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Gas masonry wall plastering crack control


The aerated block has low strength, high water absorption and large shrinkage deformation, and cracks often appear in the traditional wall masonry and wall plastering process. This quality of common disease, has been plagued by developers and construction enterprises. There are more and more complaints about wall cracks and even leakage. Therefore, analyzing the causes of cracks and seeking measures to control cracks has become the subject of common concern of all parties in the construction industry. Based on the author's many years of field construction practice, this paper discusses some experiences on the control of plastering cracks on the walls of aerated masonry.

The aerated block has low strength, high water absorption and large shrinkage deformation, and cracks often appear in the traditional wall masonry and wall plastering process. This quality of common disease, has been plagued by developers and construction enterprises. There are more and more complaints about wall cracks and even leakage. Therefore, analyzing the causes of cracks and seeking measures to control cracks has become the subject of common concern of all parties in the construction industry. Based on the author's many years of field construction practice, this paper discusses some experiences on the control of plastering cracks on the walls of aerated masonry.

I. Performance of aerated block

Aerated masonry block is a kind of light wall material, with good heat insulation performance. In hot summer and cold winter areas, aerated masonry block wall of more than 160㎜ thickness can meet the heat insulation requirements in summer without thermal insulation. The main raw materials of aerated blocks are cement, lime, sand and fly ash, etc. They are cement concrete products with large water-cement ratio and more binder, which are easy to absorb water and expand and shrink without water. Although autoclaved, the shrinkage rate can only be controlled within 0.04%-0.06% due to cost limitation, which is larger than that of sintered clay bricks.

Second, the cause analysis of cracks

There are many reasons for the plastering cracks of aerated masonry walls. For example, left over from the previous process: masonry, design and structure, foundation uneven settlement caused by chain reaction and other reasons. However, the most common more frequent or temperature change, shrinkage deformation, mix ratio is not suitable, plastering process improper operation and other factors caused. According to the causes of the most common cracks can be divided into four categories: temperature cracks, shrinkage cracks, cracks caused by design and structural defects, cracks caused by construction quality.

1. Temperature crack

Due to the temperature change caused by the temperature difference between day and night, indoor and outdoor temperature difference, and seasonal temperature difference, the material will cause thermal expansion and cold contraction. When the constraint conditions the temperature deformation caused by temperature stress is big enough, the wall body can produce temperature crack, especially in the thermal bridge of palisade structure, temperature stress, and appeared frequently, such as frame beam under along the wall at the top of the horizontal crack, Windows and doors hole Angle cracks, daughter foot horizontal cracks, vertical cracks between concrete frame column and wall, The diagonal cracks on the inner and outer longitudinal walls at both ends of the top floor of a building are in their shape. Throughout eight &; A word or & other; X” Type. Due to the different linear expansion coefficient between materials, the thermal stress will inevitably cause the internal and external expansion and contraction deformation of the structure is not consistent, and will inevitably crack the plastering layer.

The thermal insulation design of some envelope structures is too small, so that the wall is often invaded by temperature waves, repeated deformation; External insulation is not caught before internal plastering, which is also an important reason for temperature cracks.

The top layer has the greatest influence on temperature and frequent cracks. Its mechanism of action is as follows: Under the sunlight (especially the southern region in summer) roof beam slab and other thermal bridge in temperature can be as high as 60 ℃ ~ 70 ℃, and its masonry is only 30 ℃ ~ 35 ℃, such a large temperature difference, plus twice than linear expansion coefficient of concrete masonry approximation, according to the Wang Tiemeng book "the structure of the crack control", put forward the calculation theory and formula, The principal tensile stress in masonry can be calculated. When the mortar strength is M5.0 and the block strength is A7.5, the design values of axial tensile strength and shear strength along the ash joint section are only 0.14MPa and 0.12MPa, while the bending tensile strength along the ash joint (through joint) is only 0.25MPa (0.12MPa). The main tensile stress caused by temperature difference is greater than 50% ~ 300% of masonry resistance. If the lack of strengthening measures, it will inevitably crack the masonry and plaster layer.

2, dry shrinkage crack

The dry shrinkage value of aerated concrete block is 0.30 ~ 0.45mm/m, and with the loss of water, the masonry will produce large dry shrinkage deformation. In addition, the lack of maintenance of the plaster layer will also produce dry shrinkage cracks. This kind of crack in the wall is widely distributed, the number is large. For example, the horizontal cracks at the top of the beam, the corner cracks around the doors and Windows, the vertical cracks between the frame column and the filling wall. However, the above cracks are often formed under the joint action of temperature stress deformation and dry shrinkage deformation. Especially in the summer, these two factors are most influential.

