International Concrete Abstracts Portal

An unidentified vehicle recently struck the bottom of a railroad overpass and damaged one of the concrete beams in the overpass. The damaged beam was taken intact to the University of Texas where the Spectral-Analysis- of-Surface-Waves (SASW) method was used to nondestructively delineate the damaged zones. SASW measurements performed on the beam revealed a significant velocity contrast between damaged and undamaged zones. These measurements were consistent with visual inspection of the beam and also indicated the presence of cracking that was not visibly detectable. In addition, SASW measurements taken while repairing the beam revealed how surface wave velocity measurements can be used to monitor improvements in the integrity of a beam after each repair step.

Acoustic emission (AE) has the potential to be an effective tool in evaluation of concrete structures under the action of loads causing cracking. In conventional testing, several AE parameters are investigated to elucidate microfracturing behavior in concrete. To identify internal cracks, the AE location technique is available, which is based on measuring arrival time differences. By employing multi-channel AE observations, the location of a crack responsible for an AE source can be determined. To obtain quantitative information on crack kinematics, the procedure is further studied and a technique for kinematic characteristics of internal cracks is developed. The AE source is mathematically represented by a moment tensor, by which the classification of cracks into tensile and shear cracks and the determination of crack directions can be made. To implement the procedure into a conventional AE system, software named SiGMA (Amplified Green’s function for moment tensor malysis) has been developed. The analysis is readily available on an AE waveform analyzer system consisting of a digital waveform-recorder and a microcomputer (controller). The procedure is applied to a uniaxial compression test of a plate specimen with a through-thickness slit and to a tensile test of a reinforced concrete rigid frame. The crack locations, the classification of crack types, and the determination of the directions of crack motion are in good agreement with experimental findings. The results show the procedure certainly provides a new technique for kinematic identification of internal cracks.

Traditional methods of collecting data on the condition of bridges are expensive and time consuming, and the results are not always dependable. Since the early 1980’s, the Ministry of Transportation, Ontario (MT01 and the Communications Research Laboratory (CRL) at McMaster University have been involved in the use of impulse radar for the assessment of asphalt-covered bridge decks through a program called Deck Assessment by Radar Technology (DART). In this paper, a correlation study involving comparisons between the predictions of the DART survey and the actually observed conditions of the decks (after removal of asphalt during rehabilitation) is discussed with emphasis on the processing of the radar waveforms. Since a large volume of radar data is collected from bridge decks, it is important to develop strategies to extract the salient features from the reflected signal. These strategies include: first, a thresholding method and strata plot which are very effective in locating subsurface layers; and, second a differencing technique which has the capability of discriminating between sound concrete and damaged concrete. The comparisons demonstrate that there is a good correlation between the DART survey and the actual condition of bridge decks.

The paper describes a study of reinforced concrete bridge decks that addresses the need for an accurate evaluation method that is not hindered by an asphalt cover or extensive operator experience. The use of two antennas for Subsurface Interface Radar (SIR) surveys allows for more accurate evaluations by reducing false readings that may be associated with the use of a single antenna. A more accurate determination of the amount of deterioration is also achieved, because the different frequency antennas will detect different types and severities of defects. To assess the use of dual frequency radar for evaluation of bridge decks, both laboratory studies and field studies were conducted. The results of the research indicate that false readings are reduced by using dual frequency radar such that bridge deck repair jobs can be prioritized. The added information on the extent of deterioration also allows more accurate predictions of the time, effort and money necessary for bridge deck rehabilitation.

Large concrete structures such as dams and bridge abutments and piers built over 50 years ago were usually constructed of mass concrete with no or little reinforcing steel. Most of these structures are in service today, and make up a considerable part of the infrastructure. Concern about their reliability and residual life now prompts owners to initiate structural inspections. The traditional approach of coring and visual inspection alone does not necessarily provide a full over for this evaluation. The more familiar nondestructive tests (NDT) such as rebound hammer, ultrasonic pulse velocity and similar surface tests are usually unsuccessfbl in providing information about bulk concrete quality at depth. This paper reviews two novel NDT methods that can address this problem: sonic ogging and parallel seismic, and presents three case histories that illustrate their application.