ASTM C39: Mastering Compressive Strength Tests on Concrete

ASTM C39 is the go-to standard for determining concrete’s compressive strength. In the following article, you’ll learn precisely how this test is conducted and how its outcomes are critical to construction quality and safety.

Key Takeaways for ASTM C39

ASTM C 39 is a standard test method for measuring the compressive strength of cylindrical concrete specimens to ensure structural integrity, compliance with specifications, and effectiveness of the concrete mix.
The test involves specific procedures from preparing concrete cylinders to applying axial load, while factors such as initial curing conditions, temperature, and air content in the concrete mix can significantly influence test results.
ASTM C 39 test results are crucial for quality control in concrete proportioning and placement operations, acceptance testing, and evaluating the effectiveness of concrete mixes in construction.

Overview: The Importance of Compressive Strength Testing

In the realm of construction, the compressive strength of concrete is a vital determinant of its ability to withstand loads and maintain structural integrity. The ASTM C39 standard is widely accepted in the industry for measuring this key parameter. This standard, which involves testing the compressive strength of cylindrical concrete specimens, forms part of a rigorous testing regime designed to verify the soundness and load-bearing ability of concrete elements.

It deals with both molded cylinders and drilled cores, and an examination requiring performance demonstration is an integral aspect of ASTM C 39 testing. The principal terminologies used in this test encompass cylindrical concrete specimens, axial load, and compressive strength. These terms respectively denote the specific samples subjected to testing, the type of load exerted, and the capacity of a material to resist axial load. The process of testing concrete cylinders involves evaluating the compressive strength of these cylindrical specimens.

ASTM C39 testing contributes to the reliability and durability of concrete structures by:

Establishing standards for quality control of concrete proportioning, mixing, and placing operations
Assessing compliance with specifications
Evaluating the effectiveness of admixtures.

ASTM C39: Purpose

Evaluating the compressive strength of cylindrical concrete specimens is the primary purpose of the ASTM C39 test. This assessment provides valuable insights into the structural integrity and load-bearing capability of concrete, which is of paramount importance in the building industry. The test ensures that the concrete satisfies the necessary strength criteria for structural purposes.

Furthermore, the precise objectives of conducting ASTM C 39 compressive strength testing on concrete extend beyond merely measuring strength. They include:

Quality control of concrete proportioning, mixing, and placing operations
Determination of compliance with specifications
Control for evaluating the effectiveness of admixtures
Similar applications

Key Terminologies and Definitions

Familiarity with specific terminologies and definitions is vital for understanding the ASTM C39 standard. The principal term used in this standard is the determination of compressive strength of cylindrical concrete specimens, which includes both molded cylinders and drilled cores. The term ‘compressive strength’, as per the ASTM C 39 standard, represents the maximum load achieved during the test, divided by the cross-sectional area of the specimen. This parameter functions as a key control factor in concrete quality.

As per ASTM C 39, concrete cylinders are cylindrical specimens of concrete used to gauge compressive strength. The term ‘axial load’ as per ASTM C 39 describes the compressive load applied to molded cylinders or cores at a specific rate until a failure occurs. This is the final concept I wanted to cover.

Performing Test: Guidelines and Procedures

The ASTM C39 test execution involves a sequence of clearly defined steps, from preparing concrete cylinders to applying compressive axial load. Each stage of the test plays a crucial role in ensuring the reliability of the results.

The test begins with the surface preparation of concrete cylinders, followed by the setup of the testing machine, and finally, the application of compressive axial load.

Preparing Concrete Cylinders

Proper preparation of concrete cylinders for the ASTM C 39 test involves the following steps:

Remove the cylinders from the moisture room while keeping them covered with wet burlap to preserve moisture.
Measure the diameter of each cylinder.
Position the cylinder on the lower plate of a compression testing system, ensuring precise alignment with the loading axis.
Thoroughly inspect the cylinders for any defects before testing.

These steps are crucial for ensuring the accuracy of the test results.

The typical dimension of such cylinders is usually 4×8 inches or 100×200 mm. The preparation of a concrete cylinder for ASTM C39 testing also involves the use of capping compounds such as sulfur mortar.

Testing Machine Setup

According to ASTM C 39, setting up the testing machine involves:

Measuring the specimen at three separate locations and recording the average calculation.
The loading rate should ideally be within the range of 0.14 to 0.34 MPa/sec (20 to 50 psi/sec).
For safe test viewing during the setup, it is recommended to use Lexan safety doors.

Calibration and periodic maintenance are also crucial to ensure the accurate functioning of the testing machines. Lastly, adherence to the safety precautions detailed in the Manual of Aggregate and Concrete Testing is paramount to prevent accidents during the testing machine setup.

