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published online 13 October 2014Structural Health MonitoringZhiming Zhang, Ying Huang, Leonard Palek and Robert Strommen

overlay monitoringpackaged fiber Bragg grating sensors for ultra-thin unbonded concrete−reinforced polymer−Glass fiber

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Glass fiber–reinforced polymer–packaged fiber Bragg grating sensorsfor ultra-thin unbonded concreteoverlay monitoring

Zhiming Zhang1, Ying Huang1, Leonard Palek2 and Robert Strommen2

AbstractThe US Department of Transportation suggests a minimum thickness of 7 in (178 mm) to 8 in (203 mm) for unbondedconcrete overlay. Thinner overlays driven by economic advantages have attracted worldwide attention. To prove the fea-sibility of thinner overlay, especially ultra-thin unbonded concrete overlays, robust overlay assessment techniques aredemanded. In this article, glass fiber–reinforced polymer–packaged fiber Bragg grating sensors were developed anddeployed to evaluate the early performance of fiber-reinforced unbonded concrete overlay with a thickness of 3 in (76mm) placed on a 7.5-in (191-mm) concrete pavement, using a fabric interlayer as bond breaker. The early performanceof the ultra-thin concrete overlay within a year was monitored and investigated, indicating the occurrence of transversecracks located adjacent to the transverse cracks in the existing pavements. The numerical analysis and field experimentsconfirmed the accuracy and robustness of the fiber optic sensors, which showed their potential for further applicationsin concrete pavement monitoring.

KeywordsPavement monitoring, ultra-thin unbonded concrete overlays, fiber Bragg grating sensors, glass fiber–reinforced polymerpackaging, finite element modeling

Introduction

Concrete has long been recognized as a durable solu-tion for pavement maintenance and rehabilitation ofalmost any combination of existing pavement types andconditions.1 With no need for removal of existing pave-ments and minimal pre-overlay repair costs, concreteoverlays can significantly extend the existing pave-ments’ life cycle with additional load-carrying capac-ity.2 Across the United States, most state Departmentof Transportation (DOT) agencies have used concreteoverlays to maintain or rehabilitate aging pavementsthat have been in service for decades. Studies also showthat well-designed and constructed concrete overlaysprovide reliable performance and, in many cases,extend the existing pavements’ life for an additional 30years or more.1 Concrete overlays are designed to beeither bonded or unbonded to the existing asphalt, con-crete, or composite pavements for pavement rehabilita-tion (known as whitetopping if over existing asphaltpavements).1 Concrete overlays have a wide range of

thicknesses and the AASHTO3 Design Guide recom-mends a minimum thickness of 5 in (127 mm) to 6 in(152 mm).

Bonded concrete overlays for asphalt are typicallythin, ranging from 4 in (102 mm) to 6 in (152 mm) witha panel dimension of 6 ft (1.83 m) by 6 ft (1.83 m).4 Thefirst case using thin bonded concrete overlay for asphaltpavement repair in the United States was in Louisville,Kentucky, in 1991.5 This project studied two concreteoverlays with thicknesses of 2 in (51 mm) and 3.5 in (89mm) and slab sizes of 2 ft (0.61 m) by 2 ft (0.61 m) and

1Department of Civil and Environmental Engineering, North Dakota

State University, Fargo, ND, USA2Minnesota’s Cold Weather Road Research Facility (MnROAD),

Minnesota Department of Transportation, St. Paul, MN, USA

Corresponding author:

Ying Huang, Department of Civil and Environmental Engineering, North

Dakota State University, 1340 Administration Ave., P.O. 6050, Fargo, ND

58108, USA.

Email: [email protected]

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4 ft (1.22 m) by 4 ft (1.22 m), respectively. The white-topping halved the time required for resurfacing anddecreased the cost by 30%, compared with that of theremoval and replacement operations. In addition, theperformance of whitetopped pavement sections in simi-lar environments had been satisfactory over 10 years.Since the Louisville project in 1991, ultra-thin concretewhitetoppings have seen significant growth and popu-larity in the United States.6 In addition, materialimprovements have also been intensively studied to fur-ther enhance the capacity of the thin whitetoppings,such as the use of fiber-reinforced concrete.1,7 Previousstudies showed that fiber-reinforced ultra-thin concretewhitetoppings with a thickness of 4 in (102 mm) to 6 in(152 mm) exhibited acceptable performance for rehabi-litation of existing asphalt pavements.8

On the other hand, thin bonded overlays over exist-ing concrete pavements exhibited numerous problems.Typically, bonded overlays of 4 in (102 mm) to 6 in(152 mm) are applied to reduce the surface distress,enhance the loading capacity of existing concrete pave-ments, and, therefore, extend the service life for 15–20years when the existing pavements are still in good con-dition.9 With sensitivity to existing pavement surfacepreparation and joint sawing operations, however,these concrete overlays showed early failures, such aselimination at the joint corners, leading to cracking inthe overlay.10

