Introduction
Aerated concrete blocks are widely utilized in construction. Enhancing quality control during construction can prevent common defects and significantly improve the overall project quality.
1. Requirements for Laying Aerated Concrete Blocks
(1) Masonry work with aerated concrete blocks must strictly adhere to the technical requirements outlined in the national standard Code for Acceptance of Construction Quality of Masonry Structures (GB 50203-2011).
(2) Plan construction schedules reasonably; avoid rushing and try to refrain from masonry during rainy seasons whenever possible.
(3) Use special mortar with strong bonding properties for masonry. The mortar should have a minimum strength grade of M5 and excellent water retention. Adding inorganic or organic plasticizers is recommended. If available, use specialized aerated concrete masonry mortar or dry powder mortar.
(4) To prevent shrinkage cracks caused by temperature differences between the main structure and the enclosure wall, install tie bars at the junction between blocks and wall columns. Vertical spacing should be 500-600 mm. According to product height specifications, embed two Ø6 steel bars, extending at least 800 mm into the wall on both ends. Use full-length Ø6 steel bars every 1.5 m of masonry height to prevent wall shrinkage and cracking.
(5) For walls with large spans or height, install structural beams and columns. If the wall length exceeds 5 m, reinforced concrete structural columns can be placed in the middle. For walls taller than 3 m and thickness not exceeding 120 mm, or taller than 4 m with thickness at least 180 mm, reinforced concrete waist beams should be added at the wall’s high waist.
(6) The junction between window sills and walls between windows are stress concentration zones prone to cracks from masonry shrinkage. It is advisable to install reinforced concrete cast-in-place strips at window sills to resist deformation. Corners of door and window openings are also vulnerable to cracking and hollowing; using ring beams instead of lintels is recommended.
(7) Horizontal concave and convex features of exterior aerated concrete walls—such as line feet, rain covers, eaves, and window sills—should be waterproofed and designed with drip edges to prevent water accumulation.
(8) Before masonry, calculate the number of layers and rows according to block dimensions, and check and adjust the tie steel bars accordingly. A base layer of plain concrete matching the wall thickness can be poured. A common method involves building two rows of red bricks with a 20 mm gap on the top row, allowing solid auxiliary small blocks made from the same material as the aerated blocks to be diagonally laid and compacted firmly on top.
(9) Since aerated concrete blocks differ in dry density and strength grades, blocks with varying specifications should not be mixed. Additionally, aerated concrete blocks should not be combined with other types of bricks or blocks.
(10) Strictly control the moisture content during block construction. According to relevant standards, moisture content should be below 15% for concrete blocks and below 20% for fly ash aerated concrete products. Shrinkage is minimal when moisture content ranges between 10-30%, typically between 0.02 and 0.1 mm/m. Best practice is to maintain block moisture content at 10-15% during construction, with surface moisture depth of 8-10 mm. Watering should occur 24 hours before masonry, with amounts adjusted for seasonal and weather conditions. Do not use blocks saturated by rain or excessively watered.
(11) Limit daily masonry height to 1.4 m. During spring, reduce this limit to 1.2 m. Stop masonry during rainy days. After reaching approximately 200 mm below the beam, allow the masonry to stand for 7 days. Once deformation stabilizes, use solid auxiliary small blocks to diagonally build and compact the top firmly.
(12) Mortar joints must be horizontal and vertical, with staggered joints between layers. Corners should be interlocked, and mortar applied fully. Horizontal joints should not exceed 15 mm and vertical joints 20 mm in thickness. Mortar fullness should be above 90%. For prefabricated R&D vertical joints, use temporary internal and external clamps to fill the joints. After masonry, immediately tool the mortar joints inside and outside to ensure full mortar coverage.
(13) Wall construction joints must be diagonal, with a length no less than two-thirds of the wall height.
(14) After wall construction, cover it to protect from rain. Walls exposed to direct sunlight should be shaded to prevent rapid water evaporation from the mortar. If necessary, cure the wall by spraying water.
(15) At junctions between block walls and concrete beams, columns, shear walls, as well as at door and window frame corners, nail and hang steel wire mesh with 10 mm × 10 mm mesh size, extending 200 mm on each side. Overlap mesh sections flat and securely, ensuring an overlap length of no less than 100 mm.
(16) When chiseling grooves and laying pipes in walls, use specialized tools instead of axes or trowels. Pipe surfaces should be recessed 4-5 mm below the wall surface and firmly fixed without looseness or rebound. Moisten the area, fill with mortar matching the masonry strength, and level with the wall. Lay 10 mm × 10 mm steel wire mesh along the pipe direction, extending at least 50 mm on each side, and secure tightly with nails.
(17) Top brick masonry should begin 7 days after wall construction. Bricks must be laid at an oblique angle of 45° to 60°. Prefabricate top bricks according to the gap between the last brick and the beam slab, maintaining a height of 170-190 mm. Use grey sand bricks meeting technical specifications for the top layer.
2. Plastering Requirements
(1) Plastering should be performed 7 days after masonry completion. During rainy seasons, extend the interval appropriately based on wall surface dryness.
(2) Before plastering, thoroughly brush the wall with a wire brush to remove loose materials, dust, and debris that could reduce mortar adhesion. Then, wet the wall to wash away remaining powder. Level any low spots on the wall to avoid cracks and hollowing caused by uneven plaster thickness. Inspect mortar joints and fill any gaps.
(3) Keep the wall surface moist with a moisture content of 10-15% before plastering. Depending on weather, soak the wall 2-3 times overnight. In spring, when humidity is high, a light wetting is sufficient. In hot, dry weather, increase watering accordingly.
(4) Choose plastering mortar compatible with aerated concrete blocks and with good water retention. Special aerated concrete plastering mortar or cement-lime mortar is recommended. Adding organic or inorganic plasticizers can improve water retention and bonding. Mortar strength should gradually increase from inner to outer layers to balance base material and external finish requirements.
(5) Before applying the base plaster layer, treat the surface with a 1:1 cement mortar or bonding slurry to roughen the wall, or use a specialized interface agent. Apply the base plaster layer before the treatment material dries.
(6) The bottom plaster layer should have strength and expansion properties similar to the base layer. A lower strength 1:1:6 cement-lime mortar is suitable. Increase the proportion of medium coarse sand to reduce drying shrinkage. Apply the bottom plaster with a trowel at less than 5 mm thickness. Press mortar firmly into holes or gaps to create a staggered bond, enhancing adhesion and accommodating base layer deformation.
(7) Once the base plaster is partially dry, inspect for hollowing or cracking before applying the middle plaster layer, 7-9 mm thick, using a 1:1:4 cement mortar mix. If thick, apply in multiple layers spaced no more than 24 hours apart. After the middle layer is about 70% dry, apply the finish plaster layer, compacting and smoothing it thoroughly.
(8) After plastering, protect the wall with a rainproof cover to prevent direct rain exposure. Shade walls exposed to direct sunlight. If needed, spray water to cure the plaster.
Article source: Architectural Technology Magazine
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