ASTM C39: Guide to Concrete Compressive Strength Tests

The ‘astm c39’ standard is pivotal in the construction industry for testing the compressive strength of concrete cylinders. It gauges the material’s capacity to sustain structural loads, ensuring safety and reliability. This article dives into the ‘astm c39’ processes, from specimen preparation to result analysis, equipping professionals with the knowledge to certify the integrity of the concrete.

Key Takeaways

ASTM C39 standard outlines critical procedures and equipment requirements for accurately determining the compressive strength of cylindrical concrete specimens, which is crucial for assessing a material’s capacity to withstand loads and ensure structural safety.
Preparation and curing of concrete specimens must adhere to exact guidelines, with a focus on specific molding dimensions and controlled conditions (temperature and humidity), coupled with careful handling and transportation, to maintain specimen integrity for testing.
Compressive strength test execution requires calibrated equipment and carefully prepared specimens, with a focus on applying a controlled compressive axial load and understanding the factors affecting test results to evaluate the material’s capacity to withstand compressive forces.

Understanding ASTM C39

The construction industry leans heavily on concrete, an ever-reliable material. Yet, its reliability is not assured by mere intuition. It’s confirmed by rigorous testing led by the ASTM C39 standard, a systematic approach to determine and measure the compressive strength of cylindrical concrete specimens. This determination is critical in evaluating the concrete’s capacity to withstand the load of a structure.

The ASTM C39 standard covers a wide range of requirements for determining the compressive strength of different cylindrical concrete specimens, including molded cylinders and drilled cores. It specifies the procedures and equipment needed for accurate testing. This examination is more than a mere formality; it is a performance demonstration that ensures the safety and quality of concrete structures.

Significance of Compressive Strength Testing

The compressive strength of concrete is akin to the heartbeat of a building. It verifies concrete’s ability to withstand significant forces and sustain loads without failure, ensuring the structure’s quality and safety. The ASTM C39 standard provides a clear roadmap to evaluate this critical parameter. But what happens if we overlook this testing? It could lead to undetected flaws, rendering buildings unsafe and potentially uninhabitable.

Moreover, compressive strength testing also plays a significant role in the selection of concrete for diverse construction projects. It provides users with crucial insights into the material’s quality, strength, and durability, guiding the choice of concrete for specific structural applications.

Scope of ASTM C39

When we talk about the scope of ASTM C39, we refer to the materials tested, the procedures followed, and the equipment used. The standard is designed for testing the compressive strength of cylindrical concrete specimens, which include molded cylinders and drilled cores. The procedure outlined entails the determination of the compressive strength of these specimens.

As for the equipment, it includes compression testing machines that meet or exceed ASTM C39 requirements. The meticulousness of this standard ensures that every test is accurate and reliable, leading to safe and sturdy structures.

Preparing Concrete Specimens

Before initiating the testing, gaining knowledge about the process involved in preparing concrete specimens is important. Whether they are molded cylinders or drilled cores, these specimens hold the key to understanding the strength of the concrete. The dimensions for molding concrete cylinders according to ASTM C39 are specific, with the most prevalent size being 6×12-inch cylinders.

And how do we handle and transport these specimens? There’s a precise procedure for that too, outlined in the ASTM C31 standard.

Molding Cylinders

The art of molding cylinders for ASTM C39 testing is a meticulous process. The recommended dimensions are usually 4×8 inches or 6×12 inches. Craft these cylinder molds from a range of materials, including plastic, steel, and cast iron.

Carefully layer a representative concrete sample within the molds, ensuring they are positioned on a flat and stable surface and filled with the appropriate amount of fresh concrete. Such meticulous preparation paves the way for precise testing.

Curing Conditions

After molding the cylinders, it is necessary to provide an appropriate environment for their maturation. The prescribed curing conditions for concrete specimens as per ASTM C39 are defined by ASTM C511 standards. These specify maintaining specimens in a temperature range of 60 to 80°F (16°C to 27°C) and in a moisture room with a constant fog spray to facilitate proper curing.

Temperature and humidity directly affect the development of compressive strength in concrete specimens. Thus, maintaining the right curing conditions is a key step towards obtaining precise test results.

Handling and Transporting Specimens

With the specimens cured and ready for testing, careful handling and transportation become imperative. Here are some guidelines to follow:

The specimens should not be relocated within 8 hours of the initial set time.
They should be placed upright during transportation.
Protect the specimens with appropriate cushioning material to prevent damage.

The storage conditions during transportation also need to be maintained within the prescribed temperature range of 60 to 80°F (16°C to 27°C) to preserve their integrity. Attentive handling and transport of specimens guarantee their perfect condition upon reaching the testing stage.

