Section 4: Non-Destructive Evaluation of Pavement Structural Properties
Urgency of the design, importance of the highway, value of the project, and availability of non-destructive tools usually influence what testing is performed. However, proper evaluation is essential to ensure performance; there is no substitute for comprehensive planning that includes reserving the equipment when needed.
The most common structural-based non-destructive tools used in TxDOT are discussed in some detail in the Flexible Pavement Rehabilitation Training (CD/ROM). Contact the Research & Technology Implementation Office (RTI) at (512) 465-7555 to request a copy of this CD. Practical exercises in the use of analyses software for data collected by these systems are also included with this training disc.Anchor: #i1006478
List of Non-Destructive Tools in Order of Availability
The general order of availability for non-destructive tools is as follows:
- Falling Weight Deflectometer (FWD). One device usually shared between two districts; there are 15 of these devices assigned to the districts. This device is essential in establishing the in situ stiffness properties of the pavement layers through analysis of the deflection data by backcalculation of layer moduli values using MODULUS. Moduli can then be used as design inputs to FPS-19.
- Dynamic Cone Penetrometer (DCP). Many districts have purchased their own (roughly $2500 per unit), two units are available at CST-M&P. This portable device is a secondary tool used to verify unbound pavement layer thicknesses, confirm the presence of lime stabilized subgrade, and evaluate the relative stiffness of unbound layers. It can prove useful in verifying MODULUS layer inputs and moduli outputs. A conversion equation developed by the Army Corps of Engineers is used to convert DCP data to moduli values. The DCP requires the boring of a small pilot hole through bound materials.
Air-coupled Ground Penetrating Radar (GPR). There
are currently four systems located throughout the state. The systems’
- Fort Worth District
- Odessa District
- CST-M&P in Austin
- through an interagency contract (IAC) with the Texas Transportation Institute (TTI) that is managed by CST-M&P, also in Austin, and
- One additional system is scheduled for completion in FY 07.
This is a van-mounted system that provides a nearly continuous profile of layer thicknesses and dielectric variations to a maximum depth of about 24 in. beneath the pavement surface. Dielectric properties are correlated to material density and moisture content so subsurface problems (stripping, trapped moisture) in the existing HMAC and excessive moisture in base layers can be detected.
GPR can also be used to identify segregation and low density in existing HMAs. GPR testing of PCC pavements has not been as successful as testing on HMAC and surface treated pavements due to the GPR signal interference by the reinforcing steel and the attenuation of the signal through PCC materials.
- Ground-coupled Penetrating Radar (GPR) testing is available through an interagency contract with TTI that is managed by CST-M&P. GPR testing involves towing one or more GPR antennas along the ground at walking speeds. Some antennas operate at a lower frequency than the air-coupled GPR units and therefore penetrate much deeper into the pavement and underlying layers. Ground-coupled GPR has been used to locate sink holes; search for buried underground objects such as abandoned tanks or test for water damage or other anomalies that are deeper than an air-coupled GPR unit can sense.
- Seismic-based tools (Seismic Pavement Analyzer [SPA], Portable SPA [PSPA], Dirt SPA [DSPA], V-meter, Free-free Resonant Column). There is one SPA system headquartered at CST-M&P. Three of each PSPA and DSPA systems are fielded at the district level. There is one of each system headquartered at CST-M&P.
The V-meter and Resonant Column have roughly the same distribution as the PSPA and DSPA. Seismic devices are slowly gaining acceptance as evaluation tools, as familiarity with their operation increases and improvements to the analysis software have been completed.
The SPA series of devices are used in the field to measure in situ properties. These devices are used to generate waveforms in the material being tested. The elastic modulus at small strain is proportional to the velocity of the wave propagation.
The V-meter and Resonant Column are laboratory instruments used to measure properties of samples collected in the field or molded in the lab. These devices use a pin or hammer “source” to impact the pavement or sample surface. Wave propagation speed and analysis of the wave dispersion curves can be used to determine layer thickness, stiffness with depth, and presence of discontinuities (cracks, delaminations). Seismically-derived stiffness values are evaluated at very low strain and require adjustments to be used for determining values comparable to ones estimated at higher strains, for example, truck wheel loads.
- Rolling Dynamic Deflectometer (RDD). The RDD provides the capability of generating a continuous deflection profile, rather than generating deflections at discrete points as does the falling weight deflectometer (FWD). The device can be instrumental in evaluating older rigid and flexible pavements to determine the relative level of underlying support, load transfer at joints and cracks, and overall pavement stiffness.
