Because Circle Redmont has had the highest, self-imposed quality standards for decades, the CR spec exists as an example to the building industry to adapt and deploy. Unfortunately, only Circle Redmont has been able to comply with this high standard. We have the level expertise, craftsmanship, technology and dedication that no one else seems to be able to reach.
To bring the rest of the industry even close to the CR spec, Fred Sandor of Circle Redmont has been a compelling force on the ASTM committee charged with developing and updating glass testing standards. ASTM International, originally known as the American Society for Testing and Materials (ASTM), is one of the largest voluntary standards development organizations in the world-a trusted source for technical standards for materials, products, systems, and services. Known for their high technical quality and market relevancy, ASTM International standards have an important role in the information infrastructure that guides design, manufacturing and trade in the global economy.
Laminated glass is often used in glass structures. Laminated glass is a type of safety glass that holds together when shattered. In the event of breaking, it is glazed or held in place by an interlayer, typically of polyvinyl butyral (PVB), between its two or more layers of glass. The interlayer keeps the layers of glass bonded even when broken, and its high strength prevents the glass from breaking up into large sharp pieces. This produces a characteristic “spider web” cracking pattern when the impact is not enough to completely pierce the glass.
Laminated glass is normally used when there is a possibility of human impact or where the glass could fall if shattered. Skylight glazing and automobile windshields typically use laminated glass. In geographical areas requiring hurricane-resistant construction, laminated glass is often used in exterior storefronts, curtainwalls and windows. The PVB interlayer also gives the glass a much higher sound insulation rating, due to the damping effect, and also blocks 99% of transmitted UV light.
Laminated glass is produced by bonding two or more layers of ordinary annealed glass together with a plastic interlayer, usually Polyvinyl Butyral (PVB). The PVB is sandwiched by the glass which is passed through rollers to expel any air pockets and form the initial bond then heated to around 70 °C in a pressurized oil bath. The tint at the top of some car windshields is in the PVB.
Multiple laminates and thicker glass increases the strength. Bulletproof glass is often made of several float glass, toughened glass and Perspex panels, and can be as thick as 100 mm. A similar glass is often used in airliners on the front windows, often three sheets of 6 mm toughened glass with thick PVB between them.
Toughened glass is made from annealed glass via a thermal tempering process. The glass is placed onto a roller table, taking it through a furnace that heats it above its annealing point of about 720 °C. The glass is then rapidly cooled with forced air drafts while the inner portion remains free to flow for a short time.
An alternative chemical process involves forcing a surface layer of glass at least 0.1mm thick into compression by ion exchange of the sodium ions in the glass surface with the 30% larger potassium ions, by immersion of the glass into a bath of molten potassium nitrate. Chemical toughening results in increased toughness compared with thermal toughening, and can be applied to glass objects of complex shape.
The term ‘toughened glass’ is generally used to describe fully tempered glass but is sometimes used to describe heat strengthened glass as both types undergo a thermal ‘toughening’ process.
There are two main types of heat-treated glass, heat strengthened and fully tempered. Heat strengthened glass is twice as strong as annealed glass while fully tempered glass is typically four to six times the strength of annealed glass and withstands heating in microwave ovens. The difference is the residual stress in the edge and glass surface. Fully tempered glass in the US is generally above 65 MPa while Heat Strengthened glass is between 40 and 55 MPa.
It is important to note that while the strength of the glass does not change the deflection, being stronger means that it can deflect more before breaking. Annealed glass deflects less than tempered glass under the same load, all else being equal.
