Shandong chuangyao analyzes the cracking of wall mortar and its causes

1、 Five phenomena and causes of nonstandard operation of wall mortar cracking

1. Cutting corners and not adding anti cracking measures for interface as required.

Cutting corners, unreasonable mortar formula, such as redispersible latex powder, hydroxypropyl methyl cellulose, and other adhesive additives are not enough, or the products of additives are unqualified. Wood fiber and anti crack fiber and other anti cracking materials are not added or the quality is not enough! Will cause the wall mortar cracking. According to Article 4.2.4 of code for quality acceptance of building decoration (GB 50210-2001), measures should be taken to prevent cracks when plastering at the joint of different materials. When reinforcing mesh is adopted, the width between reinforcement net and each interface shall not be less than 100 mm.

Many construction workers ignore this requirement, and there are no strengthening measures at the interface of different materials, especially at the interface of concrete block, concrete flue, concrete ventilation duct and light block filled wall. The obvious vertical cracks and even penetrating cracks appear in these parts, which gives people a sense of insecurity of structural cracks.

2. Less work and lack of sequence, combined with layered plastering operation times.


分析抗裂砂浆开裂的原因

According to Article 4.2.4 of code for quality acceptance of Building Decoration Engineering (GB 50210-2001), plastering works shall be carried out in layers. When the total thickness of plastering is greater than 35mm, strengthening measures shall be taken. Because cement mortar is a kind of material with high dry shrinkage (dry shrinkage rate is 4%), the method of layered plastering can effectively prevent the drying shrinkage cracking of cement mortar plastered wall. In the standard design, there are clear requirements for the number of times of cement mortar plastering. In the specification, the cement mortar plastering of internal and external walls is required to be completed in three times, and the thickness of each time is controlled within 5-8mm.

Most construction workers combine the required plastering times in operation. The random operation phenomenon of "less work and lack of sequence" appeared. The requirement of three times of survival is reduced to two times of survival, and the interval time of separate plastering is too short. The first time of plastering and spreading mortar has not been collected and hardened, the second time is started, and some of them are even finished once. The thickness of such a plastering is more than 20 mm. Due to the uneven base course, some parts may be more than 30 mm, so that cracks will inevitably appear in the process of hardening and drying shrinkage of high dry shrinkage cement mortar plastering wall. In addition, this kind of plastering will also make the brick concrete structure of the wall in the process of plastering, the concrete surface layer will produce a fall strong crack.

3. Save effort and increase the mix proportion of plastering mortar at will.

In the field operation, it is found that the operator increases the mix proportion of cement mortar for the convenience of operation. Plastering mortar is generally measured by volume ratio, and different mix proportion is worked out according to different uses of mortar, which is very necessary to match the strength of base course and prevent hollowing and cracking of plastered wall.

The actual mix proportion is far greater than the mix proportion required by the design. Some of the mix proportion of the base leveling and plastering is as high as 1:2.5-1:2, and the surface layer mortar reaches 1:1 or even higher, and even one mix proportion is used for base plastering and surface plastering. The increase of mix proportion not only wastes materials, but also increases the compactness and strength of mortar, which not only destroys the strength matching between mortar and masonry wall, but also enhances the brittleness of mortar, which makes the mortar wall more prone to hollowing and brittle crack after hardening. If the above phenomenon occurs, it is suggested to add hydroxypropyl methyl cellulose to increase the water retention performance of mortar in the early stage and reduce the curing performance. Add redispersible latex powder to improve the strength and flexibility of mortar, prevent mortar holding and cracking!

4. Do not understand the material properties, use the mortar after setting.

The causes of hollowing and cracking of wall surface, especially backhand ash of ceiling, were investigated. After eliminating the causes of raw materials and base cleaning, it was found that the use of mortar after initial setting was the main cause of hollowing cracking.

Some construction units store the plastering mortar on the floor because of alleviating the contradiction of using mixer in the interspersed work. In summer, when the temperature is high, the cement mortar is stored for a long time, the cement has been initially set, and the mortar is set. Then the operator uses it after mixing with water, which reduces the mildness and operability of the mortar. Practice has proved that few walls, especially the ceiling, are free from hollowing cracks.

5. For smooth effect.

The plastering surface shall be plastered with plaster. In article 4.2.6 of "code for quality acceptance of building decoration" (GB 50210-2001), the requirements of smooth, clean, flat and even plastering surface layer are put forward for ordinary plastering and advanced plastering engineering.

The operator achieves this requirement by scraping the pure cement paste on the plastering surface, so as to achieve better acceptance effect. I don't know that it is this method that touches the wall that is the main reason for the cracks in plastering, especially the external wall. In addition, the mixture ratio of the surface layer mortar is too large, and the plain paste put forward by the surface layer calendering is too thick during the operation, which is also a main reason for the cracks on the plastered wall surface.

2、 Control points

There are many reasons for the above illegal operations, and the main reasons are as follows:

The first is the possibility that the operator can operate and choose at will because there are many operation procedures in plastering process;

Second, in the construction project, the process control procedure is simplified by neglecting the operator's monitoring;

Third, they do not understand the nature of the material, and simply pursue the effect of appearance.

Therefore, the operation of plastering works should pay attention to the following aspects of control.

1. Aiming at the operators' pursuit of smooth surface plastering effect. The phenomenon of large mixing ratio and surface scraping and plastering can make the surface of wall surface smooth, straight, clean and not smooth. The method is: when plastering the surface of the wall, grasp the appropriate heat, use the wood trowel to smooth out the original slurry, and then use the steel trowel to press the surface into a fine, brown eye like surface. It can also be used in the original pulp calendering, and then the sponge block dipped in water is gently wiped into a fine wool surface.

2. The plastering mortar should be mixed in a centralized way and measured by a specially assigned person, which is not affected by the operator. The supervision and quality management personnel should inspect the mortar mix proportion and correct the abnormality in time.

3. The cement mortar should be mixed as soon as possible. The storage time of mortar mixed in summer should not exceed 1.5h, and it can be used up within 2h in spring and autumn.

4. For different collective joints of plastering base, reinforcement net shall be set up according to the specification requirements. The reinforcing net shall be laid first, and the supervision personnel shall conduct concealed inspection and acceptance, and the plastering can be carried out after handling concealed inspection procedures.

For cement mortar plastering, if we can master the characteristics of large dry shrinkage rate of cement mortar and insufficient rigidity and toughness, it is mainly to select appropriate additives, hydroxypropyl methyl cellulose and redispersible emulsion powder to adjust, strengthen the control of operation process, and operate in strict accordance with the requirements of specifications and standards.


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