ASTM C1693 and C1691: AAC Standards Explained

ASTM C1693 is the US specification for autoclaved aerated concrete as a material. It defines what AAC is, which raw materials may be used, and the strength classes AAC-2 to AAC-6 with their density limits and test requirements. ASTM C1691 applies those classes to unreinforced AAC masonry units (blocks), and ASTM C1694 covers reinforced AAC elements such as panels.

If you are planning a plant to supply a market that writes ASTM into its tenders, you need to know which of these documents governs which product, which edition is current, and which numbers your quality lab will be measured against. This article sets out the family, the strength classes and what they mean for a new AAC plant. It summarizes public abstracts and published secondary sources. It does not replace the standards, which you should buy from ASTM in their current editions.

What ASTM C1693 Covers

The current edition is ASTM C1693-11(2025), the 2011 text reapproved in November 2025 (ASTM; technickenormy.cz listing). According to the ASTM scope:

• AAC is a cementitious product based on calcium silicate hydrates. Its low density comes from macroscopic voids made by a void-forming agent, and it is cured under high-pressure steam.
• The permitted raw materials are portland or blended cement, quartz sand or another siliceous fine aggregate, water, lime, gypsum or anhydrite, and the void-forming agent. Fly ash may replace part of the sand.
• The slurry is cast into steel molds, expands, sets, and is cut into units before autoclave curing.
• Inch-pound units are the standard; SI values are for information.

For a plant buyer, the scope is a useful test: it describes the gas-aerated, autoclaved process. A foam concrete block cured in air (CLC) is not AAC under this definition. The two products are compared in our article on AAC vs CLC blocks.

The process the scope describes is the one on our reference line: sand or fly ash wet-milled into slurry, dosed with lime, cement, gypsum and aluminum paste, poured, pre-cured, tilted, wire-cut on six sides and autoclaved at 1.3–1.4 MPa (about 190–200 °C).

The documents C1693 calls up

The reference list of C1693-11(2025), as shown in the normdocs catalog, points to the raw material specifications your supplier certificates will be checked against:

If your target market specifies ASTM, ask your cement, lime and fly ash suppliers early whether they can certify to these documents, and budget for a compression press whose calibration can be verified to ASTM E4.

The ASTM AAC Standards Family: Who Does What

ASTM split AAC into a material standard and separate product standards. Several older documents were withdrawn in 2013, and many web pages still quote them.

Sources: ASTM store pages for C1693, C1691, C1692, C1686, C1452; Technické normy listing for C1694; ASTM Subcommittee C27.60 jurisdiction page; normdocs catalog for the documents that reference C1693.

The practical point: if a tender or a consultant quotes ASTM C1452, it is quoting a withdrawn document. The ASTM page for C1452-00(2006) lists it as “withdrawn, no replacement”, and its scope (floor, roof, wall and stair elements in three strength classes) is now handled by C1694 for the elements, with C1693 for the material. Ask which current document they mean before you commit to a panel test program.

ASTM C1693 Strength Classes and Density Limits

C1693 classifies AAC by strength class. Each class has a minimum compressive strength and is paired with one or more nominal dry bulk densities, each with a permitted density range. The class name gives the strength in MPa: AAC-4 means a minimum of 4.0 MPa.

Strengths for all five classes: FIDE Specification 052, which classifies AAC “according to its compressive strength per ASTM C1693”. Density pairings for AAC-2, AAC-4 and AAC-6: Matthys and Barnett, ASCE Structures Congress 2004, a paper that predates the current C1693 text, so confirm the density table in the current edition. The psi values for AAC-3 and AAC-5 are conversions.

Three points that matter in production:

1. Strength and density are specified together. You cannot sell an AAC-4 block at any density you like; it must fall in one of the density ranges paired with AAC-4. The lighter the block, the harder it is to hit the strength.
2. Overall range. C1693-type AAC spans roughly 350–850 kg/m³ and 2–6 MPa.
3. Drying shrinkage. FIDE Specification 052, built on the C1693 classes, sets a maximum average drying shrinkage of 0.02%. Full autoclave curing is what keeps AAC shrinkage this low; an under-cured product is the first to fail it. That is one reason autoclave pressure and holding time are not the place to save money.