Unbonded concrete overlays usually required amuch thicker design than bonded overlays due to lesssupport from the base. Conventional unbonded con-crete overlays required at least 6 in (152 mm) in thick-ness with a panel dimension between 12 ft (3.66 m) by12 ft (3.66 m) and 15 ft (4.57 m) by 15 ft (4.57 m). Thesethick unbonded concrete overlays are widely used torehabilitate existing asphalt and concrete pavementswith a high level of distress.1 In recent years, the stateof Michigan started to investigate thinner unbondedconcrete overlays with a thickness of 4 in (102 mm) to 6in (152 mm) and a panel dimension of 6 ft (1.83 m) by 6ft (1.83 m) or smaller. Thinner unbonded concrete over-lays have been used successfully on many projects.11,12

An unbonded concrete overlay also requires a separa-tion layer between the overlay and the existing pave-ment. Currently, the separation layer is established as a1-in (25 mm) to 2-in (51 mm) hot mix asphalt layer inthe United States.1 A geo-fabric, which is sometimesused in European countries, was adopted for the designof thin concrete overlay in Missouri during 2008.9,13

Limited efforts have been devoted to the application ofultra-thin unbonded concrete overlays. Recently, theMinnesota Department of Transportation (MnDOT)initiated the research for ultra-thin unbonded concreteoverlays with a thickness of 3 in (76 mm) on top ofexisting asphalt and concrete pavement.14,15

To assess the performance of pavement overlays,infrastructure sensing technologies are necessary.Traditional sensing technologies include load cells forweigh-in-motion,16 magnetic loops for traffic capacitydetection,17 electrical strain gauges for performanceevaluation, and thermal trees for temperature gather-ing.18 In the past several years, new designs of piezo-electric sensors also gained attention for trafficmonitoring.19 However, despite their relatively lowcost, the electrical sensors have significant limitations,indicating that they are unreliable for long-term pave-ment performance monitoring. For example, these sen-sors have shown high susceptibility to electromagneticinterference (EMI) and relatively short life cycles.20

Furthermore, corrections are needed due to tempera-ture variations, and lead wires/connections are easilydegraded by high humidity in the pavementenvironment.

To overcome the shortcomings of electrical sensors,advanced fiber optic sensing technologies have beendeveloping rapidly. The optical fiber Bragg grating(FBG) sensor, which is the most common engineering-applied fiber optic sensor, has been investigated widelyfor structural health monitoring of various infrastruc-tures.21–24 Because of its unique advantages of com-pactness, immunity to EMI and moisture, capability ofquasi-distributed sensing, and long life cycle, FBG sen-sors are a potential candidate for long-term concreteoverlay performance evaluation. Nevertheless, the FBGsensor, which is made of glass, is very fragile and easilydamaged during concrete overlay construction.20

Glass fiber–reinforced polymer (GFRP) material, onthe other hand, has proven to be a strong material forcivil applications and has been used to package andprotect FBG sensors to improve their ruggedness.25,26

The FBG component was aligned in the middle of aGFRP bar, and the assembly was used as a GFRP-rein-forced FBG (GFRP-FBG) sensor. The GFRP-FBGsensors were successfully used in the performance eva-luation of asphalt pavements in 2012.27 In this study,GFRP-FBG sensors are developed and applied to eval-uate the performance of the ultra-thin unbonded con-crete overlay, and the feasibility is validated to provideDOT agencies an effective way to monitor the repaircondition of concrete pavements.

Sensor design

Operational principle of the FBG sensors

Figure 1(a) shows the operational principles of theFBG sensors. FBGs are made by laterally exposing thecore of a single-mode fiber to a periodic pattern ofintense ultraviolet (UV) light. UV light creates a fixedrefractive index modulation, called grating. At each

2 Structural Health Monitoring



periodic refraction change, a small amount of light isreflected, forming a coherent large reflection at a partic-ular wavelength known as the Bragg wavelength. Lightsignals at wavelengths other than the Bragg wavelengthpropagate through the grating with negligible attenua-tion or signal variation. The ability to accurately presetand maintain the grating wavelength is a fundamentalfeature and advantage of FBGs.27Figure 1(b) shows atypical reflected spectrum of FBG. The Bragg wave-length satisfies the Bragg condition28

lBragg = 2nL

where n is the index of refraction and L is the gratingperiodicity of the FBG.

Due to temperature and strain dependence of theparameter, L, the wavelength of the reflected compo-nent will change as a function of temperature and/orstrain. The general expression of the strain–temperaturerelationship for the FBG strain sensor and temperaturecompensation sensor can be described as27

Dl1

l1

=Dle

l+

DlT

lT

= (1� Pe)e + (a + §)DT

where l, z, a, Pe, e, and T are the resonant wavelength,thermal-optics coefficient, thermal expansion coefficient,optical elasticity coefficient, strain, and temperature,respectively. The strain after temperature compensationcan then be calculated as27

e =1

(1� Pe)

