Discussion on the Safety of Facing Tiles for External Thermal Insulation Exterior Walls
Due to the influence of climatic conditions, consumption levels, and aesthetic habits in many areas of China, decorative facade bricks are popular with many real estate developers and residents. However, there are earliest hearings about the emptying and falling of the facing bricks, which can not help but worry people. Due to the influence of climatic conditions, consumption levels, and aesthetic habits in many areas of China, decorative facade bricks are popular with many real estate developers and residents. However, there are earliest hearings about the emptying and falling of the facing bricks, which can not help but worry people.
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In the past ten years, the central building energy conservation policy has been seriously implemented throughout the country, and the external thermal insulation composite wall has become the preferred form of wall insulation. Is it safe to use decorative tiles on lightweight, low-strength insulation? This has become the focus of attention in the industry
The facing brick directly pasted on the external wall will allow the wall to bear two kinds of external loads: one is the self-weight of the facing brick and adhesive, which is about 0.25 kN/m2; the second is the wind load (mainly negative wind pressure) in the Beijing area. The calculated value of the maximum negative wind pressure at a height of 20 meters above ground is about 2.1 kN/m2, and about 3.6 kN/m2 at a height of 100 meters above the ground (in coastal areas, this value may reach 8-9 kN/m2). It depends on the bond strength between the adhesive and the exterior wall. According to the requirements of the National Industry Standard “Construction and Acceptance Regulations for Exterior Facing Tile Projectsâ€, the exterior façade bricks have been tested for the bond strength of facing tiles, and the pull-out strength is not less than 0.4 MPa, ie 400 kN/m2. Visible and acceptable The initial adhesion provided by the project has a sufficiently large safety factor to overcome external loads. Therefore, the vast majority of exterior wall facing brick projects can be used safely for a long period of time.
The reason that the problem of falling off of the facing brick in the individual project over a period of time is that the temperature and moisture migration over the years have weakened the bond strength between the facing brick and the outer wall. There are two main causes: First, severe and repeated temperature changes on the external wall surface produce large temperature differential stress at the interfaces of different thermal expansion coefficient materials; Second, due to insufficient adhesive construction, facing brick or caulking High water absorption of the agent, impeded penetration of water vapor, etc., cause moisture accumulation on the inside of the facing tile, causing frost heave damage in winter in cold regions. To avoid this problem, it is necessary to thoroughly understand and seriously implement the “Construction and Acceptance Regulations for Exterior Facing Tile Projectâ€. The "Regulations" in the design of the basic provisions of a number of issues worth noting: 1 the use of the process due to temperature changes caused by shrinkage, is often caused by external wall decoration bricks, one of the main factors. (2) Set expansion joints to prevent wall structure deformation and cracking and shedding caused by temperature deformation of the exterior wall facing tiles themselves. 3 The use of flexible waterproof material caulking, can absorb deformation, increase the impermeability of the veneer. 4 If the joints of the facing tiles are too small, they will easily fall off under the effect of temperature stress. The procedure is defined as "the width should not be less than 5mm." The "Regulations" also emphasizes that the adhesion strength of facing bricks should be inspected in accordance with the "Standard for the Inspection of the Bonding Strength of Building Facing Tiles". Other research results show that: to reduce the shear stress caused by temperature difference at the interface, to prevent tiles from falling off, flexible adhesives should be used as far as possible; appropriate adhesive layer thickness should be used; use low-elasticity modulus and small-size ceramic tiles; The spacing between the tile facing seams should be appropriately reduced; the water vapor permeability of the composite wall should be calculated to avoid the use of poorly air-permeable insulation materials and facing tiles, and should be applied to lightweight filling walls such as air-entrained mortar. Gas or air separation measures.
In the external thermal insulation composite wall, the adhesive strength of the facing brick and the external thermal insulation system still needs to reach 0.4 MPa. The self-weight of the exterior thermal insulation system with the facing brick and the negative wind pressure received are linked by the external thermal insulation system and the structural wall. Force to bear. Therefore, three problems need to be solved: First, can the adhesive strength of facing bricks and external thermal insulation systems reach 0.4 MPa? Second, whether the external thermal insulation system and the structural wall joint strength can overcome the self-weight of the external insulation system with facing tiles Negative wind pressure transmitted? The third reason is how to solve the new problems caused by the insulation system in the middle.
(1) Not long ago, we did a batch of ceramic tile pull test. One part is reinforced with a glass fiber mesh, and the glass fiber mesh used in the 8 sets of specimens has different types, specifications, and layers. The highest drawing strength is 0.46 MPa, the lowest is 0.36 MPa, and the average is 0.40 MPa. It is accurate to the decimal point according to the standard. All requirements shall not be less than 0.4 MPa. Another group of steel wire mesh reinforced with different types, specifications and wire diameters is used to reinforce the surface mortar with the highest drawing strength of 0.74 MPa and the lowest 0.51 MPa, with an average of 0.66 MPa. The question may be controversial: where does the breakage cut before the drawing? ? The "Standard for the Inspection of the Bonding Strength of Building Facing Tiles" stated that "breakages should be cut from the surface of the facing brick to the surface of the substrate". Some people emphasized that they must cut into the surface of the structural wall; others also argued that they could only cut into the surface of the composite thermal insulation wall, that is, the surface of the anti-crack mortar, because the decorative tile was attached to the base surface of the composite thermal insulation wall. Recently, disputes have tended to subside and most people are in favor of the latter view.
