Flowable fill (CLSM) is a cementitious backfill placed as a fluid and held at 8.3 MPa (1,200 psi) or less by ACI PRC-229R-13(22), with most applications at 2.1 MPa (300 psi) or less. At 0.3 to 0.7 MPa (50 to 100 psi) it bears like well-compacted fill, and below 0.7 MPa (100 psi) at 90 to 180 days it stays diggable with hand tools.
Those strength levels decide almost every mix choice. A CLSM batch is sand and water, bound by 15 to 119 kg/m³ (25 to 200 lb/yd³) of portland cement and up to 415 kg/m³ (700 lb/yd³) of fly ash in the ACI 229R-13 ranges reported by NRMCA (2021). Air-generating admixtures lower its density and raise its flow, and accelerators are allowed except calcium chloride. In CLSM, the cement content and these concrete additives are dosed to keep strength low enough for later excavation.
The sections below cover the definition and strength levels, the admixtures and powders, the mix ranges with a worked batch, excavatability and the removability modulus, density and bearing, the governing documents with a specification checklist, and two dated US price ranges from 2019 and 2026.
The table below summarizes the core values of flowable fill with the document that states each one.
| Attribute | Value |
|---|---|
| Other names | Controlled low-strength material (CLSM), controlled density fill, flowable fill, soil-cement slurry |
| Maximum compressive strength | 8.3 MPa (1,200 psi) (ACI PRC-229R-13(22)) |
| Most applications | 2.1 MPa (300 psi) or less |
| Hand-tool excavation | below 0.7 MPa (100 psi) at 90 to 180 days |
| Bearing equal to well-compacted fill | 0.3 to 0.7 MPa (50 to 100 psi) |
| Wet density | 1,842 to 2,323 kg/m³ (115 to 145 lb/ft³) (NRMCA 2021) |
| Removable when | removability modulus RE below 1.0 |
| US guide | ACI PRC-229R-13(22) Report on Controlled Low-Strength Materials |
Each value is traced to its source: standards and codes to their issuing body, product values to the data sheet named with them, prices to the dated source beside them (methodology and fact-checking).
What Is Flowable Fill (CLSM)?#
Flowable fill is controlled low-strength material (CLSM), also called controlled density fill or soil-cement slurry, a low-strength cementitious material proportioned to replace compacted fill. All four names describe one material with one strength ceiling.
The American Concrete Institute uses the term controlled low-strength material in its report ACI PRC-229R-13(22), adopted and published in June 2013, reapproved in 2022, 22 pages long, and superseding ACI 229R-99. The report caps CLSM at 8.3 MPa (1,200 psi), which separates it from structural concrete by strength rather than by ingredients: the same cement, fly ash, sand, water and admixtures appear in both.
The replacement logic is a bearing comparison. A CLSM at 0.3 to 0.7 MPa (50 to 100 psi) equals well-compacted fill in bearing capacity (ACI PRC-229R-13(22)), so a trench, void or pipe bedding filled with it needs no lift-by-lift compaction. That strength-defined purpose places flowable fill among the special concretes, next to high-strength concrete and UHPC at the other end of the strength scale.
Strength range of controlled low-strength material#
Three strength levels define CLSM: the 8.3 MPa (1,200 psi) ceiling, 2.1 MPa (300 psi) or less for most uses, and below 0.7 MPa (100 psi) long-term for hand-excavatable fill. The three levels are listed below with their meaning.
- Ceiling: 8.3 MPa (1,200 psi), the maximum compressive strength of any CLSM under ACI PRC-229R-13(22).
- Most applications: 2.1 MPa (300 psi) or less, the range NRMCA (2021) reports from ACI 229R-13 for the bulk of backfill and void-fill work.
- Hand-excavatable: below 0.7 MPa (100 psi) at 90 to 180 days, so the fill can later be dug out with hand tools.
The lowest level is the one that controls a mix most tightly. A fill that reaches 0.7 MPa (100 psi) at 28 days has already lost hand excavatability, since the limit applies at 90 to 180 days, not at 28.
