Nuclear Gauge Research Articles

English

Rarely do rock pile characterization methods allow for examination and sampling of the undisturbed interior of large rock piles in-situ. The regrading of the Goathill North (GHN) rock pile at the Molycorp Questa mine, New Mexico provided a unique opportunity to examine and sample the undisturbed, interior of a large rock pile through the construction of trenches cut into the rock pile as earth-moving progressed. Weekly during the regrading of the GHN rock pile, contractors excavated a trench to allow for sampling of rock pile material. Trenches typically had four benches, which were 1.5 m wide and did not exceed 1.2 m in height, to give an overall slope of 1.4 horizontal to 1.0 vertical within the trench. Each trench extended for a length sufficient to explore site conditions, maintain the regraded 2:1 slope, and ensure personnel safety. Once excavated, trench walls and benches were surveyed using a differential global positioning system. For every trench, maps and logs of each bench were created to describe the different mine soil units, including the thickness, dip and extent of the units. Units were defined based on color, grain size, stratigraphic position, and other physical properties that could be determined in the field. Units were correlated between benches and to both sides of each trench, and several units were correlated downward through the series of successively excavated trenches. The field sampling crew began sampling within each of the identified units after the unit boundaries were identified and mapped. The following in situ measurements were taken along either the horizontal or vertical surfaces of each exposed bench and along the base of the trench: sand cone (density), tensiometer (matric suction), gravimetric moisture content, grain size, infiltration, and nuclear gauge measurements (density, moisture content). Gravimetric water content samples were collected at the locations selected for the measurement of matric suction and infiltration tests. Samples were collected from each defined unit for geochemical, geotechnical (including shear box tests, slake durability tests), biological, and electron microprobe analyses. Channel sampling for pyrite reserve modeling was performed in short 1.5-m long horizontal slots using a rock hammer to chip material to be placed into a sample bag. Additional material from selected layers was collected for potential weathering-cell tests in the future. Typically, paste pH increases with distance from the outer, oxidized zone (west) towards the interior, unoxidized zone (east) of the GHN rock pile.

Basic considerations for Monte Carlo calculations in soil

Monte Carlo codes are extensively used for probabilistic simulations of various physical systems. These codes are widely used in calculations of neutron and gamma ray transport in soil for radiation shielding, soil activation by neutrons, well logging industry, and in simulations of complex nuclear gauges for in soil measurements. However, these calculations are complicated by the diversity of soils in which the proportions of solid, liquid and gas vary considerably together with extensive variations in soil elemental composition, morphology, and density. Nevertheless use of these codes requires knowledge of the elemental composition and density of the soil and its physical characteristics as input information for performing these calculations. It is shown that not always all of the soil parameters are critical but depend on the objectives of the calculations. An approach for identifying soil elemental composition and some simplifying assumptions for implementing the transport codes are presented.

Adjustment of Measuring Devices With Linear Models

We consider the problem of linear calibration or adjustment of two measuring devices based on a sample of replicated measurements. Linear adjustments are routinely used in the pavement industry. A simple solution based on linear regression of the average measurements of one device on the other has been used by field engineers. Toaddress the statistical concern that the regression parameter estimates are biased due to attenuation, we consider a more sophisticated multivariate model to obtain asymptotically unbiased estimates of the calibration parameters. The maximum likelihood estimates (MLEs) of the multivariate model are computed with the EM algorithm. Simulation studies and asymptotic calculations are used to compare properties of the MLE with the simple regression method. Data from two measuring devices used for determining asphalt pavement density, coring and nuclear gauge, are used in an example. We find that the superiority in parameter estimation of the MLE does not always result in better adjustments. In typical applications, such as determining pavement density, the simple regression method is highly competitive and often performs better than the multivariate MLE in adjusting the measurements from a cruder device despite its bias problem.

