Determination of Splitting Tensile Strength: Principles, Methods, Standards, and Applications

Splitting tensile strength determination is an indirect tensile test used worldwide to evaluate the tensile performance of concrete, rock, and other brittle construction materials in accordance with standards such as ASTM C496/C496M, EN 12390-6, ISO 1920-4, GB/T 50081, and ASTM D3967. By applying a diametral line load to a cylindrical or disc specimen, the method produces a controlled tensile splitting failure that is simpler and more reproducible than direct tension testing. This article explains the test principle, specimen requirements, procedure, calculation, and typical engineering applications of splitting tensile strength testing.

Technical Overview

Splitting tensile strength determination, also known as the Brazilian test or indirect tensile test, measures the tensile resistance of brittle materials such as hardened concrete, mortar, grout, and intact rock. A compressive line load is applied along two opposite generatrices of a cylindrical or disc-shaped specimen. According to classical elasticity theory, this diametral loading generates a nearly uniform transverse tensile stress distributed across the loaded vertical diameter, and the specimen fails by splitting into two halves once its tensile capacity is exceeded.

The method is widely preferred because direct uniaxial tension testing is notoriously difficult: gripping the specimen, eliminating load eccentricity, and suppressing secondary bending all introduce scatter. The splitting test, by contrast, uses standard compression testing equipment, accepts the same cylinders routinely cast for compressive strength testing, and delivers reproducible results at low cost. The principal governing standards include:

• ASTM C496/C496M — Standard Test Method for Splitting Tensile Strength of Cylindrical Concrete Specimens
• EN 12390-6 — Testing hardened concrete, Part 6: Tensile splitting strength of test specimens
• ISO 1920-4 — Testing of concrete: Strength of hardened concrete
• GB/T 50081 — Standard for test methods of concrete physical and mechanical properties
• ASTM D3967 — Standard Test Method for Splitting Tensile Strength of Intact Rock Core Specimens

The splitting tensile strength is calculated from the maximum failure load using the relationship fct = 2F / (π × d × l), where F is the maximum applied load, d is the specimen diameter, and l is the contact length of the specimen along the loading line.

Testing Samples

Standard Concrete Cylinders

Cast cylinders, typically 150 mm in diameter and 300 mm in length, or 100 mm by 200 mm where the standard permits, made and cured under controlled conditions (for example, water curing at 20 ± 2 °C) and tested at specified ages such as 7, 28, or 90 days.

Concrete Cubes

EN 12390-6 also permits cubic specimens loaded along a diametral plane. Cube and cylinder results may differ systematically, so results from different geometries should not be interchanged without engineering justification.

Drilled Cores from Structures

Cores extracted from existing structures are used for in-place strength evaluation. The length-to-diameter ratio, moisture condition, presence of reinforcement, and drilling disturbance must be documented, and the failure plane must not cross embedded reinforcing bars.

Rock Core Discs

Discs prepared from exploration cores, commonly NX size or larger, with a thickness of approximately 0.5 times the diameter per ASTM D3967, tested in dry or saturated condition depending on the project specification.

Specialty Cementitious Materials

Mortar, grout, fiber-reinforced concrete, and shotcrete specimens, either cast cylinders or cores and sawn panels, may be evaluated using the same principle with geometry and conditioning adjusted to the material.

Testing Items

A typical splitting tensile strength determination program by accredited third-party laboratories covers the following items:

• Splitting tensile strength at the specified test age, expressed in MPa
• Maximum failure load and verification of the applied loading rate
• Specimen dimensions, mass, and calculated density
• Moisture condition of the specimen at the time of testing
• Description of the failure pattern and judgment of test validity
• Statistical evaluation of replicate specimens, including mean value and comparison with the specified characteristic strength
• For rock samples, strength index determination and assessment of directional anisotropy relative to bedding, foliation, or micro-fracturing

Testing Methods

Apparatus

A calibrated compression testing machine of appropriate force capacity and accuracy class is used together with two flexible bearing strips, commonly plywood approximately 25 mm wide and about 3 mm thick per ASTM C496, positioned between the machine platens and the specimen. A centering jig is used to align the vertical diameter of the specimen with the load axis.

