Radio Frequency Identification Applications in Pavements

CHAPTER 2: FEASIBILITY EVALUATION

The feasibility evaluation encompassed the following components: 1) literature review, 2) identification of appropriate RFID technology, and 3) prototype tag development and evaluation. Each of these is described in the following sections.

LITERATURE REVIEW

The literature review focused on RFID technology and applications in the general areas of civil engineering and construction. Specific RFID applications identified in these areas include the following:

• Tracking of PCC test specimens.
  
  
• Maturity monitoring of in-place PCC.
  
  
• Tracking of construction materials at project site (e.g., fabricated pipe).
  
  
• Construction tool inventory/usage control/monitoring.
  
  
• Location identification of buried infrastructure (cables and pipes).
  
  

An annotated bibliography of the reviewed articles is provided at the end of this report.

IDENTIFICATION OF APPROPRIATE RFID TECHNOLOGY

Aspects of RFID technology of relevance to this project include the following:

• Physical size of the tag.
  
  
• Operating frequency (low, high, ultrahigh, and microwave; this influences tag size and read range).
  
  
• Type of tag (active or passive; this influences cost, power consumption, and maximum read range).
  
  
• Read range (including high-gain antenna design).
  
  
• Durability/survivability (e.g., strength of tag encapsulation; maximum/operating temperature ranges).
  
  
• Cost of the expendable tag.
  
  

Some light was shed on these aspects by articles from the literature review. However, other lines of inquiry were also pursued: 1) consultations with Dr. Marc Cohen, a research scientist at the Center for Engineering Logistics and Distribution in UMD's Institute for Systems Research and 2) exploratory laboratory studies.

Dr. Cohen has extensive experience in RFID technology. Discussions with him confirmed and expanded the conclusions reached from the literature review (see also table 1):

1. For a physical tag size on the order of 1 inch (25 mm), the UHF range is optimal.
   
   
2. Passive tags, which are preferred because of their cost differential relative to active tags, should be able to provide the requisite read range performance.
   
   
3. Commercial RFID readers are readily available for vehicle mounting (e.g., antennas on a front bumper) with read ranges on the order of 3 ft (1 m) or more, which is sufficient for this study's applications.
   
   
4. Possible attenuation of the RFID signal by the asphalt concrete could be an issue and therefore needed to be explored early.
   
   
5. Thermal and mechanical survivability are key challenges. The maximum temperature range tolerable for short periods (e.g., the time required to truck a load of asphalt from the production plant to the paver) is significantly higher than the typical rated operating and/or storage temperatures for RFID tags.
   
   

Table 1 . Characteristics of different RFID technologies.

- Indicates no information available
1 ft = 0.0305 m
KHz = kilohertz
MHz = megahertz
GHz = gigahertz

Preliminary Evaluations

Potential attenuation of the RFID signal by the asphalt surrounding the embedded tag could have a major impact on read range and thus system practicality. Because this aspect is vital to the project, it was evaluated early on in a very preliminary way as part of the identification of appropriate RFID technologies. Jaselskis et al. describe the theoretical aspects of electromagnetic wave interaction with materials, with a particular focus on asphalt concrete.⁽²⁾ The key material property governing the transmission of dielectric materials is the permittivity. A dielectric material is an electrical insulator. A vacuum is the ideal insulator, but many materials (e.g., asphalt concrete) also fall into this category. When an electric field interacts with a dielectric medium, redistribution of charges within its atoms or molecules alters the shape of the field both inside and around the medium. The absolute permittivity, , is the fundamental physical quantity governing the interaction between and electric field and a dielectric medium. It is a measure of the ability of the medium to polarize and thus reduce the strength of the field. In other words, permittivity defines a materials ability to transmit or "permit" an electric field. The relative permittivity or dielectric constant, , is the permittivity of a material relative to that of an ideal vacuum, , as shown in the equation in figure 1.

Figure 1. Equation. Definition of relative permittivity or dielectric constant ε.

According to Maxwell's equations for nonmagnetic media, the electric field E induced by the incident electromagnetic wave is defined in the equation in figure 2.

Figure 2. Equation. Electric field E.

The displacement vector D is calculated using the equation shown in figure 3:

Figure 3. Equation. Displacement vector D.

where:
ω = circular frequency of the wave.
ε₀ = permittivity of vacuum.
ε(ω) = frequency dependent dielectric constant of the medium
         = ε_a(ω)/ε₀, where ε_a(ω)= absolute permittivity.
i = √-1.

