1-30. MOISTURE REFLECTION ERRORS

Figure 1-9. B/S Density and Moisture, 4" Transmission

TM-5-6635-386-12P Density and Moisture Tester Manual
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TM 5-6635-386-12&P Compaction is obtained by coating the surfaces of the soil particles with a thin, molecular film of water to provide lubrication during our compaction effort. This is the only moisture we wish to measure with the tester. Unfortunately, however, some soils have other forms of water known as "bound water" or water of hydration, which bind themselves into the mineral matrix and become a part of the soil particle. This form of water does not contribute to the compactive effort, but is measured by the neutron channel in the soil tester. A simple corrective technique can be used to cancel this error when detected. The soil tester will always indicate a higher than actual moisture value when this error exists. It will be advisable to make periodic checks with conventional testing means to verify that the problem continues to exist in a given embankment. An infrequent form of error is occasionally observed in desert countries where a high deposit of boron may result in absorption of the moderated neutrons producing a low moisture reading. This is highly unusual. A compensating calibration curve must be prepared using oven samples for this occurrence. 1-30. MOISTURE REFLECTION ERRORS Known as "trench wall effect", this error is the result of reflection of moderated neutrons from the surface of a nearby object to the tester, usually a trench wall or a backfill wall. Knowledge of how the tester is constructed will minimize this error and a simple test can be conducted to determine the magnitude of this error in the soil tester. The moisture detector is a long, 2 inch diameter tube lying along the long axis of the tester under the display area. Its most sensitive side is the side presented to the exterior of the tester on the display side. The least sensitive area is the ends of the tube and the side of the tester facing the operator as he views the display. It is proper procedure to place the tester so the broadside of the tube "looks down" the trench rather than at the wall. Conversely, the geiger tube used for counting is located at the end of the tester furthest from the source and lies across that end. It would be best that this end be placed looking away from a wall for density tests. Thus, good practice in a confined space would be to face the tester longitudinally in the direction of the trench for density testing and across the trench for moisture determination. The operator can make a simple experiment with his nuclear tester to determine the effect of the wall reflection. Set the tester in the center of a concrete floor, at least ten feet from nearby objects. Determine a moisture reading on that spot. Fill a two cubic foot box with damp sand or soil and slowly move this box towards the tester on each of the four sides, about 6-inches at a time, taking readings each time. Note where the box is when the readings produce an error sufficient to cause an unacceptable field density conclusion. This distance from the tester will differ on the four sides, and will differ between density and moisture. In general, the tester can be used in a trench of not less than 18-inches span, preferably about 24-inches wide, for best results on moisture. The density readings can be taken with the tester nosed directly into the wall, but the geiger tube end must be kept at least six inches from the wall. Conduct this experiment with your testers to determine your own limits in your shop. 1-31. ASPHALT CONTENT DETERMINATION The tester’s moisture channel is a hydrogen analyzer. 1-20

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