Clay Brick Testing Methods: Strength, Absorption and Efflorescence

Clay bricks are tested for compressive strength, water absorption (cold-water and boiling, which together give the saturation coefficient), initial rate of absorption, efflorescence, freeze-thaw resistance, and size and warpage. The test methods sit in ASTM C67 in the United States, the EN 772 series in Europe, GB/T 2542 in China and IS 3495 in India. The product standard your market uses sets the pass limits.

That split matters for a plant. A test method tells you how to sample, prepare, condition and load a brick; a product specification such as ASTM C62, ASTM C216, EN 771-1 or the Chinese GB/T product standards tells you what result is acceptable. Results from different methods are not interchangeable, even when the property has the same name. This guide sets out the main tests, how the methods differ, and how to build a test plan for a new fired clay brick plant. Clause numbers and limits are given only where we could check them in a public source; otherwise, check the current edition.

Clay Brick Testing Methods at a Glance

ASTM section numbers follow the scope table of ASTM C67/C67M-25; the ASTM store lists C67/C67M-26 as the active edition, so check the current edition. EN 772 part titles are as listed in the normative references of EN 771-1:2011+A1:2015. GB/T 2542-2012 is listed as valid in the chinesestandard.net record; check whether a newer edition has been issued. GOST 8462-85 and GOST 7025-91 are the test standards referenced by GOST 530-2012, the interstate standard for ceramic brick and stone used in much of Central Asia.

Compressive Strength Testing

Compressive strength is the headline number on most brick specifications, but the method behind it changes the result.

ASTM C67. The specimen is a full-height, full-width piece about half a brick long, oven-dried, capped to give smooth parallel bearing surfaces, and centered under the spherical bearing block within 1/16 in. (1.6 mm). Load is applied to half the expected maximum, then at a rate that brings the specimen to failure in not less than one and not more than two minutes. Strength is the maximum load divided by the gross cross-sectional area, or the net area if net strength is required (BIA TN 39, summarizing C67 as of 2001; check the current edition).

IS 3495 Part 1. Solid bricks have their frogs and bed-face voids filled with cement mortar, are cured damp and soaked before testing; perforated bricks are soaked but not filled or capped. Load is applied at a uniform 14 N/mm² per minute (IS 3495, Parts 1 to 4: 1992; check for later revisions).

EN 772-1. The European method feeds the declared strength under EN 771-1. Eurocode 6 masonry design then works from the normalized mean compressive strength of the unit, fb, in the formula fk = K fb^0.7 fm^0.3 (Haseltine, JRC Eurocodes training material). Check the current edition of EN 772-1 for conditioning and the conversion to normalized strength.

Two differences catch buyers out:

• Gross or net area. For a perforated brick, the same failure load gives a higher net-area strength than gross-area strength. With 30% voids, as our own illustration, net-area strength is about 1.43 times the gross-area figure (1 ÷ 0.70). Always ask which area a report uses.
• Moisture state. Oven-dry, air-dry and soaked specimens give different results. A soaked brick tested to IS 3495 cannot be compared directly with an oven-dried brick tested to ASTM C67.

Strength in the plant traces back to the body and the firing. Underfired bricks are weak and absorbent; see our article on clay brick firing temperature for how the firing range is set.

Water Absorption, Saturation Coefficient and Initial Rate of Absorption

Cold-water and boiling absorption

ASTM C67 measures absorption after 24 hours in cold water and after 5 hours in boiling water. The ratio of the two is the saturation coefficient (Mason Contractors Association of America). A low ratio means part of the pore space stays empty in normal soaking, leaving room for ice to expand, which is why ASTM uses it as a durability indicator.

The limits sit in the product standards. As summarized by the BIA in 2007, ASTM C62 and C216 required:

Under the absorption alternate, the saturation coefficient limit does not apply if no single brick in a random sample of five exceeds 8% cold-water absorption (BIA TN 9A, 2007). Check the current editions of C62 and C216 before using these limits; our article on ASTM C62 and C216 explains the grades.

Other methods differ in detail. IS 3495 Part 2 dries the brick at 105–115 °C to constant mass, immerses it at 27 ± 2 °C for 24 hours and weighs it within three minutes of removal. EN 772-21 covers cold-water absorption of clay units, and EN 772-7 boiling absorption for clay damp proof course units.

Initial rate of absorption

Initial rate of absorption (IRA, or suction) measures how fast a dry brick draws water from fresh mortar. In the ASTM C67 laboratory test, five oven-dried, full-size bricks stand in 1/8 in. (3 mm) of water for one minute, and IRA = 30 W / (L × B), with W the weight gain in grams and L and B the length and width in inches. ASTM C62, C216 and C652 recommend a limit of 30 g/30 in.²/min (BIA TN 39, a 2001 summary; check the current editions). IRA is not a qualifying property in ASTM, so it must be requested if the buyer wants it (BIA TN 9A). In Europe, EN 772-11 covers IRA for clay units.

