Characterization of porous construction materials using electromagnetic radar wave

Abstract

This thesis reports an experimental effort of characterizing three porous construction/geological materials (i.e. concrete, asphalt and soils) and the establishment and formulation of a set of novel unified constitutive models for these materials by utilizing electromagnetic (EM) radar waves of 0.4 to 1.5 GHz nominal frequencies. This research is recognized, as far as the author is aware, the first study of its kind (i.e. in terms of the technique of characterizing concrete, asphalt and soils based on an inter-disciplinary approach encompassing EM wave and engineering/geological properties). An important outcome of this piece of research is that the studied materials have been assigned successfully into their rightful positions corresponding to the different regimes governed by three different EM wave properties and two engineering/geological properties of the materials. The former refers to the real part of complex dielectric permittivity, energy attenuation and drift of peak frequency of the materials. The latter refers to porosity and permeability of the materials determined with forward models or conventional testing techniques. In soil and asphalt, the material characterization was achieved by a novel in-house developed method called Cyclic Moisture Variation Technique (CMVT) which has been implemented successfully in a series of well-coordinated laboratory experiments. This CMVT is a non-invasive experimental material characterization technique for classifying material behaviour in laboratory, or as a non-invasive near-surface geophysical method in field applications. With this technique, water was used in CMVT as an enhancer or a tracer to allow differentiation of the studied materials which are difficult to differentiate when they are relatively dry. The technique is termed cyclic because the porous materials were subjected to change from partially saturated states to a fully saturated state and vice versa, via a number of cycles of water-permeation and dewatering processes. Through the CMVT, soils and asphalt with different textures of their particulate constituents were characterized by different curves exhibited in the relationship between the real part of complex permittivities and degrees of water saturation (Sw). The porosity of construction/geological materials is a subject of major concern in this research. It is a fundamental property of these porous materials, which has a bearing on their other major material behaviour. For mass manufactured materials such as concrete and asphalt, the presence of varying degrees, as well as the sizes of macro-pores and capillary pores within and between their particulate constituents affect their ultimate bulk physical behaviour, such as strength, permeability and durability. For natural particulate materials such as soils, the porosity is a useful measure to characterize the textures and compositions of soils. Values of porosities (in the ranges of 0.38 to 0.63 in soils and 0.04 in asphalt) were estimated by fitting the data of real part of complex permittivities and Sw into the well-established Complex Refractive Index Model (CRIM). By using the findings in soils from this research and the referenced sources performed by others, the curves of the real part of complex permittivity versus degree of water saturation were found to be clearly divided into three regions: very low (i.e. existence of transition moisture/wilting point), intermediate (i.e. fitted by the CRIM) and very high degree of water saturation (i.e. existence of a critical degree of water saturation). In particular in these plots, dielectric hysteresis was observed (but rarely reported in the field of ground/surface penetrating radar) in soils and in asphalt. These observed phenomena are considered to provide profound influences on the behaviours as well as the understanding of these studied porous materials. The different curing environment is known to affect both the porosity as well as the pore size distribution within mature concrete. By injecting water into concrete specimens under high pressure, the experimental technique advocated in this research allows the differentiation and non-destructive detection of the different curing history a concrete has gone through in fresh state. In particular, the curves (in the plots of the real part of complex permittivity against water saturation) for air-cured (AC) concrete were found to be very similar to those of soils. This similarity implies that the pore systems within air-cured concrete (but not normally cured concrete) and soils are rather similar. Verification of the measurements of the properties investigated by other independent methods (such as mercury intrusion porosimetry test, soil test etc.) was undertaken as time and resource was allowed. However, there were circumstances in which other methods of measurements were considered to be either too time-consuming, disruptive (to the samples) or simply the instrument concerned was not available. The presence of water in subsurface porous materials has usually been considered undesirable in most GPR applications and studies because of significant EM wave attenuation by water. However, the CMVT and the unified constitutive models advocated in this research have turned this drawback into a useful characterization technique of the porous construction and geological materials. Also taking note that the findings were obtained in well-controlled laboratory conditions but not in field conditions. However, it is believed that they have already provided the fundamental and indispensable understanding of the materials and will be able to pave the ways for future field scale application. The research is believed to have advanced the fronts of both construction material research and geophysical explorations.