A Comparison of Three Methods for Measuring the Density of a Forest Soil in New Zealand

Three methods for measuring the density of a forest soil were compared: sampling with a traditional hand-held soil sampler and thin-wall (Shelby) tubes, and testing with a single-probe nuclear moisture-density gauge. An incremental approach was used when sampling with the nuclear gauge. Values of wet density for the soil layer between the source and the sensor were then recast for individual soil layers and converted to dry densities using the water contents provided by either the hand-held soil sampler or Shelby tubes. Shelby tube and nuclear gauge derived values of dry density were strongly related (R2 = 0.80), values derived from the hand-held soil sampler and nuclear gauge less so (R2 = 0.66). The nuclear gauge proved the most economical in use by a factor of about two, and data collection with the Shelby tube sampler and nuclear gauge was quicker when compared to the traditional hand-held soil sampler by a factor of about two. Sample compression associated with the Shelby tubes was corrected for when calculating the final density values. However, the degree of sample disturbance associated with the hand-held soil sampler was uncertain. The advantages and disadvantages of each method are discussed.

Analysis of Multivariate Models for Evaluating Segregation in Hot-Mix Asphalt Pavements

Multivariate statistical techniques were used to determine the feasibility of quantifying segregation in hot-mix asphalt pavements. Data obtained by using visual ratings of segregation, nuclear density gauge readings, and laboratory tests at nine field sites as reported by the National Center for Asphalt Technology were analyzed. Data were screened, before multi variate statistical analyses were performed, to develop relationships among segregation level, change in key mixture properties (air voids, asphalt content, and percent passing various sieves), and change in mixture density. The results revealed that segregation level could not be accurately identified by visual ratings and that multivariate statistics based on actual measured properties were appropriate for evaluating segregation level. Laboratory-prepared samples were made to assess the accuracy of the model developed in the study. The verification results indicated that the model could predict segregation level of these samples with accuracy. From the analysis of the models, it was concluded that a change in mixture density could predict segregation. This allows screening of segregated pavement sections on the basis of numerical field measurements instead of reliance on subjective visual observations. Given the promising results from this work, further studies on segregation models based on density measurements are suggested.

Evaluating Nonnuclear Measurement Devices to Determine In-Place Pavement Density

The air void content of both in-place and laboratory-compacted hot-mix asphalt may be the factor that most affects the performance of a properly designed mixture. A mediocre mix, well constructed with good in-place air voids, often will perform better than a good mix that has been poorly constructed. In-place density may be monitored by using three methods: cores, nuclear density gauge measurements, and nonnuclear density gauge measurements. A study was done to evaluate and compare two non-nuclear density devices, the pavement quality indicator (PQI) Model 301 and the PaveTracker, to in-place core densities. Testing and analyses of 20 projects showed that uncorrected gauge measurements (from each gauge) provided reasonable correlation (significant at a 5% level of significance) with core density measurements approximately 75% of the time. Average coefficients of determination for the relationships between uncorrected gauge measurements and core densities were approximately .5. Simulation analyses indicated that the error in measuring in-place air voids using calibrated PQI Model 301 gauge measurements based on the constant offset method can be 1% to 2.7% air voids for the average of 10 measurements. The numerical experiments also indicated that calibration of PQI Model 301 by simple offset or by slope and offset did not improve the correlation between the gauge and core densities.

A new approach to determining the precision and bias of on-line gauges

On-line gauges are used in many industries to measure characteristics of raw material destined for a production process. In the coal industry, which is the focus of interest in this paper, the traditional method for determining coal characteristics has been the assay of coal samples. This method requires a great deal of work on a day-to-day basis. It has the further drawback that there is generally a time lag of at least 1 day between sampling of the coal and obtaining numerical measures of its quality characteristics. An interesting recent development is that nuclear gauges, which are capable of producing estimates of coal quality in real time, are now available commercially. The present paper demonstrates how bootstrap methods can be used to obtain a confidence interval for gauge precision using gauge output data alone. No specially constructed reference values from a sampling and assay operation are required. As such, the method is potentially an attractive alternative to the methodology set out in ISO 15239. It is also shown how gauge calibration can be checked using gauge output and one set of reference samples obtained under normal operating conditions. The method, which again uses the bootstrap, involves the construction of a simultaneous confidence band for a calibration function that relates gauge values to reference values.

