Research at The University of Nottingham (UK) and the University at Ningbo (China) has found that laser scanning is a viable structural safety technique to detect the damaging effects of fire on concrete. Concrete is the most extensively used construction material worldwide with an average global yearly consumption of 1 cubic meter per person. Fire is one of the most serious potential risks to concrete structures such as bridges, tunnels, and buildings.
While concrete is known to have high fire resistance and retain much of its load-bearing capacity, its physical, chemical, and mechanical properties do undergo severe modifications when subjected to high temperatures. A significant loss in strength occurs when concrete is heated above 300 °C. A structural safety assessment provides information needed to evaluate the residual bearing capacity and durability of fire-damaged concrete structures. These assessments are also used to propose the appropriate repair methods or to decide if demolition is needed.
There are several conventional on-site and off-site techniques for assessing fire-damaged concrete. Some on-site methods include visual inspections of color change and physical features, whereas off-site methods involve invasive tests such as core drilling or lab-based techniques.
The engineering teams studied the use of terrestrial laser scanning (TSL) as a non-destructive way to assess and detect fire-damaged concrete in a structural safety appraisal. The method allows scanning to be done at a distance, which improves site safety. Scanning is also quick, with millions of points measured in a few seconds, and spatial resolution acquired in a short time. This is advantageous for engineering structures, considering their scale or magnitude.
Baseline Assessments of Concrete
The study investigated the influences of scanning incidence angle and distance on the laser intensity returns. Concrete color change was also studied. Data was collected and interpreted on unheated and heated concrete to establish the baseline condition of the material.
Study experiments were carried out in a controlled laboratory and used two phase shift terrestrial laser scanners to scan the concrete specimens before heating, and then after they were cooled again. The concrete specimens were heated in a furnace to elevated temperatures of up to 1,000 °C; the temperature attained is an important factor in assessing fire-damaged concrete.
To assess color change in the heated concrete, specimen images were captured using a camera attached to the laser scanner. A flatbed scanner was also used to scan heated concrete surfaces and capture images. It is these images that were used for analysis due to their resolution.
During the experiments, the measurement of the incidence angles for the concrete blocks was found to vary with distance. As the scanning distance increased, the incidence angle decreased, and both scanners showed the same trend. A comparative analysis of the laser intensity for heated and unheated concrete showed that the recorded intensity values for heated concrete are higher than those for unheated concrete. The laser intensity values of heated concrete showed a remarkable increase in the concrete exposure temperatures from 250 °C to 1,000 °C.
Such a correlation between the intensity and the exposure temperature is of importance in assessing the condition and extent of damage to concrete. This finding implies it could be possible to use laser intensity to detect the state of concrete, whether it has been heated or not. The study has also shown that RGB data improves the visual identification of features, and provides a rough idea of the concrete condition after a fire. Laser scanners have an advantage in that most of them have either an internal or external camera that can be used to capture concrete images if good resolution can be achieved.
Although the laser scanners had different wavelengths, the results demonstrated the feasibility of using TLS as an approach to assessing levels of fire-damaged concrete, and provided an understanding of the condition of concrete in relation to the strength changes of concrete when it is heated to elevated temperatures. A structural safety assessment provides information needed to evaluate the residual bearing capacity and durability of fire-damaged concrete structures. They are also used to propose the appropriate repair methods or to decide if demolition is needed.
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