Item No. 402
Flowable Backfill
This item governs Controlled Low Strength Material (Flowable Backfill) used for trench backfill and for filling abandoned culverts, pipes, other enclosures, and for other uses as indicated on the drawings, Standard Details or as approved by the Engineer or designated representative. Flowable Backfill is a low strength, self-compacting, flowable, cementitious material used in lieu of soil backfill. It is intentionally prepared at low strength to allow for future removal using conventional excavation equipment.
The Flowable Backfill shall be composed of Portland cement or fly ash, or both, filler aggregate and water. The Flowable Backfill, specified for use in filling abandoned culverts, pipes, or other enclosures, shall contain a settlement compensator, in addition to the other ingredients, to minimize settlement of the Flowable Backfill within the enclosure.
Normal Set Flowable Backfill shall be specified whenever the material will remain uncovered or will not be subjected to traffic or other loads within 24 hours after placement. Fast Set Flowable Backfill shall be specified whenever the material will be covered, subjected to traffic or other loads within 24 hours, or needed to expedite construction.
Flowable Backfill can be used for permanent subgrade repairs below the base layer, but shall not be used for permanent pavement repairs. For temporary traffic applications, a minimum 2 inch cap composed of Hot Mix-Cold Laid Asphaltic Concrete (TxDoT Standard Specification Item 334) shall be placed on the Flowable Backfill.
The submittal requirements of this specification item (required at the discretion of the NBU engineer or inspector) include:
A. A mix design submittal including the results of unconfined compressive strength tests, air entrainment (if applicable), flow consistency, hardened unit weight, and timed Ball Drop and corresponding Penetrometer tests.
B. Certifications and test results for the cement fly ash, and admixtures.
C. Particle-size gradation and specific gravity tests on the filler aggregate.
402.3 Materials
A. Cement.
Portland cement shall conform to ASTM C 150, Type I (General Purpose).
Portland cement manufactured in a cement kiln fueled by hazardous waste shall be considered as an approved product if the production facility is authorized to operate under regulation of the Texas Commission on Environmental Quality (TCEQ) and the U. S. Environmental Protection Agency (EPA). Supplier shall provide current TCEQ and EPA authorizations to operate the facility.
B. Fly Ash
Fly ash shall conform to the requirements of Standard Specification Item No. 405, "Concrete Admixtures" and TxDOT Specification Item 437.
C. Filler Aggregate.
Filler aggregate shall consist of sand, stone screenings, pavement milling cuttings or other granular material that is compatible with the other mixture components. The filler aggregate shall be fine enough to stay in suspension to the extent required for proper flow without segregation, and, in the case of filling of enclosures, for minimal settlement. Filler aggregate shall have a Plasticity Index (TxDOT Test Method Tex-106-E) less than 15 and shall conform to the following gradation:
|
Sieve Designation US (SI) |
Percent Passing |
|
No. 200 (75mm) |
0 - 10 |
D. Mixing Water.
Mixing water shall conform to the requirements of Standard Specification, "Concrete for Structures".
E. Settlement Compensator
An air entraining admixture with a higher than usual dosage, which meets the requirements of Standard Specification, "Concrete Admixtures", shall be used as a settlement compensator. The settlement compensator may be introduced to the Flowable Backfill at the job site by placement of prepackaged admixture in capsules or bags in the mixing drum in accordance with the admixture manufacturer's recommendations.
The proportioning of Flowable Backfill shall be the responsibility of the Contractor. The Contractor shall furnish a mix design conforming to the requirements herein, for review and approval by the Engineer or designated representative. The mix design shall be prepared by a qualified commercial laboratory and then reviewed and signed by a registered Professional Engineer licensed in the State of Texas.
The Mix Design submittal must include:
A. Test results for unconfined compressive strength, air entrainment (if applicable), flow consistency, hardened unit weight, and timed Ball Drop (ASTM C-360) and corresponding Penetrometer tests (with a concrete pocket penetrometer),
B. Certifications and test results for the cement, fly ash, and admixtures, and
C. Results of particle-size gradation and specific gravity tests on the filler aggregate. The submittal shall include Penetrometer tests performed every thirty minutes until the Ball Drop test shows a 2-inch indentation, as well as the predicted Penetrometer reading that corresponds to a 3-inch Ball Drop indentation. Particle-size gradation shall be determined using a series of sieves that gives no fewer than five uniformly spaced points for graphing the entire range of particle sizes larger than a No. 200 sieve (75-µm).
The Contractor shall perform the work required to substantiate the design at no cost to NBU, including all testing. Approved mix designs shall be valid for one year, provided there are no changes in the type, source, or characteristics of the materials during that year.
At the end of one year, the mix design may be submitted for renewal, provided that:
A. field tests of the Flowable Backfill during the year have been satisfactory,
B. there have been no changes in type or source of the materials of the mix, and
C. the characteristics of the materials have not changed significantly since the original submittal.
The Contractor shall also submit certifications and test results for the cement, fly ash and admixtures, and particle-size gradation and specific gravity test results for the filler aggregate. The Contractor shall compare results of tests made on the filler aggregate at the end of the year to the results of tests reported in the original submittal. Gradation changes less than ten percent in percent passing any sieve and specific gravity changes less than five percent shall not be considered significant.
