Prefabricated exterior wall panels are currently the most widely used form of PC construction in China. Prefabricated exterior wall panels have good surface flatness, high overall accuracy, and can directly prefabricate the insulation and decorative layers of building windows and facades in factories, which has gained the favor of many developers.
Due to the fact that prefabricated exterior walls are assembled in blocks, it is inevitable to encounter waterproofing issues with connection joints. Therefore, we must attach great importance to the treatment of waterproofing nodes in prefabricated exterior walls. This article will analyze and introduce several existing waterproofing technologies for prefabricated exterior wall panels, and explore some issues that need to be taken seriously and paid attention to in the waterproofing construction of prefabricated exterior wall panels.
1、 Design concept of waterproofing for prefabricated buildings
The waterproofing engineering of buildings has always been a very important part of construction, as the effectiveness of waterproofing directly affects the future functionality of the building. Houses that frequently leak water cannot meet the needs of users for living and use.
We know that water has strong fluidity and is ubiquitous, so the main design concept of traditional building waterproofing is to block water, blocking all channels through which water can enter the interior to achieve waterproofing. This concept can still achieve ideal results when applied to traditional cast-in-place structures, but its effect may not be as ideal for prefabricated modular buildings.
Prefabricated modular construction is the process of disassembling the structural components of a building, such as wall panels, columns, beams, floor slabs, stairs, etc., according to certain specifications, and then prefabricating them in a factory before transporting them to the site for assembly. Due to the on-site assembly of components, there will be a large number of assembly joints left, which can easily become channels for water flow infiltration. Therefore, prefabricated modular buildings actually have certain inherent weaknesses in waterproofing. In addition, some prefabricated buildings are designed with exterior wall panels that can move within a certain range to resist the impact of seismic forces, which further increases the difficulty of waterproofing the wall panel joints.
In view of the above factors, the design concept of waterproofing for prefabricated and assembled buildings must be adjusted. We believe that for waterproofing of prefabricated and assembled buildings, water diversion is better than water blocking, and drainage is better than waterproofing. Simply put, it is necessary to consider in the design that there may be a certain amount of water flow that will break through the outer waterproof layer. By designing a reasonable drainage path, this part of the water that breaks through will be guided into the drainage structure and discharged outdoors to avoid further infiltration into the interior.
In addition, utilizing the principle of natural vertical flow of water under the action of gravity, the wall panel joints are designed in a groove shape with high inside and low outside, combined with a certain pressure reducing cavity design to prevent water flow from crawling back into the room through capillary action. In addition to concrete structure waterproofing measures, the use of rubber waterstops and multi-component weather resistant waterproof adhesives to improve the waterproof system of the entire prefabricated wall panel can truly achieve no leakage.
2、 Introduction to several forms of waterproofing treatment for prefabricated exterior wall panel joints
The commonly used waterproofing forms for prefabricated exterior wall panel joints in practical applications are as follows:
1. Prefabricated exterior wall panels with internal pouring and external hanging (i.e. PCF panels) mainly use outer drainage cavities and glue, and rely on the joint waterproofing form of cast-in-place concrete self waterproofing on the inner side.
This type of joint waterproofing for exterior wall panels is currently the most commonly used form. Its advantage is that the construction is relatively simple and fast, but its disadvantage is that the waterproofing quality is difficult to control, and cavity blockage often occurs. Once the inner concrete cracks, it directly leads to the failure of wall panel waterproofing.
2. The external prefabricated exterior wall panel adopts a closed line waterproof form.
There are three main waterproof measures for this type of wall panel. The outermost part is made of high elasticity weather resistant waterproof silicone, and the middle part is a pressure reducing space formed by a physical cavity. The inner part uses waterproof rubber strips pre embedded in concrete to press each other up and down to achieve waterproof effect. An additional polyurethane waterproof layer is added outside the rubber waterstop at the cross joint between the walls. Its main function is to use the good elasticity of polyurethane to seal the small gaps that may be caused by the mutual displacement of the rubber waterstop. For rooms or buildings with particularly high waterproof requirements, polyurethane waterproofing can be fully applied inside the rubber waterstop to enhance the reliability of waterproofing. Install a drainage pipe on the waterproof silicone of the exterior wall at a distance of about 3 layers, which can effectively guide the rainwater that seeps into the decompression space to the outside.
The waterproof structure of the enclosed line adopts a combination of internal and external waterproofing, as well as a combination of thinning and blocking methods. Its waterproof structure has the advantage of being very complete, so the waterproof effect is also very good. The disadvantage is that the precision requirement during construction is very high, and the wall panel displacement cannot exceed 5mm, otherwise the waterproof rubber strip cannot be compressed. BIM design uses weather resistant waterproof adhesive with high performance requirements, not only high elasticity and aging resistance, but also a service life of not less than 20 years. The cost is relatively high, and the quality requirement during structural adhesive construction is relatively high, which must be operated by a professional and experienced construction team.
