TEMPERED GLASS :

Tempered glass (also known as toughened glass) is an extremely strong glass which has been thermally heat treated to induce compressive stresses of 10,000 to 20,000 psi on the surfaces and edge compression of not less than 9700 psi.

Tempered glass is being used increasingly in architecture because of its strength and safety properties. It is usually installed in areas where safety glass is required to reduce the possibility of mechanical or thermal breakage and/ or to assure greater uniform load strength. Worldwide the use of Tempered glass or Safety glass in general has been mandated by building codes. These codes are also made for India and are in the process of being mandated.

Cut to size glass sheets are electrically heated in the furnace where it oscillates back and forth on rollers till glass reaches temperature of about 650°C. After heating, the glass is conveyed to a quenching chamber where it is cooled rapidly by a blast of air, blown through nozzles, on both sides of the glass.

Characteristics of Tempered Glass:

Strength : Toughened (or Tempered Glass) is four to five times stronger than its equivalent thickness of normal annealed float or sheet glass.

High Thermal Shock Strength : Tempered Glass provides greater thermal strength. It offers increased resistance to both sudden temperature changes and temperature differentials up to 240°C compared to normal annealed glass which can withstand temperature differentials up to 40°C only.

Safety : Tempered Glass is very difficult to break but even on breakage, it will break into small, relatively harmless fragments. This substantially reduces the likelihood of injury to people as there are no jagged edges or sharp corners which are normal in the case of breakage of annealed glass. Due to the inherent superior features of Tempered Glass, like more strength, ability to withstand sudden impacts and breaking into small pieces, it is considered as a safety glazing and so in most countries, use of Architectural Toughened Glass is mandatory for entrances in public areas or where it is to be used by general public.

Others : The Tempering process on normal glass does not alter the light transmission and solar radiant heat properties of the glass. Tempered glass cannot be further cut, ground, drilled or worked upon. Tempered Glass is prepared by completing prior to its tempering, all such works in accordance with drawings, sizes or templates.

HEAT STRENGTHENED GLASS

Heat strengthened glass is similar to tempered glass except that after heating the glass in a furnace as in the case of tempered glass, the cooling is done at a much slower pace. The heating temperature remains nearly the same for both processes. The residual stress on the edge and the glass surface differ in both cases with tempered glass having a higher level of stress. Due to the relatively lower rate of cooling during the strengthening process, heat strengthened glass develops less stress as compared to fully tempered glass.

Characteristics of Heat Strengthened Glass:

Strength: The process increases the mechanical and thermal strength of heat strengthened glass, making it twice as tough as annealed glass but nearly half of fully tempered glass. However, its breakage characteristic is similar to annealed glass, which means that it breaks down into sharp pieces like ordinary glass even though it has been put through the heat strengthening process.

Optics: Heat strengthened glass has a comparatively flatter finish than fully tempered glass. It therefore has lesser optical distortions and so can be used in places where high optical quality is required.

Safety: It can be used for general glazing where additional strength or resistance to mechanical/thermal strength is desired. The glass can also be used in high wind load areas, but cannot be used in any safety glazing applications. As Heat Strengthened glass breaks in a manner similar to that of float glass, the fragments tends to remain in the sashes, having less probability of fallout as compared to tempered glass. In comparison with tempered glass, heat strengthened glass is not easily broken spontaneously.

High Thermal Shock Strength : Heat Strengthened Glass will resist temperature difference of up to approximately 130°C.

LAMINATED GLASS

Laminated glass is two or more panes of glass with one or more layers of polyvinyl butyral (PVB) or ionomers sandwiched between them and treated. This sandwich has some unique advantages. The outer glass surfaces are hard and transparent but are strongly bonded to a transparent rubber like material on the inner surfaces of the sandwich. The glass panes can be basic float glass or tempered or heat strengthened panels. If the glass is broken, fragments remain strongly adhered to the PVB interlayer thereby nearly eliminating the risk of injury from glass and helping to resist further impact or weather damage as it does not generally fall out of the fixing.

The most common use of laminated glass is for the front windscreens of all cars. It is now finding more usage in architectural applications because of its distinct characteristics.

