The long term test is performed in order to define the actual creep coefficient and long term behaviour of the cable system.
The specimen, manufactured in Redaelli plant, can be fit with laboratory sockets or with sockets equal to permanent ones.
The specimen is installed in the testing machine with proper care, checking alignment and centering of all the elements.
The typical measurement instruments, installed on the specimen, are:
- extensometer with automatic data reading step by step, used to measure the cable elongation;
- position transducer installed at the end of the original sockets used to evaluate the cone setting.
Other geometric characteristics (cable diameter, socket geometry, ecc.) can be measured as well.
After the first pre-stretching cycles, performed to remove the initial inelastic deformation, an established load approximately equal to 40% MBF is kept constant throughout the test,
for minimum 200 hours.
The test is completed when the partial elongation between the two reading is reasonably stabilized. The gradient has to decrease for minimum 3 subsequent steps.
The graph above shows an example of results obtained from long term test (200 h). Since the function creep-time is well approximated by a logarithmic function,
it is used to evaluate the creep referred to 5 years and 10 years time.