The casting and curing of concrete cylinder and beam specimens are essential steps in building construction. These steps are crucial because the results of the tests conducted on these specimens are used for various significant purposes. For example, they determine the acceptance of the designated concrete strength, examine the adequacy of mix proportions for strength, and ensure quality control.
The values obtained from testing these specimens are also crucial for specifying the capacity of the structure to be put into service. They help check the adequacy of curing and protection of concrete structures and determine form removal time requirements. Therefore, it is essential to carry out these tests accurately to ensure the safety and stability of the building.
It is important to note that the concrete used for making the molded specimens must be sampled after all on-site adjustments have been made to the mixture proportions. This ensures that the concrete is representative of the final product used in the construction of the building. By following these procedures, builders can have confidence in the strength and durability of their structures.
Apparatus
Table 1 presents the requirements for various tools used in the process of testing concrete. These tools include cylinder molds, beam molds, tamping rods, vibrators, mallets, placement tools, finishing tools, slump apparatus, air content apparatus, and temperature measuring devices.
Cylinder molds and beam molds are used to create cylindrical and beam-shaped concrete samples for testing. Tamping rods are used to compact the concrete and ensure there are no air voids. The specific requirements for tamping rods are listed in Table 1.
Vibrators are used to ensure the concrete is properly consolidated and there are no air pockets. Mallets are used to remove any air that may have gotten trapped during the mixing process. Placement tools are used to properly place the concrete in the mold or form. Finishing tools are used to smooth and level the surface of the concrete after it has been placed.
Slump apparatus is used to measure the consistency of the concrete, specifically how much it slumps when a certain amount of pressure is applied. Air content apparatus is used to measure the amount of air in the concrete, which is important for ensuring the concrete is durable and long-lasting. Temperature measuring devices are used to monitor the temperature of the concrete during the curing process.
Table 1 Tamping Rod Diameter Requirements
| Diameter of Cylinder or Width of Beam, mm | Diameter or Rod, mm |
| <150 | 10 ± 2 |
| $150 | 16 ± 2 |
Testing Requirements
To conduct tests on concrete specimens, it is important to follow certain specifications regarding their casting and positioning. For instance, when casting cylindrical specimens, they should be set upright and allowed to harden. The number and size of the samples required for testing should be determined by the entity specifying the tests.
To ensure the accuracy of the tests, the length of the cylinder mold should be twice its diameter, and the diameter of the cylinder should be at least three times the maximum size of the coarse aggregate. Acceptance testing of specified compressive strength typically requires cylinder molds that are either 150 by 300 mm or 100 by 200 mm in size.
In contrast, concrete beam specimens should be cast horizontally and allowed to harden in that position. The length of the beam should be at least 50 mm greater than three times its depth, while the ratio of width to depth as molded should not exceed 1.5. The minimum cross-sectional dimension of the beam should be determined based on the values provided in Table 2, and the standard beam should be 150 by 150 mm in cross section.
When determining the modulus of rupture, different specimen sizes can be used. However, it is generally observed that the measured modulus of rupture increases as the specimen size decreases. Additionally, it is worth noting that the variability of individual test results tends to increase as the specimen size decreases.
Table 2 Minimum Cross-Sectional Dimension of Beams
| Nominal Maximum Aggregate Size, mm | Minimum Cross-Sectional Dimension, mm |
| Equal or smaller than 25 | 100 by 100 |
| Greater than 25 but smaller than 50 | 150 by 150 |
Slump
The task at hand is to measure and record the slump of each batch of concrete. Slump refers to the consistency or fluidity of the concrete mixture, and it is an important indicator of the quality of the concrete being produced. In order to ensure that the concrete is of the desired consistency, it is necessary to measure and record the slump of each batch. This involves placing a sample of the concrete in a cone-shaped mold, then lifting the mold and allowing the concrete to flow out and settle into a flat, circular shape. The distance between the top of the mold and the highest point of the concrete is then measured, and this measurement is recorded as the slump. By measuring and recording the slump of each batch of concrete, it is possible to ensure that the concrete being produced is of a consistent quality and meets the required specifications.
Air Content
Determine and record the air content
Molding Specimens
To ensure accurate concrete specimens, it is crucial to place the molds on a level and rigid surface that is free of any vibrations or disturbances. The number of layers required for concrete placement can be determined by referencing Table 3 and Table 4. When placing the concrete in the mold, it is recommended to use a scoop and distribute the concrete evenly while minimizing segregation.
The method of specimen consolidation can be selected based on Table 3. If the chosen method is rodding, molding requirements can be determined from Table 4. On the other hand, if vibration is chosen as the method of consolidation, molding requirements can be found in Table 5.
Each layer of concrete should be consolidated as required. For beam consolidation using vibrations, it is important to insert the vibrator at intervals that do not exceed 150 mm along the center line of the long dimension of the specimen. If the specimen is wider than 150 mm, alternating insertions along two lines should be used. Sufficient vibration is usually applied once the surface of the concrete becomes relatively smooth and large air bubbles cease to break through the top surface.
It is recommended to allow the rod or vibrator to penetrate through the layer being rodded and into the layer below approximately 25 mm. After each layer is rodded or vibrated, it is important to tap the outsides of the mold lightly 10 to 15 times with the mallet. This will help to close any holes left by rodding or vibrating and to release any large air bubbles that may have been trapped.
