ALC Panel

What is an ALC Panel

An ALC panel is a precast, factory-cured building element made from autoclaved aerated concrete with an embedded two-way welded steel reinforcing mesh. The abbreviation ALC stands for Autoclaved Lightweight Concrete. The same product is written several different ways across markets: "ALC" is the term prevalent in Japanese and East-Asian markets, while "AAC panel," "reinforced AAC," and "RAAC" are the equivalent Western and international terms for the same product family. The core material is chemically identical to AAC block material; only the reinforcement and the panel format differ. The element sits under Building Materials › Masonry & Insulation.

The single most important thing to understand about an ALC panel is what separates it from an AAC block. Both are made from the same autoclaved aerated concrete, but the block is an unreinforced masonry unit that is laid in mortar like brick, one small unit at a time. The ALC panel, by contrast, embeds a two-way welded steel reinforcing mesh inside the aerated matrix, which turns it into a large reinforced slab. That reinforced slab can span between structural members and act as a self-supporting wall, roof, or floor element rather than being stacked unit by unit. In engineering terms, the panel is a spanning element while the block is a masonry unit, and that distinction drives everything downstream: panel format, dry construction, span tables, and the need to protect the embedded steel.

Because the panel is a reinforced, factory-made spanning element, ALC construction is a dry-construction method: panels are erected and dry-jointed rather than mortared course by course, which requires far less wet plastering than a block wall. The aerated matrix simultaneously provides insulation, fire resistance, and mass while keeping weight low, and the steel mesh is the load-resisting skeleton that lets the panel carry bending loads the modest concrete strength alone could not. This dual role, structure plus insulation in one element, is the central reason ALC panels are specified for exterior walls, partitions, roofs, and floors.

A note on naming is worth keeping in mind throughout this guide, because procurement documents from different regions use different words for the same thing. ALC equals AAC panel equals reinforced AAC equals RAAC; the material is the same autoclaved aerated concrete used in AAC blocks. SpecForge lists "AAC Block" as a separate sibling entry, and the key engineering difference to communicate between the two entries is reinforcement: the panel has a mesh, a panel format, and behaves as a dry-construction spanning element, while the block is an unreinforced, mortar-laid masonry unit.

Why does the same product have so many names? It is largely a matter of which market named it first. "ALC" is the term prevalent in Japanese and East-Asian markets, where the product is a long-established construction system, and it is the abbreviation buyers will see on Asian-sourced datasheets. "AAC panel," "reinforced AAC," and "RAAC" are the equivalent Western and international terms used in Europe, North America, and the engineering literature. They describe exactly the same thing: autoclaved aerated concrete cast around a steel mesh and formed as a reinforced slab. When a specification, a tender, or a corrosion-failure report uses any one of these labels, treat them as synonyms for the same product family, and read the spec on its substance (matrix, mesh, panel format, grade) rather than on the acronym. The only sibling product you must keep mentally separate is the AAC block, which shares the matrix chemistry but has no reinforcement and is laid in mortar rather than spanned between supports.

Panel Types by Function

ALC panels are classified by the structural function they perform in the building. The function determines how heavily the panel is reinforced and how it is detailed, so getting the type right is the first selection decision. There are five functional types in common use, summarized in the table below.

Exterior or external wall panels are the most common ALC product. They can be installed in a vertical or horizontal lay-up and act as the self-supporting external wall of the building. Because a wall panel carries mainly its own self-weight plus wind and seismic loads rather than acting as a primary flexural member, it is lighter-reinforced than a roof or floor panel. Wall panels are typically supplied with tongue-and-groove edges for dry jointing, so adjacent panels interlock and can be erected quickly without wet mortar joints.

Interior partition wall panels are a form of lightweight partition panel: non-loadbearing dividing walls used to subdivide interior space. They share the wall panel's lighter reinforcement and tongue-and-groove dry-jointing approach, and they are usually thinner than exterior walls because they do not face weather or wind load. Their value is fast, dry installation of fire-rated, sound-attenuating partitions without the wet trades a masonry partition would require.

