Strength determination of non-fired blocks is a critical quality control process that verifies the load-bearing capacity of concrete masonry units, autoclaved aerated concrete blocks, and other cured building products before they reach the construction site. This article explains the sampling requirements, testing equipment, step-by-step procedures, and evaluation criteria used to measure the compressive and flexural strength of non-sintered blocks in accordance with recognized international standards.
Technical Overview
Non-fired blocks, also known as non-sintered or cured blocks, are masonry units that gain strength through hydration, carbonation, or autoclaving rather than through kiln firing. Common products in this category include concrete small hollow blocks, autoclaved aerated concrete (AAC) blocks, fly ash blocks, slag blocks, and compressed stabilized earth blocks. Because these materials harden progressively over time and are highly sensitive to mix proportion, curing conditions, and forming pressure, their mechanical performance must be verified through systematic laboratory testing.
Strength determination is the most important acceptance index for non-fired blocks. Compressive strength serves as the primary parameter for assigning strength grades such as MU3.5, MU5.0, MU7.5, MU10, MU15, and MU20, which directly govern whether a block can be used in load-bearing walls, infill walls, or non-structural applications. Testing is typically performed in accordance with standards such as GB/T 4111 for concrete small hollow blocks, GB/T 11969 for autoclaved aerated concrete, and ASTM C140 for concrete masonry units, ensuring that results are comparable, repeatable, and legally defensible.
Testing Samples
Concrete Small Hollow Blocks
Concrete small hollow blocks are sampled directly from the production line after the specified curing age, usually 28 days. Test specimens should be representative of the batch, with intact faces and no visible cracks. The number of specimens is normally no fewer than five per batch for compressive testing, and dimensions are measured to determine both gross area and net area for strength calculation.
Autoclaved Aerated Concrete Blocks
AAC block specimens are cut from production units into standard cubes of 100 mm x 100 mm x 100 mm. Because AAC strength is strongly influenced by moisture content, specimens must be dried to a controlled state, typically to a moisture content below 10 percent, before testing. Both compressive strength in the dry state and in the saturated state may be required to calculate the softening coefficient.
Fly Ash and Industrial Residue Blocks
Blocks made from fly ash, slag, or other industrial by-products are tested after natural curing or steam curing reaches its design age. These materials often exhibit strength gain beyond 28 days, so testing at both 28 days and 56 days may be specified for product development purposes. Sample conditioning must ensure uniform moisture distribution across all specimens.
Compressed Stabilized Earth Blocks
For compressed stabilized earth blocks and interlocking blocks, specimens are tested in their actual formed condition. The testing age should correspond to the stabilization period required by the cement or stabilizer used, generally 28 days. Dimensional checks are essential because thickness variation significantly affects measured strength.
Testing Items
A complete strength evaluation program for non-fired blocks typically includes the following items:
- Compressive strength: the fundamental indicator of load-bearing capacity, calculated on either gross area or net area depending on the product standard.
- Flexural strength: relevant for blocks subjected to bending stresses in pavement, flooring, and wall applications.
- Softening coefficient: the ratio of saturated compressive strength to dry compressive strength, used to assess water resistance.
- Dry density and moisture content: measured alongside strength to verify consistency between strength grade and density grade, especially for AAC products.
- Strength after freeze-thaw cycles: required for blocks intended for cold-climate exposure to evaluate durability retention.
Testing Methods
Specimen Preparation and Conditioning
Specimens are first inspected and measured for length, width, and height using a steel rule or calipers with accuracy of 1 mm or better. For concrete hollow blocks, load-bearing surfaces must be flattened by capping with a thin layer of high-strength gypsum or cement mortar to ensure uniform load application. AAC cubes are surface-ground to achieve parallel faces. Specimens are then conditioned to the required moisture state in a drying oven at 60 degrees Celsius or by air drying until mass change is below 0.2 percent within 4 hours.
Equipment Requirements
Testing is performed on a compression testing machine with an accuracy of at least grade 1, meaning the force indication error is within plus or minus 1 percent. The machine capacity should be selected so the anticipated failure load falls between 20 percent and 80 percent of the range. The bearing platens must be flat, rigid, and equipped with a spherical seat to eliminate eccentric loading. A flexural testing fixture is required for bending tests.
Loading Procedure
The specimen is centered on the lower platen with its loading axis aligned to the machine axis. Load is applied continuously and uniformly without shock. For concrete hollow blocks, the loading rate is typically maintained at 10 kN to 30 kN per second; for AAC cubes, the rate corresponds to a stress increase of approximately 0.1 MPa to 0.2 MPa per second. Loading continues until the specimen fails, and the maximum load is recorded to the nearest 1 percent.
Calculation and Evaluation
Compressive strength is calculated using the formula f equals P divided by A, where P is the failure load and A is the gross or net compressed area. Results from all specimens are averaged, and the minimum individual value is checked against the specified limits. A batch passes if the mean strength is not less than the grade requirement, for example 10.0 MPa for MU10, and no single result falls below 80 percent of the specified mean. If flexural strength is required, it is calculated from the bending failure load, span, and section modulus. Any specimen failing prematurely due to handling defects is discarded and replaced.
Applications
Strength determination data for non-fired blocks supports multiple stages of the construction supply chain:
- Factory quality control: manufacturers verify each production batch, adjust mix designs, and optimize curing schedules based on measured strength values.
- Construction acceptance: site engineers and supervision units use test reports to confirm that delivered blocks meet the specified strength grade before installation.
- Product certification: third-party testing laboratories issue compliance reports for product licenses, green building certifications, and export documentation.
- Engineering research: developers of new block formulations, including recycled aggregate and industrial residue blocks, rely on standardized strength data to validate performance.
- Structural design: designers select block strength grades based on test-verified values to ensure wall bearing capacity and seismic performance requirements are satisfied.
FAQ
How many specimens are required for one strength test batch?
For routine compressive strength testing of concrete hollow blocks, a minimum of five specimens per batch is standard. AAC blocks generally require three to six cubes per test condition, depending on whether dry strength, saturated strength, or both are being evaluated.
Should compressive strength be calculated on gross area or net area?
This depends on the governing product standard. Concrete hollow blocks are commonly evaluated on net area to reflect the actual material carrying the load, while some specifications and AAC products use gross area. The chosen basis must be stated clearly in the test report.
Why is moisture conditioning important before testing?
Non-fired blocks lose strength when saturated, and artificially high moisture can either reduce or distort measured values depending on the material. Standardized conditioning ensures that strength results represent a defined reference state, making comparisons between batches and laboratories valid.
What loading rate should be used?
Loading must be continuous and controlled. Typical rates are 10 kN to 30 kN per second for hollow blocks and a stress rate of 0.1 MPa to 0.2 MPa per second for AAC. Excessive speed inflates results, while interrupted loading can cause premature failure and unreliable data.
Can blocks be tested before 28 days?
Early-age testing at 7 days can provide process control information, but acceptance testing must be performed at the standard age of 28 days unless the product specification states otherwise, because hydration and autoclaved reactions continue to develop strength over time.