Sheathing ducts are commonly used in Pre and Post tension of concrete structures. They serve the purpose of creating a void in the concrete, which allows for the insertion of cables or tendons that remain free to stretch during the stressing operation. There are two types of sheathing ducts available in the market: metallic and HDPE.
Metallic sheathing ducts are made by rolling CRCA Steel Strips with a spiral corrugation throughout its length. This design provides better bonding with the concrete from the outside and the cement grout from the inside. The metallic sheathing ducts are subjected to several tests to determine their workability, transverse load, tensile load, and water loss.
In order to ensure that the metallic sheathing ducts are of good quality and meet industry standards, various tests are carried out. These tests help to determine the workability of the ducts, which is an important factor in their performance. The transverse load and tensile load tests are carried out to evaluate the strength and durability of the ducts, while the water loss test is done to check for any leakage issues.
Fig 1: Metallic Sheathing Duct.
Overall, metallic sheathing ducts are an important component in the construction of concrete structures. They are designed to provide a void for the cables or tendons to be inserted, which allows for the necessary stretching during the stressing operation. With proper testing and quality control, these ducts can offer reliable and long-lasting performance in concrete construction.
1. Workability Test
Fig 2: Workability Test on Sheathing Duct
In Figure 2, a workability test is being performed on a sheathing duct that has a length of 1100mm. The duct is fixed onto a base plate of a testing instrument using a soft soldering method. To conduct the test, a cable or wire is used to fasten the duct and pulled through a pulley on both sides of the instrument that are 1 meter apart from the center. The sheathing duct is then pulled up to a radius of 1800mm on both sides alternatively, and this bending process is repeated three times.
The purpose of this workability test is to ensure that the sealing joints of the duct remain intact and show no signs of failure or opening. Any openings observed in the duct during the test will be considered a failure for the sample. The visual inspection of the duct’s sealing joints is necessary to verify that the duct has successfully passed the test.
2. Transverse Load Rating Test
To conduct the transverse load rating test, a sheathing duct sample with a size of 500mm is selected and its outer diameter is measured and recorded as D1. The sample is placed on a stand that supports the duct. A loading plate with a contact surface of 12mm in length is then placed in the center of the sample. The loads are added on the loading plate based on the diameter of the duct, as specified in Table No-1. These loads are added evenly and positioned at the center of two corrugations.
During the loading process, the deformation formed in the duct is carefully measured for the outer diameter D2. To be considered acceptable, the permanent deformation of the duct should be less than 5% of its diameter. If the permanent deformation exceeds this limit, the sample will be considered unsuitable for use.
Fig 4 and Fig 5 provide a visual representation of the elevation and plan of the transverse load rating test, respectively. Overall, this test is designed to determine the ability of the sheathing duct to withstand transverse loads and maintain its shape under external pressures.
Table No 1: Transverse Load Rating Test
| Diameter of the Duct in mm | 25-35 | 35-45 | 45-55 | 55-65 | 65-75 | 75-85 | 85-90 |
| Load in N | 250 | 400 | 500 | 600 | 700 | 800 | 1000 |
3. Tension Load Test
Fig 9: Tension Load Test
A tension load test involves subjecting a test specimen to a tensile load. To ensure a circular profile during the test loading, the bellow core is filled with a wooden circular piece having a diameter of 95% of the inner diameter of the sample. Once the specimen is prepared, a coupler is screwed on and the sample is loaded in increments until reaching the load specified in Table 2.
During the test, it is important to monitor the joints and couplers for any signs of deformation or slippage. If no such issues are noticed and the specimen successfully withstands the specified load, the test shall be considered satisfactory. This ensures that the specimen is capable of withstanding the expected load without compromising its structural integrity.
Table No 2: Tensile Load Test
| Diameter of the Duct in mm | 25-35 | 35-45 | 45-55 | 55-65 | 65-75 | 75-85 | 85-90 |
| Load in N | 300 | 500 | 800 | 1100 | 1400 | 1600 | 1800 |
4. Water Loss Test
In the Water Loss Test, a sample is sealed at one end and filled with water. The other end is connected to a system that can apply a pressure of 0.05 MPa. A hand pump with a pressure gauge is used to maintain a constant pressure of 0.05 MPa for a period of 5 minutes.
During this time, the water coming out of the duct is collected and measured. The acceptability of the sample is determined by the amount of water lost, which should not exceed 1.5% of the initial water-filled volume.
This test is important for evaluating the integrity and quality of the sample. If the sample fails to meet the acceptable water loss criteria, it may indicate the presence of leaks or defects that could compromise its performance. Therefore, this test is commonly used in various industries to ensure the reliability of products and materials.
Fig 11: Water Loss Test