Efflorescence and Lime Bursting Tests

Efflorescence. The ASTM C67 test partially immerses bricks in distilled water for seven days. The bricks are then dried and compared with bricks that were not immersed, and rated “not effloresced” or “effloresced” (BIA TN 23A, 2019). The BIA notes that drying at 220 °F (104 °C) is more severe than normal service. As the BIA summarizes the ASTM specifications, the “not effloresced” requirement applies only if the specifier invokes it, and C62 has no efflorescence requirement (BIA TN 9A).

IS 3495 Part 3 grades the result instead: brick ends are stood in 25 mm of distilled water in a ventilated room at 20–30 °C until the water evaporates, the cycle is repeated, and the deposit is rated:

EN 772-5 takes a chemical route and determines the active soluble salts content of clay units. GB/T 2542 includes an efflorescence test as well.

Where the salts come from decides the fix. The BIA lists calcium sulfate, vanadyl sulfate, manganese oxide and iron oxide (linked to black core) as salts whose most probable source is the brick itself (TN 23A). Sulfates in the raw material are the usual plant-side cause; some plants add barium carbonate before extrusion to keep them from rising to the surface (US EPA AP-42, section 11.3).

Lime bursting. GB/T 2542 includes a lime bursting test. Lime particles in the body hydrate after firing, expand and blow out small pieces of the face. Ceramic Industry notes that the size of the lime particles, more than their presence, decides whether pops appear, so finer grinding and screening of lime-bearing raw material is the usual fix.

Freeze-Thaw Testing

Freeze-thaw tests show whether a brick survives repeated freezing while wet. ASTM C67 has a freezing and thawing method. Under the freezing and thawing alternative described by the BIA, the boiling absorption and saturation coefficient limits do not apply if five bricks pass the C67 test with a weight loss not greater than 0.5% of dry weight for any brick (for Grade SW) (BIA TN 9A, 2007). Check the current edition for the cycle count and procedure.

In Europe, EN 772-22:2018 determines the freeze/thaw resistance of clay masonry units in category F1 or F2, using a test panel built from the units. It superseded CEN/TS 772-22:2006. In China, GB/T 2542 includes a freeze-thaw test; in the CIS, GOST 7025-91 covers frost resistance of ceramic brick.

Climate decides how much this test matters. In the hot Gulf states, freezing is rare; ASTM provides a low weathering index alternative where the weathering index is below 50 (BIA TN 9A). In Central Asia, with freezing winters, frost resistance is often the property that decides whether a brick is accepted for facades, so plan for regular freeze-thaw testing from the first production.

Dimensions, Warpage and Appearance

Size and warpage are measured on every routine sample because they are cheap to test and tell you quickly when something has changed. ASTM C67 measures size and warpage; the tolerance tables sit in the product standards, such as the FBS and FBX tables in C216 (BIA TN 9A). GB/T 2542 measures dimensions with calipers to 0.5 mm, and EN 772-16 covers dimensions for European units.

IS 3495 Part 4 describes warpage measurement in detail: a steel rule graduated in 0.5 mm divisions or a steel wedge, against a flat surface at least 300 × 300 mm and plane to 0.02 mm. Concave warpage is the largest gap under a straight edge; convex warpage is measured with the convex face down at the four corners, and the larger of the two is reported.

In the plant, size drift usually comes from three places: a worn die, a change in drying or firing shrinkage, or a change in raw material. Die sizes and cutter spacings are calculated to compensate for normal shrinkage (BIA TN 9), so a change in firing temperature changes the fired size too.

Building a Testing Plan for a New Brick Plant

Sampling and the plant laboratory

ASTM C67 requires full-size bricks as test specimens, representative of the lot and free of foreign material (ASTM C67/C67M-25 scope). Compressive strength is reported as the average of at least five specimens, and void area is measured on a sample of ten bricks (BIA Technical Note 39, summarizing C67 as of 2001; check the current edition).

European product standards add a statistical layer. EN 771-1 sorts units into Category I, where the probability of failing to reach the declared compressive strength does not exceed 5%, and Category II, which does not reach that confidence level (EN 771-1:2011+A1:2015, definitions 3.34 and 3.35). Declaring Category I means the plant’s sampling and production control must support that confidence, not just pass one test.

A fired clay brick plant usually needs these items in its own laboratory, with an external accredited laboratory for certification tests:

1. A compression testing machine with a spherical seat, and capping materials.
2. A drying oven, a balance, a cold-water tank and a boiling tank for absorption.
3. Trays with a controlled water depth for initial rate of absorption and efflorescence.
4. Calipers, a steel rule and a flat reference surface for size and warpage.
5. Access to freeze-thaw testing, in-house or external, if your market has freezing winters.

A test plan in six steps

1. Pick the target standard per market first. A plant selling to the Gulf, Central Asia and Africa may need ASTM, EN and GOST results for the same product. Each sets its own tests.
2. Test during the raw material stage. Test firings at several temperatures should report strength, absorption, efflorescence and lime bursting, so that the firing range is known before the kiln is designed.
3. Sample across the kiln car during commissioning. Take bricks from the top, bottom, edge and center of the setting. Firing varies across the setting, and the weakest position sets the grade you can claim.
4. Set a routine plan. Size, warpage and visual checks each shift; strength and absorption per production lot; efflorescence and freeze-thaw at fixed intervals and whenever the raw material changes.
5. Link every result to production records. Record kiln car, date, firing curve and raw material batch with each test, so that a failure can be traced.
6. Use an accredited external laboratory for certification. In-house results run the plant; third-party results support declarations and tenders.

