Controlled low-strength material is a self-leveling cementitious backfill made with a little cement, often fly ash, sand and plenty of water, poured in place of soil compacted in lifts. The category tops out at 1,200 psi, and mixes meant to be dug up later are kept around 50 to 150 psi. It flows around pipes and under slabs, needs no tamping and can usually be walked on within hours.
Utility trench gets CLSM, so tell the plant the excavatable mix, not the 1,000 psi stuff, or the gas company will never dig it out.
Also heard as
- flowable fill
- slurry
- lean fill
- controlled density fill
Do not mix it up with
- SCC: SCC is full-strength structural concrete that happens to flow; flowable fill is deliberately weak backfill.
Related terms
SCC
Self-consolidating concrete, a highly fluid mix that fills forms and wraps rebar under its own weight.
Rat slab
A thin layer of lean concrete over dirt that gives a clean working surface or seals a crawlspace floor.
Mix design
The recipe for a concrete mix: the weights of cement, SCMs, water, aggregates and admixtures per cubic yard.
Fly ash
Fine powder captured from coal-fired power plant exhaust, used to replace part of the cement in concrete.