Indirect tensile strength, commonly measured through the splitting tensile test or flexural test, is a critical material property specified in the design and quality control of water-retaining structures. Unlike compressive strength, which is the primary parameter for most structural concrete applications, tensile strength directly influences crack resistance, serviceability, and long-term durability of water-retaining elements. The specification of minimum indirect tensile strength in standards such as BS 8007 and Eurocode 2 Part 3 reflects the unique demands of structures that must contain water without leakage. Understanding why tensile strength is specified separately from compressive strength is essential for engineers designing durable water infrastructure.
The Relationship Between Tensile Strength and Crack Control
The tensile strength of concrete is directly related to its resistance to cracking under restrained shrinkage and thermal movements. In water-retaining structures, where crack width limits are extremely tight at 0.1 to 0.2 mm, the tensile strength of the concrete determines the load at which first cracking occurs. Higher tensile strength concrete can withstand greater tensile strains before cracking, which delays the onset of cracking and reduces the total crack width that develops under a given restraint condition. The tensile strength also influences the spacing of cracks because higher tensile strength concrete requires greater stress buildup before the next crack forms, leading to wider crack spacing and potentially wider individual cracks. This interplay between tensile strength, crack spacing, and crack width is central to the design philosophy of water-retaining structures.
Indirect tensile strength is measured using the splitting tensile test, also known as the Brazilian test, which involves applying a compressive load along the length of a cylindrical concrete specimen. This test produces a tensile stress perpendicular to the loaded diameter, causing the specimen to split in tension. The test is preferred over direct tension testing because it is simpler to perform, uses standard cylinder specimens, and produces more consistent results. The splitting tensile strength typically ranges from 8 to 14 percent of the compressive strength for normal-weight concrete. Specifications for water-retaining structures often set a minimum splitting tensile strength of 2.5 to 3.0 MPa, even when the compressive strength required for structural adequacy is relatively modest. This explicit tensile strength requirement ensures that the concrete has adequate inherent crack resistance regardless of the compressive strength achieved.
| Property | Test Method | Typical Value Range | Role in Water-Retaining Structures |
|---|---|---|---|
| Splitting tensile strength | BS EN 12390-6 / ASTM C496 | 2.5 – 4.0 MPa | Primary crack resistance parameter |
| Flexural tensile strength (MOR) | BS EN 12390-5 / ASTM C78 | 3.5 – 5.5 MPa | Used for pavement/thin section design |
| Direct tensile strength | Specialized test (rare) | 2.0 – 3.5 MPa | Research; approximates true tensile capacity |
| Compressive strength | BS EN 12390-3 / ASTM C39 | 25 – 50 MPa | Structural capacity, not crack control |
| Modulus of elasticity | BS EN 12390-13 / ASTM C469 | 25 – 35 GPa | Determines strain under stress |
Why Indirect Tensile Strength Is Specified Separately from Compressive Strength
The relationship between compressive strength and tensile strength is not linear and varies significantly with aggregate type, water-cement ratio, curing conditions, and the presence of supplementary cementitious materials. Two concretes with the same compressive strength of 35 MPa can have substantially different tensile strengths depending on their mix proportions and material sources. For example, a concrete made with crushed granite aggregate may achieve a splitting tensile strength of 3.2 MPa at 35 MPa compressive strength, while a concrete made with rounded gravel aggregate achieving the same compressive strength may only reach 2.6 MPa. This variability means that specifying only a minimum compressive strength does not guarantee adequate tensile performance for crack control. Water-retaining structure standards therefore include explicit minimum tensile strength requirements that must be verified through testing during construction.
The specification of indirect tensile strength also serves as a quality control measure for concrete production. Tensile strength is more sensitive to curing quality, compaction, and the presence of microcracks than compressive strength. A concrete batch that achieves the specified compressive strength but shows low tensile strength may indicate problems with aggregate quality, inadequate curing, or poor compaction that could compromise watertightness. Regular tensile strength testing during construction provides an additional layer of quality assurance that complements compressive strength testing. Many project specifications require tensile strength tests at specific ages, typically 7 and 28 days, with minimum values that must be achieved before the structure can be placed into service. These requirements ensure that the concrete placed in water-retaining structures meets not only the structural strength requirements but also the durability and serviceability demands essential for long-term performance.
Practical Implications for Mix Design and Construction Quality
Achieving the specified indirect tensile strength in water-retaining structures requires careful attention to concrete mix design and construction practices. The water-cement ratio is the single most influential factor affecting tensile strength, with lower ratios producing higher tensile strengths. A maximum water-cement ratio of 0.45 to 0.50 is commonly specified for water-retaining structures to ensure adequate tensile performance and low permeability. The type and maximum size of coarse aggregate also significantly influence tensile strength. Angular, rough-textured aggregates provide better mechanical bond with the cement paste and produce higher tensile strengths than smooth, rounded aggregates. The aggregate interlock across cracked surfaces, which contributes to post-cracking shear transfer, is also enhanced by angular aggregates. Curing is perhaps the most critical construction practice for developing tensile strength, as inadequate curing can reduce the tensile strength by 20 to 30 percent compared to properly cured concrete.
Quality control testing for indirect tensile strength should include not only standard 28-day tests but also early-age tests at 3, 7, and 14 days to verify that the tensile strength development is on track. If early-age results indicate that the 28-day tensile strength target is unlikely to be met, adjustments to the mix design or curing procedures can be implemented before significant construction progress has been made. The location of test specimens is also important; samples should be taken from concrete placed in representative sections of the structure, not only from easy access locations. Temperature-matched curing, where test cylinders are cured at temperatures that simulate the thermal history of the mass concrete elements in the structure, provides the most reliable indication of in-place tensile strength development. When combined with proper design of reinforcement for crack control and attention to crack mitigation strategies, specifying and verifying indirect tensile strength ensures that water-retaining structures achieve the required durability and watertightness throughout their service life. The indirect tensile strength requirement is not a theoretical nicety but a practical specification that directly contributes to the long-term performance of critical water infrastructure.
Testing Frequency and Compliance Verification
The frequency of indirect tensile strength testing during construction of water-retaining structures is typically specified in the project quality plan. Common requirements include one set of splitting tensile tests for every 50 cubic meters of concrete placed, with a minimum of one test per day of concreting. Each test set consists of three cylinders tested at 28 days, with additional cylinders tested at 7 days for early-age verification. The acceptance criteria typically require that the average tensile strength of the three cylinders exceeds the specified minimum value by at least 10 percent, and that no individual cylinder falls below 90 percent of the specified value. If the test results fail to meet these criteria, the engineer must evaluate whether the concrete in the structure has adequate tensile capacity and may require additional testing through core sampling from the placed concrete.
When core samples are taken from the structure for tensile strength verification, the results are typically 10 to 20 percent lower than standard-cured cylinder tests due to the effects of in-place curing conditions, compaction variations, and the damage caused by the coring process. Interpretation of core test results requires engineering judgment and knowledge of the specific construction conditions. In some cases, the engineer may accept lower core tensile strengths if the structural analysis shows that the crack control requirements are still satisfied with the actual in-place concrete properties. However, the preferred approach is to prevent tensile strength deficiencies through rigorous quality control during construction, including proper curing, adequate compaction, and verification of concrete mix proportions through trial batches before production begins. By specifying, testing, and verifying indirect tensile strength, engineers ensure that water-retaining structures achieve the necessary crack resistance and long-term durability.