Dl1

l1

� DlT

lT

� �ð1Þ

Geometric design

The GFRP-FBG sensors previously developed byZhou et al.27 were modified and used in this study asshown in Figure 2. Two geometric layouts were imple-mented for the use of ultra-thin unbonded concreteoverlay monitoring, including the GFRP-FBG sensorsin three dimensions (3D) as shown in Figure 2(a) andthe GFRP-FBG sensors in one dimension (1D) asshown in Figure 2(b). For the 3D GFRP-FBG sensor,the short-gauged sensor was intended to monitor thevertical strain and the long-gauged sensors were usedto monitor the longitudinal and transverse strainsinside the concrete overlays. Limited by the thicknessof 3-in (76-mm) concrete overlay, the vertical sensorwas designed and fabricated with a total height of 2.1in (53 mm), including the length of the GFRP holder.The longitudinal and transverse components of 3Dsensors had a gauge length of 2.2 in (56 mm). The dia-meter of 3D GFRP-FBG sensors was designed as 0.2-in (5-mm) 1D GFRP-FBG sensors had a longer gaugelength of 3 in (76 mm), and they shared the same dia-meter with 3D sensors. Figure 2 also shows the geo-metric layout detail of the developed sensors.

The FBG signals from all of the strain sensors weremonitored in real time using an optical signal analyzer(OSA) and recorded by computers for post-processing.The NI PXIe-4844 Optical Sensor Interrogator wasused in this research for FBG data acquisition. Sensorcalibration also followed the approaches used by Zhouet al.27 The GFRP-FBG sensors had a strain sensitivityof 7.937 3 1024 nm/me, which is the value of 1/(1 2Pe)in equation (1).

0.2

0.4

0.6

0.8

1

1.2

1549.6 1549.8 1550 1550.2 1550.4

Nor

mal

ized

Inte

nsity

Wavelength (nm)(a) (b)

Figure 1. Sensor operation principle and typical spectrum: (a) schematic representation of signal interrogation system and sensoroperation principle and (b) typical spectrum of FBG sensors.FBG: fiber Bragg grating; UV: ultraviolet.

Zhang et al. 3



Sensor implementation in the concreteoverlay at Minnesota’s Cold Weather RoadResearch Facility

Ultra-thin unbonded concrete overlay at Minnesota’sCold Weather Road Research Facility

Minnesota’s Cold Weather Road Research Facility(MnROAD) is owned and operated by the MnDOT.MnROAD consists of two unique roadways including

a two-lane low-volume road loop that is loaded with a5-axle 80,000 lb (36,287 kg) semi and an interstate I-94‘‘mainline’’ that contains two westbound lanes with livetraffic, as shown in Figure 3(a). This study utilized cell-40, an existing pavement test section (500 ft) on thelow-volume road. Cell 40 originally had 15 ft (4.57 m)by 12 ft (3.66 m) skewed jointed concrete undoweledpanels. After 19 years of continuous simulated truckloading, Cell 40 showed significant degradation. Two

Figure 3. Layout of MnROAD and cracking mapping of existing concrete pavements: (a) layout of MnROAD,15 (b) transverse andshort longitudinal cracks, and (c) long longitudinal crack.MnROAD: Minnesota’s Cold Weather Road Research Facility.

Figure 2. Geometric layout of the GFRP-FBG sensors: (a) 3D GFRP-FBG sensor and (b) 1D GFRP-FBG sensor.FBG: fiber Bragg grating; GFRP: glass fiber–reinforced polymer; GFRP-FBG: GFRP-reinforced FBG; 3D: three dimensions; 1D: one dimension.




longitudinal cracks and one transverse crack weredocumented. Figure 3(b) shows the transverse crackand the short longitudinal crack, and Figure 3(c) showsthe long longitudinal crack. The two longitudinalcracks were identified at 18 ft (5.49 m) and 90 ft (27.43m) in lengths, respectively, while the transverse crackwas at 12 ft (3.66 m) at the east end of Cell 40. Thelong longitudinal crack occurred under the wheel pathnear the pavement edge, while the transverse crackand short longitudinal crack were located under theinner wheel path. The joint fault with an average widthof 1

4in (6 mm) was noted with a history of pumping

each spring. Based on the significant degradation, pave-ment rehabilitation was recommended by the MnDOT.

To cost-effectively rehabilitate the existing concretepavements, a 3-in (76-mm) ultra-thin fiber-reinforcedunbonded concrete overlay was an interest for bothMnDOT and the industry. The concrete material wasreinforced with engineered alloy polymer macro-synthetic structural fibers (Febermesh 650).14 The over-lay consisted of 6 ft (1.83 m) by 6 ft (1.83 m) squarejointed panels, and sealed joints were applied betweenthe overlay panels. Figure 3(b) and (c) also showed thepanel layout for original and overlay panels as theskewed jointed panel and the smaller rectangularjointed panel, respectively. Moreover, to investigate theeffectiveness of the separation layers, two differentthicknesses of fabrics were used in the study. Cell-40was split into two new test sections with each at 250 ft(76.2 m) in length. The first consisted of a thin fabricinterlayer, named as Fabric 1, a weight of 8 oz/yard2