3, mortar mix ratio is not appropriate to produce cracks

Both the design and construction of many projects are not in accordance with the requirements of the "Autoclaved aerated concrete Masonry mortar and plastering mortar" (JC890-2001) standard, the mix ratio of plastering mortar optimization, or the use of ordinary plastering mortar. Ordinary mortar shrinkage rate, hydration heat, poor workability, poor water retention, poor plasticity, dry density, with aerated block affinity is not strong, linear expansion or contraction is also big difference. So plastering and masonry. Neighbor relation. It will not be handled well, and the co-deformation ability will be weakened, causing large cracks or microcracks. Therefore, it is very important to optimize the mix ratio, which is to control the plastering cracks from the source and the mechanism of action.

4. Improper operation of wall plastering process causes cracks

(1) grassroots cleaning is not in place, resulting in mortar bonding is not strong and also prone to hollow drum cracking.

(2) The base is not wet one day in advance, because the water retention performance of plastering mortar can not meet the water absorption requirements of the block, resulting in the plastering layer shrinkage cracking.

(3) plastering a survival, or stratified plastering without appropriate interval, or plastering layer is too thick without taking strengthening measures. Large cracks or microcracks are caused by self-shrinkage, plastic shrinkage and slip of plastering layer.

(4) The four corners of the door and window holes are in the stress concentration area, and reasonable structural measures have not been taken to connect them, so they are easy to deformation and crack.

(5) No strengthening measures were taken for the joints of different materials and around the construction holes.

(6) The water is lost too quickly after plastering in summer, and the spray curing is not timely; In winter, the temperature difference between day and night makes the mortar lose its cohesive force.

(7) When plastering the wall larger than 3 meters (such as the living room wall) did not set the partition seam to take stress release, stress concentration prone to cracks.

Plaster (8) when operating strength is too light, especially when scraping the bottom ash without mortar with certain pressure to squeeze into the holes or seam formation of jagged connection, this will be a lot of problems, such as bond not firm, uneven thickness, density, loose plaster layer, is not conducive to the deformation of the bottom ash used to grassroots prone to fracture.

(9) When the surface is laminated, it is covered with a layer of plain cement slurry or dry cement ash. This layer of cement slurry is thin and brittle after air drying, easy to peel, easy to crack, and easy to sand off powder.

(10) cement mortar has not been used up in the initial setting time, has been air-dried and hardened, workability, plasticity, water retention, cohesiveness has been greatly weakened, prone to large organic irregular cracks.

(11) With cement mixed mortar, the curing period of the admixture stone ash paste is not enough, not 15 days, resulting in the expansion of the nodules inside the plastering.

(12) Improper plastering and caulking filling treatment of slotted wall installation pipelines, wall threading sleeves, door and window sides, wire boxes, sockets, air conditioners, water heaters and other parts will cause local cracking without maintenance.

3. Crack control measures

1. Seek control measures from the mechanism of crack generation

In the long-term construction practice, we found that the adjacent relationship between different building materials is difficult to deal with, often to bully the weak, easy to conflict and crack. This is mainly due to the difference in the linear expansion coefficient of different materials, physical and mechanical properties, with the exclusion, under certain conditions such as external force, temperature, humidity, deformation and other effects will produce contradictions, split, go their separate ways. I think we must stick to it. Prevention first, anti - release combination, flexible gradient, seek common ground while reserving differences. The principle of. Different materials are afraid of mutation, easy to crack when expanding, easy to crack when contracting; The same material is afraid of discrete, discrete assembly will lead to local defects in performance. It's either taken between different materials. Resistance & throughout; Measures, that is to strengthen resistance, such as nail hanging steel wire mesh, paste alkali resistant glass fiber mesh cloth, set reinforcement, Bundle & throughout; To work together in unison, in coordination and in harmony to cope with stress and strain; Or do it. Put & throughout; The measures, that is, separation and release, such as expansion joint, partition joint, partition, community harmony, well water does not violate the river, reduce conflict. Flexible gradient is to set a buffer zone between materials with large performance differences to resolve conflicts, and different materials should try to find common ground. Therefore, the following control measures are sought from the mechanism of fracture generation:

