W.L. Lai

Properties of lightweight aggregate concrete prepared with PVC granules derived from scraped PVC pipes

This paper aims to investigate the fresh and hardened properties of lightweight aggregate concretes that are prepared with the use of recycled plastic waste sourced from scraped PVC pipes to replace river sand as fine aggregates. A number of laboratory prepared concrete mixes were tested, in which river sand was partially replaced by PVC plastic waste granules in percentages of 0%, 5%, 15%, 30% and 45% by volume. Two major findings are identified. The positive side shows that the concrete prepared with a partial replacement by PVC was lighter (lower density), was more ductile (greater Poissons ratios and reduced modulus of elasticity), and had lower drying shrinkage and higher resistance to chloride ion penetration. The negative side reveals that the workability, compressive strength and tensile splitting strength of the concretes were reduced. The results gathered would form a part of useful information for recycling PVC plastic waste in lightweight concrete mixes.

Experimental determination of bulk dielectric properties and porosity of porous asphalt and soils using GPR and a cyclic moisture variation technique

This paper describes a new method for determining porosities in two porous construction and geologic materials (asphalt and soil) by using ground-penetrating radar (GPR) over a wide range of controlled degrees of water saturation ( Sw ) . We call this method a cyclic moisture variation technique (CMVT). Freshwater is used as an enhancer or a tracer to allow GPR to easily detect and differentiate amounts of water or other moisture in these materials. The CMVT is based on measuring the changes of real permittivity ( e′ ) and Sw in the test materials as they transition from partially saturated states to a fully saturated state via cycles of water permeation and dewatering. This method does not disturb the test materials, as do the methods associated with traditional laboratory testing on cored samples. It also tests a large mass of in situ material, compared with the small mass tested by the conventional or electromagnetic coaxial transmission line (EMCTL) method (also known as a dielectric cell) and the tim...

Detection of accelerated reinforcement corrosion in concrete by ground penetrating radar

This paper presents a new approach to evaluate corrosion of reinforcement in concrete by ground penetrating radar (GPR). Pulse GPRs were used to monitor the continuous corrosion process in concrete, in which the changes of amplitudes, travel times and short time fourier transform (STFT) spectrograms associated with the bar reflections were continuously measured. The yearly long corrosion process of reinforcement bar was rapidly accelerated within 10 days by impressing 2A direct current across two embedded reinforcement bars serving as anode and cathode. When corrosion started, it was found that the travel times, amplitudes and the frequency spectra from the bar reflection reached a maximum or minimum, but the trends of these parameters were then reversed when the crack became wide open. The results presented in this paper will pave the way for future corrosion characterization using GPR, both in laboratory and in field.

Foreword to the Special Issue on Advances in Ground-Penetrating Radar Research and Applications

This Special Issue in IEEE JOURNAL OF SELECTED TOPICS IN APPLIED EARTH OBSERVATIONS AND REMOTE SENSING (IEEE JSTARS) on Advances in Ground- Penetrating Radar Research and Applications is organized in the frame of the 15th International Conference on Ground Penetrating Radar (GPR 2014), held in Brussels, Belgium, from June 30 to July 4, 2014 (http://sites.uclouvain.be/GPR2014/). This International Conference is a longstanding traditional event making the premier forum of research and applications in the field of ground-penetrating radar (GPR). GPR 2014, the 15th in the series that has been held biannually since 1986, brought together about 300 high-standard scientists, engineers, industrial delegates, and end-users working in all GPR areas, ranging from fundamental electromagnetics to the so various fields of application. In total, 42 countries worldwide were represented. GPR 2014 was organized by the Universite catholique de Louvain (UCL, Belgium) and co-organized by the University of Edinburgh (U.K.), Delft University of Technology (The Netherlands), and Roma Tre University (Italy), together with EU-COST (European Cooperation in Science and Technology) via the COST Action TU1208 “Civil Engineering Applications of Ground Penetrating Radar.” The conference was sponsored by the IEEE Geoscience and Remote Sensing Society, the COST Action TU1208, the Fonds de la Recherche Scientifique(FNRS, Belgium), the European GPR Association (EuroGPR), and 14 private companies which exhibited their products and services during the conference.

Detection and imaging of city's underground void by GPR

Effective ways of underground void detection and imaging by ground penetrating radar (GPR) in urban environment were studied in this paper. Through indoor laboratory validation experiments, two criterions were suggested and validated as the tell-tale sign of underground void on GPR 2D and 3D imaging. These two criterions are (1) non-continuous strong reflections in 3D slice scans, and (2) reverberation patterns with decaying amplitude in later time windows in 2D radargrams. The lab validations were further proved in line with two field studies in Hong Kong where the overlaid pavements were made of asphalt and concrete, and actual positions of voids were confirmed by subsequent ground truthing. Sources of underground noise, levels of difficulties due to different types of overlaid pavement, reasons and solutions were summarized based on the lab and field studies reported in this paper.

Spectral shift and absorption of GPR signals in a wetted sand column

Spectral shift and absorption of dispersed GPR signals corresponding to a downward wetting front in a sand column simulated in laboratory were studied. The non-stationary GPR signals (making use of a 1000MHz antenna) were processed with short time-fourier transform (Lai et al. 2012) and wavelet transform (WT) in a time-frequency domain. Distributions of peak frequency, bandwidth and associated spectral absorption obtained by the STFT and WT are mapped and compared at (1) the direct wave across the transmitting and receiving antennae, (2) reflected wave of the downward wetting front. There are two important findings. First, the WT-processed results show a better representation of the wetting front reflections than the STFT-processed results because of its capability of multi-time and multi-frequency resolution accounting for both high-frequency wetting front and low-frequency direct wave signals. Second, the spectral content of high frequency component is continuously absorbed over time with propagation of downward wetting front. This paper shows the possibility to study the subsurface in the time-frequency domain, which gives additional information to the well-established time/spatial-domain mapping.