The clay brick manufacturing process article shows where each of these properties is set, from raw material preparation to firing.

FAQ

What tests are done on clay bricks?

The core tests are compressive strength, water absorption (cold-water and boiling, giving the saturation coefficient), initial rate of absorption, efflorescence, freeze-thaw resistance, size and warpage. Chinese practice adds lime bursting, and hollow or perforated units are also tested for void area. ASTM C67, the EN 772 series, GB/T 2542 and IS 3495 each set out the methods.

How is the compressive strength of clay bricks tested?

A prepared specimen is loaded to failure in a compression machine and the load is divided by the bearing area. ASTM C67 uses oven-dried, capped half bricks loaded to failure in one to two minutes, while IS 3495 tests soaked bricks at 14 N/mm² per minute. Always check whether the result is on gross or net area.

What is the saturation coefficient of a brick?

It is the ratio of 24-hour cold-water absorption to 5-hour boiling absorption, both measured under ASTM C67. A lower ratio leaves more pore space empty for ice to expand into. As summarized by the BIA in 2007, ASTM C62 and C216 limited it to 0.78 average for Grade SW and 0.88 for Grade MW; check the current edition.

How is efflorescence tested on bricks?

In ASTM C67, bricks are partly immersed in distilled water for seven days, dried and compared with untreated bricks, then rated effloresced or not effloresced (BIA TN 23A). IS 3495 Part 3 stands brick ends in 25 mm of distilled water until it evaporates and grades the deposit from nil to serious by the area covered.

Can test results from different standards be compared?

Only with care. Specimen size, moisture state, capping, loading rate and the area used in the calculation all differ between ASTM C67, EN 772-1, GB/T 2542 and IS 3495. Test to the method named in the product standard your buyer specifies, and state the method on every report.

Planning a Fired Clay Brick Plant?

The properties tested here are set by the raw material, extrusion, drying and firing on a fired clay brick production line. Send raw material samples and your target standard early, so that the test firing is run against the limits your market applies.

Sources

• ASTM International, ASTM C67/C67M-25 Standard Test Methods for Sampling and Testing Brick and Structural Clay Tile (scope page; C67/C67M-26 listed as active): https://store.astm.org/c0067_c0067m-25.html
• Brick Industry Association, Technical Note 9A: Specifications for and Classification of Brick (October 2007): https://www.gobrick.com/media/file/9a-specifications-for-and-classification-of-brick.pdf
• Brick Industry Association, Technical Note 39: Testing for Engineered Brick Masonry – Brick and Mortar (November 2001): https://www.gobrick.com/media/file/39-testing-for-engineered-brick-masonry–brick-and-mortar.pdf
• Brick Industry Association, Technical Note 23A: Efflorescence – Causes and Prevention (June 2019): https://www.gobrick.com/media/file/23a-tn23a.pdf
• Brick Industry Association, Technical Note 9: Manufacturing of Brick (December 2006): https://www.gobrick.com/media/file/9-manufacturing-of-brick.pdf
• Mason Contractors Association of America, ASTM Standards for Brick: https://masoncontractors.org/technical-talks/astm-standards-for-brick/
• EN 771-1:2011+A1:2015 Specification for masonry units – Part 1: Clay masonry units (iTeh Standards preview, SIST adoption): https://iteh.es/catalog/standards/sist/479eafd4-c9f5-44e4-97dd-2bcec02dae0a/sist-en-771-1-2011a1-2015
• EN 772-22:2018 Methods of test for masonry units – Part 22: Determination of freeze/thaw resistance of clay masonry units (iTeh Standards): https://iteh.es/catalog/standards/cen/13949381-023b-4598-8c48-3406c7a24a5d/en-772-22-2018
• B. Haseltine, Information for Training – EN 1996: Compressive Loading, European Commission JRC Eurocodes: https://eurocodes.jrc.ec.europa.eu/sites/default/files/2022-06/3_am_Haseltine.pdf
• GB/T 2542-2012 Test methods for wall bricks, standard record: https://www.chinesestandard.net/Related.aspx/GBT2542-2012
• Bureau of Indian Standards, IS 3495 (Parts 1 to 4): 1992 Methods of Tests of Burnt Clay Building Bricks, via Public.Resource.Org: https://law.resource.org/pub/in/bis/S03/is.3495.1-4.1992.pdf
• GOST 530-2012 Ceramic brick and stone: General specifications (normative references), via Paragraph (prg.kz): https://prg.kz/m/amp/document/31448969
• US EPA, AP-42 Compilation of Air Emissions Factors, Section 11.3: Bricks and Related Clay Products: https://www.epa.gov/sites/default/files/2020-10/documents/c11s03.pdf
• Ceramic Industry, Identifying and Correcting Clay Body Defects (2003): https://www.ceramicindustry.com/articles/ppp-identifying-and-correcting-clay-body-defects/