(0.27 kg/m2) about 14in (3 mm) and the 3-in (76-mm)

concrete overlay, which was numbered as Cell 140 (westend). The second test section consisted of a standard

fabric interlayer named as Fabric 2, a weight of 15 oz/yard2 (0.51 kg/m2) about 1

4in (6 mm) and the same 3-in

(76-mm) concrete overlay, which was numbered as Cell240 (east end). The goal of the two test cells was to bet-ter understand whether a standard (15 oz/yard2 or 0.51kg/m2) fabric could be used under ultra-thin concreteoverlays without causing too much deflection andcracking when compared to the thinner (8 oz/yard2 or0.27 kg/m2) fabric that would deflect less in theory.15

Sensor layout and implementation

GFRP-FBG sensors were installed inside the twopanels of Cell 40. The panel with sensors in Cell 140was named as West Panel, and the panel in Cell 240was as named East Panel. Figure 4(a) and (b) illustratesthe sensor layouts on the West Panel and East Panel,respectively, which were selected for monitoring basedon the layout of the longer longitudinal crack and thelocation of fabric layers. Inside each panel, four sensorswere cast inside the overlay, including one 1D GFRP-FBG sensor, one 3D GFRP-FBG sensor, one straingauge, and one thermal tree. To monitor the influenceon the overlay from the joint opening behavior of theexisting pavement, the Sensor 1D-3 was pre-deployedacross the joint of the existing pavement using high-performance polymer right below the Sensor 1D-1which was located inside the concrete overlay. Figure5(a) shows the installation of the Sensor 1D-3. In addi-tion, to track the potential growth or shrinkage of thetransverse crack, one longitudinal 1D GFRP-FBG sen-sor (1D-5) was implemented in the existing pavementusing high-performance polymer across the transversecrack as shown in Figure 5(b). Also, another 1D

Figure 4. Sensor layouts on Cell 40 (unit: inch; 25.4 mm): (a) West Panel in Cell 140 and (b) East Panel in Cell 240.3D: three dimensions; 1D: one dimension.

Zhang et al. 5



GFRP-FBG sensor (1D-4) was pre-placed inside theoverlay above the location of Sensor 1D-5 which wasplaced in the existing pavement comparison. In eachlocation, only one sensor has been located inside theconcrete overlay for monitoring purpose. Although theSensor 1D-5 was placed across the crack, it contributeslittle to restrain the further development of the cracksince the restriction was limited to one local point.

The GFRP-FBG sensors, strain gauges, and thermaltrees inside the overlay were deployed upward begin-ning from the position 0.5 in (13 mm) above bottomsurface of the concrete overlay. To assess the mechani-cal effect from truck loading, all GFRP-FBG sensorswere implemented along the wheel path. The transmis-sion connections of all the sensors were protected andcentrally connected to the instrument at the roadsidefor data acquisition and processing. As seen fromFigure 4, the developed sensor and strain gauges arenot located in the exact same location. However, thelocations of the two types of sensors were detaileddesigned with one type of sensor placed below each tireof the two tires on one side of the truck. Since the truckwas expected to deliver the loads uniformly on the twotires, although the actual loading may vary a little, thereading of the strain gauge and the developed sensor isexpected to be compatible. In addition, due to materialdifference between the host material (concrete) and thedeveloped sensors (GFRP), the strain transfer ratiobetween the two materials was derived according to theauthors’ previous study.27 Since the developed GFRP-FBG sensors have a relatively small size, the influencefrom the sensor to the host matrix has been neglected.

Finite element model development

Numerical analysis such as finite element analysis playsan important role for sensing system development andstructural health monitoring system. Validated finite

element model can verify the result of the deployed sen-sors and therefore double check the reliability of thesensing system. More importantly, due to the limitedsensors in the concrete overlay, for pavement perfor-mance prediction at locations other than the sensorlocations, validated finite element models could con-tribute significantly for concrete overlay performanceevaluation and prediction. Thus, in this study, a numer-ical analysis using commercial finite element modeling(ANSYS) is included.

Finite element model setup

ANSYS V.13.0 was selected for numerical analysis, andthe concrete overlay and the GFRP-FBG sensors weremodeled with tetrahedral SOLID 45 element.29 The tet-rahedral SOLID 45 elements can perform reliablenumerical analysis for linear, nonlinear, static, anddynamic conditions.30 Each SOLID 45 element haseight nodes, and each node has three degrees of free-dom: x, y, and z translations. Two materials, concretefor the overlay and GFRP for the sensors, were applied,and their properties are listed in Table 1.

East Panel in Cell 240 was selected for numericalanalysis. Figure 6(a) shows the general layout of themodel with all the components. The image is not toscale, due to the large difference between the sensor andthe pavement panel in dimensions. Figure 6(b) to (d)shows the finite element model for the concrete overlayand separate views of the models for the 1D GFRP-FBG and 3D GFRP-FBG sensors on the East Panel. Atotal of 232,325 tetrahedral SOLID 45 elements wereused in this study.