(1) From the aspect of mortar “ Seek common & throughout; Optimize the mix ratio of mortar. The selection of plastering mortar should be adapted to the aerated block material, I think the use of fly ash cement lime mortar is very good. Plasticizer can be added to mortar to increase the water retention and bonding ability of mortar. The mortar has low hydration heat, good workability, strong water retention, good plasticity, small dry density and strong affinity with aerated block. For example, in the No.9 building of Cangnan Times Square in Wenzhou, we suggest using fly ash cement lime mortar, which has achieved a good plastering effect with few cracks. Oujiang cup & throughout; The panel unanimously agreed to recommend the application. Zhang & throughout; . In addition, the choice of mortar strength should be from inside to outside from low to high flexibility gradient, in order to take into account the base material and external finish requirements. The strength and expansion coefficient of the bottom ash should be equivalent to that of the base, and the strength of 1:1:4 fly ash cement lime mortar can be selected with lower strength (the strength of 1:3 cement mortar can be used on the concrete wall), and the content of medium coarse sand can be appropriately increased to reduce the shrinkage of mortar.

(2) From the aspect of block; Seek common & throughout; . It is advisable to choose products with density above B07 and compressive strength not less than A5; The dry shrinkage deformation is characterized by rapid development in the early stage, and about 50% of the dry shrinkage deformation can be completed if the block is guaranteed to shrink for 28 days after discharge from the kettle. Blocks should not be stored in the open for repeated deformation.

(3) slow down or eliminate thermal expansion, cold contraction and dry contraction power sources, such as: set heat insulation layer, deformation joint; External wall and roof insulation should be done before internal plastering; After the external plastering is completed, it is necessary to do a good job to cover the rain and avoid the rain directly washing the wall; Summer to take shading measures, when the weather is extremely hot, with thin film cover; Spray water maintenance with sprayer if necessary.

(4) Take the situation as it is; Resistance & throughout; Measures to improve the plastering layer and masonry co-deformation ability. For example: nail hanging wire mesh between different materials; The top layer is made of crack resistant plastering mortar; The top wall should not be built with air block, instead of reinforced brick masonry.

(5) Take the situation as it is; Put & throughout; For example, when the wall is longer than 3 meters, the plastering layer is set up to release the stress.

2, from the plastering process operation control

Gas block wall plastering process: base treatment → Do the ash cake & rarr; Necessary parts hang reinforced net treatment. 1:1 cement mortar or construction glue mud drawing wall &rarr Wipe the bottom dust. Apply middle layer ash. Plaster layer of ash.

(1) Plastering should be carried out 7 days after the completion of the top laying. Before plastering, the wall should be kept moist, and the water content should be kept at about 10% ~ 15%. The water depth of masonry should be 8 ~ 10mm on the surface, which can be observed by scraping. Before plastering can be 1 ~ 2 days in advance of the wall water 1 ~ 2 times, do not need to pour through, as long as the appropriate watering, and then dry for about half a day before plastering.

(2) Before plastering, basic treatment should be carried out first. The wall should be fully brushed with a steel wire brush to remove the adhesion of loose objects, floating ash and dirt, and then watering and wetting the wall. Before plastering should also be filled with insufficient fullness of the ash seam.

(3) Wipe the bottom ash must be forced to facilitate the bond with the base. Plastering layer at the joint, the first wipe should be slightly thin, and then evenly connected, should not be too much contact, prevent uneven contact, to be the bottom mortar after the final setting should be moistened with water, and then wipe the next layer.

(4) plastering should be layered, the bottom thickness should be controlled within 5mm, the middle layer, surface layer thickness to 7 ~ 9㎜ is appropriate, wipe the bottom ash after 60 to 70 dry, hand pressure has a solid sense, can leave the hand pattern can be carried out, the surface layer plastering.

(5) At the intersection of two different matrix or plastering thickness of more than 35㎜, should use nail hanging wire mesh plastering or alkali resistant glass fiber mesh cloth to strengthen processing, lap width should not be less than 150㎜. The top two layers of thermal stress is larger, it is suggested that the top two layers of paint mortar should be mixed with anti-crack fiber or plastered before hanging wire mesh.

(6) plastering mortar should be used up within the initial setting time. Usually it should be controlled within 3 hours, and in summer it should be controlled within 2 hours.

 

Aerated masonry walls

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