Concrete Block Testing Machine and Methods: Strength, Absorption and the Plant Lab

A concrete block testing machine is a compression frame with a spherically seated head that crushes capped blocks to measure compressive strength, the property every block standard grades first. A plant laboratory also needs an oven, a balance and an immersion tank for absorption and density, and gauges for dimensions and drying shrinkage. ASTM C140, the EN 772 series and IS 2185 (Part 1) set the methods.

This guide is for plant managers and quality engineers at a new concrete block or cement brick plant: which machine to buy, how each standard runs the tests, how many blocks to sample and what the laboratory needs. Fired clay bricks follow different methods; see clay brick testing methods. Limits are given only where we could check them; otherwise, check the current edition.

Concrete Block Testing at a Glance

Three method families cover most export markets for concrete masonry units. The product specification your buyer names decides which one you test to.

ASTM lists C140/C140M-26a as active; its annexes cover concrete masonry units, concrete and calcium silicate brick, retaining wall units, paving units and other products (ASTM International, 2026). The EN 772 parts are those referenced by EN 771-3:2011+A1:2015. IS 2185 (Part 1) is the 2005 edition; check for revisions.

Choosing a Concrete Block Testing Machine

A concrete block strength testing machine must be large enough in two ways: force capacity and bearing area. A press built for concrete cubes or cylinders often cannot take a full block, so check both before you buy.

The standard for the machine: ASTM C1716/C1716M

ASTM C1716/C1716M, Standard Specification for Compression Testing Machine Requirements for Concrete Masonry Units, Related Units, and Prisms, is written specifically for this machine; the active edition is C1716/C1716M-24. It provides minimum design standards for testing machines used to measure compressive strength under ASTM C140/C140M and C1314 (masonry prisms). It also sets the maximum allowable specimen size and load for a given machine, based on deflection of the bearing surfaces and the machine load frame. Those limits do not replace the machine maker’s own operating limits, and the user must confirm that capacity and limits suit the sample (ASTM International, 2024). If you test to C140, ask for written confirmation that the frame and plates comply with C1716/C1716M, and for which maximum specimen size.

Force capacity: work it out from your largest product

Estimate the highest failure load you will ever test, then choose a frame whose verified force range covers it with margin. As an illustration using IS 2185 (Part 1) arithmetic: actual length and height are 10 mm less than nominal, while width is not reduced (clause 4.2.1). A 400 × 200 × 200 mm nominal hollow block is therefore 390 mm long, 200 mm wide and 190 mm high, and its gross bed area is 390 × 200 = 78,000 mm². At 15 N/mm², the top Grade A class, the failure load is about 1,170 kN (our calculation).

The table is our arithmetic (capacity ÷ 78,000 mm²), before any margin. Size the frame for the strongest product you plan to make, not the average one, and confirm the verified range on the calibration certificate, not only the nameplate.

Bearing area, plates and spherical seat

• Plate size. The bearing plates must cover the whole bed face of the largest unit you test. A 390 × 200 mm bed face is more than three times the 22,500 mm² face of a 150 mm cube (our calculation), so a frame and plates designed around cubes are usually too small.
• Plate stiffness and flatness. CMHA lists bearing plates of adequate planeness and thickness, and alignment of the specimen’s center of mass with the center of thrust, among the critical areas for accurate testing (TEK 18-02C). C1716/C1716M ties the allowable specimen size to the deflection of the bearing surfaces.
• Spherical seat. CMHA also lists compression machines with spherically seated heads among those critical areas; the seated head settles evenly onto the capped specimen.

Load control, verification and capping

• Load control and readout. IS 2185 (Part 1) loads to half the expected maximum at any convenient rate, then applies the rest in not less than one and not more than two minutes (Annex D-4.2). Load-rate control and a digital readout make that repeatable between operators.
• Verification. ISO 7500-1:2018 specifies the calibration and verification of tension/compression testing machines: a general inspection, a calibration of the force-measuring system, and confirmation that the machine meets the limits of a specified class (ISO, 2018). In the ASTM system, ASTM E4-24 covers the force calibration and verification of tension or compression testing machines by means of force measurement standards (ASTM International, 2024). Your accredited calibration provider reports the result to the standard your market uses; check the interval your certification body requires.
• Capping station. Order a capping table and capping material (sulfur, gypsum plaster or the material your standard names) with the frame. A strong frame with poor caps gives poor results.