Roof panels span between purlins or beams and carry the roof live load, which makes them flexural members rather than self-weight-only walls. Floor or slab panels span between beams and carry the floor live load, and they carry the heaviest reinforcement of all the panel types because the floor live load is the most demanding bending duty. The engineering rule that ties this chapter together is straightforward: wall and partition panels carry mainly self-weight plus wind and seismic, so they are lighter-reinforced, while roof and floor panels are flexural members and carry more and heavier reinforcement. Cladding or curtain-wall panels are a non-loadbearing facade application mounted on a steel frame, where the panel provides the weatherproof, insulating, fire-resistant skin without carrying primary structural load; in this role ALC competes with alternative facade systems such as a glass curtain wall or a metal curtain wall panel.

The reinforcement difference between the two groups is not a minor detail; it is what makes the type selection a structural decision rather than a cosmetic one. A wall or partition panel is sized mainly to stand up under its own weight and to resist lateral wind and seismic forces, so its mesh is lighter and its design checks center on out-of-plane bending under wind and in-plane behavior under seismic drift. A roof or floor panel is a flexural member first: it is loaded transversely across its span by roof or floor live load, so its mesh must be designed to carry that bending moment and limit deflection over the span. This is why the floor or slab panel carries the heaviest reinforcement of the family, and why roof and floor panels in particular must always be backed by verified manufacturer span tables and load capacity figures before they are ordered. Specifying a wall-grade panel for a floor duty, or assuming a partition panel can span a roof bay, is the kind of type error that the function-first decision sequence in Chapter 6 is designed to prevent.

Manufacturing Technology

Understanding how an ALC panel is made explains nearly every property a buyer cares about: why it is so light, why it insulates, why it resists fire, and why the embedded steel must be corrosion-protected. ALC is produced by mixing fine silica (quartz sand or fly ash), Portland cement, quicklime, gypsum, and water into a slurry, then adding a tiny dose of aluminium powder as the expansion agent, roughly 0.05 to 0.08 percent by volume. The aluminium reacts with the alkaline lime and cement to release hydrogen gas, which froths the slurry and creates the closed and open cellular pore structure that gives the material its light weight. The cured material is up to about 80 percent air by volume, which is the single fact that explains the low density covered in Chapter 5.

Once the slurry has frothed, the pre-expanded "cake" is cast around a pre-positioned, corrosion-protected welded steel mesh. While the cake is still green (uncured and soft), it is cut to size with wires, which is how panels of precise width, length, and thickness are produced before any hardening locks in the geometry. Casting the mesh into the cake before cutting is what makes the steel an integral, two-way load-resisting skeleton rather than a surface attachment.

The defining process step is autoclaving. The cut panels are cured in a pressurized steam autoclave at about 190 degrees Celsius and 800 to 1,200 kPa (8 to 12 bar) for roughly 12 hours. Autoclaving is not just drying: it converts the calcium and silica into tobermorite, a calcium silicate hydrate, and tobermorite is the crystalline phase responsible for the material's strength and dimensional stability. In other words, the heat-and-pressure cure is what chemically creates the load-bearing mineral structure. Inside the finished panel, the steel mesh is the load-resisting skeleton, while the aerated matrix provides insulation, fire resistance, and mass, all while keeping the panel weight low.

Photo: Vkansagra, CC BY-SA 3.0, via Wikimedia Commons

The pore structure created by the aluminium reaction is also the reason ALC behaves so differently from ordinary concrete on every metric a buyer checks. Because the cured material is up to about 80 percent air by volume, its dry bulk density falls to roughly one quarter that of normal-weight concrete and its thermal conductivity drops by about an order of magnitude, which is what allows a single panel to serve simultaneously as structure and as insulation. The trade-off written into the chemistry is strength: an aerated matrix that is mostly air cannot match dense concrete in compression, so AAC's overall compressive strength tops out at about 8 MPa (8,000 kPa), roughly a third to a half of even low-grade dense concrete, and ALC product grades sit lower still at roughly 3 to 5 MPa. That modest matrix strength is exactly why the embedded steel mesh is not optional reinforcement but the primary load-resisting skeleton: the panel earns its load capacity from the steel, while the matrix earns its keep through low weight, insulation, fire resistance, and mass.

