The compressive strength of concrete cylinders is a widely used performance measure in structural design, often assessed by engineers. This involves subjecting the cylinders to continuous load until failure occurs, and the test is typically conducted on a compression-testing machine.
Apparatus for Concrete Cylinder Test
The sample cylinder used for concrete testing can have one of two dimensions: a diameter of 150mm and a height of 300mm, or a dimension of 100mm x 200mm. The diameter of the cylinder must be at least three times the nominal maximum size of the coarse aggregate used in the concrete manufacturing process. The apparatus required for this testing includes a compression testing machine, a cylinder mold of the specified dimensions, and a weighing balance.
Procedure for Concrete Cylinder Test
Sample Preparation
The cylinder specimens for concrete testing are typically cast using molds made of non-absorbent materials such as steel or cast iron. It is important that these molds retain their original shape and dimensions even under severe conditions, and do not leak during the casting process. Prior to placing the concrete mix into the mold, the interior of the mold must be properly greased to ensure easy removal of the hardened cylinder after it has cured.
During the casting process, the mixed concrete is placed into the molds in layers that are at least 5cm deep. Each layer is then compacted with a minimum of 30 strokes per layer to ensure proper compaction and to allow air voids to escape from the underlying layers. The compaction process must be thorough, reaching all the way to the bottom of the mold.
Once the molds are filled and compacted, the specimens are stored undisturbed in a location with at least 90% relative humidity and a temperature of 27° ± 2°C for a period of 24 hours. After this curing period, the samples are carefully removed from the molds and submerged in clean and fresh water until they reach the desired testing age. This ensures that the specimens are properly cured and ready for testing.
Test Procedure
The concrete cylinders are cast to standard size and allowed to cure for 28 days. Three specimens of the same dimension are cast for testing. To begin the testing process, the specimens are taken out from the curing tank and excess water is wiped off from their surfaces. The specimens are then placed vertically on the platform of the compression testing machine, with pad caps at the ends to facilitate uniform load application and distribution. Prior to applying the load, it is ensured that the loading platforms touch the top of the cylinders. The load is then applied continuously and uniformly, without shock, at a rate of 315 kN/min, and the loading is continued until the specimen fails. The maximum load taken during the test is recorded. This process is repeated for the remaining two specimens.
Fig.1: Concrete Cylinder for Compression Test
Fig.2: Fractured Concrete Cylinder Specimen at Failure
Note :
In order to conduct a proper test, at least three specimens need to be used. The diameter of the casted cylinder must be measured at two locations that are perpendicular to each other, specifically at its mid-height. The average of these two diameter values is then used to calculate the cross-sectional area of the cylinder. However, if the difference between the two diameter measurements exceeds 2%, the cylinder should not be tested to ensure accurate results.
Report for Concrete Cylinder Test
The technician responsible for conducting the compression test of the cylinder is required to record observations both before and after the test. These observations are crucial for documenting the condition and performance of the cylinder during the testing process. The technician must meticulously record any relevant information and data to ensure accuracy and reliability of the results obtained from the compression test. These observations serve as valuable reference points for analysis and comparison, helping to assess the cylinder’s performance, identify any potential issues, and make informed decisions based on the test outcomes. Therefore, the technician must diligently document the observations made before and after the compression test to maintain a comprehensive record of the cylinder’s behavior and performance throughout the testing process.
Before the Test
The date of casting and the date of testing are important parameters in material testing. The date of casting refers to the specific day when the specimen was created or cast, while the date of testing is the day when the specimen undergoes the actual testing process. These dates are crucial in determining the timeline and history of the specimen’s preparation and evaluation.
Another important factor in material testing is the age of the specimen. This refers to the length of time that has elapsed since the specimen was cast. The age of the specimen is typically measured in days, weeks, or months, depending on the specific material being tested and the purpose of the testing. It is important to record the age of the specimen accurately as it can significantly affect the test results.
Curing condition is another critical parameter in material testing. It refers to the environmental conditions under which the specimen was cured or allowed to set and harden after casting. Curing conditions can vary depending on the type of material and the specific testing requirements. Common curing conditions include temperature, humidity, and time duration. Accurate recording of the curing condition is crucial as it can impact the properties and performance of the material being tested.
