Pavement Structural Evaluation at the Network Level: Final Report

CHAPTER 3. MANUFACTURERS', OWNERS', AND USERS' QUESTIONNAIRES

To augment the literature review findings, questionnaires were developed and sent to the device manufacturers as well as owners and users of the devices. Interviews were also conducted to follow up with specific questions or to pursue clarification. The completed questionnaires and highlights from the interviews conducted are presented in appendix A.

3.1 Device Manufacturers' perspectives

The purpose of this questionnaire was to gather information from the manufacturers for analysis, operation, and evaluation. Follow-up interviews were conducted with additional questions to clarify the responses in the questionnaires. The highlights of these interviews are also presented in appendix A.

The RWD utilizes laser sensors to estimate or measure the maximum deflection between the dual tires of an 18-kip (80-kN) single-axle load trailer. The deflection is estimated by comparing the measured undeflected road profile to the deflected road profile at the same location. The TSD utilizes Doppler lasers to estimate the so-called deflection velocity of the road profile that is the velocity the pavement deflects due to the moving load.

The RWD was equipped with four laser sensors, with the first three sensors used to measure the undeflected road profile and the fourth sensor used to measure the deflection 7.25 inches (184.15 mm) behind the center of dual tires at the same location that the undeflected road profile was measured. The laser sensors were spaced 8.5 ft (2.6 m) apart for this spatially coincident approach. Two additional sensors were added to measure a second deflection point at a distance of 15 inches (381 mm) in front of the maximum deflection point to estimate the radius of curvature (R) of the deflection basin. The TSD provided deflection velocities between three and nine points, with the model that was tested in the United States in September 2013 measuring six. Similar to an FWD, the offsets from the load to the measurement points on the TSD could be adjusted. The load applied to the pavement could be varied from 13.2 to 28.7 kip (58.7 to 127.6 kN) for the TSD by using sealed lead loads. Currently, only the TSD estimates the instantaneous dynamic wheel load, but the RWD can be equipped if desired. Both devices are equipped with Global Positioning System (GPS) units and temperature gauges. The TSD was also equipped with a front camera and an inertial profiler with the addition of a GPR under consideration. Although the RWD was not equipped with GPR or video/laser profiling capabilities, these options have been considered.

The recommended speed for data collection with the TSD ranges from 20–55 mi/h (32.2– 88.55 km/h). For the RWD, valid measurements can be acquired up to the maximum speed the vehicle is capable of. The minimum speed in which the RWD can acquire valid measurements is 5 mi/h (8.05 km/h).

The fixed factory setting of the Doppler laser limits the reported sampling frequency to a maximum of 1 kHz for the TSD. The recommended distance between successive readings for the RWD is 0.6 inch (15.2 mm) but can be changed by the user to 0.2–0.6 inch (5.1–15.2 mm). The recommended spatial averaging is 32.8 ft (10 m) for the TSD and 528 ft (161.04 m) for the RWD.

To date, owners have mostly only used the TSD on flexible and semi-rigid pavements. The TSD has been used to detect load transfer efficiency issues on rigid pavements to group locations as having or not having significant structural problems. The RWD is capable of working on flexible pavements with AC layers or surface treatments as well as most rigid and composite (AC over portland cement concrete (PCC)) pavements. However, field evaluations of the RWD conducted during this project showed limited application on PCC pavements. The main concern with rigid pavements is the texture as longitudinal tining can distort the data.

Calibration of the TSD includes the proper aligning of the Doppler laser angle to the pavement. RWD users should calibrate the laser sensors annually and should adjust the mounting height of each sensor relative to a flat surface beneath the device prior to each project.

Because both devices utilize a form of a laser sensor, measurements on wet pavements are problematic. Pavement texture can also magnify the random laser measurement error due to the spatially coincident method utilized by the RWD, but spatial averaging reduces this error.

Interviews conducted with manufacturers of both the TSD and RWD provided additional technical information about the devices. Both manufacturers were also open to providing the project team with the processed data for all segments tested and a limited amount of pre-processed/raw data if adequate justification for use of the data is provided and subject to execution of a nondisclosure agreement by the project team with the manufacturers.

