Radio Frequency Identification Applications in Pavements

CHAPTER 9: CONCLUSIONS

PROJECT OBJECTIVES

The original objective of this project was to demonstrate that inexpensive expendable radio RFID tags can be used to identify the spatial location along the pavement alignment of specific truckloads of HMA (or warm/cold mix asphalt) production. These tags are placed in the truckload as it leaves the production plant, pass through the paver, and are compacted into the finished mat. Cross referencing these tags with GPS latitude and longitude coordinates after construction allows spatial referencing of QA material property data measured at the production plant, enabling linkage to other spatially referenced in-place test results and PMS pavement performance data. This use of the large QA and PMS datasets already collected by highway agencies will permit more robust analyses and insights into the relationships between HMA material properties and actual pavement performance.

During the pursuit of this original objective, additional applications of RFID technologies to pavements were identified for evaluation:

• Evaluation of potential problems caused by surfaced tags.
  
  
• Exploration of SAW RFID technology for improved performance of small format tags.
  
  
• Demonstration of RFID tracking of placement of PCC loads in pavements.
  
  
• Provision of guidance to agencies on data integration.
  
  

These issues were pursued in the final phase of the project.

KEY FINDINGS

Key findings of this work are summarized here organized by the following topical areas:

• RFID tracking of HMA placement (chapters 2, 3, and 4).
  
  
• RFID tracking of PCC placement (chapter 5).
  
  
• Pavement temperature measurement using SAW RFID (chapter 6)
  
  
• Reflection crack detection (chapter 7).
  
  
• Guidance on data integration (chapter 8).
  
  

RFID Tracking of HMA Placement

The feasibility of using RFID tags to track HMA placement was evaluated via: 1) literature review, 2) identification of appropriate RFID technology, and 3) prototype tag development and evaluation. Key findings from the feasibility evaluation are as follows:

• UHF (860-960 MHz) passive RFID technology was determined to be best suited to the HMA paving application. The advantages of passive UHF RFID included small tag size, low cost, adequate read range, and wide commercial availability.
  
  
• A ThingMagic Mercury® 5 RFID system was evaluated as being most appropriate for use on this project and was therefore acquired.
  
  
• Suitable RFID tags of appropriate size were identified. These included Alien® Gen 2 1x1 and Alien® Gen 2 2x2 tags. The larger format Alien® Gen 2 2x2 tags had superior read performance and were adopted for most of the study. A UPM Raflatac® 1x2 tag was subsequently added to the study after the Alien® 1x1 tags were taken out of production by the manufacturer.
  
  
• An effective and inexpensive encapsulation system was developed that adequately protects the RFID tags from the temperatures and compaction stresses inherent in HMA transport and paving. The flexible RFID tag is curled inside a 3/4-inch (19-mm) nominal outside diameter CPVC pipe, which is subsequently filled with high-temperature epoxy (Cotronics® Durapot™ 866). An improved curing process for the epoxy was also developed that minimized the formation of air bubbles and void.
  
  
• 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.
  
  

The encapsulated RFID tags were field tested to evaluate the following issues: survivability under real-world paving scenarios, read range under actual field conditions, required redundancy, construction practicality issues, and construction quality issues. The encapsulated RFID tags were evaluated at three parking lot locations on the UMD campus and in two stages in a new pavement constructed by the MSHA for the Hampstead Bypass. Key findings from the field trials are as follows:

• In the parking lot trials, the read success rate (percentage of tags that could be read) varied from about 60 percent for the Alien® 1x1 tags to 100 percent for the larger format Alien® 2x2 tags.
  
  
• In the Hampstead Bypass trial, the overall read success rates were slightly less than 20 percent for the UPM Raflatac® tags and between 60 and 80 percent for the larger Alien® 2x2 format.
  
  
• Read success rate decreased slightly with increasing vehicle speed. The read success rate at 45 mi/h (72 km/h) was approximately 65 percent of the rate at slow speeds.
  
  
• Approximately 15 percent of the RFID tags "floated" to the surface of the mat in all of the field trials.
  
  

The field trials confirmed the very high survival rate of the encapsulated tags. Read success rates varied significantly with tag size and less significantly on other details such as antenna configuration and vehicle speed. The field trials consistently demonstrated that post-construction read success rates of 60 to 80 percent or higher are achievable from a bumper-mounted antenna array, even on a vehicle moving at traffic speeds.

