Nuclear radiation apparatus and method for dynamically measuring density of test materials during compaction

The present invention provides an apparatus and method for dynamically measuring the density of soil, asphaltic material and the like during the compaction of the material. The gauge includes a nuclear radiation source and a nuclear radiation detector means which are mounted in spaced relation from the surface of the test material to form an air gap therebetween. The gauge also includes means for measuring the size of the air gap between the test material and the source and detector system, and means to compensate for the effect of the size of the air gap to thereby obtain the density of the test material.

FIELD AND BACKGROUND OF THE INVENTION 
The present invention relates to an apparatus and method for determining 
the density of test materials, and more particularly to a nuclear 
radiation measurement apparatus and method for measuring the density of 
soil, asphaltic materials and the like during movement of the radiation 
measurement apparatus across the surface of the material which is 
undergoing measurement. 
Nuclear radiation gauges for determining the density of soil and asphaltic 
materials are well-known, as described for example in U.S. Pat. No. 
2,781,453. Such gauges employ the phenomenon of scattering of gamma rays 
and are known by those skilled in the art as "scatter" gauges. 
Such gauges typically take the form of a hand held portable instrument 
which is positioned on the surface of the test material for a 
predetermined period of time while backscattered radiation is counted to 
obtain a density reading. Devices of this type have been widely used and 
well accepted in the industry for obtaining rapid non-destructive density 
measurements of the test material. The density gauges are particularly 
useful in determining the degree of compaction of soil or asphalt during 
the construction of roadbeds and pavement surfaces, in which heavy rollers 
or compactors are rolled back and forth across the surface and density 
readings are made periodically using the portable stationary nuclear 
density gauges of the type described above. 
It has been recognized that it would be quite desirable to obtain a readout 
of density continuously during the compaction operation, rather than 
periodic spot density readings. This approach would give a density reading 
over a large area rather than an instantaneous spot reading, and would 
also make it possible to more rapidly respond to changes in the density 
readouts during the compaction operation. To this end, several movable, 
dynamically reading nuclear density gauges have been proposed. One such 
gauge is in the form of a wheeled unit and employs a pair of small 
cylindrical rolls as wheels with a nuclear source and detector mounted 
between the rolls in a suspended, noncontacting relationship with the 
underlying test surface. The gauge rides along the pavement surface and 
may be connected to and pulled by a pavement compactor vehicle. Another 
such gauge, described in published European Patent Application No. 
108,845, has the nuclear radiation source and detector mounted inside a 
cylindrical roll, and the roll may be manually pushed along the pavement 
surface or propelled therealong by connecting it to a pavement compactor 
vehicle. 
Both of these gauges assume a constant spacing (air gap) between the 
source/detector system and the underlying pavement surface. However, the 
density reading obtained from a backscatter gauge through an air gap is 
quite sensitive to variations in the size of the gap. During operation, 
any buildup of asphalt on the rolls will increase the effective diameter 
of the rolls and alter the size of the air gap between the source/detector 
system and the surface of the underlying test material, introducing error 
in the density reading. 
Mobile nuclear density gauges are also disclosed in U.S. Pat. Nos. 
3,341,706 and 3,354,310. These patents disclose a mobile nuclear gauge 
which can be towed by a vehicle along a roadway or surface to obtain a 
continuous logging of the density and moisture. The gauge is mounted in a 
trailer above the road surface. The patents recognize that in order to 
obtain an accurate measurement, the air gap between the gauge and the 
surface must be maintained as nearly constant as possible, and they 
attempt to accomplish this by providing soft tires on the trailer vehicle 
so that roughness is absorbed by the soft tires. The '706 patent mentions 
that as an alternative to maintaining a constant air gap, it may be 
desirable to monitor the air gap by suitable optic, mechanical, nuclear or 
sonic means and to compensate for the differences which may occur in the 
air gap. However, the patent gives no indication of how this compensation 
might be accomplished. 
It is an object of the present invention to provide a nuclear density 
measurement method and apparatus for dynamically measuring pavement 
density and which overcomes the above-noted disadvantages and limitations 
of the prior art. 
It is a further object of the present invention to provide a nuclear 
density measurement method and apparatus which is particularly suited to 
be used by a pavement compactor vehicle during a pavement compaction 
operation. 
