If you had a choice, would yo u

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I

f you had a choice, would yo u backfill with a fluid material that f l ows as easily as thick pancake batter and is self-leveling? Or would you use a dry, granular mate-rial that must be placed in lifts and s p read and compacted after each lift? Most contractors do have a choice between these two types of backfil1, and many are opting to use the material that flows.

This material has seve ral generi c names including flowable fill, lean-mix backfill, unshrinkable fill, flow-able mort a r, and contro l l e d - d e n s i t y fill (CDF). But the name given to it by A m e rican Co n c rete Institute (AC I ) Committee 229 seems to be gaining the most acceptance—c o n t ro l l e d l ow - s t rength material ( C L S M ) .

CLSM is not a concrete nor a soil-cement but it has pro p e rties similar to both. It’s a fluid mixture made of p o rtland cement, water, and fine ag-g reag-gate or fly ash or both. So m e-times it also contains an admixture. The consistency of CLSM is like that of a slurry or lean grout, yet seve ra l hours after placement the materi a l h a rdens enough to support tra f f i c loads without settling. Typical 28-day compre s s i ve strengths ra n g e f rom 50 to 200 psi—more than the s t rength of most compacted soil or g ranular fills. Densities range fro m 115 to 145 pounds per cubic foot.

W h e re can contractors get CLSM? Chances are it’s available from their local ready mix suppliers. The mate-rial contains many of the same components found in concrete but in different pro p o rt i o n s. There f o re, it can be batched and mixed using

the same equipment used in pro-ducing concrete and delive red to the jobsite by ready mix tru c k .

Applications

CLSM can replace compacted soil as stru c t u ral fill or backfill in many

Controlled low

strength material

A cementitious backfill that flows like a liquid,

supports like a solid, and self-levels without

tamping or compacting

BYANNESMITH

Flowing down the chute of a ready mix truck, CLSM quickly fills this deep utility trench. Because CLSM self-levels and self-compacts, labor is minimal. A worker

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a p p l i c a t i o n s. Because CLSM flow s and needs no compacting, it’s ideal for use in tight or re s t ri c t e d - a c c e s s a reas where placing and compact-ing soil or granular fill is difficult or e ven impossible. Examples include: • Filling voids under existing p a ve m e n t s, buildings, or other s t ru c t u re s

• Backfilling narrow tre n c h e s • Filling abandoned underg ro u n d

s t ru c t u res such as culve rt s, p i p e s, tunnels, storage tanks, we l l s, and sewe r s

Utility companies often specify CLSM instead of soil for backfilling a round pipes or conduits. The ma-t e rial flows under and around pipes, p roviding uniform support without leaving vo i d s. Se l f - l e veling, CLSM also eliminates the chance of work-ers accidentally damaging pipes by o p e rating compaction equipment near them. If easy access to utility lines is essential for maintenance or re p a i r s, CLSM compre s s i ve s t rengths can be specified at or be-l ow 100 psi. At these stre n g t h s, the m a t e rial can be exc a vated easily with a backhoe or other digging e q u i p m e n t .

CLSM also has applications for p a vement construction and main-t e n a n c e. Used under ro a d w a y s, imain-t s e rves as a strong, stable subbase. As a fill material for pavement sec-tion replacement, CLSM solidifies enough to support the patch as soon as 3 hours after placement, de-pending on the mixed used, we a t h-er conditions, and the depth of the t rench. If it’s not possible to place the patch right away, CLSM can t e m p o ra rily support traffic when placed up to pavement gra d e. Be-f o re placing the permanent pave-ment section, re m ove CLSM to the depth re q u i red to accommodate the section. By using CLSM with high-e a r l y- s t rhigh-ength concrhigh-ethigh-e patchhigh-es, c o n t ractors can open a re s t o re d p a vement to traffic in less than 8 hours (Ref. 1).