Performing the ASTM C39 Test

With the specimens prepared, we can now proceed to the core aspect – executing the ASTM C39 test, an examination requiring performance demonstration. This procedure requires specific equipment, including sulfur capping equipment and a testing machine fitted with protective guards.

The testing machine must also have the capability to determine the compressive strength of cylindrical concrete specimens.

Testing Machine Requirements

The testing machine plays a pivotal role in the ASTM C39 test. It needs to meet certain requirements, including regular calibration and maintenance. Humboldt’s range of end grinders and specimen grinding machines, along with equipment from Global Gilson and Controls Group, are appropriate for conducting the ASTM C39 test.

Regular calibration of the machines within 13 months of the previous calibration is advisable. Maintaining and calibrating the testing machine appropriately is key to guaranteeing the results’ accuracy and reliability.

Specimen Preparation

Before the test, the specimens need to undergo a thorough examination. The cylinders should be inspected for any anomalies, such as holes, cracks, and crumbliness. This is to ensure that no irregularities might affect the accuracy and reliability of the test results when one tests concrete cylinders.

Indeed, even the most accurate testing machine cannot produce precise results if the specimens are defective.

Compressive Axial Load Application

The moment of truth arrives when we apply the compressive axial load to the specimen. This load has to be applied at a controlled rate within a specified range. The procedure continues until the specimen fails.

Applying this load attentively is essential for obtaining precise insights into the concrete’s compressive strength.

Interpreting Test Results

With the completion of the test, it’s time to interpret the results. The compressive strength of concrete according to ASTM C39 is determined by dividing the maximum load obtained during the test by the cross-sectional area of the specimen. However, various factors could influence the test results, including the size and geometry of the specimen, as well as the presence of friction at various locations on its ends.

Calculating Compressive Strength

The compressive strength is calculated by:

Dividing the maximum load obtained during the test by the cross-sectional area of the specimen.
This calculation helps determine the ability of the material to withstand compressive forces.
You can determine the cross-sectional area of a cylindrical concrete specimen using the formula A = πr², where A is the area and r is the radius.
The maximum load represents the force or load at the point of failure of the concrete specimen, and you use it to calculate compressive strength.

The compressive strength is typically expressed in megapascals (MPa).

Factors Affecting Test Results

While interpreting the test results, it’s important to take into account various factors that might influence these results. These include:

The preparation of specimens
The reliability of testing laboratories
Factors such as human errors, mold quality, and curing conditions
The operator technique can influence the loading rate during the test, subsequently affecting the results.

Acceptance Criteria

Finally, understanding the acceptance criteria is vital while interpreting the test results. These are generally determined by the specified strength, and individual test results are evaluated against this specified strength. If the test results deviate from the acceptance criteria, it is crucial to verify if the test has failed the acceptance criteria outlined in the Code. In this context, acceptance testing plays a significant role in ensuring the quality and reliability of the product or system being tested.

When in doubt, it’s advisable to seek guidance from an independent examiner or the testing and training agency for further assistance.

Quality Control and Data Management

Ensuring quality control and effective data management are significant aspects of ASTM C39 testing. Laboratory service technicians play a pivotal role here. Their responsibilities include conducting, documenting, and communicating the outcomes of the test.

These outcomes are utilized for overseeing the quality and effectiveness of concrete proportioning, mixing, and placing operations.

Laboratory Technician Requirements

Laboratory technicians should possess a certification equivalent to either ACI Concrete Laboratory Technician, Level I, or ACI Concrete Strength Testing Technician, meeting the concrete laboratory technician requirements for the position. Their knowledge and skills are integral to the smooth and accurate execution of the test. They should have a comprehensive understanding of ASTM practices and test methods, including Capping Cylindrical Concrete Specimens (ASTM C617/C617M) and Loadbearing Concrete (ASTM C90).

Record Keeping and Reporting

Documenting the test results is as important as the test itself. Keeping a daily log of cylinder diameters and ensuring that the testing laboratory adheres to the requirements of ASTM C1077 is key.

There are also specific software and tools, such as the Geotechnical Data Management System (GDMS), Bluehill Universal software and services, and QCConcrete2 software, which can streamline the process of organizing the test results for analysis.

Ensuring Compliance with Standards

Compliance with ASTM C39 standards is essential when testing concrete specimens. From the guidelines outlined in Practices C31/C31M for the preparation and curing of cylindrical specimens to employing machinery capable of applying a compressive axial load to the specimen in a controlled manner, ensuring compliance with the standards is vital for the accurate determination of the compressive strength of cylindrical concrete specimens.

Drilled Cores vs. Molded Cylinders

Selecting the appropriate specimen type for the test is a significant decision. Should we use drilled cores or molded cylinders? Both come with their own set of advantages and considerations. We extract drilled cores from completed asphalt or concrete installations, obtaining a sample that reflects the in-situ conditions.