The displacement field D represents how the electric field E influences the organization of electrical charges in a given medium. The total current flowing within a medium is divided into conduction and displacement components. The displacement current can be thought of as the elastic response of the material to the applied electric field. As the magnitude of the applied field is increased, an increasing amount of energy is stored in the displacement field. If the electric field is subsequently decreased, the material will release the stored energy.

Owing to inertia processes in fluctuating electromagnetic fields, the displacement field D for most materials is not in phase with the electric field E. As a consequence, the dielectric constant is the complex frequency-dependent quantity ε^*(ω) defined in the equation in figure 4.

Figure 4. Equation. Complex relative permittivity (dielectric constant).

The real part of the permittivity, ε^'(ω) , is a measure of polarization and is thus a measure of the energy stored in the medium while the imaginary part,ε^''(ω), defines the average power absorbed and/or scattered by the medium and thus is a measure of energy loss. This energy loss can also be related to the electrical conductivity σ for the given frequency as shown in the equation in figure 5.

Figure 5. Equation. Relationship between imaginary portion of complex dielectric constant and electrical conductivity.

The net effect of all of the above is that electromagnetic wave propagation is impeded as the permittivity increases. Maximum RFID read range occurs when the RFID electromagnetic waves travel through a material with a low dielectric.

For asphalt concrete, both the real and imaginary permittivity components will be functions of the mixture composition, moisture content, and density. Jaselskis et al. measured the complex relative permittivity components as functions of frequency and temperature for several asphalt concrete mixtures.⁽²⁾ Values of the real and imaginary relative permittivity components for a sand asphalt mixture varied between about 4.5 and 5.5, and 0.2 and 0.3, respectively.

The theoretical description of electromagnetic wave propagation and losses in terms of relative permittivity is placed in better context through comparisons with other materials. Table 2 summarizes relative permittivity values for construction and related materials. Asphalt concrete typically has a relative permittivity of between 3.5 and 5.0, comparable to Portland cement concrete, aggregates, and other similar materials. This range lies closer to the properties of good UHF media (e.g., vacuum and air), and relatively far away from poor UHF media (e.g., water)-suggesting that attenuation losses of RFID signals through asphalt concrete should be modest.

Table 2 . Typical dielectric constant (relative permittivity) values for common materials.

- Indicates no information available
Note: Bold font indicates materials most relevant to the present study.
MHz = megahertz
GHZ = gigahertz

RFID System

Based on a careful review of the specifications and available product reviews for various manufacturers' systems, the Mercury® 5 RFID reader from ThingMagic was selected as most suitable for this project. Key specifications of the Mercury® 5 reader include the following:

The ThingMagic Mercury® 5 was purchased as a complete start-up system, including the reader, two bistatic circular antennas, four 25-ft (7.6-m) cables, a startup CD, and a selection of RFID tags.

Suitable tags compatible with the Mercury® 5 reader and meeting size limits were also identified. Key features of these tags are summarized in table 3. Sample quantities were obtained for the Alien® Gen 2 1x1 and Alien® Gen 2 2x2 tags. Attempts to purchase small trial quantities of the Avery-Dennison™ AD-812 tags were unsuccessful.

Table 3 . Candidate RFID tags (all passive).

- Indicates no information available
MHz = megahertz
°C = (°F - 32)/1.8
1 m = 3.28 ft

Linking the RFID tag identifier with GPS latitude and longitude coordinates is also an important component of this project. A Garmin™ GPS 12 transponder was initially evaluated for linking GPS with RFID. Preliminary findings suggested that a data logger would be an easier technology for linking the RFID tag data to latitude and longitude coordinates. A GPS data logger records a constant stream of latitude/longitude/time values that can then be matched post facto to the time stamps on the RFID tag data. An inexpensive Pocket Tracker Pro system from Brickhouse® Security (www.brickhousesecurity.com) was therefore acquired for use on the project. Key features of the Pocket Tracker Pro system useful for this project include: 1) location accuracy to within 8.2 ft (2.5 m); 2) read rate of one location point per second; 3) magnetic vehicle mount; 4) universal serial bus computer interface; 5) 100-h battery life (AAA batteries); and 6) low cost.

