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TS 7 – Underground Structures INGEO 2011 – 5th International Conference on Engineering Surveying Brijuni, Croatia, September 22-24, 2011

Digital Photogrammetry in Fire Testing Procedure

for Concrete Tunnel Linings

Chlepková, M.

Department of Surveying, Faculty of Civil Engineering, Slovak University of Technology in Bratislava, Radlinského 11, 81368 Bratislava, Slovak Republic Tel.: +421 2 5927 4427, E-mail: Miroslava.chlepkova@stuba.sk

Abstract Digital photogrammetry as a measuring method is more frequently used in different

applications of engineering practice. Especially high accuracy, low costs and short measuring and processing time are the main advantages which help photogrammetry to compete with methods like laser scanning and common surveying methods.

This paper provides information about using digital photogrammetry in the process of fire testing for concrete tunnel linings where the main task is to measure geometric parameters of concrete samples before and after spalling test and to obtain the quantity information about loss of material in each sample. The result helps to choose the best concrete mixture with the best attributes to build the tunnel lining to protect the tunnel from fatal damage in case of fire.

Key words: digital photogrammetry, fire test, concrete tunnel lining

1 INTRODUCTION

In recent years some very serious fires happened in tunnels where in addition to serious

structural damage, the loss of human lives occurred as well. Reconstruction work may be very expensive and also requires the closure of tunnel for a long time. Moreover, such incidents can damage the buildings or structures built above the tunnel. Therefore, the question of effective fire protection of tunnel linings comes to the fore during the construction or reconstruction of existing tunnel. Active fire protection as water sprinklers, water mists or foam systems are used next to passive fire protection systems. Passive fire protection is designed to be installed as a shield protecting the tunnel structure from fire all the time, creating a permanent part of the tunnel and does not require external activation in case of fire (Clement et al., 2007 and Promat Tunel, 2008).

In connection with the issue of passive fire protection of tunnels several tests are used in practice to obtain the most accurate information about the behaviour of the structure in case of fire. Within the frame of the spalling test the loss of material during a simulated fire is one of the studied parameters. This article discusses the exploitation of digital photogrammetry to determine the geometric parameters of samples tested in the process of passive fire protection systems testing.

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2 PASSIVE FIRE PROTECTION

Passive fire protection is a permanent part of the tunnel and requires no activation in case

of fire. Passive systems are not used to fight the fire but they are the last line of defence and maintain the stability of the tunnel structure in order to safe escape for the public and secure access to the fire brigade. They keep ventilation systems in use separated from traffic part of the tunnel by inner concrete structures. Primarily, passive systems prevent the tunnel from the collapse and the catastrophic loss of lives and property.

The role of passive fire protection in tunnels is to (Efnarc, 2006):

minimise the rate of temperature rise within the concrete and structural steel reinforcement (if present) so that structural integrity is retained during and after the fire,

to reduce or eliminate the risk of explosive spalling and loss of the concrete surface resulting from a build-up of vapour pressure within the concrete.

There are three common types of passive fire protection: an applied protective coating or render, a preformed thermal barrier or board fixed to the concrete surface, integral protection incorporated into the structural concrete, usually based on

polypropylene fibres. Especially the third option provides an optimal solution for the construction of new tunnel

projects. The fibre modified concrete shows no or less spalling losses than the unmodified concrete. This type of passive fire protection was the subject of testing during the construction of Airport Link tunnel in Brisbane, Australia, using the spalling test.

2.1 SPALLING TEST

Typical response of concrete exposed to fire is explosive spalling, which lasts until total damage of concrete slab or fire lies down. The explosive concrete spalling is a complex system of actions involving a combination of physical, chemical and mechanical processes, influencing each other. That is the reasons why scale tests or computer modelling is considered insufficient to define risks of the explosive concrete spalling. Test samples should be designed according to real situation in the tunnel together with the identical concrete mixture and possible fire control systems installed.

The quality of tunnel linings depends on several parameters and is determined by various tests. One of these tests is spalling test where the sample material is exposed to the temperature up to 1300°C in a special oven and the occurred changes are exanimated. The sample of the concrete panel after the test is shown in Figure1.

Figure1 Concrete sample after the test

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These tests are necessary for evaluating the suitability of concrete structure for application in practice and determine the probability of non-acceptable damage to concrete structures during possible fire based on the explosive spalling of the concrete surface. Among other parameters studied after spalling test, very important part is to determine the depth, resp. the amount of the material loss from the surface of concrete samples.

3 DIGITAL PHOTGRAMMETRY USED IN SPALLING TESTS

Together 16 standard size concrete samples (2600 mm x 1000mm x 250mm) were tested in CSIRO (Commonwealth Scientific and Industrial Research Organisation) area in Sydney, Australia. The concrete mixture of the samples varied as well as the percentage of polypropylene fibres used as anti-blasting component of the concrete. The aim of our measurements was to determine the geometry of the panels before and after spalling test, to compare created digital models “before” and “after” and to determine the depth of spalled material during the test. Two methods have been proposed for geodetic measurements: reflectless measurement using Leica TCRP1201 (2 +2 ppm accuracy) total station with additional software allowing automatic measuring the points on a predefined grid (similar to laser scanner) and photogrammetric measurement using a Canon 500D digital SLR camera and software PhotoModeler Scanner (PMS) (submillimeter accuracy in the circumstances). As digital photogrammetry seemed to be faster, more accurate and efficient method with possibility of increasing the density of points according to the individual requirements in post-processing, it was used in all tests.

