The selection of sealant for joint sealing on prefabricated buildings

03 / 03 / 2021

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 Article 5.3.4 of JGJ 1-2014 <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, in the south rainy areas of China, anti-alkali phenomenon of concrete may also occur, 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 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. But 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, that is, sealant joints are treated as opening seams. Besides, sealants 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. So, 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 are as follow:

 

Types of sealant Silicone Polyurethane Modified silane  
Main chain structure Si-O C-O C-N C-O C-C Si-O UV energy
Bond energy
(KJ/mol)
444 339 284 339 348 444 399



Picture 2. Damage of polyurethane sealant after UV radiation

(3)Paintability
When the joint 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

(5)Anti-movement capacity
Because of the existence of interlayer movement caused by strong wind and earthquake, expansion movement caused by hot expansion and cold contraction, dry movement caused by dry shrinkage and sedimentation movement caused by foundation sedimentation on prefabricated buildings, the force requirement of sealant is very high. Therefore, sealant must have outstanding movement capacity and elasticity. Nowadays, the joint width of domestic prefabricated buildings is usually designed as 20 mm, and the deformation of joint are mainly caused by thermal expansion and contraction of components, so we could calculate according to joint width and choose sealant with proper movement capacity.

Simplified formula for calculation of ordinary PC exterior joint width:
W>δ/ε×100%+|We|
W=designed joint width(domestic standard width is generally 20mm);
δ=temperature difference deformation of component/mm=L·α·△T;
L=length of component(deformation direction)
α= concrete linear expansion coefficient, usually taken as(10×10-6)m/ ℃
△T=limit temperature difference of the concrete interface, generally taken as 80℃;
ε=sealant movement %;
|We|=joint construction error(generally taken as 5mm)


Calculation example:
When the length of concrete component board is L=3000mm, sealant movement ε>L·α·△T/ (W -|We|)·100%=3000× 10×10-6 ×80/(20-5)×100%=16%

As the stipulation of JC/T 881 <Building sealants for joint of concrete construction>, the movement level of sealants can be divided into 7.5, 12.5, 20 and 25, so the movement capacity of sealant used in the joint of prefabricated buildings should be at least 20 or even above. Besides, sub-level of sealant should be noticed that it is divided into low modulus and high modulus. Because the surface of sealant is relatively loose and has low strength, if sealants have high modulus and cohesion strength, it’s easy to cause bond failure for sealant on the interface of concrete. But sealant with low modulus is relatively soft, the cohesion strength is lower than interface adhesion strength, it can adapt the deformation better and not easy to be destroyed, so the movement capacity of sealants used on prefabricated buildings should be 20LM and 25LM.
When building joints have permanent deformation due to earthquake or material dry and shrinkage, which will produce continuous stress on the sealant. And MS sealant exhibits excellent elasticity and stress relaxation ability, it can release prestress to the maximum when subjected to permanent deformation and ensure sealant not to be damaged.

(6)Construction performance
Sealant can be divided into one component and two components, take modified silane sealant as example, the construction performance comparison of one-component and two-component sealant as follow:

 
Construction performance
(MS sealant)
Advantage Disadvantage
One-component sealant
(Moisture curing)
  • Easy for construction operation
  • Less loss
  • Toning by factory, no difference
Relatively slow curing speed
Two-component sealant
(Reaction curing)
Fast curing speed
  • Special mixing equipment and sealant gun are needed, construction requirement is high
  • Organic solvent (flammable) is needed to clean tools
  • Adhesion seriously depends on primer
  • Toning is on site, difference would occur
 
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. As the reality, 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:

 
  Modified silane Polyurethane Silicone
Adhesion of concrete Excellent Good Bad
Paintability Excellent Excellent Bad
Stain resistance Excellent Excellent Bad
Heat resistance Good Bad Excellent
Weather resistance Good Bad Excellent
Storage stability Good Bad Excellent
Application substrate Concrete, metal Concrete, metal Glass, metal

In conclusion, modified silane sealant should be chosen for joint sealing of exterior prefabricated wall, as for sealant for joint of interior wall, modified silane sealant or polyurethane sealant could be chosen.

The technique properties of sealant used for joint of prefabricated buildings should conform to stipulation of present industry standard JC/T 881 < Building sealants for joint of concrete construction> . The depth of sealant should be between the half of the width sealant and the width of sealant, that is ½Width w ≤Depth d ≤Width w, and the depth should be not less than 8mm. And the sealant construction requirements should conform to Article 12.3.12 of JGJ 1-2014< Technical specification for precast concrete structures>: “ Waterproof construction of exterior wall panel joint should be up to the stipulation as follow:
1. Joint cavity of panels should be cleaned up before waterproof construction
2. Back material should be filled as the design requirement
3. Sealing material should be full, non-porous, homogenous, straight and have smooth surface, and the depth should meet the design requirements.”


Specific construction operation should be in accord with the construction approach offered by manufacturers.


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