| Sažetak (engleski) | Road networks are crucial to the European Union's transport links and prosperity. Safe and smooth traffic flow, i.e. the maintenance of the prescribed technical and functional level of the road, requires the timely and regular implementation of maintenance procedures. One of the main causes of surface damage to the pavement structure is the change in the properties of the unbound granular layer (UBG) due to the increase in moisture and fines. In this doctoral dissertation, the ... Više possibility of quantifying moisture and fines in the UBG of pavement structures was investigated using ground penetrating radar (GPR) with air-coupled antennas based on relative permittivity. In recent decades, GPR has been increasingly used for pavement condition assessment due to its non-destructive nature and versatility. It is based on the calculation of the velocity of reflected electromagnetic (EM) waves, from which the values of relative permittivity in a single layer of the pavement structure are calculated. The results of some researchers indicate that there is a strong correlation between variations in the value of the relative permittivity and changes in the proportion of moisture and fines. This doctoral dissertation presents results of tests and investigations designed to evaluate the effectiveness of the GPR in determining the moisture and fines content in UBG.
The aim of this work was to develop a new method to quantify moisture and fines in UBG by efficiently analysing the relative permittivity value obtained by GPR. This would enable the timely implementation of maintenance measures and thus prevent the occurrence of structural damage. An attempt was made to achieve this objective in three steps:
1. Investigation of the influence of changes in the proportion of moisture and fines on the value of the relative permittivity of the UBG.
2. Evaluate the sensitivity of the UBG to the presence of moisture with variable fines content based on the correlation of the relative permittivity value obtained by using GPR with air- coupled antennas with the values obtained by using a percometer;
3. Correlation of the relative permittivity value obtained by using GPR with air-coupled antennas in the field and in the laboratory.
Laboratory tests, field research and analysis of the results were carried out to confirm the following hypothesis: By establishing correlations between the relative permittivity values obtained by GPR with air-coupled antennas in the field and in the laboratory, it is possible to quantify the moisture and fines content in the unbound granular layer of the pavement. The research itself was carried out in four interlinked phases. In the first phase, the scientific and technical literature on methods for determining the relative permittivity of UBG using non-destructive methods was reviewed in detail: GPR, Time domain reflectometry (TDR) and percometer.
The second phase involves preparing the samples and the performance of laboratory tests to determine the value of their relative relative permittivity. Preliminary laboratory tests were carried out before the main tests were performed. The preliminary laboratory tests were used to determine the geometric characteristics as well as the physical-mechanical and chemical properties of the stone material, the additional material and the mixtures of stone and additional material. In addition, the compaction parameters for the production of the samples were determined for the defined test mixtures. Crushed rock of nominal size 0/31.5 mm was used as the main material, the grain size curve of which lies within the limit range for UBG specified in the ordinance. The additional material was obtained by crushing dried tiles. The proportion of crushed stone was constant, the proportion of additional material varied. The additional material was dosed in a mass proportion of 3 to 15%, step 3%, resulting in a total of six test mixtures. The moisture content varied in each mixture so that six different steps were used. Three samples were prepared for each specific ratio of moisture and fines content, so that a total of 108 samples being tested. Mould was made of waterproof panels, which are mainly used to make formwork for reinforced concrete structures. After the material mixture had been placed in the mould, it was compacted with a vibrating plate. Value of the relative permittivity was determined using GPR with air-coupled antennas with a centre frequency of 1 and 2 GHz. In addition, the tube suction test (TST) was used to evaluate the moisture sensitivity of the test mixtures in relation to the increase in the fines content.
In the third phase, the results of the laboratory tests were statistically analysed and presented with the corresponding data-statistical parameters. Models were created to predict the value of the relative permittivity as a function of the moisture and fines content. In addition, models were developed for the correlation of relative permittivity values, which were determined using a percometer and GPR with an air-coupled antennas. This created the prerequisite for the implementation of a procedure that enables the qualitative assessment of UBG and the quantification of moisture and fines content based on the value of the relative permittivity. The fourth phase comprised field investigations. At 30 sites, the value of the relative relative permittivity of UBG on existing pavements was determined by GPR using air-coupled antennas at a nominal frequency of 1 GHz. The UBG material was taken from the test pits at the GPR measuring point. The moisture and fines content of the sampled material was analysed in the laboratory. By correlating the results of the field research and the laboratory tests, the possibility of using GPR to quantify moisture and fines in the UBG of the pavement based on relative permittivity was evaluated.
