glass in building

Glass in BuildingGlass panes are widely used in all modern buildings, commercial or even residential asfaçade panels or decorative components:- Aesthetically good-looking- Varying colors & reflection (coating/film/tinting1)- Different methods of fixing- Tailored acoustic behavior- High compressive strength- Visco-elasticforming into desired shape by heating the glass to the plastic phase to produce complex shapesSag bending: the glass, supported peripherally and heated to the plastic phase, is allowed to sag under its own weight to the desired shape (gravity sag bending).Control is through the pattern of temperature distribution across the sheet.Between 500-600°C, viscosity falls by a factor of 10,000Glass transforms from a brittle solid to a plastic substance.- Tailored optical properties (coating or film)Daylight admissionSolar gainOpacity- Surface pattern can be easily introducedSurface patternedSand blasting and acid etchingSurface Printing / Screen PrintingFor similar reasons, glass based hardware is also getting very popular in building: washbasinsshower screen doorstables & deskscooktopsΞ1melt colorants are added for tinting and solar-radiation absorption properties. This reduces heat penetration in buildings. Coloured glass is an important architectural element for the exterior appearance of façades. It is also used in interior decoration (doors, partitions, staircase panels, mirrors,...).Float glass- is a term for perfectly flat, clear glass (basic product). The term "float" glass derives from the production method, introduced in the UK by Sir Alastair Pilkington in the late 1950's, by which 90% of today's flat glass is manufactured.Production:The raw materials (silica sand, calcium, oxide, soda and magnesium) are properly weighted and mixed and then introduced into a furnace where they are melted at 1500° C. The molten glass then flows from the glass furnace into a bath of molten tin in a continuous ribbon. The glass, which is highly viscous, and the tin, which is very fluid, does not mix and the contact surface between these two materials is perfectly flat. When it leaves the bath of molten tin the glass has cooled down sufficiently to pass to an annealing chamber called a lehr. Here it is cooled at controlled temperatures, until it is essentially at room temperature.Defects in Float Glass- Point Defects: Bubbles, Stones Knots- Tin defects, roller marks- Optical DefectsDisadvantages of Annealed Glass- glass is extremely brittle, and cannot withstand tensile load- forms dangerous sharp edges when broken- Flat glass can be further processed to enhance its service performance or to form into desired shape:Tempered / Toughened GlassCoated glassLaminated glassInsulated Glass UnitPatterned GlassΞToughened GlassToughened glass is two or more times stronger than annealed glass. When broken, it shatters into many small fragments which prevent major injuries. This type of glass is intended for glass façades, sliding doors, building entrances, bath and shower enclosures and other uses requiring superior strength and safety properties.Production:There are two different methods used to produce tempered glass:- Heat treating: Where the annealed glass is subjected to a special heat-treatment in which it is heated to about 680°C and afterwards cooled. If it is cooled rapidly, the glass is up to four times stronger then annealed glass and its breaks into many small fragments(fully-tempered). If it is cooled slowly, the glass is twice as strong as annealed glass and the fragments of the broken glass are linear and more likely to remain in the frame(Heat-strengthened).- Chemical Strengthening: The glass is covered by a chemical solution which produces a higher mechanical resistance. Chemically-strengthened glass has similar properties to thermal-treated glass. The product is not generally used for window glass, but more commonly seen in industries where thin, strong glass is needed.Characteristics- The toughening process created residual compressivestresses on the surface and equivalent tensile residualstresses at the core.- when the glass breaks, it shatters into small fragments thatremarkably reducesthe likelihood of injury to people asthere are no jagged edges or sharpshards.Toughened (Tempered) Glass Stress Measurement- Destructive Method: Break the glass and measure the size of fragments- Non-Destructive MethodBirefringence is proportional to the difference of principal stresses in the plane perpendicular to the ray of lightDifferential RefractometryPolarimetric MesurementsSpontaneous BreakageIn various situations fully-tempered glass may break for no reason. Many factors might cause such spontaneous breakages, but the most common are nickel sulphide inclusions. Nickel sulphide inclusion, also known as NiS, occurs during the manufacturing process for float glass. In the glass batch, nickel-rich contaminants such as stainless steel might be present,and then combine with sulphur form nickel sulphide inclusions. Glass manufacturers take extraordinary steps to minimize the potential for nickel sulphide inclusions. Considering that a large furnace may produce up to 600 tons of glass per day, total elimination of contaminants is extremely difficult. In normal float glass production, nickel sulfide inclusions formed at about 1 per 8 tonnes of raw glass, or 1 gram of nickel sulfide can produce approximately 1000 inclusions of 0.15 mm diameter in glass panes.When glass is heat-treated during successive stages, in order to obtain fully-tempered glass, nickel sulphide inclusions change in size from what is known as a low-temperature (LT) structure to a high-temperature (HT), crystalline structure. When cooled quickly the NiS particle is unable to change back to its original form(LT). Over a certain period of time NiS will slowly convert to the original form (LT) phase with an increase in volume of about 2 - 4%. Such increase of NiS will cause glass breakage, i.e., caused by volumetric expansion (due to phase transformation) of nickel sulfide NiS at ambient temperature from alpha, a high-temperature form, to beta phase, the low temperature form of NiS.