Several factors can contribute to the failure of the concrete cylinder test at 28 days. These factors may include improper casing and curing of specimens, as well as an incorrect mix of concrete used for construction. Therefore, it is crucial for engineers to identify the cause of the failure to determine the appropriate actions that need to be taken.
The first step towards identifying the cause of failure involves checking whether inappropriate casting, curing, or testing procedures of the specimen were followed. If these procedures were correct, then the fault may lie with the mix, which could have produced low-strength concrete. Engineers need to conduct tests on the structure, such as a core cutter test, to ensure that the concrete used in the building achieves the designated strength.
The strength of the concrete cylinder at 28 days is an essential indicator of whether the concrete used in the construction of a structure is strong enough. If the strength of the concrete used is low, engineers need to use suitable strengthening and rehabilitation techniques to increase the load-carrying capacity of the structure. Thus, proper testing and evaluation of concrete strength are crucial for ensuring the safety and longevity of any construction project.
1. Failure Criteria of Cylinder Specimens
ACI 318-19 specifies two criteria that must be met for concrete strength to be considered acceptable. Firstly, the average compressive strength of three consecutive cylinder specimens must be equal to or greater than the designated compressive strength. Secondly, for a specified compressive strength of 35 MPa or less, the single compressive strength should not be less than the designated compressive strength by more than 3.5 MPa. For a specified compressive strength higher than 35 MPa, the individual compressive strength should not be less than the specified compressive strength by more than 10%. These criteria must be met to ensure the strength of the concrete is adequate for its intended use.
2. Cylinder Test Failure at 28 days
To investigate the possible reasons behind specimen failure, it is important to analyze two different scenarios. The first scenario involves instances where the concrete strength is high, but the cylinder strength is weak. There are several factors that can contribute to this phenomenon, such as inadequate cylinder compaction, leaving cylinder specimens in unfavorable environmental conditions, improper alignment of molds, insufficient filling of molds, testing wrong samples, errors in testing machine, errors during sample testing, and concrete cylinders taken from the wrong mix. In order to avoid such issues, it is crucial for engineers to closely monitor all procedures of cylinder preparation and curing to ensure that the specimens properly represent the concrete of the structure.
It is also possible for both concrete and cylinder strength to be low, which constitutes the second scenario. To properly address this issue, it is important to implement a thorough sampling procedure and testing steps. One key factor to consider is the timing of testing, which should take place at the appropriate age of the concrete to ensure accurate results. Additionally, the sampling location should be carefully selected to represent the structure as a whole. It is also important to ensure that the specimens are properly cured and handled to avoid any damage or adverse effects. By taking these measures, engineers can minimize the potential for specimen failure and obtain reliable data on the strength of the concrete.
Fig. 1: Concrete Cylinder Test
2.1 When Concrete Strength is High, but Cylinder Strength is Weak
There are several factors that could result in high concrete strength but weak cylinder specimen strength. These factors include inadequate compaction of the cylinders, exposure of cylinder specimens to adverse environmental conditions such as extreme heat or cold, improper alignment of molds leading to distorted sample production, insufficient filling of molds, testing of incorrect samples, errors in testing machine, errors during sample testing, and using concrete cylinders taken from the wrong mix. To address this issue, it is essential for engineers to closely monitor all procedures involved in cylinder preparation and curing to ensure that the specimens accurately represent the concrete used in the structure. By diligently overseeing these processes, engineers can ensure that cylinder specimens reflect the true strength of the concrete in the structure.
2.2 Both Concrete and Cylinder Strength are Low
Further tests and calculations should be conducted to determine the cause of cylinder failure if it is not due to the previously discussed scenario. One possible reason could be an incorrect mix design, where the designated mixture cannot achieve the required strength. Another potential cause could be errors during the mixture preparation process, which may be caused by mechanical or software faults.
In this situation, it is important to take appropriate actions. These actions should include conducting thorough investigations to identify the root cause of the cylinder failure. Based on the results of the investigation, steps can be taken to address the problem and prevent it from happening again in the future.
2.2.1 Hammer Test
To evaluate the compressive strength of concrete, a hammer test can be conducted. However, if the strength is still low in cylinder specimens after conducting the hammer test, then it may be necessary to go for a core cutter test. The hammer test involves striking the concrete with a hammer and observing the resulting sound to estimate the strength of the material. On the other hand, the core cutter test involves drilling into the concrete to extract a cylindrical sample, which can then be tested to determine the compressive strength. If the results of the hammer test are not satisfactory, it may be necessary to resort to the more invasive and destructive core cutter test to obtain a more accurate measurement of the concrete’s strength.
2.2.2 Core Cutter Test
To evaluate the concrete strength of a structure, it is recommended to perform a core cutter test. According to the ACI 318-19 specifications, if the specified compressive strength is not met by a certain limit, then three cores should be taken for each strength test.
The core cutter test involves taking samples of concrete from the structure using a special tool called a core cutter. These samples are then tested for their compressive strength. To ensure accuracy, it is recommended to take three core samples for each strength test.
If the average strength of the three core samples is at least 85% of the designated compressive strength, and no individual core sample strength is lower than 75% of the specified compressive strength, then the concrete area represented by the core samples is considered structurally sufficient according to ACI 318-19.
However, if the concrete fails to meet the acceptance limit of the core cutter test, then it is necessary to consider a proper strengthening technique. This means that the concrete in the structure may not be strong enough to withstand the intended load, and measures must be taken to reinforce it.