For the boundary conditions of the unbonded con-crete overlay, it is worth noting that the concrete over-lay was actually supported by the separation layer andexisting pavements in an elastic manner. To reflect theelastic behavior of the overlay’s support, spring

Figure 5. Photographs for sensor deployments in the existing pavement: (a) deployment of 1D-3 across the joint and (b)deployment of 1D-5 across transverse cracks.1D: one dimension.




elements, COMBIN 14, were applied to support eachnode at the bottom surface of the overlay. The stiffnessof the spring elements (KSpring), the key characteristicof COMBIN 14, will be determined using one field cali-bration test in next subsection. In the transverse direc-tions and outer side of the longitudinal direction, theoverlay panel was set as free from supports based onthe weak constraint from adjacent panels and the roadedge. On the inner boundary of the longitudinal direc-tion, the overlay was assumed to be symmetrically con-strained based on the physical alignment of the overlayonsite. Free longitudinal boundary conditions may alsobe applied since no additional connections were usedbetween the ultra-thin overlay panels. Analysis showednegligible difference between the symmetrically con-strained and the free boundary conditions for the innerlongitudinal side. Thus, in this study, considering the

potential transverse supports from the adjacent panel,the inner longitudinal boundary was set as symmetri-cally constrained. In addition, all the sensors embeddedinside the overlay were assumed to be perfectly bondedto the overlay.

The loading on the ultra-thin unbonded concreteoverlay was performed based on the geometry andgross weight of the MnROAD truck as shown in Figure7(a). The truck has five axles and a gross weight of 80kips (36.29 ton). Figure 7(b) illustrates the weight distri-bution on each axle of the MnROAD truck.

Model calibration of KSpring through field testing

In order to determine the value of KSpring, the influenceof KSpring on the strain of sensors was first parametri-cally analyzed with KSpring ranging between 1 lb/in

Table 1. Material properties.

Material Modulus of elasticity, klbf/in2 (6.89 MPa) Weight density, lb/ft3 (16.02 kg/m3) Poisson’s ratio

Concrete 2175.57 156.07 0.2GFRP 4351.14 152.62 0.3

GFRP: glass fiber–reinforced polymer.

Figure 6. General layout and finite element model of overlay and sensors: (a) general layout, (b) concrete overlay, (c) 1D GFRP-FBG sensor, and (d) 3D GFRP-FBG sensor.FBG: fiber Bragg grating; GFRP: glass fiber–reinforced polymer; GFRP-FBG: GFRP-reinforced FBG; 3D: three dimensions; 1D: one dimension.

Zhang et al. 7



(175.13 N/m) and 1015 lb/in (1.75 3 1017 N/m). Figure8 shows the variation of strains of Sensor 3D-1 underthe loading condition with one tire of the truck’s firstaxle located on the 3D-1. It can be seen that significantvariations of the sensors’ strains are noted when KSpring

ranges from 102 lb/in (17,512.68 N/m) to 107 lb/in (1.753 109 N/m). When the spring stiffness goes over 107 lb/in (1.75 3 109 N/m), the springs tend to act as a fixedboundary, resulting in the longitudinal and transversestrains of the overlay to be constrained and only verti-cal strains being possible.

For further determination of KSpring of the test field,one field calibration test with the static truck load waslocated in the same position. Figure 9 shows the com-parison on the result from the static field tests and thatfrom the finite element analysis with different values ofKSpring. Figure 9 indicated that a value between 104 lb/in (1.75 3 106 N/m) and 105 lb/in (1.75 3 107 N/m) forKSpring is a good estimation of the foundation stiffness,which will be applied for further numerical analysis.

Overlay performance evaluation and fieldtests

Early performance evaluation

The ultra-thin unbonded concrete overlay was pavedand cured for 21 days (10 June 2013 to 1 July 2013).After curing, it was opened to simulated low-volumetraffic with an 8 kips (3.63 tons) semi-truck as shown inFigure 7 driving 50 laps a day for 5 days each week.The early performance of the overlay for the first yearwas presented in this article. Table 2 shows the mea-sured raw strains at 21 days (1 July 2013, 100�F(38�C)), 52 days (1 August 2013, 92�F (33�C)), 85 days(4 September 2013, 88�F (31�C)), 162 days (22November 2013, 22�F (26�C), and 308 days (18 April2014, 34�F (1�C)) after paving. The accumulatedstrains, which were temperature compensated usingequation (1) and with the consideration of strain trans-fer rate based on previous study,27 were calculated by

subtracting strains obtained at 21 days after pavingfrom strains at 52 days, 85 days, 162 days, and 308days after paving. The developed sensors survived theoverlay construction process and showed a 100% sur-vival rate throughout the early performance duration,which demonstrated their robustness for the concretepavement monitoring.

