As one of the earliest decorative materials, the quality awareness and safety performance of stone have not been widely concerned by people. However, with the continuous development and improvement of the stone industry and application, especially the large number of applications of stone high-rise curtain walls, the safety problems and related quality problems have become increasingly prominent.
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The safety and quality problems of stone materials mainly involve material selection and application, stone products, construction and installation, etc. The safety and quality of stone products are related to their material properties, production process, selected adhesives, etc.
natural quality
Stone is a kind of natural material. It pursues the integrity of natural materials. Its performance varies greatly. There are no two products that are completely consistent. The product contains a large number of natural defects, such as cracks, hidden cracks, argillaceous lines, and sutures, which can not be completely solved in the production process. Behind the beautiful stone pattern is often the weak link of its strength, which is very easy to break after removal and installation, causing casualties.
Natural stone products are characterized by thick and heavy. Each product can range from tens of kilograms to hundreds of kilograms. There is a one in ten thousand chance that it will fall down and cause unsafe factors for the whole building.
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physical property
Natural stone materials are divided into five categories according to major categories: granite, marble, limestone, sandstone and slate. The physical performance indicators of each category are different, and there are thousands of different types of stone materials. The physical performance and technical indicators of each type vary greatly. Even the same variety, due to different mining locations, mining levels, cutting directions and processes, will result in volume density, water absorption, compression strength Dispersion of physical performance indicators such as bending strength.
These physical performance indicators are essential elements for the design and calculation of stone bearing capacity and curtain wall safety factor. If the physical performance of the actual product is lower than the design requirements, or the physical performance of some stone products is lower due to natural factors, it will cause potential safety hazards for the stone curtain wall.
In addition, the physical properties of stone exterior curtain wall under natural conditions such as temperature difference deformation, wind vibration, freezing and thawing will directly affect its safety performance.
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chemical composition
The chemical composition of various natural stones is different, and the structure of rocks and minerals is also different. Different varieties and categories should be selected under different use environments.
Granite is mainly composed of silicate minerals such as quartz and feldspar. It has high hardness, high strength, acid and alkali resistance, and is suitable for various environments.
Marble, limestone and other stones, mainly composed of carbonate minerals, are not acid resistant and are only suitable for indoor use, not for external curtain walls in acid rain environment.
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Glue filling, repair glue and surface glue treatment process of stone
During the processing of marble, limestone and other plates, glue shall be poured, repaired and coated to improve glossiness, make up defects, enhance physical and mechanical properties or meet special requirements.
In this process, from the performance of the adhesive to the processing technology and other aspects, there are usually different degrees of quality problems.
The main problems are: domestic adhesive products can not meet the requirements of foreign products, which make the stone have a soft mirror luster; The transparent surface adhesive required for stone materials such as cave stones is lack of practical application, and many surface adhesives have quality problems such as shrinkage and discoloration after use;
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The performance of surface adhesive and repair adhesive does not conform to the durability of stone materials. Many marble and limestone products have surface adhesive peeling, slag dropping and fracture after use, resulting in serious damage to the appearance quality of the products and affecting the project quality;
The high viscosity of the top glue or the absence of vacuum glue filling process led to the failure of effective treatment of some hidden cracks and covered up the existence of defects, resulting in the phenomenon of fracture and personal injury during handling, installation and use.
Application process of back screen adhesive
According to the characteristics of stone, the back mesh adhesive is generally divided into three types:
One is that there are relatively few defects in the stone. Considering the adhesive problem between the back mesh of the stone with adhesive and the cement adhesive, the back mesh and adhesive will be removed during the stone construction stage, and the pavement construction will be carried out after brushing the stone protective agent. At this time, the back mesh adhesive is required to have a short-term effect and easy to remove;
The second is that for some special stone varieties, the back mesh and glue of the stone should be retained during construction due to the fragmentation of the stone plate after the back mesh is removed. At this time, the back mesh glue is required to have the performance of structural glue and good water and alkali resistance;
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The third is the reinforced adhesive and back mesh used for dry-hanging stone, which is required to have sufficient bonding strength, weather resistance and compatibility to enhance the positive wind pressure resistance of the plate and the safety factor of the curtain wall, and ensure the safety performance of the stone curtain wall.
Adhesive process
A large amount of adhesive is required for the bonding of stone products due to cracks or fractures, as well as the bonding required by the design process (such as the combination splicing of special-shaped stone and stone carving, the reinforcement bonding of dry-hanging stone, the composite bonding of stone mosaic and ultra-thin stone composite plates, etc.).
In the relevant stone standards and specifications of our country, it is required to strictly use epoxy resin adhesive for such occasions and cooperate with metal connectors to ensure the safety of stone products.
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The quality problems often occur in practice are as follows: due to the long solidification time of epoxy resin adhesive, many enterprises use unsaturated resin adhesive for fast bonding of such products to achieve the purpose of rush time;
Due to price reasons and market competition, some enterprises use fake and inferior epoxy adhesive products for bonding, while others use adhesives with a large amount of additives, resulting in a decline in bonding strength and aging resistance, which has seriously affected product performance;
Due to the consideration of production cost and unfamiliar with stone installation technology, many stone bonding and splicing are not carried by metal connectors, and are suspended in the air only by the adhesive force.
Bond strength of stone and adhesive
There are many reasons for this kind of problem, which can lead to hollowing and movement of stone, and wet sticking of stone on the wall can also lead to falling off and hurting people.
The main reason is that the stone protective agent does not select the bottom type, and the solvent-based protective agent with obvious waterproof effect is used to protect the stone, resulting in the decrease of the bonding strength between the stone bottom and the cement-based adhesive;
The cement-based adhesive is not added with effective adhesive components, resulting in the overall bond strength failing to meet the requirements that the bond strength of ground stone should not be less than 0.5MPa and the bond strength of wall stone should not be less than 1.0MPa specified in relevant standards and specifications;
When bonding with stone with back mesh, reactive resin adhesive is not used, or reinforcement measures are not taken when using cement-based adhesive, such as sand bonding process; When the point sticking method is used, the bonding area is too small; When reactive resin adhesive is used, epoxy adhesive is not used.
The bond strength of diamond disc saw cutting surface granite and ordinary cement mortar is generally 0.1MPa, and that of rough surface granite and ordinary cement mortar can be increased to about 0.2MPa. The bond strength of stone varieties with different density and water absorption is slightly different, and the bond strength of stone with cement mortar will generally decline after coating with protective agent.
Therefore, the special protective agent for the bottom surface shall be strictly used for the stones wet pasted on the ground and wall. The special cement-based adhesive with relevant resin adhesive shall be used for the stone adhesive to improve the bonding strength, so as to meet the standard requirements and ensure the safety of the wall stone engineering.
For stones with poor firmness and need to be constructed with back mesh, it is recommended to use reactive resin adhesive for pasting; If cement-based adhesive is used for pasting, it is necessary to use epoxy adhesive to paste a layer of sand on the back mesh adhesive, and the sand particles are half exposed;
When the wall is constructed by wet pasting method, other anti-falling measures should also be considered to ensure safety. In practical projects, if the adhesive area is too small or the epoxy resin adhesive is not used, the stone on the wall will fall off and the stone on the ground will be hollowed out. The stone will fall off after a period of use.