3. Rehabilitation Methods
When it comes to increasing the load-carrying capacity of structural members in order to achieve the desired strength, various rehabilitation methods can be utilized. However, it is important to thoroughly examine each strengthening technique to determine its suitability and consider various aspects before selecting the most suitable, economic, and effective method for the specific structure being considered.
Some of the rehabilitation methods that can be used to increase load-carrying capacity include fiber-reinforced polymer sheet bonding, concrete jacketing, external steel bonding, near-surface mounted FRP, steel casing methods, shotcreting (spray concrete), and attaching pre-tensioned cables to the concrete members. Each method has its own unique characteristics and advantages, so it is important to evaluate each option in order to select the most appropriate technique for the specific rehabilitation project.
FAQs
1. What is a concrete cylinder test?
Ans: A concrete cylinder test, also known as a concrete compression test or a concrete strength test, is a standard method used to determine the compressive strength of concrete.
During the test, a cylindrical concrete sample is cast and cured under controlled conditions, typically for 28 days. After the curing period, the concrete cylinder is placed in a testing machine that applies a compressive force to the sample until it fractures. The maximum force that the sample can withstand before failure is recorded as the compressive strength of the concrete.
This test is essential in construction projects as it helps to ensure that the concrete used in the construction meets the minimum strength requirements necessary to withstand the expected loads and stresses during its service life. The compressive strength of concrete is also an important factor in determining the durability and overall quality of the concrete.
2. How do you test a concrete cylinder sample?
Here is a detailed step-by-step procedure on how to test a concrete cylinder sample:
- Remove the concrete cylinder from the curing tank and wipe off any excess water from its surface.
- Place the cylinder vertically on the compression testing machine’s baseplate, ensuring that it is centered and level.
- Place the pad caps on both ends of the cylinder to ensure uniform load application and distribution.
- Adjust the compression testing machine’s platen so that it is in contact with the top pad cap.
- Apply a gradually increasing compressive load on the cylinder until it fails.
- Record the maximum load that the cylinder can withstand before failure.
- Calculate the compressive strength of the concrete by dividing the maximum load by the cross-sectional area of the cylinder.
It is important to note that the cylinder must be tested at the appropriate age, typically at 28 days of curing. The testing machine must also be calibrated to ensure accurate results. The testing process must be conducted by trained personnel following the appropriate safety protocols.
3. What ASTM is concrete cylinder testing?
ASTM C39 is the standard test method for compressive strength of cylindrical concrete specimens. This test method has been widely used in the industry for more than 80 years and covers the determination of the compressive strength of molded cylindrical concrete specimens and drilled cores.
ASTM C39 provides specific guidelines for preparing and curing the concrete specimens, as well as for conducting the compression test. The standard also includes requirements for the testing machine, including the load capacity and accuracy, and the testing procedure, including the rate of loading and the number of specimens to be tested.
By following the guidelines set forth in ASTM C39, test results can be compared across different laboratories and projects, ensuring consistency and accuracy in the testing process.
Why do we test concrete cylinder at 28 days?
Concrete is a material that becomes stronger over time after it has been cast. However, it can take a significant amount of time for concrete to reach its maximum strength, and the exact time required is still unknown. Typically, the rate at which concrete gains compressive strength is highest during the first 28 days after casting, after which it gradually slows down. This phenomenon is well-documented and understood by professionals in the construction industry.
Does concrete gain strength after 28 days?
The strength of concrete is known to increase gradually after its initial setting. However, it is typically after 28 days that concrete gains strength at a noticeably slower rate. This means that although the strength of concrete continues to increase beyond 28 days, the rate of increase is not as significant as during the initial period.
The slow rate of strength gain after 28 days is an important consideration in the construction industry. It means that structures made of concrete may not reach their full strength potential until several weeks after they are built. Therefore, it is crucial to allow sufficient time for the concrete to cure and gain strength before subjecting it to any significant loads or stresses.
Various factors can affect the rate at which concrete gains strength, such as the mix design, curing conditions, and the type of cement used. Understanding these factors can help engineers and builders optimize the strength and durability of concrete structures. Additionally, testing the strength of concrete at different intervals can provide valuable insights into its properties and inform decisions about when it is safe to begin using the structure.
How do you test a concrete cylinder?
The process of determining the compressive strength of concrete cylinders involves applying a continuous load to the cylinder until it fails. This test is carried out using a compression-testing machine. If the cylinder is able to withstand the load without fracturing, the load is gradually increased until it ultimately breaks. The compressive strength of the cylinder is then calculated based on the amount of load it was able to withstand before failing. This test is an important way to evaluate the quality and durability of concrete structures.
What are the criteria for concrete acceptance according to ACI 318-19?
The ACI 440.2R-08 specifies that the average compressive strength of three consecutive cylinder specimens should be equal to or greater than the designated compressive strength. Additionally, for a specified compressive strength of 35MPa or less, a single compressive strength should not be less than the designated compressive strength by more than 3.5MPa. For a specified compressive strength higher than 35MPa, the individual compressive strength should not be less than the specified compressive strength by more than 10%. These guidelines are relevant for the design of FRP axial strengthening of RCC columns, CFRP laminates for shear strengthening of RC beams, as well as repairs and rehabilitation of concrete structures for failure and defects.