(2) The joints between external insulation systems with decorative tiles and structural walls are generally based on the combination of bonding and mechanical anchorage. For new construction, bonding is often the mainstay. For the most widely used post-polystyrene board insulation system, the polyphenyl board is attached to the structural wall with an adhesive. The relevant standards require that the tensile bond strength between the adhesive and the structural wall be not less than 0.40 MPa. Adhesive and polystyrene board tensile bond strength of not less than 0.10 MPa, bonding area rate of not less than 40%; polystyrene board tensile strength of not less than 0.10 MPa; trowel mortar and polystyrene board The tensile bond strength is not less than 0.10 MPa. It can be seen that the weakest link between the external insulation system and the structural wall is at the interface between the adhesive and the polystyrene board, where the minimum bond strength is not less than 0.04 MPa, ie 40 kN/m2. Obviously, there is a great safety factor to overcome the negative wind pressure at a height of 100 meters above the ground. The self-weight of the external insulation system with a facing brick is generally about 0.5 kN/m2, not to worry too much. The role of mechanical anchors has not been included.
(3) Insulation boards and plastering mortars sandwiched between them have brought two new problems:
The first is that since the presence of the reinforcement network plays a key role in the drawing strength of the tiles, the durability of the reinforcement network becomes the key to preventing the facing tiles from falling off over time. The glass fiber is susceptible to alkali corrosion, and the cement mortar it is located in is an alkaline environment. To prevent the glass fiber from being greatly reduced in strength due to alkali corrosion, the glass fiber mesh that meets the requirements of the standard should be selected. Although the reinforcing effect of the uncorroded steel wire net is more prominent, its anti-rust problem needs special attention. Sometimes the wire mesh that is seen on the construction site is zinc plated, but it can be observed without the wall. This is very dangerous. According to our experimental study, hot-dip galvanized steel wire mesh or galvanized wire woven mesh should be selected first; the thickness of the galvanized layer must exceed a minimum value; if possible, choose a coating material with better rust-proof effect; Avoid bends during transportation and construction; Both sides of steel wire mesh should be covered with a certain thickness of mortar protective layer.
Second, under the long-term effect of the self-weight of facing bricks, adhesives, and plasters, the shear deformation of polystyrene panels causes horizontal cracks in the mortar, and the intrusion of water will affect the durability of the bonding of the facing tiles. The prevention and control method is to leave horizontal expansion joints at a certain height, and to install the intermittent brackets in the joints to bear their own weight; the density of the polyphenyl board can also be appropriately increased to reduce the deformation.
In the past ten years, the central building energy conservation policy has been seriously implemented throughout the country, and the external thermal insulation composite wall has become the preferred form of wall insulation. Is it safe to use decorative tiles on lightweight, low-strength insulation? This has become the focus of attention in the industry
The facing brick directly pasted on the external wall will bear two kinds of external load on the wall: one is the self-weight of the facing brick and adhesive, about 0.25 kN/m2; the second is the wind load (mainly negative wind pressure) in the Beijing area. The calculated value of the maximum negative wind pressure at a height of 20 meters above ground is about 2.1 kN/m2, and about 3.6 kN/m2 at a height of 100 meters above the ground (in coastal areas, this value may reach 8-9 kN/m2). It depends on the bond strength between the adhesive and the exterior wall. According to the requirements of the National Industry Standard “Construction and Acceptance Regulations for Exterior Facing Tile Projectsâ€, the exterior façade bricks have been tested for the bond strength of facing tiles, and the pull-out strength is not less than 0.4 MPa, ie 400 kN/m2. Visible and acceptable The initial adhesion provided by the project has a sufficiently large safety factor to overcome external loads. Therefore, the vast majority of exterior wall facing brick projects can be used safely for a long period of time.
The reason that the problem of falling off of the facing brick in the individual project over a period of time is that the temperature and moisture migration over the years have weakened the bond strength between the facing brick and the outer wall. There are two main causes: First, severe and repeated temperature changes on the external wall surface produce large temperature differential stress at the interfaces of different thermal expansion coefficient materials; Second, due to insufficient adhesive construction, facing brick or caulking High water absorption of the agent, impeded penetration of water vapor, etc., cause moisture accumulation on the inside of the facing tile, causing frost heave damage in winter in cold regions. To avoid this problem, it is necessary to thoroughly understand and seriously implement the “Construction and Acceptance Regulations for Exterior Facing Tile Projectâ€. The "Regulations" in the design of the basic provisions of a number of issues worth noting: 1 the use of the process due to temperature changes caused by shrinkage, is often caused by external wall decoration bricks, one of the main factors. (2) Set expansion joints to prevent wall structure deformation and cracking and shedding caused by temperature deformation of the exterior wall facing tiles themselves. 3 The use of flexible waterproof material caulking, can absorb deformation, increase the impermeability of the veneer. 4 If the joints of the facing tiles are too small, they will easily fall off under the effect of temperature stress. The procedure is defined as "the width should not be less than 5mm." The "Regulations" also emphasizes that the adhesion strength of facing bricks should be inspected in accordance with the "Standard for the Inspection of the Bonding Strength of Building Facing Tiles". Other research results show that: to reduce the shear stress caused by temperature difference at the interface, to prevent tiles from falling off, flexible adhesives should be used as far as possible; appropriate adhesive layer thickness should be used; use low-elasticity modulus and small-size ceramic tiles; The spacing between the tile facing seams should be appropriately reduced; the water vapor permeability of the composite wall should be calculated to avoid the use of poorly air-permeable insulation materials and facing tiles, and should be applied to lightweight filling walls such as air-entrained mortar. Gas or air separation measures.
