Breaking strength and alkali resistance strength retention rate
Breaking strength and alkali resistance strength retention rate are important properties of glass fiber mesh cloth. Breaking strength is inseparable from the number or weight of glass. At present, the weight of glass fiber mesh cloth used for external wall insulation system in the Chinese market is generally around 130g per square meter, and the strength is generally low. Its normal breaking strength is only about 1000N/5cm. In Europe or the United States, where the external wall insulation technology and regulations are relatively mature, the breaking strength requirements of glass fiber mesh cloth are much higher than those in my country. The German Building Technology Committee (DIBt) requires that the normal breaking strength of glass fiber mesh cloth used in external wall insulation system is ≧1750N/5cm, and the alkali resistance retention rate is ≧50%. If domestic products meet this requirement, the weight of glass fiber mesh cloth per square meter must be above 160g.
It is undoubtedly necessary to improve the breaking strength standard, especially for polymer cement plastering mortar with high cement content and cement strength grade. The damage of these things to the fiberglass mesh is not only chemical corrosion, but also the crystallization pressure corrosion of calcium hydroxide and calcium sulfoaluminate. At the same time, the high water absorption rate of mineral materials also has an adverse effect on the fiberglass mesh. Experiments have shown that after soaking in distilled water, the strength of the fiberglass mesh will be reduced by about 30%.
My country still lacks standards for fiberglass mesh used in exterior wall insulation systems, especially the alkali resistance strength retention rate and alkali resistance test methods. The alkali solutions used by testing agencies or manufacturers in various places are different. Some use NaOH solution, some use 80℃ cement slurry, and some use NaOH, KOH, Ca (OH) 2 mixed alkali solution. Since the formulation of this performance indicator has a great impact on the performance trend, price and market tolerance of products on the market, relevant research units must conduct a unified survey of the performance of domestic fiberglass mesh, refer to foreign standards, and formulate standards suitable for my country's national conditions.
Cement-free pure polymer plastering mortar has no alkali corrosion and low water absorption rate. Fiberglass mesh cloth can function normally for a long time, which is undoubtedly the safest for the long-term crack resistance of the protective surface layer.
Fiberglass mesh cloth and plastering mortar should be closely related
When subjected to tensile deformation, the plastering mortar must work in a coordinated manner with the fiberglass mesh cloth. The two materials must have a good correlation. The mesh cloth with poor correlation with the plastering mortar will still produce cracks in the early stage, no matter how high the strength is.
When the protective surface layer begins to bear tension, the mesh cloth should be able to be in a stress state as soon as possible to exert its tensile resistance and disperse the stress. Although the mesh cloth cannot prevent the plastering mortar from generating the first crack when a certain tensile strain is reached, it can effectively delay the development time of the first crack and limit the development width of the first crack, and distribute stress through its warp and weft nodes, so that the entire plastering mortar is evenly stressed and the cracks are evenly distributed.
In Europe and the United States, it is generally believed that cracks with a width of less than 0.2mm are harmless. For the external wall insulation system, the more important thing is not to prevent the occurrence of cracks (just like steel bars cannot prevent the cracking of concrete, whether the plastering mortar will eventually crack depends on its cracking strain), but to delay the deformation time as much as possible, thereby delaying the cracking, and at the same time limiting the width of a single crack.
Therefore, the mesh cloth with good correlation with the plastering mortar should have the following properties:
1. Higher node strength
If the node of the mesh cloth slips when it is pulled, the stress in the initial stage of force will be borne by the plastering mortar and cracks will occur.
The test method of node strength is: cut a 330mm×50mm test strip (the weft yarn must be an odd number) along the weft direction, cut all the yarns on both sides about 150mm from the end of the test strip, and only leave one weft yarn in the middle; after 50mm (or 6 nodes apart), cut the middle weft yarn. Put the test strip on the tensile test machine for tensile test, clamp the weft yarn at one end of the test machine, and clamp all the yarns at the other end, and read the force (N) of the yarn withdrawal. The external wall insulation system generally requires the node strength of the fiberglass mesh cloth to be ≥3.0N/single wire.
2. Lower tensile strain
If the mesh cloth does not slip at the node when it is tensile, and the tensile strain is large, it will also cause the plastering mortar to crack at an early stage.
The tensile strain of the fiberglass mesh cloth includes the strain at the early stage of tensile strain and the strain at break. It can be expressed by the tensile force at elongation of 0.25%, 0.5%, and 1.0%, as well as the tensile force (breaking strength) and breaking strain at break.
3. Smaller wire deflection
The wire will inevitably deviate when the mesh cloth is woven. If the deviation is too much, it will cause eccentricity when subjected to force, which will also affect the use effect. Generally speaking, within a length of 1m, the wire deflection shall not exceed 3 times the wire's diameter.
