lessons learned

1.

Conduct F/HWD testing within the first year of construction to establish a baseline for subsequent comparative analyses during future testing periods. It is advisable to utilize the same F/HWD device or brand for testing when the objective is to periodically monitor the changes in layer properties resulting from seasonal variations, aging, or long-term damage similar to the LTPP program.

2.

When conducting F/HWD tests on flexible pavements, it is crucial to conduct repeat testing at the end of the day or shift, covering a minimum of ten locations assessed earlier in the day or shift. This approach ensures the presence of F/HWD data for the same test locations at different temperatures, facilitating the assessment of temperature sensitivity of the asphalt concrete layer(s) and enhancing the robustness of the backcalculation.

3.

When conducting F/HWD tests on rigid pavements, it is crucial to conduct the testing during the lowest temperature conditions, ideally at night or in the early morning. This strategy enhances the likelihood of identifying critical or poorly performing joints and ensures proper contact between the centers of the slabs and the support layers.

4.

Conduct testing at nearby test points, which collectively represent a test location, to augment the reliability of the load and deflection data and comprehensively capture the pavement response under loading. In addition, employing extra deflection sensors could potentially yield benefits by collecting additional data for every test point.

5.

Conduct F/HWD testing at multiple load levels by applying a minimum of one, preferably two or three drops at each load level. This data is crucial for assessing the nonlinearity of unbound materials. The approach also ensures the availability of replicates when outliers are encountered.

6.

Save F/HWD load-deflection time histories for all drops.

7.

Ensure the selected F/HWD produces reliable load and deflection time histories over the entire test duration for every drop, e.g., over 100 msec., without any presence of noise or excessive/unreliable negative deflections. Reviewing data collected during recent F/HWD roundups (round robins) for the same device/brand could be helpful. Even if a device has demonstrated reliability, it remains essential to conduct limited testing and have an engineer review the load-deflection data before each project.

8.

Follow a standard procedure for F/HWD set up, warm up and testing. The LTPP FWD manual (Schmalzer, 2006) serves as an excellent reference for this task. Minor adjustments to the procedures outlined in the manual may be necessary, such as modifying load levels and plate diameter for testing at ports and airports, and adopting a different setup of deflection sensors for evaluating the load transfer efficiency of jointed PCC pavements.

9.

Ensure the F/HWD undergoes an annual reference calibration and regular relative calibrations, preferably involving the entire load-deflection time histories and not just the peak responses.

10.

Obtain accurate thickness measurements for both the pavement surface and subsurface layers, and identify any stiff or rigid layers present, including their depths. Employ Ground Penetrating Radar (GPR), coring/boring techniques, and conduct a thorough review of construction and maintenance records. Consulting with a GPR expert for the selection of antennas and their corresponding frequencies is crucial to attain reliable results.

11.

Employ nonlinear dynamic backcalculation to accurately determine the layer properties and evaluate the pavement's structural capacity. Nonlinear dynamic backcalculation allows the incorporation of the actual load applied versus time into the analysis, irrespective of the shape and duration of the load pulse, and enables the modeling of layers according to their true behaviors, encompassing linear elastic, linear viscoelastic, and nonlinear elastic properties.

12.

Collect and preserve pavement materials during construction for potential laboratory testing in the future, as needed.