Currently, there is only one of these devices and is available for use through an interagency contract (IAC) with the Center for Transportation Research (CTR) at UT Austin. This IAC is managed by CST-M&P.Anchor: #i1006575
Falling Weight Deflectometer (FWD)
The falling weight deflectometer (FWD) is a trailer mounted device that places an 11.8 in. (300 mm) diameter load plate in contact with the highway at each test location. The testing interval is set at 0.1 mi. (maximum) or 30 locations per project. A load column above the load plate carries a stack of weights that are dropped to impart a load to the pavement similar to that imparted by a passing dual truck tire set. A series of seven geophones spaced away from the load plate at 12 in. increments measure the surface deflection, generating a “deflection bowl.” Measurements are generally acquired in the right wheel path of the outside lane.
Figure 4-5. Falling Weight Deflectometer (FWD).
Figure 4-6. Falling Weight Deflectometer, detail view.
Where a divided roadbed exists, surveys should be taken in both directions if the project will include improvements in both directions. Some care in the placement of the load plate and sensors is required by the survey crew, especially where the highway surface is rutted or cracked. The load plate should lay on a flat surface; the load plate and all geophones should lie on the same side of any visible cracks. A human spotter should be used to ensure placement in these case.
Temperature data collected at the time of testing is necessary for all flexible pavements since the modulus of bituminous materials is temperature-dependent. The FWD has a built-in thermocouple to measure ambient air temperature and records this temperature at each drop location. In addition, for pavements with bituminous surfaces at least 3.0 in. thick, in-pavement temperature measurements should be made at the beginning and end of each survey, at a minimum. Longer surveys or surveys made on days where rapid temperature changes occur will require temperature checks at additional intermediate locations. A portable drill with masonry bit is provided to accomplish this; holes are drilled to mid-depth or 3.0 in. maximum. Alternatively, surface temperatures measured by infrared devices can be made with correlations available for mid-depth temperature estimation.
Liberal use of comments placed in the FWD data file at the time of data collection is encouraged. Comments pertaining to proximity to reference markers, bridge abutments, patches, cracks, etc., are all important documentation for the individual evaluating the data.
Description of the FWD with conceptual discussion on basic operation is given in the Flexible Pavement Rehabilitation Training (CD/ROM). Contact the Research & Technology Implementation Office (RTI) at (512) 465-7555 to request a copy of this CD.
Backcalculation of Deflection Data
TxDOT currently uses version 6.0 of the MODULUS software for backcalculation of deflection data collected by the FWD. Version 6.0 is a Windows compatible program with many added analyses and graphing features compared to the previous version. The software and user’s manual in PDF document format is available from the TTI on-line pavement design training site. Also, basic operation and discussion of inputs, cautions, and example problems is presented in the Rehabilitation Strategies for Flexible Pavements (CD/ROM). Contact the Research & Technology Implementation Office (RTI) at (512) 465-7555 to request a copy of this CD.
The raw deflection file, pavement layer thicknesses, layer Poisson ratios, probable layer moduli ranges, and asphalt temperatures at the time of testing are all required inputs to perform backcalculation. The backcalculation process works on the assumption that the pavement structure can be modeled as a linear-elastic layered system. If the parameters of layer thickness, deflection, and Poisson ratio are known, the modulus can be approximated. A likely range of “probable” layer moduli provided by the program user facilitates the process by forming the basis of a small internal database against which mathematically generated deflection bowls are compared to the actual measured deflection bowl by the software. Once a reasonable match is made, the moduli that allow this match are reported as the individual layer moduli. In addition, the program reports a depth to stiff layer or bedrock.
There are precautions and limitations to the backcalculation procedure that the user must consider. In the end, engineering judgment will be needed to decide on the viability of solutions generated. The following are some pointers when using MODULUS 6.0:
- The modulus for layers thinner than 3.0 in. cannot be backcalculated. This situation arises most often for thin-surfaced flexible pavements. The user must assign a reasonable modulus to this layer (minimum and maximum are input as the same value in the program) based on thickness, level of distress, temperature, etc.
