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Flowable Fill, aka ... CLSM, Controlled Low Strength Material

Aka…Soil Replacement, K-Crete, Lean Mix Backfill, Flowable Mortar, Liquid Dirt, Controlled Density Fill etc…

Controlled Low Strength Material is a mixture of Portland Cement and/or fly ash, aggregates, water, and possibly chemical admixtures that, as the cement hydrates, forms a soil replacement material. CLSM is a self-compacting, flowable, cementitious material that is primarily used as a backfill or structural fill in lieu of compacted fill or as unsuitable native soil. Depending on the amount of water used in the CLSM mixture, it can also be placed as a non-flowable compacted material or as a mortar. (Defined in by American Society for Testing and Materials ASTM)

CLSM/Flowable Fill is not concrete. It is delivered in a ready mix truck just as concrete is, and has some of the same ingredients concrete has, but it is very different.

Backfill   Fast
Utitlity Trenches   Economical
Retaining Walls   Eliminates Settlement
Mud Jacking   Shrink Resistant
Float/Footing Support   Non-Corrosive
Pavement   Easily excavatable with proper mix design
Bases   Year round availability & placment
Bridge Conversion   Self-leveling & self-compacting
Erosion Control   Increased worker safety
Pipe Bedding   Can be color coded easily for utility identicication
Sub Base Stabilization   Can vary psi expectatations with mix design
Abandoned Tank Fills   Reduce in place costs-less labor, equipment & time
Sink Hole Fills    

Important Questions ....The Right Mix for the Right Project!

  • What is the application?
  • Job location? Where will it be used?
  • What are the surrounding soil conditions?
  • Does it need to be excavated in the future? If so, by hand or by machine?
  • Are there multiple locations to pour from?
  • What is your desired time frame for a workable surface?
  • What is your psi requirement?
  • Is the site accessible for a ready mix truck to drive to it easily?
  • Will it need to be pumped at the site?
  • How do you plan on testing the CLSM?
  • What are the climate conditions?
  • Are there any other requirements?
  • Are you working with in a specification design?

Testing CLSM:

Since CLSM is not concrete, the typical cylinder testing methods for concrete are not appropriate for testing CLSM.  The American Society for Testing and Materials has identified the following as appropriate for CLSM.

ASTM Designation: PS 31-95
Provisional Standard Test Method for Ball Drop on Controlled Low Strength Material to Determine Suitability for Load Application.

ASTM Designation:  PS 30-95
Provisional Standard Practice for Sampling Freshly Mixed Controlled Low Strength Material.

ASTM Designation:  PS 28-95
Provisional Standard Test Method for Flow Consistency of Controlled Low Strength Material.

ASTM Designation:  PS 29-95
Provisional Standard Test Method for Unit Weight, Yield, and Air Content (Gravimetric) of Controlled Low Strength Material


Flowable Fill a Self-Leveling Soil Replacement, Provides an Economical & Safe Choice to Filling Trenches

Excavatable and Non-excavatable Flowable Fill

Flowable Fill “Liquid Dirt” flows into place
Two Trial Mix Designs: #1 is more permanent, #2 is excavatable

Guideline for Flowable Fill or CLSM-Controlled Low Strength Material

Flowable Fill is an earthlike material to be used as soil replacement, it is self compacting and has a flowable consistency.

Typical uses include: utility trench backfill, mud jacking, erosion control, pipe bedding, subbase stabilization, sinkhole fills, construction mud mats, abandoned tank fills, and more.

The biggest benefits for using flowable fill instead of earth material is safety, ease of placement and compaction,economics and superior results. The alternative puts a worker in a trench to fill the hole in a series of earthcompacted lifts. This puts the worker in a potentially unsafe position. This is also a time consuming process and typically yields call backs when settlement occurs.


ACI 229 Controlled Low Strength Material (www.concrete.org) ASTM D6103, C939, C143, D6023, C1152,D4380, D1556, D2922, D6024, C403, D4832, D1196, D4429 NRMCA Guide Specification for Controlled Low Strength (www.nrmca.org)

Flowable Fill Strengths
A broad definition of flowable fill includes materials that have up to a 1200 psi compressive strength.
Type of Flowable: range for psi:
Excavatable or Removable 35-200 psi
Non-excavatable or Non-removable: 300-1200 psi (some at 200-300 psi are mechanically excavatable)


The Guideline below refers to Excavatable or Removable Flowable Fill:
Materials: Mix design parameters:
Cementitious Materials(Portland Cement Type I or II, Fly Ash or Slag) 100 - 300 lb.
(Maximum of 200 lbs of straight cement)
Sand 2000-3000 lbs(depends on air, water & cementitious materials)
Water water to cementitious ratio = 1.0 to 1.5
Air 10-30% (10-15% is common)
Unit Weight 118 pcf +/- 8

Important Notes:

  • Fly ash will increase the long‐term strength gain.

  • Fly ash will lower the early strength gain.

  • Increasing the fly ash will typically lower the water demand.

  • Ambient temperature WILL affect the placement and performance of the product.

  • The higher the air the more likely it will be removable in the future.

  • Flowable fill can be placed by belt. Pumping is difficult but the mix can be modified (increasing air) to make it more pumpable.

  • Be careful to not float a pipe when placing flowable fill. The pipe should be anchored down securely. Fill the trench to about the halfway point on the pipe and let it settle and cure. Then continue to fill the trench.

  • Remember the water in the mix must have somewhere to bleed off for the flowable fill to set up. In high plasticity clay soils caution must be taken to allow for the bleed water to rise and evaporate or migrate elsewhere.

  • The 3, 5 or 7 day compressive breaks are not necessarily indicators for the 28 day breaks.

  • If the material falls apart when removing the test cylinder, it is mostly likely excavatable or removable.

A Performance Specification Option: Maximum 200 psi compressive strength (at 28 days)