The Backscatter Density Method is very similar to the Direct Density Transmission Method except that the rod is only lowered to the first notch position, also known as the backscatter notch.
a. The backscatter notch positions the bottom of the source rod just above the surface.
b. It is not necessary to drill a hole for backscatter tests.
c. Backscatter tests are usually made just for asphalt.
d. The radiation emitted from the bottom of the source rod can generally only penetrate the depth of the asphalt to a few inches before being reflected back to the detector tubes.
e. Regular backscatter measurements can be used to measure thicker lifts of asphalt.
f. Some manufacturers incorporate a thin lift mode that allows you to focus your measurement on a distinct depth of asphalt, say 1 ½ or 2 inches. The thin lift mode will aid in filtering out the density levels below the chosen depth.
The gamma rays pass through the material and are counted by the detector tubes or they are absorbed, never making it far enough to be counted.
a. The denser the material becomes under the gauge the lesser the amount of gamma ray radiation there is to be counted.
b. The material becomes more compacted, and therefore more dense, with each pass of the roller.
c. You can track this densification by measuring after each pass of the roller.
Remember that radiation is present at all times and travels in all directions. The gauge is designed to detect and interpret that portion of the radiation that travels to the detector tubes. Individuals must be aware that radiation is also traveling undetected in other directions, including areas that they may be occupying. That is why you will want to maintain a distance that is safely outside of this radiation zone.
The gauge works by counting the radiation that is received over a given period of time, generally one minute. So when you push the button to take a test the gauge counts the radiation for one minute and calculates the density based on the depth of measurement. Remember, even though the test has finished, the radiation is still present.
In a general sense, compacting asphalt is not all that different than compacting soil. You are moving new material to a given surface and then compacting that new material. With asphalt, you just need to think of the aggregate as soil and the asphalt binder as the moisture.
Asphalt moves through the screed and is placed in a uniform nature on a surface that is being prepared as a roadway. The type of road has already been determined and the design criteria have already identified the aggregate blends and liquid bitumen. Lab tests (Marshall for percent asphalt and Rice for specific gravity) will give you the target values that should be used in the gauge. You need to compact the asphalt to a predetermined and/or specified compaction level.
For example, specifications may call for you to compact the asphalt mat to 95% of the target value (Rice Test). As you begin to roll the asphalt you will want to place the gauge on the mat and test for compaction. Every time you test you will be looking at a result based on a percentage of your lab standard (Rice Test). Keep rolling the asphalt until you achieve your desired level (95%). If it takes 5 passes you will now know that your rolling pattern is 5.
When working with a gauge on asphalt it is wise to follow a few guidelines:
1) Always have as much predetermined information already entered into the gauge. If applicable, have the following information pre-loaded:
a) Date & time
b) Target value (Marshall or Rice)
c) Test length (Typically one minute)
d) Project number
e) Special calibrations
2) When testing on asphalt always place the gauge long-ways in the direction of the pavers. This will reduce the chance that unseen creases will impact your reading.
3) Always take 3-4 tests and average the results at each testing site. This will give a more representative indication of density.
4) If the mat has a lot of surface voids you may want to fill in the surface with fine sand or sediment. This will dampen the effects of the voids.
5) Always make sure that the source rod is well seated in the backscatter position notch. Any misalignment will impact the results.
6) Always practice good ALARA by returning the source to its safe position before recording your results.
7) Always practice good ALARA by maintain your distance during the test.
8) Always practice good security by never taking your eyes off the gauge while in operation. This will prevent possible damage from heavy equipment.
9) Never take a test while within 30 feet of another gauge.
10) Never take a test adjacent to large vertical objects, including vehicles and people.
11) After use, always return the gauge to its case and secure the case to the vehicle.
12) You can do a quality control test for your gauge by comparing your gauge readings to a core sample. If needed, you can do a simple offset of the gauge’s result to match those of the core results.
13) After the test, never leave the gauge sitting on a hot asphalt mat. Doing so might “cook” the electronics.
If you are performing work for the state you will probably be required to take and pass that state’s certification program. Some regions of the country have certification programs that cover working in all states of that region.
Reasons for Errors in Gauges
There may be nothing more frustrating than a gauge that isn’t measuring correctly and there can be plenty of reasons for malfunctioning gauges:
a. Environmental factors – Environmental errors are typically out of the control of the operator.
1. Natural hydrogen content – Some soils contain naturally bound hydrogen. The gauge views this natural bound hydrogen as moisture. The natural hydrogen may give a false low dry density reading, which in turn can lead to false low percent compaction. The Proctor procedure may likewise be fooled by the hydrogen. An additional oven dry or microwave test will be needed to recalculate the values. They can be time consuming but necessary.