Compressive Strength Testing: ASTM C140, EN 772-1 and IS 2185-1

Results are only comparable when specimen, moisture state, capping and loading follow one method.

IS 2185 (Part 1), Annex D. Blocks are tested at 28 days, counted from when water was added to the dry ingredients, and within 72 hours of reaching the laboratory. Bed faces are capped with sulfur compound (poured to about 6 mm in the mold) or gypsum plaster (average cap not more than 3 mm), and caps age at least 2 hours. Strength is the maximum load divided by the gross cross-sectional area, reported to 0.1 N/mm² for each block, with the average of 8 blocks (IS 2185 (Part 1):2005).

ASTM C140. A standard set is six identical units: three for compression and three for absorption. Full-size units are tested where possible; unsupported projections are saw-cut, and where that does not give a compliant specimen, coupons are cut from the face shells. Specimens are capped to ASTM C1552, and hollow units are tested with the cores vertical. For concrete brick, the specimen’s height divided by its least lateral dimension must be 0.6 ± 0.1 (CMHA TEK 18-02C). Units used for absorption must not be reused for compression, because moisture changes the result.

EN 772-1. The European method requires at least six specimens, unless the product standard asks for more. Bed faces are ground flat and parallel or capped with a cement/sand mortar about 5 mm thick, and specimens are conditioned air-dry, oven-dry, to 6% moisture content or by immersion (EN 772-1:2011+A1:2015). Results are converted to a normalized strength for masonry design; check the current edition for the shape factors and conversion.

A common plant practice is to also crush blocks at an early age, such as 7 days, and track how that relates to 28-day results; it supports control but does not replace the specified age.

Gross area vs net area in block strength results

The same block, crushed at the same load, gives two different strengths depending on the area used, so every report must state which one it uses.

• Net area is the solid area under load. ASTM C90 specifies a minimum net area compressive strength of 2,000 psi (13.8 MPa) for loadbearing units (CMHA CMU-TEC-001). The International Code Council’s Building Safety Journal reports that the minimum was raised from 1,900 psi in 2011 (2026). C140 derives the net area from the unit’s net volume, which is measured by water displacement.
• Gross area is the full area of the section, including cores. IS 2185 (Part 1) reports strength on gross area.

As an illustration: for a hollow block whose solid material is 55% of the bed area, a net area strength of 13.8 MPa corresponds to about 7.6 MPa on gross area (13.8 × 0.55, our calculation). A buyer comparing an ASTM report with an IS report without this conversion can wrongly reject a good block or accept a weak one.

IS 2185 (Part 1) also defines what counts as hollow: solid material between 50 and 75% of the total volume. Above 75%, it is a solid block.

Water Absorption, Density and Moisture Content Tests

Absorption and density tell you how well the block was compacted, and they are quicker to test than strength.

ASTM C140. Each absorption unit is weighed at least five times: as received, immersed, saturated surface-dry, and oven-dry at least twice. Immersed and saturated weights are taken after 24 to 28 hours of immersion. From the same weighings, C140 calculates absorption, oven-dry density, net volume, percent solid and equivalent thickness (CMHA TEK 18-02C). ASTM C90 sets maximum absorption by density class; lightweight units are allowed more than medium-weight or normal-weight units (CMHA CMU-TEC-001). Check the current edition of C90 for the limits.

IS 2185 (Part 1). Absorption (Annex E) and block density (Annex C) are measured on three blocks each. The pass limits, 10% average absorption and density classes by grade, are set out in our hollow block machine article; Grade C solid blocks, not covered there, need a density of at least 1,800 kg/m³.

EN 771-3. Producers declare gross dry density of the unit and net dry density of the concrete (EN 772-13), and water absorption by capillarity (EN 772-11) where relevant.