Sensor 1D-3 was placed across the joints in the exist-ing pavements, and Sensor 1D-1 was placed in longitudi-nal direction directly above 1D-3 inside the overlay.Comparing the data of Sensor 1D-3 on different days, itcan be seen that the joint in the existing pavements hadextensive tension strain (2746 me) at 162 days. Since theconcrete usually can only hold tension strains up to sev-eral hundred according to the research by Liners,31

potential further joint opening might be expected beneaththe overlay after 6 months of overlay casting. Furthercontinuous close monitoring is needed to track the devel-opment of these potential further opening of the beneathjoints. With the unbonded design of the concrete overlay,the overlay above the existing joints behaved well withlarge tension (130 me) in summer and returned to minortension (45 me) or compression in winter (293 me).Considering that concrete materials are strong for com-pression, which can hold compression strains up to 2000or 3000 me depending on the mix design of the con-crete,31 no micro-cracks were expected inside the concreteoverlay during the monitoring period.

Sensor 1D-5 was placed across the existing trans-verse crack, and Sensor 1D-4 was placed in the longitu-dinal direction right above 1D-5 inside the overlay. Themeasured strains of Sensor 1D-5 showed that althoughthe transverse crack inside the existing pavement sealedwith 2197 me right after the overlay’s casting, it reopensa little in summer with a tension strain around 415 meat 162 days. This significant extension also caused theconcrete overlay above to be in tension in summer (10me) and in compression in winter (2316 me) at 162 days,as indicated from strain recorded by Sensor 1D-4. At308 days, an abnormal large compression strain wasrecorded on Sensor 1D-5, indicating that the crack

Figure 7. Photographs of loading truck and load distribution: (a) MnROAD truck and (b) load distribution.MnROAD: Minnesota’s Cold Weather Road Research Facility.




turned to be closing. This closing of the crack in exist-ing pavements may be induced by through-cracks in thecurrent or the adjacent concrete overlay panels. Avisual inspection was then performed, and a large trans-verse crack was notified on the west side of the panel asshown in Figure 10, mostly induced by the freezing andthawing effect in spring. The sensors successfullydetected abnormal events on the concrete overlays.Continuous close monitoring of the further opening ofthe transverse crack inside the existing pavements andtheir influence related to reflective cracking in the over-lay was recommended.

Sensor 1D-2 was installed in the transverse directioninside the overlay. As indicated in Table 2, continuouscompressive strains from this sensor indicated an accep-table performance of the overlay in the transverse direc-tion. The longitudinal and transverse components ofthe two 3D GFRP-FBG sensors showed tension strainsin summer and compression strains in winter. A tensionstrain of 181 me was noted in 3D-2-L at 85 days, indi-cating that careful attention should be paid on the long-itudinal direction. Early longitudinal cracks may be

developed with continuous simulated traffic after sev-eral season changes. In summer, the overlay in Cell 240was noted to deform more than Cell 140 with a tensionstrain difference of around 100 me in longitudinal direc-tion. The regular fabric with 1

4in (6 mm) of thickness

induced a significant tension strain (181 me) in longitu-dinal direction on the overlay in summer, which maypotentially bring up micro-cracks in the near future andneeds continuous close monitoring. In winter, Cell 140compressed more than Cell 240 in longitudinal direc-tion with the significant temperature changes. In trans-verse direction, no significant difference was notedbetween the Cell 140 and Cell 240. The monitoringresults from the 3D fiber optic sensors indicated thatthe thinner fabric, Fabric 1 with 8 oz (236.5 mL) about14in (3 mm), performed better than the thicker fabric,

Fabric 2 with 15 oz (443.6 mL) about 14in (6 mm). The

test results approved the theory that thinner fabric willcause less deformation for the overlay, which alsoproved the potential support effect from the existingpavements to the overlay although the overlay wasunbonded.

Figure 8. Influence of spring stiffness on strains.Figure 9. Field calibration of the KSpring.

Table 2. Measured raw and accumulated temperature (environmental)-compensated strains.

Cell no. Sensor no. Raw strain, me Accumulated strain after temperature compensation, me

21 days 52 days 85 days 162 days 308 days 52 days 85 days 162 days 308 days

140 1D-1 214 38 29 2532 2497 125 134 45 293240 1D-2 237 204 122 2655 2516 9.6 255 2352 2388140 1D-3 21251 2956 2927 934 2186 504 413 2746 1463240 1D-4 106 73 29 2825 2410 224 10 2364 2218240 1D-5 2197 2211 2179 2350 15 45 108 415 2614140 3D-1-L 327 291 209 2414 2219 66 23 2267 2190140 3D-1-T 272 267 246 2451 2278 100 73 2153 2142240 3D-2-L 348 420 447 2303 234 151 181 290 25.5240 3D-2-T 388 423 388 2282 249 127 77 2133 253

Zhang et al. 9



Static field testing

Test setup. In addition to the continuous evaluation ofthe overlay’s performance, static field loading tests wereperformed on 1 August 2013 and 18 April 2014 to detectthe overlay’s early behavior and to validate the sensors’effectiveness. The MnROAD truck was used as theloading truck (Figure 7). Eight loading positions weretested using the MnROAD truck as shown in Figure11(a) to (h). Loading Position 1 placed the right tire ofthe front axle on top of Sensor 1D-1, Position 2 loadedthe same tire on top of Sensor 3D-1, Position 3 locatedthe right tire of the second axle on top of Sensor 3D-1,Positions 4 and 5 situated the right tire of the front axleon top of Sensors 1D-2 and 3D-2, Positions 6 and 7placed the right tire of the second and third axles on topof Sensor 3D-2, and Position 8 loaded the right tire ofthe fourth axle on top of Sensor 3D-2.

