why tight-radius (tr) duct

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2010 PTI TECHNICAL CONFERENCE & EXHIBITION

IMPROVED PERFORMANCE WITH TIGHT-RADIUS PLASTIC DUCT

Larry B. KrauserGeneral Technologies, Inc.

Stafford, TX

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What is tight-radius plastic duct?

Corrugated plastic post-tensioning duct that can be bent to a tight radius while still

achieving fib and FDOT wear resistance requirements through the use of specially formulated, proprietary composite, high-

performance materials.

WHY TIGHT-RADIUS (TR) DUCT

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Why use tight-radius plastic duct?

• Light-weight, corrosion resistant materials with superior bonding properties.

• Prevents concrete spalling and deterioration due to expansion of corroding elements (metal or galvanized metal duct).

• Eliminates corrosion potential between highly stressed post-tensioning strand and galvanized metal pipe.

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Excerpt from: Potential for Hydrogen Generation and Embrittlement of Prestressing Steel in Galvanized

Pipe Voided Pile by Hart and Suarez

“The possibility of hydrogen embrittlement of prestressing strand in prestressed concrete … using galvanized pipe cannot be discounted. Recognizing

that alternative materials to galvanized pipe are available and that these have proven satisfactory, it is recommended that galvanized pipe not be used...”

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TIGHT-RADIUS (TR) DUCT

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DUCT TESTING

Duct Testing:

• fib Bulletin 7, Corrugated Plastic Ducts for Internal Bonded Post-Tensioning

• FDOT 462-4.2.5.5 Corrugated Plastic Duct• Difference is primarily the residual plastic thickness

after “Wear Resistance” testing.– fib ⟨ 0.040 in (1mm) for all sizes– FDOT ⟨ 0.060 in (1.5mm) for duct up to 3.35 in (85mm)– FDOT ⟨ 0.080 in (2mm) for duct over 3.35 in (85mm)

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DUCT TESTING

Duct Testing:

1. Dimensional Tolerance of Duct2. Flexural Behavior of Duct3. Flexibility of Duct4. Lateral Load Resistance of Duct5. Longitudinal Load Resistance6. Leak Tightness of Duct System7. Wear Resistance of Duct8. Bond Behavior of Tendon

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DUCT TESTING

Wear Resistance of Duct

The test confirms that the duct is resistant to wear caused by the prestressing steel during stressing when the tendon is bent to the minimum specified

radius of curvature.

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DUCT TESTING

Tendon Profile

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DUCT TESTING


• Test Specimen– Two strips cut out of production duct

• 1/8 of duct circumference (45 out of 360 degrees)• Not less than 100 mm in length• Contain a full rib/corrugation spacing

– Thickness recorded at six locations each– Strips mounted into a supporting body

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DUCT TESTING


• Test Specimen

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DUCT TESTING

CUT DUCT SAMPLES PRIOR TO TEST


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DUCT TESTING


• Procedure– Strips mounted into a supporting body– Length of strand stressed to 70%– Clamp specimens around strand to force Q– Move specimen 750mm (29.5 in)

• Maintain clamping force• Do not allow specimen to rotate around strand• Movement within 2 min. of applying clamping force• Hold clamping force 3 min. after movement complete

– Measure remaining wall thickness

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DUCT TESTING


• Force Q – simulates curvature of duct• Calculated using

– Guaranteed ultimate tensile strength of strand– Cross sectional area of strand– Cable factor to account for effect of actual

number of strands within one duct– Length of specimen– Minimum specified radius of curvature

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DUCT TESTING


• Calculations – Cable Factor

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DUCT TESTING


• Procedure

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DUCT TESTING

CLAMPED DUCT SAMPLES MOVING IN APPARTUS 750 mm


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DUCT TESTING

FDOT Bending Radius Test

• Same apparatus• Procedure

– Strips mounted into a supporting body– Length of strand stressed to 70%– Clamp specimens around strand to force Q– Do not move specimens to simulate wear– Maintain force Q for a duration of 7 days– Measure remaining wall thickness

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DUCT TESTING

DUCT SAMPLES CLAMPED TOGETHER


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DUCT TESTING

DUCT SAMPLES SHOWING WEAR FROM TEST


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MINIMUM BENDING RADII PER FDOT

Minimum Bending Radii (MBR) for GTI Corrugated Plastic Duct*48mm

59mm

76mm

85mm

100mm

115mm

130mm

130mm

Quantity of 0.6” Strands 5 7 12 15 19 27 31 37

MBR for GTI Standard Duct (ft)

10.9 13.7 14.2 17.1 19.7 24.6 26.4 28.3

MBR for GTI Tight-RadiusDuct (ft)

7.8 9.8 10.2 12.3 14.7 15.7 15.1 17.3

* Based 0.06” residual thickness up to 85mm and 0.080” > 85mm duct

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APPLICATION FOR TIGHT-RADIUS (TR) DUCT

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Concrete Deviator Application at External Tendons

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Stressing Blister Application

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Stressing Blister Application

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TIGHT-RADIUS (TR) DUCT

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ADVANTAGES/BENEFITS OF TIGHT-RADIUS (TR) DUCT

• Eliminates corrosion potential between highly stressed pt strand and galvanized metal pipe.

• Allows use of light-weight, corrosion resistant materials with superior bonding at external tendon deviators.

• Provides material cost savings versus galvanized pipe.

• Permits use of light-weight material that is easy to transport, handle, and install.

• Allows a greater radius for tendons into stressing blisters increasing construction flexibilities.

• Provides positive connections to HDPE duct of external post-tensioning tendons.

why tight-radius (tr) duct

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