Specimen Preparation

Specimens are made and cured in accordance with ASTM C192 or ASTM C31, EN 12390-2, GB/T 50081, or the applicable project specification. Before testing, dimensions are measured to the precision required by the standard, the specimen is weighed, and its surface condition is recorded. Unlike compression testing, end preparation such as capping or grinding is generally not required.

Test Procedure

• Place the specimen with its longitudinal axis horizontal, oriented transverse to the direction of loading.
• Position the bearing strips along the full contact length at the top and bottom of the specimen so that the load is applied along a vertical diametral plane.
• Align the specimen carefully so that the loaded diameter passes through the center of the cross section.
• Apply the load at a constant rate within the range prescribed by the governing standard, for example 0.04 to 0.06 N/mm² per second per EN 12390-6, until sudden failure occurs, and record the maximum load.
• Examine the fracture surface: a valid failure exhibits one dominant crack propagating along the loaded diameter, with the specimen split into two halves.
• Test the required number of replicate specimens and evaluate the results statistically as specified by the standard.

Calculation and Reporting

The splitting tensile strength is computed as fct = 2F / (π × d × l) and reported in N/mm² (MPa). The test report should include specimen identification, dimensions, casting and curing history, age at testing, moisture condition, density, maximum load, calculated strength, failure description, any deviations from the standard, testing date, and the calibration status of the equipment. Reports from accredited third-party laboratories provide full traceability for contract acceptance and regulatory submission.

Applications

• Acceptance testing and mix proportioning of structural concrete against specified tensile requirements
• Crack control design of mass concrete, such as dams and thick raft foundations, where thermal and shrinkage tensile stresses must remain below the tensile capacity
• Concrete pavement and airport runway engineering, where slab behavior is governed by tensile and flexural resistance
• Quality control of tunnel linings, shotcrete, and precast products including pipes, sleepers, and segmental liners
• Rock mechanics characterization for excavation design, pillar and support stability, and anisotropy assessment
• Condition assessment of existing structures using drilled cores, including evaluation of fire-, frost-, or chemically damaged concrete
• Research and development of fiber-reinforced concrete, high-performance concrete, and rapid-hardening or early-age strength systems

FAQ

What is the difference between splitting tensile strength and direct tensile strength?

Splitting tensile strength is measured indirectly through diametral compression, while direct tensile strength is measured by pulling a specimen along its axis. Because of the biaxial stress state near the loading strips, splitting values are generally slightly higher than direct tensile values on comparable specimens, and design codes treat the two parameters as distinct.

Why is the splitting test preferred over direct tension testing?

Direct tension tests are sensitive to gripping arrangements, load eccentricity, and secondary bending, which cause high scatter. The splitting test avoids these problems by using simple compression equipment and standard cylinders, giving reproducible results suitable for routine quality control.

How does splitting tensile strength relate to compressive strength?

For normal-weight concrete, splitting tensile strength is typically about 8 to 12 percent of the compressive strength. Empirical relationships exist in design codes, but they should only be used for estimation and never as a substitute for measured values in acceptance or design verification.

Which specimen size should be used?

The 150 mm by 300 mm cylinder is the reference specimen for concrete in most standards. Smaller 100 mm by 200 mm cylinders are acceptable for many purposes provided the maximum aggregate size is proportionally limited, but specimen geometry must remain consistent when comparing data sets.

What invalidates a splitting test result?

Common causes of invalid results include failure planes that do not follow the loaded diameter, local crushing beneath the bearing strips, specimen misalignment, loading rates outside the specified range, and specimens containing reinforcement or defects in the failure plane. Invalid results should be discarded and the test repeated.

How many specimens are required?

The number of replicates is defined by the governing standard and project specification, typically a set of two or three specimens per age per mix. Testing replicate sets reduces statistical uncertainty and is essential for acceptance decisions on production concrete.