Failure Analysis and Solution of Aluminum Sheet Curtain Wall Seal Failure

【China Aluminum Network】 curtain wall is composed of metal framework and plate, does not assume the main structure of the load and the role of the building envelope structure, with beautiful, energy-saving, easy maintenance, etc., is the modern high-rise buildings and special-shaped building of the preferred program [1]. The main forms of modern building curtain walls are glass curtain walls, stone curtain walls, metal curtain walls, and artificial plate curtain walls. The metal panels used are aluminum panels, fireproof panels, stainless steel panels, and color coated steel panels. Aluminum curtain wall has the following characteristics: 1) Rich color, long-lasting, diversified appearance shape, perfect combination of glass and stone curtain wall through spray painting process; 2) Light weight, only 1/5 of stone curtain wall, 1/1 of glass curtain wall 3, significantly reducing the weight of the building's exterior wall; 3) low maintenance costs and high cost performance. Because of the advantages of aluminum curtain walls, they are widely favored by the owners. At present, aluminum plate curtain walls in China's market are mainly made of aluminum alloy single-layer aluminum plate, aluminum-plastic composite plate and aluminum honeycomb plate.

With the extensive use of aluminum curtain walls, more and more sealing failures have occurred. The damage caused by the failure of aluminum panel curtain wall sealing is the leakage of the curtain wall, which will easily cause damage to the interior of the building, corrode the anchorage of the curtain wall, and affect the safety of the building. At the same time, the seal failure will increase the energy consumption of the building. This article analyzes the common causes of sealing failures in aluminum panels and finds effective solutions.

2 Causes of Seal Failure of Aluminum Curtain Walls

There are many reasons for the sealing failure of aluminum panel curtain walls, such as unreasonable design, improper selection of sealant, poor adhesion, improper construction operations, and so on.

2.1 Interface design and sealant displacement capability do not match

People often find that when the sealant is used in the aluminum panel curtain wall interface, cracking phenomenon occurs (see Figure 1). Especially when the season changes, the temperature difference between day and night is particularly large. When the temperature decreases, the sealer overstretches after the shrinkage of the sheet. . This is mainly due to the fact that the displacement capacity of the sealant does not meet practical requirements. When calculating the width of the interface sealant, the designer must consider the following factors in addition to the thermal expansion and contraction of the sheet, such as displacement caused by the dynamic load on the floor, installation errors, etc. [2]. The smaller width required by the interface is generally calculated by the following formula:

Smaller interface width=(100/X)*(Mt+Ml)+Tc

Among them, X: Displacement capacity of sealant (%)

Mt: Displacement due to thermal expansion (mm)

Ml: Displacement due to dynamic load (mm)

Tc: Construction Error (mm)

Designers need to use a reasonable displacement of the sealant in the design, to avoid cracking caused by the lack of sealant displacement capacity, sealant displacement capacity should also be the report of the National Authority Test Center as the basis. It should also be noted at the time of design that the sealant should be bonded on both sides in the interface instead of three sides. When the three-sided bonding occurs, the sealant can withstand only about 15% of the design value. For deeper interfaces, PE foam sticks should be used to control the thickness of the sealant; for shallower seams, use an anti-adhesive tape to isolate the sealant from the bottom. Because the use of anti-stick tape or PE foam stick can effectively avoid the three sides of the bond. Otherwise, the sealant is easily torn when subjected to an external force and will lose its sealing and anti-leakage effect.

2.2 Improper selection of sealant

At present, there are many sealant products for weatherproof and waterproof in the market: polyurethane sealant, polysulfide sealant and silicone sealant. In the case of engineering, many people did not pay attention to the difference between several sealants, and randomly choose to apply in the aluminum curtain wall, cracking and chalking of the sealant surface often appear (Figure 2).

Since the main chain structure of the base polymer used in the silicone sealant is a Si-O bond, the main chain structure of the polyurethane sealant is a CO bond, a CC bond and a CN bond, and the polysulfide sealant has a main chain structure of CS. , SS button. In Table 1, we compare the bond strengths of different sealant chemical bonds with those of strong ultraviolet rays in sunlight. In addition to Si-O bonds, the bond strengths of other chemical bonds are all lower than the energy of 300 nm UV light. This is the silicone sealant energy. The long-term performance under ultraviolet irradiation still maintains good performance. After other sealing materials are used for a period of time, cracks will occur at different levels, leading to water leakage, and the root cause of the sealing and waterproofing function will be lost.

In the selection of sealant for the sealing of aluminum curtain walls should be preferred silicone sealant, but also need to choose quality assurance products, not for free, choose to add a lot of "white oil" silicone sealant. Because "white oil" is inexpensive and can make the surface of the sealant that has just been used to be more glossy, some irresponsible manufacturers even use it as a selling point. However, as the white oil gradually volatilizes, the colloid itself hardens and is easily cracked. When using silicone sealant, you should also pay attention to the shelf life of the product. Use of the product after the expiration of the product can easily lead to bubbles in the seam, degraded performance, or lack of cure.