PROTOTYPE TAG DEVELOPMENT AND EVALUATION

Fabrication

Encapsulation of the RFID tags to protect them from temperature extremes and compaction stresses is key to their use in asphalt paving applications. Two candidate encapsulation media were considered: ceramics and high-temperature epoxy. Upon evaluation of these two materials, the ceramics were discarded because of high cost and fabrication difficulties. A high temperature epoxy, Durapot™ 866 from Contronics, Inc. (www.contronics.com), was selected as a suitable encapsulating material. Durapot™ 866 is a thermally and electrically insulating compound that forms a low-density nonporous foam for high-temperature applications. This two-part epoxy, after proper curing at room temperature for 24 h followed by oven curing at 248 °F (120 °C) for 4 h, has an upper temperature limit of 500 °F (260 °C), more than adequate for asphalt applications.

Suitable molds for encapsulating the RFID tags in the high-temperature epoxy are also required. CPVC pipe was found to be suitable for this purpose. CPVC is a thermoplastic commonly used for cold and hot piping systems in building construction. It has a glass transition temperature of approximately 230 °F (110 °C) and a melting point of 414 °F (212 °C), which is sufficient for use with hot mix asphalt. The Alien® 1x1 and 2x2 tags were curled respectively inside 1-inch (25-mm) and 2-inch (50 mm) long sections of 3/4-inch (19-mm) nominal diameter CPVC pipe. The inside diameter of this pipe is sufficiently large that the ends of the curled tags did not overlap. Curling the tag with the antenna facing outward or inward had no influence on the read range. The pipe with tag was then filled with the high-temperature epoxy and cured. Because the CPVC pipe is transparent to the RFID signal and has a melting point higher than the HMA compaction temperature, there was no need to extrude the tag after curing of the epoxy.

The epoxy molding and curing process was further refined to minimize problems caused by air bubbles during curing. The original procedure followed the epoxy manufacturer's suggestions: the mold was filled with epoxy at room temperature, then cured at room temperature for 24 h followed by oven curing at 248 °F (120 °C) for 4 h. This process often resulted in voids between pipe wall and tag during filling (figure 6) and air bubbles within the epoxy during the room temperature curing (figure 7). These defects would then expand and cause tag failure during the subsequent oven curing (figure 8 and figure 9). A post-mortem examination of tags compacted into gyratory HMA cylinders also found bulges along the edges of the encapsulated tags (figure 10). However, the compaction force of 88 psi (608 kPa) over 160 gyrations did not appear to cause any significant damage to the tag encapsulation.

In the refined encapsulation method, the epoxy resin is first preheated for 15 to 30 min at 104 °F (40 °C) to reduce its viscosity. The hardener is then added to the preheated resin. The tags are placed in CPVC pipe molds in a circumferential orientation (see figure 16 later) and the bottom pipe opening is sealed with duct tape. Epoxy is then poured in three lifts into the pipe, with the pipe tapped vigorously on the laboratory bench after each lift to eliminate any trapped air bubbles. The encapsulated tags are then placed upright in the oven at 122 °F (50 °C) for up to 1 h until hardened. Occasionally, escaping air bubbles could be observed during this stage, and in a few cases (fewer than 1 in 10), some of the epoxy was pushed out of the pipe owing to entrapped air. After hardening, the tags are removed from the oven, the duct tape seal removed, and the tags cooled for 2 h at room temperature. The tags are then checked for survival through the first stage of the curing process. Observed survival rates at this stage were very high, especially for the Alien® 2x2 tags. Surviving tags are then cured in a second stage at 257 °F (125 °C) for 4 h.

Figure 6. Photo. Voids between epoxy and CPVC pipe wall after filling at room temperature.

Figure 7. Photo. Air bubbles at ends of epoxy filler after room temperature curing.

Figure 8. Photo. Extrusion of epoxy from end of pipe after oven curing.

Figure 9. Photo. Bulge in sidewall of encapsulated tag after oven curing.

Figure 10. Photo. Post-mortem examination of tags retrieved from compacted HMA with bulges in encapsulated tags circled.

Evaluation of Read Range

Initial Evaluation

Read range was evaluated initially using the Alien® 2x2 tags curled in the CPVC pipe sections. During this initial evaluation, it was discovered that there are several sources of interference that can alter the measured effective read range as follows:

1. There appears to be a "body antenna" effect. Tags held by hand were discovered to have different read ranges than tags placed on an inanimate object with all persons removed to a distance. Even body movements 8 ft (2.5 m) away from the tag would trigger a read when none had been observed before.
   
