Applications of AAC Blocks

Walls made from AAC blocks can be used as partition walls and perimeter walls. Perimeter walls are subjected to wind and earthquake loads and must be capable of withstanding impact loads, whereas partition walls are affected by seismic motions and must respond to smaller impacts. Accordingly, the design and execution criteria for these walls differ, as outlined in these guidelines.

Advantages of AAC Blocks in Building Product Categories

Due to their unique properties, AAC blocks are recognized as a superior choice in the classification of building products. Key advantages include:

• Lightweight and Fast Execution: The low weight of the blocks (up to 80% lighter than conventional concrete) reduces the building's dead load and increases wall construction speed by up to 3 times compared to clay bricks or hollow clay blocks. This makes them ideal for residential and commercial projects.
• Thermal and Acoustic Insulation: Low thermal conductivity (approx. 0.1 W/mK) and a porous structure place AAC blocks in the category of thermal insulating materials. According to the Building and Housing Research Center, these blocks reduce noise pollution by up to 40 decibels.
• Earthquake and Fire Resistance: With a compressive strength of 2–5 MPa, AAC blocks are highly flexible against seismic activity and are fire‑resistant for up to 4 hours.
• Economic Efficiency: Larger dimensions (e.g., 60×25×15 cm) reduce mortar consumption by up to 50% and extend the useful life of the building to over 80 years.

Compared to similar products such as Leca blocks or Siporex, AAC blocks (Heblex) have higher dimensional accuracy (deviation less than 1 mm) due to the autoclave process and fall into the category of "advanced materials".

Raw Materials Used in AAC Blocks

AAC blocks are made from silica, cement, gypsum, lime, aluminum powder, and water. The combination of these materials creates millions of tiny air cells, which results in low weight, sound insulation, and thermal insulation.

Binder Material and Its Requirements

Polymer‑modified dry mortar is a mixture of a binder (organic or inorganic), polymer, filler, and water. When mixed with water, it undergoes physical and chemical reactions and becomes a strong adhesive. In some contexts, the mortar plays a more significant role than the binder in the masonry unit. This has rapidly increased over the past two decades with the introduction of various chemical additives. The choice of mortar must consider the nature of the work and the existing units, as the consequences are significant, and all design and physical aspects of the building must be taken into account.

Regarding the selection of AAC mortars, attention must be paid to whether the building structure is reinforced or non‑reinforced. Using mortars with compressive strengths different from those specified in the design is not permitted.

The materials used in polymer‑modified dry mortar, including cementitious materials and aggregates, must comply with Iranian National Standards.

The water used must be clean and free of any oil, acid, alkali, salt, organic matter, or substances that could damage the mortar or the metal used in construction – same as water used in concrete processing.

The use of additives in mortar is not permitted unless included in the mix design. The soluble chloride content of the additive must not exceed 0.009% of the total chloride content of the mortar. The additive must not damage the mortar or the metal used in masonry units. When using the recommended maximum amount of polymer in the mortar, the soluble chloride content of the polymer must not exceed 0.009% of the total chloride content of the mortar. The polymer used must not damage the mortar, the metal used, or the masonry units.

Calcium chloride in the mortar as an accelerator must not exceed 1%, as it has harmful effects on metals.

For designed mortars, the compressive strength must be declared by the manufacturer. The manufacturer may state the compressive strength class according to the table, where the compressive strength is indicated by the symbol "M" followed by the minimum class in Newtons per square millimeter. If the amount of air lime (calculated as calcium hydroxide) in the mortar equals or exceeds 50% of the total binder mass, this must be declared by the manufacturer.

Execution Details for AAC Blocks

Connections of walls consisting of AAC blocks must be made in such a way that the intended wall performance is ensured under phenomena such as deflection of beams below the wall and the roof, floor drifts, out‑of‑plane forces including wind, impact during use, and earthquakes. Due to the relatively low tensile strength of all masonry blocks, including AAC blocks, it is essential that connection details be executed with high precision. It should be noted that because AAC blocks are bonded to each other with adhesive, and the adhesive has high strength and adhesion to the block compared to cement‑sand mortar bonding to other types of masonry blocks, if the provided details are not followed, tensile failure may occur within the AAC block (in other types of block walls, due to low mortar strength, this failure generally occurs in the connecting mortar).

Connecting AAC Walls to Reinforced Concrete and Steel Columns

The structure is designed assuming that partition and perimeter walls do not contribute to the building's stiffness. Therefore, the vertical edge connection of walls to columns, shear walls, or any other vertical load‑bearing element must not resist relative displacement. The connection of the AAC wall to columns and other elements (beams, wall ties, etc.) must follow the design specifications.

