AAC Block Compressive Strength: Classes and Test Method

AAC block compressive strength usually falls between about 1.5 and 7 MPa (N/mm²), depending on density and the standard. China’s GB/T 11968-2020 uses classes A1.5 to A5.0, ASTM C1693 uses AAC-2 to AAC-6, India’s IS 2185 (Part 3) sets 1.5–7.0 N/mm² by density and grade, and EN 771-4 has no classes: the manufacturer declares a value.

The number on a data sheet only means something if you know which standard it was tested to, on what specimen, and at what moisture content. A block that tests at 4 MPa oven-dry can test noticeably lower when wet. This guide sets out the strength requirements in each standard, how the test is run, and what decides strength in the plant.

AAC Block Compressive Strength in MPa: Standards at a Glance

GB/T 11968-2020 Strength Classes (A1.5–A5.0)

GB/T 11968-2020 replaced the 2006 edition. It removed the A7.5 and A10.0 classes and the B08 density grade, so the current range is A1.5 to A5.0 and B03 to B07. Each strength class has a minimum mean and a minimum for the weakest single group of cubes, and each class can only be paired with certain density grades.

These figures are as cited in the T/CASME 2022 draft group standard, which is built on GB/T 11968-2020 (the 2023 Hunan draft standard for high-precision blocks gives the same values for A2.5 to A5.0); check the current edition of GB/T 11968 before using them in a contract. For the density side of the table, see AAC block density.

On our reference line, the product range is B03–B07 and A1.5–A5.0 to GB/T 11968-2020. In practice, the class you can reach at a given density depends on your raw materials and autoclave curing, which is why we test local materials before fixing the formula.

EN 771-4 and EN 772-1: Declared Strength, Not Classes

EN 771-4 does not set strength classes. The manufacturer declares the compressive strength of the unit in N/mm², as a mean or a characteristic value, and says whether the units are Category I or Category II. Category I units have a declared strength with a low, controlled probability of not being reached, backed by factory production control; Category II units do not meet that confidence level. For load-bearing units the declared value must be at least 1.5 N/mm².

Points that matter when you read a European data sheet:

• Normalized compressive strength. Results are converted to the strength of an equivalent 100 mm cube in the air-dry condition, using the shape factor and the conditioning factor in EN 772-1. This is the value masonry design to EN 1996-1-1 (Eurocode 6) uses.
• Consignment checks. In a consignment test, no individual unit may fall below 80% of the declared mean, or 90% of the declared characteristic value.
• Test method. Compressive strength is tested to EN 772-1. EN 679 is the separate European method for the compressive strength of AAC itself.

As an example of a published European declaration, H+H UK declares a mean compressive strength of 7.3 N/mm² (Category I) for its High Strength Celcon block at a gross density of 730 kg/m³. The GCC adopted EN 771-4:2011 as GSO EN 771-4:2021, so the same declaration system applies to suppliers working to the Gulf standard. More detail is in our EN 771-4 AAC masonry unit standard guide.

Strength Classes in ASTM C1693, IS 2185 and DMS 1

ASTM C1693 (United States)

ASTM C1693 (current edition C1693-11, reapproved 2025) is the US specification for AAC itself. It defines strength classes named AAC-2, AAC-3, AAC-4, AAC-5 and AAC-6. Each class has a minimum compressive strength and is paired with nominal dry bulk densities. Check the current edition for the strength and density values of each class and for the moisture condition at test. ASTM C1691 covers unreinforced AAC masonry units. See ASTM C1693 and C1691 explained.

IS 2185 (Part 3): strength by density band

India’s IS 2185 (Part 3):1984 (reaffirmed) sets two grades in each density band. The value is the minimum average compressive strength of the units.

The average is taken over twelve units, and the standard adds a check on test groups: the mean minus 0.6 times the average range must still meet the table value. The test method is IS 6441 (Part 5).

