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Dieses Kapitel untersucht den Einsatz von faseroptischen Sensoren (FOS) zur Überwachung der strukturellen Gesundheit von Asphaltdecken. Die Studie untersucht die Dauerhaftigkeit von FOS unter rauen Baubedingungen wie hohen Temperaturen und Verdichtungskräften sowie deren Auswirkungen auf die Qualität von Straßenbelägen. Im Rahmen des Laborprogramms wurden verschiedene Arten von FOS in Asphaltproben eingebettet und verschiedenen Tests unterzogen, darunter Integritätsmessungen und einachsige zyklische Kompressionstests. Die Ergebnisse zeigten, dass der Verdichtungsprozess die Kabel nicht beschädigte und die Haftung zwischen den Kabeln und Asphalt gut war. Der Verformungswiderstand des Asphalts nahm jedoch mit der Vergrößerung des Kabeldurchmessers ab. Das Kapitel kommt zu dem Schluss, dass FOS ohne Schutzmaßnahmen zur Überwachung des Asphaltbelags verwendet werden kann, aber eine weitere Validierung auf voll befestigten Straßen erforderlich ist.
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Diese Zusammenfassung des Fachinhalts wurde mit Hilfe von KI generiert.
Abstract
Implementing fiber optic sensors (FOS) in asphalt pavements provides a wealth of data with multiple applications. Successful integration of FOS into asphalt pavements depends on two key requirements. First, the sensor embedded in the asphalt must withstand the paving process without damage. Second, the cable must neither compromise the performance nor the durability of the asphalt.
These requirements were rigorously evaluated in a joint effort between the Technical University of Darmstadt and RINA Consulting S.p.A. Using realistic forces and material temperatures, asphalt samples were compacted and cable functionality and integrity were non-destructively evaluated. Standardized mechanical tests were used to conduct asphalt performance under dynamic loading. Void distribution within the asphalt specimens were evaluated using asphalt petrology techniques.
Results confirmed the integrity of nearly all cables tested, with minimal impact on asphalt void content and structure. Mechanical tests provided insight into the durability and performance of FOS-containing specimens.
1 Introduction
Asphalt pavements are a critical component of modern transportation networks and are subjected to severe and varied stresses from vehicular traffic, climatic variations, and environmental factors. As a result, continuous monitoring and evaluation of the structural health and performance of these pavements is essential to ensure road safety, optimize maintenance strategies, and extend pavement life. Traditional pavement monitoring methods often fall short in providing real-time and comprehensive data, requiring innovative approaches that overcome traditional limitations.
FOS (Fiber Optic Sensors) are a relatively new approach to pavement condition monitoring. Embedded in the asphalt structure, they allow accurate assessment of strain and stress distribution across the pavement structure, which, when combined with temperature data, is of great importance in understanding pavement performance and failure mechanisms [1]. During the installation of the FOS in an asphalt pavement, they are usually exposed to difficult conditions such as high temperatures, humidity, strong compaction forces, frequent heavy loads and other similar factors. However, it must be checked to what extent the cables can withstand these conditions and whether this measure actually results in the asphalt’s performance properties not deteriorating.
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2 Literature Review
2.1 FOS in the Field of Transportation Infrastructure
FOS have been used in numerous applications in transportation infrastructure. Various methods have been used to protect the cables.
One implementation is in the realm of smart cities, where FOS cables have been embedded within asphalt layers to continuously monitor traffic loads, temperature fluctuations, and strain distribution. This approach serves for the optimization of maintenance schedules and the enhancement of inner-city road durability [2]. The cables were laid in the ground next to the road to protect it. The quality of the road surface does obviously not suffer, but the measurement process is not as direct.
At an airport pavement test facility in the USA a study was conducted involving the installation of an inventive polymeric plate technology on four distinct pavement test sections to assess the strain response under traffic-induced loads [3]. However, the production of the plates is very complex and it is uncertain to what extent the measurements are still accurate.
