Prestressed concrete is widely used in construction for its ability to carry higher loads and resist cracking and deformation. However, over time, the prestressing force applied to concrete structures can decrease due to various factors. This reduction in prestress, known as prestress loss, can significantly impact the load-carrying capacity of the structure, making it essential to monitor and measure the loss to ensure safety and longevity. This article explores the various methods and instruments available for measuring the loss of prestress, with a particular focus on Vibrating Wire (VW) Load Cells and Vibrating Wire Strain Gauges.
Introduction to Prestress Loss
Prestressed concrete involves the use of steel tendons or cables that are tensioned before concrete is poured. This initial prestressing force is intended to counteract the tensile forces that develop when the concrete is subjected to external loads. However, over time, the prestressing force decreases due to several factors:
- Elastic Shortening: When the concrete is stressed, it undergoes elastic deformation, which causes the prestressing force to reduce.
- Anchorage Losses: Some of the prestressing force is lost at the points where the tendons are anchored.
- Frictional Losses: The friction between the prestressing tendons and the ducts or sheaths through which they pass also contributes to prestress loss.
- Creep and Shrinkage: Over time, concrete continues to deform under stress (creep), and as it dries, it shrinks, which reduces the initial prestress.
- Temperature Effects: Changes in temperature can also lead to expansion or contraction of the prestressing tendons, further affecting the prestress force.
- Relaxation of Steel: The steel tendons used in prestressing undergo relaxation over time, reducing their tension.
These losses, if not properly monitored, can significantly reduce the load-bearing capacity of the structure. Therefore, accurately measuring the loss of prestress is critical for ensuring the safety and serviceability of prestressed concrete structures.
Methods for Measuring Loss of Prestress
Currently, there is no direct method to measure the loss of prestress in concrete. However, indirect methods are used to track the change in stress levels within the prestressing tendons. By monitoring these changes, the loss of prestress can be inferred. The most common approach involves periodic measurements of the stress in the prestressing cables using specialized instruments.
Among these instruments, Vibrating Wire Load Cells and Vibrating Wire Strain Gauges are two of the most widely used devices to measure the force in prestressed tendons.
Vibrating Wire (VW) Load Cells
1. Overview of VW Load Cells
A Vibrating Wire Load Cell is a highly effective tool for measuring the force in prestressing tendons. It consists of a set of vibrating wire gauges mounted in parallel, within a cylindrical steel housing. These load cells are specifically designed to measure the force in the prestressing steel. They come in a variety of capacities, ranging from 100 to 5000 kN, making them suitable for a wide range of applications in prestressed concrete structures.
2. Working Principle
The operation of a Vibrating Wire Load Cell is based on the principle that the tension in a vibrating wire is directly proportional to its natural frequency of vibration. When pressure is applied to the diaphragm of the load cell, it changes the tension in the wire. This change in tension alters the frequency at which the wire vibrates.
The natural frequency of the vibrating wire can be expressed mathematically as: f=12LTmf = \frac{1}{2L} \sqrt{\frac{T}{m}}
Where:
- ff = Natural frequency of the wire
- LL = Length of the wire
- TT = Tension in the wire
- mm = Mass per unit length of the wire
By measuring the frequency of the vibrating wire, the tension in the wire and, consequently, the force applied to the prestressing tendon can be determined.
3. VW Load Cell Setup
The load cell is typically installed at the prestressing end of the girder, with the vibrating wire gauges attached to a diaphragm. The load cell is then connected to a readout unit that measures the natural frequency of the wire. The readout unit can be connected to a personal computer (PC), where custom-made software allows for data analysis.
The process involves taking an initial reading of the load cell immediately after installation. Subsequent readings are taken at various stages of prestressing during construction. However, it’s important to note that after the cable duct is grouted with cement, measuring the force using VW load cells becomes difficult because the force transfer to the load cell is hindered. If the duct is filled with grease or gel instead of cement, the load cell can continue to measure the force during service.
4. Limitations and Use Cases
The VW load cell system is particularly suited for use in new construction projects. It is not as effective in existing structures where the cable ducts are cement grouted, as the grout prevents proper force transfer to the load cell. However, if the ducts are filled with grease, oil, or other suitable substances, the load cell system can be used to monitor prestress force even after grouting.
In new construction, VW load cells can be used to ensure that the prestressing force is within desired limits, preventing the risk of overstressing the tendons during the prestressing process.
Vibrating Wire (VW) Strain Gauges
1. Overview of VW Strain Gauges
Another widely used instrument for measuring prestress loss is the Vibrating Wire Strain Gauge. This device measures the strain (deformation) in the concrete around the prestressing tendons. By measuring the strain and knowing the modulus of elasticity of the material, the stress can be calculated. The prestress force is then determined by multiplying the stress with the cross-sectional area of the prestressing cable.
2. Working Principle
The Vibrating Wire Strain Gauge operates on a similar principle to the VW load cell. It consists of a wire that is tensioned between two end flanges. When the concrete experiences force, the distance between the flanges changes, which in turn alters the tension in the wire. This change in tension affects the frequency of vibration of the wire.
The frequency of the vibrating wire is proportional to the strain in the concrete, which can be calibrated to determine the change in prestress force over time.
3. Types of VW Strain Gauges
There are two types of VW strain gauges:
- Weldable Type: These gauges are fixed to the surface of the structure and are not suitable for embedded applications.
- Embedment Type: These gauges are embedded in the concrete during construction, making them ideal for monitoring the prestress force in prestressed concrete elements.
4. Installation and Measurement
For new construction, the embedment type strain gauge is installed in the concrete near the prestressing cable during the concrete placement process. The leads from the strain gauge are connected to a readout or data logger unit. The initial readings are taken immediately after installation, and subsequent readings are taken at different stages during construction and throughout the service life of the structure.
For existing structures, a hole is drilled in the concrete at the desired location, and the strain gauge is installed. After installation, the hole is filled, and the leads from the gauge are connected to the data logger. The strain is measured periodically at predetermined intervals to monitor the changes in prestress force.
Conclusion
Monitoring the loss of prestress in prestressed concrete is essential for maintaining the safety and structural integrity of concrete elements over time. While there is no direct method to measure prestress loss, instruments like Vibrating Wire Load Cells and Vibrating Wire Strain Gauges provide effective means of monitoring changes in stress and strain within the prestressing tendons.
These instruments are particularly useful in new construction projects but can also be adapted for use in existing structures with proper installation techniques. By periodically measuring the force and strain, engineers can detect early signs of prestress loss and take corrective measures, ensuring the longevity and stability of prestressed concrete structures.