Sheathing ducts play a crucial role in pre-tensioning and post-tensioning systems used in concrete construction. These ducts are installed within concrete members to create voids through which cables or tendons are later inserted. This allows for the free movement of tendons during stressing operations, ensuring proper load transfer and structural integrity.
There are two main types of sheathing ducts: metallic and high-density polyethylene (HDPE). While both serve the same purpose, this article focuses specifically on metallic sheathing ducts and the various tests conducted to evaluate their performance under different stress conditions.
Description of Metallic Sheathing Ducts
Metallic sheathing ducts are manufactured by spiral-forming cold-rolled copper-killed steel (CRCA) strips with corrugations along their entire length. These corrugations enhance the bond between the duct and the surrounding concrete on the outside, while also improving adhesion with cement grout injected inside the duct after tendon installation.
This dual bonding ensures that the duct remains securely embedded in the concrete structure while allowing the tendon to move freely when tension is applied.
Workability Test
Objective
To assess the flexibility and structural integrity of the duct when subjected to bending forces, simulating real-world installation conditions.
Procedure
- A 1100mm long duct sample is fixed to a base plate using soft soldering.
- The duct is fastened with a wire or cable and pulled through pulleys placed 1 meter apart from the center.
- It is bent alternately to a radius of 1800mm on both sides, repeated three times.
- After the test, the sealing joints are visually inspected for any signs of opening or failure.
Acceptance Criteria
- If any opening or damage is observed in the duct, the sample is considered failed.
- If the duct remains intact without joint separation, it passes the test.
Transverse Load Rating Test
Objective
To determine the duct’s resistance to lateral loads and its ability to maintain shape without permanent deformation.
Procedure
- A 500mm long duct sample is selected, and its outer diameter (D1) is measured.
- The sample is placed on a support stand.
- A loading plate with a 12mm wide contact surface is positioned at the center of the duct.
- Loads are applied incrementally as per Table 1, based on the duct’s nominal diameter.
- After unloading, the new outer diameter (D2) is measured to calculate permanent deformation.
Table 1: Required Load Based on Duct Diameter
| Duct Diameter (mm) | Required Load (N) |
|---|---|
| 25–35 | 250 |
| 35–45 | 400 |
| 45–55 | 500 |
| 55–65 | 600 |
| 65–75 | 700 |
| 75–85 | 800 |
| 85–90 | 1000 |
Acceptance Criteria
- The duct is acceptable if the permanent deformation does not exceed 5% of the original diameter.
Tensile Load Test
Objective
To evaluate the duct’s strength and joint stability under axial tensile forces.
Procedure
- A wooden core, 95% of the inner diameter of the duct, is inserted to maintain its circular shape during testing.
- A coupler is screwed onto the ends of the duct.
- The assembly is subjected to increasing tensile loads as specified in Table 2 until the required load is reached.
Table 2: Required Tensile Load Based on Duct Diameter
| Duct Diameter (mm) | Required Load (N) |
|---|---|
| 25–35 | 300 |
| 35–45 | 500 |
| 45–55 | 800 |
| 55–65 | 1100 |
| 65–75 | 1400 |
| 75–85 | 1600 |
| 85–90 | 1800 |
Acceptance Criteria
- The duct is considered acceptable if there is no visible deformation of the joints or slippage of the couplers during or after the test.
Water Loss Test
Objective
To verify the watertightness of the duct, ensuring it can effectively contain grout and prevent leakage.
Procedure
- One end of the duct sample is sealed.
- The duct is filled with water and connected to a pressurized system capable of applying 0.05 MPa pressure.
- Pressure is maintained for 5 minutes using a hand pump equipped with a pressure gauge.
- Any water escaping from the duct is collected and measured.
Acceptance Criteria
- The duct is deemed acceptable if the water loss does not exceed 1.5% of the initial volume of water filled.
Conclusion
The performance of metallic sheathing ducts is critical to the success of pre-tensioning and post-tensioning applications in concrete structures. Conducting these four key tests—workability, transverse load rating, tensile load, and water loss—ensures that the ducts meet the required standards for flexibility, strength, and sealing capability.