A high viscosity asphalt mortar was used in Japan on a slope of an under-construction asphalt-faced dam to protect the installed cables. The FOS were fixed to the asphalt base with a primer and finally built over. Preliminary assessments demonstrated FOS survival during hot mix asphalt fabrication, involving compaction at temperatures exceeding 170 ℃ [4].
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2.2 Feasibility of FOS for Planned Application
Fiber optic (FO) are essentially a core normally made of glass surrounded by a transparent cladding of glass with a lower index of refraction. The difference in the index of refraction of the two layers of glass causes the total reflection phenomena, which permits the light to travel in the core. FOS are based on waveguides where deviations in the physical characteristics of light waves that propagate from external stimuli/signals through the optical fibers are sensitive to numerous external signals. Being by their nature very fragile, FO are normally coated with further materials to achieve higher resistance. Introducing such coating materials, it is necessary to ensure a proper transmission of external signals (i.e., strains and temperature changes) to the FOS through the coating’s materials. For this reason, a study on commercially available FOS was performed to identify the best solution in terms of coatings materials able to transmit properly external signals and to protect the FO during the asphalt construction process. Among the different possibilities, SOLIFOS FO were selected. Such cables are armoured with central metal tube and structured polyamide (PA) outer sheath. They can resist up to 30 MPa of hydrostatic pressure and for short periods (about 15 min) can operate at very high temperature (180 ℃). In particular, being necessary to monitor both strain and temperature changes three different solutions were identified: BRUsens DSS 3.2 mm V9 grip: 1 strain sensing cable with strain range up to 1% ⌀–3.2 mm, [ID: DSS V9]; BRUsens DTS STL PA: 2 temperature sensing cables with operating temperatures between −30 ℃ and +70 ℃ ⌀–3.8 mm, [ID: DTS PA]; BRUsens DSTAS V13: 2 strain sensing cables with strain range up to 1% and 4 temperature sensing cables with operating temperatures between −30 ℃ and +70 ℃, ⌀–6.5 mm [ID: DSTAS V13].
3 Laboratory Program
3.1 Preparation of Specimens
To simulate the paving process with FOS placed directly on wearing course, a special preparation method had to be developed. After several tests, a variation of the compaction process using the Marshall Impact Tester was used. At first, base specimens with a thickness of minimum 3.5 cm were produced. The Marshall compaction mould has a height of 9.7 cm. Hence, 6 cm thickness to pave over the base with the binder layer were left. Both, the base and the binder specimens were built out of asphalt concrete AC 22.
Before the paving process, the base was applied with a suitable amount of bitumen emulsion to ensure a sufficient layer bonding. After the breaking process of the emulsion was finished, a cable part with the length of roughly 50 cm is placed inside the compaction mould. The mould was prepared in such a way that the integrity measurements could be conducted afterwards. After the cable positioning, hot mix asphalt is put into the compaction mould. The asphalt is then compacted using the Marshall impact tester. After cooling, the produced Marshall-cable-specimen is pressed out of the mould. In total, six specimens were produced.
3.2 Integrity Measurement
Once embedded in the asphalt the integrity of the FOS was checked by injecting a laser light into the fibre cable and verifying the light passing through in the FOS. In particular, a beam of laser light with a wavelength around 650 nm, which is visible to the human eye, was used. Being FOS waveguides, they can transmit light inside only if intact, without excessive bending or compressive zones, otherwise the light would come out from the waveguide. Therefore, seeing the light exiting from the opposite extremity of the FO cable means that the waveguide is properly working (Fig. 1).
Fig. 1.
On the left - samples realized for the experimental campaign; on the right - one of the performed tests.
Six samples for each typology of FOS cable were manufactured and tested. All the tested FOS resulted to have been correctly embedded in the asphalt (cf. Table 1).
Table 1.
Summary of tested cables and rate of survival of tested cables.