Which Admixtures and Cementitious Materials Make Flowable Fill?#
Flowable fill is made from portland cement, fly ash and other powders plus two admixture functions: air or foam generation, and acceleration. ACI PRC-212.3-16 treats this use in its chapter on miscellaneous admixtures, section 16.7 "cellular concrete/flowable fill", and the UK CAA covers foamed concrete and CLSM together in Admixture Technical Sheet ATS 10.
The table below compares the components of a CLSM batch by their role.
| Component | Role in flowable fill | Source |
|---|---|---|
| Portland cement | Binder; content sets strength and excavatability | ACI 229R-13 via NRMCA 2021 |
| Fly ash, Class C or F | Filler and binder; ASTM C618 conformance preferred, not necessary | ACI 229R-13 via NRMCA 2021 |
| Cement kiln dust, silica fume, slag | Acceptable cementitious materials | ACI 229R-13 via NRMCA 2021 |
| Fine aggregate | Bulk of the mass | ACI 229R-13 via NRMCA 2021 |
| Air-entraining admixture or foaming agent | Lowers density, raises flow and yield | ACI PRC-212.3-16 section 16.7; product data |
| Accelerator | Speeds hardening; calcium chloride excluded | ACI 229R-13 via NRMCA 2021 |
The cement and ash decide strength, while the admixtures adjust density, flow and the time before loading.
Air-generating admixtures for lighter, more fluid fill#
Air-generating admixtures lower density and raise flow; the CLSM foaming agent Aerflow is dosed at 116 to 193 mL/m³ (3 to 5 fl oz/yd³) of fill without dilution or foam generator, as a typical product value. A second product, Stable Air, reports air contents up to 80 % in a flowable fill mix.
How large is the effect? The table below lists the published values of two products, each a product value and not a standard requirement.
| Product (data sheet, edition) | Published value |
|---|---|
| Aerflow (Aerix Industries foaming agents comparison chart, November 2023) | 116 to 193 mL/m³ (3 to 5 fl oz/yd³), no dilution, no foam generator; CLSM density 1,602 to 1,842 kg/m³ (100 to 115 lb/ft³) |
| Aerflow (Aerix Industries product page) | 116 mL/m³ (3 fl oz/yd³) raises slump from 38 mm (1.5 in) to 178 to 229 mm (7 to 9 in); yield +20 to 25 % |
| Stable Air (Cellular Concrete Technologies data sheet, 2016) | up to 80 % air in a flowable fill mix; lifts up to 3.0 m (10 ft) without collapse |
The yield gain is the economic point: 20 to 25 % more volume from the same powders lowers the cement and ash per cubic meter of placed fill.
Accelerating admixtures in flowable fill#
Accelerators may be used in CLSM, but calcium chloride is excluded, as NRMCA (2021) reports from ACI 229R-13. A producer who needs earlier loading therefore selects a non-chloride accelerator and states the exclusion in the mix submittal.
Cement, ash and other powders#
Portland cement, Class C or F fly ash, cement kiln dust, silica fume and slag are all acceptable in CLSM, and ASTM C618 conformance of the ash is preferred but not necessary (ACI 229R-13 via NRMCA 2021). That tolerance lets producers use ash that misses a concrete-grade requirement.
The calcium content of the ash matters more than its class. For excavatable mixes, NRMCA (2021) describes fly ash with a CaO content above 10 % as a concern, which is guidance, not a specification limit. ASTM C618-26, active since 2026-08-11, sets the Class F boundary at a CaO maximum of 18 %, with Class C above 18 %. A Class F ash can therefore carry 10 to 18 % CaO and still raise the concern. Our editorial recommendation is that a specification for excavatable fill states a CaO limit chosen by the owner, not only an ash class.
