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55 Equipment and Operation A typical hand-held portable IE device consists of a source and a receiver, and it requires about 15 to 30 s to place the device, conduct the test, and move to the next location. The actual IE measurement occurs in milliseconds. For the pur- poses of scanning a larger area of pavement, an automated system is preferred. The system consists of an array of rotat- ing sensor wheels, each equipped with impact sources and motion sensors around the perimeter. The purpose of the sensor wheel is to carry out IE tests while the wheel is con- tinuously rolling. Use of multiple sensor wheels spaced trans- versely across the width of the pavement allows coverage of an area of pavement while traveling at a slow walking speed. The purpose of the wheel array is to collect equally spaced parallel lines of data simultaneously so that coherent areas of delamination can be identified and mapped. Figure C.1 shows two examples of IE equipment. In those examples, each wheel is approximately 1 ft in diameter, and is mounted with six pairs of a displacement sensor and an impact source. Each sensor/source pair is spaced at 6-in. inter- vals around the circumference of the wheel. Every two sensor wheels are coupled together with a rubber isolated axle. Data are collected continuously at 6-in. intervals while the system rolls along the surface of the pavement. The data collection for each wheel is independently triggered, and the position of the collected data is obtained by using a distance measuring instru- ment (DMI) mounted to the vehicle wheel or to an external distance wheel. Table C.1 lists equipment specifications for IE. Data Output and Display The field operation and playback software should be capable of the following displays: ⢠Direct time domain waveforms from each source-receiver pair; Basic Principle Impact echo (IE) is a point test method that transmits a high-frequency mechanical (sound) wave into the pavement and measures the P-wave reverberation (echo) between the top and bottom surfaces. An impact source and receiver are placed adjacent to each other on the pavement surface. The amplitude of the reverberation detected by the receiver is converted into the frequency domain as amplitude ver- sus frequency. For a homogeneous pavement layer, there is a resonant or dominant frequency directly proportional to the thickness of the pavement layer, according to the equation below. This resonant frequency is referred to as the âthickness resonance.â The frequency data are typically converted into thickness by using the following equation with an assumed P-wave velocity. T V f= 2 where T = thickness, V = P-wave velocity in pavement, and f = frequency. For a uniform pavement with no delaminations, the cal- culated thickness resonance will be relatively uniform. How- ever, when there is delamination, the reverberation will be disrupted and other, lower frequency modes of vibration will occur. This lower frequency will lead to abnormally high calculated thickness values at delamination locations. For a series of IE tests conducted over an area, the calculated thickness can be plotted, and areas where the thickness is unreasonably high (i.e., not expected in the pavement struc- ture) are interpreted as delaminated. Automated equipment has been developed to carry out this test continuously at a slow walking speed. Software to analyze the data is partially automated. Technical Brief: Impact Echo A P P E n D i x C
56 ⢠Running amplitude/thickness plot, or equivalent b-scan, for each sensor wheel; and ⢠Output format should be a two-dimensional (2-D) array of data with thickness as a function of x (longitudinal dis- tance) and y (transverse offset). Data Collection Protocol Because of this systemâs relatively low collection speed, this system should be used exclusively for project-level evaluation, with a focus on specific locations and areas of concern. The start and end points of data collection should be referenced with mile markers or other fixed reference points. Roadway features encountered during the survey (e.g., intersections, bridge decks) should be annotated with some type of man- ual markers in the data. The data should be observed visu- ally during collection to ensure that the system is operating properly and that the expected features are appearing in the data. Data Analysis The objective of the analysis is to identify locations where a significant increase in computed thickness is associated with debonding or delamination of an asphalt layer from the layer below. To accomplish this task, the amplitude/thickness plots created at each location are assembled as a 2-D array of data, where x and y are the coordinates of the test point, and z is the resonant thickness value. These thickness values can be contour plotted, and thickness values exceeding a speci- fied threshold can be highlighted as those associated with debonding and delamination. An example of such a plot is shown in Figure C.2. Equipment Availability and Cost The IE scanning system discussed in this technical brief is not available commercially. A prototype system has been developed and tested by Olson Engineering. Also, the data interpretation procedures and analysis software are reason- ably well developed. An estimated cost of a fully operational system is not available. (a) (b) Synchronized transducer Automated Impactors Cable to data acquisition PC Vehicle ball hitch mount Distance wheel Transducer Wheel Data Acquisition Computer Figure C.1. Current IE/SASW scanning systems: (a) six-sensor wheel array and (b) two-sensor wheel unit. Table C.1. IE Equipment Specifications System Type Array of Six Rotating Sensor Wheels, Lined Up Transverse to the Direction of Travel Sensor frequency response Up to 50,000 Hz Impact source input frequency Up to 50,000 Hz Lateral spacing of sensor wheels 2.0 ft (typical) Lateral coverage per pass 12 ft (full lane width) Longitudinal data collection rate One test per foot (minimum) Travel speed during data collection 1 to 2 mph Travel speed during mobilization Posted speed limit Real time display Resonant frequency System monitoring and control From within or outside survey vehicle Data collection rate Based on speed and sensor spacing on sensor wheel Spatial reference Vehicle DMI or external distance wheel
57 Note: dark blue/cyan areas = anomalously high thicknesses; light green = expected AC thickness. Figure C.2. Contour plot of resonant thickness. Advantages and Limitations The primary advantage of an IE system is its ability to directly detect debonding and delamination within asphalt pavement layers. The principal limitations are temperature and speed. The detection capability is limited to periods of colder temperatures, where the asphalt has a high enough stiffness to generate the reverberations detected by this method. The system can operate only at slow walking speed and its application is thus limited to project-level analysis and diagnostic investigations.