Gulf requirement: DMS 1 Part 3 (Dubai)

Dubai Municipality’s DMS 1: Part 3: 2014 sets a single minimum for AAC masonry units: an average normalized compressive strength of at least 3.2 N/mm² and at least 2.6 N/mm² for any individual specimen, tested in the air-dry condition to BS EN 772-1. Designers can specify higher values. Six specimens are tested for every lot of 20,000 blocks or part of a lot. A plant that sells into Dubai therefore needs a density and mix that clear 3.2 N/mm² normalized with a margin, not a mean that just reaches it.

AAC Block Compressive Strength Test: Step by Step

The details differ by standard, but the sequence is the same.

1. Sample at random from the lot as the standard requires (for example, 6 specimens per 20,000 blocks under DMS 1 Part 3).
2. Prepare specimens. GB/T 11969-2020 cuts 100 mm cubes from the center of the block along the rise direction at three heights: the top cube 30 mm below the top face, one at the center, and the bottom cube 30 mm above the bottom face. EN 772-1 allows whole units or cut specimens.
3. Check faces. Loaded faces must be flat and parallel; GB/T 11969-2020 sets a ±1 mm dimensional tolerance and 0.5 mm flatness for cut specimens (check the current edition).
4. Condition to the required moisture. This is the step most often skipped. GB/T 11969-2020 brings specimens to a set moisture content; EN 772-1 uses air-dry, oven-dry or a set moisture content and converts the result; IS 2185 (Part 3) and DMS 1 Part 3 refer to their own methods.
5. Load to failure at the rate and in the loading direction set in the method.
6. Calculate. Strength = failure load / loaded area. Apply shape and conditioning factors where the standard requires normalization.
7. Assess conformity using the standard’s rule: mean and single-group minimum (GB/T 11968), mean minus 0.6 × range (IS 2185), declared value and 80%/90% rule (EN 771-4), or mean and individual minimum (DMS).

A 2024 paper in Jiangsu Building Materials (Huang) notes that GB/T 11969-2020 does not describe in detail what to do when specimens are below 8% moisture, and that labs commonly add water to bring them up, which can leave the water unevenly spread and affect results. If two labs disagree, ask both how they conditioned the specimens.

Why Moisture and Specimen Shape Change the Result

AAC is about 70–80% air voids by volume (as cited by Silva and Antunes, Materiales de Construcción, 2021), so the water in those pores changes the measured strength a lot.

• Moisture. Jasiński, Drobiec and Mazur (Materials, 2019) tested 127 AAC specimens and found up to about a 30% reduction in strength between 0% and 30% moisture, and up to about 40% at higher moisture. Saturated specimens reached only about 0.57–0.62 of their dry strength. They also warn that the 0.8 moisture factor in EN 772-1 can overestimate the strength of very wet AAC above about 20% moisture.
• Specimen type. Ferretti, Michelini and Rosati (Cement and Concrete Research, 2015) measured 3.1 MPa on standard 100 mm cubes but 2.43 MPa on whole blocks and 2.39 MPa on small walls: whole blocks and walls were about 20% weaker than cubes.
• Rise direction. The same study found blocks weaker on one edge because of the rise during manufacture. Cubes from the top of the cake usually test differently from cubes from the bottom, which is why GB/T 11969 samples three heights.
• Wet masonry. A 2024 study in the ASCE Journal of Materials in Civil Engineering found wet AAC masonry prisms lost about 50–60% of their strength compared with dry ones.

For a buyer, this means a strength figure without the standard, specimen and moisture condition cannot be compared with another supplier’s figure.

What Sets Compressive Strength in the Plant

• Density. Strength rises with density. Every standard above ties strength to a density grade or band.
• Mix and raw materials. The silica source, lime reactivity and cement content decide how much binder forms in the autoclave.
• Pore structure. Even, fine pores give higher strength at the same density than large or merged pores; aluminum dosing and slurry temperature control this.
• Autoclave curing. On our reference line, blocks are cured at 1.3–1.4 MPa (about 190–200 °C) with 7 hours at full pressure. A shorter hold or wet steam gives weaker blocks.
• Plant laboratory. Testing every shift to your market’s standard is the only way to hold a class, not just reach it once.