SUMMARY OF THE INVENTION 
In accordance with the present invention an apparatus and method is 
provided in which the nuclear source/detector system is mounted for 
movement in spaced relation above the surface of the test material and 
wherein the spacing between the nuclear source/detector system and the 
underlying test surface is monitored as the nuclear density reading is 
being taken, and any variations in the spacing are taken into account and 
corrected for during the measurement. 
In accordance with the invention, a method is provided for measuring the 
density of a test material using a nuclear radiation backscatter gauge 
including a nuclear radiation source and nuclear radiation detector means 
positioned in spaced relation to the source, and wherein the method 
comprises the steps of: moving the source and detector along the test 
material while maintaining the source and detector in spaced relation 
above the surface of the test material to form an air gap therebetween, 
obtaining a count of the photons which are backscattered to the detector 
by the test material, obtaining a measurement which represents the size of 
the air gap between the surface of the test material and the 
source/detector means, and calculating the density of the test material as 
a function of the count of backscattered photons and said measurement of 
the air gap. 
The method and apparatus of the present invention are especially suited for 
use during the compaction of soil or pavement material using a compactor 
vehicle to obtain a real-time readout of pavement density so that the 
operator of the compactor vehicle can determine when the optimum amount of 
compaction has been achieved. To this end, the gauge is provided with a 
computer means for performing real-time calculations, and the method of 
the present invention comprises: 
moving the source and detector relative to the test material while 
maintaining the source and detector in spaced relation above the surface 
of the test material to form an air gap therebetween, 
obtaining a count of the photons which are backscattered to the detector 
means by the test material storing said count as a value in a 
predetermined memory location in said computer means, 
obtaining a measurement which represents the size of the air gap between 
the surface of the test material and the detector means and storing said 
air gap measurement as a value in said computer means, and 
storing in said computer means, values defining an empirically derived 
relationship between the air gap measurement and the calibration constants 
A, B and C for the density equation 
##EQU1## 
calculating with said computer means, values for the calibration constants 
A, B and c for said density equation using said stored values defining 
said empirically derived relationship and using said stored value for the 
air gap measurement; and 
calculating with said computer means, the density of the test material, 
using the thus calculated calibration constants A, B and C and the stored 
photon count in the density equation 
##EQU2## 
In accordance with a further aspect of the present invention there is 
provided a nuclear radiation gauge which is mounted on a compactor vehicle 
for providing the operator of the vehicle with a readout of the density or 
degree of compaction during operation of the vehicle. The compactor 
vehicle has rollers for compaction of materials such as soil, asphalt 
pavement and the like and a vehicle chassis connected to the rollers. In 
combination with this there is provided a nuclear density gauge means for 
measuring the density of the material during operation of the compactor 
vehicle. The gauge means comprising a nuclear radiation source, nuclear 
radiation detector means mounted in a predetermined spaced relation from 
the source, and means for suspendingly mounting said source and said 
detector means from said vehicle chassis with the source and detector 
means being located in spaced relation from the surface of the material 
undergoing compaction so that the source is positioned for emitting 
nuclear radiation through the air space beneath the gauge means and into 
the material and the detector means is positioned for detecting 
backscattered radiation from the material, means cooperating with said 
detector means for obtaining a count of the photons which are 
backscattered to the detector means by the test material, means for 
measuring the size of the air gap between the surface of the test material 
and the detector means, and means for calculating the density of the test 
material as a function of said count of backscattered photons and said 
measurement of the air gap.

DESCRIPTION OF THE ILLUSTRATED EMBODIMENT 
Referring now more specifically to the drawings, the reference character 10 
generally indicates a compactor vehicle of the type which is 
conventionally used for rolling and compacting soils, paving materials and 
the like. The compactor vehicle includes a chassis 11 and large diameter 
smooth surfaced rollers 12, 13, mounted to the chassis 11 and serving as 
the wheels of the compactor vehicle. As illustrated, a driver's seat 14 is 
located in the central portion of the vehicle chassis, and suitable 
controls 15 are provided to enable the driver to control the direction and 
speed of the vehicle. As is conventional, suitable motor means (not shown) 
is provided for propelling the vehicle along the pavement. 