GEANT simulation of the gamma nuclear gauge

The gamma nuclear gauging technique used for monitoring the sediment load suspended in water, is based on the detection of /spl gamma/ rays emitted by a radioactive source. The GEANT321 Monte Carlo simulation tool, originally developed at CERN for high energy physics experiments, is used for the evaluation and calibration of gamma nuclear gauges. A set of parameters, principally the source energy, the source-detector separation, the lead block thickness and the energy threshold below which the sediments elemental composition affects the measurement or the energy corresponding to the Compton and photoelectric windows separation, are discussed and evaluated in the case of the gamma scattering gauge. For the gamma transmission gauge, the GEANT321 code has been used to define the optimal source detector distance interval, particularly for the Moroccan sediment samplers, and to check the influence of the radionuclide existing in the suspension, on the gauge response accuracy. Experimental calibration was also carried out using a gamma transmission gauge and a good agreement was found between experimental and simulated results.

Validation of Vibration-Based Onboard Asphalt Density Measuring System

The nuclear density gauge served the industry well for almost forty years as project owners performed quality control on the asphalt paving and compaction process. Recently, however, the responsibility for quality control of these processes has largely been given to the contractor. This shift in responsibility comes at a particularly unfavorable time for the average paving contractor. The construction industry is faced with the worst labor shortage in history, limiting the number of qualified quality control technicians and equipment operators. In addition, this is the dawn of the age of Superpave, a family of hot-mix asphalt mixes designed to combat pavement rutting by increasing the quantity of large aggregate in the mixes. Understandably, these are more difficult to compact. The Onboard Density Measuring System, a model patented by Penn State University, offers density measurements in real time at a rate of one per second during the compaction process, thereby affording the constructor the opportunity to recognize and correct compaction problems immediately while maintaining a permanent record of the entire compaction process.

Field Measurement of Water-Cementitious Material Ratio Using Nuclear Gage

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Methods for determining representative density–depth profiles using nuclear density gauges

Most fills when compacted in layers will, to some degree, experience variations in density with depth. When measured using a nuclear density gauge in direct transmission mode, such variations in density through the layer will lead to unrepresentative density–depth profiles. This is because each measurement used to generate the profile is the mean density between the surface and the depth of measurement, and no correction for this is currently made. Uniform sands and silts, and pulverised fuel ash (PFA), often go through a process of over-stressing during compaction, leading to significantly lower densities in the upper part of a compacted layer. Under these circumstances the errors recorded in nuclear density gauge determinations of the density–depth profile may become significant. In extreme cases the usual guidance is to remove the upper part of the compacted layer prior to density testing. This paper describes numerical techniques to determine the mean density between successive measurement depths for the cases of both equal and unequal depth increments, thus giving a density–depth profile that is much closer to reality.

Methods for determining representative density–depth profiles using nuclear density gauges

Most fills when compacted in layers will, to some degree, experience variations in density with depth. When measured using a nuclear density gauge in direct transmission mode, such variations in density through the layer will lead to unrepresentative density–depth profiles. This is because each measurement used to generate the profile is the mean density between the surface and the depth of measurement, and no correction for this is currently made. Uniform sands and silts, and pulverised fuel ash (PFA), often go through a process of over-stressing during compaction, leading to significantly lower densities in the upper part of a compacted layer. Under these circumstances the errors recorded in nuclear density gauge determinations of the density–depth profile may become significant. In extreme cases the usual guidance is to remove the upper part of the compacted layer prior to density testing. This paper describes numerical techniques to determine the mean density between successive measurement depths for the cases of both equal and unequal depth increments, thus giving a density–depth profile that is much closer to reality.

Too many notes?

