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Tips for Developing Statistical Limits for Light Weight Deflectometers

14. July 2017

Light Weight Deflectometers have come to replace the conventional methods used for compaction testing. Offering faster and more accurate measurements without causing a considerable drain on resources, the LWDs have turned out to be a boon to the paving industry. Having received enhanced traction among the industry, it is important to develop statistical limits for the LWD. Here is a quick look at some of the tips that will help to develop the limits that are important to understand the maximum permissible deflections.

Proper preparation of the test area

The material being tested should ideally be in full contact with the undersurface of the load plate. Material that is found to be loose and protruding needs to be removed. Areas that are not found to be even, need to be leveled by filling in fine sand. The test for developing the statistical limits should ideally be conducted when the temperature is not very cold, as this may cause errors in the readings recorded. It is important to ensure that the test area is larger in size, to the tune of 150% of the load plate.

The three seating drops

Three seating drops are mandatory during the testing. The free falling weights need to be raised to the release mechanism and the hammer needs to be permitted to fall freely onto the load plate thrice.  The weight need to be successfully snared or caught after it rebounds on impact with the load plate.  The three seating drops need to be followed up with three drops during which time the data will be recorded.

Reasons to consider invalidation

The test may be considered invalid if the conditions of testing are not met. The plate should not move laterally on impact. Any lateral movement of the load plate will automatically render the test invalid as the readings will go haywire. Similarly, the weight that rebounds post impact with the load plate needs to be caught cleanly. Any failure to do so, will render the test invalid.  An invalid test will then necessitate the development of a new test site, not less than feet away from the existing site. And readings that show a variation of more than 10% are indicative of errors, which will then require the tests to be conducted afresh.