- NiS inclusions in general present no harm to annealed glass- In toughened glass, If the inclusions are situated in the core of the tempered glass panes, the expansion resulted from the phase transformation may induce additional tensilestresses that superimpose on the residual tensile stresses at the core.- When the combined stresses exceeded the strength of the glass, the glass may brake spontaneously with no apparent reason.- Chances of spontaneous breakage depends onType and nature of inclusionsPosition of inclusionsNumber of inclusionsSize of inclusions(Critical size 0.03 mm diameter which isfar too small be detected visually.)Measures to tackle NiS ProblemHeat-soak treatment (Heat Soak Test)A destructive process to expedite the phase transformation of NiS during the manufacturing stage to minimize chances of breakage after installation.- Even after heat soaking, toughened glasses are liable to a few apparently spontaneous failures:incomplete conversion of nickel sulphide inclusionsUneven temperature distribution in heat soak furnace;Inconsistent heat soak operation;Improper control of heat soaking parameters;even with full conversion, inevitably some of these inclusions are too small to fail in the heat soak, but may cause failures at unexpectedly low loads in service.A. Kasper: “Although the HST is in use now since more than 30 years, it seems that its scientific background is not yet fully understood. In the last years there were several trials to shorten the test, e.g. by raising the temperature level (so-called short HST) or by trying to stop the temperature during the cooling phase of the tempering process (so-called on-line HST), but these modifications were not successful and, when tried out in practice, caused epidemic failure of glass panes on the buildings concerned. The reason for these erroneous technical developments is a basic lack of knowledge about the real behaviour on heating or cooling of the glass surrounded NiS particulates”.Use of Heat-Strengthened Glass to replace Toughened Glass- Lower strength- Need thicker pane- NiS spontaneous fractures have been known to occur in HS glass, but the occurrence is very much lower than in fully tempered.- When fracture, the glass would not shatter into small fragmentsModification of the manufacturing processAdding Zinc sulfate and nitrate to reduce the formation of NiS.- Decomposition of NiS by adjusting oxidation-reduction reaction.Static FatigueApart from NiS, the other common cause of spontaneous breakage is due to the presence of surface defects on the surface of glass- due to moisture attack at pre-existing surface defects induced by:- Poor edge grinding at the manufacturing process- Mishandling / Accidental shock loading during transportation, storage and installation- crack propagates progressively until it approaches the tensile zoneWater & Chemical AttackApart from static fatigue, water may also stain glass and affect its appearance:Flat glass furnished for buildings is uniquely resistant to chemical attack and other degradation. Substances and conditions which may cause many building materials to deteriorate have no important effect on glass. The basic characteristics of glass are not altered by the principle environmental factors of solar radiation, rain, or wind. In view of this, it is surprising to find buildings with serious glass staining problems. On occasion, the staining evolves into etching of the glass surface, requiring replacement of the glass. Staining normally implies a chemical change or degradation of the glass surface.Glass is inert to most acids and a wide variety of other chemicals. Only phosphoric and hydrofluoric acid are harmful. Either of these in concentrated form will rapidly etch a glass surface and are commonly used for this purpose. Among many other uses, strong solutions of hydrofluoric acid are used for cleaning new concrete. There is a possibility that this acid may be splashed onto the glass during the construction of the building. The staining can become severe in an hour or less. The recorded cases of this occurring are rare, however. Alkaline materials, unlike acids, will attack glass surfaces to varying degrees. In concentrated forms, etching of glass surfaces can occur very rapidly, within a few hours in some cases. Diluted alkaline solutions are often used as glass cleaners and attack on the glass surface is extremely slow. However, even though the concentration may be low, the glass surface should always be thoroughly rinsed after cleaning to avoid staining.Some of the more common alkaline solutions are ammonia, trisodium phosphate (used in diluted form as a commercial glass cleaner), most cleaners and detergents and, run-off from