In the design and use of architectural glass, the responsible design professional must consider carefully the performance characteristics of the six basic types of glass and their differences as they relate to the various construction requirements.
Types of Glass
In addition to annealed float glass there are five other types of float glass available: heat-strengthened glass, fully tempered glass, laminated annealed glass, laminated heat-strengthened glass and laminated fully tempered glass. These glass types can be used individually, or in combinations, for various architectural applications. Each has its own specific properties and performance characteristics that can be related to the requirements established by the design community.
Annealed glass has the surface strength that provides the wind-load performance and thermal-stress resistance needed in most architectural applications. In areas of high wind loads, or in conditions where higher-than-normal thermal stresses occur, heat-treated glass may be required.
Annealed glass in standard thickness does not meet the safety glazing standards of the Consumer Product Safety Commission (CPSC) 16 CFR 1201 or the American National Standards Institute (ANSI) Z97.1.
ASTM C1036 “Standard Specification for Flat Glass” is the standard that specifies the required thickness, dimensional tolerances and characteristics of annealed glass.
Heat-strengthened glass is produced by heat-treating annealed glass under regulated thermal conditions. In this process, annealed glass that has been cut-to-size is carefully heated in a furnace that is controlled between 1100–1500 degrees Fahrenheit (593–815 degrees Celsius) and then quickly air-cooled. This sudden cooling causes a compression envelope around the glass surface and edges, along with a balanced tension stress within the glass itself. This equilibrium of stresses increases the strength of the glass to approximately two times that of the original annealed product when tested under uniform pressure such as wind loads. In addition, when broken, glass that has a low to moderate degree of heat strengthening will generally exhibit few cracks and tends to break into large pieces that initially may remain in the glazed opening. (Note: glass should be removed and replaced as soon as possible after breakage.) As the degree of heat-treating increases, the break pattern of the glass will more closely resemble that of tempered glass.
A significant advantage of heat-strengthened glass is its ability to withstand high thermal stresses due to partial shading and heat build-up from solar loading. With its edge compression levels in excess of 5500 pounds per square inch (38 MPa) and surface compression levels in the 3500 to 7500 psi range, heat-strengthened glass has performed well in demanding architectural applications, such as in direct contact with insulation or with dark applied frit (durable, colored ceramic material) in spandrel areas. This ability to withstand high thermal stresses, and its wind-loading resistance, make heat-strengthened glass a preferred choice in many architectural applications.
The increased toughness of heat-strengthened glass also reduces the likelihood of glass breakage during shipment, handling, installation and in-service use. Heat-strengthened glass, because of its break pattern, does not meet the safety glazing standards of CPSC 16 CFR 1201 or ANSI Z97.1.
ASTM C1048 “Standard Specification for Heat-Treated Flat Glass” is the standard that specifies the required tolerances, characteristics and compression levels for heat-strengthened glass.
Fully Tempered Glass
Fully tempered glass is created in a process that is similar to heat-strengthened glass. Cut-to-size, annealed, float glass is heat-treated and air-cooled, creating an edge compression greater than 9700 psi (67 MPa) and a surface compression greater than 10,000 psi (69 MPa). Fully tempered glass may show more visual distortion of reflected images than heat-strengthened glass. Its key performance characteristics are increased strength and the ability to meet the requirements of safety glazing standards (i.e., CPSC 16 CFR 1201 or ANSI Z97). Fully tempered glass when fractured tends to break into small irregular shaped fragments that meet the criteria of the aforementioned safety glazing standards.
Heat Tempered Glass - Relative Strength
The strength of heat tempered glass is proportional to the surface compression introduced by processing. A surface compression level of 10,000 PSI yields a part that is approximately twice the strength of an equivalent heat strengthened part and four (4) times that of an annealed glass part of similar thickness, size and fabrication processing when placed under uniform static pressure loads.
Heat Tempered Glass - Mechanical Properties
Bow and warp are introduced due to the slight differences in the heating and quenching parameters of the glass surfaces. These differences are also influenced by tinting of the material, coatings and the fabrication of holes, slots or notches in the glass. Thinner materials as well as larger sizes of glass will have more bow and warp introduced than thicker materials and smaller parts. This increased bow and warp is due to the parts heating and cooling from the edges as well as the surfaces and in addition, heating and cooling airflow creates uneven boundary layers on the surfaces during heat tempering.
Under uniform static loads, fully tempered glass is about four times stronger than annealed glass of the same thickness, and twice as strong as heat-strengthened glass of the same thickness. It also has significant resistance to breakage by blunt projectiles. The increased strength of fully tempered glass (due to its compression stresses) makes it an option for almost any exposure.
The increase in compression stresses and equilibrium center tension stress in fully tempered glass also contribute to infrequent occurrences of spontaneous breakage (see related article April 1998 USGlass magazine page 66). All heat-treated glass will break when the compression layer is penetrated. Surface or edge damage, which does not completely penetrate the compression layer, may be propagated by thermal or wind loads, building creep and static fatigue, resulting in spontaneous breakage. This breakage may occur days or even months after the initial damage, therefore the cause is not readily apparent. Spontaneous breakage may be the result of one or more of the following: surface or edge damage to the glass; deep scratches or gouges in the glass surface; severe weld splatter on the glass surface; glass to metal contact; thermal loading; and nickel sulfide inclusions.
Nickel sulfide inclusions refer to the existence of certain types of rare and very small, undissolved nickel sulfide stones that are extremely difficult to detect. Glass manufacturers take extraordinary steps to minimize the potential for nickel sulfide inclusions. Considering that a large furnace may produce up to 600 tons of glass per day, total elimination of contaminants is impossible.
There are several laminated glass manufacturing processes. The first calls for two or more lites of glass and one or more interlayers of plasticized polyvinyl butyral resin permanently bonded together under heat and pressure. The second is two or more lites of glass and polycarbonate, bonded together with aliphatic urethane interlayers under heat and pressure. The third type of laminate utilizes a cured resin as the interlayer material.
This bonding of materials provides a variety of performance benefits in architectural applications. The most important characteristic is the ability of the interlayer(s) to support and hold the glass when broken. This provides for increased protection against fall-out and penetration of the opening. Most building codes require the use of laminated glass for overhead glazings as monolithic lites, or as the lower lite in multiple glazed units. Other applications include acoustical insulation, resistance to smash-and-grab burglaries, security, bullet resistant and safety glazing.
Laminated glass is 75 percent to 90 percent as strong as annealed glass of the same thickness depending on exposed temperatures, aspect ratio, plate size, stiffness and load duration. The edges of laminated glass are less resistant than annealed glass to handling and installation damage. Laminated glass, however, can be made with both heat-strengthened and fully-tempered glass for additional benefits, such as resistance to additional wind loading strength, increased impact resistance or resistance to thermal stress. Quality standards for laminated glass are defined in ASTM C1172 “Standard Specification for Laminated Architectural Glass.”
Anti-reflective (A/R) glass is a coated glass with several purposes. It may be used to reduce glare, improve the brightness (signal strength) of transmitted /images or light beams, or to improve the contrast of an image (signal-to-noise ratio of a beam).
The purpose of these coatings is to reduce the reflected light from the surfaces of glass and/or increase the amount of light transmitted through the glass.