The doctoral dissertation is organised into ten chapters. The introductory chapter lays the foundations for the research and clearly defines the questions and aims of the dissertation. By presenting the current state of the road infrastructure, the importance of road maintenance is emphasised, particularly in relation to the change in the properties of UBG due to the increase in the proportion of moisture and fines. The problem is concretised by the need to develop new procedure for non-invasive, i.e. non-destructive, monitoring of these changes to enable timely road maintenance. The study recommends the use of GPR as a key tool for quantifying moisture and fines content and emphasises the advantages of this method over conventional destructive testing methods such as core drilling. The research hypotheses are clearly defined and include the identification of correlations between the value of relative relative permittivity obtained with the percometer and GPR and the actual moisture and fines content in the UBG. The research objectives were formulated with a particular focus on developing a procedure that would allow the condition of UBG to be effectively analysed in terms of moisture and fines content.
The second chapter provides a detailed overview of the current state of knowledge on the role and importance of UBG in pavement. The most important functions of UBG were highlighted, such as the transfer of traffic loads into the subgrade, the adaptability of the layer to different settlements of the underlying soil and the resistance to permanent deformations. By analysing the influence of the granulometric composition, in particular the proportion of fines and moisture, it was shown how decisive changes in these parameters are for the stability and bearing capacity of the UBG. It is concluded that an excessive proportion of fines and an increase in moisture can significantly deteriorate the mechanical properties of UBG, leading to damage to the pavement structure such as potholes, cracks and rutting. This finding emphasises the need for careful monitoring of UBG to ensure the longevity of pavement structures.
The third chapter is dedicated to GPR as a key tool for assessing the condition of transport infrastructure. The historical development of GPR, the principle of operation and the parameters that influence the propagation of electromagnetic waves through the medium are explained. The importance of the choice of antenna frequency for measurement accuracy is particularly emphasised. The analysis of previous research work has shown that GPR has considerable potential not only for layer determination of thickness, but also for the detection of moisture and fines in layers. However, the application of the GPR needs to be further adapted and researched to increase its accuracy under real field conditions.
In the fourth chapter, TST is presented as an important method for assessing the quality of materials in terms of their moisture resistance. It describes in detail how the test is performed and how the materials are categorised according to their relative permittivity. The results of previous studies confirm that materials with a higher fines content are more sensitive to moisture, which can lead to significant problems with UBG, such as reduced bearing capacity and increased sensitivity to freezing. Based on previous research, it was concluded that the TST method is reliable in assessing potential moisture problems in UBG and that it can be useful for the qualitative categorisation of materials.
The fifth chapter deals with the application of TDR. TDR enables the volumetric determination of moisture content in unbound fine-grained materials. Various design solutions of the device are presented and the basic principles of its operation are explained. The limitations of the application in practise are pointed out, with particular emphasis on the placement of the probes in the sample. Some of the most suitable empirical, theoretical-empirical and physical models for determining the moisture content as a function of the relative permittivity are listed. In addition, the use of TDR in UBG is documented in detail.
The sixth chapter refers to the realisation of the previous laboratory tests. The previous laboratory tests comprised the determination of the geometric characteristics as well as the physical-mechanical and chemical properties of crushed stone, additional material and material mixtures of crushed stone and additional material. The tests were carried out with the aim of evaluating the material properties that influence the performance of GPR data acquisition and to ensure the reproducibility of the results and the possibility of comparison with the results of other research.
The seventh chapter presents the main laboratory tests. The procedure for selecting the moulds and the methods for determining the moisture content as well as the method for preparing the samples are described in detail. The collection and processing of GPR data is introduced and the results are presented. The main laboratory studies were carried out with the aim of determining the influence of changes in fines content and moisture content on the value of the relative relative permittivity of UBG. In order to achieve this goal, the values of the relative relative permittivity of UBG at different fines and moisture contents were determined using GPR.
The eighth chapter describes the procedure for conducting the field research. The aim of the field research was to test the proposed method in a real environment. The results of field tests are shown, in which the value of the relative permittivity was determined on real pavements using GPR. In addition, the moisture and fines content were determined in laboratory tests. In the ninth chapter, additional statistical analyses of the results of laboratory tests and field research are carried out. The mathematical models developed are presented, which make it possible to determine the value of the relative permittivity as a function of the moisture and fines content. In addition, a model for the correlation of the relative permittivity determined with GPR and percometer is presented. The developed model has the potential to quantify the moisture and fines content in the UBG of the pavement structure based on the relative permittivity, but additional research is needed.
The final chapter summarises the most important findings and results of the research work. The research objectives achieved are presented and the scientific and practical contributions are emphasised. Finally, the shortcomings and limitations identified are listed and recommendations and guidelines for future research are given. Sakrij dio sažetka |
