Casting and Curing Concrete Specimens in Field Based on ASTM C31

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Casting and curing of concrete cylinder and beam specimens from representative samples of fresh concrete is crucial in building constructions.

This is because the results of the tests conducted on these specimens are employed for many significant purposes. For instance, acceptance of designated concrete strength, examine adequacy of mix proportions for strength, and quality control.

Moreover, the values of these specimen tests are also employed for specifying capacity of the structure to be put into service, check the adequacy of curing and protection of concrete structure, and form removal time requirements.

Lastly, the concrete used to make the molded specimens shall be sampled after all on-site adjustments have been made to the mixture proportions.

Apparatus

1. Cylinder Molds
2. Beam Molds
3. Tamping Rod for which requirements are presented in Table 1.
4. Vibrators
5. Mallet
6. Placement Tools
7. Finishing Tools
8. Slump Apparatus
9. Air Content Apparatus
10. Temperature Measuring Devices

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 cylinder specimens should be casted and allowed to set in an upright position.
• The number and size of samples are determined by the specifier of the tests.
• Cylinder mold length should be twice its diameter, and the cylinder diameter should be minimum 3 times maximum size of coarse aggregate.
• Cylinder molds should be 150 by 300 mm or 100 by 200 mm for acceptance testing of specified compressive strength.
• Concrete beam specimens should be made and hardened in the horizontal position.
• The length shall be at least 50 mm greater than three times its depth.
• The ratio of width to depth as molded shall not exceed 1.5.
• The minimum cross-sectional dimension of the beam should be in accordance with values presented in table 2.
• The standard beam shall be 150 by 150 mm in cross section.
• The modulus of rupture can be determined using different specimen sizes. However, measured modulus of rupture generally increases as specimen size decreases.
• The variability of individual test results increases 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

Measure and record the slump of each batch ofconcrete

Air Content

Determine and record the air content

Molding Specimens

• The molds should be placed on level, rigid surface, free of vibration and other disturbances.
• Determine the number of layers for concrete placement for specimens based on Table 3 and Table 4.
• Placing the concrete in the mold, move the scoop around the perimeter of the mold opening to ensure an even distribution of the concrete with minimal segregation.
• Select method of specimen consolidation based on Table 3.
• If the method of consolidation is rodding, determine molding requirements from Table 4.
• If the method of consolidation is vibration, determine molding requirements from Table 5.
• Each layer of concrete shall be consolidated as required.
• For beam consolidation using vibrations, insert the vibrator at intervals not exceeding 150 mm along the center line of the long dimension of the specimen.
• For specimens wider than 150 mm, use alternating insertions along two lines.
• Usually sufficient vibration has been applied as soon as the surface of the concrete has become relatively smooth and large air bubbles cease to break through the top surface.
• 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, tap the outsides of the mold lightly 10 to15 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.
• In placing the final layer, add an amount of concrete that will fill the mold after consolidation, avoid overfilling by more than 6 mm.

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

• Finish specimen surface and prevent depressions or projections larger than 3.3 mm.
• Strike specimen surface off with the tamping rod or with a handheld float or trowel.
• Mark the specimens to identify them and the concrete they represent.

Initial Curing

1. After finishing, specimens should be stored for up to 48hours in a temperature range of 16 to 27°C or 20 and 26°C for specified concrete strength of 40MPa or higher,
2. Specimens should be protected from direct sun light, radiant heating devices to prevent the loss of moisture. Heating and cooling machineries may be used for this purpose.

Final Curing

Standard Curing

• Standard curing is used when the specimens are employed for several purposes such as acceptance testing for specified strength, checking adequacy of mixture proportions for strength, and quality control.
• Cylinder and beam specimens need to be moved into water storage tanks to cure them with free water within 30 min after removing the molds.
• Drying of the surfaces of the beam specimens shall be prevented between removal from water storage and completion of testing otherwise surface drying of flexural specimens can induce tensile stresses in the extreme fibers that reduce the indicated flexural strength.
• Standard curing temperature is not required provided that free moisture is maintained on the cylinders and ambient temperature is between 20 and 30°C.

Field Curing

• Field curing is used if the specimens are used for determination of whether a structure is capable of being put in service, comparison with test results of standard cured specimens, adequacy of curing and protection of concrete in the structure, and form or shoring removal time requirements.
• Place the concrete specimens in or on the structure as near to the point of deposit of the concrete represented as possible.
• Protect all surfaces of the specimens from the elements in as near as possible the same way as the formed work.
• Provide the concrete specimens with the same temperature and moisture environment as the structural work.
• At the end of the curing period, leave the specimens in place exposed to the weather in the same manner as the structure.
• Ensure the same moisture on all specimens before testing by submerging them in water for twenty-four hours.

Report

The following data should be provided to the lab where the specimens are tested:

• Identification number
• Location of concrete represented by the samples,
• Date, time and name of individual molding specimens,
• Slump, air content, and concrete temperature, test results and results of any other tests on the fresh concrete.
• Curing method
• For standard curing method, report the initial curing method with maximum and minimum temperatures and final curing method.
• For field curing method, report the location where stored, manner of protection from the elements, temperature