   
2. Placing the tags on the floor and then stepping some distance away gave more controlled conditions and consistent read data. The disadvantage of placing the tags on the floor and then reading them in a horizontal direction to determine maximum read distance is that the tags are slightly off the centerline of the antenna. This tends to decrease the measured read range. The advantage, of course, is that placing everything on the laboratory floor is much easier than elevating everything several feet off the floor.
   
   
3. When a half-filled bucket of aggregate was accidentally placed behind one of the tags, the read range increased considerably. This may have beneficial implications for field versus laboratory performance of the tags.
   
   

The read range tests were designed to minimize these compounding influences. The 1 by 2 ft (0.3 by 0.6 m) RFID antenna was oriented perpendicular to the floor, supported along its long edge directly on the floor. A grid was laid out on the floor in front of the antenna. Tags were placed at each grid point, and the observer stepped aside before tag read success was assessed. Replicate specimens and multiple reads were employed to improve the robustness of the test results.

The first series of tests employing this protocol were conducted on flat Alien® 2x2 tags before curling or encapsulation. Six read attempts were made at each grid point. Three of these were with the tag facing the antenna, and three were with the tag facing away. The tag orientation was found to have no effect on the read success rate. Each grid point was assigned a value of 1 if the read success rate was greater than 50 percent for the six read attempts, a value of 0.5 if the tag could be read but the success rate was less than 50 percent, and a value of 0 if the tag could not be read at all. A total of six tags were tested using this scheme. The results from this test series are summarized in figure 11. Each block in the figure represents a 1 by 1 ft (0.3 by 0.3 m) cell on the floor as viewed from above. The antenna is perpendicular to the floor (i.e., vertical), resting along its 2-ft (0.6-m) long edge. Note that the antenna assembly contains separate transmit and receive antennas along its length, as indicated by the "T" and "R" designations in the figure. The numeric values in figure 11 represent the total score in each cell for the six tags, and the color coding depicts the read success rate category (all tags read, >50 percent tags read, <50 percent tags read, no tags read). Overall, the results in figure 11 suggest an effective read range of approximately 7 ft (2.1 m) for the flat Alien® 2x2 tags in air. There is some slight asymmetry in the read success pattern that may be due to experimental variability, local interference, and/or the asymmetric antenna design.

The next series of tests was conducted on the curled Alien® 2x2 tags inside the CPVC pipe but before encapsulation in epoxy. This test series was designed to evaluate the effects of the curved tag geometry on read success. The pipes containing the curved tags were placed on the floor and parallel to the plane of the antenna in all tests. However, two orientations of the tags relative to the antenna were considered: 1) the pipe axis horizontal with respect to the floor and parallel to the long axis of the antenna and 2) the pipe axis vertical and parallel to the short axis of the antenna. The results from this test series are summarized in figure 12 and figure 13 for the horizontal and vertical orientations, respectively. It is clear from these figures that curling the tags does reduce the read range, and by a larger amount for the vertical orientation. Maximum effective read range in air in front of the antenna is approximately 5 ft (1.5 m) for the horizontal orientation and 3 ft (1 m) for the vertical, compared with approximately 7 ft (2.1 m) for the flat tag configuration.

All distances in ft (1 ft = 0.305 m)
T=transmit side of antenna
R=receive side of antenna
Figure 11. Diagram. Read range for flat Alien® 2x2 tags in air with tags oriented parallel to reader antenna.

All distances in ft (1 ft = 0.305 m)
T=transmit side of antenna
R=receive side of antenna
Figure 12. Diagram. Read range for curled Alien® 2x2 tags in CPVC pipe before encapsulation and curing with tags oriented in horizontal direction.

All distances in ft (1 ft = 0.305 m)
T=transmit side of antenna
R=receive side of antenna
Figure 13. Diagram. Read range for curled Alien® 2x2 tags in CPVC pipe before encapsulation and curing with tags oriented in vertical direction.

Read ranges after encapsulation with epoxy and curing, the next stage in the fabrication process, are summarized in figure 14 and figure 15 for the horizontal and vertical tag orientations, respectively. Effective read range limits are reduced even further by the epoxy and curing process to approximately 2 ft (0.6 m) for the horizontal orientation and 1 ft (0.3 m) for the vertical. This may be because the Alien® 2x2 tags were curled with the antenna and chip facing inward and the paper backing facing outward. It is possible that there may have been some signal leakage from the antenna into the epoxy that is responsible for the reduced read range. Unfortunately, the initial supply of tags was nearly exhausted when this phenomenon was discovered.