3.1 FIELD WORK

The measurement was carried out in two phases, before and after the test. Using a crane,

each sample panel was positioned enabling to capture the surface to be tested. Within the testing area there were subsequently installed special surveying nails resistant to high temperature with a length of min. 80 mm (max. estimated value of material loss was 50 mm) which were used to determine the identical reference system before and after the test. The distribution on the nails is shown in Figure 2. They were used for scale definition as well.

Figure 2 Distribution of the control points

In addition these control points were signalized by coded targets for speeding up the

subsequent processing in PMS software. Up to 20-30 additional coded targets were located on each sample framing the measured area to increase the accuracy of model. Images were captured using digital camera Canon 500D EF-S 18-55 mm, which was calibrated before measurement using calibration grid supplied by the PMS software for a focal length of 18 mm at a distance roughly comparable to imaging one. Camera positions for images taking were selected to fulfil the condition of the highest accuracy for both convergent photogrammetry and automatic generation of point cloud – photogrammetric scanner:

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for convergent photogrammetry: the optimal angle of ray intersection 60°-120° (to determine the exact model using control and tie points signalised by coded targets),

for photogrammetric scanner: Baseline should be ¼ of the object distance and the angle of ray intersection less than 30° (for the automatically generation of points based on the principle of image correlation).

Figure 3 Distribution of the camera stations

Photogrammetric measurements were complemented by measuring minimum of 3

distances using steel tape, one to define the scale of the model and the other two for the control.

3.2 PROCESSING OF PHOTOGRAMMETRIC MEASUREMENTS

As mentioned above, the Photomodeller Scanner software was used for photogrammetry

measurements processing, which in addition to the measurement of discrete points allows the automatic generation of point cloud using the image correlation. The condition for generating high-quality point cloud is to create very accurate model based on subpixel measuring all control and tie points signalised by coded targets. Their spatial position in the model was estimated with the precision 0.2-0.4mm. Subsequently there was point cloud (grid 10x10mm) generated in the testing area of the panel and created a 3D TIN model (Figure 4).

Figure 4 TIN model created from the point cloud

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3.3 COMPARISON

Digital surface models "before" and "after" test were exported from PMS software to

exchange formats (.dxf and .txt) and then processed using the graphical software AutoCAD Civil 3D 2009. The “after” test measurement data were affected by deflections, which occurred due to warm-up of concrete panels together with its own weight action. Data of the deflection has been provided directly from the executor of the test and also checked by measuring position of control points 2, 5 and 7. Deflections were numerically removed using software Terramodel.

The aim was to determine the quantity of material loss on the exposed surface of the sample. As the coordinate systems of digital models "before" and "after" test were identical using the same control points, the result is a differential model of two surface models.

3.4 DATA VISUALISATION

The results were visualized as both the differential hypsometric models (Figre 5) where

the colours represent the value range of material loss (Tab. 1) as well as the plan with the differences values (depth of material loss) on the grid 100x100mm (Figure 6).

Figure 5 Hypsometric represantation of differential model

Tab.1 Spalling Values Table

Figure 6 Numerical interpretation of differential model

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Graphical results were supplemented by a statistical data with the values of minimum, average and maximum values of material loss (Tab.2).

Tab.2 Statistic results

As can be seen on the graphical results as well as in the table, the measurement also

indicates positive values, material gain. This phenomenon can be explained by the fact that at a certain temperature with horizontal position of concrete samples over the oven, the concrete began to melt and drain slightly, as evidenced by the visible stalactites on the sample surface.

4 CONCLUSION

The digital photogrammetry is progressively becoming used in wide variety of

engineering practical applications. High accuracy, low cost and relatively high efficiency of field and office works are the main characteristics in competition with other methods. Digital photogrammetry has also found its application in testing the passive fire protection of tunnels. Here it should be noted that the determination of depth of spalled concrete using digital photogrammetry is only a partial tasks throughout the whole testing procedure and its results are an integral part of the decision-making process on the suitability of the concrete for the construction.

When measuring irregular surfaces, as was the case of concrete samples after the fire test measurements, where the surface is irregular, the photogrammetry again appears in advantage comparing to terrestrial geodetic methods. Main reason is the possibility of choosing the grid of measured points in post-processing to ensure to identify and measure all the details of measured surface.

Acknowledgement

The article has been written within the solving the granted project VEGA 1/0142/10.

REFERENCES

CLEMENT, F. - ZÁME NÍK, M.: Alternatívy protipožární ochrany betonového ost ní tunelú. Tunel, Vol.16,No.1/2007.

PROMAT TUNEL: Centre for fire safety: Fire testing procedure for concrete tunnel linings, 2008 http://www.promat-tunnel.com/en/2008-EfectisR0695_Fire_testing_procedure_concrete_tunnel.pdf

EFNARC: Specification and guidelines for testing of passive fire protection for concrete tunnels linings, March 2006. www.efnarc.org.