- The surface layer is always the layer that the load plate is in contact with, so a thickness must be entered. Where the surface is a bituminous surface treatment, it is allowable to use a nominal thickness such as 0.5 in. and assign a nominal modulus such as 250 ksi. Alternatively, the surface treatment can be combined with the underlying layer as the “surface” reducing the total number of layers by one.
- The maximum number of layers for which the modulus can be backcalculated is four (one of which is always the natural subgrade) in MODULUS 6.0. For circumstances where more layers are known to exist, the user must either consolidate or ignore layers. Consolidation is recommended for materials that are more likely to have a similar modulus and shear strength properties (i.e., different types of HMAC or flex base). Ignoring layers may be reasonable in certain cases where the material’s contribution to the overall stiffness of the structure is minimal (i.e., “foundation course,” or lime treated subgrade – constructed as a working platform).
- There are times when a more reasonable solution is obtained
modeling your pavement structure as a 3-layer system even if you
know there are four layers present. This situation may develop for
a number of reasons, such as variable stabilization (leaching),
variable depth to bedrock, etc.
Reasonableness is related to the in-place stiffness characteristics of the layers being modeled and not necessarily to the size of the average errors reported by the software in comparing the mathematically generated bowls to the measured bowls. While the 4-layer solution will generally give lower overall errors, the backcalculated material moduli may be unrealistic with respect to the in-place material. When there is doubt of reasonableness, the user should perform backcalculation runs using both 3- and 4-layer solutions (employing guidelines given in bullet 3). Additional field testing such as with the dynamic cone penetrometer (DCP) along with engineering judgment is necessary to ensure a valid, reliable solution.
- A check of the MODULUS summary
table should be made to detect outliers that skew the average value
reported. Outliers may be the result of full-depth patches (different
pavement structure) or very weak areas.
For the purpose of using MODULUS-reported values as input to FPS-19, adjustment of the average modulus should be considered; otherwise the performance of any pavement design solution based on these inputs could be jeopardized. As a rule-of-thumb, consider removing values that exceed one standard deviation from the unadjusted average, and then re-average. This should always be done for modulus values that are much higher than values that are more typical for the section. Consideration can be given to eliminating very low values only if the intention is to include a bid item for repair of weak areas (i.e., Item 351 “Flexible Pavement Structure Repair”) as part of the job.
- Shallow bedrock (typically less
than 60 in. deep) will almost always result in underestimation of
the subgrade modulus and over-estimation of the flexible base modulus.
The recommended workaround is to fix the depth to bedrock at 120
in. or alternatively 240 in. if the solution using the program-generated
depth to bedrock produces suspect subgrade/base moduli. Another
clue that the default solution is suspect would be if the ratio
of the flexible base modulus (unstablized layers only) to the subgrade
modulus is very high (>5).
MODULUS can perceive a shallow depth to bedrock in high water table situations (water is incompressible) as can be the case in east Texas. It may be beneficial to override the program-generated depth-to bedrock value, by using a fixed value of 120 in.
- Soft upper subgrade can also lead to high errors in the backcalculation process. In these cases, the use of a 4-layer solution where the soft portion of the subgrade is modeled as the subbase layer (fix depth at 12 in.) may provide better fit and more realistic backcalculated values for the base and deep subgrade. Verification with a dynamic cone penetrometer (DCP) may be warranted.
Dynamic Cone Penetrometer (DCP)
This portable device complies with ASTM D 6951 – 03 and consists of a 2-piece rod; the lower rod is about 39 in. long (1 m) and is fitted with a replaceable cone tip at the penetration end and an anvil at the upper end. The anvil houses a threaded receptacle for attaching the upper rod. The upper rod carries the 17.6-lb. sliding hammer and has a handle for steadying the device during testing. The hammer free-fall distance on the upper rod is 22.6 in.
Figure 4-7. Dynamic Cone Penetrometer (DCP).
DCP Operation Instructions
The operator drives the DCP tip into the soil by lifting the sliding mass (hammer) with one hand to the handle then releasing it. The other hand holds the instrument by the handle to maintain an approximate vertical position. If a bound layer (HMAC, PCC, cement-stabilized base) is present above the non-bound layers, a 7/8 in. hole should be first bored through the bound layer(s).
DCP Readings and Data Collection
An initial depth reading is made using a measuring stick between the bottom of the sliding mass and a stationary surface (pavement surface, ground level, etc.). The total penetration for a set of blows is measured and recorded by an assistant. A plot of depth vs. cumulative blows is generated and a trend line is fitted to the data points for each tested layer. The penetration rate in mm/blow is determined as the slope of the trend line.