2. Bad luck of the draw – Some DOT’s specify the spot, through random selection, the exact spot where a test must be performed. If a “soft” spot is selected it can give a bad representation of the overall job. But, then again, some believe that the job is only as good as its weakest spot.
3. Trench & vertical object errors & corrections – Sometimes you’re put in a spot where there is no escape from adjacent vertical walls or objects. These surfaces can reflect back the neutron radiation and give a higher moisture count than actual. You can correct for trench factors by performing a standard count on a normal surface setting and compare to a trench standard count. The difference should be subtracted from the actual trench moisture count.
b. Operator errors – All gauges have small degrees of errors evident in their systems. You can never achieve a perfect level of precision and accuracy because of the very slight mechanical imperfections and electronic drift. But the last thing you need is additional errors due to operator oversights. Don’t compromise your readings due to these errors:
1. Make sure the source rod is well seated in the depth notch.
2. Make sure, once the source rod is extended into the hole, that you push the gauge and inserted rod against the side of the hole. This will eliminate any open air space between the rod and soil.
3. Make sure the base of the gauge is sitting flush against the ground surface. Use the scraper plate to properly smooth the ground, removing any protruding objects and filling any air voids.
4. Make sure to check for active offsets stored in your gauge.
5. Make sure you do your standard counts every day you use the gauge.
c. Other errors:
1. Don’t skimp on the gauge calibration. If you want a truly calibrated gauge send it to a service center that uses a minimum of a 3 block calibration. Lesser verification devices can only serve to increase error in your gauge. Too much is riding on the results of your gauge to allow cut-the-corner calibration devices or methods.
2. Gauges are subject to wear and tear. A good service center will check for and replace worn parts. Likewise, circuit boards, detector tubes and batteries can all impact the accuracy and precision of your gauge. These items should be checked during a gauge calibration.
Many government agencies and engineering firms will send inspectors to the job to verify specified compaction. Don’t let errors in your methodology compromise your work to the point that the inspector’s findings do not match specifications.
The following is an example of test methods:
Sample Test Methods for Determining Percent of Moisture and Density of Soils and Asphalt
This method covers the procedure to be used in determining the percent of moisture and density of soil embankments, base, sub-base, and select materials, and the percent density for asphalt concrete.
The apparatus required shall consist of the following:
A. Portable Nuclear Moisture-Density Gauge
B. Transport case (Type “A” Package)
D. Reference Standard Block
E. Transport Documents (Bill of Lading)
F. Leveling Plate/Drill Rod Guide
G. Drill Rod w/extraction tool
H. Hammer (4 lbs.) used for Driving the Pin
I. Safety Glasses
J. Square-Point Shovel
K. No. 4 sieve
L. Set Balance Scales
M. Drying Apparatus
N. Miscellaneous Tools such as Mixing Pans and Spoons
There are two different methods to determine percent density and percent moisture using the portable nuclear density gauge. The methods are direct transmission and backscatter.
The direct transmission method requires punching a hole into the surface of the material being tested and lowering the source rod to the desired depth of test. This method is used to test soil and aggregate materials. Please note that when testing soils, the backscatter position shall not be used as a means of acceptance for density.
In the backscatter method, the source rod is lowered to the first notch below the safe position placing the source and detectors in the same horizontal plane. No hole is required for the probe since it is flush with the bottom of the gauge. This method is used to test aggregate (sub-base and base course) and asphalt materials.
The Roller Pattern is performed first. The purpose is to determine the number of passes to be made by the roller in various combinations of static and/or vibratory rolls to achieve maximum density for that depth of material using that roller. The data collected from the gauge when properly plotted, will provide a graphical comparison of the number of roller passes necessary to produce a properly compacted product. Once completed this information is used to establish a Control Strip(s).
The Control Strip determines the target values for density that will define the acceptance criteria for the material placed and compacted using the previous determined roller pattern. The values determined by the control strip will not change until a new roller pattern is required. The Control Strip provides an accurate method of evaluating materials, which are relatively uniform and exhibit smooth surfaces.
The Roller Pattern is constructed on the same material being placed and once established, will be used for the remainder of the project. The Roller Pattern is 75 feet in length plus some additional area to accommodate the lateral positioning of the roller. The width and depth of the material depends on the projects design.
Listed below are the steps used to construct a Roller Pattern:
1. Establish an area at least 10 feet from any structure, and 33 feet from other radioactive sources (another gauge) to take standard counts. This area can be concrete, asphalt, or a well compacted soil with a minimum density of 100 lb/ft3. Do not set the gauge on truck beds, tailgates, tabletops, etc when taking a standard count. Turn the gauge on and allow it to warm up. At this time, standard counts can be taken and recorded.
Note: A standard count will be taken each day of use. If count fails, refer to the gauge
Manual of Operation and Instruction guide for further instructions.
