The size of cylinder for concrete testing is pivotal for accurate strength assessments. Commonly, cylinders are 4×8 inches or 6×12 inches in size, aligning with ACI and ASTM standards. This article will succinctly clarify these dimensions’ importance, the size of the cylinder for concrete testing, and how they impact the validity of test results.
Size of Cylinder for Concrete Testing: Key Takeaways
- Select the size of the cylinder for concrete testing based on the aggregate size. Ensure the diameter is at least three times the nominal maximum size of the coarse aggregate and maintains a length-to-diameter ratio of 2:1, following ACI and ASTM standards.
- Proper cylinder specimen preparation is critical, with non-absorbent materials for molds, greased interiors for easy removal, and stringent procedures for filling, layering, and striking off the top surface, per ASTM C31.
- Concrete cylinders require careful initial curing under controlled temperature and humidity, as well as final curing in a moist room or water storage tank to accurately measure the concrete’s strength.
Choosing the Right Size of Cylinder for Concrete Testing
The initial step in gauging concrete’s compressive strength is to select a composite sample of the appropriate cylinder size for testing concrete test specimens. The size of the cylinder is influenced by the project’s specifications and the nominal maximum size of the aggregate used in the concrete mix.
The American Concrete Institute (ACI) and the American Society for Testing and Materials (ASTM) specify that the cylinder should have a diameter at least three times the nominal maximum size of the coarse aggregate, with a length-to-diameter ratio of 2:1. Typically, 4×8 inch or 6×12 inch molds are used for casting concrete cylinders, with the smaller size and lighter weight of 4×8 inch cylinders proving popular due to their ease of use.
Nominal Maximum Size of Aggregate Considerations
The nominal maximum size of the aggregate plays a significant role in determining the cylinder mold size. Accurate compressive strength readings mandate a mold size that is at least triple the maximum size.
For concrete mixes with aggregates larger than 2 inches, technicians use wet sieving to remove these large materials, ensuring the mold size fits the remaining size. This standard practice maintains a mold length-to-diameter ratio of 2:1, which is vital for obtaining precise strength readings from test cylinders.
ACI and ASTM Standards
ACI and ASTM standards provide comprehensive guidelines for concrete testing, including acceptance testing of high-strength concrete and choosing the correct size of the cylinder for concrete testing.
ASTM C31 and ASTM C470/AASHTO M 205 provide standards for concrete cylinder molds that are suitable for the 4×8 inch and 6×12 inch sizes, ensuring compliance with the prescribed norms.
Preparing and Molding Test Cylinders
Following the determination of the size of the cylinder for concrete testing, the test process proceeds with the preparation and molding of the concrete cylinders cast. The molds used for creating the test cylinders must be made of a non-absorbent material, such as steel or cast iron, ensuring they do not deform or react with the concrete.
Proper procedures for curing concrete test specimens are essential to ensure accurate test results.
The molding process for composite concrete samples involves the following steps:
- Initiate the process promptly within 15 minutes after creating the sample.
- Layer the concrete mix using a scoop and compact it uniformly in the mold.
- Rod the concrete 25 times per layer, with the number of layers and rodding count varying based on the cylinder size.
- Grease the interior of the molds to facilitate easy removal of the hardened concrete cylinders.
Greasing and Layering
To ensure easy removal of the hardened concrete, workers apply a thin layer of form oil or mineral oil to the interior surface of the concrete cylinder molds. After this greasing process, they carefully layer the concrete mix.
They meticulously carry out the layering, ensuring each layer is not less than 5cm deep. This step prevents void formation and confirms that the concrete is uniformly compacted across the cylinder structure.
Striking Off the Top Surface
After the careful layering and rodding of the concrete mix and after the selection of the correct size of cylinder for concrete testing, the next step is to ensure a flat top surface of the cylinder. This flat surface is pivotal for accurate strength testing. A variety of tools such as a tamping rod, float, or trowel may be used for this purpose. However, a wooden float is specifically recommended to ensure a level surface. The process is guided by ASTM C31, which ensures that the surface is ready for transportation and further testing without the risk of disturbances.
Initial Curing of Concrete Cylinders
After the preparation and molding of the structural concrete cylinders and selecting the correct size of cylinder for concrete testing, the process of curing concrete test specimens begins with initial curing. This step is crucial for the accurate measurement of the concrete’s true strength.
The initial curing conditions should maintain a controlled temperature and relative humidity. Studies have shown that deviations in these conditions can lead to significant variations in the strength of concrete. ASTM C31 specifies the temperature range for the initial curing process, which varies depending on the design strength of the concrete mix.
To prevent moisture loss for up to 48 hours, the test cylinders should be stored in water storage tanks during the initial curing process, ensuring a temperature-controlled environment.