Rising absorption with falling density on one product usually points to under-compaction: a mix too dry, short vibration, worn molds or changed aggregate grading. Our article on cement brick mix ratio explains how mix water and grading set density.

Drying Shrinkage, Moisture Movement and Dimensional Tests

Drying shrinkage and moisture movement

Shrinkage tests show whether blocks are likely to crack walls after laying; they take longer, so they are run less often.

• ASTM C90 limits the average linear drying shrinkage of three units to 0.065%, measured to ASTM C426 (as read in ASTM C90-23 and CMHA CMU-TEC-001; ASTM now lists C90-24a as active, so check the current edition). CMHA notes that under C90, the shrinkage test is generally repeated every two years if materials and processes have not changed (TEK 18-02C).
• IS 2185 (Part 1) tests drying shrinkage (Annex F) and then moisture movement (Annex G) on the same three blocks; the moisture movement limit is 0.09%, and the shrinkage limit and drying period are covered in the hollow block machine article.
• EN 771-3 requires moisture movement to be declared, tested to EN 772-14.

Shrinkage needs gauge studs, a length comparator, an oven and controlled drying; smaller plants often outsource it.

Dimensions, tolerances and face shell checks

Dimensional checks are the cheapest test in the laboratory and the first to catch a worn mold or a pallet problem.

• ASTM C140 measures overall length, width and height, and face shell and web thicknesses, which are checked against the unit specification. ASTM C90 sets minimum face shell thickness by unit width, for example 1¼ in. (32 mm) for units 8 in. wide and wider, and a minimum normalized web area (as read in ASTM C90-23; check the current edition).
• IS 2185 (Part 1) measures dimensions to Annex B; its tolerances and nominal sizes are listed in the hollow block machine article.
• EN 771-3 asks the producer to declare a tolerance category from D1 to D4; D4 units for thin-layer mortar also need declared flatness and plane parallelism of the bed faces (EN 772-16 and EN 772-20).

Height is the dimension to watch on a vibration-and-press block machine. Uneven height across one pallet points to pallet wear or a tamper head problem; height drift over a shift points to mix moisture. The hollow block machine article explains how pallets and compaction affect block dimensions.

How Many Blocks to Test: Sampling Plans

IS 2185 (Part 1) takes a sample of 20 blocks from each lot of 5,000 blocks or part thereof, of the same grade, size and batch. Of these, 3 are used for block density, 8 for compressive strength, 3 for water absorption and 3 for drying shrinkage followed by moisture movement; the remaining 3 are kept for retests (clause 11).

ASTM C140 works from a set of six identical units, three for compression and three for absorption, plus units for shrinkage where required. CMHA stresses random sampling: picking units from one part of a pallet, or picking the best or worst, misrepresents the lot (TEK 18-02C).

EN 771-3 adds a statistical declaration: Category I units have a probability of failing to reach the declared strength not exceeding 5% (3.1.19), which a plant must support with enough production control results.

A practical plant routine:

1. Every shift: dimensions and visual checks on blocks from several pallets, and mix moisture at the mixer.
2. Every production lot: density and absorption; compressive strength at the age your standard specifies, with early-age tests for control.
3. At fixed intervals and on any material change: drying shrinkage, and a full set to the product standard.
4. For certification and tenders: an accredited external laboratory, using the method the buyer names.

Setting Up a Concrete Block Testing Laboratory

Record each result against production date, mold, pallet position and mix batch so that failures can be traced. For pressed fly ash bricks under IS 12894, see the fly ash brick manufacturing process; for dry-stack products, see interlocking brick machine.

FAQ

What is a concrete block strength testing machine?

It is a compression testing machine with a spherically seated upper head, bearing plates that cover the whole bed face of the block, and a calibrated load readout. It crushes capped blocks to failure to give compressive strength. The force capacity must cover the strongest full-size product you make, which is usually far more than a concrete cube press is built for.

How is the compressive strength of a concrete block tested?