FAQ

What is the minimum compressive strength of AAC blocks?

It depends on the standard and density. GB/T 11968-2020 starts at class A1.5 (1.5 MPa mean); EN 771-4 requires at least 1.5 N/mm² declared for load-bearing units; IS 2185 (Part 3) starts at 1.5 N/mm² (Grade 2, 451–550 kg/m³); Dubai’s DMS 1 Part 3 requires 3.2 N/mm² average and 2.6 N/mm² individual.

What is AAC block compressive strength in kg/cm²?

Multiply MPa by about 10.2. So 3.5 MPa is about 36 kg/cm², and 5.0 MPa is about 51 kg/cm². Some product listings quote kg/cm², but GB/T 11968, EN 771-4, IS 2185 and DMS 1 all use N/mm² or MPa (1 N/mm² = 1 MPa), so convert before comparing a listing with a standard.

Why do AAC test results differ between labs?

Mainly moisture conditioning, specimen type and sampling position. Wet specimens test lower than dry ones, whole blocks lower than 100 mm cubes, and cubes from the top of the cake differ from those at the bottom. Agree the standard, specimen and moisture condition in the contract before comparing results.

Is AAC strong enough for load-bearing walls?

AAC is used for load-bearing walls in low-rise buildings in Europe and elsewhere, typically at higher density and strength. EN 771-4 requires a declared strength of at least 1.5 N/mm² for load-bearing units, but the wall design, done by an engineer to the local code, decides the class needed. Where AAC is used for infill walls, the frame carries the load and the block class is set by the local code and the designer.

How is AAC block compressive strength tested?

Specimens are sampled from the lot, cut into cubes or tested as whole units, conditioned to the moisture content the method requires, loaded to failure, and the result is divided by the loaded area. Shape and moisture factors are applied where the standard normalizes results, and conformity is judged on means and minimums.

Planning an AAC Block Plant?

Holding a strength class every shift depends on raw materials, autoclave curing and laboratory control. See our AAC block production line for the process and equipment, and the AAC block plant setup guide for the full planning sequence.

Sources

• GB/T 11968-2020, Autoclaved aerated concrete blocks (China) — SAMR national standards catalog entry
• GB/T 11968-2006 / 2020 revision notes — preview (Chinese Standard)
• GB/T 11969-2020, Test methods of autoclaved aerated concrete — preview (Chinese Standard)
• T/CASME draft (2022), Lightweight compressive and durable aerated block — National Group Standards Platform
• DBJ 43/T (draft, 2023), Technical standard for application of high-precision AAC blocks — Hunan Province
• Huang Qianpei (2024), A new approach to compressive strength testing of AAC blocks, Jiangsu Building Materials (3)
• EN 771-4:2011+A1:2015, Autoclaved aerated concrete masonry units — CEN catalogue entry (iTeh)
• GSO EN 771-4:2021 — GCC Standardization Organization
• EN 679:2005, Determination of the compressive strength of autoclaved aerated concrete — EVS
• ASTM C1693-11(2025), Standard Specification for Autoclaved Aerated Concrete — BSI Knowledge
• ASTM C1693 scope — ASTM
• IS 2185 (Part 3):1984 — Bureau of Indian Standards
• IS 2185 (Part 3):1984, text — Internet Archive
• DMS 1: Part 3: 2014, Autoclaved aerated concrete masonry units — Dubai Municipality
• H+H UK, High Strength Celcon Block declaration of performance
• Jasiński, Drobiec, Mazur (2019), Validation of Selected Non-Destructive Methods for Determining the Compressive Strength of Masonry Units Made of AAC, Materials 12(3), 389
• Ferretti, Michelini, Rosati (2015), Cracking in autoclaved aerated concrete: Experimental investigation and XFEM modeling, Cement and Concrete Research 67
• Impact of Moisture Content on the Compressive and Shear Behavior of AAC Masonry (2024), Journal of Materials in Civil Engineering 36(6)
• Silva, Antunes (2021), Materiales de Construcción 71 (343) — AAC porosity and block properties