Sliding Connection Using Steel Angles

One suitable method for connecting the wall to columns is to use a sliding connection at the contact surface using discontinuous steel angles. For this connection, the following points should be noted.

- The angle sections can be made from cold‑formed galvanized steel sheet.

- No connection (e.g., welding or nailing) should be made between the angle flange and the AAC wall.

- The gap between the wall and the column must be filled with flexible material such as polyurethane foam or polystyrene.

- When the column is steel, the angle is welded to the column.

Connection with Resilient (Spring) Clips

Among the connections recommended for similar situations are sliding connections to provide out‑of‑plane restraint while allowing in‑plane freedom of movement. In this case, the use of cold‑formed steel angles on both sides of the wall, properly attached to the reinforced concrete or steel column, is recommended.

Regarding the above connection, the following points should be recalled:

- In interior walls, one resilient clip should be used for every three block courses.

- In exterior walls, one resilient clip should be used for every two block courses.

- Using a clip with a bent (resilient or radical) shape is essential to allow limited horizontal movement of the wall; connections using conventional angle sections cannot prevent cracks caused by bending or other deformations.

Wall‑to‑Wall Connection

Although technical literature always recommends adequate interlocking of masonry units in orthogonal walls, for AAC walls it is recommended that because of the possibility of tensile stresses within the plane of orthogonal walls, metal clips similar to those used for column connections be employed.

Wall‑to‑Slab Connection (Top Connection)

The connection of the wall to the underside of the slab can be either a sliding connection or essentially no direct connection, using out‑of‑plane restraint with angles. The choice depends on the panel situation between vertical elements (columns, walls, or wall ties). If it is not possible to provide proper sliding connections above the wall due to the type of slab, and if wall ties are provided at appropriate intervals determined by wall analysis, then a mechanical connection between the slab and the top edge of the wall may be omitted. In this case, the minimum gap between the top of the wall and the slab must be greater than the maximum slab deflection along the wall. The top connection can be achieved in the following ways:

Sliding Connection Using Angles (Top)

The top edge of the wall can be restrained using two angles properly attached to the structural slab. The angles must not be nailed or welded to the AAC blocks. This connection allows free in‑plane movement of the wall, and no stresses (due to shrinkage, floor drift, etc.) are induced in the wall, thus eliminating the potential for cracking. The gap between the top of the wall and the slab must be sufficient to allow the beam to deflect freely without contacting the wall. The angles are installed first on one side, and after masonry placement and positioning of the topmost block, the second angle is attached. The figure below shows the execution of a perimeter wall.

Connection Using Resilient (Radical) Clip

Connection to Wall Ties (Vertical Ties)

According to Clause 1‑4‑3 of the Iranian Code for Seismic Design of Buildings (Non‑structural components and their supports), non‑structural components and their supports must be anchored to the structure so that they can transfer forces from the non‑structural component to the structure and accommodate the resulting deformations. The load path in these components must have adequate strength and stiffness, and the connection point to the structure must be able to withstand the local effects of loads. Welded or bolted connections and the like are permitted, but frictional resistance from gravity loads must not be used. Based on this clause, AAC wall panels must be checked against applied loads and the boundary conditions at the top (under the slab), the two vertical edges, and the bottom (on the floor), and accordingly the restrained length of the panel is calculated. Although existing rules allow a distance of up to 40 times the thickness between vertical supports for non‑structural walls, the spacing of wall ties can be determined by calculating the flexural capacity of the wall panel under assumed edge support conditions and applied loads.

It should be noted that the connection details provided in the connections chapter must be modeled as hinged conditions; if the wall is not connected to the slab or vertical elements (columns or walls), it must be modeled as free. The execution of wall ties includes the following steps:

- First, locate the connection plates on the floor and ceiling according to the positions marked on the drawings.

- After installing the connection plates, weld the metal wall tie (vertical tie) to the bottom and top plates.

- On one side, cut the block completely; on the other side, create a recess inside the block with a saw to accommodate the wall tie.

- After placing the blocks, use resilient clips every two or three courses to connect the blocks to the wall tie.

- Ensure that no large gap or cavity remains at the connection point.

Execution of Lintels and Window Installation

When perimeter AAC walls contain doors or windows, the lintel must be installed first, followed by the remaining details as shown below. For openings larger than 2.5 meters, according to Code 2800, horizontal and vertical ties (reinforced concrete confining elements) are required around the opening.

