Carbide Bomb (CM) Test
The Carbide Bomb test is also known as the CM test. It was designed to measure the moisture content of a wide range of products. However, it is used extensively for screed and concrete. The measurement is the percentage moisture of a given sample before the laying of sensitive floor coverings.
It can be used for all screed and concretes, even fast-drying screeds that act by chemically binding the majority of construction moisture where a relative humidity or electrical impedance device cannot be used.
Typical Target Measurements Are:
- Concrete/sand cement slabs/screeds: • 2.5% CM without underfloor heating installed • 1.5% CM with underfloor heating
- Anhydrite/Hemi-Hydrate screeds: • 0.5% CM without underfloor heating installed • 0.3% CM with underfloor heating installed.
The test is carried as follows:
- The sealed vessel should be thoroughly cleaned before each test is carried out. This will prevent any discrepancies with the results
- The CM test involves removing a sample of the slab/screed and crushing using a mortar and pestle, then weighing the required amount. The sample is then placed into the pressure vessel in a horizontal position.
- Calcium carbide reagent is then added to the vessel. The cap is inserted into the pressure vessel, and the clamp is tightened to seal the cap to the unit.
- The vessel is then turned to a vertical position and is vigorously shaken in a rotating motion for 20 seconds.
- It is then rested for 20 seconds and then continue again for a further 20 seconds, with the process continuing for 3 minutes.
- Calcium Carbide, when in contact with moisture, produces acetylene gas. The higher the moisture concentration, the more gas is produced, which is read as pressure from the devices gauge.