2.1 Scope, Significance, & Specimen Preparation
Key Takeaways
- ASTM C39 is applicable to cylindrical specimens with a density greater than 50 lb/ft³ [800 kg/m³], including molded cylinders and drilled cores.
- Compressive strength results are used as a basis for quality control of concrete proportioning, mixing, and placing; compliance with specs; and evaluating admixtures.
- Specimens must be tested in a moist condition; drying of specimens leads to micro-cracking and a substantial reduction in measured compressive strength.
- Moisture loss must be prevented during transportation and wait times at the laboratory using wet burlap or plastic coverings.
- Concrete cylinders must be cast and cured in accordance with ASTM C31 (field) or ASTM C192 (laboratory), and cores must be prepared under ASTM C42.
2.1 Scope, Significance, & Specimen Preparation
The compressive strength of concrete is its primary mechanical property, serving as the benchmark parameter for structural design, durability estimation, and compliance with structural specifications. Engineers specify concrete based on its design compressive strength ($f'_c$) at a particular age—typically 28 days. Standardizing the method by which this strength is measured is paramount. ASTM C39 / C39M, Standard Test Method for Compressive Strength of Cylindrical Concrete Specimens, establishes the standard procedure for applying a continuous compressive axial load to molded cylinders or drilled cores until failure.
To obtain accurate, repeatable, and legally defensible test results, concrete strength technicians must understand the physical and mechanical boundaries of the test, why the test is performed, and the exact protocols for curing and preparing specimens. Minor errors in the pre-test handling of specimens, particularly related to moisture loss, can introduce critical errors that invalidate the test results.
Scope and Specimen Boundaries of ASTM C39
ASTM C39 is specifically designed for testing cylindrical concrete specimens. The standard is restricted by material density and specimen geometry:
- Density Restrictions: The test method applies to concrete mixtures having a unit weight (density) of greater than 50 lb/ft³ [800 kg/m³]. Concrete mixtures below this unit weight limit are considered ultra-lightweight and fall outside the scope of this standard.
- Specimen Types: The standard accommodates both molded cylinders and drilled cores. Molded cylinders are typically standard sizes of $6 \times 12\text{ inches}$ [150 \times 300 mm] or $4 \times 8\text{ inches}$ [100 \times 200 mm], cast from fresh concrete in accordance with ASTM C31/C31M (field) or ASTM C192/C192M (laboratory). Drilled cores are obtained from hardened in-place concrete and prepared in accordance with ASTM C42/C42M.
The test method is not applicable to concrete cylinders that do not have plane, perpendicular ends unless they are capped or ground flat. Standard dimensions and aspects are verified before testing.
Significance and Practical Applications
The compressive strength measured under ASTM C39 is used for several critical engineering and quality control purposes:
- Compliance with Specifications: Verifying that the concrete delivered and placed in a structure meets the design strength ($f'_c$) specified by the engineer.
- Quality Control of Production: Tracking the uniformity of concrete mixtures during production, mixing, and placing at the batch plant and job site.
- Mix Proportioning: Evaluating the performance of different cementitious materials, water-cement ratios, aggregates, and chemical admixtures during laboratory trial batches.
- Evaluating Curing and Protection: Assessing the adequacy of field curing and cold-weather protection by testing field-cured cylinders (under ASTM C31).
- Structural Evaluation: Determining the load-carrying capacity of existing structures, estimating when structural elements can be put into service, or determining when shoring can be safely removed.
It is important to remember that compressive strength measured by ASTM C39 is a property of the concrete mixture under standardized specimen conditions. The measured strength of a cylinder does not represent the exact in-place strength of the concrete in the structure, which is influenced by consolidation, curing temperature, member dimensions, and environmental conditions.
Moisture Conditioning: The Non-Negotiable Requirement
The moisture state of the cylinder at the time of testing has a profound effect on the measured compressive strength. ASTM C39 mandates that specimens must be tested in a moist condition.