Cladding (Veneer) Execution Details

Building facades are either curtain walls or conventional wall veneers. This section presents details related to the installation of stone, brick, cementitious, and ceramic veneers on walls built from AAC blocks, which are suitable for wall‑veneer type facades. Additionally, the installation of curtain‑wall facades on these walls must comply with the guidelines in Publication No. 714 of the Management and Planning Organization and the Building and Housing Research Center, titled "Structural Design Guidelines, Performance Requirements, and Execution of Building Exterior Facades".

Stone Veneer Installation

A stone veneer system has five main interrelated parts that must be considered when designing the cladding system:

1. The stone pieces or panels forming the cladding.

2. Anchors, which connect the cladding to the backup structure.

3. Secondary frames, which act as an interface when the anchors are not directly attached to the building structure.

4. Joints between the stone pieces or panels.

5. The backup wall or the main building structure (depending on the cladding system), to which loads on the stone are transferred via anchors or adhesives.

Anchoring Details for Anchored Stone Veneer

There are different methods for executing anchors in anchored stone veneer. These details can be implemented using the following approaches:

Direct installation methods, where each stone is directly attached to the backup wall to resist gravity and lateral loads.

Combination of gravity and lateral load anchors into a single clip.

Installation of stone panels using vertical channel‑shaped pieces. Details and execution considerations for each of the above methods are fully presented in Publication No. 714 of the Management and Planning Organization.

Brick Veneer Installation

Brick veneers consist of single‑layer brick walls with a maximum thickness of 10 cm. The backup wall on which the brick veneer is installed may be load‑bearing or non‑load‑bearing. In buildings of up to three stories (10 meters), the backup wall may be load‑bearing; otherwise, it is non‑load‑bearing. Backup walls may be reinforced concrete or AAC.

Anchored Veneers

In these conditions, the masonry units used must have a minimum thickness of 67 mm. The gravity support for this veneer must be chosen such that the weight of the anchored masonry veneer is transferred to the concrete foundation. Control of anchor deflection under gravity loads is essential. In masonry veneers attached to concrete backup walls, the connection of the veneer to the concrete supports must be made using adjustable anchors. The maximum distance between the inner surface of the veneer and the outer surface of the concrete backup must be 115 mm. A minimum air gap of 25 mm must be provided.

Adhered Veneer

a) Loads from the veneer must be transferred to the backup wall by suitable mechanical connections.

b) Out‑of‑plane bending must be limited to prevent separation of the veneer from the backup wall.

Requirements for this type of veneer include:

- The dimensions and thickness of each masonry unit in an adhered veneer must not exceed 65 mm; each dimension must not exceed 900 mm; the area must not exceed 0.45 m²; and the weight per unit area must not exceed 700 N/m².

- There are no restrictions on wall dimensions (height, length, area) for adhered veneers, except when stress control between the veneer and the backup wall is required.

- The backup wall must provide a continuous, moisture‑resistant surface for bonding the veneer. Before installing the veneer, a layer of Portland cement mortar is applied to the surface of the backup wall.

Connections for Lateral Load Resistance

Walls with an AAC base are considered a suitable substrate for cementitious veneer because they have favorable rigidity. Moreover, the roughness and porosity of the masonry material create a good bond between the cementitious veneer and the backup wall. Therefore, no metal lath is needed for applying cementitious veneer on AAC walls. The bond between the cementitious veneer and the AAC wall is inherently strong because both are cement‑based materials.

Anchoring Veneer Using AAC Blocks

Cementitious veneer applied to an AAC wall typically consists of two layers (a base layer to smooth any voids on the wall surface and a finish layer) with a total thickness of 16 to 19 mm. To preserve the natural roughness of the AAC block, the mortar joints in the masonry wall are not tooled. Obviously, the wall surface must be clean and free of defects that would interfere with the bond between the cementitious veneer and the backup material. Because the outer surface of AAC walls is porous, it may absorb water from the mix, leaving insufficient water in the cementitious veneer. Therefore, the wall surface should be saturated before applying the base layer.

Applying Cementitious Veneer on AAC Blocks

When applying cementitious veneer on AAC walls, control joints can be placed at wider intervals. In this case, the recommended area of cementitious veneer between control joints is 22 m², and other requirements can be derived from Publication No. 714. As far as possible, control joints and expansion joints in the cementitious veneer should be located at positions corresponding to joints in the base layer. Control joints and other openings are attached to the wall using steel nails.

Buying Tips for AAC Blocks

• Always purchase from reputable manufacturers with the National Standard mark.

• Pay attention to the density and compressive strength grade of the block.

• Estimate transportation and loading costs before purchasing.

• Purchase the special AAC mortar (AAC block adhesive) together with the blocks.