The penetration resistance test is a commonly used method for determining the initial and final setting times of a concrete mixture. This test is carried out in both laboratory and project sites, and it involves measuring the time at which the cement begins to harden and lose its plasticity, as well as the time at which the concrete loses its plasticity completely and hardens. To conduct this test, a sample of sieved mortar is used, and variables such as water content, type and amount of cementitious material, and admixtures can affect the time of setting of the concrete.
According to ASTM C403M-16, the initial and final setting times of concrete can be determined when the penetration resistance reaches 3.5 MPa and 27.6 MPa, respectively. However, there are several factors that can influence the setting times of concrete. For example, the fineness of the cement, the presence of salts in the sand, and atmospheric conditions can all have an impact on the setting times.
The temperature is also an important factor that affects the setting times of cement. Cement requires a temperature of 27°C to complete its hydration process. During winters, the temperature is low, and as a result, the hydration process takes longer. Therefore, it is essential to consider these factors when determining the setting times of concrete, as they can significantly impact the final product.
Apparatus
1. Container
The provided requirement specifies that the lateral dimension of the object must be a minimum of 150 mm, and the height must also be at least 150 mm. This means that the object must have a base that measures at least 150 mm in both directions, and it must also stand at a minimum height of 150 mm. It is important to adhere to this requirement to ensure that the object meets the specified size and shape parameters. By meeting these requirements, the object can effectively fulfill its intended purpose and function as desired. Therefore, it is crucial to carefully consider these specifications during the design and construction process.
2. Penetration Needles
Loading apparatus often employs penetration needles, which come with different sizes of bearing areas. These needles are typically used to test the mechanical properties of materials by penetrating them to a certain depth. The bearing area of a needle refers to the surface area of the tip that makes contact with the material being tested.
In this particular case, the penetration needles attached to the loading apparatus come in six different sizes of bearing areas: 645, 323, 161, 65, 32, and 16 mm2. The size of the bearing area is an important consideration when selecting a penetration needle for a particular testing application.
The larger the bearing area of a needle, the more force it can exert on the material being tested without causing damage. Conversely, a smaller bearing area allows for greater precision in measuring the material’s mechanical properties.
The choice of needle size also depends on the type of material being tested and the intended use of the test results. For example, a large bearing area may be suitable for testing the strength of a metal alloy, while a smaller bearing area may be more appropriate for testing the hardness of a ceramic material.
3. Loading Apparatus
A device is required to measure the force needed to cause penetration of needles. The accuracy of this device must be within 10 N and it should be able to measure penetration force with a capacity of at least 600 N. In other words, the device needs to accurately and precisely measure the force needed to penetrate needles with a high degree of accuracy and have a large enough capacity to handle a wide range of needle types and sizes. This device will be an important tool for researchers and manufacturers who need to evaluate and test the quality and performance of different needles for various applications.
4. Tamping Rod
The object being described is a steel rod that has a round shape and a straight structure. Its diameter measures 16 mm while its length is approximately 600 mm.
5. Pipet
The given context mentions a specific use case where a tool is employed to remove excess water from the surface of a test specimen. To further elaborate on this, it is common practice in various industries such as construction and material testing to conduct experiments on samples to analyze their properties and characteristics. One critical aspect of such testing is to ensure that the test specimen is in an optimal state for accurate observations.
However, during the preparation of the test specimen or the testing process itself, water can accumulate on its surface. This is known as bleed water and can affect the accuracy of the results obtained from the test. Therefore, to address this issue, a tool has been developed specifically for the purpose of drawing off bleed water from the surface of the test specimen.
The use of this tool is particularly important because manually removing excess water can be time-consuming and can also lead to errors in the testing process. By using this specialized tool, the process of removing bleed water can be done more efficiently and effectively, resulting in more accurate test results.
6. Thermometer
A thermometer must have the ability to measure the temperature of fresh mortar up to 60.5°C.
7. Stop Watch
Sample Preparations
To conduct tests under field conditions, it is necessary to obtain a representative sample of fresh concrete from which three specimens will be prepared for the test. On the other hand, for tests under laboratory conditions, the concrete must be made, and its slump and air content should be specified. The concrete that has passed through a 4.75-mm sieve should be used for preparing the specimens.
