Carbide Bomb Test

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.