2. To prepare a Roller Pattern, place the material on a section of roadway approx. 75 feet in length for the typical application width (an area of at least 100 yards), and at the proper loose depth before any rolling is started. (The Contractor should be allowed to place 100 feet of material prior to the 75 foot section for plant mix stabilization, adjustment, and compaction purposes, with testing to be conducted at the completion of the roller pattern.)
The compaction is to be completed uniformly and in the same manner for the remainder of the job. (It is also recommended that a 50 foot section be placed before and after the roller pattern section for positioning of the roller.)
The moisture content of aggregates should be kept as near optimum as possible throughout the rolling operation. Water must be added when needed to maintain optimum moisture. To speed up operations, select the 15-second mode on the read out panel and record the density and moisture readings. When testing the control strip and test section, select the 60 second mode for acceptance.
3. Make two (2) passes (1 pass is counted each time the roller crosses the test site) with the roller over the entire surface of the Roller Pattern. Make sure the previous passes have been completed over the entire surface before the next pass is started. When testing asphalt materials, take a nuclear test for density only, using the Backscatter Method. The above test on aggregates and asphalt materials should be made at three randomly selected points within the area to be tested.
Choose points with good surface conditions, and try to spread the 3 tests over most of the 75 foot section, making sure not to place the gauge closer than 18 inches to an unsupported edge. Be sure to mark the exact location where the gauge is placed. (If using spray paint to mark the locations, do not spray the gauge with paint.) The gauge, when in use, shall always be positioned parallel with the roadway, with the source end toward the direction of the paver.
Obtain the total and average for both moisture and density.
All further tests for the Roller Pattern must be made in the same 3 locations, with the gauge source rod pointing in the same direction as the first test. Plot the average dry density versus the number of roller passes on the graph.
4. Make additional passes with the roller over the entire surface of the Roller Pattern, and again obtain and record the 3 readings for density and moisture in the same location as the previous set of readings. Calculate the average from the readings.
Continue the rolling and testing of the section until the Roller Pattern reaches its maximum density before decreasing or the curve levels off. To be certain this is a sufficient degree of compaction, make one additional roll over the entire surface and test again.
Note: The number of passes that are indicated do not necessarily have to be set at two (2) every time. It may be found that in some instances one pass would be sufficient between readings and, in other instances, 3 or 4 passes would be required. An accurate count of the required passes should be maintained and may vary, depending on sub-grade conditions, roller efficiency, type of materials and moisture content.
Note: When testing aggregates, upon completion of the control strip, perform a direct transmission test to validate that compaction has been obtained comparing the result to AASHTO T-99.
Notes on determining Maximum Attainable Density with Roller Pattern/Control Strip Technique
The Control Strip shall be rolled until maximum dry density for granular materials or maximum density for asphalt materials is obtained. Materials compacted to maximum density provide a solid platform on which to construct pavement. Materials at maximum density increase pavement load carrying capacity and pavement life. Opportunities for future pavement distress will be greatly decreased.
In the interest of good construction procedure and practice, the inspector should use these guidelines to the best of his/her ability.
In brief, the change in density in a typical Roller Pattern, for example, on Aggregate Base Material, Type I, Size 21B, may look as shown below:
|Number of Passes
|Change in Density, lb/ft3
It can be seen from the above that continued rolling after 10 passes resulted in diminishing returns. This is typical for many Roller Patterns. Based on an analysis of this type, the following is recommended as a guideline for granular materials.
In the event that the increase in dry density for a Roller Pattern on granular material is less than 1 lb/ft3 one additional pass shall be required.
For asphalt base, the same guidelines as for granular materials should be used, with the exception that after the increase becomes less than 0.5 lb/ft3 per pass, one additional pass shall be required. If the density does not increase by 1 lb/ft3 with the additional pass, rolling should be discontinued.
Occasionally, there will be instances where a decrease in density rather than a small increase will occur. This usually occurs for two reasons: a false break, where the density levels off well before maximum density is achieved, and over rolling. In this case, consideration should be given to the number of passes already made and the materials involved, making certain that the break occurring in the Roller Pattern curve is not greater than 1.5 lb/ft3.
When the break is greater than the above value, re-compact the material to the maximum dry density based on the peak of the roller pattern.
A new roller pattern should be established whenever there are multiple lifts of material or there is a change in the following:
Source of material
Visual change in subsurface conditions
Gradation or type of material
Nuclear Density Gauge
Test section readings are significantly above the target values by more than 8 lb/ft3
Another Control Strip will be established.
To prepare a Control Strip, an additional 300 ft. of roadway is required extending from the Roller Pattern area (same spreaderbox width at the same designed depth). This area is to be rolled the same number of passes from the Roller Pattern.
In order to determine the maximum dry density of the Control Strip, 10 readings for density and moisture should be performed and recorded over the entire 300 ft. section. Calculate and enter the data. The Target Values of 98% and 95% of the average dry density can now be determined. The dry density determined from the average of the Control Strip should compare within 3 lb/ft3 of the roller pattern’s maximum dry density. This applies to both aggregate and asphalt materials.