Static testing results. Figure 12 shows the measuredstrains after temperature compensation from the staticfield loading tests on 1 August 2013. On 1 August2013, a static loading of 5.8 kips (2.63 ton) on the over-lay developed a tension strain of around 50 me in thetransverse direction, a tension strain of around 40 me inthe longitudinal direction, and a compression strain ofaround 235 me in the vertical direction. All the sensors

were recovered after the removal of static loading. Thestatic loads did not induce cracks inside the concreteoverlay with all the tension strains below 100 me, vali-dating the strength of the mix design of the overlay.However, the truck loading induced relatively large ten-sile strains in the longitudinal and transverse directions,which increases the possibility of fatigue cracks withlong-term truck traffic.

Table 3 shows the comparison of measured strains ofthe static testing in two different seasons on 1 August2013 (seasons of summer and fall) and 18 April 2014(seasons of winter and spring). It is clearly demon-strated that the pavement overlay and its foundationacted much stiffer in the winter season than in the sum-mer season with smaller strains in general for all load-ing cases. The phenomenon of stiffer concrete overlaywas also attributed to the continuous dynamic loadingwith heavy vehicles on the pavements. In the winter sea-son, the strain differences between two fabric types aresmaller than in the summer season.

Sensor validation with comparison to the numericalanalysis

To verify the effectiveness of the developed GFRP-FBG sensors for concrete pavement monitoring, wedid the finite element analysis on the concrete overlaywith deployed sensors and compared the Sensor 3D-1’s responses from the field tests with responses mea-sured from numerical simulation for LoadingPositions 2 and 3 in the summer and winter seasons.The simulation data are presented with spring stiff-ness ranging between 104 lb/in (1.75 3 106 N/m) and105 lb/in (1.75 3 107 N/m) as stated in section ‘‘Finiteelement model development’’ (for comparison, weshowed a wider range of KSpring in the result). It canbe seen from Figures 13 and 14 that the field testingdata match well with the simulation result with a rela-tive error below 25% in most cases. The consistenceof the field test results with those from the finite ele-ment analysis indicates that the developed sensors arereliable for the structure health monitoring of theultra-thin unbonded concrete overlay.

Table 3. Comparison of measured strains on 1 August 2013 and 18 April 2014.

Position Strains on 1 August 2013 (me) Strains on 18 April 2014 (me)

3D-1-V 3D-1-T 3D-1-L 3D-2-T 3D-2-L 3D-1-V 3D-1-T 3D-1-L 3D-2-T 3D-2-L

Position 2 229 49 40 0 0 217 35 18 0 0Position 3 230 53 48 0 0 220 42 26 0 0Position 5 0 0 0 90 60 0 0 0 62 33Position 6 0 0 0 85 76 0 0 0 51 45

Figure 10. The occurrence of transverse cracks on adjacentpanel of crack sensors (1D-4 and 1D-5).




Sensor validation with comparison to commercialstrain gauges

To validate the performance of the developed sensorsfor concrete overlay monitoring, commercial straingauges were also implemented for comparison purposeas shown in Figure 4 for both panels. Data from strain

gauges and the GFRP-FBG sensors on the West Panelwere recorded simultaneously. Table 4 shows the mea-sured strains from the strain gauge and the transversecomponent of the 3D GFRP-FBG sensor, respectively.A maximum difference of 3.16% was noted when com-paring the test data from the strain gauge and that ofthe GFRP-FBG sensor, part of which can be accounted

Figure 11. Field static testing setup: (a) Loading Position 1, (b) Loading Position 2, (c) Loading Position 3, (d) Loading Position 4, (e)Loading Position 5, (f) Loading Position 6, (g) Loading Position 7, and (h) Loading Position 8.

P 1 P 2 P 3 P 4 P 5 P 6 P 7 P 8

-30

-15

0

15

30

45

60

Stra

in (μ

ε)

Position

1D-13D-1-V3D-1-T3D-1-L

P 1 P 2 P 3 P 4 P 5 P 6 P 7 P 8

0

15

30

45

60

75

90

105

Stra

in (μ

ε)

Position

1D-23D-2-L3D-2-T

(a) (b)

Figure 12. Measured temperature–compensated strains throughout static testing using the MnROAD semi parked on the sensorson 1 August 2013: (a) West Panel in Cell 140 and (b) East Panel in Cell 240.3D: three dimensions; 1D: one dimension; MnROAD: Minnesota’s Cold Weather Road Research Facility.

Zhang et al. 11



for by the positional difference between these two sen-sors. The variance within 5% proved the effectivenessof the developed GFRP-FBG sensor for concrete over-lay monitoring. The comparison approved the effective-ness of the developed GFRP-FBG sensor for concreteoverlay monitoring.

