Steel Strand Stay Cable Testing

Steel strand stay cable inspection mainly conducts scientific testing and analysis on the cable force testing, appearance and corrosion testing, anchorage system testing, protection system testing and geometric alignment and vibration characteristic testing, objectively assessing the product quality grade and issuing a detailed test report, providing a basis for manufacturers' quality control and downstream users' product acceptance.

Testing Items

Cable force testing: Through the non-destructive means such as the vibration frequency method, pressure transducer method or magnetic flux method, the actual cable force values of the steel strand stay cables under the static and dynamic loads are determined. The cable force is the core parameter of evaluating the working states of the stay cables and the structural safety. The deviation between the testing results and the design cable forces is the important basis of judging whether the tension adjustment or replacement is needed.

Appearance and corrosion testing: The visual or endoscope inspection is carried out on the parts such as the steel strands, anchorages, sheaths and damping devices, emphatically recording the grades, areas and depths of the broken wires, wear, cracks, pits, mechanical damages and corrosion. The corrosion testing is often combined with the technologies such as the ultrasonic thickness measurement and corrosion potential measurement, quantitatively evaluating the material loss conditions.

Anchorage system testing: The integrity, relative displacements and corrosion conditions of the anchorages (including the anchor cups, anchor plates and wedges) are checked. The compactness of the anchoring is checked through the tapping method. When necessary, the ray or ultrasonic flaw detection is used to detect the internal defects such as the cracks and slag inclusions of the anchorages, evaluating whether their anchoring efficiencies are decreased due to the fatigue or corrosion.

Protection system testing: The integrity of the high-density polyethylene (HDPE) sheaths or other anti-corrosion coatings is evaluated, checking whether there are the signs of the cracks, bulges, aging, scratches and UV degradation. The sealing performances of the protection systems are tested, and whether the internal water accumulation occurs is judged, which is directly related to the corrosion environments of the steel strands.

Geometric alignment and vibration characteristic testing: The total stations, GPS or image recognition technologies are used to measure the spatial alignment and sag of the stay cables, comparing with the design alignment. At the same time, the vibration signals of the stay cables under the environmental excitation or artificial excitation are collected, and their natural frequencies, damping ratios and mode shapes are analysed, used for the cable force inverse calculation and abnormal state identification.

Testing Scope

Bridge stay cable systems: Widely applied to the stay cables, hanger rods, tie rods and main cable strands of the cable-stayed bridges, suspension bridges, arch bridges and tied-arch bridges. The testing covers the full length ranges from the anchorages, pylon anchoring areas to the mid-span cable clamps, including the exposed sections and embedded sections (through the embedded transducers or inspection holes).

Building and space structure stay cables: Covering the roof cable nets, curtain wall support cables, chord cables of the beam string structures and prestressed cable-rod structures of the large-span buildings such as the stadiums and airport terminals. The testing needs to consider the effects of the complex building internal environments on the testing accessibility.

Geotechnical and special engineering anchor cables: Including the prestressed anchor cables for the slope reinforcement, foundation pit support and cavern stability, as well as the bearing cables of the mine hoisting equipment and cable cranes. This type of testing particularly needs to pay attention to the compactness of the grouting bodies in the anchoring sections and the corrosion protection states of the steel strands in the free sections.

In-service and newly constructed stay cables: The in-service stay cable testing focuses on the safety evaluation and life prediction, belonging to the periodic or emergency inspections. The newly constructed stay cable testing focuses on the construction quality control, such as the tension process monitoring, anchoring acceptance and construction quality verification of the protection systems.

All stages of the whole life cycle: The testing runs through all the key stages of the whole life cycle of the stay cables such as the manufacturing ex-factory inspection, on-site installation acceptance, periodic operation monitoring, inspection after the special events (such as the overload, earthquake and impact) and scrap appraisal.

