Air-Modified Controlled Low-Strength Material

By Hank Hauge, Technical Sales Engineer, Master Builders Technologies

ACI 116 defines controlled low-strength material (CLSM) as a material that results in a compressive strength of 1200 psi or less.¹ Falling within this strength limit is a broad range of highly fluid mixtures that traditionally have been referred to in different geographical areas by terms such as flowable fill, controlled density fill, backfill, soil-cement slurry, foamed concrete, and cellular concrete.

The use of CLSM is attractive because of a wide range of benefits that the materials provide. Several of the benefits are listed in ACI 229R-94 and include the following: versatile, readily available, easy to place, limited settlement, strong and durable, can be excavated, reduces equipment need, and reduces excavation costs.² These benefits can be realized in applications such as backfills, structural fills, insulating and isolation fills, pavement bases, conduit bedding, and void filling.

ü Is the void completely filled?
ü Will there be any settlement?

ü Will there be any shrinkage?
ü Is the material homogeneous throughout, insuring consistent engineering properties?
ü Can it be excavated in the future?
ü Is the CLSM ready to receive the final wear surface?

The performances of traditional water based CLSM mixtures fall short in addressing these issues. Typical problems with water based CLSM mixtures develop due to the amount of extra mechanical lubrication water necessary to obtain fluid properties.

These problems include:

Air-Modified CLSM

The engineering and material properties of CLSM can be altered through the use of air-modifying admixtures, reducing the water content of the mix while still maintaining fluid properties. Air-modified low-water content CLSM retains the compressibility, shear, load-bearing, and flowability properties of water based CLSM, while improving permeability, shrinkage, bleeding and freeze-thaw properties. Because of the lower water content, air-modified CLSM maintains a homogeneous matrix during placement and setting, with minimal segregation and bleeding.³ An added benefit of low water content is that the use of other chemical admixtures such as water reducers, accelerators, and dyes are applicable. These additives do not become overly diluted or washed out of the mixture when incorporated into the material mix.

CLSM can be classified as either regular or low-density, depending on the density of the material. Mixtures that have a density of 50 lb/ft³ or more are generally classified as regular CLSM (Reg-CLSM) and those with a density less than that are classified as low-density CLSM (LD-CLSM). The air content of Reg-CLSM mixtures typically does not exceed 35 percent, whereas LD-CLSM mixtures can have air contents of up to 80 percent. The density and compressive strength requirements for a given application will influence the choice of mixture constituents and their proportions.

The use of air-modifying admixtures allows for the design of specific engineering properties such as predictable strengths and densities, while eliminating concerns regarding settlement, shrinkage, segregation, high ultimate strengths, and bleed water traditionally associated with "flowable fill". Required engineering properties are obtained by varying the mix designs and the quantity of air-modifying admixtures.

CLSM Testing

Test methods are continuing to be developed and refined as more experience is gained. This is leading to an increase in confidence on the part of CLSM producers, encouraging them to gain more product experience and to create a higher quality product. As engineers and architects also become familiar with CLSM, and their confidence in it increases, they will include CLSM in more project specifications.

Traditional testing procedures evolved from the point of view that CLSM is "weak concrete". Therefore, traditional concrete tests were modified to accommodate CLSM, and the following ASTM Test Methods were established in 1996:

Suitability for Load Application"

The above tests are useful. However, the temptation to overtest this material is great. It must be remembered that CLSM is merely "strong dirt". The testing that is required should parallel that for ordinary compacted granular back fill, answering the questions:

ü Is the void completely filled?
ü Will there be any settlement?
ü Will the material be excavatable?

Much discussion has ensued on the topic of additional test methods for CLSM. The construction industry is realizing that procedures and results of several soil tests and other applicable backfill tests are of considerable value in determining the quality of "strong dirt". Useful soil properties tests include compressive strength, direct shear, California bearing ratio (CBR), triaxial shear, incremental consolidation, water permeability, freeze-thaw, and bleeding. Results from these tests more closely quantify and qualify the engineering properties that are of concern for fill materials.

Summary

Air-modified CLSM is frequently used as a backfill and void filling material in many locations across the United States and Canada. The predictable engineering properties and labor saving attributes of air-modified CLSM are making it the material of choice for numerous applications. Air-modified CLSM provides the engineer and contractor with another tool to help solve the many challenges of constructing and maintaining today's civil infrastructure.

References

1. ACI 116R-90, "Cement and Concrete Terminology," ACI Manual of Concrete Practice,

American Concrete Institute, Detroit, MI

2. ACI 229R-94, "Controlled Low-Strength Materials (CLSM)," ACI Manual of Concrete Practice,

American Concrete Institute, Detroit, MI

3. Gianetti, F., Rear, K., and Callender, I., "Non-Shrink Flowable Fill: A Revolutionary Cementitous

Backfill Mixture Manufactured by Ready Mix Concrete Producers," Presented at the

11^th European Ready Mix Concrete Congress, Istanbul, Turkey