On the static fatigue limit of cellular glass

logo_smallThis topic is important when the strength of cellular glass becomes an issue. Cellular glass has a much larger strenth than organic foams and in a lot of cases, a glass foam is selected for this reason.

A very important basic paper in that perspective is written a long time ago by S. M. WIEDERHORN and L. H. BOLZ . They discuss the static fatigue limit of glass already in 1970. Recent measurements with an atomic force microscope are confirming the original ideas.

staticfatiguelimitBoth papers show that glass has a static fatigue limit, which means that below this limit, a crack does not grow at all. It is also shown that this limit is decreased with water on the crack tip. Soda lime glass is the weakest glass, borosilicate is better while aluminosilicate is the best. The following figure became the standard in this field.

As a consequence, cellular glass is able to resist a load for ever if the static fatigue limit is not exceeded. This safe limit can be measured with acoustic emission like shown in another paper.

The graph also shows that between very slow (or no) crack growth and fast crack growth about a factor 2 is present for soda lime glass. This could be seen as the material factor between the short time compressive strength of GLAPOR cellular glass and the possible load on the long term. 1000 kPa compressive strength ware resists in principle a load of 500kPa forever.

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PhD thesis at Rijksuniversiteit Groningen, The Netherlands by Cihan Tekoğlu

logo_smallIf we think about steel or wood or any other material, the dimensions of the sample should have only a negligible effect on the measurement result. The reason is that the sample is orders of magnitude larger than the grain size of the steel or the cell size of the wood. In other words, the continuum theory applies.

However, if we consider cellular solids with a cell size of the order of millimeters, a sample of a few centimeter may show side effect, influencing largely the mesurement.

In this theoretical thesis, the different calculation methods are discussed to handle this problem. This may be important as a theoretical base for writing standards, EN or ASTM, where the dimensions of the samples are defined. It gives also an answer  which deviations like holes can be accepted.

Master thesis at Imperial College, London by Song, Xiaoguang

logo_smallI found a first draft of this  work but I am still looking for the final version. The student did some foaming tests according to the powder method without clear description of the foaming agent. He investigated the effect of the particle size distribution with fast and slow heating and obtained remarkable results.

He worked with fine and coarse powder but also with a mixture. He found that mixture gives a higher density for the green sample.  But also that a much better foaming is present during fast heating of this mixture compared to the fine and coarse particle powder.

xongThe last observation is remarkable and he used the following picture to explain the phenomenon. It suggests that working with a mixture of fine and coarse powder is giving a better cell structure. I think this really a new idea: working with a two peak particle size distribution to improve the cell structure.

Nice review article from the university of Padova, Italy

logo_smallI already mentioned an interesting book with a very important chapter. The authors G. Scarinci, G.Brusatin and E.Bernardo made a nice work about the history of glass foams with the focus on glass recycling.

  • Different techniques are mentioned but the powder process is today the favorite method.
  • They make reference to a number of patents and are describing the currently employed methods.
  • A list of different starting glasses are given with emphasis on recycled glass
  • The particle size of powder and foaming agent are discussed in relation to the cell structure but unrealistic thermal conductivities are given.
  • Foaming temperatures and time are given with carbon black as foaming agent
  • The cooling rate is discussed without taken into account the thickness, which is not logic.
  • Different foaming agents are discussed based on thermal decomposition and by reaction. Even SO2 is mentioned as foaming gas.
  • The different glass foam products are listed: gravel, blocks and pellets with their properties.
  • Alternative processes are discussed with a chapter about foaming CRT-tubes.
  • Last but not least, an impressive list of references is present with many interesting patents.

This review article is a fantastic introduction for people interested how glass foaming improves ecology.

The happy marriage of cellular glass and natural hydraulic lime

logo_smallNatural hydraulic lime is much less known than cement although it is much older. It has the same function but sand particles, bounded with cement form stronger parts than bounded with natural hydraulic lime (NHL). However a wall, made from bricks and NHL lime is able to deform after many years without developing a crack. Walls made with a cement mortar always generate a crack in case there is deformation due to insufficient foundation. Today, most people have forgotten that stable buildings can be made with bricks and NHL-mortar without using any cement. A very good book, written by the Prof. Dr. Ir. Koen van Balen, KU Leuven is an introduction in this matter, although written in Dutch. It handles about all kinds of lime in comparison with cement and remembers us that NHL-mortars without any cement are the only tool to renovate old historical buildings. Indeed cement mortars on old bricks are inducing cracks in these bricks.

