Separation (debonding) of the top, thin course is a common mode of failure in modern road and pavement construction and rehabilitation. Freeze-thaw environmental cycles, exacerbated by repeated and wide-ranging traffic loads, cause expansion and pushing of the top layer up from its base. Repeated traffic loads break this layer and, if no remediate action is taken, the latter disintegrates into loose fragments (chippings, grit) revealing a pothole. These chippings have no cohesion with the lower layer whatsoever, reducing friction and skid resistance, affecting vehicle handling, stability and compromising safety. Often, loose chippings are caught between the tread patterns of vehicle tyres and then catapulted onto other vehicles, or bikers. It is clear that this can result in loss of concentration at best, but can also become exceptionally dangerous, often leading to very serious consequences. All this comes at a time when Government and local authorities around the globe promote and support sustainable and environmentally friendly transport means.
A relevant research project started over three years ago at the School of Energy, Construction and Environment, Coventry University, under the title: Non-invasive Damage Detection and Classification of the Bond beneath Thin Overlays in Rigid Pavements. This project is the first of a series and is regarded as a pilot study.
A critical literature review has shown that a number of non-destructive testing and evaluation NDT&E techniques have been deployed for the detection and evaluation of the flaw bond beneath thin overlays in pavements. However, no technique has been accepted universally that can both ‘identify’ and ‘quantify’ the flaw-bond reliably.
In essence, this project endeavours to develop a ‘’new’’- NDT&E method and demonstrate, how the combination of two initially incompatible non-destructive testing and evaluation (NDT&E) techniques, such as Infrared Thermography (IRT) and Impact Hammer Testing (IHT) can team up to provide an accurate and reliable account of the problem and evaluate and predict the degree of substandard bond. These two methods have never been used together for the purpose of assessing debonding in pavement structures. However, prior to that the suitability of both methods to large scale pavement testing had to be tested and several modifications and improvements had to be considered.
Both NDT&E techniques (IRT and IHT), were explored thoroughly, by conducting laboratory scaled experiments on a number of purposely designed concrete slabs (robust and flawed), while replicating overlays and support/boundary conditions in real life. Practical tests were accompanied by corresponding numerical studies. Some tests (IRT) were also carried outside the laboratory, to benefit from the real environmental conditions and calibrate the numerical models accordingly.
The designed concrete specimens with deliberate implanted voids/delaminations, were assessed ‘qualitatively and quantitatively’ by the IRT and IHT techniques. Finally, both techniques were integrated together with some aid from finite element analysis and the guidelines for a new ‘NDT&E method’ emerged. Conclusively, the developed procedures were demonstrated, envisioning its real time implementations.
It was found that as the defects became deeper (>30mm) and thinner (<3mm), the assessment through IRT became more complex, and IHT was called-in, to evaluate those dubious areas. Briefly, in this new ‘NDT&E method’, a special ‘three-way’ system analogous to traffic lights (Green, Amber, Red) was proposed and demonstrated (through a case study) to classify the flaw-bond, where each colour was associated with the severity of the defects (delamination/voids). Red, represented ‘severe delamination’, necessitating a quick remedial action, Green showed a sound, or ‘flawless’ region with no defects, whereas, ‘Amber’ was used for the ‘uncertain regions’, impossible to classify with IRT, and therefore, handed over to IHT for further investigation. The latter, with the help of numerical modelling was able to classify those uncertain regions with acceptable accuracy (error below 3%).
Conclusively, both techniques showed promising results in detecting and classifying the flaw-bond non-destructively. However, some limitations emerged during the research and these are reported at the end of the thesis. Recommendations for further research are also reported, based on these limitations, together with the need for full-size long stretch road testing before the method gains acceptance as a NDT&E technique, for the evaluation of flaw bond beneath thin road courses. At the end, a possible implementation scenario is also presented, demonstrating the full-scale implementation of the method proposed.
|Date of Award||2021|
|Supervisor||John Karadelis (Supervisor), Adegoke Olubanwo (Supervisor) & Luigi Simeone (Supervisor)|