In l988, the De p a rtment of Pu b l i c Se rvices of Pe o ria, Il l i n o i s, in coop-e ration with thcoop-e Illinois Co n c rcoop-e t coop-e Council, studied the field perf o r-mance of CLSM as a backfill for

util-ity repairs in the cutil-ity’s streets (Re f . 2). Having seve re problems with set-tlement of soil backfill in utility t re n c h e s, the department was look-ing for an altern a t i ve backfill mater-ial. It conducted seve ral experi-ments using a CLSM mix containing

50 pounds portland cement, 200 pounds fly ash, 2,990 pounds fine a g g re g a t e, and 49 to 57 gallons of water per cubic yard. CLSM was placed in trenches with depths ranging from 3 to 9 feet. Tre n c h widths also va ried. In all cases, CLSM achieve d densities of more than 120 pounds per cubic foot

This pro j e c t , completed in 1987, used CLSM as backfill for the c a s t - i n - p l a c e c o n c re t e

foundation walls of a building addition. Because the addition was built about 10 feet fro m the existing building, the e x c a v a t i o n between the buildings was not accessible by t ruck or loader. T h a t ’s one re a s o n the pro j e c t manager chose to use CLSM instead of soil backfill. Workers could place CLSM easily with a crane-mounted bucket. Safety was another reason for using CLSM. Workers did not have to stand in the more than 1 4 - f o o t - d e e p t rench to compact the fill. Pouring a total of 700 cubic y a rds of CLSM, workers backfilled the trench in just 2 days, After backfilling, the g round floors of the addition and the existing building w e re connected over the backfilled t rench by a 6-inch-thick concre t e s l a b .

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and shrinkage was minimal. The material set quickly and could support a per-s o n’per-s weight within 2 to 3 h o u r s. Ex p e riments placing p a vement patches on CLSM within 3 to 4 hours p roved successful. In one test, a pavement patch was successfully placed on a s e wer trench immediately after the trench was back-filled. Convinced by these e x p e riments of CLSM’s su-p e rior su-perf o rm a n c e, the de-p a rtment changed its back-filling pro c e d u res to re q u i re use of the materi a l .

The most common use of CLSM by the Iowa De p a rt-ment of Tra n s p o rt a t i o n ( D OT) is to repair old b ridges by conve rting them into culve rts (Ref. 3). Fi r s t , enough culve rt pipes are placed under the bridge to handle water flow re q u i re-m e n t s. Wrapping the pipes in polypro pylene sheeting and placing dirt dams at their ends pre vents infiltra-tion of CLSM as it’s placed. Ready mix trucks then place CLSM until the material is only a few inches from the bottom of the bridge deck. A few days later, more CLSM is pumped thro u g h c o re holes in the deck until the material comes out the holes at the deck’s highest point. Wo rkers then re m ove

railings from the sides of the bri d g e and widen the deck as if it we re a slab on gra d e. Using this method, the Iowa DOT can conve rt about four bridges for the price of building a new one.

Mix design and performance A typical CLSM mix contains ce-ment, water, fly ash, and fine aggre-g a t e. But ready mix producers can combine these components in va ry-ing pro p o rtions to meet specific p e rf o rmance re q u i rements and to take advantage of locally ava i l a b l e m a t e rials (see chart). Some mixes,

for example, use all fly ash or all fine a g g regate instead of both, depend-ing on material cost and ava i l a b i l i-t y. He re are i-the roles each compo-nent plays in CLSM perf o rm a n c e :

Wa t e r—The large quantity of wa-ter used in CLSM allows the mawa-teri- materi-al to flow re a d i l y, self-consolidate, and self-level. Water contents va ry depending on flowability and s t rength re q u i re m e n t s. In c re a s i n g the water-to-solids ratio incre a s e s C L S M ’s flowability but reduces its c o m p re s s i ve stre n g t h .