The nuclear density measurement apparatus of the present invention 
comprises two units, a sensor unit 20 mounted beneath the compactor 
vehicle and located close to the surface of the pavement, and a console 
unit 40 accessible to the driver's seat 14 at the top of the vehicle. The 
sensor unit 20 and the console unit 40 are interconnected by a cable 21. 
The console unit 40 includes a keyboard 41 by which the operator may 
control the operation of the gauge and a display 42 by which the density 
reading obtained by the gauge as well as other information, is 
communicated to the operator. The mounting of the components of the sensor 
apparatus in this manner makes it possible to measure the pavement density 
during the operation of the compactor vehicle, and to rapidly provide the 
operator of the vehicle with a readout of pavement density. This makes it 
possible for the vehicle operator to immediately know when he has 
completed a sufficient number of rolling passes to achieve a desired 
degree of compaction. In the preferred embodiment illustrated in the 
drawings, the console unit is equipped with a pair of signal lights, such 
as a green light 44 and a red light 45. The operator can enter into the 
console unit via the keyboard 41, a setpoint representing the desired 
target density to be achieved in the material by operation of the 
compactor vehicle. As the compactor unit is being operated, the density 
which is calculated by the system is continually compared to the setpoint 
value. As long as the actual density is less than the setpoint, the green 
light 44 is illuminated, indicating to the operator that continued rolling 
and compaction is required. When the actual density equals or exceeds the 
setpoint density, then the red light 45 is illuminated, giving the 
operator an immediate visual indication that the desired density has been 
achieved, and that he can now move the compactor to another area. 
As best seen in FIG. 1, means is provided on the underside of the compactor 
vehicle for mounting the sensor unit 20 in suspended relation a short 
distance, above the pavement surface. In the particular embodiment 
illustrated, the mounting means includes an elongate beam 22 mounted to 
the chassis 11 of the compactor vehicle and extending transversely 
thereacross. The sensor unit is suspended from the beam 22 by three 
adjustable mounting brackets 24. Brackets 24 are pivotally mounted to the 
beam, and in turn to the sensor unit 20 to provide freedom of movement to 
the sensor unit in all directions. Thus in the event that the sensor unit 
strikes an obstruction protruding above the pavement surface, it can 
freely swing out of the way and then return to its normal suspended 
operative position. As seen in FIG. 4, the mounting brackets 24 are 
constructed in the form of turnbuckles, to permit adjusting the height of 
the measurement unit and the spacing between the undersurface of the 
measurement unit 20 and the pavement surface. Nominally, this distance is 
about 1/2 inch. 
As is also best seen in FIGS. 1 and 4, means is provided for raising the 
sensor unit 20 from its lowered operative position to a retracted non-use 
position where the sensor unit is safe from accidental contact with 
obstructions and the like. In the embodiment illustrated this takes the 
form of a cable 25 connected to measurement unit and an actuator handle 26 
associated with the cable 25 and which is positioned at a location 
accessible to the operator. Prior to further discussion of the structure 
and operation of the gauge, it will be helpful to review some of the 
underlying principles of nuclear density gauge operation, particularly as 
applied to soil density measurement through an air gap. 
The simplest method of measuring density involves the so-called backscatter 
method. To make this measurement, the source and detectors are both on the 
surface of the soil and gamma photons passing into the soil are scattered 
back to the detectors. If the soil had a density of zero, there would be 
nothing to cause scattering and the number of photons backscattered to the 
detector would be essentially zero. As the density of the soil increases, 
the number of backscattered photons increase with increase in density to a 
point where the backscattered photons are approximately equal to the 
losses due to additional scattering and absorption. The quantity of 
backscattered photons detected then becomes an approximate negative 
exponential function as the count decreases with increasing density of the 
soil. 
The backscatter method, while simple to perform, is subject to error from 
various factors including surface roughness, soil composition, etc. Soil 
composition errors can be particularly significant. In an effort to 
improve accuracy and reduce soil composition error, the so-called "air gap 
density" method was developed. In this method a measurement is made with 
the gauge raised above the soil a predetermined height to form an air gap 
and a reading is obtained based upon both the thickness of the air gap and 
the composition of the soil. The air gap measurement can be used in 
combination with a flush, backscatter measurement to partially cancel out 
the composition error. Prior measurement methods have not been able to 
obtain a sufficiently accurate direct measurement of soil density through 
an air gap. Among other reasons, this is due to the fact that the counts 
are very sensitive to the size of the air gap. As the size of the gap 
increases, the count rate increases rapidly, since the soil density 
becomes a much less significant part of the average density seen by the 
detector. 
