The prevention of leakage of underground works and the waterproofing of urban underground power, thermal power, communication tunnels, and inspection wells are not only issues of concern in the field of waterproofing, but also the impermeability and water repellent treatment of such projects.
1 Difference of impermeability of waterproof material
According to the water-proof material to the water shielding method, we can divide the waterproof material into two kinds, one kind is the closed type waterproof material, one kind is the waterproof material with the capillary action. The closed type waterproof material is based on an organic polymer material, which is a dense hydrophobic material, completely impervious to water, and has a capillary action waterproof material, and is a mortar and a concrete, a polymer formed using various types of water repellents. Cement waterproof material is the main body. The characteristics of these materials are that they have a certain degree of water absorption, they can firmly connect with the base layer to form an interface, allow the water to penetrate through the interface of convergence, and depend on the difference in the density of the material itself, in different thicknesses. The water is gradually shielded to achieve the purpose of impermeability and waterproofing. The ability of a watertight material with capillary action to resist pressure water, i.e., the ability to resist water seepage, and the duration of this ability are distinguishable, and it obviously depends on the density and thickness of the material itself.
Most of the flexible materials are hermetically sealed waterproof materials. They have excellent impermeability on the surface of the water, but have low impermeability on the back surface. According to data, the impermeability of flexible coatings and coils is very different between the water surface and the back surface. See Table 1.
Table 1 Coating Waterproofing and Coil Waterproofing Impermeability MPa
Types of
Water surface impermeability
Back surface impermeability
Silicone rubber coating
Two felt three oil
Three felt four oil
1.0
0.4
l. 5
0.3
-
-
As can be seen from Table 1, with the same coating material, the impermeability of the water-facing surface is about three times higher than that of the backwater surface. Even if it is used as a common linoleum waterproofing surface, its water impermeability is obviously dependent on the thickness of the material. Change and produce mutations within a certain thickness range. When using a waterproof material that meets the impermeability requirements, consider the time that impermeability of the impervious material can be maintained. It is an important indicator to measure the long-term water resistance of the impervious material. That is, the impervious material must have a certain thickness reserve in order to talk about waterproofing. The impermeability of the layer. For materials with the same density, the thickness determines the durability of the impermeability.
Water resistance and impermeability are conceptually different, and impermeability is the ability of the waterproof material to resist water under pressure. The water resistance index of flexible materials is generally measured under a water pressure of 0.2-0.3 MPa for 30 minutes to observe the water permeability of the material. The water resistance characteristic of this experimental method is suitable for reference when the flexible material is used for water-repellent surface usage. However, when used on the back surface under long-term pressure water, the resistance to water seepage is determined due to the material properties of the flexible material. Below the water surface. The main reason is that: under the effect of pressure water, the waterproof layer is easy to be separated from the base layer, and the flexible material is completely impermeable, and pressure water is gathered from the seepage surface to the completely impermeable surface. Secondly, the increasing pressure water between the interfaces will exert a great pressure on the flexible waterproof layer, which will cause the waterproof layer to bulge, stretch, thin, and crush and lose its waterproof function.
For the rigid waterproof material mixed with a water repellent agent and the polymer cement type waterproof material with hardness and softness, there is no difference in the impermeability between the front surface and the back surface.
2 back surface waterproof special
The underground construction of the building is like a ship immersed in water. If the water breaks through the lines of defense of the walls, floors, deformation joints, etc. and enters the room, it cannot be processed from the outside and can only be treated from the back surface. Backwater surface treatment and flood control surface water treatment are very different, because of the different locations of the waterproof layer, the selection of materials and construction methods are also different, such as still facing the water surface treatment methods will often fail. Due to the special location of the back surface of underground engineering, it is difficult to use traditional sealed waterproof materials because:
1) Flexible materials (solvent type or reactive type coating films, macromolecules, modified bituminous coils themselves are far from mortars and concrete concretes, and it is difficult to achieve a solid connection with the base surface.
2) When the flexible material is waterproof on the water front surface, it can exhibit good water resistance, but its ability to resist water seepage can not reach the water surface effect for the backwater surface with water seepage pressure.
Due to improper material selection, there are many examples of failure to manage leaks. For example, a residential building in Beijing Shengguzhuang, the basement of the building for the framework of brick-concrete structure, due to the destruction of the outer waterproof layer, every rainy season, the groundwater level rises, indoor multiple leaking water, the area of ​​water up to tens of centimeters. In the treatment, polyurethane waterproofing and brick wall protection were selected. After the implementation of precipitation, according to the above scheme, after the brick wall was laid for a week, it was found that there were many wet traces in the newly built brick wall, and the brick wall was opened in the wet trace area. It was found that many holes appeared in the polyurethane film. Lots of drums.
The failure of the project is caused by improper material selection: 1) Backwater surface waterproofing adopts closed type waterproof material, and the pressure water outside the structure will cause the waterproof layer to peel off from the base surface and even play a drum; 2) The weak sealed waterproof layer under the action of pressure water , will be squeezed by the water getting more and more between the interface and failure.
This project example shows that the backwater surface is particularly waterproof under the pressure of water seepage.
3 materials and materials
Each type of waterproof material has its own characteristics. When we implement the waterproof material for the back surface of the underground envelope structure, we must consider the important factor that the envelope structure is subjected to pressure water.
The waterproof materials that can be applied on the back surface include the following types: 1) water-based coating materials and cement polymer-based composite coating materials; 2) mortar and concrete mixed with water repellents; 3) polymer cement-based waterproof materials.
The use of various materials should pay attention to the length and length, in order to properly and reasonably use the material, to achieve durable, reliable waterproof purposes.
The following points should be noted on the material selection:
1) When selecting water-based coating materials, the environment, humidity and ventilation of the construction should be fully considered because the water-based coating material will naturally show a water-repellent property after moisture evaporation, and the degree of evaporation of water will greatly affect the waterproof performance of the film. . In the past, many underground construction projects had used neoprene asphalt latex paint as the basement waterproofing layer. Although the water resistance of these coatings was very good, they were often due to the lack of time required for construction. In the case of back-to-earth soils, the coating layer with un-evaporated water, when immersed in groundwater, will undergo water swelling and lose its waterproof performance. Therefore, when using water-based coating materials, it is necessary to ensure the drying time, so that the moisture in the film completely evaporated. In addition, when using water-based coating materials, it is also necessary to consider the waterproof properties of various types of materials after long-term soaking. There are many materials, and the water absorption rate after 168 h soaking is as high as 20% to 40%. At this time, the waterproof material has lost its waterproof function.
2) When selecting a cement polymer-based composite coating, attention should be paid to the mechanical properties of the material after 168 h of water immersion.
3) When selecting water repellent products, the volume shrinkage of the mortar during hardening and hardening should be fully considered.
4) When using polymer cement waterproof mortar and concrete, the volume shrinkage and adhesive strength of different types of polymer cement should be considered, as well as the suitability of polymer latex and hydraulic material during the mixing process.
When considering backwater maintenance, in addition to careful selection of materials, the construction process must also be considered.
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