Field experimental study on the retrofit of cracked onshore wind turbine foundations using externally prestressed anchor bolts

Inserted rings are considered as a crucial element of onshore wind turbine foundations to resist uplift and compressive forces at the interface of the steel towers and concrete foundations. These inserted rings can be deficient because of the crushing of the concrete and the occurrence of cracks as the result of the vibration of the tower and the shrinkage of the concrete. Therefore, it is very important to develop an efficient retrofitting system in order to restore the lost capacity and/or even outperform it. The objective of this paper is to develop a retrofit strategy for the deficient foundations using external prestressing technique and verify it using field data. A field test was conducted on two onshore wind turbine foundations with almost the same level of cracks. One of them was retrofitted using externally prestressed anchor bolts after the appearance of cracks, and the other was the control foundation. Strain sensors were installed in the foundation and strain signals were recorded in the period of construction, prestressing, and operation. Field test results showed that the prestressing technique can limit the crack width and improve the section stiffness.