The diameter of the cylinder samples is also an important parameter in material testing, especially for cylindrical specimens used in compression or tensile tests. The diameter of the samples is typically measured in millimeters and is an essential parameter in determining the load and stress applied during testing. Accurate measurement and recording of the sample diameter is critical to ensure consistent and reliable test results.
Lastly, the testing age is an important parameter that indicates the length of time that has elapsed since the specimen was cast until the actual testing is conducted. It is crucial to accurately record the testing age as it can affect the interpretation of the test results. This parameter is usually expressed in days, weeks, or months, depending on the specific material and testing requirements.
After the Test
The maximum load applied, also known as the compressive strength, is the highest force that can be applied to a cylinder or cap without causing failure. This measurement indicates the ability of the material to withstand compression without collapsing or fracturing. The type of fracture that may occur depends on the material and the conditions under which the load is applied. Common types of fractures include brittle fractures, ductile fractures, and mixed mode fractures.
Defects, if any, in cylinders or caps can significantly affect their compressive strength. These defects can include manufacturing imperfections, material inconsistencies, voids, cracks, or other structural irregularities. These defects can weaken the material and reduce its overall compressive strength. It is important to carefully inspect cylinders or caps for any defects before conducting compressive strength tests to ensure accurate results. Proper handling, manufacturing, and storage practices can help minimize the occurrence of defects in cylinders or caps and ensure reliable and accurate testing results.
Calculations for Compressive Strength of Concrete Cylinder
The compressive strength of a material is a measure of its ability to withstand applied loads without breaking or deforming. It is typically calculated by dividing the maximum load a material can withstand before failure by its cross-sectional area. In other words, compressive strength is the ratio of the maximum load a material can bear to the area over which the load is applied. This parameter is often used in engineering and construction to assess the structural integrity and durability of materials under compression, such as in buildings, bridges, and other load-bearing structures. A higher compressive strength indicates a material that is capable of withstanding larger loads, while a lower compressive strength suggests a material that is more prone to deformation or failure under pressure.
Result of Cylinder Test
The given context pertains to the 28th day compressive strength of a cylinder, expressed in units of N/mm2.
Compressive Strength of Concrete at Various Ages
The strength of concrete is known to increase over time as it ages. Table-1 provides data on the strength of concrete at various ages, comparing it to the strength of concrete at 28 days after casting. This table displays how the strength of concrete changes as it progresses in age, with specific measurements at different time intervals. This data allows for a comparison of the relative strength of concrete at different stages of its curing process, providing insight into how concrete develops its desired strength characteristics over time.
| Age | Strength per cent |
| 1 day | 16% |
| 3 days | 40% |
| 7 days | 65% |
| 14 days | 90% |
| 28 days | 99% |
Compressive Strength of Different Grades of Concrete at 7 and 28 Days
Table.2: Compressive Strength of Different Grades of Concrete at 7 and 28 Days
| Grade of Concrete | Minimum compressive strength N/mm2 at 7 days | Specified characteristic compressive strength (N/mm2) at 28 days |
| M15 | 10 | 15 |
| M20 | 13.5 | 20 |
| M25 | 17 | 25 |
| M30 | 20 | 30 |
| M35 | 23.5 | 35 |
| M40 | 27 | 40 |
| M45 | 30 | 45 |
Precautions for Compressive test on Concrete Cylinder
It is essential to prevent the cylinder from drying out completely before conducting the compression test. This is because a completely dried out cylinder may not provide accurate test results. To ensure the cylinder retains its moisture content, appropriate measures should be taken, such as covering it with a damp cloth or storing it in a sealed container until the test is performed.
In addition, it is crucial to wear gloves and protective goggles when handling the cylinder during the compression test. This precaution is necessary to protect oneself from potential harm that may result from specimen breakage. The cylinder may fracture or shatter under the applied load during the test, which can pose a risk of injury from flying debris or sharp edges. Wearing gloves and protective goggles helps to mitigate these risks and ensures the safety of the person performing the test.