Thomas Kind

A study of concrete hydration and dielectric relaxation mechanism using ground penetrating radar and short-time Fourier transform

Ground penetrating radar (GPR) was used to characterize the frequency-dependent dielectric relaxation phenomena in ordinary Portland cement (OPC) hydration in concrete changing from fresh to hardened state. The study was experimented by measuring the changes of GPR A-scan waveforms over a period of 90 days, and processed the waveforms with short-time Fourier transform (STFT) in joint time-frequency analysis (JTFA) domain rather than a conventional time or frequency domain alone. The signals of the direct wave traveled at the concrete surface and the reflected wave from an embedded steel bar were transformed with STFT, in which the changes of peak frequency over ages were tracked. The peak frequencies were found to increase with ages and the patterns were found to match closely with primarily the well-known OPC hydration process and secondarily, the evaporation effect. The close match is contributed to the simultaneous effects converting free to bound water over time, on both conventional OPC hydration and dielectric relaxation mechanisms.

Measurement of Accelerated Steel Corrosion in Concrete Using Ground-Penetrating Radar and a Modified Half-Cell Potential Method

A new approach is presented to evaluate corrosion of steel bars in concrete by 1.5- and 2.6-GHz ground-penetrating radar (GPR) and a modified half-cell potential method. Changes in time-lapsed travel times, amplitudes, and peak frequencies that are associated with short-time Fourier transform spectrograms of the bar reflections were continuously measured. The year-long corrosion process of the reinforcement bar rapidly accelerated within a few days by impressing direct current across a pair of embedded reinforcement bars, which served as the anode and cathode. When corrosion started, the travel times, amplitudes, and frequency spectra of the bar reflection changed. The results were analyzed by dividing the material’s response into three phases (NaCl contamination, depassivation, and corrosion). The writers attribute the phenomena of the first two phases to the ionic conduction and interfacial polarization effect, described in the low-frequency regime of complex dielectric permittivity outlined in the Maxwell-Wagner effect. The remaining phase corresponds with the appearance of large and multiple interfaces among steel, concrete, corrosion product, and cracks, in addition to the upward movement of the corrosion product to the concrete surface that intercepts wider radar footprints. The findings, based on time lapse measurements, provide a basis to further apply the GPR technique to spatial measurements in laboratory and field studies.

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.

Nondestructive Investigation of Wet Building Material: Multimethodical Approach

Building stones are porous media and they can deteriorate through moisture ingress and secondary damage such as crystallization of soluble salts. Not only is this due to the increasing number of flood events in the past years but also structural damages of houses from activity such as leakage or rising moisture (groundwater) are the main causes. The potential benefit of several nondestructive testing methods to assess water damage in building stone has been studied in a field-scale experiment. Three testing walls made of fired clay brick, sandstone, and spongilite were flooded and their drying behavior monitored using infrared thermography, complex resistivity, ground penetrating radar, and ultrasonics. The results were compared to the average moisture content determined by gravimetric weighing of the specimens. Qualitatively, the results of the different nondestructive testing methods matched well. But in terms of quantitative data, some scatter was observed and the results should be viewed with care. Co...

Application of impulse radar for nondestructive investigation of concrete structures

The application of impulse radar for structural investigation of concrete elements has been increased during the last years related to technical developments, i. e. of high frequency antennas. In this paper, case studies will be presented related to the location of reinforcing bars, tendon ducts and repaired concrete areas in concrete bridges, anchors and dowels in concrete highways and delaminations of layered structures as they are used for non-ballasted railway tracks. It is shown that impulse radar can be applied in case of regular inspection and for searching the cause of damages but also for quality assessment in civil engineering.

Positioning accuracy of an automatic scanning system for GPR measurements on concrete structures

Most GPR measurements use hand-held survey wheels for positioning, something not feasible in some scenarios, such as measuring a tall retaining wall, tunnels or if tedious and repetitive measurements are required. For situations like these an automated positioning system is required and was therefore developed at BAM. Such a system was tested to calibrate the positioning of a GPR grid measurement which was performed in regular one-direction traverse mode and in a meandric mode. The preliminary results show a systematic error in the positioning, which depends on the traveling speed of the scanning system, because there is an electronic delay of the trigger signal. These errors are duplicated in meandric mode compared to the errors occurring in one-direction traverse mode. This may be insignificant when larger objects are investigated, but can be important for the identification and location of small objects which show weaker reflections, like e.g. compacting faults with a size of a honeycomb.

Antenna Development for Impulse Radar Applications in Civil Engineering

The application of impulse radar for nondestructive testing in civil engineering is diversified and requires the development of specified broadband antennas. In this paper, antenna development for this purpose is discussed. Typical testing problems and results of radar applications in civil engineering will be presented.

Detection of granite landmarks in soil with GPR

In Germany plots of land were often marked with landmarks. They are typically made from granite stones in a defined shape and have been buried closely below the surface. The suitability of GPR for the detection of landmarks has been tested. GPR laboratory experiments in a sandbox under ideal conditions as well as field trials have been carried out. The influence of the water content of soil on the GPR measurements was investigated in laboratory. In these experiments suitable characteristic parameters have been identified to distinguish granite landmarks from other reflectors.