For how the strength figure is tested and what moves it in the plant, see AAC block compressive strength; for the density side, see AAC block density.

ASTM C1691: The Block Standard

ASTM C1691-21 covers solid, unreinforced masonry units made from AAC. Its abstract states that the units must be made entirely of AAC material and classified by strength class, and that they must meet compressive strength, dry bulk density and drying shrinkage requirements determined under the referenced ASTM documents, in practice C1693. On top of that, C1691 adds:

• dimensional tolerances for the units;
• maximum limits for drying shrinkage;
• shipping and handling requirements.

So a block offered “to ASTM C1691” is a block whose material meets a C1693 strength class and whose dimensions, shrinkage and handling meet C1691. When you write a sales specification, name both, for example “AAC-4, 600 kg/m³ nominal, units to ASTM C1691”.

On dimensions, the cutting machine decides what you can promise. On our reference design the six-side wire cutter works to ±3 mm in length and ±1 mm in width and height, about 6 minutes per mold. Compare that with the C1691 tolerance table in the edition you buy before you commit to a thin-bed system. The cutter is therefore part of the standards decision, not only a production choice.

Reinforced AAC: C1694 and Panels

ASTM C1694 covers reinforced solid elements made from AAC that meets C1693. It adds dimensional tolerances, drying shrinkage limits, requirements for the steel reinforcement and shipping and handling. ASTM C1686 covers installation and testing of reinforced units.

For a plant, panels use the same mix, pouring, cutting and autoclaving as blocks, with steel mesh placed in the mold before pouring. The extra steps are mesh preparation, corrosion protection and panel handling, and the extra testing covers the reinforcement. Most export plants start with blocks and add panels once the block line runs steadily.

How ASTM Classes Compare with GB/T 11968 and EN 771-4

Lines built in China are usually designed around GB/T 11968-2020, which uses density grades B03–B07 (300–700 kg/m³) and strength classes A1.5–A5.0. Europe uses EN 771-4, where the producer declares a mean compressive strength and a unit category instead of a fixed class (see EN 771-4 explained).

Do not convert between them on paper. Specimen size, moisture condition at test and acceptance rules differ, so a block that meets “A3.5” on a GB/T test will not automatically meet AAC-3 on an ASTM test. The reliable route is to run trial batches on your own raw materials and test them to the standard your buyers will use.

Our reference line is configured for strength classes A1.5–A5.0 (GB/T 11968). If your market specifies a higher class, such as AAC-6, state it in your requirement brief, because it affects the mix design and the raw material tests.

What ASTM Compliance Means When You Build an AAC Plant

Compliance belongs to the product and the producer, not to the machinery. A line can make blocks that meet C1693 and C1691, but the plant earns that by controlling raw materials, process and testing. A practical sequence:

1. Buy the current editions of C1693, C1691 and, if you will make panels, C1694. ASTM sells them online; free “PDF” copies on the web are usually old or unlicensed versions.
2. Fix the target classes and densities you will sell, for example AAC-2 at 400 kg/m³ for partitions and AAC-4 at 600 kg/m³ for load-bearing walls.
3. Test your raw materials. Send 30–50 kg of your sand or fly ash, lime and cement for laboratory tests and trial batches before the formula and line configuration are fixed.
4. Equip the quality lab for compressive strength, dry bulk density, moisture content and drying shrinkage, with a press whose calibration can be verified to ASTM E4.
5. Arrange independent testing of first production at a laboratory your customers accept, and keep the reports.
6. Write the sales specification naming the class, the nominal density and the unit standard.

FAQ

Where can I get the ASTM C1693 PDF?

From ASTM International’s online store, or through a subscription service your company or a university library holds. The current edition is C1693-11(2025). PDFs found on file-sharing sites are often superseded editions, and using them risks quoting withdrawn numbers. The ASTM product page shows the scope free of charge; the strength class table and test methods are in the full document.

What is the difference between ASTM C1693 and C1691?

