When the numbers don’t make sense…

Whilst carrying out shielding integrity testing for a client, measurement results indicated that the ceiling – which doubles as the floor for the room above – had the shielding equivalent of 3 to 4 cm of concrete. This made no sense!

The results indicated that the ceiling/floor is either very dense wood or very thin concrete. Both though, would be equally unlikely – wood is not renowned for its shielding properties and a very thin concrete floor (the thickness of a garden paving slab), would be structurally insufficient. But you can’t argue with the numbers!

The picture below shows what materials were actually present once the floor boards were lifted. Wooden joists infilled with a plaster/screed made from a weak lime mortar and straw mix varying between 50 – 90 mm in across most of the floor. The combination of wood and screed result in the concrete equivalent figures produced. .

Mystery Solved.

The infill is known as pugging, which is very early acoustic separation between floors and also has some very minimal fire resistance. It becomes more effective over time as it is usually laid loose and dry, then it compacts and hardens. Unfortunately, it is rather limited in terms of providing radiation shielding.

Shielding Integrity Testing
/ʃiːldɪŋ ɪnˈtɛɡrɪti ˈtɛstɪŋ/

verb

A service provided by Aurora to determine, through non-destructive means:

the location of any gross weaknesses in radiation shielding, and
the amount of radiation shielding present; quoted as an equivalent thickness of concrete or lead.
“Aurora were able to demonstrate that the radiation shielding installed was the correct code lead – but also discovered that there was a gap in between the lead sheets!”