Po rtland cement—As in con-c re t e, the portland con-cement in CLSM f o rms a paste with water to bind the

a g g regate and fly ash. Al-though CLSM contains much smaller amounts of p o rtland cement than does c o n c re t e, sufficient hyd ra-tion occurs to produce a h a rdened mass that will not settle. Use Type I or Type II portland cement c o n f o rming to ASTM C 150 or blended cements con-f o rming to ASTM C 595. Ge n e ra l l y, a higher cement content produces gre a t e r c o m p re s s i ve stre n g t h .

Fly ash—The pri m a ry role of fly ash in CLSM is to i m p rove flow a b i l i t y. It also i n c reases strength slightly and reduces bleeding, s h ri n k a g e, and perm e a b i l i-t y. Specify ASTM C 618 Class C or Class F fly ash. If fly ash is not readily ava i l-a b l e, the rel-ady mix supplier can produce CLSM using only cement, fine aggre-g a t e, and water. In open-t rench backfilling, flow a b i l-ity is not as critical as when filling a cavity through a small opening. There f o re, t renches can be backfilled e f f e c t i vely using CLSM that d o e s n’t contain fly ash. Bu t if fly ash is plentiful and a vailable at low cost, fly ash can be used alone without fine aggre g a t e.

Fine aggre g a t e— A g g re-gate increases the density of CLSM but reduces its flow a-b i l i t y. CLSM is more economical to p roduce with local aggre g a t e. Aggre-gate meeting ASTM specifications or nonspecification aggregate can be used as long as the material isn’t ex-p a n s i ve or re a c t i ve. Aggregates used successfully include (Ref. 4):

• Pea gra vel with sand • 3_⁄

4-inch minus aggregate with

s a n d

• Na t i ve sandy soils with more than 10% passing a #200 sieve • Qu a r ry waste products (genera

l-ly 3_⁄

8-inch minus aggre g a t e )

• ASTM C 33 specification aggre-gate within specific gra d a t i o n s

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Most chemical admixtures can be used in CLSM including water re-d u c e r s, superplasticize r s, anre-d accel-e ra t o r s. Using admixturaccel-es usually is

not cost-effective, so only add them to solve unique placement pro b-l e m s. Air- e n t raining agents can be specified to enhance the flow a b i l i t y

of CLSM and reduce its density. Bu t air contents greater than 6% can in-c rease segregation of the mix.

Like concre t e, CLSM develops its s t rength through cementitious and p oz zolanic re a c t i o n s. CLSM is not as d u rable as concre t e, but that’s usu-ally not a drawback since the mater-ial is used only to replace compact-ed soil or granular fill. CLSM is not designed to resist fre ezing and thawing, abrasion, or aggre s s i ve c h e m i c a l s. But CLSM usually is b u ried in the ground or otherw i s e confined, so even if it deteri o rates in place it still perf o rms effectively as g ranular fill.

When specifying CLSM compre s-s i ve s-stre n g t h s-s, cons-sider how the m a t e rial is to be used. CLSM 28-day c o m p re s s i ve strengths can ra n g e f rom 50 to 1200 psi. At 50 to 100 psi, the bearing capacity of CLSM is equal to that of soil having a bear-ing capacity of 3,500 pounds per s q u a re foot (Ref. 1). If CLSM needs to have greater bearing capacities, such as when used as a perm a n e n t s t ru c t u ral fill or to tempora rily sup-p o rt traffic loads, ssup-pecify a high-s t rength, high-cement-content mix. If using CLSM to bed utility lines re-q u i ring future maintenance, keep c o m p re s s i ve strengths below 100 psi so the fill will be easy to exc a va t e. At 100 psi or less, CLSM can be re-m oved with a backhoe or other dig-ging equipment. At strengths ex-ceeding 150 psi, CLSM genera l l y re q u i res re m oval by jackhammers or breakers (Ref. 5).