The Physics of Drying and Strength Loss
When a concrete cylinder is removed from water storage or a moist room and allowed to dry, moisture evaporates from the outer surface first. This differential moisture loss creates a moisture gradient from the dry exterior to the wet core. As the outer cement paste dries, it undergoes drying shrinkage. However, the wet interior core resists this shrinkage. This restraint generates significant tensile stresses on the outer shell of the cylinder, leading to the development of microscopic shrinkage cracks.
When an axial compressive load is applied to a dry or partially dried cylinder, these pre-existing surface micro-cracks act as stress concentrators. They rapidly propagate under load, causing the cylinder to fail prematurely.
- Strength Reduction: A cylinder that has been allowed to dry can exhibit a 10% to 25% reduction in measured compressive strength compared to an identical cylinder tested in a moist condition.
- Technician Directive: Technicians must keep specimens moist between removal from curing and the start of testing. During transportation from the curing tanks to the testing machine, or while waiting in the lab, cylinders must be protected from drying.
Curing and Handling Procedures
Cylinders must be cured in accordance with either ASTM C31/C31M or ASTM C192/C192M. Standard curing requires the specimens to be placed in a moist room with free water on the surfaces at all times, or fully submerged in lime-saturated water tanks at a temperature of $73.5 \pm 3.5^\circ\text{F}$ [$23.0 \pm 2.0^\circ\text{C}$].
During transport and preparation:
- Cylinders must be transported in sealed plastic bags or tightly sealed containers containing wet rags or water to prevent evaporation.
- While waiting to be tested in the laboratory, specimens must be covered with wet burlap or plastic sheeting to prevent dry drafts from initiating surface drying.
- If a specimen's surface dries out prior to testing, it must be noted in the final test report.
Testing Age and Tolerances
Concrete gains strength over time due to the hydration of cement. Therefore, testing must occur at the precise ages specified in the project contract documents. To account for laboratory schedules and logistical constraints, ASTM C39 establishes strict tolerances for the age of the specimen at the time of testing:
| Designated Test Age | Permissible Testing Age Tolerance | Percentage Tolerance |
|---|---|---|
| 24 Hours | $\pm 0.5\text{ Hours}$ | 2.1% |
| 3 Days | $\pm 2\text{ Hours}$ | 2.8% |
| 7 Days | $\pm 6\text{ Hours}$ | 3.6% |
| 28 Days | $\pm 20\text{ Hours}$ | 3.0% |
| 90 Days | $\pm 2\text{ Days}$ | 2.2% |
If a cylinder is tested outside of these tolerances, the actual time of testing must be recorded and reported, and the test results may be rejected by the engineer of record due to non-compliance.
Summary of Curing and Preparation Standards
The following table summarizes the different curing environments, handling protocols, and standards referenced:
| Parameter | Molded Field Cylinders (ASTM C31) | Molded Lab Cylinders (ASTM C192) | Drilled Concrete Cores (ASTM C42) |
|---|---|---|---|
| Initial Curing | 24 to 48 hours in a controlled environment at $60–80^\circ\text{F}$ [$16–27^\circ\text{C}$] ($68–78^\circ\text{F}$ for $f'_c \ge 6000\text{ psi}$). | 20 to 28 hours in molds at $73.5 \pm 3.5^\circ\text{F}$ [$23.0 \pm 2.0^\circ\text{C}$]. | In-situ curing inside the structure until drilling occurs. |
| Standard Curing | Submerged in lime-saturated water or in a moist room at $73.5 \pm 3.5^\circ\text{F}$ [$23.0 \pm 2.0^\circ\text{C}$] until test. | Moist room or lime-saturated water at $73.5 \pm 3.5^\circ\text{F}$ [$23.0 \pm 2.0^\circ\text{C}$] until test. | Wrapped in plastic or placed in sealed bags for 5 days minimum prior to test to balance moisture. |
| Moisture at Test | Tested in moist condition; surface must be visibly damp. | Tested in moist condition; surface must be visibly damp. | Tested in moisture condition matching structural state or standard wet. |
What is the lower concrete density limit for cylindrical specimens tested under ASTM C39?
Why is keeping concrete specimens moist until the moment of testing critical under ASTM C39?
Under ASTM C39, what is the permissible testing age tolerance for concrete specimens designated to be tested at 28 days?