Testing Methods

Vibration frequency method: The most commonly used non-destructive cable force testing method. The vibration signals of the stay cables under the environmental wind or artificial excitation are collected through the acceleration transducers, and their fundamental frequencies and higher-order frequencies are obtained through the spectrum analysis. Combined with the parameters such as the boundary conditions, linear densities and lengths of the stay cables, the cable forces are inversely calculated according to the string vibration theory. This method is fast and convenient, but is sensitive to the boundary condition assumptions.

Magnetoelastic method (magnetic flux method): Based on the principle that the magnetic permeability of the ferromagnetic material is related to its stressed states. The transducers are permanently installed on the stay cables, and the cable forces are directly and continuously monitored by measuring the magnetic flux changes through the steel strands. This method has the high precision, can carry out the long-term online monitoring, and is not affected by the boundary conditions and vibration environments.

Ultrasonic testing method: Using the characteristics such as the sound time, attenuation and frequency changes of the ultrasonic waves propagating in the steel strands, the internal broken wires, corrosion section losses and anchoring area grouting defects are detected. The phased array ultrasonic and guided wave technologies can realize the fast scanning of the long-distance cable bodies, positioning the damage positions.

Ray and industrial CT testing: The X-ray or γ-ray is used for the perspective imaging of the key parts such as the anchorages and anchoring connection areas, which can intuitively find the internal defects such as the wedge cracks, steel strand misalignments and grouting voids. The computed tomography (CT) can provide the three-dimensional images for the fine testing of the complex structures.

Corrosion electrochemical testing: Through the measurement of the parameters such as the corrosion potentials, polarization resistances and electrochemical impedance spectra of the steel strands or anchorages, the corrosion rates and states are qualitatively or quantitatively evaluated. Suitable for evaluating the activity degrees of the steel strands in the corrosive media after the protection system failures.

Testing Standards

Domestic Standards:

1. GB/T 17101, Hot-dip galvanized steel wires for bridge cables

2. GB/T 18365, Hot-extruded polyethylene high strength steel wire stay cables for cable-stayed bridge

3. GB/T 5224, Steel strand for prestressed concrete

4. JT/T 449, Anchorage, grip and connector for prestressing steel strand in highway bridges

International Standards:

1. EN 1993-1-11, Eurocode 3: Design of steel structures — Part 1-11: Design of structures with tension components

2. PTI Recommendations for Stay Cable Design, Testing and Installation

Note: The applicable standard can be selected according to the product use and customer trade requirements; standards agreed in the contract take precedence over general standards.

Testing Instruments and Equipment

Cable force dynamic tester: Integrating the high-sensitivity acceleration transducers, data acquisition modules and dedicated analysis software. The equipment is light and portable, can be quickly deployed, collects the vibration signals through the wireless or wired methods, and automatically calculates and displays the cable force values, frequencies and waveforms. It is the core equipment of the on-site inspection.

Magnetic flux transducer system: Composed of the excitation coils, detection coils, data collectors and analysis software. The transducers need to be designed for the specific specification stay cables. After the installation, the long-term monitoring nodes are formed, and the data can be remotely transmitted to the monitoring centres through the wired or wireless networks.

Ultrasonic flaw detector: Including the routine ultrasonic pulse-echo instruments and advanced phased array ultrasonic instruments. Equipped with the angle-beam probes and roller probes of the various angles to adapt to the curved surface testing of the steel strands. The guided wave testing systems use the low-frequency ultrasonic waves to realize the single-point excitation and long-distance testing.

Digital industrial endoscope: The video endoscopes with the high-resolution cameras and bendable catheters, used to check the hidden areas that cannot be directly observed with the naked eyes such as the sheath interiors and anchorage interiors. With the image storage, measurement and marking functions, convenient for the recording and comparative analysis.

Comprehensive testing robot/cable climbing device: An automatic climbing device equipped with the high-definition cameras, ultrasonic transducers, corrosion detection probes and other equipment. It can automatically travel along the stay cables, carrying out the continuous and high-precision appearance, damage and section loss testing of the full cable lengths, especially suitable for the ultra-long stay cable testing of the large-span bridges.