U.S. Department of Transportation
Federal Highway Administration
1200 New Jersey Avenue, SE
Washington, DC 20590
202-366-4000
Federal Highway Administration Research and Technology
Coordinating, Developing, and Delivering Highway Transportation Innovations
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| This report is an archived publication and may contain dated technical, contact, and link information |
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| Publication Number: FHWA-HRT-14-061 Date: August 2014 |
Publication Number: FHWA-HRT-14-061 Date: August 2014 |
The original objective of this project was to demonstrate that inexpensive expendable radio frequency identification (RFID) tags could be used to identify the spatial location along the pavement alignment of specific truckloads of hot mix asphalt (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 Global Positioning System (GPS) latitude and longitude coordinates after construction allows spatial referencing of quality assurance (QA) material property data measured at the production plant, enabling linkage to other spatially referenced in-place test results and pavement management system (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:
The evaluation of these additional topics was the focus of the additional and final Phase III of the project.
This report documents all work done over all three phases of the project.
Two types of RFID technology were employed in this study: conventional passive ultrahigh frequency (UHF) RFID tags and the newer SAW RFID tags. The passive UHF 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 radio frequency (RF) energy to transmit its encoded digital signature back to the reader. These RFID tags, although small, can be read several yards away from the reader's antenna.
SAW RFID tag consists of an interdigital transducer (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 direct current (DC) power (i.e., battery). 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. 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.
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. Specific RFID applications identified in the literature review include the following:
Several new applications of RFID technology to pavements were evaluated in this study: RFID tracking of HMA and PCC placement, measurement of pavement temperature with depth and time during intelligent compaction, and early detection of reflection cracks in HMA overlays.
Key findings of this work are summarized here, organized by the following topical areas:
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 chlorinated polyvinyl chloride (CPVC) pipe provided a hardened tag of appropriate size and adequate read range.
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, construction practicality issues, and construction quality issues. The encapsulated RFID tags were evaluated at three parking lot locations on the University of Maryland (UMD) campus and in two stages in a new pavement constructed by the Maryland State Highway Administration (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.
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.
The successful application of RFID tags to track placement of HMA immediately suggested the parallel use for PCC placement. Preliminary laboratory evaluation of the passive UHF RFID tags in concrete cylinders showed promise. 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 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.
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 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.
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. 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.
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. 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.
RFID tracking of HMA placement was the most successful application in this study and the one with the most potential for immediate commercial implementation. RFID tracking of PCC placement was unsuccessful, at least with the RFID systems evaluated in this study; the high dielectric constant of the hydrated cement paste severely attenuates the RFID signals. Real-time measurement of pavement temperatures with depth and time during intelligent compaction shows promise, but further work is required to develop faster and more reliable reader software/hardware and RFID tags. Laboratory and limited field evaluation of an RFID-based sensor for early detection of reflection cracks in HMA overlays also shows promise, but additional development work and field trials are required.
Recommendations for additional research and development include the following:
