The casting and curing of concrete cylinder and beam specimens from representative samples of fresh concrete play a crucial role in the construction of buildings. This is because the results of the tests conducted on these specimens serve many significant purposes in the building process.
One such purpose is the acceptance of designated concrete strength, which is necessary for ensuring that the concrete used in the construction meets the required strength specifications. Additionally, the adequacy of mix proportions for strength is examined through the tests conducted on these specimens, which is important for maintaining the quality of the building.
The values obtained from these specimen tests are also used to specify the capacity of the structure to be put into service. This is important for ensuring that the building can safely accommodate the intended load and usage. Moreover, the tests help in checking the adequacy of curing and protection of the concrete structure, which is crucial for maintaining the durability and longevity of the building.
Lastly, it is essential that the concrete used to make the molded specimens is sampled after all on-site adjustments have been made to the mixture proportions. This ensures that the sample collected is representative of the actual concrete used in the construction, and the tests conducted on the specimens accurately reflect the properties of the concrete in the building.
Apparatus
Table 1 outlines the requirements for cylinder molds, beam molds, and a tamping rod that are commonly used in concrete testing. The cylinder molds are used to create cylindrical samples of concrete, while the beam molds are used to create beam-shaped samples. The tamping rod is used to compact the concrete in the molds and ensure that it is evenly distributed.
Vibrators are also commonly used in concrete testing to ensure that the concrete is properly compacted and to remove any air pockets. A mallet can be used to strike the sides of the molds to remove any air bubbles that may have formed during the pouring process. Placement tools and finishing tools are also used to help shape and smooth the concrete as needed.
Slump apparatus is used to measure the consistency or workability of the concrete mixture. This test is important to ensure that the concrete is suitable for the intended application. The air content apparatus is used to measure the amount of air that is trapped in the concrete mixture. This can have an impact on the strength and durability of the final product.
Finally, temperature measuring devices are used to monitor the temperature of the concrete as it sets and hardens. This is important because temperature can affect the strength and durability of the final product. By closely monitoring the temperature, the testing team can ensure that the concrete is setting and curing properly.
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
Concrete specimens are required to be casted and allowed to set in an upright position in order to prepare them for testing. The number and size of the samples that need to be prepared are determined by the person or organization that specifies the tests. The cylindrical mold used for casting should be twice its diameter in length, and the cylinder diameter should be a minimum of three times the maximum size of the coarse aggregate. In acceptance testing of specified compressive strength, the recommended size for the cylinder molds is 150 by 300 mm or 100 by 200 mm.
In contrast, concrete beam specimens should be made and hardened in the horizontal position. The length of the beam should be at least 50 mm greater than three times its depth. Additionally, the ratio of the width to depth as molded should not exceed 1.5. The minimum cross-sectional dimension of the beam must be in accordance with the values presented in table 2. The standard size for the beam is 150 by 150 mm in cross-section.
When determining the modulus of rupture, different specimen sizes can be used. However, it should be noted that the measured modulus of rupture generally increases as the specimen size decreases. Moreover, the variability of individual test results increases as the specimen size decreases as well.
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. This means that for every batch of concrete that is produced, it is necessary to determine the slump value and keep a record of it. The slump is a measure of the consistency and workability of the concrete, which is important to ensure that the final product is of the desired quality. By measuring and recording the slump value for each batch, any deviations or inconsistencies can be detected and addressed in a timely manner. This helps to ensure that the concrete is of the required quality and meets the necessary specifications. Therefore, it is essential to carefully monitor the slump values for every batch of concrete that is produced.
Air Content
Determine and record the air content
Molding Specimens
To ensure the accuracy of concrete specimens, it is important to place the molds on a level and sturdy surface that is free of vibrations and other disturbances. The number of layers for concrete placement should be determined based on Table 3 and Table 4. When placing the concrete in the mold, it is important to move the scoop around the perimeter of the mold opening to ensure even distribution with minimal segregation. The method of specimen consolidation should be selected based on Table 3. If the method of consolidation is rodding, the molding requirements should be determined from Table 4. If the method of consolidation is vibration, the molding requirements should be determined from Table 5. Each layer of concrete should be consolidated as required. For beam consolidation using vibrations, the vibrator should be inserted at intervals not exceeding 150 mm along the center line of the long dimension of the specimen. For specimens wider than 150 mm, alternating insertions along two lines should be used. Sufficient vibration is usually applied when the surface of the concrete becomes relatively smooth and large air bubbles cease to break through the top surface. The rod or vibrator should be allowed to penetrate through the layer being rodded and into the layer below approximately 25 mm. After each layer is rodded or vibrated, the outsides of the mold should be tapped lightly 10 to 15 times with the mallet 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, an amount of concrete that will fill the mold after consolidation should be added, avoiding overfilling by more than 6 mm. Fig. 2 shows how to cast cylinder and beam specimens in the field.
