With the rise of glass curtain wall in modern architecture, these advantages are widely recognized due to the decoration of the coated glass, which makes the building colorful, bright and generous, and has energy-saving and consumption-reducing effects. However, it has attracted the series of "light pollution" and safety of curtain wall glass. The problem, where security is highly visible!
The safety of unqualified curtain walls is mainly concentrated on the falling off of the glass and the falling of the glass after the crack. The main reason for glass shedding is that the quality of materials such as glue and double-sided stickers is not good, the curing time is not long enough, the glue is incompatible with the glass, the quality of the glue is not good, the construction is not standardized, the wind and the earthquake are affected, so it is necessary to design and purchase. , construction, supervision, these aspects to check, prevent, prevent the fall of glass.
However, the use of glass in the curtain wall, especially in the process of using coated glass, is more common in the occurrence of cracks or cracks. People are not clear about this phenomenon, and they are simply blamed for the poor quality of the glass, and the so-called "air timing" Bombs, so it is necessary to analyze and explore the characteristics, causes and prevention of this phenomenon, so that the public can understand the curtain wall coated glass more comfortably.
There is no crack in the normal use of coated glass, and most of the cracking of coated glass is thermal stress cracking. Under the strong sunlight, the glass absorbs the radiant energy in the sunlight and converts it into heat energy in the glass body, so that the temperature of the irradiated part of the glass is relatively elevated and is in a state of thermal expansion, while the glass edge area inside the aluminum frame structure cannot be subjected to the same solar radiation. Or shadow effect, uneven heat dissipation, resulting in uneven temperature distribution of the whole glass, resulting in internal thermal stress. The thermal expansion in the middle region of the glass creates tensile stress on the edge of the glass. This tensile stress exceeds the tensile strength of the edge region, which causes the glass to rupture. The phenomenon is called the thermal stress cracking of the glass.
Because the heat absorption of the coated glass is more significant, the difference in solar radiant energy causes the glass temperature difference to be larger, and the thermal stress cracking phenomenon is more. In contrast, coated glass used for windows and bright-framed curtain walls is more likely to occur than for hidden frame curtain walls. The characteristics, causes and prevention of thermal stress cracking of coated glass are further discussed below.
1. Characteristics of thermal stress cracking: the fracture line is at right angles to the edge of the glass, starting from the edge or corner of the glass, and splitting into two or more strips at a distance of about 50 mm from the glass or corner, and the shape is an irregular zigzag single line. It extends in the middle and usually forms a curved line in the middle of the glass.
2. Bulk absorption of coated glass: After the glass is coated, its chemical properties are basically unchanged, but its physical properties are greatly changed, especially optical properties and thermal properties. The absorption rate of solar radiant energy of coated glass is much larger than that of ordinary transparent glass and bulk colored glass. Generally, the solar radiant energy absorption rate of coated glass is several times that of ordinary transparent glass and bulk colored glass, so under the same conditions of use, coated glass The endotherm is much more than ordinary glass, and the temperature is much higher, which means that the temperature difference between different parts of the glass is relatively much larger. Therefore, for the advantages of coated glass, in the process of installation and use, take appropriate measures different from ordinary glass to ensure the safe use of coated glass.
3. Orientation of the building: In view of the sensitivity of the coated glass to the thermal energy, the orientation of the coated glass is very important, especially for the glass with large absorption, the glass absorption of the east, southeast, south and southwest is relative to other There are many more faces, and the thermal stress cracking of the glass should be considered as an important factor in design. It is recommended to use a light-colored glass (such as white glass) coating product to reduce the heat absorption of the glass, and the safer method is to strengthen the glass. , processed into tempered, semi-tempered coated glass.
4. Climatic conditions: In areas or seasons where the temperature difference varies greatly in the early morning, the coated glass is a high absorber of solar radiant heat, and the temperature rise of the glass is very fast. The aluminum frame is an excellent heat sink. After cooling through the day and night, the temperature of the glass and aluminum frame has been minimized. In the morning, the solar radiant heat quickly heats up the glass, while the temperature rise of the aluminum frame is relatively small, such as glass and aluminum frame. Contact with each other, resulting in uneven distribution of glass temperature difference, local climate temperature difference, glass orientation, sudden showers and showers and air conditioning systems can easily cause the temperature difference of the glass to exceed the limit and rupture. The glass is selected according to the above conditions to determine whether the glass is strengthened. The color and size of the glass to be used, and the coated glass shall be constructed in strict accordance with relevant regulations.