When placing the final layer of concrete, it is crucial to avoid overfilling by more than 6 mm. An amount of concrete that will fill the mold after consolidation should be added to ensure accuracy in the specimen.
Table 3 Method of Consolidation Requirements
| Slump, mm | Method of Consolidation |
| Equal or greater than 25 | rodding or vibration |
| Smaller than 25 | vibration |
Table 4 Molding Requirements by Rodding
| Specimen Type and Size | Number of Layers of Approximately Equal Depth | Number of Roddings per Layer |
| Diameter of Cylinder specimens, mm | ||
| 100 | 2 | 25 |
| 150 | 3 | 25 |
| 225 | 4 | 50 |
| Width of beam specimens | ||
| 100 to 200 | 2 | One rodding for for each 14 cm^2 of the top surface area of the beam. |
| Greater than 200 | 3 or more equal depths, each not to exceed 150 mm | One rodding for for each 14 cm^2 of the top surface area of the beam. |
Table 5 Molding Requirements by Vibration
| Specimen Type and Size | Number of Layers | Number of vibrator insertions per Layer | Approximate Depth of Layer, mm |
| Diameter of Cylinder specimens, mm | |||
| 100 | 2 | 1 | one-half depth of specimen |
| 150 | 2 | 2 | one-half depth of specimen |
| 225 | 2 | 4 | one-half depth of specimen |
| Width of beam specimens | |||
| 100 to 200 | 1 | Use rodding or vibration | depth of specimen |
| Greater than 200 | 2 or more | Use rodding or vibration | 200 as near as practicable |
Finishing and Marking
To ensure the quality of concrete specimens, it is important to properly finish the surface and prevent any depressions or projections larger than 3.3 mm. This can be achieved by using a tamping rod or a handheld float or trowel to strike the surface of the specimen. It is crucial to mark the specimens with identifying information to accurately track the concrete they represent. By following these steps, you can ensure that your concrete specimens are of high quality and suitable for testing.
Initial Curing
The storage of finished specimens is a critical step in the testing process. It is recommended that specimens be stored for up to 48 hours within a specific temperature range. For concrete with a strength of 40MPa or higher, the temperature range should be between 20 and 26°C. For all other types of concrete, the temperature range should be between 16 and 27°C.
It is essential to ensure that specimens are protected from direct sunlight and radiant heating devices during storage. These can cause the loss of moisture and ultimately affect the accuracy of the test results. To prevent this from happening, heating and cooling machinery may be used to maintain the appropriate temperature range and prevent fluctuations.
Overall, proper storage of specimens is crucial to obtain accurate test results. By following these guidelines, the specimens can be protected from external factors that may influence their quality and durability.
Final Curing
Standard Curing
Standard curing is a commonly used method for preparing specimens intended for a variety of purposes, such as acceptance testing for specified strength, verifying the adequacy of mixture proportions for strength, and quality control.
For the curing process to be effective, it is necessary to transfer the cylinder and beam specimens into water storage tanks within 30 minutes after removing them from their molds. This step ensures that the specimens are cured with free water and promotes the development of the desired strength characteristics.
It is important to prevent the surfaces of beam specimens from drying out during the curing process, as surface drying can induce tensile stresses in the extreme fibers, which can ultimately reduce the indicated flexural strength. To prevent this, the specimens must be kept moist from the moment they are removed from the water storage tanks until the testing process is complete.
While a standard curing temperature is not strictly required for the curing process, it is essential to maintain free moisture on the cylinders at all times. Additionally, the ambient temperature during the curing process should be kept within a range of 20 to 30°C. Adhering to these guidelines helps to ensure that the curing process is effective and that the resulting specimens meet the desired strength requirements.
Field Curing
Field curing is a technique that is utilized when concrete specimens are needed to determine whether a structure is fit to be put in use, to compare with standard cured test results, to assess the adequacy of curing and protection of concrete in the structure, and to determine the timing for form or shoring removal. To achieve accurate results, the concrete specimens should be placed as close to the point of deposit of the concrete as possible, either within or on the structure.
It is essential to protect all surfaces of the specimens from the elements during the curing period. This protection should be provided in the same manner as the formed work, to ensure consistency. The concrete specimens should be exposed to the same temperature and moisture environment as the structural work, to create a similar curing condition.
After the curing period ends, the specimens should be left in place and exposed to the weather in the same way as the structure. This will ensure that any changes in the environment will be reflected in the specimens, allowing for an accurate comparison of the performance of the structural work with the concrete specimens.
Before testing, all specimens should have the same level of moisture. To achieve this, it is recommended to submerge all specimens in water for twenty-four hours. This will ensure that the moisture content is consistent across all specimens and the testing results will be reliable.
Report
In order to have concrete specimens tested at a lab, certain information must be provided. Firstly, an identification number should be assigned to each specimen. The location of the concrete represented by the samples should also be specified. The date, time and name of the individual responsible for molding the specimens should be recorded as well.
In addition to these details, the lab will need information regarding the fresh concrete, including its slump, air content, and temperature. Any test results should be included, along with the results of any other tests performed on the fresh concrete.
Furthermore, the curing method used for the concrete should be specified. For standard curing methods, the initial curing method should be reported, including the maximum and minimum temperatures used, as well as the final curing method. For field curing methods, the location where the specimens were stored and the manner in which they were protected from the elements should be described. The temperature during field curing should also be recorded.