Orphan sources are now recognized as a radiation protection problem needing attention. Government and private sector initiatives have been mounted in response. Should more be done? There are promising possibilities. Under the ICRP 60 system of radiation protection, any use of radiation should be justified. Key NRC licensing regulations predate ICRP 60 and, consequently, if a use is authorized by license, then the use is presumed to be justified even if alternatives are available. As an example, x-ray gauges can be successfully substituted for nuclear container fill level gauges in the food and beverage industry. But this development is not widely known to potential users and regulators, and current regulatory policies do not address this. Limiting the number of radioactive devices would limit the number of potential orphan sources. From an operational radiation safety point of view, persons wanting to dispose of unneeded or unwanted radioactive sources are faced with a system loaded with disincentives, in part because disposition options are not well known and in part because options have become limited, complex, and expensive. New approaches are needed if the orphan source problem is to be effectively addressed. Paramount are the needs to (1) update U.S. regulatory policies to incorporate the ICRP principle of justification, (2) create a system that encourages prompt, safe dispositioning of unneeded and unwanted radioactive sources, and (3) make information on these points universally known to users of radioactive sources.

Analysis of TransTech Model 300 Pavement Quality Indicator: Laboratory and Field Studies for Determining Asphalt Pavement Density

The pavement quality indicator (PQI) produced by TransTech Systems, Inc., uses principles of electrical impedance and the dielectric constants of materials to determine the densities of asphalt pavements. The Model 300 PQI device was recently evaluated in Illinois as a potential alternative to nuclear density gauges for quality control/quality assurance of asphalt pavement construction. Both laboratory and field testing were conducted to meet these objectives. The PQI has several appealing features relative to the nuclear density gauge, including its light weight and rapid operation in the field without the need for special licensing, handling, or training. The density determined with the nuclear density gauge generally exhibited a closer correlation with the density determined from pavement cores, as denoted by significantly lower squared error of the regression values in the regression analyses performed. Also, although the Model 300 PQI has been improved from earlier models to account for moisture and temperature, the data suggest that the device is still influenced by environmental conditions. On the other hand, the PQI proved to be more operationally convenient for the field technician, being significantly lighter and less time-consuming to operate in the field. On the basis of analysis of the information obtained from the studies, the Illinois Department of Transportation has chosen to base acceptance and statistical pay factors on density values obtained by the AASHTO T166 method with pavement cores. At present, the nuclear density gauge is still the specified device for quality control/quality assurance of asphalt pavement projects in Illinois.

Evaluation of New Nonnuclear Pavement Density Gauges with Data from Field Projects

The introduction of new devices to measure pavement density requires an evaluation method that is both accurate and fair. Evaluations are commonly done by taking a density measurement with the gauge and comparing this density to the density obtained from a core taken at the same location. This approach must meet two requirements; first, enough cores must be taken at each location to make the comparisons meaningful, and second, evaluation of these devices must be done with as many projects in as many locations as possible to account for all materials used in pavement construction. Unfortunately, given the difficulties in obtaining field cores, large numbers of cores that can be used for evaluations are not available from all projects. Therefore, any evaluation of density gauges must balance rigor with practicality. The correlation coefficient was selected to evaluate the new nonnuclear pavement density gauge. However, because only a limited number of cores are available at each site for development of the correlation coefficient, this parameter by itself cannot be used to evaluate the new gauge. Instead, results from side-by-side comparisons with the accepted nuclear density gauge were used to aid in the evaluation of the new density-measuring device. It was concluded, on the basis of data from 76 projects in six different states, that the proposed nonnuclear density gauge must be further developed before it can be used to determine pavement density. In its current form, the densities obtained with the device do not correlate with the measured densities to the same degree that the densities measured with existing devices do. Use of this nonnuclear device will introduce more uncertainty in pavement density measurements than that which already exists.

Evaluation of Measurement Techniques for Asphalt Pavement Density and Permeability

A pavement quality indicator device developed through an NCHRP project, the Corelok device, and laboratory and field permeability index devices were evaluated for their potential to improve the Virginia Department of Transportation’s density specification or to be designated for use in a replacement specification. Fifteen samples were taken from six projects representing three nominal maximum aggregate sizes. Densities measured by a nuclear gauge correlated well with core densities measured in accordance with AASHTO T166 and by the Corelok device. Pavement quality indicator readings appeared to be repeatable but did not correlate with the densities measured in accordance with AASHTO T166 or with the Corelok device. A fair correlation was obtained for two projects. Laboratory permeability and field permeability indices were correlated with pavement density. A good correlation was found between the laboratory permeability and field permeability indices. The relationship appears to be linear in the range agencies typically specify.