Applying Compressive Axial Load

Applying compressive axial load is the central component of the ASTM C 39 test. This load is applied to the prepared concrete cylinders or cores until failure occurs, thereby assessing the specimen’s ability to withstand loads and determining its compressive strength. The testing machine and equipment used in this process include:

Steel bearing blocks, which enable uniform load distribution on the surfaces of the specimen
Hydraulic or mechanical loading systems, which apply the compressive axial load
Load cells, which measure the applied load
Displacement transducers, which measure the deformation of the specimen

These tools, software and equipment are instrumental in applying the compressive axial load and gathering information about the specimen’s behavior under load. This information can be used to assess the material’s load-bearing capacity and make informed decisions about its use in construction projects.

When applying a compressive axial load, users need to adhere to the recommended safety precautions:

Proper alignment of the specimen within the test frame
Gradual and uniform application of the load
Following the manufacturer’s safety guidelines for the testing equipment.

Factors Affecting Compressive Strength Results

Various factors influence the compressive strength test results from ASTM C 39. Primarily, the initial curing conditions, temperature range during testing, and air content in the concrete mix can significantly impact the results.

Initial Curing Conditions

The compressive strength test results can be significantly affected by initial curing conditions. Variations in temperature and humidity during the initial curing can lead to different strength outcomes. For instance, curing at 37 ˚F and 100% RH had no effect on the 28-day compressive strength while curing at 73 ˚F and 60% RH resulted in approximately a 10% loss of strength. Factors such as specimen size, shape, and friction at its ends during initial curing also play a crucial role in altering the compressive strength results.

Curing plays a fundamental role in ASTM C 39 concrete compressive strength testing by creating the ideal conditions required for the hydration process and the formation of strength in concrete samples. To manage initial curing conditions for ASTM C 39 testing, it is advisable to ensure that the concrete samples are controlled, protected, and retrieved for final laboratory curing within 48 hours.

Temperature Range: ASTM C39

The accuracy of ASTM C39 testing results is also heavily influenced by the temperature range. The recommended temperature range for conducting ASTM C39 concrete compressive strength testing is between 60 and 80°F (16 and 27°C). A deviation from this range may compromise the concrete’s compressive strength. For instance, a rise to 100°C may result in a 10-20% reduction in the original compressive strength.

The temperature upholding during ASTM C 39 testing, including adherence to the prescribed curing room conditions and ensuring conformity with ASTM C 511 standards, is an important aspect of the entire testing process. If the temperature deviates from the specified range during ASTM C 39 testing, it may lead to less-than-ideal curing conditions and possibly produce inaccurate test results.

Air Content

The air content in the concrete mix also plays a significant role in influencing the results of ASTM C39 testing. It significantly impacts the workability and durability of concrete mixtures, rendering it a crucial factor in evaluating the overall quality of concrete. In the context of ASTM C 39 testing, accurate evaluation of air content is essential for assessing compressive strength as it directly affects the behavior of concrete under load. Variability in air content can have an impact on the compressive strength of the concrete being tested, resulting in variations in ASTM C39 test results.

High air content and inaccuracies in the testing process can cause inconsistencies in data confirming the strength of the concrete specimen.

Quality Control and Compliance: Ensuring Accurate Test Results

The integrity of the ASTM C 39 test heavily relies on quality control and compliance. The requirements for concrete laboratory service technicians, performance demonstration, and the role of an independent examiner all contribute to ensuring accurate test results.

Concrete Laboratory Technician Requirements: ASTM C39

A concrete laboratory technician’s qualifications and responsibilities are crucial to the success of ASTM C39 testing. Certified technicians under the ACI Concrete Laboratory Testing Technician—Level 1 program, are qualified to perform, record, and report the results of compressive strength tests on concrete specimens. These technicians must possess an understanding of ASTM practices and test methods, including C617/C617M – 15 (Capping Cylindrical Concrete Specimens) and C1231/C1231M, which are crucial for evaluating the compressive strength of concrete cylinders.

Certification programs like the ACI Concrete Laboratory Testing Technician—Level 1 certification are available to validate a technician’s training and understanding of ASTM and AASHTO practices and tests, encompassing details related to ASTM C 39 compressive strength testing of concrete specimens.

Performance Demonstration and Independent Examiner

Performance demonstration and the role of an independent examiner are essential components of the ASTM C39 testing protocol. Assessing performance involves determining the compressive strength of cylindrical concrete specimens, encompassing molded cylinders and drilled cores. The independent examiner, who should ideally be certified equivalent to the minimum guidelines for an ACI Concrete Laboratory Technician, Level I, or ACI Concrete Strength Testing Technician, is responsible for assessing the demonstration process and ensuring that concrete proportioning, mixing, and placing operations adhere to the established procedure.

This process collectively contributes to the verification of the accuracy and reliability of the test results.