The block is capped to give flat, parallel bearing faces, centered under the spherical head and loaded to failure. IS 2185 (Part 1) tests 8 blocks at 28 days, loading the second half of the expected load in one to two minutes, and divides by gross area. ASTM C140 uses three units and C90 assesses net area strength.

What is the minimum strength of a concrete block?

It depends on the standard. ASTM C90 sets a minimum net area compressive strength of 2,000 psi (13.8 MPa) for loadbearing units. IS 2185 (Part 1) uses grades on gross area, from 3.5 N/mm² up to 15 N/mm² for Grade A hollow blocks. EN 771-3 has no fixed minimum; the producer declares the strength.

What is the water absorption limit for concrete blocks?

IS 2185 (Part 1) limits the average of three blocks to 10% by mass. ASTM C90 sets limits by density class, with higher absorption allowed for lightweight units; check the current edition for the values. EN 771-3 asks producers to declare capillary water absorption where relevant, tested to EN 772-11.

How many concrete blocks are needed for testing?

IS 2185 (Part 1) takes 20 blocks from every lot of 5,000 or part thereof: 3 for density, 8 for strength, 3 for absorption, 3 for shrinkage and 3 spare. A standard ASTM C140 set is six identical units, three for compression and three for absorption. EN 772-1 requires at least six specimens for strength.

Planning a Concrete Block Plant?

The properties tested here are set by the mix, the compaction and the curing on a concrete block and brick making machinery line, where a vibration-and-hydraulic block machine makes hollow blocks, bricks and pavers by changing the mold. Name your target standard before the molds are made, so that block dimensions and the laboratory are set up for the method your buyers specify. Send us your local material information and we will check the mix.

Sources

• Standard Test Methods for Sampling and Testing Concrete Masonry Units and Related Units, ASTM C140/C140M-26a – ASTM International, 2026: https://store.astm.org/c0140_c0140m-26a.html
• Standard Specification for Compression Testing Machine Requirements for Concrete Masonry Units, Related Units, and Prisms, ASTM C1716/C1716M-24 – ASTM International, 2024: https://store.astm.org/c1716_c1716m-24.html
• Standard Specification for Dry-Cast Loadbearing Concrete Masonry Units, ASTM C90-24a (active edition; shrinkage and face shell values read in C90-23) – ASTM International, 2024: https://store.astm.org/c0090-24a.html
• TEK 18-02C, Sampling and Testing Concrete Masonry Units – Concrete Masonry and Hardscapes Association, 2024: https://www.cmha.org/resource/tek-18-02C/
• CMU-TEC-001-23, Concrete Masonry Unit Specifications – Concrete Masonry and Hardscapes Association, 2023: https://www.cmha.org/resource/cmu-tec-001/
• What’s Changed in ASTM C90 and TMS 602 Since 2000 and Why It Matters – International Code Council, Building Safety Journal, 2026: https://www.iccsafe.org/building-safety-journal/bsj-technical/whats-changed-in-astm-c90-and-tms-602-since-2000-and-why-it-matters
• IS 2185 (Part 1):2005 Concrete Masonry Units – Specification, Part 1: Hollow and Solid Concrete Blocks – Bureau of Indian Standards, 2005: https://law.resource.org/pub/in/bis/S03/is.2185.1.2005.pdf
• EVS-EN 771-3:2011+A1:2015 Specification for masonry units – Part 3: Aggregate concrete masonry units (dense and lightweight aggregates) – Estonian Centre for Standardisation and Accreditation (national adoption of the CEN standard), 2015: https://evs.ee/en/evs-en-771-3-2011-a1-2015-consolidated
• DIN EN 772-1:2016-05 Methods of test for masonry units – Part 1: Determination of compressive strength (EN 772-1:2011+A1:2015) – DIN, 2016: https://din.de/en/wdc-beuth:din21:247931644
• Standard Practices for Force Calibration and Verification of Testing Machines, ASTM E4-24 – ASTM International, 2024: https://store.astm.org/e0004-24.html
• ISO 7500-1:2018 Metallic materials – Calibration and verification of static uniaxial testing machines – Part 1: Tension/compression testing machines – ISO, 2018: https://www.iso.org/standard/72572.html