How Is a Flowable Fill Mix Proportioned?#
A flowable fill mix contains fine aggregate at 1,483 to 2,077 kg/m³ (2,500 to 3,500 lb/yd³), with 237 to 297 kg/m³ water, 15 to 119 kg/m³ cement and 0 to 415 kg/m³ fly ash. NRMCA (2021) reports these ranges from ACI 229R-13 with 2 to 5 % entrapped air.
The table below compares the ACI 229R-13 ranges with a worked starting batch.
| Constituent | ACI 229R-13 range, kg/m³ (lb/yd³) | Worked batch, kg/m³ (lb/yd³) |
|---|---|---|
| Fine aggregate | 1,483 to 2,077 (2,500 to 3,500) | 1,780 (3,000) |
| Water | 237 to 297 (400 to 500) | 267 (450) |
| Portland cement | 15 to 119 (25 to 200) | 30 (50) |
| Fly ash | 0 to 415 (0 to 700) | 178 (300) |
| Entrapped air | 2 to 5 % | not batched |
| Total | 2,255 (3,800) |
Worked example (calculated from the ACI 229R-13 ranges and the NRMCA starting point). Inputs: fine aggregate 1,780 kg/m³ (3,000 lb/yd³), water 267 kg/m³ (450 lb/yd³), cement 30 kg/m³ (50 lb/yd³) and fly ash 178 kg/m³ (300 lb/yd³). The cement and ash sit inside the NRMCA (2021) cementitious starting point of 15 to 59 kg/m³ (25 to 100 lb/yd³) cement with up to 178 kg/m³ (300 lb/yd³) fly ash. The batch masses add up to 1,780 + 267 + 30 + 178 = 2,255 kg/m³ (3,800 lb/yd³, or 141 lb/ft³). That sum equals the wet density only if the batch yields exactly 1 m³, so it does not prove the yield. The yield is checked on a trial batch: the batch mass divided by the measured wet density gives the volume actually produced, and the 2 to 5 % entrapped air adds volume, so the sand content is adjusted until the batch yields 1 m³. Strength is measured on the same trial batches, because the ranges set no strength by themselves.
Excavatable versus non-excavatable mixes#
Excavatable mixes cap cement at 59 kg/m³ (100 lb/yd³), and non-excavatable mixes start there, in the North Carolina DOT (NCDOT) 2018 example. ACI 229R-13 guidance via NRMCA (2021) gives the excavatable range as 24 to 59 kg/m³ (40 to 100 lb/yd³) cement with Class F fly ash up to 208 kg/m³ (350 lb/yd³).
The table below compares the two NCDOT 2018 mix types.
| Type (NCDOT 2018 example) | Strength limit | Cement limit |
|---|---|---|
| Excavatable | max 1.0 MPa (150 psi) at 56 days | max 59 kg/m³ (100 lb/yd³) |
| Non-excavatable | more than 0.86 MPa (125 psi) at 28 days | min 59 kg/m³ (100 lb/yd³) |
The 59 kg/m³ (100 lb/yd³) line is a cement threshold, not a strength guarantee: the excavatable mix still has to meet its 1.0 MPa (150 psi) limit at 56 days, and NRMCA (2021) describes fly ash CaO above 10 % as a concern next to the cement cap.
How Is the Excavatability of Flowable Fill Controlled?#
Excavatability is controlled by capping long-term strength below 0.7 MPa (100 psi) and checking the removability modulus, which must stay below 1.0. Both limits follow ACI PRC-229R-13(22) as reported by NRMCA (2021).
The three control steps are listed below in the order a mix designer applies them.
- Cap the binder: 24 to 59 kg/m³ (40 to 100 lb/yd³) cement with Class F fly ash up to 208 kg/m³ (350 lb/yd³), and treat ash CaO above 10 % as a concern (NRMCA 2021 guidance).
- Calculate the removability modulus RE from the dry density and the 28-day strength, and accept the mix only with RE below 1.0.
- Confirm that strength stays below 0.7 MPa (100 psi) at 90 to 180 days where hand-tool excavation is required.
