What is Compressive strength testing?
The compressive strength of concrete is its most vital engineering property providing information about nearly all its other characteristics and deterioration processes. Concrete is specified by its compressive strength and this test alone is commonly used as a benchmark to prove the success of concreting operations. Compressive strength results are essential for assessing the load bearing capacity, structural safety, and durability of concrete structures, significantly reducing the risk of structural collapse.
How does Compressive strength testing work?
Concrete specimens are cast in a lab or cored from existing concrete structures then loaded to failure in a compression-testing machine (Figure 2).
The maximum load sustained by the specimen is recorded and divided by the cross-sectional area of the sample to calculate the compressive strength of the concrete. The test is performed in accordance with BS EN 12390-3:2019.
What is Compressive strength testing used for?
Deterioration process | Defects | Control of repairs |
Applicable to all deterioration processes inducing loss of concrete strength or where the compressive strength evaluation is useful | Can be applied to all defects when is required a correlation defect - deterioration process. |
How do I carry out Compressive strength testing?
To conduct a compressive strength test of a concrete structure in line with BS EN 12390-3:2019, BS EN 12390-4:2019 and BS EN 12504-1:2019, integrating procedural steps with essential considerations ensures a comprehensive understanding and adherence to standards for accurate results.
Prepare specimens for testing: If coring samples BS EN 12504-1:2019 should be followed and the number and location of cores chosen strategically to accurately reflect the concrete's condition without compromising structural integrity. If preparing samples in the lab then BS EN 12390-3:2019 should be followed. Concrete is commonly cast in 150mm cube moulds and cured to specification.
Ensure the specimens and testing environment are conditioned according to the standards requirements with controlled moisture content and other conditions that might affect the results.
Set up and calibrate the test device to BS EN 12390-4:2019. The device must be able to apply load at a specified rate until concrete failure.
Place the specimen centrally in the testing machine with cast or cored face pointed out towards the operator.
Apply load gradually until failure, recording the maximum load sustained.
Analyse the test data in context, considering specific project requirements and the concrete's structural role. Unexpected results may necessitate further investigation, additional testing, or both.
Satisfactory failure modes in compressive strength tests, like uniform crushing or diagonal shearing, indicate proper concrete compaction and curing, reflecting good structural integrity.
Unsatisfactory modes, such as vertical splitting or edge-initiated cracks, suggest issues with specimen preparation, loading alignment, or mix deficiencies like excessive water, poor curing, or subpar aggregate quality. These unsatisfactory patterns highlight potential structural performance problems, necessitating mix design and construction practice reviews to address and correct the identified issues.
What equipment and expertise are required for Compressive strength testing?
If concrete sample is to be cored from the existing structure see Sampling.
If casting concrete samples in the lab, then clean and debris free cube or cylinder moulds must be used with mould oil applied for ease of removal. A tamping rod or vibrating table may be required for consolidation of non-self-compacting concrete and scrapers , scoops, floats etc required for placing.
A wide range of commercial compressive test machines are available for testing both lab made cubes and cores taken from existing structures that conform to BS EN 12390-4:2019. Examples include those from Impact test or ELE.
Third party facilities are often used for compressive strength of concrete such as CTS or Socotec to ensure the impartiality of test operator and no conflicts of interest when reporting results.
What are the advantages of Compressive strength testing?
Offers quantifiable and accurate measurements of concrete's compressive strength, crucial for structural analysis and safety evaluations.
Compressive strength tests are the most accurate, reliable method for assessing the strength and durability of concrete.
Ensures testing and results adhere to internationally recognized standards, facilitating global benchmarking and quality assurance.
Vital for assessing the load-bearing capacity of concrete, directly impacting structural repair, reinforcement, or demolition decisions.
Helps diagnose specific issues like premature concrete deterioration, providing a basis for targeted interventions.
Accurate strength data supports the design and implementation of retrofitting measures for aging or compromised structures.
What are the disadvantages of Compressive strength testing?
This is a destructive test which requires physical removal of concrete sections, potentially weakening the structure or necessitating cosmetic repairs.
Offers detailed data but only for sampled areas, which may not represent the entire structure's condition, leading to potential oversight of critical weaknesses.
The accuracy of results can be compromised by improper sample handling, preparation, or testing procedure errors, impacting the reliability of structural assessments.
The process generates waste material and may require additional resources for repair and restoration
Once a sample is destroyed in testing, it cannot be used for further analysis, limiting the utility of each sample.
While providing definitive strength data, its destructive nature makes it less suitable than non-destructive techniques for ongoing monitoring or when preservation is paramount.
How accurate is Compressive strength testing?
The BS EN 12390-3 standard provides precision data for concrete's compressive strength tests, indicating how closely results from repeated tests should align under ideal conditions. It defines repeatability (r) for tests by the same operator using identical conditions, and reproducibility (R) for tests under varying conditions across operators or equipment.
Specifically, for 100 mm and 150 mm cubes, the standard notes a repeatability limit of 3.2% and reproducibility limits of 9.0% and 13.2%, respectively. For cylinders (160 mm by 320 mm), repeatability is set at 2.9%, with reproducibility at 11.7%.
These statistics, derived from experiments, ensure the reliability of compressive strength measurements when conducted properly.
What are the limits of Compressive strength testing?
The compressive strength test, is essential for assessing concrete's structural integrity but has limitations.
Its destructive nature necessitates the removal of concrete samples, which may compromise structural integrity, especially in critical areas. Compressive strength results only reflect the condition of the specific locations or concrete element sampled so carefully planned sampling schedules are essential.
Safety concerns arise when extracting samples from compromised structures; the risks of further damaging the structure or to the health and safety of those carrying out the sampling must be effectively mitigated. In such cases, non-destructive methods like ultrasonic pulse velocity or rebound hammer tests may be preferable, offering valuable insights without the need for invasive sampling.
These considerations underscore the importance of carefully assessing the suitability of compressive strength testing, particularly in structures where safety or integrity could be jeopardized.
Ancillary information
Maturity of test: > 10 years
Qualification & interpretation : Specialised lab
Service disruption: No
Preliminary works: Yes
Time consumption Medium (one day)
Cost Medium
Access to element 1 face
References and further information