Sample
Number of available cables
Number of tested cables
Tested cables (%)
Number of survived cables
Survived cable (%)
ID: DSS V9
Strain FO
6
6
100
6
100
ID: DTS PA
Temperature FO
12
9
75
9
100
ID: DSTAS V13
Strain FO
12
10
83
10
100
Temperature FO
24
11
46
11
100
3.3 Impact on Pavement Quality
Asphalt Petrology
Asphalt petrology is a geology science-based method for analysing the internal structure of asphalt samples. The sample to be examined is first cut to the desired dimension. The prepared specimen is placed in a vacuum chamber with pigmented epoxy resin that can penetrate the smallest cavities under negative pressure. Finally, the samples are scanned with a UV-scanner for image-analytical evaluation (Fig. 2).
Fig. 2.
Asphalt petrological sections of the three FOS samples (DSTAS V13, DTS PA, DSS V9)
Figure shows examples of the asphalt petrological sections for each FOS sample and a reference sample without cable. A total of three sections were produced and examined for each cable, each showing an equivalent pattern. The sensors can be seen in the green boxes with the voids in yellow. It can be noted that the cable with the smallest diameter (ID: DTS PA) has the fewest cavities, which also correlates with the results of the uniaxial cyclic compression test. Overall, however, it can be assumed that the bond between asphalt and FOS is very good, as no significant accumulations of voids around the cable can be detected.
Uniaxial Cyclic Compression Test
The uniaxial cyclic compression test describes the behaviour of asphalt under continuous load cycles and a temperature of 50 ℃. Specimens are prepared according to [5]. During testing the samples are loaded with 0.35 MPa lasting a loading time of 0.2s and relaxed with a load of 0.025 MPa lasting 1.5 s. This cycle is repeated until the stop criteria of a strain of 80‰ is reached or 10,000 cycles have passed. The influence of the cable on the asphalt quality can be determined by comparing the samples with each other (cf. Table 2). Three tests were carried out in each case and the outliers were neglected.
Table 2.
Results of the cyclic compression test
Sample ID
Sample
Impulses [n]
End strain [‰]
Strain Rate
[‰10–4/n]
Mean Strain Rate
[‰10–4/n]
DSTAS V13
A
4250
80
72.7
75.0
B
5450
80
77.2
DTS PA
A
10000
14.4
3.6
2.5
B
10000
9.3
1.3
DSS V9
A
10000
34.6
33.4
34.0
B
8600
80
34.7
The strain rate at the inflection point is suitable as a decisive factor for describing the resistance to permanent deformation. This is given as the mean value from two individual measurements for the tested samples in Table 2. The smaller the strain rate, the more resistant the specimen. A decrease in deformation resistance can be seen from sample DTS PA to sample DSTAS V13. Sample DSS V9 is in the middle range. The test results thus indicate that the deformation resistance decreases with the increase of the cable diameter.
4 Conclusion
As part of this research, a laboratory program was developed to investigate the effect of compaction on the integrity of FOS specimens, as well as to determine the effect of different FOS specimens embedded in asphalt on the quality of an asphalt pavement. The integrity tests showed that the compaction process replicated in the laboratory did not result in any damage to the cables. The subsequent analysis by asphalt petrology revealed that the cable with the largest diameter has a slightly void-richer microstructure. Since no significant void accumulation around the cables is visible, a good adhesion between cable and asphalt pavement is assumed. Finally, the uniaxial cyclic compression test showed that the cable thickness affects the deformation resistance. The specimens with the thinner cables led to a better deformation resistance. However, since the cores used for testing represent only a small area in relation to the total area of a road pavement, it is necessary to validate to what extent this applies on a full-sized paved road construction. Nevertheless, the presented research is a first step towards the usage of FOS inside the asphalt pavement without protective measures.
Acknowledgment
This paper was carried out in the framework of the InfraROB project (Maintaining integrity, performance and safety of the road infrastructure through autonomous robotized solutions and modularization), which has received funding from the European Union’s Horizon 2020 research and innovation program under grant agreement No. 955337. It reflects only the author’s view. Neither the European Climate, Infrastructure and Environment Executive Agency (CINEA) nor the European Commission is in any way responsible for any use that may be made of the information it contains.
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