Radio Frequency Identification Applications in Pavements

CHAPTER 1: INTRODUCTION

PROJECT OBJECTIVES

The original objective of this project was to demonstrate that inexpensive expendable RFID tags can be used to identify the spatial location along the pavement alignment of specific truckloads of HMA 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.

Phases I and II of this project addressed the development of techniques for making the RFID tags sufficiently rugged to withstand the harsh thermal and mechanical conditions of HMA paving and for evaluating the survival and read performance of the tags after construction. Phase I focused on identifying feasible RFID devices for HMA tracking, identifying candidate projects for field testing, and formulating a field evaluation work plan. Phase II executed the work plan developed in Phase I.

During the Phase I and II work, some 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.

The evaluation of these additional topics was the objective of the additional and final Phase III of the project.

REPORT ORGANIZATION

This report documents the work and findings from all three phases of the project. The subsequent chapters, along with brief descriptions of the work in each and highlights of the findings, are described below. Each chapter is intended as standalone document of its respective work area.

Chapter 2: Feasibility Evaluation. 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. The feasibility evaluation identified UHF passive RFID technology as best suited to the hot mix asphalt paving application. After considerable research of available products, a ThingMagic Mercury® 5 RFID system and related hardware were acquired for use in this project. Suitable RFID tags were identified and a feasible encapsulation system was developed that adequately protects the RFID tags from the temperatures and compaction stresses inherent in HMA transport and paving. An Alien® Gen 2 2x2 RFID tag curled and encapsulated in high-temperature epoxy inside a ¾-inch (19-mm) nominal outside diameter CPVC pipe provided a hardened tag of appropriate size for adequate read range.

Chapter 3: RFID Tracking of HMA Placement.The encapsulated RFID tags were field tested to evaluate the following issues: survivability in real-world paving scenarios, read range under actual field conditions, required redundancy (i.e., how many tags must be added to a truckload to locate the mix reliably along the pavement alignment; this is related to the survivability issue, but also includes delays in passing through the paver), construction practicality issues (e.g., do the tags interfere with normal paving operations), and construction quality issues (e.g., do the tags impair the quality of the compacted mat by inducing segregation or other defects). 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 MDSHA. 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.

Chapter 4: Evaluation of Surfaced Tags. 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. These findings confirm that the encapsulated RFID tags are a successful low-impact and inexpensive technology for tracking placement of HMA in the roadway.

Chapter 5: RFID Tracking of PCC Placement. The successful application of RFID tags to track placement of HMA immediately suggested the parallel use for PCC placement. The reconstruction of a section of I-90 near Syracuse, NY, provided an opportunity to evaluate this in the field. 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 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. The penetration depth (i.e., read range) of the UHF RFID radio waves diminishes sharply as the dielectric constant increases. A different technology is therefore needed for the PCC tracking application.

Chapter 6: Pavement Temperature Measurement Using SAW RFID. SAW RFID technology has the intrinsic capability to measure thermal expansion and thus temperature within a pavement layer as a function of depth and time. This is particularly useful for intelligent compaction, where estimates of HMA stiffness using sensor data from the vibrating roller must consider the instantaneous distribution of temperature in the mat. The temperature measurement capability of SAW RFID technology 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 was judged to have high potential for in situ temperature measurement in HMA pavement layers.

Chapter 7: Reflection Crack Detection. Reflection cracking is a dominant distress in rehabilitation overlays. Early detection of these cracks before they reach the pavement surface can be important to highway agencies both for rehabilitation planning purposes and for enforcing construction contract warranties. 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. A prototype reflection crack detector was developed and tested in the laboratory and, under more limited conditions, in the field. The prototype detector was shown in the laboratory 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. This approach shows considerable promise, but it will require additional and more rigorous field evaluation that was beyond the scope and resources of the present project.

Chapter 8: Guidance on Data Integration. The motivation for this entire study was the application of RFID technology for tracking HMA placement. This is also the application that is most suited to near-term commercial implementation. The major benefit of this application is the integration of agency QA materials testing data with their pavement management database. This integration could provide insights into the relationships between good material properties and construction techniques and superior pavement performance as well as enable more powerful forensic investigations of poorly performing pavements. This integration could also permit much more powerful forensic investigations of poorly performing road sections by enabling easy and quick access to the associated material properties measured during construction. However, several steps are needed to effect this data integration, and given the wide range of materials and pavement management systems in use among the States, these steps will be slightly different for each agency. General guidelines are provided for the data integration steps that can be easily adapted to each agency's specific databases and policies and procedures.

Chapter 9: Conclusions. The key findings and recommendations from each topic area are consolidated and summarized.

Chapters 2 and 3 summarize the work completed in Phases I and II, and chapters 4 through 8 document the work completed in Phase III.

RFID TECHNOLOGIES IN THIS STUDY

RFID technology is widely used today for supply chain inventory management, security, equipment tracking, among other applications. However, in construction, RFID technology is primarily used for tracking equipment and materials. General surveys of RFID applications in construction are provided by Jaselskis et al., Jaselskis and El-Misalami, and Sawyer.^(1,2,3) Two types of RFID technology were employed in this study: conventional UHF passive RFID tags and the newer SAW RFID tags. Each of these technologies is briefly described in the following subsections.

UHF RFID Technology

The UHF RFID technology employed in this study is the basis for the work in Phases I and II. The RFID technology encodes a digital signature on a small microchip attached to a copper foil antenna-the RFID "tag." This passive tag receives energy from UHF radio waves transmitted by a RFID "reader"; the tag harvests this incoming RF energy to transmit its encoded digital signature back to the reader. These RFID tags, although small, may be read several yards away from the reader's antenna. The primary objectives of Phases I and II were to evaluate the feasibility of using UHF RFID tags in laboratory asphalt specimens and in the field during paving construction.

SAW RFID Technology

One of the major objectives of Phase III was to evaluate the feasibility of using SAW-based RFID technology to measure HMA temperatures. SAW-based RFID sensors are inherently capable of measuring and transmitting temperatures as well as a digital signature. This is in contrast to the conventional RFID technology used in Phases I and II, which is capable of only transmitting digital signatures. In addition, the SAW RFID technology is inherently capable of operating successfully at lower energy inputs than the conventional silicon-based integrated circuit RFID technology.

SAW RFID tag consists of an IDT and a series of acoustic reflector traps etched into a piezoelectric substrate. The tag reader emits a radio wave pulse to the IDT that is converted piezoelectrically into a nanoscale acoustic wave. The wave travels past the reflectors to produce a unique pattern of reflected pulses. These travel back to the IDT, where they are piezoelectrically converted into an encoded electronic reply signal to the reader. The SAW chip operates in a purely passive mode and does not require supplementary DC power (i.e., battery).⁽⁴⁾

Overall, the principal advantages of the SAW RFID technology for pavement construction include better inherent ruggedness, smaller formats, and longer read ranges for a given tag antenna size compared with conventional RFID. Unlike conventional RFID tags, the incoming RF signal does not need to be converted to DC in SAW RFID. Therefore, the incoming RF signal strength does not need to exceed the minimum threshold required for the rectifier operation. This is the principal theoretical reason for the inherently longer read ranges in the SAW RFID technology.

In addition, and most relevant to this study, SAW RFID technology is capable of wireless measurement of temperature via the perturbation of the return wave signal caused by the influence of thermal strains on the spacing of the acoustic reflectors.