A nuclear gauge is used to measure the amount of compaction of a substance. In this case it is gravel being placed under and around a concrete culvert that will be buried beneath an exit ramp from a highway. It must be compacted to at least 95% of it's maximum capability so that it can carry the load of the road without settling or rutting.
The gauge contains a small amount of cesium 137 which is a radioactive material that is used to measure the density of an object. The gauge can also determine how much moisture is in the granular material as well.
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