3.2 TSD Owners'/Users' Perspectives

A questionnaire was also developed for owners and users of the TSD, including those in Italy, Poland, South Africa, Australia, and the United Kingdom. The purpose of the questionnaire was to gather information regarding how long each agency has had the device and the main objectives for using the device.

All agencies that purchased the TSD cited that the specific reason was to identify segments of a pavement network for more detailed follow-on structural evaluation using FWDs or other methods. They also indicated that they purchased the TSD to help with the planning and budgeting of major rehabilitation/reconstruction of a pavement network. One responding agency uses the structural indices (SCI₃₀₀ and SCI_sub (i.e., SCI subgrade)) to evaluate the bearing capacity of the upper and bottom layers to establish rehabilitation and reconstruction needs. TSD owners expressed that the TSD met their expectations for the intended purpose in terms of operation and data collection. Owners of the TSD also felt that network-level data collection using the TSD should occur every 2–3 years.

Although the system does not currently allow for user calibration, agencies suggested storing angle and odometer calibration values for use in verification during post-processing. Pavement and air temperature are also recorded during measurements, which can be used during analysis such as adjusting SCI₃₀₀ with air temperature.

Overall, owners of the TSD are satisfied. Several suggested improvements include the potential addition of more sensors, faster exporting of data with more custom options, and more details on the built-in analytical model with options for calibration. As a result, the TSD manufacturer will include more details on the analytical model and also output in standard F25 file format that can be used as input to backcalculation software as with traditional pavement analysis in a future software release.

3.3 RWD Users' perspectives

A questionnaire was developed for users of the RWD including agencies in Virginia, Kansas, Connecticut, and Louisiana. Since the RWD is not currently commercially available, it has mostly been limited to pilot projects and device evaluation projects. Therefore, the purpose of the questionnaire was to gather information regarding how agencies view the RWD and potential applications for it within their agencies. Interviews were conducted to follow up with additional questions.

All agencies participating in the evaluation of the RWD cited the specific reasons were to assess the general network-level structural capacity of the pavements in terms of delineating weak and strong pavement structures within the network. Identifying segments of the pavement network for more detailed structural evaluation using FWDs or other methods was also reported by most agencies.

Operationally, the RWD met the expectations of its intended purpose for all agencies. However, one of the concerns regarding data collection includes loss of accuracy along roadway segments with sharp curves. Another concern expressed is that the lasers are triggered temporally (in time domain) instead of spatially (at set distances), which can increase the likelihood of the successive lasers not measuring the exact same pavement location and also results in varying number of data points per unit length as vehicle speed changes. The data analysis/interpretation currently performed by the manufacturer's staff provides temperature corrected deflection data at 0.1-mi (0.161-km) increments.

The suggested improvements to the RWD included improving data collection and reducing data variability. In terms of data collection, a quicker processing rate of deflection data was preferred. It was also desired to build correlations with FWD data and existing PMS data in order to use RWD data to describe pavement condition by some agencies. Several agencies do not believe that RWD data should be compared to FWD data but instead used to screen locations where pavement structure is changing significantly which can then be followed by other means of structural data collection, such as FWD.

Pavement texture is believed to impact the deflection measurements of the RWD, with one agency noting that the standard deviation of deflection measurements changed significantly with pavement surface type. Wet pavements have also posed problems during data collection as it masks the reflections from the lasers. In addition to pavement texture, it is believed that pavement condition, especially roughness and cracking, also impacts the deflection measurements.

3.4 Summary

Table 4 provides a summary of the RWD and TSD. This represents the initial information provided by the manufacturers and was the basis for the decision of moving forward with the evaluation of the RWD and TSD. Since this time, some updates have been made, including the RWD having two sensors and the TSD having six sensors.

1 mi/h = 1.61 km/h

1 inch = 25.4 mm

1 mil = 0.0254 mm

1 kip = 80 kN

^aIf the manufacturer has added the second sensor, this would be possible.