C1693 specifies AAC as a material: raw materials, strength classes, density limits and test requirements. C1691 specifies unreinforced masonry units made from that material and adds dimensional tolerances, drying shrinkage limits and shipping and handling rules. A block specification should normally cite both: the C1693 class and density, plus C1691 for the unit.

What does AAC-4 mean?

AAC-4 is the ASTM C1693 strength class with a minimum compressive strength of 4.0 MPa (580 psi). Published summaries pair it with nominal dry bulk densities of 500 to 800 kg/m³, each with its own permitted range. AAC-4 is a common choice for load-bearing walls in low-rise buildings, while AAC-2 is typical for lighter partition blocks.

Is ASTM C1452 still valid for AAC panels?

No. ASTM lists C1452-00(2006), the specification for reinforced AAC elements, as withdrawn in 2013 with no named replacement. Reinforced AAC elements are now covered by ASTM C1694, with the material in C1693 and installation in C1686. If a tender cites C1452, ask the engineer to confirm the current document before you plan testing.

Can a line built in China produce ASTM-compliant AAC blocks?

Yes, if the plant controls its raw materials and process and tests its product to ASTM methods. The standard does not specify where equipment comes from; it specifies the product. What matters is the mix design proven on your raw materials, full autoclave curing, cutting accuracy and a lab that tests every production lot.

Planning an AAC Block Production Line?

See how lines from 20,000 to 300,000 m³ a year are configured on our AAC block production line page, and read the AAC block plant setup guide for the planning sequence from raw material tests to commissioning.

Sources

• ASTM International, ASTM C1693-11(2025) Standard Specification for Autoclaved Aerated Concrete (AAC): https://store.astm.org/c1693-11r25.html
• Technické normy, ASTM C1693-11R25 listing (publication date, replaces C1693-11R17): https://www.technickenormy.cz/en/astm-c1693-11r25-standard-specification-for-autoclaved-aerated-concrete-aac/
• normdocs catalog, ASTM C1693-11(2025): referenced documents and documents that reference it: https://catalogue.normdocs.ru/catalog/com.normdocs.astm.card.c1693-11.2025./Standard-ASTM-C1693-11-2025?tab=links
• ASTM International, ASTM C1691-21 Standard Specification for Unreinforced Autoclaved Aerated Concrete (AAC) Masonry Units: https://store.astm.org/c1691-21.html
• Technické normy, ASTM C1694-09R25 Standard Specification for Reinforced Autoclaved Aerated Concrete (AAC) Elements: https://www.technickenormy.cz/en/astm-c1694-09r25-standard-specification-for-reinforced-autoclaved-aerated-concrete-aac-elements
• ASTM International, ASTM C1692-18(2025) Standard Practice for Construction and Testing of AAC Masonry: https://store.astm.org/c1692-18r25.html
• ASTM International, ASTM C1686-09(2025) Standard Practice for Installation and Testing of Reinforced Autoclaved Aerated Concrete (AAC) Units: https://store.astm.org/c1686-09r25.html
• ASTM International, ASTM C1452-00(2006) Standard Specification for Reinforced Autoclaved Aerated Concrete Elements (Withdrawn 2013): https://store.astm.org/c1452-00r06.html
• ASTM International, Subcommittee C27.60 on Precast Autoclaved Aerated Concrete, jurisdiction and withdrawn standards: https://www.astm.org/get-involved/technical-committees/committee-C27/subcommittee-C27/jurisdiction-C2760
• J. H. Matthys and R. E. Barnett, “New Masonry Product for the US Designer Emerges: Autoclaved Aerated Concrete”, ASCE Structures Congress 2004 (Table 1, AAC compressive strengths and densities): https://engineering.purdue.edu/~frosch/ftp/Talbott/11%20-%20References/files/ASCE%20Structures%20Congress%202004/PDFs/1st04-3638.pdf
• FIDE, Especificación Sello FIDE No. 052, Concreto Celular Autoclaveado (AAC), Rev. 3: https://www.fide.org.mx/wp-content/uploads/sello-fide/especificaciones/ESP052_03.pdf