construction materials such as concrete, mortar, plaster, and gravel.Probably the most common cause of staining after the glass is installed is from the run-off of rain from unsealed concrete panels or mortar joints. The resulting alkaline solution can be concentrated and cause serious etching in a few days. Another common cause is the use of a concentrated liquid cleaner as a lubricant for gasket-glazed installations. In this case, even though the glass is rinsed, some cleaner is trapped between the gasket and the glass. This will seep out onto the glass over a period of time and a serious attack of the glass surface mayresult. Only lubricants recommended by the gasket manufacturer should be used.Glass Staining in Storage or ShipmentStaining of glass in storage or shipment is almost always caused by an alkaline attack triggered by the presence of water. Special conditions often occur when glass is packed close together in crates or racks. These conditions do not exist for glass in practical uses, such as in windows, curtain walls, and automobiles.For glass in storage, conditions are markedly different. Water may collect between the panes of glass. This can be from a leaky pipe or a roof for glass stored indoors and from rain, snow, or ground water for glass stored outdoors. Most often, however, it is caused by condensation of water vapor on the glass. This is possible whenever the glass temperature is below the dew point of the surrounding air. For glass stored outdoors or in an unheated warehouse, this is very likely to occur.The glass temperature will change more slowly than the air temperature because of the bulk and consequent temperature lag of the glass. A crate of glass stored all night at a cool temperature may be close to the air temperature early in the morning. The air temperature and moisture vapor content often increase rapidly after sunrise. The glass will remain at a much lower temperature for a number of hours. Under these conditions, condensation of water vapor is likely. This water is trapped and usually cannot drain away. It evaporates slowly and often will only partially evaporate during a 24-hour period. If the same temperature cycle reoccurs a number of times, the glass surface may remain wet for a number of weeks or months. As a portion of the water evaporates, the alkaline solution becomes more and more concentrated. It will almost always become sufficiently concentrated to etch the glass. Severity of Glass StainingDuring the first stage of staining a transparent white film will appear on the glass. Under some lighting conditions, this is observable on installed glass which has not been washed in some time and almost always uniformly covers the entire glass surface. Visually, it cannot be readily separated from surface contamination from cigarette smoke, atmospheric dust and other materials.The second stage of staining appears as an iridescence or discoloration of the glass surface and has the appearance of an oil film. This is multi-colored with a very irregular pattern. It is readily observable when daylight or other light sources are viewed by reflection from the glass surface.The final stage of staining produces an irregular translucent white discoloration which is readily discernible under most lighting conditions. It can reach the stage where objects cannot be clearly distinguished through the glass.Erosion or Mechanical ScratchingCoated glass is also vulnerable to erosion or mechanical scratching:- By air borne particles- Accidental or Vandalism- Use of abrasive cleaning agentCoated GlassOrdinary float glass can be coated to achieve different properties:Production- Pyrolitic (On-Line): in this process, semi-conducted metal oxides are directly applied to the glass during the float glass production while the glass is still hot in the annealing lehr.These are hard coatings which are relatively harmful to the environment.- Vacuum (magnetron) Process (off-line):in this process one or more coats of metal oxide are applied under a vacuum to finished glass. The coatings applied by this technique are soft and must be protected against external influences and are therefore used for theinterior side of glass panes.There are some other techniques for the OFF-LINE coating:- Immersion Process- Chemical Process- FoilFor coated glass, the coating on the glass may be more susceptible to chemical attack:- Nature of coating (soft or hard)- Location of coating (outside, inside, between panes)Low Emissivity CoatingThe use of a low emissivity (Low E) coating on the glass provides the possibility of reducing the long wave radiation exchange between the panes. In air spaces with uncoated surfaces, the long wave radiation exchange between those enclosing glass surfaces is high, amounting to about 60% of the total heat exchange across the space.With one of the glass surfaces having a coating with emissivity less than 0.2 (compared with 0.84 for the uncoated glass surface), the radiation exchange is reduced by approximately 75% and consequently the U value is reduced. The term "Low E" is now generally taken to refer to coatings with an emissivity less than 0.2. At ambient temperatures, the long wave radiation lies between 5,000 - 50,000nm where the reflection of Low E coating is high - extending beyond the wavelength coordinate.The higher insulating effect (lower U value) provide by a Low E coating in a double glazed unit is due to the high