Key points for construction of autoclaved aerated concrete blocks

Abstract: With the continuous advancement of wall material innovation, autoclaved aerated concrete blocks have gradually been widely used in construction projects as a lightweight, sound-insulating, The application of new building wall materials with good thermal insulation properties and autoclaved aerated concrete blocks has good development prospects. However, the reality is that due to the characteristics of the material itself and construction factors, there are several problems after wall construction, which affects the promotion of this material. Now I would like to put forward some personal opinions on the key points that should be paid attention to in the construction of aerated concrete blocks, hoping to serve as a starting point.
At present, the problems that are more common in the construction of steaming pressure and gas concrete blocks are: gray seam cracking, cracking of plastering layers, empty drums, leakage, and reducing sound insulation effects. The entire construction process is tracked and controlled.
First of all, the selection of materials must be strictly controlled. The raw materials of autoclaved aerated concrete blocks are mainly cement, lime, sand, fly ash, etc. It is a cement concrete product containing calcium silicate hydrate colloid generated by water curing. It has a large water-cement ratio and a lot of cement. The aggregate is air. Although it has been autoclaved, the shrinkage value can only be controlled within the range of 0.04% (4/10,000) to 0.06% (6/10,000) due to cost factors, which is larger than traditional sintered clay bricks. Moreover, due to differences in raw materials and process conditions, the dry shrinkage of blocks from various manufacturers varies greatly. In order to ensure quality, products from manufacturers with mature production processes and good maintenance conditions should be selected; try to choose products with grade 07 or above or with a compressive strength level of not less than 5Mpa. The higher the strength, the better the density of the material. Generally speaking, The drying shrinkage value will also be reduced; the drying shrinkage value of the selected aerated concrete blocks is required to be no more than 0.5mm/m, and a 28-day curing period must be guaranteed after leaving the kettle before the wall can be built. Since the shrinkage rate of aerated concrete blocks within 5-7 days after leaving the kettle usually accounts for about 50% of the total shrinkage, if it can be guaranteed to be put into use 28 days after leaving the kettle, the chance of cracks caused by dry shrinkage can be greatly reduced. .
After the blocks enter the site, rainproof measures must be taken. Blocks should not be stacked in the open during construction, especially in spring and summer. They should be stacked in a covered place; if conditions limit the possibility of stacking in the open, they should be stacked in a higher place and drainage treatment should be done.
Masonry requirements:
(1) The masonry of aerated concrete blocks must strictly comply with the technical indicators of the national standard “Code for Acceptance of Construction Quality of Masonry Projects” (GB
50203-2002) Require.
(2) Reasonably arrange the construction period and do not rush the work blindly. If possible, construction should be avoided during the perennial rainy season.
(3) The masonry mortar should be a special mortar with good bonding performance. Its strength grade should not be less than M5. The mortar should have good water retention. Inorganic or organic plasticizers can be added to the mortar. If possible, special aerated concrete masonry mortar or dry mortar should be used.
(4) In order to eliminate shrinkage cracks caused by temperature changes between the main structure and the surrounding wall, tie bars must be left where the blocks and wall columns meet, with a vertical spacing of 500 to 600 mm (according to Determined by the height specification of the selected product), 2Ф6 steel bars are buried, and the two ends extend into the wall not less than 800mm; in addition, when each masonry is 1.5m high, 2Ф6 full-length steel bars should be used to tie them to prevent shrinkage and cracking of the wall.
(5) Set structural beams and columns in walls with larger spans or heights. Generally, when the wall length exceeds 5m, reinforced concrete structural columns can be set up in the middle; when the wall height exceeds 3m (≤120mm thick wall) or 4m (≥180mm thick wall), reinforced concrete waist beams can be added at the middle of the wall. .
(6) The intersection between the window sill and the wall between the windows is a location where stress is concentrated and is easily affected by the shrinkage of the masonry to cause cracks. Therefore, it is advisable to set up reinforced concrete cast-in-situ strips at the window sill to resist deformation. Cracks and hollows are also prone to occur at the upper corners of door and window openings, so it is advisable to use ring beams instead of lintels here.
(7) The horizontally concave and convex parts of the aerated concrete exterior wall (such as moldings, rain covers, eaves, window sills, etc.) should be flooded and dripped to avoid water accumulation.
(8) Before masonry, calculate the number of skins and rows according to the size of the blocks, check and correct and complete the tie steel bars. A certain height of plain concrete with the same thickness as the wall can be poured at the root of the wall in advance. The current common practice is to lay two skins of red bricks, leaving a gap of about 20mm high in the upper skin, so that the same material as the original bricks can be used. Use small solid auxiliary blocks to build them diagonally and squeeze them tightly.
(9) Since the performance indicators of aerated concrete blocks with different dry densities and strength levels are different, aerated concrete blocks with different dry densities and strength levels should not be mixed, nor should aerated concrete blocks. It should be mixed with other bricks and blocks.
(10) Strictly control the moisture content of aerated concrete blocks when building walls. According to relevant specifications and regulations, the moisture content of aerated concrete blocks during construction should be less than 15%, and for fly ash aerated concrete products, it should be less than 20%. The drying shrinkage law of aerated concrete shows that the shrinkage value when the moisture content is between 10 and 30% is relatively small (generally between 0.02 and 0.1mm/m). According to experience, it is more appropriate to control the moisture content of aerated concrete blocks at 10 to 15% during construction, and the water content depth of the blocks to be 8 to 10 mm on the surface. The surface water content depth can be observed by scraping with a knife or tapping a small edge. Determine based on experience. Under normal circumstances, watering is done 24 hours before masonry construction. The amount of watering should be based on the season at the time of construction.The joints are determined by the dry and wet temperature conditions and are controlled by the surface moisture. It is prohibited to directly use blocks that are saturated with rainwater or overwatered.
