British engineers have concluded laser scanning is a viable structural safety technique to detect the damaging effects of fire on concrete.
While concrete is known to be a material with high fire-resistance, capable of retaining much of its load-bearing capacity, a significant loss in strength occurs when concrete is heated above 300°C.
Existing techniques for assessing fire-damaged concrete include visual inspections of colour change and physical features, and invasive tests such as core drilling or lab-based techniques. However, they all have merits and drawbacks.
The researchers from The University of Nottingham, UK and Ningbo, China (UNNC) studied the use of terrestrial laser scanning (TLS) as a non-destructive way to assess and detect fire-damaged concrete in a structural safety appraisal.
They investigated the influences of scanning incidence angle and distance on the laser intensity returns. Concrete colour change was also studied. Data was collected and interpreted on unheated and heated concrete to establish the baseline condition of the material.
Experiments were carried out in a controlled laboratory using two-phase shift terrestrial laser scanners (Leica HDS7000 and FARO Focus 120) to scan the concrete specimens before heating and after they were cooled again. These concrete specimens were heated in a furnace to temperatures of up to 1000°C.
To assess colour change in the heated concrete, specimen images were captured using the M-Cam attached to the Leica HDS7000 laser scanner.
A flatbed scanner (HP Scanjet G2410) was also used to scan heated concrete surfaces and capture images. It is these images that were used for analysis due to their better 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 used showed the same trend.
PhD student Wallace Mukupa said 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 of unheated concrete.
"In fact, the laser intensity values of heated concrete showed a remarkable increase in the concrete exposure temperatures from 250°C to 1000°C," he said.
“Such a correlation between the intensity and the exposure temperature is of cardinal 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 FARO Focus 120 laser scanner. Photo: Faro]