Suitable Curing Environments
The use of boxes, plastic bags, or caps on the plastic mixed concrete cylinder mold enables the maintenance of a moist environment for the initial curing of concrete cylinders. These measures, along with practices such as ventilation or the use of ice in hot conditions, help manage the temperature during the initial curing period.
Commercially available curing boxes, such as the Thermocure II, can ensure the maintenance of correct temperature and moisture conditions. These curing boxes are especially beneficial when the ambient air temperature is likely to fall outside the correct range.
Final Curing Practices Before Testing
After initial curing, technicians subject the concrete cylinders to a final curing process before testing. This includes making and curing concrete test specimens. Perform this process in a moist room or water storage tank with a constant temperature, ensuring free water remains on the cylinders’ surfaces at all times.
Maintaining standard curing conditions in both the field and the laboratory is critical. This helps assess the inherent properties of concrete independently from variable job site temperatures. Proper final curing practices ensure:
- Accurate assessment of the strength
- Optimization of concrete mixtures
- Reduction of destructive testing
- Overall cost reductions.
Proper curing concrete test specimens ensures the accuracy of the strength measurements.
The transition from Field to Lab
The transition of test cylinders from the field to the lab is critical and requires careful attention. Protect concrete specimens against mechanical damage and moisture loss during transportation. Ensure the transportation time does not exceed 4 hours to maintain specimen integrity.
Use insulation materials like plastic, wet burlap, or wet sand to prevent moisture loss. During transport, use suitable cushioning material to prevent damage from jarring. This helps protect the specimens.
Compressive Strength Testing and Analysis
Now we venture into the final stage of the strength development process – the compressive strength testing and analysis. This process follows the established standard of ASTM C39/AASHTO T 22.
Ensure the concrete testing cylinder retains moisture from the curing environment before testing. Apply a continuous load to the cylinder until it fails to determine strength. This process helps understand the material’s ability to withstand pressure. Measure the diameter of the concrete cylinder at two perpendicular points at mid-height and average the measurements for an accurate calculation of strength.
Ensuring Axial Load Application
Apply the load evenly during strength testing to prevent localized stress and false failure indications. Achieve uniform load distribution using unbonded caps.
Correct application of these caps leads to higher strength results, indicating a more accurate reflection of the concrete’s quality. Regular maintenance and adherence to ASTM C 1231 guidelines for replacement are critical to prevent inaccuracies in test results caused by worn or damaged unbonded caps.
Interpreting Test Results
The raw data from strength testing can be overwhelming. However, with the right knowledge, interpreting these results is straightforward. According to ACI 318-19, the average of any three consecutive strength tests must equal or exceed the specified strength. No single test should fall below the specified strength by more than the allowed margin.
Evaluators assess a concrete batch by averaging the results of at least two cylinders. They determine the average strength by dividing the failure load by the cross-sectional area. Evaluators compare the cylinder strength results to the specified strength of concrete (f’c) to determine if the concrete mixtures meet job specifications.
Handling and Reporting Data
Following testing, data handling and reporting take precedence. During this phase, the technician records detailed observations, such as:
- The date of receipt and testing in the lab
- Specimen identification
- Cylinder Size
- Test age
- Maximum load applied
- Strength
- Type of fracture
- Any defects or anomalies in the cylinders or caps
These reports are crucial documents that should be quickly forwarded to the concrete producer, contractor, and owner’s representative.
Routine analysis of concrete strength results includes:
- Correlations with factors such as slump or air temperature
- Acting as an early warning system
- Ensuring quality control and production consistency over time.
Summary
In conclusion, choosing the right cylinder size for concrete testing is crucial in the construction industry. The process includes selecting the right size, preparing and molding test cylinders, initial and final curing, and compressive strength testing. Each step follows established standards and project specifications. Handling and reporting data offer valuable insights for quality control, ensuring safe and durable concrete structures. Remember, the strength of concrete is in the details!
Frequently Asked Questions
What size cylinder is used for the compression test?
A cylinder size of 4x8in or 6x12in (100x200mm or 150x300mm) in diameter by length is used for compression testing. This allows for testing a range of strengths.
What size are concrete sample tests?
Concrete sample tests typically use 6-inch-diameter by 12-inch-tall plastic molds. Some projects also use 4-inch-diameter by 8-inch-high cylinders.
What is the ASTM standard for testing concrete cylinders?
The ASTM standard for testing concrete cylinders is ASTM C39. It has been the industry standard for testing the strength of cylindrical concrete specimens for over 80 years.
Does cylinder size affect the compressive strength of concrete?
The size of the cylinder can impact strength in small-scale models. Standard cubes generally show higher strength than standard cylinders with a height/diameter ratio of 2.0. However, for normal-strength concrete, different cylinder sizes do not affect compressive strength tests at various time intervals.
Why is it important to choose the right cylinder size for concrete testing?
Choosing the right cylinder size for concrete testing is important. It ensures the sample represents the concrete’s in-place strength, especially for strength testing. This is crucial for obtaining accurate results (2023).