In the external thermal insulation composite wall, the adhesive strength of the facing brick and the external thermal insulation system still needs to reach 0.4 MPa. The self-weight of the exterior thermal insulation system with the facing brick and the negative wind pressure received are linked by the external thermal insulation system and the structural wall. Force to bear. Therefore, three problems need to be solved: First, can the adhesive strength of facing bricks and external thermal insulation systems reach 0.4 MPa? Second, whether the external thermal insulation system and the structural wall joint strength can overcome the self-weight of the external insulation system with facing tiles Negative wind pressure transmitted? The third reason is how to solve the new problems caused by the insulation system in the middle.
(1) Not long ago, we did a batch of ceramic tile pull test. One part is reinforced with a glass fiber mesh, and the glass fiber mesh used in the 8 sets of specimens has different types, specifications, and layers. The highest drawing strength is 0.46 MPa, the lowest is 0.36 MPa, and the average is 0.40 MPa. It is accurate to the decimal point according to the standard. All requirements shall not be less than 0.4 MPa. Another group of steel wire mesh reinforced with different types, specifications and wire diameters is used to reinforce the surface mortar with the highest drawing strength of 0.74 MPa and the lowest 0.51 MPa, with an average of 0.66 MPa. The question may be controversial: where does the breakage cut before the drawing? ? The "Standard for the Inspection of the Bonding Strength of Building Facing Tiles" stated that "breakages should be cut from the surface of the facing brick to the surface of the substrate". Some people emphasized that they must cut into the surface of the structural wall; others also argued that they could only cut into the surface of the composite thermal insulation wall, that is, the surface of the anti-crack mortar, because the decorative tile was attached to the base surface of the composite thermal insulation wall. Recently, disputes have tended to subside and most people are in favor of the latter view.
(2) The joints between external insulation systems with decorative tiles and structural walls are generally based on the combination of bonding and mechanical anchorage. For new construction, bonding is often the mainstay. For the most widely used post-polystyrene board insulation system, the polyphenyl board is attached to the structural wall with an adhesive. The relevant standards require that the tensile bond strength between the adhesive and the structural wall be not less than 0.40 MPa. Adhesive and polystyrene board tensile bond strength of not less than 0.10 MPa, bonding area rate of not less than 40%; polystyrene board tensile strength of not less than 0.10 MPa; trowel mortar and polystyrene board The tensile bond strength is not less than 0.10 MPa. It can be seen that the weakest link between the external insulation system and the structural wall is at the interface between the adhesive and the polystyrene board, where the minimum bond strength is not less than 0.04 MPa, ie 40 kN/m2. Obviously, there is a great safety factor to overcome the negative wind pressure at a height of 100 meters above the ground. The self-weight of the external insulation system with a facing brick is generally about 0.5 kN/m2, not to worry too much. The role of mechanical anchors has not been included.
(3) Insulation boards and plastering mortars sandwiched between them have brought two new problems:
The first is that since the presence of the reinforcement network plays a key role in the drawing strength of the tiles, the durability of the reinforcement network becomes the key to preventing the facing tiles from falling off over time. The glass fiber is susceptible to alkali corrosion, and the cement mortar it is located in is an alkaline environment. To prevent the glass fiber from being greatly reduced in strength due to alkali corrosion, the glass fiber mesh that meets the requirements of the standard should be selected. Although the reinforcing effect of the uncorroded steel wire net is more prominent, its anti-rust problem needs special attention. Sometimes the wire mesh that is seen on the construction site is zinc plated, but it can be observed without the wall. This is very dangerous. According to our experimental study, hot-dip galvanized steel wire mesh or galvanized wire woven mesh should be selected first; the thickness of the galvanized layer must exceed a minimum value; if possible, choose a coating material with better rust-proof effect; Avoid bends during transportation and construction; Both sides of steel wire mesh should be covered with a certain thickness of mortar protective layer.
Second, under the long-term effect of the self-weight of facing bricks, adhesives, and plasters, the shear deformation of polystyrene panels causes horizontal cracks in the mortar, and the intrusion of water will affect the durability of the bonding of the facing tiles. The prevention and control method is to leave horizontal expansion joints at a certain height, and to install the intermittent brackets in the joints to bear their own weight; the density of the polyphenyl board can also be appropriately increased to reduce the deformation.
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