The compressive strength test results of concrete cylinders play a crucial role in determining the acceptance of ready mix concrete for construction purposes. These results provide important information about the quality and strength of the concrete produced. During the compression testing of concrete cylinders, if low-strength breaks occur, it indicates that the compressive strength of the concrete is below the desired level. This can be a cause for concern as it suggests that the produced concrete may not have the required strength for its intended application. Therefore, accurate and reliable compressive strength test results are essential for ensuring the quality and durability of concrete used in construction projects.
Compression Test of Concrete Cylinder
Image Credits: Concrete Construction
Concrete cylinders used for standard compressive tests are created from fresh concrete samples that are cured under standard conditions and tested at a specific age, typically 28 days. However, there are instances where these cylinders may exhibit low-strength breaks, which can be attributed to various reasons such as excess water and air content, or unforeseen errors during sample preparation. In this article, we will delve into the ACI reference on low-strength cylinder test results and the major causes behind them.
Low-Strength Results of Concrete Cylinder- ACI 318 & ACI 301
According to the ACI Building Code ACI 318 and the Standard Specifications for Structural Concrete ACI 301, it is possible for a concrete mix to have low-strength results during compression testing due to normal variability. In fact, such results may occur about once or twice in every 100 tests, even if the mix is proportioned to meet the standards’ requirements.
The ACI and CSA have established criteria for determining when a cylinder break during a compression test indicates low strength. Specifically, if the individual compressive strength test of one or two cylinders falls more than 500 psi below the specified strength, or if the average of three consecutive tests is lower than the specified strength, the concrete is considered to have low strength.
Causes of Low-Strength Concrete Breaks
The six key causes of low-strength concrete cylinder breaks are:
1. Poor Concrete Mix Design
If the wrong mix of concrete is prepared, it can have a negative impact on the strength gain of the material within the designated time frame. As a consequence, the compressive strength test may yield a lower value than anticipated. This delay in strength gain can be problematic, as it can lead to issues with the overall structural integrity of the project. Therefore, it is crucial to ensure that the correct concrete mix is used during the construction process to avoid such complications. Proper attention to detail and following the recommended guidelines can help ensure that the concrete is mixed accurately and meets the necessary strength requirements.
2. Incorrect Production of Test Sample
When concrete cylinders are cast improperly, the resulting test specimen is not suitable for compression testing. The improper casting process may have introduced defects or inconsistencies that could affect the strength and integrity of the specimen. This can compromise the accuracy and reliability of the compression test results, which are crucial for determining the quality and durability of the concrete. Therefore, it is essential to ensure that concrete cylinders are cast properly to obtain reliable and meaningful compression test results.
3. Improper Handling
Concrete test samples are prone to cracking if they are not handled and transported properly. Improper handling and transportation of these samples can cause significant damage and compromise their structural integrity. As a result, it is essential to ensure that these samples are treated with the utmost care and caution during their handling and transportation. Any negligence or mishandling of the samples could lead to cracks in the concrete, which would render the test results inaccurate and unreliable. Therefore, it is critical to follow the appropriate protocols and guidelines for the handling and transportation of concrete test samples to prevent any potential damage or cracking.
4. Improper Curing Conditions
The strength values of in-situ concrete samples cannot be accurately reflected by field-cured test specimens. Failing to address this issue will result in the strength value obtained through sample testing not accurately representing the strength of the actual concrete structure. Therefore, it is crucial to correct this discrepancy to ensure that the strength value obtained from sample testing accurately reflects the strength of the concrete in the actual structure.
5. Improper Specimen Preparation
The preparedness of the test specimen is crucial for it to break properly under the applied test load. If the cured specimen’s ends are not properly ground or capped, it may not break as intended. This suggests that the process of preparing the specimen is just as important as the test load itself. Any misstep in the preparation process could lead to inaccurate results or render the test useless. Therefore, it is essential to ensure that the specimen is adequately prepared before subjecting it to any test load.
6. Calculation Errors
To ensure accurate results in a compression test, it is necessary to properly calibrate the compression testing machine beforehand. This is important as low-strength concrete breaks can occur if the machine is not calibrated correctly. Additionally, if the sample being tested shows low strength, it is recommended to wait for the concrete to completely cure before conducting another test on the next break day. If the sample still exhibits low strength, it is necessary to investigate and address any potential causes of the problem.
Recently, wireless maturity sensors have become increasingly popular for monitoring the strength gain of concrete samples that have failed the compression test. These sensors allow for the continuous monitoring of concrete strength over time, particularly for ditched cylinders that have failed initial tests. For acceptance purposes, standard-cured cylinders are typically made and tested at 28 days, and the strength of concrete with age is determined through the use of maturity sensors. Overall, these technological advancements have significantly improved the monitoring and testing of concrete strength, leading to more accurate and reliable results.
FAQs
What are low-strength cylinder breaks?
During compression testing, if the concrete cylinder experiences low-strength breaks, it indicates that the produced concrete has low compressive strength. This means that the concrete may not be able to withstand significant loads or forces, and may not be suitable for use in applications that require high strength and durability. It is important to identify low-strength breaks during compression testing in order to ensure that the concrete being used meets the required specifications and can perform its intended function safely and effectively. By detecting and addressing low compressive strength early on, it is possible to avoid potential failures or damage in the future.
What are the causes of low-strength cylinder breaks?
Low-strength breaks in concrete cylinders can happen for various reasons. These reasons may include excess water and air content, as well as errors that occur during sample preparations. Despite the best efforts of those involved, unforeseen issues can still arise that impact the strength of the concrete samples. When these issues occur, it can result in weaker breaks than expected, which can be a concern for those involved in construction projects. Understanding the causes of low-strength breaks is essential for preventing them in the future and ensuring that concrete samples are as strong as possible.
How do curing conditions influence low-strength cylinder breaks?
When concrete samples are left to cure in the field, the resulting strength values may not accurately reflect those of the concrete structure they are meant to represent. This can be problematic, as it can lead to lower-than-expected strength readings when testing concrete samples taken from the structure. In order to avoid this issue, it is important to correct for any discrepancies between field-cured specimens and in-situ concrete samples, so that accurate strength values can be obtained through cylinder break testing.