One of the most important activities of experts in forensic engineering is the investigation of structural damages and the reasons for them. Such investigations usually consist of a theoretical part, including different structural analysis and design checks, and estimation of physical properties of materials, elements and structural systems conducted via experimental investigations.
The presented Non-Destructive Testing (NDT) themed issue of Forensic Engineering focuses on methods and techniques for on-site experimental investigations, most of them consisting of NDT, a traditional area that has been subject to fast and exciting developments in recent years. With this second part of NDT themed issue, the editors of the journal are aiming to present an excellent collection of latest achievements and applications in this amazingly dynamic and colourful area. The issue includes four papers, two of them focused on increasing the effectiveness of NDT applications for reinforced concrete (RC) bridge decks, one focused on the improving visual inspection techniques for bridges and one focused on satellite radar investigations on land surface subsiding.
1 Understanding the performance of bridge decks through full scale accelerated testing
This investigation was developed by team of USA-based researchers. The presented results were part of a Federal Highway Administration project utilising accelerated long-term testing of bridge superstructure systems. For the specific investigation, a combination of construction information with non-destructive evaluation (NDE) techniques was used. The full-scale decking was composite, and the experiments were divided in two phases—phase one with a continuation of 22 months which focused on assessing long-term performance of the RC part of the bridge deck, and phase two, which, after observing significant deterioration, saw an overlay system designed and tested. This paper focused on phase one of the project.
During this phase, the bridge decking was subjected to environmental loading, including thermal cycling, and chloride ions with 5 different chloride contents were introduced. In addition, live loading as a type of highway load was applied during both phases of the project. Seven different types of NDE techniques were used—impact echo (IE), ultrasonic surface waves (USW), ground penetrating radar (GPR), half-cell potential (HCP), electrical resistivity (ER), infrared thermography (IRT) and high-resolution imaging (HRI).
Due to the specific problem during the construction process, and as result of the application of a range of NDE methods, a significant durability problem connected with reduced concrete cover for reinforcement was detected and investigated. The conducted experiments showed the capability of NDE techniques to monitor long-term effects on new constructed RC decks and suggested further effective usage of NDE data for service-life modelling of bridges.
2 Development of reference specimens for NDE of concrete bridge decks
The paper is developed by team of researchers from USA and Germany. The aim was to develop laboratory specimens without using external materials. The range of defects, subject to physical modelling, included cracks, delamination, honeycombing and rebar corrosion. The paper describes the design and fabrication of corresponding reference specimens.
Construction procedures for indicated types of defects have been described.
The generation of cracks is suggested to be developed via introduction of expanding mortar in pre-defined boreholes in the concrete. The size and positioning of the cracks could be controlled. For evaluation of the obtained cracks, crack-gauges, macrophotography and ultrasonic pulse velocity approaches are utilised.
For the development of delamination, similar techniques using expanding mortar were utilised. Evaluation of the effect of delamination was conducted with ultrasonic tomography.
Modelling of honeycombing was suggested to be created using modified concrete mixes that allowed production of hemisphere-shaped blocks with significantly lower density of the concrete. C-scan and corresponding contour mapping were used for evaluation in this case.
Corrosion reference specimens were developed via the application of an electric current over different periods of time. Assessing the corrosion effect in terms of NDE was done with GPR following extraction of rebars at a later stage.
The destructive and non-destructive testing demonstrated that the proposed procedure for creating defects in RC specimens was successful. Using such reference specimens will allow for more effective and reliable application of NDE methods.
3 Analysis of visual inspection data for a sample of highway bridges in the UK
This paper discusses the effectiveness of visual inspections (VI) of highway bridges in the UK and the reliability of the obtained data. The authors’ team consisted of both UK-based and worldwide recognised experts in bridge design, monitoring and maintenance. Results from the monitoring of principal inspections (PI) and testing of a random representative sample of 200 highway bridges from Highways England’s network were presented. Each inspection was conducted in parallel by the original inspection team (appointed by service providers) and the research team of experienced inspectors to assess the variability of the results due to human error and other factors. The obtained data was coupled with preliminary data from PIs provided by Highways England. The condition of each of the bridges was assessed via PIs, which are conducted once every six years at a small distance to each point of the bridge structure. It was established that there is strong correlation between the two sets of data (72%), but that there were some differences as well.
The leading standard for conducting PIs at the time of investigation has been BD 63/07. 93% of observed inspections were deemed to be corresponding to the spirit of BD 63/07, and 81% were fully compliant with it. The most common reason for lack of full compliance was the lack of proper access to some areas of the bridge.
The influence of the following factors was investigated:
Evidence of poor quality of construction, split into categories of ‘age group’, ‘condition’, ‘construction type’, ‘maintenance area’ and ‘structure type’
Water management, split in categories of ‘age group’ and ‘maintenance area’
Performance of repairs, split in categories of ‘maintenance area’
Additional factors, such as effects of time of the day and weather conditions were also analysed, and areas of improvement noted.
The main conclusions drawn include the following: the linking of structural defects to issues related to construction process; that there is significant variation regarding quality of construction and water management in different maintenance areas; that most conducted repairs are poor-quality; and that improved access to the structures being inspected is needed to improve the quality of PIs.
The results of this study will be beneficial for further improvement of instructions and standards in this area.
4 Estimation of land subsidence hazard using interferometry of satellite radar images
The radar interferometry method, as indicated in the paper, is the use of radar wave phase information to detect changes in ground point coordinates as result of comparison of two radar images taken at two different times in the same area. For the specific investigation presented in this paper, the area around Kerman City in Iran was monitored. To obtain radar images, the satellite Sentinel -1 was used, and corresponding analysis with Snap software was conducted.
The monitoring period was from 2014 to 2020. During this period, the subsidence changed between 3.3 and 13.2 cm, and one of the main reasons for intensifying the process of developing subsidence is the intensive usage of underground water resources. Significant subsidence effects resulted in a range of structural problems, including damage to infrastructure, such as road surface damages, illustrated in the paper. The relationship between the human activities influencing water level and the increase in the subsidence rate was analysed and the analytical expression of this relationship presented.
The conducted research shows that identifying and measuring the subsidence rate can be done with high precision using the suggested approach.
