If you had a choice, would yo u

Controlled low strength material A cementitious backfill that flows like a liquid, supports like a solid, and self-levels without tamping or compacting BY ANNE SMITH If 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 material 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, leanmix backfill, unshrinkable fill, flowable 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 soilcement but it has pro p e rties similar to both. It s a fluid mixture made of p o rtland cement, water, and fine agg regate 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 material 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 producing 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 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

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 compacting 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 mat 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 workers 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 bel 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 maint e n a n c e. Used under ro a d w a y s, it s e rves as a strong, stable subbase. As a fill material for pavement section replacement, CLSM solidifies enough to support the patch as soon as 3 hours after placement, depending 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. Bef o re placing the permanent pavement section, re m ove CLSM to the depth re q u i red to accommodate the section. By using CLSM with highe a r l y- s t rength concrete patches, 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 coope ration with the Illinois Co n c re t e Council, studied the field perf o r- mance of CLSM as a backfill for utility repairs in the city s streets (Re f. 2). Having seve re problems with settlement of soil backfill in utility t re n c h e s, the department was looking for an altern a t i ve backfill material. It conducted seve ral experiments 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 cranemounted 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-inchthick concre t e s l a b.

and shrinkage was minimal. The material set quickly and could support a pers o n 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 backfilled. Convinced by these e x p e riments of CLSM s sup e rior perf o rm a n c e, the dep a rtment changed its backfilling 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 rtment 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 rem e n t s. Wrapping the pipes in polypro pylene sheeting and placing dirt dams at their ends pre vents infiltration 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 cement, water, fly ash, and fine aggreg a t e. But ready mix producers can combine these components in va rying 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, depending on material cost and ava i l a b i l i- t y. He re are the roles each component plays in CLSM perf o rm a n c e : Wa t e r The large quantity of water used in CLSM allows the material 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 conc re t e, the portland cement in CLSM f o rms a paste with water to bind the a g g regate and fly ash. Although CLSM contains much smaller amounts of p o rtland cement than does c o n c re t e, sufficient hyd ration 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 conf 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 ready mix supplier can produce CLSM using only cement, fine aggreg a t e, and water. In opent 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 regate 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. Aggregate meeting ASTM specifications or nonspecification aggregate can be used as long as the material isn t exp 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 aggregate within specific gra d a t i o n s

Most chemical admixtures can be used in CLSM including water red u c e r s, superplasticize r s, and accele ra t o r s. Using admixtures usually is 15 REASONS TO USE CLSM 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 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 advantages, described below, facilitate the entire backfilling process, from order and delivery of materials to clean up. Readily available Using locally available materials, ready mix suppliers 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. Because CLSM is self-leveling, it needs little or no spreading or compacting. This speeds construction and reduces labor requirements. Versatile CLSM mix designs can be adjusted to meet specific fill requirements. 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 permanent structural fill, CLSM can be designed to achieve 28-day compressive 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 downtime 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 significant if the backfill is to be covered by a pavement patch. Soil or granular 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 because it eliminates having to widen trenches to accommodate compaction equipment. Improves worker safety Workers can place CLSM in a trench without 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 unnecessary. 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 containing fly ash benefits the environment by making use of this industrial waste material. of CLSM and reduce its density. Bu t air contents greater than 6% can inc 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 usually not a drawback since the material is used only to replace compacted 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 stre n g t h s, consider 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 bearing 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 supp o rt traffic loads, specify a highs t rength, high-cement-content mix. If using CLSM to bed utility lines req 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 rem oved with a backhoe or other digging equipment. At strengths exceeding 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, embankments, 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 insulating qualities. Using a foamed mix reduces the lateral pre s s u res pro-

duced by freshly placed CLSM for such applications as backfilling retaining walls or foundations. Because 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 delive red 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, conve 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 backfilling retaining walls, placing CLSM in lifts pre vents lateral pre s s u re s f rom exceeding the loading capacities 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 using the same methods for heating 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 depending 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 material can be made from a va riety of agg re g a t e s, including nonspecification 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 ine x p e n s i ve, the ready mix supplier may use large amounts of fly ash in the mix. Chemical admixtures can imp rove the pro 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 mixing and material storage may be m o re cost-effective. Onsite mixing can reduce delive ry costs and eliminate 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 stand a rds used to test concre t e. But the unique pro p e rties of CLSM sometimes call for modifying these proc e d u re s. For example, using a stand a rd 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 material spread of at least 8 inches indicates acceptable flow (Ref. 1 ). To test the strength of CLSM, foll 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 conc re t e, capping the 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 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-thea rt re p o rt cove ring the applications, p ro p o rtioning, handling, placement, 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 Association (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 Lowstrength Material, The Construction Specifier, December 1990, Construction Specifications Institute, 601 Madison St., Alexandria, VA 22314. 2. Bill Dunham, Controlled Lowstrength Material: Sample Specifications and Project Evaluation, November 1988, City of Peoria Department of Public Services, City Hall Bldg., 419 Fulton St., Peoria, IL 61602. 3. Ronald L. Larsen, Use of Controlled 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 Development contract by Scientific Service 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 Placement), 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 Concrete Association, 900 Spring St., Silver Spring, MD 20910. 8. Utility Cuts and Full-depth Repairs in Concrete Streets, IS235.01P, Portland Cement Association, 5420 Old Orchard Rd., Skokie, IL 60077. 9. Unshrinkable Fill for Utility Trenches in Streets, CP004.01P, 1989, Canadian 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 Co py right 1991, The Ab e rd e e n Gro u p. All rights re s e rve d