CHAPTER 3

LTPP Seasonal Monitoring Program

The SMP intended to quantify and validate relationships between climatic conditions and in situ material properties, and their impact on pavement performance (FHWA 2017). The following summarizes the instrumentation, and performance, inventory, and materials testing data for the SMP sections included in the LTPP program.

The SMP includes 85 GPS and SPS sections (Figure 15). The sections consist of thin and thick asphalt pavements, JPCPs, and reinforced concrete pavements, over fine and coarse subgrade soils, and subjected to wet/dry and freeze/no freeze conditions.

SMP section data were collected between 1993 and 2004. The majority of SMP sections (32), have 2 to 3 years of available seasonal testing data (Figure 16).

Most of the SMP sections were placed out-of-study in 2004; however, seven SMP sections are currently active. For the seven active sections, data collected after 2004 followed the standard LTPP testing frequency (i.e., seasonal monitoring was not conducted). A summary of each LTPP SMP section is provided in Appendix B and the LTPP data and table reference information are summarized in Appendix C.

Instrumentation

Data collected from the SMP instrumentation included surface and subsurface temperature, precipitation, frost penetration, and depth to water table (Figure 17). Hourly average surface temperature and total precipitation were collected using air temperature probes and tipping bucket rain gauges, respectively. Subsurface temperatures were collected using thermistor probes installed at varying pavement depths. TDR was used to measure subsurface moisture content. Temperature and electrical resistivity instrumentation was used to estimate frost penetration. Piezometers were used to manually read the water table within 33 ft of the pavement surface. Appendix D provides summary statistics of available climatic data for SMP sections.

Ambient Air Temperature and Precipitation

For 64 of the 85 SMP sections, the LTPP data included average hourly ambient air temperature, total hourly precipitation, daily air temperature (i.e., average, minimum, maximum), total daily rainfall, and total hours of data collected per day.

Table 6. Air Temperature and Precipitation Summary Statistics

Subsurface Temperature

Subsurface temperature data were available for 82 of the 85 SMP sections (excluded Sections 04-1017, 04-1018, and 46-3010). Only one section, 30-8129, had manual temperature measurements and these included 1 to 6 hours of data over a period of 10 non-consecutive days. Pavement subsurface temperatures were characterized according to:

• Hourly temperature measurements from multiple thermistors at varying depths in the pavement and from manual measurements when the automatic measurement equipment was out-of-service.
• Hourly subsurface temperature measurements (daily average, maximum, and minimum subsurface temperatures), and the corresponding hours in which the maximum and minimum temperatures were measured.

Table 7 shows that most of the subsurface temperature data were collected as part of a full 24-hour data collection window. The hourly temperature measurements were limited to only the first five thermistor depths. However, the number of thermistors installed at each SMP section varied, with some locations having as many as 18 (at depths typically within the subgrade). Daily temperature statistics such as average, minimum, and maximum temperature and time of minimum and maximum temperature were available for these thermistors. There was little deviation in average monthly temperature from year to year. The standard deviation of average monthly temperature was typically between 3°F and 5°F. However, at some sections, the standard deviation could be as high as 16°F (at Thermistor No. 1, closest to the pavement surface) on certain months (Figure 18).

Table 7. Subsurface Temperature Summary Statistics

Subsurface Moisture Content

The in situ volumetric moisture content was measured using 10 TDR probes. At most sections, the TDR installation hole was located approximately 2.5 ft from the outside edge of the white (fog) stripe and at least 4 ft away from existing joints and/or cracks to avoid unrepresentative surface moisture infiltration.

Subsurface moisture content measurement was typically recorded every one to 3 months (55.7 days on average). For most SMP sections, the total number of days with moisture content measurements ranged from 10 to 50 days. Typically, one to two measurements were recorded by each probe (at the installed depth) with 5 to 7 minutes between each measurement. The TDR probes were typically installed 5 to 7 in. apart, with some of the deeper probes installed 11 to 13 in. apart. The maximum install depth (depth of the bottom-most probe) ranged from 36 to 92.5 in. with an average of 77.2 in., noting the range for most sections was 70 to 85 in. A summary of subsurface moisture measurements is provided in Table 8.

Table 8. Subsurface Moisture Content Summary Statistics

Frost Penetration

The state of soil freeze was based on the average soil temperature at an interpreted depth. The soil temperature was calculated based on electrode resistivity measurements and moisture content trend analysis. Normalized resistivity data were available for 41 of the 85 SMP sections. Many of the SMP sections were in non-freeze areas. Subsurface temperature data were also available on 96 percent of days with freeze-state measurements (of the 41 sections where freeze-state data were available).