Note 1: When testing Asphalt Concrete, the gauge should be programmed to the asphalt mode.
Note 2: When testing aggregates a verification test will be performed at the completion of the control strip using the direct transmission method or other methods approved by the engineer.
To complete a test section, 5 readings are required. Each test section for asphalt concrete will be one quarter mile in length for the full width of the roadway or one half mile in length or half the width of the roadway. Each test section for aggregate base, sub-base, and select materials will be one half mile in length per application width. The length of test sections for shoulders will be the same as the mainline. If possible, test alternating sides.
Five (5) readings will be made on each test section for both density and moisture using the same method of test used on the Roller Pattern and Control Strip. Rolling is continued until none of the 5 readings is less than 95% of the Control Strip density, and the average of the 5 readings is equal to or greater than 98% of the Control Strip density.
This does not apply to aggregate shoulder material, which requires an average density of 95 ± 2 percentage points of the control density, with individual densities within 95 ± 5 percentage points of the control density. No other test will be required, unless specified by the engineer.
When test section readings are significantly above or below the target values by more than 8 lb/ft3, another Control Strip will be established.
When testing turn lanes, acceleration lanes, deceleration lanes, and crossovers, take 2 or 3 readings on each, whichever is needed, to complete the full test section.
Note: For sections of roadway less than 900 feet, the direct transmission method or other approved testing methods for density determinations may be used. If obvious signs of distress are observed while rolling, cease rolling and evaluate the area of distress. Such signs include cracking, shoving, etc. Structural failures will cause the gauge to give an erroneous reading indicating more compaction is needed, when actually over-compaction is causing the failure. If this situation occurs, it should be brought to the attention of the engineer.
Note: When taking tests for Asphalt Concrete only record the wet density from the gauge.
Direct Transmission Method
Establish an area at least 10 feet from any structure and 33 feet from other radioactive sources (another gauge) to take standard counts. This area can be concrete, asphalt, or a well compacted soil with a minimum density of 100 lb/ft3. Do not place the gauge on truck beds, tailgates, tabletops, etc when taking a standard count.
Turn the gauge on and let it warm up (10 minutes). At this time, standard counts can be taken and recorded.
When testing soil, level off an area with the scraper plate. The surface of this area should be as smooth as possible. Care should be taken not to additionally compact the surface during its preparation.
All density tests on embankments and sub-grade will be tested using the direct transmission method.
Place the guide plate on the surface. Make a hole in the material with the drill rod, using the guide plate to be sure the hole is straight and vertical.
Extend the rod to the desired depth of test making sure the device is sitting flush on the surface and the rod is pulled back tight against the back side of the hole. Take a one-minute count in this position.
When the test is complete record the results.
If the material tested is represented by a predetermined proctor test the dry unit weight should be entered into the gauge as a target value. This allows the gauge to calculate the percentage of compaction.
When it is apparent that the material being placed is different from the material that is described, such as color, texture, rock size, etc., another proctor may need to be performed.
In the event the material contains appreciable amounts of material retained on the No. 4 sieve a correction shall be performed to determine the correct Proctor Density.
If the material being placed is determined to be “rock fill” make an entry showing location and elevation of rock.
Aggregate material shall be compared to the theoretical maximum density. The density shall conform to the following:
|% Retained on No. 4 Sieve
|Minimum % Density
Note: Percentages of material will be reported to the nearest whole number. The requirements for percent density referenced above apply only to the direct transmission method for aggregate.
Background Calculations for Trench and Sidewall Moisture Testing
When a gauge is operated within 24 inches of a vertical structure, the density and moisture counts will be influenced by the structure.
Due to the hydrogen-bearing materials in trench walls, on occasion, a higher moisture reading will be observed when testing backfill materials around pipe, culverts, abutments, etc. It is necessary, therefore, to determine the “background” effect and apply this correction to the observed moisture count readings. The background correction count should be determined each day of testing and when trench wall conditions (distance from wall, moisture content,
material composition, etc.) vary.
Moisture in certain soil properties containing high amounts of hydrogen rich compounds such as ash, mica, organics, cement, boron and cadmium, will give inaccurate readings and as a result a moisture offset should be performed. The moisture offset should be a minus for ash, mica, organics and cement and a plus for boron and cadmium. Other alternative methods to determine moisture content are the speedy moisture tester and hotplate method.
The procedure to determine the background effect and apply the necessary correction is as follows:
Take a standard count with the gauge on the standard block outside the trench and record these values.
Place the gauge on the standard block inside the trench in the testing area and select trench offset. The density and moisture trench offset constants will be calculated and stored. Always disable the offset when the gauge is not being used for trench measurements.