All distances in ft (1 ft = 0.305 m)
T=transmit side of antenna
R=receive side of antenna
Figure 14. Diagram. Read range for curled Alien® 2x2 tags in CPVC pipe after encapsulation and curing with tags oriented in horizontal direction.

All distances in ft (1 ft = 0.305 m)
T=transmit side of antenna
R=receive side of antenna
Figure 15. Diagram. Read range for curled Alien® 2x2 tags in CPVC pipe after encapsulation and curing with tags oriented in vertical direction.

Additional Evaluation

After receiving more RFID tags, additional evaluations were performed to examine the effects on read performance of orientation of the tag within the CPVC mold cylinder and of the encapsulated cylindrical tag relative to the antenna geometry. All tags were now curled with the antenna surface outward and against the CPVC pipe wall. As shown in figure 16, two orientations can be defined for the tags depending on whether the antenna dipoles are oriented in an axial or circumferential direction with respect to the CPVC pipe axis (CPVC pipe axis is vertical in figure 16). In addition, the cylindrical encapsulated tag itself can have varying orientations relative to the antennae. Two limiting cases were considered: both had the axis of the cylindrical encapsulated tag parallel to the plane of the antenna unit, but in one orientation, the axis was parallel to the line connecting the centers of the transmit and receive antennae (i.e., parallel to the long dimension of the antenna unit) and in the other, the axis was perpendicular to this line (i.e., parallel to the short dimension of the antenna unit). Note that in reality, one can only control the orientation of the RFID tag within the CPVC mold; the orientation of the cylindrical tag relative to the antenna geometry will depend on the final random orientation of the encapsulated tag within the compacted pavement.¹

Table 4 summarizes the influence of tag and cylinder orientation on maximum read range for the encapsulated Alien® 1x1 tags after oven curing at 248 °F (120 °C) for 24 h and subsequent oven heating to 247 °F (175 °C) for 1.5 h. Table 5 presents similar data for the Alien® 2x2 tags. Recall that the maximum read range for the nonencapsulated flat tags through air ranged between 2.5 to 4 ft (0.6 to 1.2 m) and 6 to 13 ft (2 to 4 m) for the Alien® 1x1 and 2x2 tags, respectively. Conclusions that can be drawn from these data include the following:

• The maximum read range of the Alien® 1x1 tags is substantially less than that of the Alien® 2x2 tags under all conditions.
  
  
• Regardless of orientation details, curling the RFID tag inside the CPVC pipe and/or encapsulating it in high-temperature epoxy significantly degrades the maximum read range. The 4-ft (1.2-m) maximum read range of the flat Alien® 1x1 tag decreased to a maximum of 2-ft (0.6-m) when encapsulated; the 13-ft (4-m) maximum read range of the flat Alien® 2x2 tag decreased to a maximum of 9 ft (2.7 m) when encapsulated.
  
  
• In nearly all cases the circumferential orientation of the RFID tag inside the CPVC pipe gives longer maximum read ranges. The differences between circumferential and axial orientation are modest to nonexistent for the smaller Alien® 1x1 tags but significant for the larger Alien® 2x2 geometry.
  
  
• The effect of orientation of the tag axis relative to the T-R antenna line appears to be quite small, with no consistent trends except for a small benefit of the perpendicular alignment for the circumferentially oriented Alien® 2x2 tags. It is fortunate that this influence is small because there is no way to consistently control the orientation of the tags relative to the antennae in the field.
  
  

Figure 16. Drawing. Orientation options for RFID tags inside CPVC pipe molds (axis of CPVC pipe mold is vertical in all drawings).

Table 4 . Maximum read range (ft) for Alien® 1x1 tags after 24-h oven cure at 248 oF (120 °C), cooling, then reheating for 1.5 h at 247 °F (175 °C).

Note: Cylindrical tag axis parallel to plane of antenna unit in all cases.

Table 5 . Maximum read range (ft) for Alien® 2x2 tags after 24-h oven cure at 248 °F (120 °C), cooling, then reheating for 1.5 h at 247 °F (175 °C).

Note: Cylindrical tag axis parallel to plane of antenna unit in all cases.