In accordance with the present invention the density measurement is taken 
through an air gap, and means is provided for accurately measuring the air 
gap during the count and for adjusting the count rate in accordance with 
the measured air gap distance to obtain an accurate indication of soil 
density. The measurement of the air gap distance can be accomplished in 
various ways in accordance with the broad aspects of the present 
invention, including the use of ultrasonic means, laser means and 
capacitance, as well as nuclear methods. The preferred means and method as 
employed in the embodiment illustrated herein employs ultrasonic methods. 
As best seen in FIG. 5, the measurement unit 20 comprises a housing 30 
having a relatively smooth planar undersurface which is oriented generally 
parallel and in spaced relation to the pavement surface. The housing 30 
encloses a suitable radiation source 31 and a detector means 32. The 
radiation source may be a CS-137 source of gamma radiation and the 
detector means may take the form of Geiger-Mueller tubes sensitive to 
photons. As illustrated, the source 31 is located adjacent to one end of 
the housing and the detector means 32 is mounted adjacent to the opposite 
end of the housing. Shielding 34 is provided around the source and around 
the detector means 32, as is conventional, to prevent radiation from 
reaching the detector means in a direct path from the source. 
Additionally, means (not shown) is provided for shielding the radiation 
source when the gauge is not being used for measurement. 
As noted earlier, in an air gap measurement technique, the amount of 
radiation reaching a detector is a function of both the size of the air 
gap and the density of the material. The attenuation relationship for a 
given air gap for a single detector means, (e.g. for the detector means 32 
illustrated) may be expressed as follows: 
EQU CR=Ae.sup.-BD -C (I) 
where: 
CR is the count ratio, the detector count normalized by a standard 
reference count; 
D is the density; and 
A,B,C are the exponential curve fit parameters for the density vs. count 
ratio relationship for the given air gap. 
The parameters A, B, and C are dependent on the size of the air gap between 
the gauge and the test material. If density measurements are taken on 
calibration standards of known density at a number of given air gap 
distances, one can derive a series of curves for the count ratio vs. 
density relationship at each air gap distance. Using suitable 
curve-fitting techniques, one can derive relationships for the density 
equation constants A, B, and C as a function of air gap such as the 
following: 
##EQU3## 
where A, B, C=constants from density equation I; 
a.sub.1, a.sub.2, a.sub.3, a.sub.4 =constants determined empirically by 
gauge calibration; 
b.sub.1, b.sub.2, b.sub.3, b.sub.4 =constants determined empirically by 
gauge calibration; 
c.sub.1, c.sub.2, c.sub.3, c.sub.4 =constants determined empirically by 
gauge calibration; and 
G=Air gap distance between the gauge and the test material. 
Knowing the air gap distance, one can calculate A, B, C for that air gap 
distance using equations (II) above and the empirically derived constants 
a.sub.1, a.sub.2, a.sub.3, a.sub.4, b.sub.1, b.sub.2, b.sub.3, b.sub.4, 
c.sub.1, c.sub.2, c.sub.3, and c.sub.4. Then density D can be calculated 
from the density equation (I) above, or from the alternative form of the 
equation: 
##EQU4## 
In the illustrated embodiment of the invention ultrasonic means is utilized 
for measuring the air gap G between the test material and the measurement 
unit 20. As shown in FIG. 5, the ultrasonic means includes an ultrasonic 
transmitter 35 and a cooperating ultrasonic receiver 36 which are mounted 
in a recessed area or well on the underside of the housing 30 and aimed 
downwardly for measuring the distance to the underlying pavement surface. 
Additionally, the ultrasonic means also includes a reference transmitter 
37 and cooperating receiver 38 mounted in the recessed well a 
predetermined distance apart and oriented generally horizontally toward 
one another for measuring the speed of sound in the air above the heated 
pavement or other underlying surface and producing a reference signal. 