It helps to read the manufacturing sequence as a chain of cause and effect. The aluminium powder dose controls the pore structure, which controls density and therefore both weight and insulation. The embedded mesh controls bending capacity, which is what lets the modest-strength concrete behave as a spanning slab. The autoclave cure controls the mineralogy (tobermorite), which controls strength and dimensional stability. And because the aerated matrix is permeable, the cure does not give the embedded steel the same alkaline passivation that dense concrete provides, which is precisely why the mesh has to be corrosion-protected before casting, the topic of the next chapter.

Materials and Reinforcement

An ALC panel is made of three things working together: the aerated concrete matrix, the embedded steel reinforcement, and the panel's jointing detail. Each contributes a distinct property, and each carries its own selection implication.

The matrix is autoclaved aerated concrete: cement plus lime plus silica (sand or fly ash) plus gypsum plus water, expanded with an aluminium powder expansion agent. This is the same material chemistry as an AAC block. The matrix is what delivers the panel's low weight, its insulation, its fire resistance, and its mass. Because it is largely air by volume, it is also permeable, and that permeability is the root cause of the durability consideration that dominates this chapter.

The reinforcement is a bidirectional (two-way) welded steel wire mesh that is anti-corrosion treated. The mesh is dipped or coated with a rust-inhibiting agent before casting, and the reason is specific to ALC: the aerated matrix is permeable and offers less alkaline passivation than dense concrete, so the steel cannot rely on the surrounding concrete to protect it the way rebar in dense concrete can. Corrosion protection of the embedded steel is the critical long-term durability factor for the panel. This is not a theoretical concern: legacy RAAC panels from 40 to 50 years ago have shown reinforcement-integrity problems, which is exactly why the coating spec and in-service inspection matter when the product is expected to last 50 years or more.

The jointing detail on wall and partition panels is the tongue-and-groove edge, which lets adjacent panels interlock for dry jointing. Tongue-and-groove edges are part of what makes ALC a dry-construction method: panels key into one another and are fixed to the frame rather than being bedded in mortar like masonry. When you compare ALC to a block wall, the tongue-and-groove dry joint is a large part of why ALC requires far less wet plastering on site.

The practical takeaway for materials selection is that the matrix and the steel have opposing failure logics. The matrix is durable, non-combustible, and chemically stable thanks to its tobermorite mineralogy, but it is permeable. The steel is strong and is what makes the panel a spanning element, but it is vulnerable to corrosion precisely because the matrix around it is permeable. The whole long-term performance of the panel therefore hinges on the anti-corrosion treatment of the mesh, which is why that single spec deserves explicit confirmation on every datasheet.

Key Specification Parameters

Reading an ALC panel datasheet means checking a defined set of physical parameters: dry bulk density, compressive strength, thickness, standard dimensions, thermal conductivity, fire resistance, sound insulation, and service life. The table below collects the cross-verified ranges so they can be compared at a glance; each parameter is then explained beneath.

Dry bulk density is typically 425 to 650 kg/m3, with common product grades clustering around 500 to 625 kg/m3. That is about one quarter the density of normal-weight concrete (about 2,400 kg/m3) and about one third that of fired clay brick, which is the headline reason ALC reduces structural dead load. In the Chinese standard GB/T 15762, densities are expressed as B-grades: for example B04 is about 400, B05 about 500, and B06 about 600 kg/m3.

Compressive strength is roughly 3 to 5 MPa, expressed as A-grades such as A2.5, A3.5, and A5.0, meaning 2.5, 3.5, and 5.0 MPa respectively. AAC's overall compressive strength runs up to about 8 MPa (8,000 kPa), roughly a third to a half of even low-grade dense concrete. The point to internalize is that the concrete's own compressive strength is modest; ALC panels resist bending via the steel mesh, not via concrete strength alone, which is why reinforcement, not concrete grade, governs flexural panels.

Thickness is commonly 50 to 200 mm, with 50, 75, 100, 125, 150, 175, and 200 mm being typical increments. Partitions tend to be about 75 to 125 mm, exterior walls about 100 to 150 mm, and floor or roof panels 100 to 200 mm. Standard dimensions are usually a width of about 600 mm (a 600 mm module, which some sources cite as roughly 2 ft) with a length up to about 6 m, held to dimensional tolerances on the order of plus or minus 3 mm on width and thickness and plus or minus 5 mm on length.

