4. Condition of Test Specimen
Concrete compressive strength is typically determined through testing cylindrical or cube specimens at specified ages (e.g., 7, 14, or 28 days). However, the test results depend not only on concrete quality and production but also on the physical and environmental condition of the specimen during testing. Key influencing factors include:
4.1. Shape and Size
In Indonesia, standard test specimens are cylinders 150 mm in diameter and 300 mm in height, while cubes (150 mm sides) are common in Europe and the UK. The shape and size affect measured strength values.
- Specimen size: Larger specimens may contain internal flaws (microcracks, voids), yielding lower strength.
- Cylinder vs. cube: Cylinders generally yield lower strength than cubes due to less lateral confinement. Typical ratio: fcube′≈1.25×fcylinder′f’_{\text{cube}} ≈ 1.25 × f’_{\text{cylinder}}fcube′≈1.25×fcylinder′
| Specimen Type | Standard Size | Relative Strength | Remarks |
|---|---|---|---|
| Cylinder | Ø150 mm × 300 mm | 1.00 | Common in SNI and ASTM |
| Cube | 150 mm × 150 mm | 1.25 | Used in Europe, UK, etc. |
4.2. Moisture Content of Test Specimen
The moisture content of the test specimen during testing significantly affects the compressive strength:
Therefore, SNI 2493:2011 recommends that the specimen be immersed in water for at least 24 hours before testing.
- Saturated specimens generally produce higher compressive strength results, as the presence of water helps in achieving a more uniform stress distribution.
- Dry or partially dry specimens may lead to the formation of micro-cracks during testing due to unbalanced tensile stresses, thus reducing the recorded compressive strength.
4.3. Temperature of Test Specimen
The temperature at the time of testing can affect the concrete’s response to loading. Concrete tested under cold (<10°C) or hot (>30°C) conditions may yield varying results:
- High temperature can reduce compressive strength due to internal water evaporation and weakening of the cement hydration crystal structure.
- Low temperature may increase the concrete’s density temporarily but can also make it more brittle.
The ideal temperature of the specimen during testing is between 20–25°C for accurate and representative results.
4.4. Surface Condition of the Specimen Ends
The loading surfaces of the specimen must be flat, perpendicular, and free from defects. Uneven or inclined surfaces lead to non-uniform stress distribution, early cracking, and premature failure.
Improperly leveled surfaces can reduce the recorded compressive strength by 10–20% of the actual value.
The ends can be leveled using grinding or by applying leveling compounds such as sulfur capping or epoxy capping.
4.5. Loading Method
The method of applying load in a compression testing machine also influences the results:
- Load centering: The load must be applied precisely at the center of the specimen surface. Off-center loading generates local bending moments and leads to inaccurate test results.
- Loading rate: Should follow the standard, typically around 0.25 MPa/second. Excessively fast loading may cause sudden failure without the gradual development of micro-cracks.
Conclusion
Compressive strength is one of the key parameters determining the quality and reliability of structural concrete. As the most commonly tested mechanical parameter, it directly reflects the concrete’s quality and suitability for structural load-bearing applications.
This article has explained that compressive strength is influenced by a wide range of factors—from the characteristics of constituent materials such as water, cement, coarse and fine aggregates, and admixtures, to production processes like mixing, stirring, casting, and compaction. Additionally, curing procedures and the condition of specimens during testing also have a significant impact on the final test results.
A thorough understanding of these factors is not only essential during the planning and execution phases of construction projects but also crucial in field quality assessment and overall concrete performance enhancement. By paying attention to these details, engineers and construction personnel can produce concrete with optimal performance, safety, and durability.
We hope this article is beneficial for civil engineering students, construction practitioners, and all parties involved in concrete quality monitoring and control in the field.