2.3 Poor adhesion

Aluminum panel curtain wall also often appears to be a poor sealant and aluminum plate. As the aluminum surface treatment methods are anodized aluminum, fluorocarbon spraying and powder spraying, different treatment methods and different manufacturers' treatment processes will affect the surface structure and performance of the aluminum plate, thus affecting the adhesion of the sealant. . If the project does not conform to the specifications before the construction of the corresponding adhesion test and compatibility test, the result is that the silicone sealant is incompatible with aluminum plates, strips, and other materials, and chemical changes affecting the adhesion occur, and at the same time, Sealing effect. Therefore, in accordance with GB "Building silicone structural sealant" Appendix A and B for silicone sealant and contact material compatibility test, adhesion test, in order to ensure the system's seal.

During construction on site, in order to ensure good adhesion between the sealant and the aluminum plate, it must be ensured that the construction process is performed in strict accordance with the specifications: 1) The cleaning solvent recommended in the adhesion test report is used for cleaning. Alcohol solvents may not be effective. Remove contaminants from the polyester powder coating material; 2) Use a two-step process for cleaning. Use a white, clean, soft, absorbent, or non-linting cotton cloth. First wipe with a piece of cloth moistened with a solvent. Then use Wipe with two clean cotton cloths; 3) Apply sealant to fill the entire interface, and firmly stick to the substrate surface that needs to be bonded with sealant.

2.4 Other improper construction operations

In order to avoid scratching during the processing and transportation of the aluminum plate unit, the surface of the aluminum plate is usually covered with a PE protective film. Therefore, after the aluminum plate is installed, the PE protective film needs to be cleaned before sizing. If the PE protective film is not cleaned and sizing, the sealant is only adhered to the PE protective film. As the joints of the sheet are continuously stretched and compressed, the phenomenon of detackification between the sealant and the substrate occurs (see Fig. 3(a). )).

In the sizing process of the sealant, it is usually necessary to use a PE foam stick to control the sizing thickness. When installing a PE foam stick, it needs to be prevented from being punctured by a sharp object (such as a knife, a scraper, a rivet, etc.) so as not to cause blistering during curing of the sealant (see FIG. 3(b)). The construction thickness of the sealant should be controlled at about 6mm. If the injection glue is too thin (2 - 3mm), stress concentration can easily lead to cracking of the sealant (Figure 3(c)). At the same time, the sealant sizing should not be too thick, otherwise it will lead to a significant decrease in the displacement of the sealant. Under normal circumstances, the construction thickness of the sealant is 50% to 100% of the actual seam width.

The coefficient of thermal expansion of the aluminum plate is relatively large. When the temperature difference is large, the thermal expansion and contraction of the aluminum plate easily leads to a druming phenomenon on the surface of the uncured sealant, especially a transverse seam (see FIG. 4 ). When the drumming phenomenon occurs, the interior of the sealant is solid and will not affect the sealing effect, mainly affecting the appearance of the joint. For this phenomenon, the solution can be divided into two sizings, or the sizing operation can be performed at a relatively constant ambient temperature.

When the sealant cracks, if it is not convenient to clean the glue joint, the repair interface design scheme shown in Figure 5 can be used: 1. Size A is at least 6mm; 2. Size B is at least 3mm; 3. Must use anti-adhesive Tape, separate the new construction sealant from the failed sealant so that the repaired sealant can move with the interface.

Conclusion:

Aluminum curtain wall has the advantages of rich color, long-lasting, diversified appearance shape, low maintenance cost and high cost performance. It has been widely used in curtain wall projects, but the sealing failure is the main problem facing aluminum curtain wall. Through the analysis of this paper, joint design, sealant selection, construction quality, and environment are the key factors that affect the perfect sealing of aluminum curtain walls. At the same time, the adhesiveness test is required before the sealant is applied. The construction personnel should follow the standard construction process. After the construction is completed, on-site sampling and tapping experiments are carried out to ensure that the sealant satisfies the requirements of the design drawings and the substrate is well adhered to ensure that the entire aluminum panel curtain wall is well sealed.

references:

[1] China Academy of Building Research. JGJ 102—2003 Technical Specifications of Glass Curtain Wall Engineering[S]. Beijing: China Building Industry Press, 2004.

[2] Zhou Wenliang Matters needing attention when choosing sealant for design of curtain wall interface [C] Architects and Building Materials, 2010

[3] Qiu Zemin, Yuan Sulan, Wang Youzhi, Wang Zuhua. Application of silicone sealant on doors and windows[J] Silicone Materials, 2008,22(5): 296-299