Characteristics of Laminated Glass :

Safety : Laminated glass doesn’t shatter like ordinary glass. It absorbs impact, resists penetration, and remains intact even if broken, holding glass fragments in place and lowering the risk of injury. Global building standards increasingly specify stricter safety requirements, especially for overhead glazing where any breakage could mean a major hazard from falling glass.

Security : The rise in urban crime and terrorism also points to laminated glass as increasingly desirable as a material of construction. Laminated glass resists intrusion because the interlayer continues to safeguard the building even after the glass itself is broken. Security glass cannot be cut from only one side, so ordinary glass-cutters are useless as break-in tools.

Laminated glass tends to resist impact. In multiple configurations, it can even resist bullets, heavy objects, or small explosions. In most cases, it takes many blows, all in the same spot, to penetrate the glass.

Sound Reduction : Laminated glass is an excellent barrier to noise. The sheer damping performance of the plastic interlayer makes laminated glass an effective sound control product. This makes it ideal for airports, hotels, data-processing centers, recording studios, and any building near airports, highways, or train lines.

UV Control : Ultraviolet light is the leading cause of deterioration and fading of furnishings, pictures, and fabrics. Laminated glass screens out 99% of the sun’s damaging rays, protecting interior furnishing,displays or merchandise from fading effects of UV radiation.

Earthquakes & Natural Disaster Protection: Earthquakes often produce a fallout of extremely dangerous shards of broken glass. But laminated glass remains in the frame, maintaining a protective envelope around the home or building to keep the weather out and deter glass shards from flying. Similarly, the heavy winds of tornados and hurricanes easily shatter conventional glass, causing injuries from flying fragments and damage to interiors exposed to the devastating weather outside.

Durability: Laminated glass retains its colour and strength for the life of the building, yet is as easily cleaned as any conventional glass.

Fire Retardacy : While standard laminated glass does not meet code requirements for fire-resistant glass, it will not disintegrate readily when exposed to heat, so it confines the fire longer, giving more time for evacuation and control of the fire. It is important though that in case of complete glazing with laminated glass, some panels for fire escape should be glazed with tempered glass with a prominent sign “Break in case of fire”.

Insulated Glass

Insulated glass is a prefabricated unit made of two or more glass panes, which have been separated by an air gap and edge-sealed together. This is also known as double glazed unit (DGU) or Insulated Glass unit (IGU). The two panes of glass are kept separated by a hollow aluminium tube spacer filled with a moisture absorbing material (desiccant) and is hermetically sealed to ensure that the air or gas in the cavity or hollow space between the two glass panes does not escape during hot climate, or outside air is not sucked in during winter or cold climate as a result of expansion & contraction of trapped dry air or gas due to change in temperature. This edge seal not only binds the individual sheets of glass together to maintain the mechanical strength of the joint but also protects the space between the glasses from outside influences. The air enclosed between the two glass panes is dried with a desiccant. Because of the low heat conductivity of the enclosed dry air between the glass panes, heat transmission through the window is drastically reduced. The sealants delay the moisture getting into the air space and the desiccant absorbs the moisture coming through the seal and during the manufacturing process. Insulated glass can also be made with three panes of glass & two air gapes. This type is known as triple glazed units & are used only in regions having extremely cold climate.

Characteristics of Insulating Glass :

Insulation : Saves on heating and cooling, by reducing air to air heat transfer. The enclosed layer of air makes the insulating capability about twice that of monolithic glass. Because of this, the load on the air conditioning is greatly reduced. The use of heat absorbing or heat reflective glass will further reduce the load on the cooling system.

Prevention of Dew Condensation : With monolithic glass, the temperature difference between the outside and inside of a room will often lead to condensation. However, the insulating effect of the air layer makes it difficult for the glass to become cold enough to have condensation on its surface giving a pleasant view through in rainy or high humidity climates.

Retards Sound Transmission : Insulating glass can significantly save on exterior noise pollution. The amount of sound reduction depends on the combination of the insulating glass. Using one of the panes as laminated glass will drastically reduce sound transmission. Using dissimilar thickness of glass panes will also help combat noise.

Pleasant Room Temperature : It offers increased personal comfort and aids in energy conservation. Because of its high insulation properties, the lack of cold or warm droughts leads to a pleasant internal environment even when seated close to a glass surface.

Strength : DGU glass will marginally increase the overall strength against wind load pressure.