The monitoring period for the freeze-state varied from 0.8 to 10.2 years and the frequency of measurements varied from 1.7 to 55.3 days (Table 9). There was consistent variability in the site-to-site freeze-state monitoring period and measurement frequency. Some sections where freeze-state was more consistent or less of a concern (such as in DNF environments), may not have necessitated extensive freeze-state monitoring or may have had less time in the SMP.

Table 9. Frost Penetration Summary Statistics

Depth to Water Table

The depth to the water table was measured manually and recorded along with the measurement date and time. Water table depth measurements were recorded for 65 of 85 SMP sections. Table 10 summarizes the statistics associated with the depth of the water table measurements. On average, the water table depth was manually measured every 62.5 days, with one to two measurements recorded daily. When there were two or more daily measurements, the average time between measurements was 4.2 hours (typically, 2 to 6 hours). Water table depth was monitored for 1 to 10 years, similar to other SMP data. The standard deviation in the depth of the water table within the same day is nearly zero. However, the standard deviation for all water table depth measurements, across the monitoring period, varied between 0 and 1.6 ft.

Table 10. Depth to Water Table Summary Statistics

Performance Data

The performance data collected at the SMP sections included distress surveys, longitudinal profile testing, and FWD testing. All 85 SMP sections had profile and FWD data collected. On average, there were 24 longitudinal profiles and 31 FWD test dates per section. Relevant pavement monitoring data included:

• IRI for the left wheel path, center lane, and right wheel path, and mean roughness index.
• Elevation data for the left wheel path, center lane, and right wheel path.
• FWD peak deflection measurements at varying drop heights, locations, and stationing.
• Temperature measurements at varying depths during FWD testing.
• Average backcalculated layer moduli for asphalt sections and backcalculated layer moduli using the best-fit method for concrete sections.
• Maintenance and rehabilitation records.

Longitudinal Profile

Longitudinal profile elevation data contained elevation measurements for the left and right wheel paths (1 in. intervals) and center of lane (6 in. intervals), and calculated IRI (left and right wheel path and mid-lane), mean roughness index (MRI), and the root mean square vertical acceleration at base lengths of 1, 2, 4, 8, 16, 32, 64, and 128 ft. Longitudinal profile measurements were recorded approximately every 2 to 20 months. Appendix B includes MRI box-and-whisker plots and time-series graphs of the average MRI for each SMP section.

Deflection

LTPP deflection testing requirements on asphalt pavements included specified drop sequences and sensor spacing. Required target loads included 6,000, 9,000, 12,000, and 16,000 lbs, along with acceptable load ranges of ±10 percent (e.g., acceptable load range for a 6,000-lb drop is 5,400 to 6,600 lbs). FWD testing was performed at various locations (mid-lane and outer wheel path) relative to the travel lane and dependent on pavement type. Figure 19 illustrates an example testing scheme for SPS-1 (asphalt pavement).

FWD testing included deflection measurement at nine sensors for LTPP-owned FWDs and at seven sensors for non-LTPP-owned FWDs, and four drop heights (resulting in loads of approximately 6,000, 9,000, 12,000, and 15,000 lbs). The deflection sensors were offset from the center of the applied load as summarized in Table 11.

Table 11. Deflection Sensor Offset

Note: na = not applicable.

FWD testing was performed at specified intervals on asphalt pavement sections and at varying intervals on concrete pavement sections (depending on the length and number of concrete slabs). In addition to deflection data, subsurface pavement layer temperatures were measured during FWD testing.

Inventory Data

LTPP inventory data provided details of the section at the time of construction; however, many of the SPS sections did not have inventory data. If available, this information included layer-specific properties of joints, mixtures, compaction, reinforcing steel, and placement, as well as information regarding bound and unbound base and subbase layers and subgrade soils.

Materials Testing Data

Materials testing data was not available for every SMP section. Where available, materials testing data included:

• Concrete: compressive, flexural, splitting tensile, and shear strength, thermal expansion, static modulus, density, and air content.
• Asphalt: bulk and maximum specific gravity, moisture susceptibility, resilient modulus, and dynamic modulus.
• Asphalt binder: penetration, specific gravity, viscosity, and test results from shear rheometer, bending beam, direct tension, multiple stress creep recovery, and recycled engine oil.
• Asphalt extracted materials: specific gravity, aggregate gradation, and particle shape.
• Unbound layers (including subgrade): gradation, hydroscopic moisture content, layer type, AASHTO soil classification, Atterberg limits, moisture density, unconfined compressive strength, resilient modulus, unbound base permeability, natural moisture content, dynamic cone penetrometer, and subgrade stabilizing agency.
• Subgrade: modulus of subgrade reaction, soil density, unconfined compressive strength, soil permeability, and potential vertical rise.

Layer thickness for asphalt and concrete pavement was based on cores and available field sampling, core evaluation, and inventory data for determining the representative thickness and material type for each layer and each construction event.