Q: We are about to bid on a project calling for controlled low-strength materials (CLSM) for backfilling sewers and other underground utilities. The designer is hoping to use this material to earn LEED credits.
We have been asked to verify the density of CLSM we plan to provide. Does CLSM's density in place vary? Why is this important to the design engineer? And how can we control the density?
A: The industry defines CLSM as a hardened material that results in a compressive strength of 1200 psi or less. Since CLSM is most often used as an alternative to soil or rock bedding, engineers are concerned about the possibility of digging out the material in case the pipe needs to be exposed for repair. Thus, the specifications for most current CLSM applications require unconfined compressive strengths of 200 psi or less.
Since CLSM mixes are more often sold as performance specifications than prescriptive specifications, product attributes can vary by producer. Producers have used a wide range of acceptable ingredients to create successful CLSM mix designs.
The engineer's concern with the density and weight is probably related to construction activity rather than the service life after the project is completed. According to the engineers at NRMCA, designers should focus on the weight of CLSM in relation to the size and design of the underground pipes or culverts the material will surround. Engineers must determine if the pipe or culvert can support the weight of the CLSM during placement. The project engineer should consult with the pipe's manufacturer or a structural engineer to confirm the pipe's capacity using the CLSM's unit weight.
According to ACI 523.3R-93, Guide Specification for Controlled Low Strength Materials, wet density of normal CLSM in place is in the range of 115 to 145 pounds per cubic foot, which is greater than most compacted materials. A CLSM mixture with only fly ash, cement, and water should have a density between 90 to 100 pounds per cubic foot. Ponded ash or basin ash CLSM mixture densities are typically in the range of 85 to 110 pounds per cubic foot. Lower unit weights can be achieved by using lightweight aggregates, high entrained air contents, and foamed mixtures.
If the engineer is concerned about your product's wet density, you might consider offering a special class of CLSM. Low-density CLSM mixtures (LD-CLSM) are produced using pre-formed foam as part of the mixture proportioning. Preformed foam is made up of air cells generated from foam concentrates or gas-forming entrainment admixture. Air content up to 30% and unit weights as low as 90 to 100 pounds per cubic foot may be produced. Using CLSM air-generating admixtures with normal weight aggregates may result in air contents up to 40% and unit weights as low as 80 to 90 pounds per cubic foot.
When proposing the use of CLSM, producers would benefit by referencing the Guide Specification for Controlled Low Strength Materials (CLSM), prepared by NRMCA. The Guide is available at their website, www.flowablefill.org.
Proposed test methods for hardened density and compressive strength of pervious concrete are moving forward.
ASTM WK29212 will provide a useful way to reliably determine the void content and resulting density of pervious concrete, particularly regarding quality and functionality related to stated goals or specifications. “ASTM WK29213 will provide designers of pervious concrete pavements a value to use in their designs,” says Charles Mitchell, a principal at Specialized Engineering and C09.49 member. “It would also provide an owner with a standard to be used to verify pervious concrete material strengths on their projects.” Anyone with an interest in pervious concrete is encouraged to participate in the development of ASTM WK29212 and ASTM WK29213.
“ASTM F2659 is very useful as a fast and easy preliminary standard because the user can take multiple tests, getting instant readings across the slab in a short time,” says Sean Fallon, an F06.40 member. Fallon notes that the new standard can be used by anyone who needs to be concerned with the moisture condition of a floor slab, including flooring installer, consultants and inspectors, and material manufacturers and suppliers.
For more information on becoming an ASTM member, visit www.astm.org/join.
Public comment period on ACI 318 is closing.
ACI Committee 318, Structural Concrete Building Code, is finalizing the upcoming edition of the Code, ACI 318-11, Building Code Requirements for Structural Concrete and Commentary. In accordance with ACI's standardization procedures, the draft is available for public review for 45-days.
Changes of interest to producers: Test records for determining standard deviation of mixture design may now be up to 24 months old; testing agencies performing acceptance testing on concrete are now required to comply with ASTM C1077; criteria for overhead adhesive anchors, seismic requirements for anchoring to concrete, installation and inspection of adhesive anchors, and certification of adhesive anchor installers; enhanced reinforcement detailing requirements for seismic applications.
Enhancements include: Rules for identifying and detailing distinct segments of special structural walls, detailing of horizontal bars at special boundary elements, and confinement of beam flexural hinge regions; ASTM D516 and D4130 are identified as tests for sulfate ions in water, brackish water, or seawater; ASTM C1580 is identified as a test for water-soluble sulfate in soil; new deformed bars are included; Grade 80 deformed bars per ASTM A615 and A706 are allowed for non-seismic applications; and zinc and epoxy dual-coated reinforcing bars per ASTM A1055 are now allowed.