Thermal conductivity (lambda) is approximately 0.10 to 0.16 W/(m·K), with measured ALC data as low as about 0.11 W/(m·K). That is roughly an order of magnitude lower than dense concrete and roughly four to six times lower than fired clay brick (about 0.6 to 1.0 W/(m·K)), and it is the low conductivity that lets ALC act as both structure and insulation in one element, often reducing or removing the need for a separate insulation board layer on the wall. Fire resistance is non-combustible, A-class: a roughly 100 mm panel typically achieves around 3 to 4 hours of fire rating, because the mineral matrix has a melting or break-down point well above about 1,000 degrees Celsius and releases no smoke or toxic gas, which is why an ALC partition is often paired with a fire-rated door to complete a fire-rated compartment.

Sound insulation is roughly 40 to 50 dB (Rw or STC), depending on thickness and finish; measured panel data run about 40 dB at 100 mm, 45 dB at 120 mm, and 48 dB at 150 mm. Service life is commonly cited as a design life of 50 years or more, but with an explicit caveat: it is subject to reinforcement corrosion protection. Legacy RAAC panels from 40 to 50 years ago have shown reinforcement-integrity problems, which is why corrosion protection and inspection matter for actually achieving the design life.

Selection Decision Factors

To turn the preceding five chapters into a specific panel order, follow the decision sequence below. As with most engineering selection, mistakes tend to come not from one wrong step but from deciding lower-level details before the higher-level function is fixed. These eight steps can serve as a fixed RFQ template for ALC panels.

1. Function and orientation: First choose wall versus partition versus roof versus floor panel. Flexural panels (roof and floor) need verified span tables and load capacity, because they carry live load in bending rather than just self-weight.
2. Strength and density grade: Match the A-grade (MPa, e.g. A2.5 / A3.5 / A5.0) and the B-grade (density, e.g. B04 / B05 / B06) to the load and the required thermal and acoustic performance. Remember the trade-off: higher density means more strength but worse insulation.
3. Thickness: Drive thickness from the fire rating, the thermal target (U-value), the acoustic target, and the span. Thicker panels give better fire, thermal, and acoustic performance but are heavier, so do not over-specify beyond what the duty requires.
4. Reinforcement corrosion protection: Confirm the anti-corrosion coating spec on the mesh. This governs long-term durability and is the historic failure mode of RAAC, so it is the one spec never to leave unstated.
5. Connection system: Specify and detail the fixing method to the frame: L-hook bolts (welded, used mainly with steel structures), slip-type or sliding crossing connectors, and U-shaped clips for seismic drift accommodation. Connection seismic performance (stiffness, energy dissipation, hysteresis) is an active research area and a real selection differentiator in seismic zones.
6. Span versus support spacing: Verify the chosen panel against the manufacturer's span and deflection tables. The span the panel can achieve is bounded by support spacing and allowable deflection, not just by raw strength.
7. Standard and certification: Require conformity to GB/T 15762, JIS A 5416, EN 12602, or a local equivalent, together with a real datasheet rather than a marketing sheet.
8. Finish and jointing: Choose tongue-and-groove versus flat edge, and decide whether the surface is for direct render or for cladding. ALC is a dry-construction method requiring far less wet plastering than block walls, which is part of its installed-cost advantage.

One last point worth keeping in view is the supply and installation chain, because ALC panels are erected by specialist installers, not laid like loose blocks. Recognized names across the ALC family include Asahi Kasei Construction Materials of Japan, whose "Hebel" ALC panels trace back to a Hebel licensee (Hebel originated in Memmingen, Germany, in 1943); Siporex, whose AAC/ALC technology was established in Sweden in 1939 and licensed widely (Sumitomo and Asahi Glass acquired Siporex licensees in Asia); and Xella International, which owns the "Ytong" and "Hebel" AAC brands in Europe and globally. In the Chinese market, Nanjing Asahi New Building Materials (Nasahi), established in 1996, is a major ALC panel maker marketing JIS-compliant wall and roof panels, and Eastland Building Materials (Nanjing) manufactures AAC/ALC wall and floor panels. Shanghai Yitong and SMB Kenzai (a Japanese Siporex distributor and installer) are also recognized names in the ALC supply and installation chain. Confirm that the supplier provides both a real datasheet and a workable connection and installation detail before committing.