Cement mortars on cellular glass are always a risk due to the very high drying stress exceeding the tensile strength of cellular glass. Organic additives can be added to cement mortars to induce some flexibility or the cellular glass surface cells can be filled with an organic flexible material like bitumen before applying cement mortar.  NHL-mortars have even in the dry state a kind of plasticity, probably generated by micro cracks, allowing deformation of the walls of old historical buildings and avoiding too much drying and thermal stress on cellular glass. Such a mortar could be the TD1320, produced by Tassulo in Italy.

Such a mortar has a compressive strength exceeding 2.5 N/mm². GLAPOR high density cellular blocks with compressive strength > 2 N/mm², sawed on the dimensions of typical bricks can be mounted on the foundation of one stock houses with this TD1320 mortar to insulate the foundation cold bridge and to avoid diffusion of soil water into the wall.

Another application could be to cover and strengthen the cellular glass with a NHL-based coating in order to have a hard surface with a minimal thermal expansion mismatch. GLAPOR, to our knowledge the only supplier of 3m x 1.5m cellular glass plates, can in that case replace wood and other materials when weight or / and combustibility are an issue. This is typically the case in ships, air planes and other places with a low fire loading.

But last but not least, NHL-based rendering products are a perfect solution for the finishing of non-combustible outside thermal insulation like GLAPOR cellular glass. This was already mentioned in a previous post.

It is my personal believe that the cellular glass world has to replace any organic accessory by a ceramic equivalent, where the flexibility comes from load distributing micro cracks like products with an NHL-bonding agent.

Improving molds for glass foaming

Glass can be foamed in a mold or on belt (continuous foaming). A mold is typically madelogo_small from stainless steel because foaming temperatures may be up to 950°C. Heating transport at these temperatures is primary by radiation while stainless steel acts as a mirror for radiation, it has a low emissivity. On top of that, glass at 800°C has a tendency to stick on steel. This sticking can be eliminated by a coating based on kaolin or another fine refractory oxide but remains always critical. In case of even minor sticking, stripping the block from the mold may become difficult.
BELGLASCZ suggests to perform an experiment with a mold coated at the inside and outside with EMISSHIELD. This ceramic coating is nanotechnology with as primary function to increase the emissivity to 0.95 . It is available for refractory ceramics, steels and aluminium. The glass world knows this product today for the coating of the superstructure in glass melting furnaces to improve the energy efficiency. CNUD EFCO uses this product on stainless steel to improve the efficiency of the heat exchangers (patented) and to lengthen the life time of a NiCr heating coil (patented). An EMISSHIELD coated heat exchanger absorbs the radiated heat much better while the coating also protects against corrosion like published in Innovating flat glass lehrs.
An EMISSHIELD coated mold can be considered as transparent for heat and this should allow to work with a lower furnace temperature and more homogeneous heating. In this case, the flue gases will be colder and less energy will be consumed. We also expect that coating protects against the furnace atmosphere lengthening the life time of the steel. If also the walls of the foaming furnace are coated, we can expect an energy saving of minimum 5% next to a longer life time of the mold due a more homogeneous heating and protection against the furnace atmosphere. In an experiment, CNUD EFCO showed that float glass does not stick on coated heating wire at 1000°C and it may be expected that sticking of a glass foam on steel will be less likely. CNUD EFCO has all the equipment to construct and coat molds for the foaming of glass. More information can be obtained at hans.strauven@cnudefco.com.

A well documented paper about foamed glass gravel

Cellular Glass Aggregate Serving as Thermal Insulation and a Drainage Layer is a nice logo_small
article written by Andreas Zegowitz. It handles generally about the production, use and opportunities of foamed glass gravel (or cellular glass aggregate).The paper is rather objective because it mentions the leaching out of certain heavy metals,

Factory-made aggregates of cellular glass with a typical lump size of 10 to 75 mm represent a new type of thermal insulation with drainage properties being applied in Switzerland, Germany, and other European countries. Cellular glass aggregates are used as insulating filling material at the perimeter of buildings as well as under load-bearing foundations. They can serve as insulation drainage layers of garden roofs. The insulation material is manufactured from recycled glass and mineral additives in a thermal process. The aggregates form when slabs of cellular glass crack while cooling down. In order to obtain the required hygrothermal properties, the manufacturing process must be carefully controlled. Despite its low density of approximately 120 to 250 kg/m³, cellular glass aggregate has a high pressure resistance, absorbs hardly any water, and is fireproof. The expected service life is at least 50 years. Since its first appearance on the market, this insulation material has been thoroughly tested and the effect of water clinging
to the aggregate has been investigated in the laboratory. To confirm the assumptions, the average moisture content and the thermal conductivity of the material in service was also determined by material sampling on existing buildings. This paper gives an overview of the different tests that must be performed in order to obtain a German and European Technical Approval. It summarizes the aggregate properties of different manufacturers and reports the practical experience gained by in-situ investigations.