To test the compliance of a material with performance requirements, it is necessary to make at least three separate concrete batches for each variable under investigation. Additionally, one-time setting tests should be performed on each batch. However, for other tests, three test specimens should be prepared from one batch of concrete for each test variable.
It is important to record the time at which the initial contact was made between cement and mixing water. The mortar should be thoroughly mixed using hand methods on a non-absorptive surface, and the temperature of the mortar should be measured and recorded.
Once the mortar is ready, it should be placed in a container, or containers, in a single layer. It is essential to remove any air pockets in the specimen by rocking the container, tapping the sides of the container with the tamping rod, rodding the mortar, or placing the container on a vibrating table. Finally, the top surface should be leveled.
Procedures of Concrete Setting Time Test
To eliminate any bleed water from the surface of the specimen, use a pipet. The appropriate size of the needle should be selected based on the degree of mortar setting time. The selected needle should then be inserted into the penetration resistance apparatus. Make sure that the bearing surface of the needle comes into contact with the surface of the mortar. Gradually and uniformly apply a vertical force downward on the apparatus until the needle penetrates the mortar to a depth of 25±2 mm. Take note of the force needed to penetrate the needle to this specified depth, and also record the time of load application. This time is measured as the elapsed time after the initial contact of cement and water. To calculate the penetration resistance, divide the recorded force by the bearing area of the needle.
Precautions For Subsequent Tests
When conducting subsequent penetration tests on concrete samples, it is important to take precautions and avoid areas where the mortar has been disturbed by previous tests. To ensure accurate results, the minimum distance between needle impressions should be 15 mm. Additionally, there should be a clear distance of at least 25 mm and no more than 50 mm between any needle impression and the side of the container.
For conventional concrete mixtures tested at laboratory temperatures of 20 to 25 °C, the initial test should be conducted after 3 to 4 hours have elapsed since the initial contact between cement and water. Subsequent tests should be conducted at intervals of 1 to 2 hours. When testing concrete mixtures containing accelerators, the initial test should be conducted after 1 to 2 hours, with subsequent tests at intervals of 1 to 2 hours. For concrete mixtures containing retarders, the initial test may be postponed until 4 to 6 hours have elapsed.
To ensure a satisfactory curve of penetration resistance versus elapsed time, at least six penetrations should be made for each time-of-setting test. The time intervals between these penetrations should be of sufficient duration to provide accurate results. Testing should continue until at least one penetration resistance reading equals or exceeds 27.6 MPa.
Graphing Test Results
To create a graph of penetration resistance versus elapsed time, we will plot penetration resistance on the y-axis and elapsed time on the x-axis. The scale of the graph will be such that 3.5 MPa and 1 h will each be represented by a distance of at least 15 mm. This will ensure that the graph is visually clear and easy to read.
Once the graph is created, we can visually determine the times of initial and final setting by looking at the points where the penetration resistance equals 3.5 MPa and 27.6 MPa, respectively. These points will represent the times at which the material starts to harden (initial setting) and reaches its full hardness (final setting). By analyzing the graph, we can determine how quickly the material sets and how long it takes to reach its full strength.
Overall, the plot of penetration resistance versus elapsed time is an important tool for understanding the setting characteristics of materials. By creating a clear and accurate graph and analyzing it visually, we can gain valuable insights into the properties of different materials and how they perform over time.
Sources of Errors
When conducting a data analysis, it is crucial to ensure that the results accurately represent the sample being studied. However, certain factors may lead to results that are not truly representative. In such cases, the operator must use their judgement to identify any points that should be excluded from the data analysis.
One factor that can affect the accuracy of results is the interference of larger particles in the mortar being studied. Additionally, the presence of large voids within the penetration zone can also impact the accuracy of results. These issues must be identified and taken into consideration during the data analysis process.
Another potential problem that can arise is interference from impressions generated by adjacent penetrations. This can lead to errors in the results, which can be problematic if not identified and addressed.
It is also important to ensure that the instrument used during the study is maintained perpendicular to the test surface during penetration. Failure to do so can result in inaccurate results that do not truly represent the sample being studied.
Finally, errors in reading the load variations in rate of loading can also impact the accuracy of results. It is essential to be aware of this possibility and to take measures to minimize the risk of such errors occurring during the data analysis process. Overall, identifying and addressing these potential issues is crucial for obtaining accurate and meaningful results from any study.