A separate trend line should be generated for each layer containing only data points measured in that layer. Dividing the data into discrete segments in this manner ensures that only valid data points can influence the trend line's slope.
The number of blows in a set can vary; for softer soils 1-3 blows may be a set, whereas for stiffer soils 5-10 blows may be a set. Some soils may be so stiff that little to no penetration is recorded in a given set. If this circumstance persists, it is better to halt testing and bore down to a lower level, then restart the test. Any time drilling is done before the start of a DCP test, the first data point should be at the correct depth and the person processing the data will not include (0,0) as a data point, which would taint the penetration rate (mm/blow) calculation.
The instrument is extracted by tapping the mass against the handle or by using a jack attached beneath the anvil. Both methods have some risk of damage to the instrument; in softer soils using a disposable cone, tapping the mass against the handle is the most expeditious method.
The Army Corps of Engineers has developed a number of correlations relating rate of penetration to soil stiffness in terms of California Bearing Ratio (CBR). For most applications, the following relationship is adequate for approximating in situ stiffness.
- CBR = California Bearing Ratio
- PR = penetration rate, mm/blow.
This relationship has been further correlated to elastic modulus (E) using the relationship:
- E = elastic modulus.
Plots can be made to show the relationship of stiffness with depth. This procedure is useful for determining the effective depth of a granular base, the effectiveness of lime stabilized subgrade, or whether there are soft pockets of material with depth. Visual examination of the DCP shaft once extracted can also reveal the presence of free moisture. Basic operation and example problems are presented in the Flexible Pavement Rehabilitation Training (CD/ROM). Contact the Research & Technology Implementation Office (RTI) at (512) 465-7555 to request a copy of this CD.Anchor: #i1006816
Air-coupled Ground Penetrating Radar (GPR)
This van-mounted self-contained system carries a data acquisition boom-mounted antenna off the front of the vehicle, suspended above the roadway. This allows for uninterrupted data collection at near-highway speeds. The system sends pulses of radar energy into the pavement and captures returned reflections from each perceived layer interface within the structure. A survey is generally run in the right wheel path with data summarized at 10-ft. intervals; the effective width of the scanning is about 8.0 in. If data is needed at other transverse locations along the lane, additional parallel passes must be made.
Figure 4-8. Air-coupled Ground Penetrating Radar (GPR).
Figure 4-9. Detail view during calibration.
The 1-GHz antenna has a maximum penetration depth of about 24 in. The amount of energy returned and the time delay between reflections are used to calculate layer dielectrics and thickness. The dielectric constant of a material is an electrical property that is most influenced by moisture content and density. As the density and moisture content go up, the amount of energy reflected increases (and the penetrating ability decreases). Conversely, if the air voids increase, the amount of energy reflected decreases. Typical dielectric values for various pavement materials are given in Table 4-1, below:
Analysis of GPR Data
Analysis of raw GPR data is accomplished using the software COLORMAP. A user’s manual is available in TTI report 1702-4, “Colormap Version 2 User’s Guide with Help Menus.” To obtain the Colormap User’s Guide, contact CST-M&P at (512) 506-5808. Also, basic operation and discussion of inputs, cautions, and example problems is presented in the Flexible Pavement Rehabilitation Training (CD/ROM). Contact TxDOT’s Research & Technology Implementation Office (RTI) at (512) 465 -7555 for information about obtaining a copies of each of these.
Because of the capabilities to compute material dielectric properties and layer thicknesses, GPR has become a valuable tool to the pavement engineer to assist in determining the following:
- layer thicknesses, section changes, full-depth patches
- location and extent of potential problems such as elevated moisture levels or stripping in the HMAC and
- location/extent of segregation and low joint density.
Verification of suspected problem areas should be made with targeted coring and sampling. These sites should be selected based upon the analysis results from COLORMAP. Conclusions drawn from this limited sampling can then be applied with more confidence to the remainder of the survey.Anchor: #i1006904
Ground-coupled Penetrating Radar (GPR)
GPR systems using lower-frequency antennas are able to discern anomalies deeper with the pavement structure. Resolution is improved when the systems are placed in contact with the surface, so the antenna is dragged behind a van which houses the data acquisition system.