3. External prefabricated exterior wall panels also have a joint waterproofing form called open line waterproofing.
This waterproof form is basically the same as the two waterproof measures on the inner side of the closed line waterproof, namely the pressure reducing space of the tongue and groove type and the pressure sealing waterproof rubber strip on the inner side. However, in the waterproof measures on the outer side of the wall panel, the open line waterproof does not use glue, but instead uses a curtain like rubber strip that is pre embedded in the wall panel at one end and extends out of the wall panel at the other end to overlap with each other up and down for waterproofing. At the same time, stainless steel air ducts are set at a certain distance on the outer rubber strip to balance internal and external air pressure and drainage.
The outermost waterproof form of the open line adopts pre embedded rubber strips, which has the advantage of easier control and inspection of product quality. During construction, workers do not need to apply glue on the outside of the wall panel, eliminating the need for scaffolding or hanging baskets and other construction measures, making it safer and more convenient. However, the disadvantage is that it requires high product protection requirements, and once the pre embedded rubber strips are damaged, it is difficult to replace them. The BIM learning cost of weather resistant rubber waterstops is also relatively high. Open line waterproofing is currently the most advanced form of external wall waterproofing joint treatment, but it is a patented technology developed by foreign companies. Due to the impact of patent usage costs, there are still very few projects in China that use this technology.
3、 Construction points for waterproofing treatment of prefabricated exterior wall panel joints
At present, the waterproofing treatment technology for prefabricated exterior wall panel joints is still relatively complex in terms of process, so there are also significant construction difficulties during construction. In actual construction, we should develop targeted construction plans and measures based on different exterior wall panel joint design requirements. Specifically, we should pay attention to the following construction points during construction:
1. Conduct product quality inspection before wall panel construction
The processing accuracy of prefabricated wall panels and the quality of concrete curing directly affect the accuracy and waterproofing of the wall panels. Before installing the wall panels, it is necessary to carefully review the geometric dimensions and flatness of the wall panels, check whether the concrete on the surface of the wall panels and around the embedded window frames is dense, and whether there are through cracks. Wall panels with unqualified concrete quality are strictly prohibited from use.
In addition, we also need to carefully inspect the quality of the pre embedded rubber strips around the wall panels, check whether the rubber strips are firmly embedded, whether there is any damage at the corners, and whether there is concrete slurry leaking into the rubber strips, causing them to harden and lose elasticity. The rubber strips must be strictly inspected to ensure that there are no defects. If there are quality problems, they must be replaced before lifting.
2. Strictly control the installation accuracy during wall panel construction. Before lifting the wall panels, carefully carry out measurement and setting out work.
Not only should the baseline be set, but also the position lines of the wall panels should be laid out for easy positioning during lifting. The adjustment of wall panel accuracy is generally divided into two steps: coarse adjustment and fine adjustment. The coarse adjustment is to place and unhook the wall panel according to the control line as the standard. The fine adjustment requires adjusting the axis position and verticality deviation of the wall panel to within the allowable deviation range of the specification. In actual construction, it is generally required not to exceed 5mm.
3. During the waterproofing construction of wall panel joints, strictly follow the process flow and conduct quality inspections for each process.
The outer side of the wall panel joint should be glued strictly according to the design process, and the base layer and reserved cavity must be cleaned with high-pressure air. Before applying glue, the depth of the backing should be carefully checked, and the thickness of the glue must meet the design requirements. The wall panels at the glue application site should be reinforced with a primer to enhance the bonding force between the glue and the concrete wall panels. When the glue application is interrupted, a construction joint should be left, with the inside of the joint being higher than the outside, and the overlap should not be less than 5cm.
The connecting iron parts and cross joints on the inner side of the wall panel are sealed with polyurethane. As there is no rubber waterstop in the iron parts, the rust and rust prevention work of the iron parts should be carefully done before the polyurethane construction. The polyurethane should be tightly sealed without any gaps. After the construction is completed, a water splashing test should be carried out to ensure no leakage before sealing the cover plate.
4. After the construction is completed, conduct a waterproof effect test to promptly, properly, and effectively address leakage issues.
After the waterproof construction of the wall panel is completed, a water spraying test should be carried out in a timely manner to test the effectiveness of the waterproofing. The focus of the water spraying is on the cross joint of the wall panel, the connection between the prefabricated wall panel and the cast-in-place structure, and the window frame. When spraying water, it is advisable to use a fire hose to spray the test area. The external inspection should check whether there is any delamination in the glued area, whether the drainage pipe is draining smoothly, and carefully observe whether there are watermarks and water marks on the inside. If local leakage is found, it must be carefully recorded, the cause must be identified and dealt with in a timely manner. If necessary, a polyurethane waterproof layer can be added to the inside of the wall panel to improve the safety factor against leakage.
4、 Summary
Prefabricated modular construction is a new technology that is currently being vigorously promoted in the construction industry. From a technical perspective, by adopting a series of improvement measures, its external wall waterproofing performance can be effectively guaranteed. On site construction personnel should be proficient in the construction requirements of prefabricated external wall waterproofing, strictly follow the relevant specifications and procedures, and ensure the quality of waterproofing. Only in this way can the waterproofing of prefabricated walls be guaranteed to be foolproof, and prefabricated modular residential products can gain recognition from users and the market.
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