Steps 2 and 3 answer different questions. The removability modulus uses the 28-day result and the density together, since a dense fill resists digging at a lower strength than a light one; the long-term check catches strength gain after 28 days.
Removability modulus (RE) worked through#
The removability modulus combines dry density and 28-day strength into one number, and RE below 1.0 means the fill is removable. The formula exists in two unit sets with different coefficients:
- US units: RE = W^1.5 x 104 x C^0.5 / 10^6, with W the dry density in lb/ft³ and C the 28-day unconfined compressive strength in psi.
- SI units: RE = W^1.5 x 0.619 x C^0.5 / 10^6, with W in kg/m³ and C in kPa.
The coefficient in the US form is 104, not 10^4; a misread exponent inflates RE by a factor of about 96.
Worked example (calculated from the NRMCA 2021 formula). Inputs: dry density W = 1,922 kg/m³ (120 lb/ft³), 28-day strength C = 345 kPa (50 psi). In US units, 120^1.5 = 1,314.5; multiplied by 104 and by the square root of 50 (7.07), the product is 966,700, so RE = 0.97, removable. At the same density and C = 690 kPa (100 psi), the square root rises to 10, and RE = 1.37: not easily removed. The SI form gives the same two results, 0.97 and 1.37.
What Density and Bearing Capacity Does Flowable Fill Reach?#
Flowable fill weighs 1,842 to 2,323 kg/m³ (115 to 145 lb/ft³) wet and bears like well-compacted fill at 0.3 to 0.7 MPa (50 to 100 psi). Mixes of fly ash, cement and water without sand are lighter, at 1,442 to 1,602 kg/m³ (90 to 100 lb/ft³) (NRMCA 2021).
The table below compares density and strength values by mix type and source.
| Property | Value | Source |
|---|---|---|
| Wet density, normal CLSM | 1,842 to 2,323 kg/m³ (115 to 145 lb/ft³) | NRMCA 2021 |
| Wet density, fly ash-cement-water | 1,442 to 1,602 kg/m³ (90 to 100 lb/ft³) | NRMCA 2021 |
| Wet density, CLSM with Aerflow | 1,602 to 1,842 kg/m³ (100 to 115 lb/ft³) | product value, Aerix Industries 2023 |
| Bearing equal to well-compacted fill | 0.3 to 0.7 MPa (50 to 100 psi) | ACI PRC-229R-13(22) |
| Maximum compressive strength | 8.3 MPa (1,200 psi) | ACI PRC-229R-13(22) |
Density and strength are separate design targets: a lighter fill lowers the load on a pipe or soft subgrade, while 0.3 MPa (50 psi) already matches well-compacted soil in bearing.
Which Standards and Guides Govern Flowable Fill?#
ACI PRC-229R-13(22) is the governing US guide for CLSM; no ASTM or EN product specification for CLSM as a material is listed in the sources cited here. The table below covers the guides and the ingredient standards a CLSM specification draws on; field test methods are set in the project specification.
The table below lists the documents a CLSM specification draws on, with edition and scope.
| Document | Edition and status | What it covers for CLSM |
|---|---|---|
| ACI PRC-229R-13(22) | Adopted June 2013, reapproved 2022, supersedes ACI 229R-99 | Definition, 8.3 MPa (1,200 psi) ceiling, proportioning guidance |
| ACI PRC-212.3-16 | Current admixture report (formerly ACI 212.3R-16) | Section 16.7, cellular concrete and flowable fill admixtures |
| CAA ATS 10 | Admixture Technical Sheet | Foamed concrete and CLSM admixtures (UK) |
| ASTM C618-26 | Active since 2026-08-11 | Fly ash classes; Class F CaO max 18 % |
| ASTM C869/C869M-24 | Active | Foaming agents for preformed foam |
| ASTM C796/C796M-25 | Active | Test method for foaming agents |
| NCDOT 2018 | State DOT example | Excavatable and non-excavatable strength and cement limits |
A project specification combines these: strength levels from ACI, ash limits from C618 plus a CaO cap, and acceptance limits from the owner, as in the NCDOT example.
