“Will we be able to dig through this after it sets?” is one of the most common questions we’re asked about CJFill-UL Ultra Lightweight cellular concrete at CJGeo. Qualitatively, the answer is a solid “yes!”. The American Concrete Institutes’ Bulletin 229R-13, Report on Controlled Low-Strength Materials (CLSM), provides a methodology for quantifying the ease of digging through cured CLSM. ACI classifies cellular concrete as Low Density – Controlled Low-Strength Material (LD-CLSM)
Traditional Methods for Determining Excavatability
Traditionally, it has been up to owners to set a benchmark for “excavatable” CLSM vs “non-excavatable” CLSM. For example, North Carolina DOT specifies a maximum 150psi at 56 day compressive strength for excavatable flowable fill, and a maximum cement content of 100lb/cuyd. For non-excavatable material, the 28 day compressive strength just needs to be greater than 125psi, and a minimum of 100lb/cuyd of cement.
Limitations of Traditional Methods
Have you ever tried to dig through cement-stabilized soil? It can be really difficult to dig through even low strength material. The density of the material can have a huge impact on how easy it is to excavate something. Most reduction in density comes from the addition of air content. This makes sense–it’s way easier to dig through air bubbles than it is to dig through fine aggregate.
Cellular concrete is primarily made of air bubbles, uniformly distributed throughout a cement paste matrix. This chart shows the relationship between wet cast density & air content for a 0.60 w:c ratio neat mix cellular concrete.
Calculating the Removability Modulus
You need to know two physical properties of a material to calculate its removability modulus: Density, and 28 day unconfined compressive strength. Once you know those, the math is easy. The formula is RE = (W1.5 x 104 x C0.5 ) / 106, where W = dry mass density (lb/cuft), and C = 28 day unconfined compressive strength (lb/sqin). The following chart shows the removability modulus for CJFill cellular concrete. The red line is ACI 229’s benchmark value of 1. Where an RE less than one is considered removable, and an RE greater than one is considered not easily removable.
Applying the Removability Modulus
Applying the removability modulus in practice to evaluate different backfill materials is straightforward. The two required properties, strength & density, are readily available for any manufactured backfill material. This includes cellular concrete and flowable fill. The lower the removability modulus of a material, the easier it will be to excavate through in the future.
For example, on a recent project in Virginia, the designer specified 350psi flowable fill. This would have a density around 125lb/cuft, and was concerned about future excavatability. Because the material had to be pumped nearly 250 feet from the nearest point of access, but then placed near ground water levels, CJGeo proposed 65lb/cuft CJFill-ST, which has a compressive strength around 1100psi at 28 days. The specifier was initially concerned that the CJFill-ST would be more difficult to remove than the 350psi flowable fill. However, a removability modulus review showed an RM of 2.71 for the 350psi flowable fill, and of 1.8 for the 1100psi CJFill-ST. So, while both values were above the benchmark of RM 1 for easily removable, the CJFill-ST would be significantly easier to remove than the originally-specified 350psi flowable fill.
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