ASTM C39: Analyzing and Interpreting Test Results

A fundamental aspect of the ASTM C 39 test process is the analysis and interpretation of test results. Determining the maximum load, calculating compressive strength calculations, and identifying failure patterns are all crucial steps in analyzing and interpreting test results.

Determining Maximum Load

In the ASTM C39 test system, the maximum load is determined by dividing the peak load achieved during the test by the specimen’s cross-sectional area. The primary equipment utilized in determining the maximum load during the ASTM C39 test is a compression machine. However, the determination process can sometimes be marred by errors, such as the omission of essential information in the test report, including the cylinder’s diameter, cross-sectional area, maximum load, compressive strength, and type of fracture.

Calculating Compressive Strength

In ASTM C 39 testing, calculate compressive strength by dividing the maximum load by the specimen’s cross-sectional area, determined by averaging measured diameters or cube side dimensions. The loading rate can impact the determination of compressive strength. For instance, high loading rates surpassing ASTM C39 limits can result in a 20% increase in compressive strength of high-strength concrete.

It is essential to consider the size and shape of the cylinder to ensure accurate calculation of the compressive strength. However, it is crucial to be mindful of common mistakes, such as the failure to adhere to the standard test method, ASTM C39, for evaluating the compressive strength of cylindrical concrete specimens.

Identifying Failure Patterns

The identification of failure patterns plays a significant role in the analysis and interpretation of ASTM C39 compressive strength test results. Various failure patterns, such as cone failure, side fractures, and shearing off at corners, can offer valuable insights into specimen and test conditions that could impact the compressive strength of concrete.

The recommended approach for analyzing and interpreting failure patterns in ASTM C39 testing involves examining the various forms of failures observed in the concrete cylinders. This provides crucial insights into the specimen and test conditions, including:

Atypical crack patterns that might have emerged during testing
Friction at the testing machine platens
High sand content in the concrete
Size and geometry of the specimen

Understanding the typical reasons behind these failure patterns is essential for comprehending and addressing issues for subsequent tests.

Practical Applications of Test Results

In the construction industry, the ASTM C39 test results find multiple practical applications. They provide a quality control foundation for concrete proportioning, mixing, and placing operations, thereby ensuring compliance with design specifications and structural integrity. The results of the ASTM C39 test quantify the compressive strength of concrete, providing a standard measure for evaluating the quality and dependability of concrete in construction endeavors.

Through the application of a compressive axial load to concrete samples, the ASTM C39 test results facilitate the assessment of a concrete mix’s efficacy, thereby ensuring its ability to meet the specified strength criteria. Engineers can utilize the results of the ASTM C39 test to:

Ascertain whether the concrete meets the necessary strength for its intended application
Guarantee compliance with specification requirements
Confirm the suitability of the concrete for the intended structural application.

Concrete Proportioning

Concrete proportioning refers to the method of determining the quantities of cement, sand, coarse aggregate, and water in concrete. The compressive strength of concrete is affected by the proportioning of aggregates, specifically the compressive strength of the aggregates utilized. Furthermore, the water-cement ratio plays a crucial role in determining the strength of concrete.

The determination of the maximum load in ASTM C39 testing involves testing the cylinders in compression and plotting the test results to produce a strength versus water-cement ratio curve. This curve can then be utilized to determine and optimize the cementitious materials content in the concrete mixture in order to achieve the desired compressive strength.

The findings from the ASTM C39 test play a crucial role in the quality control of concrete proportioning and mixing, ultimately contributing to the safety and durability of concrete structures.

Acceptance Testing

In ASTM C39, acceptance testing entails applying a compressive axial load to molded cylinders or cores at a defined rate until failure. Subsequently, the compressive strength of the specimen is determined using the maximum load recorded during the test and the cross-sectional area of the specimen.

The criteria for acceptance testing for concrete as per ASTM C39 dictate that the concrete strength tests should be the average of at least two 6 x 12 in. (150 x 300 mm) cylinders. Acceptance testing plays a pivotal role in ensuring the quality of concrete. It verifies that the concrete adheres to the specifications of the purchaser and fulfills the quality assurance standards for ready mixed concrete.

The construction utilizes acceptance testing results to validate that the project meets specified requirements for client or owner acceptance. Testing under ASTM C39 standards should occur at specific intervals: two cylinders at 7, 14, and 28 days.

Evaluating Mix Effectiveness

Effectiveness assessment of a concrete mix via ASTM C39 testing takes into account factors like:

Specimen size and shape
Batching and mixing procedures
Sampling
Molding and fabrication methods
Age, temperature, and moisture conditions during curing

The ASTM C39 test plays a crucial role in evaluating the quality of a concrete mix by ascertaining the compressive strength of cylindrical concrete specimens, including molded cylinders and drilled cores.