5. Thermal insulation between glass and surrounding environment: Glass is a poor conductor of heat. Because of the temperature difference caused by heat absorption and uneven heat dissipation, it is difficult to achieve balance by itself. Therefore, when installing glass, use elastic materials with low thermal conductivity as much as possible. The mat separates the glass from the aluminum frame with good thermal conductivity, the cold wall, etc., and reduces the formation of uneven temperature difference of the glass.
Generally, there is a relatively small number of cracks in the hidden frame curtain wall. The reason for this is that the glass material is softly contacted with the aluminum frame by the heat-conducting elastic material. There is no heat exchange between the glass and the aluminum frame, and the glass is reduced due to the heat dissipation of the glass. On the contrary, for the coated glass used in bright frames, fixed windows, sliding windows, floor-standing glass, etc., the central heat absorption causes the temperature to rise, while the periphery is clamped, and the heat energy is not absorbed to form a low temperature band, such as a glass heel. When the aluminum frame is in contact, the glass is thermally conductive to the aluminum frame, not only the glass in the aluminum frame, but also the glass around the aluminum frame is kept at a low temperature. In many cases, the temperature in the middle of the glass reaches 60-70 ° C, and the edge is only 25-30 ° C. A few tens of degrees apart, even if it is noticeable by hand, a large thermal stress is generated inside the glass, which easily causes the glass to break.
The most common thermal stress cracking of the currently coated glass avoids the above problems, and tries to thermally insulate the glass from the aluminum frame. When the glass is installed, the lower balance pad has two rubber blocks, which not only ensures the elastic contact between the glass and the lower part, but also avoids local stress. Moreover, they are thermally insulated, and the gap between the two sides of the glass and the upper part of the glass is kept sufficient, and the rubber strips are sandwiched between the two sides of the glass and the gap of the aluminum frame, and then the glass glue is applied, so as to ensure their absolute separation, elastic contact and Thermal insulation.
6. Edge quality of glass: Glass is a brittle material, and the allowable stress at the edge is closely related to the quality of the edge of the glass. The edge is the most vulnerable part of the glass. There are many micro-cracks and defects (such as chipping, chipping, uneven trimming, etc.), which will reduce the allowable stress of the edge by a dozen times. It is easy to encounter excessive thermal stress and external impact. It is caused to break from here, so the coated glass is best cut with a cutting machine. The cutting quality of the glass edge must be strictly checked when cutting by hand. A more effective method is to machine the edge after the glass is cut, which can greatly reduce the edge microcrack. .
7. Size and shape of the glass: For coated glass, the greater the thickness of the glass, the greater the absorption of the radiant energy of the sun, the greater the thermal stress of the glass, and the more easily the glass breaks. Similarly, the larger the glass area Larger, the more likely it is to break; the larger the ratio of the long and short sides of the glass, the easier it is to form bending stress, which increases the probability of thermal stress cracking. In addition, the irregular shape of the glass, the uneven force of each part and direction, the same Increasing the probability of thermal stress cracking, a reasonable design is a critical factor in reducing this impact.
8. The influence of shadows: Outdoor objects or building structures themselves may leave shadows on the glass, resulting in different heat absorption in different parts of the glass, and the temperature difference is different. The more irregular the shape of the shadow, the more easily the glass breaks.
9, the impact of indoor sunshade: dark curtains, blinds and indoor close to glass objects (including stickers, paint on the glass), the absorption of solar radiation energy is higher, and generally has a higher emissivity. Therefore, in addition to the direct radiation of the sun, the central zone of the glass is also re-radiated by the absorption of heat by the indoor objects, and also reduces the heat dissipation of the glass. Therefore, the indoor sunshade device increases the glass temperature non-uniformity and also increases the probability of self-explosion of thermal stress, and the above device should be avoided as much as possible.
10. Reprocessing effect of glass: Reprocessing of coated glass increases the factor of glass cracking. After the hot bending of the coated glass, the annealing quality is not good, and the glass stress is not uniform; the coated glass is processed into a hollow product, and the gas in the middle expands with the heat absorption of the glass, and is pressed against the edge of the glass to increase the probability of self-explosion of the glass, so The glass coated glass is preferably edging or reinforced.
Although coated glass has its inherent disadvantages, it has many other materials that cannot replace the advantages. In order to promote the glare of coated glass in the construction industry, coated glass suppliers provide users with more product performance parameters, provide users with more reasonable design and construction plans, clarify people's misunderstanding of coated glass, and promote the use of coated glass. .
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