It was observed during the HMA field trials that some of the encapsulated RFID tags "floated" to the surface of the compacted mat. This raised the possibility of a decrease in in-place compacted density or an increase in the in situ permeability or both in the local region around the tag. Extensive density and permeability testing was performed to evaluate these possibilities. No localized decrease in density, increase in permeability, or any detrimental effects of the surfaced tags were found. There is the possibility for long-term deterioration around the tags due to degradation of the bonds between the asphalt binder and the CPVC pipe encapsulation. However, qualitative observations of the surfaced tags in Lot EE at the time of this writing show no evidence of deterioration over the intervening 6 years. These findings confirm that the encapsulated RFID tags are a successful low-impact and inexpensive technology for tracking placement of HMA in the roadway.

RFID Tracking of PCC Placement

The successful application of RFID tags to track placement of HMA immediately suggested the parallel use for PCC placement. Preliminary laboratory evaluation of concrete cylinders suggested that the Alien® Gen2 2x2 tags could be read through about 2 inches (50 mm) of concrete at a distance of up to 5 ft (1.5 m). The reconstruction of a section of I-90 near Syracuse, NY, provided an opportunity for field evaluation. A large number of RFID tags similar to those used in the HMA application were placed in the PCC delivery trucks as they left the batch plant near the paving site. These tags were deposited with the PCC ahead of the slip form paver and incorporated into the concrete slabs. Unfortunately, on the return visit to the site about 1 month after paving, none of the tags could be read. The causes of this surprising and disappointing result were investigated in the laboratory. The influences of aggregate, water, and cement on read range and read success were systematically investigated. The most plausible explanation supported by the laboratory results is that the hydrated cement paste causes the cured concrete to have a very high dielectric constant (about 20). The penetration depth (i.e., read range) of the UHF RFID radio waves diminishes sharply as the dielectric constant increases.

Pavement Temperature Measurement Using SAW RFID

SAW RFID technology has the intrinsic capability to measure thermal expansion and thus temperature. Real-time measurement of pavement temperature distributions is necessary to interpret the stiffness feedback data in intelligent compaction because the stiffness of HMA is extremely temperature dependent.

The temperature measurement capability of SAW RFID was evaluated through laboratory and field testing. For the field testing, various configurations of SAW RFID tags were encapsulated in thermally conductive epoxy, attached with quick-set epoxy to the surface of a milled existing pavement, and then covered with a 1.5-inch (38-mm) thick compacted HMA overlay. Although limitations of the SAW RFID reader system made it difficult to measure the temperature versus time trends during the first few minutes after placement, mat temperatures were recorded starting at about 10 min after placement until the mat was too cool for continued compaction. The measured temperatures versus time compared favorably with predictions from analytical/ numerical models, and an approach for using the SAW RFID field temperature measurements for calibrating these analytical/numerical models is proposed. Overall, the SAW RFID technology has potential for in situ temperature measurement in HMA pavement layers.

Reflection Crack Detection

The development of an early onset reflection crack detector was originally intended as another application of the SAW RFID technology. However, it was subsequently determined that conventional RFID was a more appropriate technology choice, in part because of the current relatively high cost of SAW RFID tags.

The reflection crack sensor concept is to implement a switch in the antenna circuit of the RFID tag: when there is no crack, the RFID tag signal can be read at a distance, but after a crack forms, the signal becomes too weak to be read. A prototype reflection crack detector was developed and tested in the laboratory and, under more limited conditions, in the field. In the laboratory, the prototype detector was shown to be capable of detecting a reflection crack before it reached the pavement surface. The limited field trials also showed that the detector had the required survivability and read range for real-world applications. Seventeen of 18 tags tested in the field survived the paving operations, and the tags could be read at distances of more than 5 ft in the optimal antenna configuration. This approach shows considerable promise, but will require additional and more rigorous field evaluation that was beyond the scope and resources of the present project.

Guidance on Data Integration

Maximum use of material property data collected during construction and pavement management data collected during service life is possible only when both sets of data can be related to a common spatial referencing system. Using RFID tags to track loads of sampled and tested material enables geospatial identification of the location of the delivered material in the pavement. These material properties can then be linked with the corresponding spatially located pavement performance data from the PMS.

Because of the wide variety of MMSs and PMSs used by agencies, it is possible only to outline the general steps needed to link these two datasets via geospatial coordinates. Generic examples are provided for the following necessary operations:

• Linkage of one or more RFID identifiers to material samples.
  
  
• Linkage of GPS coordinates to RFID identifier/material sample.
  
  
• Conversion of latitude/longitude to roadway location.
  
  
• Combining of MMS and PMS data by location.
  
  
• Display of extracted data.
  
  

The details of implementing these operations will depend on the particular MMS and PMS used by an agency.