Densities of in-place CLSM ra n g e f rom 115 to 145 pounds per cubic foot—higher than the densities of most compacted soils. Be c a u s e CLSM is heaviest when wet, duri n g placement it exerts a high fluid pre s-s u re on form s-s, embankments-s, or walls used to contain the fill. If the job re q u i res a lightweight fill or a fill with insulating pro p e rt i e s, specify air entrainment, foaming agents, or l i g h t weight aggre g a t e. Fo a m i n g agents introduce air voids into the mix, lowe ring its density. The air voids also improve the mix’s insulat-ing qualities. Usinsulat-ing a foamed mix re-duces the lateral pre s s u res

pro-Although CLSM costs more per cubic yard than most soil or granular backfill materials, its many advantages result in lower in-place costs. For some applications, contractors can’t afford not to use CLSM. Its advan-tages, described below, facilitate the entire backfilling process, from or-der and delivery of materials to clean up.

• Readily available—Using locally available materials, ready mix suppli-ers can produce CLSM to meet most project specifications.

• Easy to deliver—Ready mix trucks can deliver specified quantities of CLSM to the jobsite whenever the material is needed.

• Easy to place—Depending on the type and location of void to be filled, CLSM can be placed by chute, conveyor, pump, or bucket. Be-cause CLSM is self-leveling, it needs little or no spreading or com-pacting. This speeds construction and reduces labor requirements. • Versatile—CLSM mix designs can be adjusted to meet specific fill

re-quirements. Add more water to improve flowability. Add more cement or fly ash to increase strength. Admixtures can be added to adjust setting times and other performance characteristics. Adding foaming agents to CLSM produces a lightweight, insulating fill.

• Strong and durable—Load-carrying capacities of CLSM typically are higher than those of compacted soil or granular fill. CLSM also is less permeable, thus more resistant to erosion. For use as a perma-nent structural fill, CLSM can be designed to achieve 28-day com-pressive strengths as high as 1200 psi.

• Can be excavated—CLSM having compressive strengths of 50 to 100 psi is easily excavated with conventional digging equipment yet is strong enough for most backfilling needs.

• Requires less inspection—During placement, soil backfill must be tested after each lift for sufficient compaction. CLSM self-compacts consistently and doesn’t need this extensive field testing.

• Allows fast return to traffic—Because CLSM can be placed quickly and can support traffic loads within several hours, it minimizes down-time for pavement repairs.

• Won’t settle—CLSM does not form voids during placement and won’t settle or rut under loading. This advantage is especially signifi-cant if the backfill is to be covered by a pavement patch. Soil or gran-ular fill, if not consolidated properly, may settle after a pavement patch is placed and form cracks or dips int he road.

• Reduces excavating costs—CLSM allows narrower trenches be-cause it eliminates having to widen trenches to accommodate com-paction equipment.

• Improves worker safety—Workers can place CLSM in a trench with-out entering the trench, reducing their exposure to possible cave-ins. • Allows all-weather construction—CLSM will displace any standing

water left in a trench from rain or melting snow, reducing the need for dewatering pumps. To place CLSM in cold weather, heat the material using the same method for heating ready mixed concrete.

• Reduces equipment needs—Unlike soil or granular backfill, CLSM can be placed without loaders, rollers, or tampers.

• Requires no storage—Because ready mix trucks deliver CLSM to the jobsite in the quantities needed, storing fill material onsite is unnec-essary. Also, there is no leftover fill to haul away.

• Makes use of a waste by-product—Fly ash is a by-product produced by power plants that burn coal to generate electricity. CLSM contain-ing fly ash benefits the environment by makcontain-ing use of this industrial waste material.

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duced by freshly placed CLSM for such applications as backfilling re-taining walls or foundations. Be-cause air voids have no strength, the c o m p re s s i ve strength of a foamed mix is generally less.

Mixing and placing procedures Usually CLSM is batched and mixed at a central mixing plant in a c c o rdance with ASTM C 94 pro c e-d u res for concre t e, then e-delive ree-d to the jobsite by ready mix truck. Fo r small jobs, howe ve r, a mobile mixer can be used. It delivers dry CLSM m a t e rials to the jobsite for onsite mixing with water right before p l a c e m e n t .

