Long-Term Pavement Performance Computed Parameter: Frost Penetration

CHAPTER 8. SUMMARY AND RECOMMENDATIONS

SUMMARY

A comprehensive review of the previous LTPP frost penetration analysis methodology and an assessment of frost depth estimates provided in the LTPP database were conducted, followed by recommendations for improvements. As a result of these recommendations, an enhanced methodology and the accompanying E-FROST research analysis tool were developed for determination of frost penetration in unbound pavement layers and subgrade soil for LTPP SMP sections.

The enhanced methodology uses electrical resistivity, moisture, and soil temperature data collected for instrumented SMP sections to predict frost depth in unbound pavement layers. In addition, the EICM model was used to fill in the gaps in the measured soil temperature data.

Using the enhanced analysis methodology and E-FROST, in-situ data were analyzed to determine freeze conditions and frost depths in the unbound pavement layers. The results of the frost penetration analysis for LTPP SMP sections were assembled in the LTPP computed parameter tables described in this report.

The results presented in this report demonstrate how frost penetration beneath the pavement structure was predicted for LTPP SMP sites using a combined empirical and mechanistic technique. This technique utilizes data from LTPP in-situ measurements and thermodynamic modeling.

Study findings stress the importance of using all three different types of in-situ measurements for accurate frost penetrations prediction: temperature, electrical resistivity, and moisture content. The EICM has proven useful for filling in the gaps of measured subsurface temperatures and for understanding the thermodynamic processes that occur in pavement layers. This information could help practitioners and researchers design seasonal monitoring field experiments and analyze field data to determine frost penetration under pavement layers.

RECOMMENDATIONS FOR FUTURE RESEARCH

Future E-FROST Development to Support M-E PDG Implementation

The E-FROST research tool developed during this study could be very useful for analysis of seasonal changes in unbound pavement layers. In the future, this tool could become particularly useful for implementation of the M-E PDG, which emphasizes estimating seasonal changes in pavement layer moduli.

We recommend that LTPP consider further development of this tool into an LTPP software product similar to the LTPP profile viewer software so that the pavement research and practicing community at large can have easy access to ER, temperature, and moisture data, as well as frost penetration profiles for LTPP SMP sites. In addition, data from FWD tests can be added to this software to relate changes in mechanistic properties of pavement layers and in pavement responses and to cross reference this information with frost penetration data.

Future LTPP Data Analysis Study of Pavement Responses under FWD Loading during Freeze-Thaw Conditions

During spring thaw, sunshine and warm air temperatures result in a top-down thawing of the pavement system. The water released by the melting ice can be trapped by deeper, still-frozen material, creating saturated or supersaturated conditions that weaken the pavement structure. The change in pavement strength could be observed by FWD measurements.

The database tables developed under this study provide detailed information about the periods and the depth of freeze-thaw based on continuous temperature, ER, and moisture data analyzed. This information could be utilized to cross reference with and analyze FWD data collected during the thaw periods to capture the conditions of the supporting layers during the weakest period and to correlate these conditions with pavement responses. This task can be accomplished through mechanistic modeling of pavement responses under FWD loading based on the inventory, climatic, testing, and FWD data from the LTPP database.

The result of the proposed study could contribute to understanding pavement deterioration, as triggered by seasonal changes in pavement layer moduli and could be utilized in the development of spring load restrictions.

Future Development of In-Situ Frost Measurement Devices

One of the challenges in LTPP frost penetration analysis was the interpretation of the data from ER measurement devices, as the following describes:

• ER measurements were expected to be high during cold winter months when temperatures plunge below ground water freezing point. However, a number of ER readings during warm summer months (with temperatures high above 0 °C) were found to be as high as readings during cold winter months. These were unexpected trends.
• LTPP collected in-situ data to evaluate three types of ER parameters: voltage, resistance, and resistivity. All three parameters are expected to follow a similar trend when soil goes through freeze and thaw cycles. However, it was not uncommon that the data showed opposite trends, or one of the trends would be nearly flat (indicating no changes in the soil freeze state). This inconsistency was unexpected.

We highly recommend that LTPP promote the need for future research and development of in-situ frost measurement devices. Perhaps the next generation of such devices would have multifunctional sensors capable of monitoring temperature and moisture changes in the soil, in addition to electrical resistivity measurements, and use output of all three types of measurements to determining frost penetration.