Deviations in the ASTM C39 test results that fall below the specified strength, f’c, may indicate an unsuccessful concrete mix. Engineers can utilize the results of the ASTM C39 test to ascertain whether the concrete meets the necessary strength for its intended application and to guarantee compliance with specification requirements.

ASTM C39: Common Challenges and Solutions in Testing

Several challenges may emerge during the execution of the ASTM C39 test. These include handling molded cylinders and drilled cores, ensuring proper placing operations, and maintaining specimen integrity. Each of these challenges has potential solutions, which involve adhering to the detailed guidelines outlined in ASTM C39 for the preparation and handling of these specimens, the correct methods for placement, and the necessary measures for maintaining specimen integrity.

Handling Molded Cylinders and Drilled Cores

The process of handling molded cylinders and drilled cores demands delicate care and precision. Until steam curing is complete, subject cylinders intended for assessing adherence to 28-day strength specifications to the same curing process as the structural member. Furthermore, it is crucial to inspect the cylinders for imperfections such as holes, cracks, and crumbliness before conducting tests.

Inadequate handling can influence the integrity of molded cylinders and drilled cores in ASTM C39 testing. Adhering to the standard test method for evaluating the compressive strength of concrete specimens is crucial for obtaining precise and reliable results. When handling molded cylinders in ASTM C39 testing, it is crucial to:

Ensure that the cylinders are stored in a dry and dust-free environment
Regularly inspect them for indications of wear and tear, and replace any worn or damaged molds
Thoroughly clean the molds before and after each use to prevent contamination

Following these guidelines will help ensure accurate and consistent results in your testing.

ASTM C39: Ensuring Proper Placing of Operations

Improper placement affects C39 testing. It impacts strength due to mishandling, curing, and testing issues. This can lead to inaccurate compressive strength values.

Applying effective techniques ensures proper placing operations. It involves adhering to relevant practices such as C31/C31M. This includes specimen preparation and curing. It also involves meticulously choosing, preparing, and mixing materials. This is done in the laboratory. The aim is to ensure precise and dependable results.

ASTM C39: Maintaining Specimen Integrity

Maintaining specimen integrity is essential for guaranteeing accurate and dependable results in ASTM C39 testing. Careful mold selection, specimen handling, and proper curing procedures greatly influence test quality, ensuring specimen integrity. Each step plays a crucial role in maintaining test accuracy and reliability, highlighting the importance of attention to detail in the testing process. Human errors, mold quality, and procedural lapses can jeopardize ASTM C39 specimen integrity. Maintaining precision at every step is crucial for accurate results. Vigilance throughout the testing process is essential to ensure reliable outcomes.

The prescribed methods for specimen preservation in ASTM C39 testing involve ensuring proper batching, mixing procedures, sampling, and molding. Additionally, measurements of the specimen should be taken at three locations, with the average being calculated and recorded. Errors in handling and storage can have a significant impact on specimen integrity in ASTM C39 testing.

Summary

In conclusion, the ASTM C39 test is an invaluable tool for evaluating the compressive strength of concrete, thereby ensuring the quality and safety of construction projects. By adhering to the guidelines and procedures outlined in the standard, handling challenges effectively, and accurately interpreting the test results, engineers and construction professionals can gain vital insights into the quality of their concrete mix. Despite the complexities and challenges associated with ASTM C39 testing, its role in ensuring the durability and structural integrity of concrete structures is undeniable. The importance of the ASTM C39 test in the construction industry is a testament to the significance of rigorous testing and quality control in building safe, reliable, and long-lasting structures.

Frequently Asked Questions

What are ASTM C39 standards?

ASTM C39 standards are the industry standard for testing the compressive strength of cylindrical concrete specimens, including molded cylinders and drilled cores, and have been in place for over 80 years.

What is the difference between ASTM C31 and C39?

The main difference between ASTM C31 and C39 is that C31 is about making and curing concrete test specimens in the field, while C39 is about testing the compressive strength of cylindrical concrete specimens. These are two separate standards for different aspects of concrete testing.

What is the ASTM for cylinders?

The ASTM standard for cylinders is ASTM C39, which determines the compressive strength of cylindrical concrete specimens such as molded cylinders and drilled cores. It is limited to concrete having a unit weight over 50 lb/ft3 (800 kg/m3).

How does the ASTM C39 test work?

The ASTM C39 test works by subjecting prepared concrete cylinders or cores to a compressive axial load until failure, and then determining the compressive strength by dividing the maximum load by the specimen structure’s cross-sectional area.

What factors can influence the results of ASTM C39 testing?

The results of ASTM C39 testing can be influenced by factors such as initial curing conditions, temperature range during testing, and air content in the concrete mix. These factors should be carefully controlled to ensure accurate and reliable test results.

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