On the other hand, we form and cure molded cylinders under controlled conditions, providing a more standardized approach to testing.

Advantages of Drilled Cores

Drilled cores offer the following advantages in concrete testing:

They represent in-situ conditions, making them highly effective.
They provide superior samples for conducting laboratory tests.
They offer insights into material quality and adherence to standards.
They provide access points for a range of testing methods.

However, the process of obtaining drilled cores is more time-consuming and costly compared to molding cylinders.

Advantages of Molded Cylinders

Molded cylinders offer several advantages over drilling cores:

Standardization and controlled conditions
Less time-consuming and less costly process
More control over the test results due to the ability to control the curing conditions.

However, molded cylinders do not represent the in-situ conditions that drilled cores do.

Choosing the Right Specimen Type

The decision between using drilled cores or molded cylinders depends on several factors, including:

The required strength
Durability
Corrosion resistance
Size and geometry of the core
Coring direction
The presence of reinforcing bars or other materials
Presence of voids or defects.

The specimen type critically influences the observed compressive strength results and should align with the project’s specifications and the desired level of testing.

Common Issues and Troubleshooting

Even the most carefully planned tests can face issues. Typical challenges during ASTM C39 testing include:

Not conducting the tests accurately
Misalignment of the cylinder with a small load
Absence of documentation for cleaning and lubrication of the ball and socket
Non-planar bearing surfaces

Other issues can arise when failure occurs, such as specimen failures, equipment and computer malfunctions, and deviations from the test method.

Specimen Failures

Specimen failures might arise if laboratory technicians prematurely terminate the test after the load exceeds the requirement, without subjecting the specimens to failure. This can lead to inaccurate calculations and unreliable results. Therefore, laboratory technicians should ensure that they carry out the test until the point of failure.

Other causes of specimen failures include incorrect specimen size, geometry, excess friction at the ends, and irregularities in specimen fabrication, configuration, and capping.

Equipment Malfunctions

Several factors can lead to equipment malfunctions, such as deviation from correct test procedures, faulty fracture patterns of the specimen, or sudden rupture due to the testing machine’s limited flexibility.

To troubleshoot equipment malfunctions, it is important to ensure before that the testing machine is properly calibrated and maintained. This will minimize any flexibility or instability during the test, thereby reducing the risk of sudden ruptures and inaccurate results.

Deviations from the Test Method

Straying from the ASTM C39 method might yield inaccurate results. These can occur when individual compressive strength test results are lower than the specified strength. Other deviations include the use of cylinders smaller than the standard size, which can lead to increased within-test variability.

To ensure rigorous compliance with the ASTM C39 testing protocol, it is crucial to adhere to and utilize the standard curing method specified in ASTM C31 / AASHTO T 23 and to strictly follow the testing standards and procedures outlined in ASTM C172, ASTM C31, and ASTM C39.

Summary

The journey through the world of ASTM C39 and concrete compressive strength testing has been illuminating. We’ve walked through the importance of the ASTM C39 standard, the preparation of concrete specimens, the actual testing procedure, interpretation of results, quality control, and data management. We’ve also compared drilled cores and molded cylinders and discussed common issues and troubleshooting. The importance of each step in this process cannot be overstated. After all, the strength of our buildings, bridges, and other structures depends on it. The meticulousness of the ASTM C39 standard ensures that we can rely on the concrete we use, making our structures safe and resilient.

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Frequently Asked Questions

What is ASTM C 39?

ASTM C 39 is the industry standard for testing the compressive strength of cylindrical concrete specimens, including molded cylinders and drilled cores.

What is the difference between ASTM C31 and C39?

The main difference between ASTM C31 and C39 is that ASTM C31 covers the making and curing of concrete test specimens in the field, while ASTM C39 focuses on the compressive strength testing of cylindrical concrete specimens.

Why is it important to test the compressive strength of concrete?

Testing the compressive strength of concrete is important because it verifies the ability of concrete structures to withstand significant forces and sustain loads without failure, ensuring their safety and quality.

What are the main duties of a laboratory technician in ASTM C39 testing?

The main duties of a laboratory technician in ASTM C39 testing are conducting, documenting, and communicating the outcomes of the test. These responsibilities are crucial for maintaining accurate records and ensuring clear communication of test results.

What factors should be considered when choosing between drilled cores and molded cylinders for concrete compressive strength testing?

When choosing between drilled cores and molded cylinders for concrete compressive strength testing, it’s important to consider factors such as required strength, durability, corrosion resistance, size and geometry of the core, coring direction, presence and location of reinforcing bars or other materials, and presence of voids or defects.