Fig. 2: Casting Cylinder and Beam Specimens in Field
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 finish their surfaces properly and prevent any depressions or projections larger than 3.3 mm. This can be achieved by striking the surface of the specimen with either a tamping rod or a handheld float or trowel.
After finishing the surface, it is necessary to mark each specimen in order to identify them and the concrete they represent. This is crucial for proper record keeping and analysis of the concrete’s strength and durability. By marking the specimens, any potential issues or variations can be traced back to specific batches of concrete and addressed accordingly.
Overall, proper finishing and marking of concrete specimens is essential in ensuring the quality and reliability of concrete structures. By taking these steps, any potential issues can be identified and addressed early on, ultimately resulting in safer and more durable structures.
Initial Curing
To ensure the accuracy of concrete strength test results, it is important to properly store the specimens after they have been finished. The recommended storage time is up to 48 hours, during which the specimens should be kept within a specific temperature range. For concrete with a specified strength of 40MPa or higher, the ideal temperature range is between 20 and 26°C. For all other concrete strengths, the range is between 16 and 27°C.
It is crucial to protect the specimens from direct sunlight and any sources of radiant heat that could cause moisture loss. To achieve this, heating and cooling machinery can be used to regulate the storage environment and maintain a consistent temperature range. By properly storing the specimens in this manner, their strength can be accurately tested and evaluated.
Final Curing
Standard Curing
Standard curing is a common practice for preparing specimens used in various applications, such as acceptance testing for specified strength, evaluating the adequacy of mixture proportions for strength, and ensuring quality control. To achieve proper curing, it is essential to transfer the cylinder and beam specimens into water storage tanks within 30 minutes after removing the molds. This process helps to cure the specimens with free water, preventing surface drying that could compromise their quality.
It is important to note that drying of the surfaces of beam specimens must be prevented between removal from water storage and completion of testing. Surface drying of flexural specimens can induce tensile stresses in the extreme fibers, leading to reduced flexural strength. Therefore, it is crucial to maintain free moisture on the cylinders and beams throughout the curing process.
Standard curing temperature is not mandatory, as long as the ambient temperature falls between 20 and 30°C and free moisture is maintained on the specimens. This approach allows for flexibility in the curing process while still ensuring quality and accurate strength evaluation.
Field Curing
Field curing is a technique utilized when concrete specimens are required for testing the capability of a structure to be put into service, comparing results with standard cured specimens, determining the adequacy of curing and protection of concrete within the structure, and for evaluating form or shoring removal time requirements. To ensure accuracy, the concrete specimens should be placed as close to the point of deposit of the concrete as possible, either on or within the structure.
It is important to protect all surfaces of the specimens from the elements in the same way as the formed work. This helps to provide the same environmental conditions for the specimens as the structural work. Additionally, the concrete specimens should be given the same temperature and moisture environment as the structural work to ensure consistency.
At the end of the curing period, it is necessary to leave the specimens in place exposed to the weather in the same way as the structure. This will provide uniform conditions for both the structural work and the concrete specimens. Before testing, it is important to ensure the same moisture content on all specimens by submerging them in water for twenty-four hours.
By following these procedures for field curing, the concrete specimens can be accurately tested and compared with standard cured specimens. This helps to determine the adequacy of curing and protection of concrete within the structure and ensures that the structure is capable of being put into service.
Report
In order to have concrete samples tested in a laboratory, certain information must be provided. This includes an identification number for the samples, as well as the location of the concrete they represent. It is also important to provide the date and time when the specimens were molded, along with the name of the individual who performed the molding.
The fresh concrete should also be tested for several properties, including the slump, air content, and temperature. Any other relevant test results should also be included with the specimens.
It is crucial to specify the method of curing for the concrete. If a standard curing method was used, the initial method with the maximum and minimum temperatures should be reported, along with the final curing method. For field curing methods, the location where the samples were stored and the manner in which they were protected from the elements should be provided, along with the temperature at which they were cured.