(11) The daily masonry height should be controlled within 1.4m. During spring construction, the daily masonry height should be controlled within 1.2m. Masonry should be stopped on rainy days. When the masonry is about 200mm from the bottom of the beam, it should be stopped for 7 days. After the deformation of the masonry has stabilized, solid auxiliary small blocks of the same material should be used to lay diagonally and squeeze tightly to hold them firmly.
(12) During masonry, the mortar joints should be horizontal and vertical, the upper and lower layers should be cross-staggered, the corners should bite each other, the mortar should be full, the horizontal mortar joints should not be larger than 15mm, and the vertical mortar joints should not be larger than 20mm. The mortar fullness is required to be above 90%. Vertical joints should be grouted with temporary internal and external splints. After masonry, the original mortar should be used immediately inside and outside the mortar joints to ensure the fullness of the mortar.
(13) The construction joints of the wall must be built with diagonals, and the length of the diagonals should not be less than 2/3 of the height.
(14) After the wall is laid, it should be covered with rain to prevent rainwater from directly washing the wall; the wall on the sunny side of the exterior wall should also be protected from the sun to avoid excessive evaporation of water in the mortar caused by high temperatures. Quickly, if necessary, use a sprayer to spray water for maintenance.
(15) Nail and hang steel wire mesh with a mesh size of 10mm×10mm at the intersection of the block wall and the concrete beams, columns and shear walls, as well as at the borders and inner corners of the door and window openings. Each side is 200mm wide. The overlap of materials should be smooth and firmly connected, and the overlap length should not be less than 100mm.
(16) When cutting grooves and laying pipes on the wall, special tools should be used. Do not use axes or trowels to cut and cut randomly. The surface of the pipe should be 4 to 5mm lower than the wall surface, and the pipe should be stuck with the wall. Firmly, without loosening or rebounding, then water and moisten, the filling strength should be equal to the mortar used for masonry, make it level with the wall, and lay 10mm × 10mm steel wire mesh along the pipe laying direction, its width should span the notch. Each side should be no less than 50mm, and should be tightened and nailed firmly.
Plastering process:
(1) Aerated concrete wall plastering process: remove floating dust on the wall→correct and fill the joints→sprinkle water to moisten the base layer→make ash cake→hang netting on necessary parts→ 1:1 cement mortar or construction glue slurry roughen the wall → apply the bottom layer of dust → apply the middle layer of dust → apply the surface layer of dust → clean.
(2) The plastering time should be controlled to be carried out 7 days after the completion of the masonry. If construction occurs in the rainy season, the interval between the completion of the masonry and the plastering should be appropriately extended depending on the dryness of the wall. .
(3) Before plastering, the wall should be fully brushed with a wire brush to remove loose objects, floating dust and dirt that affect the adhesion between the mortar and the wall, and then water to moisten the wall. Rinse away the remaining powder. In order to avoid cracking and hollowing caused by large differences in the thickness of the plastering mortar, the low recesses on the wall should be corrected and leveled. Check the mortar joints before plastering and fill any mortar joints that are not full enough.
(4) Before plastering, the wall should be kept moist, with the moisture content kept at about 10 to 15%. Before plastering, the wall can be sprayed with water 2 to 3 times overnight. The specific situation depends on the climate at that time. Generally speaking, the humidity is high in spring and the wall itself has a high moisture content. You only need to wet the wall slightly. When encountering high temperature and dry weather, you need to increase the amount of water.
(5) The selection of plastering mortar should be compatible with the material of aerated concrete blocks and have good water retention. It is advisable to use special plastering mortar for aerated concrete, or cement lime mortar. If conditions permit, the plastering mortar can be used at the construction site. Organic or inorganic plasticizers are added to the mortar to increase the water retention and bonding capabilities of the mortar. The choice of mortar strength should be transitioned from low to high from the inside to the outside to take into account the requirements of the base material and external finishes.
(6) Before applying the bottom layer of ash, perform base surface treatment. You can use 1:1 cement mortar or construction glue slurry to roughen the wall surface, or use a special interface agent for base surface treatment. After the base surface is treated, apply the primer before the base surface treatment material dries and solidifies.
(7) The strength and expansion coefficient of the bottom layer of ash should be equivalent to that of the base layer. You can choose 1:1:6 cement lime mortar with lower strength, and at the same time appropriately increase the ratio of medium-coarse sand to sand in the mortar mix ratio. To reduce the drying shrinkage of mortar. The bottom layer of plaster should be scraped onto the wall with a trowel, and the thickness should be within 5mm. The mortar with a certain pressure is squeezed into the holes or seams to form a staggered connection, which is conducive to the joint work of the plaster layer and the wall, and allows the bottom layer to Gray adapts to the deformation of the base layer.
(8) After the bottom layer of ash is slightly dry and checked to see if there are no hollows or cracks, the middle layer of plastering should be carried out. The thickness should be 7 to 9mm. The mortar can be a cement mixed mortar of 1:1:4. If the middle layer is plastered If the ash is too thick, it should be applied in layers, and the time interval between each layer should be within 24 hours. After the middle layer of plaster is 70% dry, the top layer of plaster can be plastered. The plaster must be compacted and smoothed.
(9) After the plastering is completed, a rain-proof covering should be done to prevent rainwater from directly washing the wall and direct sunlight on the wall. Sunshade treatment should be done, and if necessary, use a sprayer to spray water for maintenance.
The above measures can minimize wall cracks. Considering that there are many reasons for cracks, it is difficult to eliminate wall cracks. In view of the situation that most of the crack expansion speed is concentrated within 1 to 2 years after the completion of the project, if it is found during the inspection, it is best to focus on the repair work during this period.

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