Evaluation of the tag read performance continued to be plagued by sporadic problems with RF interference/reflections, proximity of the floor and/or people in the laboratory, and other factors. Consequently, an improved procedure for evaluating read performance and range in the laboratory was standardized. Nonconductive supports were constructed for the RFID antenna and tags to elevate them approximately 3.28 ft (1 m) above the laboratory floor. All laboratory personnel move several meters away from the antenna and tags prior to conducting read tests to minimize body interference.

Figure 17 and figure 18 summarize typical read rate performance as measured using the new standardized procedure. The two samples represent Alien® 2x2 tags encapsulated in CPVC molds and compacted into a single gyratory HMA cylinder. At each position, the cylinder was oriented horizontally (axis parallel to floor) and then rotated through six different angles relative to the antenna. The figure summarizes the read success rate over all angles at various positions from the antenna for the two embedded tags. In general, the tags in these trials could be read up to a distance of approximately 5 ft (1.5 m) from the antenna and at angles off the antenna axis. Note that there are some slight differences in the read performance between the two samples; variability in individual tag read performance has been observed consistently throughout this study. Note also that the read success rate is biased toward the transmitter ("T") side of the antenna. Apparently, it is important to position the tag closer to the transmitter antenna so that it receives maximum impinging RF energy.

All distances in ft (1 ft = 0.305 m)
Figure 17. Diagram. Read performance for sample 7 as evaluated using standardized laboratory procedure.

All distances in ft (1 ft = 0.305 m)
Figure 18. Diagram. Read performance for sample 8 as evaluated using standardized laboratory procedure.

Thermal Survivability

Thermal survivability is the first hurdle for the RFID tags because typical mixing and compaction temperatures in HMA of up to 356 °F (180 °C) are well above their operating and long-term storage temperature limits. Thermal survivability was investigated in a preliminary way using the Alien® 1x1 encapsulated tags. Only one replicate was tested for each condition. All tags were left in the oven for 1.5 h at the target temperature. The time duration was chosen to duplicate the longest time between placement of the tags in the truckload of hot mix at the plant to final cooling of the compacted mat in the field. Immediately after removal from the oven, none of the tags could be read. However, all tags could be read to a distance of 3 ft (1 m) in air after being allowed to cool for 20 min. This is very similar to the baseline read for the Alien® 1x1 encapsulated tag prior to heating.

The preliminary investigation of thermal survivability was repeated with two encapsulated Alien® 2x2 tags. The first was heated for 1.5 h at 302 °F (150 °C); as with the Alien® 1x1 tags, the 2x2 tag could not be read immediately after removal from the oven but was readable up to a range of 10 ft (3 m) after 20 min of cooling. The second Alien® 2x2 tag was heated to 347 °F (175 °C) and appeared to fail because it could not be read even after cooling. However, visual inspection suggested that the encapsulation may have been flawed, directly exposing part of the tag to the high temperatures.

Mechanical Survivability

Mechanical survivability was independently evaluated via gyratory compaction of the encapsulated tags into a cold mix asphalt product. Mechanical survivability was investigated in a preliminary way using the Alien® 1x1 encapsulated tags. Each tag was placed in the loose cold mix near the center of the mold and then compacted to between 50 and 100 gyrations in the compactor. This is intended to simulate the stresses that the tags would feel during compaction in the mat in the field. All of the encapsulated tags survived the compaction. The average read range of 26 inches (670 mm) represents an approximately 30-percent decrease from the greater than 3 ft (1 m) baseline read range prior to compaction. It is speculated that this reduction is due to the additional attenuation of the RFID signals through the asphalt rather than any effect from the compaction itself.

The preliminary investigation of mechanical survivability was repeated with one encapsulated Alien® 2x2 tag compacted in the cold mix asphalt to 100 gyrations. Read range after compaction was approximately 10 ft (3 m). Again, this represents a decrease from the baseline read range through air of approximately 25 ft (7.6 m) for the 2x2 tag.

One of the cold mix gyratory plugs was saturated in water after initial testing to evaluate the influence of pavement moisture on signal attenuation. No significant difference in read range was discerned between the dry and wet conditions.

Combined Thermal and Mechanical Survivability

Once the survivability of the tags under only thermal or mechanical torture had been established, the final test was survivability under combined conditions in the laboratory. This test is intended to simulate the actual field scenario of transport of the tag in the truckload of hot mix asphalt followed by compaction into the mat behind the paver.

