Non-contact surface wave measurements on pavements

Sammanfattning: In this thesis, nondestructive surface wave measurements are presented for characterization of dynamic modulus and layer thickness on different pavements and cement concrete slabs. Air-coupled microphones enable rapid data acquisition without physical contact with the pavement surface.Quality control of asphalt concrete pavements is crucial to verify the specified properties and to prevent premature failure. Testing today is primarily based on destructive testing and the evaluation of core samples to verify the degree of compaction through determination of density and air void content. However, mechanical properties are generally not evaluated since conventional testing is time-consuming, expensive, and complicated to perform. Recent developments demonstrate the ability to accurately determine the complex modulus as a function of loading time (frequency) and temperature using seismic laboratory testing. Therefore, there is an increasing interest for faster, continuous field data evaluation methods that can be linked to the results obtained in the laboratory, for future quality control of pavements based on mechanical properties.Surface wave data acquisition using accelerometers has successfully been used to determine dynamic modulus and thickness of the top asphalt concrete layer in the field. However, accelerometers require a new setup for each individual measurement and are therefore slow when testing is performed in multiple positions. Non-contact sensors, such as air-coupled microphones, are in this thesis established to enable faster surface wave testing performed on-the-fly.For this project, a new data acquisition system is designed and built to enable rapid surface wave measurements while rolling a data acquisition trolley. A series of 48 air-coupled micro-electro-mechanical sensor (MEMS) microphones are mounted on a straight array to realize instant collection of multichannel data records from a single impact. The data acquisition and evaluation is shown to provide robust, high resolution results comparable to conventional accelerometer measurements. The importance of a perfect alignment between the tested structure’s surface and the microphone array is investigated by numerical analyses.Evaluated multichannel measurements collected in the field are compared to resonance testing on core specimens extracted from the same positions, indicating small differences. Rolling surface wave measurements obtained in the field at different temperatures also demonstrate the strong temperature dependency of asphalt concrete.A new innovative method is also presented to determine the thickness of plate like structures. The Impact Echo (IE) method, commonly applied to determine thickness of cement concrete slabs using an accelerometer, is not ideal when air-coupled microphones are employed due to low signal-to-noise ratio. Instead, it is established how non-contact receivers are able to identify the frequency of propagating waves with counter-directed phase velocity and group velocity, directly linked to the IE thickness resonance frequency.The presented non-contact surface wave testing indicates good potential for future rolling quality control of asphalt concrete pavements.