How to specify flowable fill#
Five items make a CLSM specification: strength at a stated age, excavatability, a cement cap, the ash CaO limit, and the admixtures allowed. The five items are listed below as specification actions.
- State the required compressive strength and its test age; the ACI PRC-229R-13(22) ceiling is 8.3 MPa (1,200 psi), and most applications need 2.1 MPa (300 psi) or less.
- Define excavatability as below 0.7 MPa (100 psi) at 90 to 180 days, with the removability modulus RE below 1.0.
- Cap cement at 59 kg/m³ (100 lb/yd³) for excavatable fill.
- State a fly ash CaO limit for excavatable fill instead of naming only Class F under ASTM C618-26; NRMCA (2021) describes CaO above 10 % as a concern.
- Allow air-generating admixtures and accelerators, and exclude calcium chloride.
What Does Flowable Fill Cost?#
A 0.34 MPa (50 psi) flowable fill was bid at USD 89 to 109 per m³ (USD 68 to 83 per yd³) in the 2019 bid tabulation of St. Charles County, Missouri. That 2019 range is a public record for one US county, not a national price. A US producer calculator listed flowable fill at USD 144 to 183 per m³ (USD 110 to 140 per yd³) in 2026, a single producer's figure for its own market.
A quote for flowable fill depends on four factors, listed below.
- Cement content: 15 to 119 kg/m³ (25 to 200 lb/yd³) in the ACI 229R-13 range, capped at 59 kg/m³ (100 lb/yd³) for excavatable fill.
- Fly ash content and grade: 0 to 415 kg/m³ (0 to 700 lb/yd³), with a CaO limit narrowing the usable sources.
- Air-generating admixture: a product yield gain of 20 to 25 % (Aerflow product value) cuts the powder per cubic meter placed.
- Strength level and excavatability: a non-excavatable mix above 0.86 MPa (125 psi) needs more cement than an excavatable one.
How Does Flowable Fill Differ from Cellular Concrete and SCC?#
Flowable fill is defined by its strength ceiling, cellular concrete by its density, and SCC by consolidation without vibration. Low-density cast-in-place cellular concrete has an oven-dry density of 800 kg/m³ (50 lb/ft³) or less under ACI PRC-523.1-06, far below the CLSM range. Foam-generated density is the defining attribute of cellular concrete, while strength defines CLSM.
The search term "flowable concrete" belongs to neither group. Flowing concrete is structural concrete made fluid with a high-range water reducer of ASTM C494 Type F or G; ASTM C1017, its former specification, was withdrawn in 2022.
Slab lifting and void injection under existing slabs are repair topics outside this page.
Self-compacting concrete is structural concrete that ACI CT-25 defines as highly flowable and non-segregating, spreading into place and filling formwork without mechanical consolidation. CLSM also flows into place, but it is held below 8.3 MPa (1,200 psi), while the flow, stability and fresh tests of self-compacting concrete (SCC) serve full-strength structural work.
Flowable fill FAQs#
Is CLSM the same as flowable fill?#
Yes: CLSM, controlled low-strength material, controlled density fill and soil-cement slurry name one material. ACI PRC-229R-13(22) uses the term controlled low-strength material.
Does flowable fill get hard?#
Yes: flowable fill hardens to a compressive strength of up to 8.3 MPa (1,200 psi), and most mixes reach 2.1 MPa (300 psi) or less. Excavatable mixes are held below 0.7 MPa (100 psi) at 90 to 180 days.
What are the different types of flowable fill?#
Flowable fill comes in two types by excavatability, excavatable and non-excavatable, plus air- or foam-lightened mixes of either type. In the NCDOT 2018 example, excavatable fill stays at or below 1.0 MPa (150 psi) at 56 days, and non-excavatable fill exceeds 0.86 MPa (125 psi) at 28 days.