To place CLSM, use a chute, con-ve yo r, bucket, or pump depending on the type of void to be filled and its accessibility. Because CLSM f l ows and self-leve l s, it’s possible to d i s c h a rge the material from one spot to fill re s t ricted-access are a s. When filling larg e, open tre n c h e s, m oving the discharge point helps s p read the material. To contain CLSM when filling open-ended s t ru c t u res such as tunnels, block the ends of the stru c t u re with sandbags or dirt dams.

For most applications, CLSM can be placed continuously. So m e t i m e s, though, it’s necessary to place the m a t e rial in lifts. In its fluid state, CLSM usually weighs 125 to 135 pounds per cubic foot. When back-filling retaining walls, placing CLSM in lifts pre vents lateral pre s s u re s f rom exceeding the loading capaci-ties of the wall. Allow each lift to h a rden before placing the next lift. For pipe bedding, placing CLSM in lifts pre vents floating the pipe. Sometimes sandbags or other weights are used to ballast the pipe until CLSM sets.

Standing water in a trench does not have to be pumped out before filling the trench with CLSM. CLSM will displace the water and force it out. Any loose debris or rubble in the trench can remain too. CLSM will encapsulate it.

CLSM needs no compacting and little or no spreading or finishing. When using it as fill for pave m e n t

section replacement, smooth the fill s u rface with a square shovel if the s u rface is below pavement gra d e. If placing the fill up to pave m e n t g rade for use as a tempora ry dri v i n g s u rf a c e, finish it with a wood float ( Ref. 5). Because of its high water content, CLSM will bleed. This bleedwater is usually not a pro b l e m and can be allowed to run off or e va p o ra t e.

When placing CLSM in open t renches in cold we a t h e r, heat it us-ing the same methods for heatus-ing ready mixed concrete to pre vent the m a t e rial from fre ezing before it h a rd e n s. The top layer may fre eze but it can be scraped off later. Costs

Costing about two-thirds to t h re e - q u a rters the price of standard ready mixed concre t e, CLSM is m o re expensive per cubic yard than most soil or granular fills. Still, the a d vantages of using CLSM more than compensate for its higher cost (see box ) .

The total cost of CLSM va ries de-pending on:

• Ma t e rials used

• How it’s mixed and tra n s p o rt e d • Placing methods

Most ready mix suppliers have d e veloped mix pro p o rtions for CLSM that make economical use of local materi a l s. This demonstra t e s an important advantage of CLSM: its mix design flexibility. The mater-ial can be made from a va riety of ag-g re ag-g a t e s, includinag-g nonspecifica-tion aggre g a t e. Also, the pro p o rt i o n s of aggregate and fly ash in the mix can va ry. In areas where fly ash is in-e x p in-e n s i vin-e, thin-e rin-eady mix suppliin-er may use large amounts of fly ash in the mix.

Chemical admixtures can im-p rove the im-pro im-p e rties of CLSM, but their use is not cost-effective unless they are necessary to meet specific fill perf o rmance re q u i re m e n t s.

On small pro j e c t s, it’s usually m o re economical to use re a d y mixed CLSM or CLSM delive red dry in a mobile mixer and mixed onsite with water. For projects re q u i ri n g l a rge amounts of CLSM, onsite

mix-ing and material storage may be m o re cost-effective. Onsite mixing can reduce delive ry costs and elimi-nate delays.

The costs of placing CLSM are l ow. Some jobs may re q u i re placing CLSM in lifts or some manual s p reading. Even so, CLSM is faster and less labor intensive to place than soil fill. Its use eliminates the time and manpower needed for compaction. And less inspection is needed during placement.

Testing

Many pro c e d u res for testing CLSM follow the same ASTM stan-d a rstan-ds usestan-d to test concre t e. But the unique pro p e rties of CLSM some-times call for modifying these pro-c e d u re s. For example, using a stan-d a rstan-d slump cone test to ve rify CLSM f l owability is not ve ry accura t e. On e method for measuring flow uses a 3x6-inch cylinder, open at both e n d s. Place the cylinder on a flat, h a rd surface then fill it with CLSM. When the cylinder is lifted, a mater-ial spread of at least 8 inches indi-cates acceptable flow (Ref. 1 ).

