1002182.1 Fluidized Thermal Backfill
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Revision 2
Apr 11, 2024
1. Scope
This specification covers the technical requirements for formulation, production, installation and testing of high-strength and low-strength fluidized thermal backfill (FTB) used in the construction of encased electrical power conduits (duct banks).
2. Reference Standards
Unless otherwise stated in this specification, fluidized thermal backfill shall comply with the latest revisions of the following standards:
District Standards
T&D Guideline 4-24-6.0 Encasement of Non-Metallic Conduit
Industry Standards
ASTM C31/C31M - 08a Standard Practice for Making and Curing Concrete Test Specimens in the Field
ASTM C33-07 Standard Specification for Concrete Aggregates
ASTM C39/C39M - 05e1 Standard Test Method for Compressive Strength of Cylindrical Concrete Specimens
ASTM C94/C94M - 07Standard Specification for Ready-Mixed Concrete
ASTM C136 - 06 Standard Test Method for Sieve Analysis of Fine and Coarse Aggregates
ASTM C143/C143M - 08 Standard Test Method for Slump of Hydraulic-Cement Concrete
ASTM C150 - 07Standard Specification for Portland Cement
ASTM C172 - 08 Standard Practice for Sampling Freshly Mixed Concrete
ASTM C595 - 08 Standard Specification for Blended Hydraulic Cements
ASTM C618 - 08 Standard Specification for Coal Fly Ash and Raw or Calcined Natural Pozzolan for Use in Concrete
ASTM C989/C989-18a Standard Specification for Slag Cement for Use in Concrete and Mortars
ASTM C1157 - 08 Standard Performance Specification for Hydraulic Cement
IEEE 442 Guide for Soil Thermal Resistivity Measurements
3. General
Fluidized thermal backfill (FTB) is used to encase and cover medium or high voltage underground cables. FTB has a lower thermal resistivity than conventional concrete or controlled density fill (CDF) which allows power cables to run at higher ampacities without exceeding their temperature rating.
3.1 Low-Strength FTB
Low-strength FTB, like CDF, is used wherever a flowable, self-compacting backfill is required. With a maximum compressive strength of 150 psi, low-strength FTB may be easily excavated but provides little mechanical protection.
3.2 High-Strength FTB
High-strength FTB is equivalent in strength to 4,000 psi concrete and provides a very high level of mechanical protection. In general, high-strength FTB is more thermally conductive than low-strength FTB.
4. Mix Design
4.1 Low-strength and high-strength mix designs given in Table 4.1 may be used by any ready-mix concrete supplier to produce District approved FTB mixes. Alternate mixes are allowable provided the mix meets the requirements of Table 5.1 via laboratory tests outlined in Section 8.
| Table 4.1-District Approved FTB Mix Designs | ||
|---|---|---|
| Component | Low-Strength | High-Strength |
| Medium Aggregate | 1,800 lb/yd3 | 1,515 lb/yd3 |
| Fine Aggregate | 1,400* lb/yd3 | 1,422* lb/yd3 |
| Medium/Fine Ratio | 56/44 % | 52/48 % |
| Fluidizer | 300 lb/yd3 | 100 lb/yd3 |
| Portland Cement | 50 lb/yd3 | 520 lb/yd3 |
| Water | 305 lb/yd3 | 360 lb/yd3 |
| Red Dye | 5-10 lb/yd3 | 5-10 lb/yd3 |
| *may be adjustable for unit yield | ||
4.2 Medium aggregate shall meet ASTM C33-07 grading requirement for number 8 aggregate.
4.3 Fine aggregate shall meet ASTM C33-07 sieve analysis requirements for fine aggregate.
4.4 Fluidizer shall be Class F fly ash per ASTM C618-05. Class C fly ash in not acceptable.
4.5 Portland Cement shall be type I per ASTM C150-07.
4.6 Water, as required, shall be clean and potable.
4.7 Total air content of any FTB mix shall not exceed 2% by volume.
4.8 Admixtures
4.8.1 TheDistrict requires the use of non-chloride accelerators:
4.8.2 No air entraining admixtures may be used under any circumstance.
5. FTB Properties
FTB mechanical and thermal property benchmarks are listed in Table 5.1.
| Table 5.1-Mix Design and Field Test FTB Properties | ||||
|---|---|---|---|---|
| Property | Low-Strength FTB | High-Strength FTB | ||
| Mix Design* | Field Test | Mix Design* | Field Test | |
| Thermal Resistivity
— Maximum at 0% Moisture Content |
100 (C°-cm)/W | 110 (C°-cm)/W | 75 (C°-cm)/W | 75 (C°-cm)/W |
| Thermal Resistivity
— Maximum at Critical Moisture Content |
70 (C°-cm)/W @
3% moisture content |
80 (C°-cm)/W @
3% moisture content |
60 (C°-cm)/W @
2% moisture content |
65 (C°-cm)/W @
2% moisture content |
| Minimum 28-day Compressive Strength | none | none | 3,000 psi | 3,000 psi |
| Maximum 28-day Compressive Strength | 100 psi | 150 psi | none | none |
| Minimum Slump | 4 in | 4 in | 4 in | 4 in |
| Maximum Slump | 6 in | 6 in | 6 in | 6 in |
| *Mix design properties are equal to or stricter than field test criteria because of expected variation among production batches. | ||||
6. Placement
FTB shall be placed per District T&D Guideline 4-24-6.0, Encasement of Nonmetallic Conduit.
7. Delivery
Each truckload of FTB shall include a Manufacturer's Certificate of Compliance and delivery ticket. The ticket shall include:
- Delivery location.
- Quantity of water added to mix after batching.
8. Testing
Testing for thermal resistivity and/or compressive strength may be required. All testing will be performed by a District approved testing laboratory in accordance with IEEE 442 and ASTM C39.