The selection of sealant for joint sealing on prefabricated buildings

There will be many joints after the splicing of prefabricated exterior wall panels or exterior wall components, so the waterproof sealing for joints is a very important problem that can’t be ignored. As it stipulated in the <Technical specification for precast concrete structures>: “ The joints of prefabricated exterior panels, openings of windows and doors and other waterproof weak parts should adopt a combination of material waterproof and structural waterproof methods.” In some areas’ standards, there is the way of “material waterproof plus structural waterproof” for vertical joints. But no matter what kind of design methods, sealant is the first waterproof precaution, so you have to pay attention to the election of sealants.

For sealants used on prefabricated buildings, there are mainly 6 technique difficulties:

(1)Adhesion of concrete
Concrete is alkaline material, it’s hard to bond with it for ordinary sealants. Also, the surface of concrete is loose and porous, which reduces the effective adhesive area. So, it is required that the sealants have excellent adhesion to concrete.
Besides, anti-alkali phenomenon of concrete may also occur, especially in the south rainy areas, which can cause serious damage to the adhesive interface of sealant. So, adhesion of concrete is the first factor when it comes to choose sealants for prefabricated buildings.
Among the concrete sealants in market nowadays, one component modified silane sealant and polyurethane sealant have good adhesion to concrete, two component modified silane sealant must be equipped with primer to form adhesion. However, the adhesion to concrete of traditional silicone sealant is bad.

• Picture 1. Cohesion failure and adhesion failure of sealant

(2)Weather resistance
The joints of prefabricated exterior panels are usually used as dividing seams of the decorative surface, sealant joints are treated as opening seams. Besides, precast panel joint sealant have to withstand the long-term sunlight and rain erosion, so excellent weather resistance of sealant is required. The weather resistance of sealants depends on its molecular structure, the bigger the main chain bond energy , the better the weather resistance. However, the weather resistance of silicone sealant is the best, then the MS sealant, and polyurethane sealant is the worse, which is easy to crack and chalk outside.
The main chain structure, bond energy and UV energy comparison of all kinds of sealant for precast concrete panelsare as follow:

• Picture 2. Damage of polyurethane sealant after UV radiation

(3)Paintability
When the joint of precast panels is big due to hoisting installation error and sealant needs to be painted with coatings, the compatibility of sealant and coating is vital. Because of the hydrophobic and oleophobic surface of silicone sealant, the coating is difficult to be wet on its surface, and it cannot form a paint film or causes the coating to crack and peel after drying.
But modified silane sealant and polyurethane sealant have excellent compatibility with coatings, the surface of it could be painted with coatings.

• Picture 3. Silicone sealant cause coatings to crack and peel

(4)Stain resistance
Concrete is porous materials, which is easy to be polluted, so stain resistance should be concerned while choosing sealants. Because ordinary silicone penetrates into the material pores due to the migration of plasticizers, it will cause permanent penetration pollution (can’t be cleaned); meanwhile, the surface of silicone sealant has electric charge, which is easy to absorb dust in the air, it will cause vertical pollution below the sealant joint after rain’s brush. The pollution (can be cleaned) not only influence the appearance of buildings, but it is also difficult to be cleaned. At the same time, it will increase the maintain cost. But MS sealants don’t contain silicon oil and small molecule plasticizer and exhibit excellent stain resistance.

• Picture 4. Pollution caused by silicone sealant

 
At present, it is still lack of professional worker in the construction aspect for sealant used on prefabricated buildings. Also, the construction environment on site is complex. So, how to make ordinary worker grasp sealant operation quickly and ensure construction quality also deserve to be considered. In fact, one-component sealant is more proper for the construction on site.



Picture 5. Special mixing machine and sealant gun for two-component sealant

In addition, primer should be paid more attention to. Because there may be mold discharge agent on the surface of components, and it’s hard to avoid that dust can’t be cleaned up during construction. But it is vital for primer to enhance adhesion, seal dust and promote waterproof property. So, brushing primer should be a necessary step when it comes to construct on site.

For the selection requirement of sealant used on prefabricated buildings, the detailed performance comparison of all kinds of sealants in market as follow: