In construction, sheathing ducts play a crucial role in the pre and post-tensioning of concrete structures, creating spaces within which cables or tendons can freely stretch during the stressing operation. This article delves into the examination of metallic sheathing ducts, encompassing tests evaluating workability, transverse load resistance, tensile load capacity, and water loss prevention.
Metallic sheathing ducts come in two main types: metallic and HDPE. Our focus here is on the metallic variant, manufactured by rolling CRCA steel strips with a spiral corrugation along its length. These corrugations enhance bonding with concrete externally and facilitate the flow of cement grout internally.
To assess workability, a sheathing duct measuring 1100mm is affixed to a base plate using a soft soldering method. The duct is then fastened with a cable and pulled through a pulley fixed on both sides of the instrument. This process involves bending the duct three times up to a radius of 1800mm on both sides. Visual inspection of sealing joints ensures the absence of failures or openings, with any observed openings indicating test failure.
For the transverse load rating test, a 500mm sheathing duct is measured for outer diameter (D1). The sample is then placed on a stand, and a loading plate with a 12mm contact surface is centered on the duct. Loads are incrementally added, as per Table No-1, ensuring even distribution. Permanent deformation less than 5% of the sheathing diameter is considered acceptable.
Table No 1: Transverse Load Rating Test
| Diameter of the Duct (mm) | 25-35 | 35-45 | 45-55 | 55-65 | 65-75 | 75-85 | 85-90 |
|---|---|---|---|---|---|---|---|
| Load (N) | 250 | 400 | 500 | 600 | 700 | 800 | 1000 |
The tension load test involves subjecting a test specimen to incremental tensile loads. The bellows core is filled with a wooden circular piece, ensuring a circular profile during loading. Couplers are used, and the sample is loaded until reaching the specified load from Table No 2. Satisfactory results are indicated by the absence of joint deformation or coupler slippage.
Table No 2: Tensile Load Test
| Diameter of the Duct (mm) | 25-35 | 35-45 | 45-55 | 55-65 | 65-75 | 75-85 | 85-90 |
|---|---|---|---|---|---|---|---|
| Load (N) | 300 | 500 | 800 | 1100 | 1400 | 1600 | 1800 |
In the water loss test, one end of the sample is sealed, and water is filled. The other end is connected to a system capable of applying 0.05 MPa pressure, maintained for 5 minutes using a hand pump. Water loss is collected and measured, with a threshold of 1.5% of the initial water-filled volume for acceptance.