Conclusion and future work

This article presented an innovative robust sensing sys-tem based on the GFRP-packaged FBG sensors forperformance evaluation of concrete pavements. Several1D and 3D GFRP-FBG sensors were deployed at

MnROAD, MnDOT, to monitor ultra-thin unbondedconcrete overlay behavior under the truck load. Thedeveloped sensors showed a 100% survival rate afterthe concrete overlay was cast in place. The deployedGFRP-FBG sensors successfully monitored the closingbehavior of the crack and joint in the existing pavementduring the overlay casting and a reopening behavior 6months later.

Within 1 year of service, in transverse direction, theoverlay exhibited cracks induced by freezing and thaw-ing; while in longitudinal direction, the overlay exhib-ited a weaker performance in summer, which indicatespotential cracks in the future. Fabric 1 in Cell 140 with

Table 4. Comparison between developed sensor and strain gauges.

Position Strain gauge (me) 3D-1-T (me) Relative difference (%)

Position 5 60.17 62.07 3.16Position 6 52.46 51.18 22.44

100 102 104 106 108 1010 1012 1014

0

10

20

30

40

50

Stra

in (με)

Kspring (lb/in.)

Numerical Analysis Field Data at Summer Season Field Data at Winter Season

(a)

Stra

in (με)

100 102 104 106 108 1010 1012 1014

0

10

20

30

40

50

60

70 Numerical Analysis Field Data at Summer Season Field Data at Winter Season

Kspring (lb/in.)

(b)

Stra

in (με)

100 102 104 106 108 1010 1012 1014-10

-15

-20

-25

-30

-35

-40 Numerical Analysis Field Data at Summer Season Field Data at Winter Season

Kspring (lb/in.)

(c)

Figure 13. Comparison on result of field tests and numerical analysis for Loading Position 2: (a) 3D-1-L, (b) 3D-1-T, and (c) 3D-1-V.3D-1-L, 3D-1-T, and 3D-1-V represent the longitudinal, transverse, and vertical components of the 3D-1 GFRP-FBG sensor, respectively.




8 oz (236.59 mL) about 14in (3 mm) fabrics indicated

better performance than Fabric 2 in Cell 240 with 15 oz(443.6 mL) about 1

4in (6 mm) fabrics. The test results

approved the theory that thinner fabric will cause lessdeformation for the overlay. With significant tensionstrain induced in longitudinal direction in the summerseason, longitudinal cracks are also expected to occurin Cell 240 with the standard fabric of 1

4in (6.35 mm) in

thickness and further continuous close monitoring isneeded to confirm this prediction. The static load test-ing results showed that the concrete overlay’s mixdesign had a sufficient strength. The comparisonbetween the simulation result, strain gauge data, andthe strain of developed sensor validated the reliabilityof the developed sensing technology for concrete over-lay monitoring.

The validation of the GFRP-FBG sensors not onlyproved their feasibility for monitoring ultra-thinunbonded concrete overlay but also showed their capa-bility and potential for monitoring all types of concretepavements during construction, maintenance, andrepair. The minimum thickness that the developed

sensors can apply is determined by the dimension ofthe sensors, for example, 2.1 in (53 mm) for 3D sensor’svertical component, to achieve an effective monitoringstrategy. The thickness of the pavement, as long as it isbigger than the dimensions of the sensor, is notexpected to influence the sensor’s accuracy based onprevious studies.27 In addition, multiple sensors in dif-ferent heights of the overlay cross section will also bestudied in future for pavement curvature derivation.Thus, with high durability and long-term stability, theGFRP-FBG sensors can be further applied for real-time pavement monitoring in the near future.

Acknowledgements

The findings and opinions expressed in this article are those ofthe authors only and do not necessarily reflect the views of thesponsors. The authors would like to thank Dr Genda Chenfrom MS&T for valuable technical supports and MnDOT fortremendous facility supports.

Declaration of conflicting interests

The authors declare that there is no conflict of interest.

100 102 104 106 108 1010 1012 1014

0

10

20

30

40

50

60


Kspring (lb/in.)100 102 104 106 108 1010 1012 1014

0

20

40

60

80 Numerical Analysis Field Data at Summer Season Field Data at Winter Season

Kspring (N/m)

100 102 104 106 108 1010 1012 1014-32

-30

-28

-26

-24

-22

-20

-18

-16

-14


Kspring (lb/in.)

Stra

in (μ

ε)

Stra

in (μ

ε)

Stra

in ( μ

ε)

(a) (b)

(c)

Figure 14. Comparison on result of field tests and numerical analysis for Loading Position 3: (a) 3D-1-L, (b) 3D-1-T, and (c) 3D-1-V.3D-1-L, 3D-1-T, and 3D-1-V represent the longitudinal, transverse, and vertical components of the 3D-1 GFRP-FBG sensor, respectively.

Zhang et al. 13



Funding

Financial support to complete this study was provided par-tially by the USDOT MS&T Center for InfrastructureEngineering Studies under Agreement No. 00041896-01 andUSDOT MPC under Agreement No. DTRT12-G-UTC08.

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