Eyeing the surface

Segregation may be defined as lack of homogeneity of constituents in hot-mix asphalt (HMA) pavements such that accelerated pavement distresses occur. The most common form of HMA segregation has been identified as gradation segregation. Gradation segregation is the non-uniform distribution of coarse and fine aggregate materials in the finished HMA pavements. Gradation segregation can occur as the result of aggregate stockpiling and handling, production, storage, truck loading practices, construction practices and equipment adjustments.

Localized pavement areas rich in coarse aggregate are typically associated with high air voids and low asphalt contents; these conditions can lead to moisture damage as well as to durability-related pavement distresses such as fatigue cracking, pothole formation and raveling.

Conversely, pavement areas rich in fine aggregate are associated with low air voids and high asphalt contents, making them susceptible to rutting and flushing.

The correlation between air voids and pavement durability is well documented. In New Jersey, some projects have experienced high air voids and segregation of the surface mixes due to poor construction practice or equipment problems. Hence by establishing a relationship between surface texture measurements, surface segregation and air voids, the New Jersey Department of Transportation (NJDOT) wanted to establish a screening tool to identify variations in surface texture that are typical of segregation and potentially locate pavement sections with high air void content.

A research team consisting of the New Jersey Institute of Technology, Abatech Inc. and the NJDOT conducted an extensive research program. Available technologies were evaluated to develop a screening tool for assessing surface texture as a means of locating segregated areas of the surface pavement and potentially high air void locations. The work described here was specifically focused on the use of laser technology to develop procedures or screening tools to determine segregated areas.

Low tech, high tech

There are several traditional and emerging methods to detect and quantify texture so that quality control/quality assurance program can be built on to the design and construction of HMA pavements. The following is a description of those methods.

The traditionals: There are three commonly used methods to detect segregation. They are visual identification, sand patch testing, and nuclear density gauges.

Visual identification: Visual identification of non-uniform surface texture has been used to locate segregation. This is a subjective approach which can lead to disagreements between agency and representatives of the contractor. Usually visual detection of non-uniform areas is used as the baseline against other quantitative approaches.

Sand patch testing: The sand patch test has been used to quantify visual observations of differences in the surface macro-texture. The ASTM E965 test method (ASTM 2001) indicates that the precision of the test method is approximately 1% of the measured depth in millimeters and the between operator variation is about 2%.

Nuclear density gauges: Nuclear density gauges can be used to identify segregated areas by profiling the longitudinal density of the pavement mats. The assumption is that segregation will be seen as low density. However, literature indicates limited success. There are two reasons for the erratic success. First, the common assumption for using these gauges is that density decreases with increasingly coarse aggregate segregation. However, this assumption does not consider the relationship of the gradation to the maximum density line. If the job mix formula (JMF) begins above this line, separation of the coarse aggregate in this type of mix may result in a higher density as the gradation shift towards the maximum density line. Second, different types of aggregates have different effects on gauge variability. Limestone, a commonly used aggregate source, substantially increases testing variability. Gravel, on the other hand, have much less of an effect on variability. If a mixture is composed of coarse limestone and fine gravel stockpiles, the resulting change in testing variability in coarse aggregate-rich and fine aggregate-rich areas may make it difficult to adequately detect or measure segregation.

The innovations: Three new technologies have been identified as having potential to selectively identify HMA segregation. They are thermal imaging, ground penetrating radar and laser surface texture measurements.

Thermal imaging: All objects emit infrared radiation in the form of heat, which can be detected by an infrared scanner. These natural impulses are converted into electrical pulses and then processed to create a visual image of the object's thermal energy. The colors used to represent the thermal imaging can be user-selected to represent surface temperature changes, such as blue for colder regions and red for warmer regions. The thermal imaging technology will indicate high void regions, as thermal capacity of air is minimal when compared to that of aggregates and asphalt cements. If one assumes, high segregation causes high void ratios then the technology can be easily adopted to detect segregation.

The primary component of any thermal imaging system is an optical scanner, a unit that is used to detect infrared radiation from an object. Other essential components of the system are a display monitor, video camera and computer with appropriate software for data acquisition, analysis and storage.

A full-lane width can be surveyed at one time with an appropriately placed camera. Usually liquid-nitrogen-cooled scanners provide improved resolution over other methods of cooling. Although current technology is vehicle-mounted, operation at highway speeds (50 mph) tends to blur the image. Resolution is improved substantially by operating the equipment at slower speeds (40 mph).

Ground penetrating radar (GPR): The basic theory used in GPR is a measurement of the dielectric constant, E, (or permittivity). The electrical permittivity of air is different from that of aggregate and asphalt cement. Hence in highway applications, the travel time of an electromagnetic pulse through a structure can be used to compute the layer thickness.

Continuous longitudinal surface texture profiles can be obtained quickly because the technology can be operated at normal highway speeds. However, slower speeds are needed for higher resolution. The equipment is portable and reasonably affordable and can be mounted to any vehicle.

Laser surface texture measurements: Over the past 20 years the use of laser technology to define surface texture has been gaining wide popularity. The technology uses a rapidly pulsing semiconductor laser to produce an infrared light that is projected onto the pavement surface. The light is scattered off of the surface and a receiving lens focuses this scattered light onto a linear array of photodiodes. The diode receiving the most light corresponds to the distance to the surface. Using mathematical algorithms the distance to the surface at a discrete point is obtained. The measurements are conducted very rapidly as the vehicle drives along the pavement enabling measurements at points that can be typically separated by 1mm defining a surface profile.

Based on the subjectivity of measurements, if the visual observation is eliminated, sand patch and laser methods are the most appropriate methods to quantify the segregation based on surface texture. It seems that out of the three emerging technologies discussed above, the laser technology is quite capable of quantifying the surface texture and hence the detection segregation of HMA pavements.

Table 2: Comparison of Surface Texture Measuring technologies