Date of this Version
Zhong, B., Zhu, J., Morcous, G. (2020). Development of an NDT Tool for in-Situ Assessment of Prestressed Loss. NDOT Research Report SPR-P1(17) M074.
The research objective is to develop a non-destructive testing (NDT) method to evaluate the prestress loss in prestressed concrete bridge girders using ultrasonic waves. The work principle is based on acoustoelastic effect - ultrasonic wave velocity varies with stress level in prestressed concrete. A self-reference test setup was proposed to measure wave velocity in two orthogonal directions (prestress and unstressed directions) in the girder. This setup will be able to reduce effects of material variation and temperature change.
The concept was first validated on small concrete specimens (cylinders and beams) in laboratory. A signal analysis algorithm was developed to reliably measure P wave velocity change with stress, i.e. the acoustoelastic coefficient. Then the proposed technique was applied to a full-scale prestressed concrete bridge girder (131 ft long) to monitor the stress release process. The stress change monitored by the ultrasonic test showed good agreement with the result from the strain measurement. In both the small beam test and the large girder test, the measured acoustoelastic coefficients were in the range of 0.7%/ksi.
The temperature effects on acoustoelastic coefficient were investigated on two prestressed concrete members. Experimental results showed a slight difference between temperature induced velocity changes in the prestress and unstressed directions. Although temperature variation can cause large change of velocity, the self-reference setup will be able to correct about 80% of temperature effect. The relationship between relative wave velocity changes and stress changes in two orthogonal directions after temperature correction can be used to predict the stress level in concrete and reduce environmental influences.