The first application of cellular glass was a floating device. During the war, harbors must be protected with curtains against submarines. The Germans had occupied Portugal, the major source of cork and closed cell cellular glass was the best alternative.

Later on, cellular glass became popular as a thermal insulation and this is today the real market. But it can also used as an acoustic absorber, today sold under the name REAPOR®. An acoustic absorber avoids echo, which is for example always a problem in open offices.

A less known application is to use it as kinetic energy absorber. Kafka, the famous Czech writer invented the principle of a helmet during his work for an insurance company. But a helmet can not avoid that the brains have an internal recoil, although your head is well protected. This recoil is a consequence of the elastic behavior of the helmet.

But if this helmet should have been constructed from cellular glass, your head will crush the cellular glass but there will be no internal  recoil and brain damage would be less likely. The use of cellular glass as a kinetic energy absorber was already published in  US2981317 patent of 1961 about safety seats at line 49. The specification non-elastic crushable is clearly given on line 46.  The same is needed when a Formule 1 car looses control and hits the tires along the road. The recoil of the brains of the pilot can be the reason of medical problems afterwards although the outside damage can be relatively small. I am curious to know   what density the cellular glass needs to have and which thickness is requested to let stop a car at 120 km/h without even a headache  for the driver. I guess we will find reasonable values. The calculation can not be difficult: the kinetic energy of the car is used to crush the cellular glass = thickness * compressive strength * cross section of the car.  The maximum allowed deceleration gives the compressive strength you need. Solution will be given in a next post.

Cellular glass is in the first place known as the best  thermal insulation in the roof but is  less popular on (in) facades. And within the facade world, it was for a long time not supported as thermal insulation under a rendering. The reason for this is quite clear:

Most (if not 99%) rendering products for application on walls and thermal insulation contain cement as bonding agent. Cement based products hardens in a totally not flexible way and have a tendency to let shear off the cellular glass. During drying, the rendering shrinks and induces an enormous tensile stress on the cellular glass. In case the layer is thicker than a few millimeter, the cellular glass shears off because the tensile strength of the cellular glass is exceeded. In reality, a rendering must be at least 1 cm thick to get a long life and cellular glass became obsolete in this application.

(Portland) cement changed our life with its great strength and short settling time and let us create strong but still elegant constructions. But there was a time before Portland cement where almost pure hydraulic lime was used as bonding agent. Buildings made with this bonding agent were able to deform without creating a crack, they were (are) in a certain way flexible. Foundations were much smaller or non-existent while today a house without proper foundations will show important cracks when the masonry is performed with ordinary Portland cement.

Rendering, properly based on this old recipe, don’t shear off from cellular glass, even if the layer has a few cm thickness. A thick  layer with coarse grained sand is put on the cellular glass and the finishing is done with fine grained layer. The fine grained layer will dry the wet thick base layer by the difference in surface tension.

Both systems are a perfect solution for those customers who want a rendering on thermal insulation which resists a foot ball, feels like a real wall and does not burn. Both solutions are guaranteed by an ETA and are insured.