To test the strength of CLSM, fol-l ow pro c e d u res given in ASTM D 4832-88, “St a n d a rd Test Method for Pre p a ration and Testing of So i l - c e-ment Sl u r ry Test Cy l i n d e r s.” Us e 6xl2-inch plastic cylinder molds or peel-off, wax-coated card b o a rd m o l d s. Fill them to ove rf l ow i n g , then tap their sides lightly. Be c a u s e CLSM is lower in strength than con-c re t e, con-capping the con-cylinders with sulfur compounds can break the m a t e rial. Instead, use neopre n e c a p s. Also, be careful when stri p p i n g these fragile cylinders.

ASTM C 403 penetration re s i s-tance tests can assess the setting and early strength development of CLSM. Use these tests to decide if the fill is ready to be cove red with a patch or strong enough to support equipment, traffic, or constru c t i o n l o a d s.

Standards being developed Although many city public work s d e p a rt m e n t s, utility companies, and state departments of tra n

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s-p o rtation have been backfilling with CLSM since the 1970s, no unive r s a l s t a n d a rds have been developed for its use. ACI Committee 229, form e d in 1985, is working on a state-of-the-a rt re p o rt cove ring the state-of-the-applicstate-of-the-ations, p ro p o rtioning, handling, place-ment, and perf o rmance of CLSM. It’s also pre p a ring a bibliography on CLSM re s e a rch which should be a vailable later this ye a r.

The National Ready Mixed Co n-c rete Asson-ciation (NRMCA) and many state ready mixed concre t e associations have published re c o m-mended mix designs and placement p ro c e d u res for CLSM. Co n t ra c t o r s, e n g i n e e r s, and ready mix pro d u c e r s i n t e rested in more information can contact these associations.

References

1. Joseph A. Amon, “Controlled Low-strength Material,” The Construction Specifier, December 1990,

Construc-tion SpecificaConstruc-tions Institute, 601 Madi-son St., Alexandria, VA 22314. 2. Bill Dunham, “Controlled Low-strength Material: Sample Specifica-tions and Project Evaluation,” Novem-ber 1988, City of Peoria Department of Public Services, City Hall Bldg., 419 Fulton St., Peoria, IL 61602. 3. Ronald L. Larsen, “Use of Con-trolled Low-strength Materials in Iowa,” Concrete International, July 1988, American Concrete Institute, Box 19150, Detroit, MI 48219. 4. Roger Tansley and Ronald Bernard, “Specification for Lean Mix Backfill,” October 1981, prepared under U.S. Department of Housing and Urban De-velopment contract by Scientific Ser-vice Inc., 517 E. Bayshore, Redwood City, CA 94063.

5. “Flowable Fill,” May 1989, South Carolina Department of Highways and Public Transportation.

6. “Fly Ash Design Manual and Site Applications (Volume 2: Slurried Place-ment),” October 1986, prepared by GAI Consultants Inc. for the Electric

Power Research Institute, 3412 Hillview Ave., Palo Alto, CA 94304. 7. “What, Why & How? Flowable Fill Materials,” Concrete in Practice, No. 17, 1989, National Ready Mixed Con-crete Association, 900 Spring St., Sil-ver Spring, MD 20910.

8. “Utility Cuts and Full-depth Repairs in Concrete Streets,” IS235.01P, Port-land Cement Association, 5420 Old Orchard Rd., Skokie, IL 60077. 9. “Unshrinkable Fill for Utility Trench-es in Streets,” CP004.01P, 1989, Ca-nadian Portland Cement Association, 116 Albert St., Ste. 609, Ottawa, Canada, K1P 5G3.

10. “Technical Tip: Controlled Density Fill,” February 1989, Ohio Ready Mixed Concrete Association, 1900 E. Dublin Granville Rd., Columbus, OH 43229.

Publication # C910389