In this section, non-geophysical methods developed for pavement evaluations in the transportation and geotechnical fields are briefly discussed.
Falling Weight Deflectometer
The falling weight deflectometer is a nondestructive device that sends an impulse (with known magnitude and duration) to the pavement and/or base. The system response is measured using accelerometers and geophones. The recorded response is then related to design parameters including the sub-grade reaction and dynamic response of flexible and rigid pavement pavements (i.e., the in-situ resilient elastic moduli). These data are being used by state department of transportations to evaluate the structural health, aging, and use life of pavements.
There are deflectometers with impulse capacities that range from a few killograms force (for example the Keros) to several tons (for example, Dynatest FWD/HWD Test Systems). This wide range of impulses permits the use of deflectometers to monitor responses of systems with varying stiffnesses. For example, large deflectometers permit the evaluation of complete structural pavements, while lightweight deflectometers permit the evaluation of subsystems, such as bases and sub-bases.
Pavement Quality Indicators
The Pavement Quality Indicator (PQI) is an instrument available from TransTeck (http://www.transtechsys.com) and it is intended f or the nondestructive measurement of in-situ mass density of asphalt/concrete mixtures. It measures the change in capacitive impedance of a toroid electric field and it relates the measured impedance to the dielectric permittivity. The measurement technique then correlates the dielectric permittivity of the pavement to the asphalt or concrete density using volumetric relationships.
However, the technique has some drawbacks including that the measurement is highly sensitive to moisture content, temperature, and aggregate mineral. In spite of these problems, the PQI system may provide a rapid indication of sudden changes and discontinuities in the mass density and quality of asphalt concrete pavement.
There are other similar types of instruments that use similar physical measurement concepts to evaluate in-situ pavement density. One of these instruments is the newly released Troxler's PaveTracker (non-nuclear device) used for the rapid measurements of segregation and overall pavement uniformity. The measurement results do not need to be corrected for changes in moisture or temperature.
Nuclear densimeters are a set of devices that measure the decay of a radioactive source and they correlate this decay to the density and water content of soils. Their measurements are accurate and they have been used to monitor the quality of compaction in bases and sub-bases for the construction of dams, embankments, and roads. However because of their radioactive nature, they require special certifications and permits for their use and transport.
Troxler is a leading manufacturer of nuclear densimeters. Several systems are commercialized that can be used to monitor the density of soil, aggregates, asphalt, and concrete along with the moisture of soils and aggregates. The instrument makes counts of backscattered gamma rays from a cesium source in the instrument. That information is used to compute the average density of the material through which the gamma rays (photons) passed. Most instruments also have an americium source on board for making similar measurements from which moisture content can be determined. The instrument is simply placed on the surface to be tested and a measurement procedure of warm-up and counting time is followed for each station. Measurement takes a few seconds to a few minutes at each station.
Moisture Engineering Gauge
Commonly used moisture techniques relate the measurement of dielectric permittivity to the volumetric water content in granular materials. These techniques include coaxial transmission line, time domain reflectometry, and radio-frequency capacitance. These dielectric techniques have been applied to soil measurement and found wide applications in agricultural and soil science community. However, the geotechnical and transportation engineering communities have not widely adopted these techniques as they do not directly relate the volumetric water content to the engineering gravimetric water content.
In spite of this limitation, volumetric water content techniques can provide reliable measurements of point water content and help in the evaluation of homogeneity of bases and sub-bases. Furthermore, several correlations have been proposed to directly relate volumetric and gravimetric water contents.
Purdue Time Domain Reflectometry (TDR) Method
This method has been developed by Dr. V. Drenvich (Purdue University) and it is intended for the rapid determination of in-situ water content and density. Drnevich and co-workers rely in several independent sets of measurements for the evaluation of unit weight and water content:
- In-situ soil unit weight: the relative dielectric permittivity of the in-situ soil, the relative dielectric permittivity of the Proctor mold compacted soil, the Proctor mold compacted soil unit weight and the water content.
- In-situ soil water content: the relative dielectric permittivity of the in-situ soil, the soil dry unit weight, and experimentally determined correlation parameters.
In spite of depending on correlation parameters, this is a promising procedure and it is now being tested throughout the country t o establish its applicability. The advantages of this methodology include its relative quick collection of results and its straightforward interpretation (once the correlation coefficients have been determined). An important drawback is that the technique yields unit weight measurements only at discrete points.
Humboldt Stiffness GeoGauge
This portable instrument is designed to rapidly determine the in-situ stiffness of bases and sub-bases, and to evaluate the engineering quality of the compacted soil. The instrument works by measuring the mechanical impedance/stiffness of soils under the annular footing of the instrument. This is done by monitoring the displacement of the soil under the action of a harmonic force.
The instrument was designed with the intent of replacing nuclear devices and other density measurement systems for the quality evaluation of compacted bases and sub-bases. However, the GeoGauge stiffness is not directly related to dry unit weight. As expected the stiffness values decrease with increasing water (due to the change of capillary forces. Therefore, the instrument cannot, at this time, replace traditional protocols to evaluate the quality control and acceptance of compacted soil for pavement constructions, but it can be used to nondestructively evaluate the homogeneity of the base and sub-base systems. New stiffness-based design practices and protocols are being proposed. These new protocols may take advantage of stiffness measurements of granular systems and their application in engineering design.
Infrared (IR) Thermography
Infrared thermography is increasingly being used in laboratories and in in-situ for the measurement of pavements properties. Infrared thermography (IRT) scanning can be used to evaluate delamination detection in rigid pavements, segregation in flexible pavements, and in general for the monitoring of deficient construction and quality of construction of buildings. researchers have used thermographic techniques to monitor subsurface anomalies in sewer lines and tunnels.
Thermographic techniques rely on the relative thermal emisivitty of the different components of the pavement system to render a thermal image that helps monitor possible structural problems. This technique can be used in rapidly moving vehicles and it can evaluate a large area in a few seconds. However, thermography can only capture information from the pavement surface; and, therefore, it is commonly combined with other nondestructive methods for the evaluation of pavement quality (e.g., GPR).