Proper fabrication, care, and curing of concrete test cylinders is critical for the results to be valid and meaningful.
Concrete cylinders are cast for a variety of applications to ensure that the compressive strength requirements for a project are being met. Did you know, however, that there are standards that govern the casting, curing and testing of these cylinders?
Proper molding and curing of a concrete cylinder that is representative of the concrete being placed is a task that requires training, care and effort. While the making of a concrete cylinder in itself is not difficult, there are specifications - ASTM C94, Standard Specification for Ready-Mixed Concrete, ACI 318, Building Code Requirements for Structural Concrete, and the Illinois Department of Transportation (IDOT) - that require the individual casting the cylinder be certified as an American Concrete Institute (ACI) Grade I Field Testing Technician.
Standard vs. Field Cured Cylinders
Once verified, ASTM C31, Standard Practice for Making and Curing Concrete Test Specimens in the Field, describes in detail the equipment, the casting procedures and the acceptable method for curing the cylinder. C31 also describes two cylinder sizes - the standard 6 x 12-inch cylinder and a smaller sized 4 x 8-inch cylinder. Next, the decision is made whether to standard cure or field cure the cylinders. Unless otherwise specified, cylinders used for acceptance purposes (compared to f´c) are standard cured.
1. Standard cured cylinders are maintained within specified moisture and temperature conditions from the time of molding until compression testing. The report from the laboratory must include the initial curing method with minimum and maximum temperatures and method of final curing. Strength results of standard cured cylinders can then be used for the following purposes:
acceptance testing for the specified strength, f´c
verifying mixture proportions for strength
quality control by the ready mixed concrete producer
2. Field cured cylinders are provided the “same temperature and moisture environment as the structural work”. These cylinders are stored as close as possible to the point of concrete placement. Strength results of field cured cylinders can then be used for the following purposes:
determining the time at which a structure may be put into service
evaluating the adequacy of curing and protecting concrete in the structure
scheduling removal of forms or shoring
The majority of cylinders cast at the job site will be standard cured with the results compared to the specified compressive strength (f´c) for the project. These cylinders are not intended to represent the in-place strength of the concrete but rather the potential strength of the concrete that is discharged from the ready-mixed concrete truck. In addition, a strength test is defined as the average of at least two standard cured specimens cast from a composite sample of concrete and tested at the age specified.
Cylinders that are standard cured can be stored in the field for up to 48 hours provided that the curing temperature remains between 60-80ºF. To maintain the temperature during the 48 hour initial curing period, C31 describes various methods including the use of ventilation, ice, thermostatically controlled heating or cooling devices and heating methods such as light bulbs. For cold weather concreting this may require the use of an insulated curing box.
For hot weather concreting this may require the use of a water storage tank cooled with ice. For concrete with a specified compressive strength of 6000 psi or greater, the curing temperature is further restricted to between 68-78°F.
When cylinders are cast to indicate whether the producer has delivered concrete of the required compressive strength, then standard curing is mandatory.
Once the cylinders are picked up from the project, the travel time cannot exceed 4 hours. During transportation, cylinders must be protected with suitable cushioning material to prevent damage from jarring. During cold weather, cylinders must be protected from freezing with suitable insulation materials. Whether hot or cold, cylinders must always be protected from moisture loss which is critical for hydration and strength development. Cylinders returned to the lab will be stripped from their molds and final cured at 73 +/- 3ºF until the age of testing using either water storage tanks saturated with lime or moist cure rooms.
Both ASTM C94 and C31 emphasize that deviation from referenced methods for casting and curing of cylinders must be documented on the report summary. C94 further states that deviation from standard moisture and temperature curing conditions is often a reason for low strength test results and may invalidate their use for rejection of the concrete. Strength may be reduced by up to 25% due to deviations in the initial curing of the cylinders. At the end of the day, the role of testing is to evaluate the material at the time of placement to ensure that the project specifications are being met. Standards outline the methods for performing the tests and describe in detail the equipment to be used. If everyone involved in the construction process follows the required standards and specifications, then the end result for the owner will be the best possible product that can be constructed.
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