This reference signal is utilized by the associated circuitry 54 to 
compensate for variations in the speed of sound due to changes in air 
temperature in the measuring region. These components and the associated 
electronic circuitry which is used in association therewith, as indicated 
at 54, are commercially available and their selection and use are within 
the capabilities of persons skilled in this art. 
It should be understood that the foregoing description is intended as an 
illustration of one of a number of possible ways in which a density 
measurement can be obtained from the detector count data, and persons 
skilled in the appropriate arts will recognize that other particular 
solutions are possible within the broad scope and spirit of the present 
invention. For example, in commonly owned U.S. Pat. Nos. 4,525,854 and 
4,641,030 techniques are disclosed whereby it is possible to obtain 
density readings which are weighed toward predetermined strata within a 
test material, and these principles may be utilized for obtaining density 
measurements in the present invention. 
Referring now to FIG. 6, the detector 32 is electrically connected with a 
corresponding amplifier 52. Additionally, as is required, the detector is 
connected with a source 60 of high voltage. Output from the amplifier 52 
is directed to an input/output circuit generally indicated at 62 and is 
available through such circuitry to an electronic computing device, shown 
in the form of a microprocessor means 66, and to display 42. The 
microprocessor means 66 includes a commercially available microprocessor 
chip, together with associated memory means, such as an EPROM or EEPROM 
for storing program instructions and data constants, as well as random 
access memory (RAM) for storing other information, intermediate data and 
the like. The ultrasonic transmitter 35 and receiver 36 are connected to 
an ultrasonic driver which provides as an output a signal representative 
of the air gap distance. This signal is provided to the microprocessor 
through the input/output circuit 62. Power to the entire device is 
supplied by a power controller 68. 
The microprocessor means 66 performs in the circuit of the present 
invention (as schematically illustrated in FIG. 6) a number of functions 
including governing time intervals for gauging in both "standard" and 
"measure" modes. The microprocessor means 66 also serves the function of a 
counter and recorder operatively associated with the detector for 
separately counting and recording the measured radiation information from 
the detectors and the distance signal from the ultrasonic device. In this 
regard, the radiation information preferably takes the form of a total 
radiation count for each Geiger-Mueller detector per time interval. In 
other embodiments the radiation information may take other forms, such as 
radiation count rates. 
The microprocessor means 66 also serves to store the empirically derived 
constants a.sub.1, a.sub.2, a.sub.3, a.sub.4, b.sub.1, b.sub.2, b.sub.3, 
b.sub.4, c.sub.1, c.sub.2, c.sub.3, and c.sub.4 which are needed for 
calculating the density equation constants A, B and C for a given air gap 
distance. In addition the microprocessor means stores, in appropriate 
form, the instructions needed for taking the radiation counts from 
detector means 32 and for calculating therefrom, using the density 
equation (III) above, the correct density value and for displaying the 
density values to the operator. These operations are indicated in 
flowchart form in FIG. 8. From this flowchart, persons of ordinary skill 
in the programming of microprocessors can readily produce the specific 
detailed instructions required to perform these operations in a particular 
microprocessor. Other functions, generally known to persons appropriately 
skilled in the art, are also performed by the microprocessor. 
It is significant that the microprocessor means 66 operates in "real-time", 
so that the operator of the compactor vehicle rapidly receives a display 
of the density or degree of compaction which has been achieved. The 
"real-time" operation also provides a substantially instantaneous 
determination of the air gap distance, so that the correct density 
equation constants can be used in calculating density. When compacting hot 
bituminous paving mixes for example, the paving mix is somewhat resilient 
and spongy during the early stages of compaction, such that the rigid 
rollers of the compactor vehicle sink into the pavement to some degree. 
Later, as the paving material cools and hardens, the rollers of the 
vehicle do not sink as much. Thus the air gap between the measurement unit 
and the pavement surface varies during the compaction operation. A 
"real-time" instantaneous air gap measurement ensures that the density 
calculation remains accurate throughout the compaction operation. 
In the drawings and specification, there has been set forth a preferred 
embodiment of the invention, and although specific terms are employed, 
they are used in a generic and descriptive sense only and not for purposes 
of limitation.