☰
Application of Geophysical Methods
to Geotechnical Related Problems
Bibliography Glossary Geophysical
Methods Engineering
Applications About
About Engineering Applications Geophysical Methods Glossary Bibliography
>HOME | ENGINEERING APPLICATIONS | PAVEMENTS | CONDITION EVALUATION OF EXISTING PAVEMENTS
Condition Evaluation of Existing Pavements

Pavement deteriorates with time, due to increases in loads and demand (loading cycles), variations in moisture within its base and subgrade that can damage its foundation, and exposure to the elements (freeze-thaw cycles or even de-icing chemical application). A proper diagnosis and preventive maintenance schedule-where smaller and less expensiverepairs are made at properly-timed intervals throughout the structure's life-significantly extends its life and reduces the costs associated with its ownership. (See earlier discussions on baseline condition assessments and QA verification of construction related to this topic.)

Segregation In Hot Mix Asphalt

A proper gradation of aggregate is essential for performance of a mix. Sometimes during plant mixing or placement of Hot Mix Asphalt (HMA), smaller aggregates are separated from larger aggregates leaving a ragmented mix. For example segregation occurs when a patch crew throws shovels of HMA in a hole rather than placing the HMA. Segregation reduces the performance of the mix by changing the stability of the product and compromising the air void content.

Somewhat related to segregation and evaluated with the same methodology is stripping in HMA. Stripping occurs when the asphalt covering the aggregate no longer adheres to the aggregate. This may happen with moisture and heat or be related to the aggregate. Observing the patterns from geophysical images, measuring them, and interpreting the results can often identify suspect stripping. GPR can often provide detailed screening, which can be followed up by other means, including geophysical NDT methods and destructive sampling.

Techniques Recommended

Surface NDT Methods:

A brief note on Infrared Thermography: It can successfully be applied to this application, particularly if variations in paving processes during initial construction are being identified. Variations in temperature within the asphalt as it is being unloaded from dump trucks, routed beneath the paving equipment and rolled, or "shoveled-in" HMA are areas that appear as thermal variations of significance in the infrared image. IR can be used very effectively for both QC and initial baseline condition assessment (QA of sorts). While other methods are available or under development, the ones listed provide cost-effective determination of pavement thickness.

Moisture Variation

Water enters a pavement structure from a variety of sources. This water may drain away from the pavement or pond in or under the pavement layers, weakening the pavement and causing premature failure. Tracking the presence of moisture or the variation of moisture within a pavement structure allows pavement engineers to correct the source of the water and/or repair the distressed area. Moisture variation is determined by first establishing a benchmark from the driest pavement. Geophysical methods will then locate changes from this benchmark, and they can also be useful in identifying where to look for an appropriate benchmark location, itself.

Techniques Recommended

Surface NDT Methods:

While other methods are available or under development, the ones listed above provide cost-effective ways to screen for locations with suspect moisture variation which can then be verified by other means, including in-situ sampling and laboratory testing-or instrumentation of suspect sites that are identified using a screening tool, such as GPR.

Rock Pockets

Rock pockets and other anomalies in a concrete pavement can be determined by skillful use of geophysical methods. Rock pockets occur when aggregate collects without a proper amount of binding cement. This may happen in placement or vibration of the concrete. The presence of rock pockets weakens the concrete structure. Rock pockets can be found with high frequency GPR and Impact Echo.

Techniques Recommended

Surface NDT Methods:

While other methods are available or under development, the ones listed above provide cost-effective determination of pavement thickness.

Voids Beneath Pavements

Concrete pavements experience daily and seasonal variations in joints opening and closing. During the open periods, water may enter the base. The weakened base material may "pump" up through the joints with loading and therefore cause a void under the joint. Other sources of water such as springs and water main leaks may also erode the material under the slab. It is important to find and repair these voids before catastrophic failure leading to expensive repairs to the concrete slab. Deflection testing is sometimes used to locate the voids and determine load transfer efficiency. This method is not discussed here. Methods that have been found successful in locating voids under concrete are listed below.

Techniques Recommended

Surface NDT Methods:

While other methods are available or under development, the ones listed above provide cost-effective determination of pavement thickness.

Cracking

See Pavement Condition/Integrity (below).

Crack-mapping can be performed using lasers/accelerometers, and/or profilometers (IRI). High-resolution GPR (1.5 GHz ground-coupled and 1.0 GHz horn antennas) can be used as a screening tool to look for subsurface indications of cracking beneath pavements already showing early signs of distress-in order to locate regions where more in-depth testing should be performed. However, these results are possible only if very high spatial sampling density (40 scans/meter using horn antenna to upwards of 100 scans/meter using the GC antenna) is used along each pass of the GPR antenna.

Pavement Condition/Integrity

Over time the forces such as freezing and thawing, exposure to sun, water and traffic combine to deteriorate a pavement structure. One measure of pavement integrity is strength (measured by physical methods such as Falling Weight Deflectometer (FWD). Another measure of integrity is the variance in homogeneity. Wave propagation techniques such as GPR, seismic, and Impact/Echo can be used to map integrity.

For concrete pavements, there is an excellent discussion of GPR and SASW/Impact Echo Methods discussed under Bridge Decks in a section under a similar title.

Techniques Recommended

Surface NDT Methods:

While other methods are available or under development, the ones listed above provide cost-effective determination of pavement integrity.

Structural Changes

With repeated loading from traffic or weather, a pavement will fatigue. It is important to track the structural quality of a pavement in order to timely apply a corrective activity. Simply stated, the change in integrity over time is one measure of structural changes. Another measure is tracking the change in stiffness over time.

For concrete pavements, there is an excellent discussion of GPR and Surface Wave/Impact Echo Methods discussed under Bridge Decks. Although the discussion is directed toward a concrete structure, the same general approaches toward problem-solving and initial screening, followed up by point data collection (NDT or limited destructive sampling) at identified, or targeted, locations, should be used for all pavement types.

Techniques Recommended

Surface NDT Methods:

While other methods are available or under development, the ones listed above provide cost-effective determination of pavement structural changes.