reflectance of long wavelength radiation at ambient temperatures.Emissivity and reflectance in the far infrared are related by Kirchhoff's law.The development of hard low emissivity coatings widens the possibility of including coated monolithic glazing in secondary frames applied to existing windows; the earlier, softer low emissivity coatings were restricted to protected use in sealed glazed units only. Metallic surfaces, including some metallic oxides, provide low emissivity.Water on the coated surface of the glass, perhaps as a result of condensation, will cancel out the effect of the Low E coating because of the high emissivity of water.The present technology of on-line, hard Low E coatings can provide slightly higher solar heat transmission than that exhibited by soft coatings, giving improvements to passive solar gain applications. In cold climates the higher temperature of the inner glass surface of double glazed units using Low E coatings diminishes the effect of colder long wave radiation causing discomfort near the window.Self-Cleaning glassSelf-cleaning glass is an ordinary float glass with a special photocatalytic coating. It is made by chemically bonding and integrating a microscopically-thin surface layer to the exterior surface of clear glass. The integrated coating reacts to the sun’s ultraviolet rays to gradually and continuously break down organic dirt through what is called a photocatalytic effect. In other words photocatalytic means that the active integrated coating on the outside of the glass absorbs the sun’s ultraviolet rays. This causes a reaction on the surface which breaks down dirt and loosens it from the glass. This type of glass also has hydrophilic properties, meaning that rain flows down the pane as a sheet, washing away the dirt instead of, as with normal glasses, leaving the dirt behind. As a result of these two effects, the special self-cleaning coating keeps the glass cleaner for a longer period than with normal glass in applications where it is exposed to the rain.Strength of GlassIn theory, glass is very strong - much stronger than many metals.In practice, Glass has very high compressive strength but behaves as a brittle substance (low fracture toughness) with a tensile strength only 0.4 per cent of its theoretical value.- it is severely weakened by fine cracks caused by abrasions through handling and corrosion by water vapour- Can be easily damaged by a point loadHeat strengthened and tempered glass in general possesses better fracture toughness, but there exists weak points in these glasses.The actual load that a glass component could withstand in real service situation is affected by:- Condition of the glass (amount of pre-existing flaws, edge quality, …)- Nature of load (impulsive/point or distributed? …)- Support frame & mounting- Thermal differential (indoor and outdoor / daily change in temperature …)Thermal CrackingAll type of glasses are susceptible to thermal cracking- Lack of allowance for thermal expansion- Requires proper supports with allowances for expansionStructural/Shape Stability of GlassCan a glass pane or component retain its shape after breakage?- Nature of attack- Type of support frame and mounting- Presence of additional load (e.g. Wind loading)- Type of glass- Use of Laminated GlassLaminated GlassLaminating procedure:- pre-press stage: the laminate is sandwiched between two plates of glass panes and de-aired,- Autoclaving: the pre-pressed panes are autoclaved under pressure at an elevated temperature.- Once sealed together, the glass "sandwich" behaves as a single unit and looks like normal glass. Annealed, heat strengthened or tempered glass can be used to produce laminated glass.- Polyvinyl butyral (PVB) interlayers are commonly used for glass laminationThe toughness and resiliency of laminated glass makes it an excellent safety glazing. If the glass is broken, fragments will adhere to the PVB, reducing the risk of personal injury and property damageThe standard two-ply construction provides resistance to penetration when subjected to attempted force entry. In multi-ply configurations, laminated glass 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.The shear damping performance of the PVB makes laminated glass an effective sound control product.The PVB in laminated glass helps reduce solar energy transmittance to reduce cooling loads.The ultraviolet (UV) filtering performance of the plastic interlayer helps protect valuable furnishings, displays or merchandise from the fading effects of UV radiation.Common Problems:DelaminationThe adhesion between the glass and the interlayer was affected by some meansPVB will readily absorb & vaporize water- during transportation with exposed glass edges or Storage in warehouses without humidity control- Lowered the adhesive bond strength between the PVB interlayer and the glass Chemical Damage to PVB laminate:- Laminate attacked by chemicals, e.g. sealant, this also affects adhesiveness of the laminatesthermal stresses- Difference in indoor/outdoor temperature resulting in significant differential movement of the glass panes- Most pronounce when thickness of indoor/outdoor panes differ significantly (thermal mass)trapped air- excess air trapped in laminate during production may expand and affect the adhesion of laminateResidual stresses in laminateUneven compression of laminate during the lamination process induced localized residual stresses。