in the 115 to 145 lb./cu. ft. range for non-air-entrained or
conventionally air-entrained mixtures. These densities are typically higher than most compacted fills.
If lightweight fills are needed to reduce the weight or to provide greater thermal insulation, high entrained
air (greater than 20%) mixtures, preformed foam, or lightweight aggregates may be used.
Flowability of flowable fill is important, so the mixture will flow into place and consolidate due
to its fluidity without vibration or puddling action. The flowability can be varied to suit the placement
requirements of most applications. Hydrostatic pressure and floatation of pipes should be considered by appropriate
anchorage or by placing in lifts.
Subsidence of some flowable fill mixtures with high water content is on the order of 1/4 inch per foot
(20 mm per meter) of depth as the solid materials settle. Mixtures with high air content use less water
and have little or no subsidence.
Permeability of flowable mixtures can be varied significantly to suit the application. Most mixtures
have permeability similar to or lower than compacted soil.
Durability: Flowable fill materials are not designed to resist freezing and thawing, abrasive or most
erosive actions, or aggressive chemicals. If these properties are required, use a high quality concrete.
Fill materials are usually buried in the ground or otherwise confined. If flowable fill deteriorates in
place it will continue to act as a granular fill.
HOW is Flowable Fill Delivered and Placed?
Flowable fill is delivered by ready mixed concrete truck mixers and placed easily by chute in a flowable
condition directly into the cavity to be filled. To avoid segregation, the drum should be kept agitating.
Flowable fill can be conveyed by pump, chutes or buckets to its final location. For efficient pumping, some
granular material is needed in the mixture. Due to its fluid consistency it can flow long distances from the
point of placement.
Flowable fill does not need to be cured like concrete, but should be protected from freezing until it has hardened.
Testing Flowable Fill Mixtures
Quality assurance testing is not necessary for pre-tested standard mixtures of flowable fill. Visual checks of mixture
consistency and performance have proven adequate. Test methods and acceptance criteria for concrete are generally not applicable.
Testing may be appropriate with new mixtures, or if non-standard materials are used.
- Obtain samples for testing flowable fill mixtures in accordance with ASTM D 5971.
- Flow consistency is measured in accordance with ASTM D 6103. A uniform spread diameter of at least 8 in.
without segregation is necessary for good flowability. Another method of measuring flowability is with a flow
cone (ASTM C 939). The mixture tested should not contain coarse aggregate retained on the No. 4 (4.75-mm) sieve.
An efflux time of 10 to 26 seconds is generally recommended.
- Unit weight, yield, and air content of flowable fill are measured by ASTM D 6023.
- Preparing and testing cylinders for compressive strength is described in ASTM D 4832. Use 3 x 6 in. (75 x 150 mm) plastic cylinder
molds, fill to overflowing and then tap sides lightly. Other sizes and types of molds may be used as long as the length to diameter ratio is
2:1. Cure cylinders in the molds (covered) until time of testing (or at least 14 days). Strip carefully using a knife to cut plastic mold off.
Capping with sulfur compounds can damage these low-strength specimens. Neoprene caps have been used, but high strength gypsum
plasters seem to work best.
- Penetration resistance tests such as ASTM C 403 may be useful in judging the setting and strength development. Penetration resistance
numbers of 500 to 1500 indicate adequate hardening. A penetration value of 4000, which is roughly 100 psi (0.7 MPa) compressive
cylinder strength, is greater than the bearing capacity of most compacted soil. Another method of testing for adequate hardening after
placement is the ball drop test, ASTM D 6024. A diameter of indentation of less than 3 in. (75 mm) is considered adequate for most load
applications. A relationship between the strength gain of the flowable fill and the penetration resistance can be developed for specific
mixtures.
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