402.5 Strength
The Flowable Backfill mix designs shall meet the unconfined compressive strength requirements outlined in the table below. The compression tests shall be conducted in accordance with TxDOT Method Tex-418-A, using approved unbonded caps on specimens with four-inch diameter and eight-inch height [or three-inch diameter by six-inch high specimens if a smaller capacity loading device gives more accurate results].
|
Unconfined Compressive Strength, psi |
||
|
Age |
Normal Set FLOWABLE BACKFILL |
Fast Set FLOWABLE BACKFILL |
|
3 hours |
— |
35 minimum |
|
24 hours |
35 minimum |
— |
|
28 days |
300 maximum |
300 maximum |
402.6 Flow Consistency
Flow consistency shall be established in tests involving the use of a six-inch length by three-inch diameter open-ended straight tubing made of steel, plastic or other non-absorbent material that is non-reactive with cement or fly ash. The tube shall be placed with one end on a horizontal flat surface and held in a vertical position. The tube shall then be filled to the top with FLOWABLE BACKFILL. The top surface shall be struck off with a suitable straight edge and any spillage shall be removed from the base of the tube. Within five seconds thereafter the tube shall be raised carefully, using a steady upward lift with no lateral or torsional motion. The entire test, from the start of filling until removal of the tube, shall be completed within 1˝ minutes without interruption.
After removal of the tube, the spread of the Flowable Backfill shall be measured immediately along two diameters that are perpendicular to one another. The average of those two measurements is defined as the flow consistency of the mix. The flow consistency of the Flowable Backfill shall be considered satisfactory if a circular-type spread of the mix occurs without segregation and a flow consistency (average diameter of spread) of 8 inches or more is achieved.
402.7 Air Entrainment
Air entraining admixture shall be added as a settlement compensator, whenever the Flowable Backfill will be used to fill an enclosure. The dosage shall be sufficient to result in an air content of 15 to 25 percent (as determined by TxDOT Method Tex-416-A) at the time of placement of the Flowable Backfill.
402.8 Field Strength Tests
Ball Drop or Penetrometer tests shall be used to determine, when the Flowable Backfill has developed sufficient strength to be covered or subjected to traffic or other loads as approved by the Engineer or designated representative.
The Ball Drop test shall be performed according to the latest version of ASTM C-360. An indentation diameter of three inches or less, and the absence of a sheen or any visible surface water in the indentation area shall indicate that the Flowable Backfill has achieved the desired strength. Because trench width and depth may affect the test results, the Contractor may perform this test on a control sample of Flowable Backfill in a two-foot square by six-inch deep container.
Penetrometer tests using a hand-held, spring reaction-type device commonly called a concrete pocket penetrometer, shall be performed on the surface of the CLAMS. A Penetrometer reading, equal to or greater than the value established in the mix design for a Ball Drop test indentation of 3-inches, shall indicate that the Flowable Backfill has achieved the desired strength.
402.9 Construction Methods
A. General
The height of free fall placement of the Flowable Backfill shall not exceed four feet. Since Flowable Backfill is considered to be self-compacting, a vibrator shall not be allowed. The Flowable Backfill shall not be covered with any overlying materials or subjected to traffic or other loads until the Ball Drop test or the Penetrometer test shows acceptable results or until the Flowable Backfill has been in place a minimum of 24 hours for Normal Set Flowable Backfill and a minimum of 3 hours for Fast Set FLOWABLE BACKFILL. Curing of the Flowable Backfill will not be required.
B. Utility Line Backfill
After the utility pipe has been placed and the proper bedding material placed in accordance with the details on the drawings, the trench may be immediately backfilled with the Flowable Backfill to the subgrade level shown on the Standard Details or as directed by the Engineer or designated representative.
C. Culvert Backfill
Care shall be taken to prevent movement of the structure. If the pipe or structure moves either horizontally or vertically, the Flowable Backfill and the structure shall be immediately removed and the pipe or structure re-laid to proper line and grade.
D. Other Backfill
Flowable Backfill may be used for backfill material in lieu of soil as shown on the drawings, Standard Details or as approved by the Engineer or designated representative.
E. Filling Abandoned Culverts, Pipe, or other Enclosures
The Flowable Backfill shall be placed in a manner that allows all air or water, or both, to be displaced readily as the Flowable Backfill fills the enclosure.
402.10 Acceptance Testing During Construction
The Engineer or designated representative may perform flow consistency, air entrainment, and unconfined compressive strength tests to determine if the Flowable Backfill meets the specification requirements. The number and frequency of acceptance tests will be determined by the Engineer or designated representative.
Work performed and materials furnished as prescribed by this item will not be measured nor paid for directly but shall be considered subsidiary to the various bid items in the contract.
End