Maximum penetration will vary with antenna frequency, soil type and degree of saturation. Under the right conditions, the 100 MHz antenna can penetrate up to 30 ft.; however, there is poor near surface resolution of layers. These systems have been used successfully in locating buried fuel tanks, archeological burial sites, sink holes, and springs within the right-of-way. The systems and the required data analysis remain specialized; services are available through an interagency contract with the Texas Transportation Institute (TTI).
Figure 4-10. Ground-coupled Penetrating Radar.Anchor: #i1006929
Seismic Evaluation Tools
Seismic tools can be divided into two groups: those used in the field for evaluation of in situ pavement properties and those used to evaluate samples in the laboratory.
Field Seismic Tools
Seismic Pavement Analyzer (SPA). The SPA is a trailer-mounted device with an appearance similar to the FWD. The device uses a combination of two impact hammers, three geophones and five accelerometers to measure the velocity of shock waves that pass through the pavement. Data reduction is accomplished by using specialized software developed at the University of Texas El Paso (UTEP).
Figure 4-11. Seismic Pavement Analyzer (SPA).
Figure 4-12. Seismic Pavement Analyzer, detail view.
Portable and Dirt Seismic Pavement Analyzer (PSPA, DSPA). The PSPA was developed as a supplemental tool to the SPA, with an enhanced capability to evaluate the upper strata (i.e., HMA lift) of the pavement structure. The DSPA is similar in configuration, but has a broader hammer surface for use on unbound surfaces. These devices have been fielded in three districts in a trial quality control/assurance capacity by measuring material stiffness. The piece that contacts the surface is an inactive anvil that is struck with an internal striker; the contact piece acts as a wave transmitter.
Figure 4-13. Portable Seismic Pavement Analyzer.
Laboratory Seismic Tools
V-meter. The V-meter is an ultrasonic laboratory device that is particularly useful for testing AC briquettes (lab-compacted specimens or field cores). In this device, a transmitting transducer is securely placed on the top face of the specimen. The transducer is connected to the built-in high-voltage electrical pulse generator of the device. The electric pulse is transformed into a mechanical vibration which is applied to the specimen. A receiving transducer is securely placed on the bottom face of the specimen, opposite the transmitting transducer. The receiving transducer, which senses the propagating waves, is connected to an internal clock of the device. The clock automatically displays the travel time of compression wave. By dividing the length of the specimen by the travel time, the compression wave velocity and as such modulus of the material is determined.
Figure 4-14. V-Meter.
Free-free Resonant Column. The resonant column device uses laboratory-prepared soil specimens that may be prepared using the Proctor (ASTM D-698), modified Proctor (ASTM D-1557) or any other procedure adopted by the agency. Since the test is non-destructive, a membrane can be placed around the specimen so that the specimen can be tested later for strength (static triaxial test) or stiffness (resilient modulus or cyclic triaxial test). An accelerometer is securely placed on one end of the specimen, and the other end is impacted with a hammer instrumented with a load cell. In less than 3 min., a specimen can be tested, and the test result can be obtained. The process has been automated and simplified so that a technician can perform the test, interpret the results, and generate a report almost immediately.
Figure 4-15. Resonant Column.Anchor: #i1007017
Rolling Dynamic Deflectometer (RDD)
The Rolling Dynamic Deflectometer (RDD) is a truck-mounted deflectometer that applies large cyclic loads to the pavement and measures the induced cyclic deflections with up to four rolling deflection sensors as it moves along the pavement. These sensors consist of a lightweight, rigid, three-wheeled cart supporting a vertically-sensitive velocity transducer (geophone). Speed while collecting data is 1-2 mph.
The condition and load transfer capacity of all transverse joints and cracks in rigid pavements can be determined by observing the measured deflections as the RDD approaches and crosses each joint or crack. The RDD has also been used successfully to evaluate flexible pavement structural condition. The RDD is a valuable tool for rapidly identifying regions of pavement requiring rehabilitation.
Critical regions of pavement identified with the RDD for possible rehabilitation can be studied further using traditional, discrete testing methods (FWD, DCP, seismic). Continuous deflection profiles provide valuable information about the pavement condition without backcalculating pavement moduli. For example, the relative deflections of heavily-trafficked pavement regions such as the outside wheel path, outside lane and lightly-trafficked regions (center slab, inside lane) give a very good indication of pavement degradation.
Figure 4-16. Rolling Dynamic Deflectometer (RDD), drawing with labeled parts, loading rollers, and photograph.