Measuring Soil Compaction on Construction Sites: A Review of Surface Nuclear Gauges and Penetrometers

This paper reviews two techniques of determining soil compaction on construction sites. The surface nuclear gauge is found suitable for measuring soil compaction in soils with less than 5% organic matter by weight and at a depth of no more than 0.15 m (6 in.). Penetrometer readings are often unreliable on compacted soils, as well as in dry and stony soil conditions. Therefore, the penetrometer is rarely a valuable device on construction sites as a definitive measurement instrument, but it may be useful as an indicator of compacted areas. Recommendations to measure soil compaction on construction sites are given.

Determination of Aluminum in Copper-Aluminum Alloy Using Gamma-Ray Spectrometry

In this work, a nuclear gauge, based on single and dual energy γ-ray transmissions, has been developed and used to determine the concentrations of the aluminum component in copper-aluminum alloy. Disk foils of 7.07 cm 2 area and thicknesses ranging from 0.48 to 0.834 cm have been prepared from the alloy to be used as targets for the γ-ray photons. Criteria for the best choice of the suitable photon energies for both single and dual energy γ-ray transmissions have been investigated. In this work, aluminum concentration values with an accuracy of 98.53% and 99.09% have been obtained by using both single and dual γ-ray energy transmission techniques, respectively. A comparison of the present results with those from the conventional chemical analysis indicates a fair agreement. A criterion for the best choice of the suitable γ-ray transmission technique for the aluminum determination has been investigated.

Identifying Segregation in Hot Mix Asphalt Pavements Using Rolling Nuclear Gage Measurements and Infrared Imaging

Abstract Previous research indicated that density, gradation, and asphalt content change due to segregation. A rolling nuclear gage capable of measuring both density and hydrogen count was evaluated for its ability to detect and measure both density and asphalt content. Eight projects (four recently constructed pavements and four during construction) were used to evaluate this instrument. Longitudinal density profiles were obtained for each project at transverse quarter points across the lane. Additional testing included infrared thermography and traditional destructive testing of cores. Visual observations of texture differences were used to identify areas with no, low, medium, and high levels of segregation. At least three core locations were then identified from each of these areas; cores were taken always from the longitudinal paths tested with the nuclear gage. Cores were used to determine changes in air voids, asphalt content, aggregate gradation, and mix stiffness as a result of the various levels of segregation. Results indicate it was difficult to use air voids alone to detect and measure segregation, regardless of how densities (air voids) were determined (i.e., nuclear density or core properties). The rolling nuclear density measurements showed that while densities may be uniform longitudinally, there were significant differences in densities transversely for several projects. A second type of temperature segregation was found using this gage in combination with the infrared thermographs. During paving operations, the paver may be stopped to change or wait for the haul truck or to adjust the equipment. The mix immediately behind the paver cools off but cannot be compacted since the paver is in the way of the roller. This results in a localized transverse region of very low density.

ILLISIM Program for End-Result Specification Development

A new computer simulation program, called ILLISIM, is presented. ILLISIM allows the user to conduct a comprehensive assessment of agency and contractor risks over a wide variety of end-result specification (ERS) parameters. ILLISIM was created to assist the Illinois Department of Transportation (IDOT) in developing an ERS for asphalt pavement construction. The focus is on the use of ILLISIM to explore the possibility of using the nuclear density gauge as a basis for acceptance testing of as-constructed pavement density. Preliminary simulation results from ILLISIM suggest that it may be feasible to use the nuclear gauge for acceptance if stratified random sampling techniques and adequate test replications are used. However, this is true only if test bias is kept under control, for example, if a representative nuclear gauge-to-core correlation is maintained throughout the project. ILLISIM can be used to study the effects of changing device bias, which should be investigated in future work. The results also indicate that a larger sample size will be necessary to increase accuracy and minimize disputes when marginal quality levels are detected.