The hot mix asphalt used for this evaluation was a dense graded mix with a PG 64-22 unmodified binder. Tags were placed in an oven along with the loose mix for 1.5 h at a temperature of 320 °F (160 °C) before compaction. Two tags were placed in the loose mix in each gyratory mold near the axis at approximately the third points of the mold height. After compaction, the gyratory plugs were extruded from the molds and then scanned in the axial direction by the RFID reader.

Figure 19 summarizes the read tests for three gyratory plugs, each containing two tags that had been encapsulated in the CPVC pipe. For two of the plugs, only one of the two tags survived the heating and compaction. During the read tests, the gyratory plug was rotated through six different positions (12:00, 2:00, 4:00, 6:00, 8:00, and 10:00). Each of the tags was read in each position. Given that there were only four surviving tags among the three gyratory plugs, there was a maximum of 24 possible successful reads. For the one plug in which both tags survived, there were no trends in read success or range between the closer versus more distant tag. As shown in the figure, the maximum effective read range was approximately 2 ft (0.6 m). This is roughly consistent with the effective read range for this tag type through air after encapsulation and curing (figure 14 and figure 15). In other words, the additional heating and compaction for the tags depicted by figure 19 did not significantly degrade the effective read range.

Six tags in all were oven heated and compacted into gyratory plugs. Of these, four survived and were able to transmit their identification numbers to the RFID reader when scanned. This corresponds to a survival rate of 75 percent. This is sufficient for field application because it is envisioned that multiple tags will be tossed into the truckload of HMA for redundancy.

All distances in ft (1 ft = 0.305 m)
T=transmit side of antenna
R=receive side of antenna
Figure 19. Diagram. Read range for curled Alien® 2x2 tags in CPVC pipe after HMA oven heating and gyratory compaction.

CONCLUSIONS FROM FEASIBILITY EVALUATION

• Conventional passive UHF (860-960 MHz) RFID technology was determined as the most promising for paving applications. The advantages of passive UHF RFID included small tag size, low cost, adequate read range, and wide commercial availability. Potential attenuation of the UHF radio signals by HMA and survivability of the tags in the harsh thermal and mechanical environment during paving were identified as the two key challenges to be addressed in the research.
  
  
• Based on theoretical considerations and data from the literature, it was determined that HMA was likely a good UHF medium in terms of attenuation losses. This was confirmed through laboratory studies, which concluded that UHF RFID signal attenuation by asphalt concrete should not be a serious issue in the field.
  
  
• Based on a careful review of the specifications and available product reviews for various manufacturers' systems, a Mercury® 5 RFID reader system from ThingMagic, Inc. was purchased for this project. Alien® Technologies Gen 2 1x1 and 2x2 tags were selected for initial evaluation because of their compatibility with the Mercury® 5 reader and their suitable dimensions.
  
  
• Various encapsulation schemes were evaluated for hardening the RFID tags against the thermal and mechanical stresses of paving. The final scheme consisted of encapsulation of rolled tags inside ¾-inch (19-mm) nominal outside diameter CPVC pipe subsequently filled with cured Cotronics Durapot™ 866 high-temperature epoxy.
  
  
• The read ranges of the encapsulated tags were thoroughly evaluated. The read range for the Alien® 1x1 tags in the optimal circumferential orientation inside the CPVC pipe varied between 0.5 and 2 ft (0.15 and 0.6 m), which is inadequate for the intended application. The corresponding read range for the Alien® 2x2 tags varied between 5 and 9 ft (1.5 and 2.7 m), which is more than adequate for the HMA tracking application.
  
  
• Thermal and mechanical survivability of the encapsulated tags was evaluated in the laboratory via oven heating and gyratory compaction. The encapsulated tags survived oven heating for 1.5 h at temperatures up to 347 °F (175 °C), adequate for paving applications. Survival rate for combined thermal and mechanical effects was approximately 75 percent. This is sufficient for field application because multiple tags will be placed into each truckload of HMA for redundancy.
  
  
• One gyratory plug was saturated in water after initial testing to evaluate the influence of pavement moisture on signal attenuation. No significant difference in read range was discerned between the dry and wet conditions.
  
  

¹During the subsequent field trials (chapter 3), it was observed that most of the in-place tags appeared to lie within a horizontal plane, i.e., parallel to the plane of the antenna unit during field reading.