Primary application of CLSM [Controlled Low Strength Material] is as a structural fill or backfill in lieu of compacted soil. Compaction is not required and is ideal for use in tight or restricted-access areas where placing and compacting fill is difficult. Low density CLSM is especially advantageous where weak soil conditions are encountered and the weight of the fill must be minimized. Provides superior thermal insulation and shock mitigation properties.
Flowable Fill or Controlled Low Strength Material [CLSM], Low Density Controlled Low Strength Material [LD-CLSM] and Controlled Low Strength Material-Controlled Density Fill [CLSM-CDF] are all Engineered Geotechnical Fills. Common names for this material are flowable fill, controlled density fill, unshrinkable fill and soil-cement slurry.
Flowable fill is an engineered backfill material used as an alternative to compacted fill that can make backfill faster, is self leveling, and can obtain total compaction within a few hours of placement. Compressive strengths can be adjusted according to the project requirements. Placing as a permanent material or permitting re-excavation at a later date.
There are two types:  normal with a density or weight in the range of 1842-2323 kg/m³ [115-145 lb/ft³], which is greater than most compacted materials.  As a controlled low density/medium density fill using lightweight aggregates and/or with a pre-formed foam. CLSM is defined by the ACI Committee 229 as a cementitious material that is in a flowable state at the time of placement and having a specified compressive strength of 8.3 MPa [1,200 psi] or less at the age of 28 days. Flowabilty can be varied from very stiff to very fluid depending upon requirements.
Mixture proportions consists primarily of a filler, a binding agent of some type, such as ash cement or Class C or Class F fly ash and water that is usually twice of normal concrete. Some mixes contain a fine aggregate, fly ash (optional) , cement (small amount) and hydration. When a controlled density is required, foam or lightweight aggregates are added to lower the density or weight. Cement based mixes generate highest long-term strengths. Superior resistance to freezing and thawing and thermal cycles. Foam also eliminates segregation and settlement. Recycled and non-standard materials are excellent fillers for the use of/for the aggregate.
These to include or a combination of fly ash, slag, cinders, light expanded clay. Non-standard materials consist of material not meeting ASTM C33-99ael [Standard Specifications for Concrete Aggregates] that drastically reduce the cost of CLSM mixtures can be expanded polystyrene beads [extremely light material, about 100 times lighter than ordinary filling material], plastic waste, shredded tires, wood fiber, glass, sludge waste.
Before allowing the use of recycled waste materials, the producer of the Geotechnical fill must first evaluate the potential environmental effects of using these candidate materials to ensure compliance with federal and state environmental laws. Even if the use of recycled waste materials complies with environmental laws, materials that are determined to be non-hazardous wastes can still be toxic in the environment. Use of recycled materials that result in measurable environmental toxicity could result in the producer becoming liable for future clean up and/or damages to nearby third party landowners. To manage these potential environmental risks, you should develop a risk bioassay method that evaluates the relative toxicity of current used transportation materials compared to their proposed replacements. Currently there are methods to encapsulate or treat these materials but this will bring an additional cost to the engineered fill depending on the application. This would be particularly true if the fill is to be placed near water containment areas: wells, streams, ponds or lakes.
Using foam produced from a surfactant is not an environmental issue. However in some countries this can be a religious concern/significance. This would be the case when using hydrolyzed protein based surfactants.
CLSM is also being used as a foundation material where soft soil and weak organic deposits are to the extent of being very poor quality, greatly reducing stability or settlement problems. This system can reduce or balance loads, or transfers them to more resistant layers, improve or replace problem soils. Variations and combinations of the basic methods mentioned here may be applicable.
Typical mix proportions for CLSM are: fine aggregate known as the filler aggregate which makes up the major portion, 72% of a typical CLSM mixture and water 17% and portland cement providing cohesion at 3% - 7%.
Moderate strength in the 3.54 MPa [500 psi] to 10.3 MPa [1,500 psi] range is similar to that of many naturally occurring bedrock formations. 0.35 [50 psi] to 0.7 MPa [100 psi] compressive strengths designs is suggested for removability or re-excavation.
Lightweight foamed concrete fill has been used successfully to prevent increased loads on embankment foundations. By removing a quantity of existing fill or natural material and replacing it with an equal weight of lighter fill to the required grade lines, no additional load is transferred to the foundation soil. For example, if 0.3 meter of existing material, with a density of 1920 kg/m³ [120 lb/ft³], is excavated, 0.9 meters of lightweight fill with a density of 641 kg/m³ [40 lb/ft³] can be placed without introducing any additional loading on the foundation soils. It also used as a backfill to prevent increased lateral loading on existing abutments and retaining walls. In some placements a dense layer of lightweight foamed concrete is used as a footing base. Some placements also involve a dense top lift upon which a reinforced concrete pavement is placed directly.
Density/compressive strength, permeability, absorption, buoyancy and durability are all physical and chemical properties of foamed concrete which are of concern when it is used as an engineered construction material.
Water absorption is a great concern for a lightweight foamed concrete that has a density under 320 kg/m³ [20 lb/ft³]. Since 80 % of the mix is air, this can make it very porous. This high degree of absorption can lead to a 50-percent loss of compressive strength after saturation and additional significant deterioration in freeze and thaw cycle.
Design for these variables can be controlled by additional specifications by eliminating the water from the surface, encapsulation, or additional filler aggregates resistant to surface/subsurface hydrology. Expanded polystyrene [EPS] beads, which is an extremely light material, about 100 times lighter than ordinary filling material are a excellent choice. Not only does it have sufficient strength to sustain the loads in a given structure, it also is a good insulator. Another solution is placing a thin denser lightweight concrete on top of the less dense lightweight foamed concrete fill. Another consideration on ground water is since the behavior of lightweight foamed concrete is particularly sensitive to inundation, the groundwater level, as well as any seeps or springs, should be identified in the subsurface investigation. Where seeps and springs are present which may introduce water into the bottom or sides of the foamed concrete fill, drains or a drainage blanket of sized and graded granular material should be provided to carry the water away from the fill. Geotechnical mats are a good choice.
Recently airports in the United States as well as other countries world wide are placing a Low Density Controlled Low strength Material [LD-CLSM] as a Soft Ground Arresting System or Engineered Materials Arresting Systems [EMAS]. This system is constructed of high-energy-absorbing materials of selected strength located in the safety area, or overrun, of the runway. They are designed to crush under the weight of commercial airplanes to provide controlled deceleration force on the landing gear in case of an overrun. More information concerning EMAS is in Federal Aviation Administration Advisory Circular AC 150/5220-22, Engineered Materials Arresting Systems [EMAS] for Aircraft Overruns.