Preventive measures for cracks in autoclaved aerated concrete block wall
Compared with the composite external wall insulation and energy saving system, the autoclaved aerated concrete block wall has the following advantages: the base wall and the insulation layer are combined into one, and they will not deform or fall off under the wind load; the fire performance meets the requirements of the specifications ; Durability meets full requirements; strong machinability; construction process is relatively simple; adjust indoor environment, etc. One of its main disadvantages is that it is prone to cracks. This article introduces some specific measures to prevent cracks in autoclaved aerated concrete block walls.
1. Add construction column
The integrity of the block wall is poor, and it is easy to cause cracks in the wall. One of the measures to improve the integrity of the block wall is to add structural columns at the middle of the elongated wall, at the corners of the wall and at the junction. The structural columns in the enclosure wall or partition wall of autoclaved aerated concrete blocks are not completed into large teeth. The tie bars between the masonry wall and the structural columns are preferably 3.5mm mesh and 20mm welded steel bars Net, hot-dip galvanized steel sheet with thickness of 2mm can also be used. The end that protrudes into the structural column is made into a dovetail shape, which is conducive to the compaction of the concrete. The steel plate built into one end of the masonry should be drilled with two holes and nailed into the block with cement steel nails. Reinforced mesh or steel plate knots should be installed along the wall every 500mm or between two leather blocks. In addition, reinforced concrete horizontal tie beams can also be added.
2. Reserved seam treatment
In the routine practice of masonry engineering, when filling the wall until it is close to the beam and the bottom of the slab, a certain gap should be left. After the filling wall is completed and at least 7 days apart, it will be squeezed and squeezed. . With this method, cracks may occur on the bottom of autoclaved aerated concrete masonry walls and reinforced concrete beams (slabs). It is best to reserve gaps in the autoclaved aerated concrete block-filled wall, which can be controlled at 10mm ~ 25mm, the gaps can generally be filled with expanded cement mortar (with UEA expansion agent added with 12% cement weight), and the gaps can be copied over 25mm. Expanded fine concrete embedded filling. When embedding an external wall, you can embed it on the inside first, and after the final setting of the expanded cement mortar or expansive fine stone concrete is finalized, then embed the outer side, so that it can be solid. Expanded cement mortar or expanded fine stone concrete after embedding should be moisturized and cured. Flexible materials can also be used for embedding, such as glass fiber wool, mineral wool and PU foaming agent (strip), and then elastic glass tape is applied on the surface for protection.
3. Knotted steel bars or L-shaped iron parts
The cracks at the junction of the block wall and the reinforced concrete frame column or shear wall can be controlled by the use of tie bars or L-shaped iron parts. It is best to use 3.5mm mesh and 20mm welded steel mesh for the tie bars. When L-shaped iron parts are used, the thickness should be kept to 2mm, and a gap of 10mm to 15mm should be reserved between the block wall and the reinforced concrete frame column or shear wall. After the wall is completed, the gap is embedded with flexible materials fill.
4. Strengthen the treatment of doors and windows
When the width of the door opening is greater than 2m, reinforced concrete frames should be set on both sides of the opening to avoid the vibration of the block wall at the contact portion with the door frame caused by the opening vibration of the door leaf. Reinforced concrete window sill beams should be used for the windows, with both ends extending into the wall 600mm each. The bottom of the skin block under the window should be placed with 3Ф6.5mm longitudinal reinforcement, which is suitable for welding! Welded reinforcement mesh with 3.5mm mesh size of 20mm, the length of the two ends extending into the wall should be ≥700mm.
5. Temperature influence
Reduce the outer surface of the roof and wall concrete components. Because the linear expansion coefficient of reinforced concrete is greater than the linear expansion coefficient of autoclaved aerated concrete blocks, under the action of thermal expansion and contraction, the expansion and contraction between the reinforced concrete components and the block wall Not synchronized, will cause cracks in block walls or reinforced concrete beams and columns. In view of this situation, the exposed reinforced concrete columns and beams can be designed as special-shaped structures to minimize the effect of temperature changes on concrete members.
6. Grey seam control
To improve the thermal insulation performance of the wall, it is not necessary to reduce the dry density of the light-weight blocks, but also to avoid the thermal bridge caused by large ash joints and uneven ash joints. The thermal conductivity of ordinary masonry mortar is about 3 times that of lightweight blocks. If the thickness of the ash joint can be reduced to about 3mm, the thermal bridge generated by the ash joint can be ignored. To reduce the thickness of the ash joint, the masonry must use a conversion binder that matches the material properties of the autoclaved aerated concrete, and its main technical indicators should be in accordance with JC 890-2001 In addition to the requirements for masonry mortar in the “Plastering Mortar”, it should also meet the relevant requirements.
7. The cracking of the plaster layer
(1) At the junction of block masonry and reinforced concrete columns (beams, walls), alkali-resistant glass fiber mesh cloth or hot-dip galvanized steel wire mesh ≥500mm wide should be laid.
(2) The decorative construction of block masonry should be carried out at least 7 days after the completion of the filling of the voids in the top of the masonry wall.
(3) Doors, windows, and side walls of various types of boxes should be filled and stricken layer by layer to avoid hollows and cracks at the junction of the side walls of the frame and the masonry. Around the frame where sealant is required, a gap of 7mm deep and 5mm wide should be left when plastering, so that caulking can be applied. The gap between the window board, watch box, fire hydrant box, telephone box and the masonry should be filled with PU foaming agent.
(4) Before plastering the block walls, the interface agent needs to be painted. If there is no special-purpose interface agent, it can be prepared with 999 strong-strength glue: cement: fine sand = 1: 2: 2 (mass ratio).
(5) Special-purpose plastering mortar shall be used for plastering of block walls, and its technical index shall meet the requirements of plastering mortar in JC 890-2001 “Masonry Mortar and Plastering Mortar for Autoclaved Aerated Concrete”.
(6) If there is no special method for plastering the external walls of the block-use plastering mortar, polymer cement mortar should be used for the first plastering, the thickness is 6mm, the mixing ratio of polymer cement mortar is 1: 4, cement mortar is mixed with water The amount of 801 glue is 20%.
(7) If there is no special-purpose plaster for internal wall plastering, polymer cement mixed mortar should be used. Its mixing ratio is cement: lime: yellow sand = 1: 1: 6, and the water consumption of mixed mortar mixed with mortar is 20% Of 801 glue. After drying, the second polymer cement mixed mortar plastering and calendering; can also be used trowel ash, paper bar ash or alkali-resistant short-fiber mortar plaster. In order to imitate and reduce cracking, elastic putty and elastic paint are used for both the inner and outer walls.
The above are some effective measures to control the cracks in the autoclaved aerated concrete block wall, but taking these measures can not completely eliminate the cracks in the masonry, and we need to further accumulate experience in the future application process, Continuous improvement.
