The modulus of elasticity of concrete is a measure of the relationship between the stress applied to the material and the corresponding strain that it undergoes. To determine this value accurately, a compression test is conducted on a cylindrical concrete specimen in a laboratory setting. During this test, the deformation of the specimen is analyzed as it is subjected to different loads, and the resulting data is used to construct a Stress-Strain graph, also known as a load-deflection graph.
By analyzing the Stress-Strain graph, the modulus of elasticity of the concrete can be determined. This is done by drawing a line on the graph from a stress value of zero to the compressive stress value of 0.45f’c, which is the working stress. The slope of this line gives the modulus of elasticity of the concrete.
The compression test is an important tool for accurately determining the modulus of elasticity of concrete. It provides valuable data that can be used to better understand the behavior of this material under various types of stress and strain. By conducting this test, engineers and scientists can develop more accurate models of how concrete behaves in different applications, which can help to improve the safety and reliability of structures that use this material.
Procedure
The test procedure can be divided into two stages. The first stage involves setting up the compressometer. This is an important step that needs to be carried out correctly to ensure accurate and reliable test results. Once the compressometer is set up, the second stage of the test procedure can be carried out.
During the second stage, a load is applied to the compressometer and testing is performed. This stage of the test is crucial in obtaining data that can be analyzed to determine the properties and characteristics of the material being tested. It is important to ensure that the testing is conducted according to established standards and procedures to ensure the validity and reliability of the results.
Overall, the test procedure involves careful and precise execution of both stages to obtain accurate and reliable data. The success of the testing procedure is dependent on the skill and attention to detail of the person carrying out the test, as well as the quality of the equipment used.
Setting Up Compressometer
The compressometer is an apparatus that is utilized in the compression test of a concrete cylinder to determine its characteristics related to strain and deformation. This device is composed of two frames, a top and bottom frame, which are initially assembled using spacers that hold them in position.
The assembly process involves placing the pivot rod on screws that are locked in position. The tightening screws of the top and bottom frames are left in a loose condition during this procedure.
Once the compressometer is set up, it is placed on the concrete specimen, which is located on a flat surface. The compressometer is centrally positioned on the specimen for accuracy. After positioning, the screws are tightened to secure the compressometer to the specimen.
Finally, the spacers that were used during the assembly process can be removed by unscrewing them. This completes the setup of the compressometer for the compression test of the concrete cylinder.
Testing the Specimen
The test procedure for compressive strength involves placing the specimen with the compressometer set up over the compression testing machine platform and centering it properly. The load application is then performed continuously at a rate of 140 kg/cm²/minute without any obstruction. The load application is continued until a stress value equal to (c+5)kg/cm² is attained, where c is the 1/3rd of the average compressive strength of the cube. This strength value of the cube is calculated to the nearest of 5kg /cm², which is a load of 12.4T.
Once the stress value of (c+5)kg/cm² is reached, it is maintained for a period of 60 seconds and then reduced to the stress of 1.5 kg/cm², which is a load value of 0.3T. The load is then further increased until the stress of (c+1.5) kg/cm² is reached, which is a load of 11.8T. At this point, the compressometer reading is recorded.
The load is then gradually reduced, and the readings are recorded at 1T intervals, starting from 11.8T and going down to 0.3T, in increments of 1T. This process is repeated for a third time, where the load is applied again, and the compressometer readings are recorded at an interval of 1T, starting from 10.8T and going down to 0.3T.
Read More: Compressive Strength of Concrete Cylinders
Load-Deflection Graph
A load deflection graph has been created based on the loading conditions that were observed. To further analyze this graph, tangents have been drawn at two specific points. The first tangent is drawn at the beginning of the graph, while the second tangent is drawn at the point where the value equals the working stress of the concrete mix.
Once the two tangents have been drawn, they are used to form a line. This line is created by joining the two points where the tangents intersect with the graph. The purpose of this line is to provide additional insight into the behavior of the load deflection graph. By analyzing the angle and slope of the line, it is possible to gain a deeper understanding of the properties and performance of the concrete under the given loading conditions.
Calculation and Results
Calculation
The slope of the initial tangent represents the initial tangent modulus, which is equal to the ratio of stress and strain at the beginning of the deformation process.
On the other hand, the slope of the tangent at the working stress point represents the tangent modulus at the working stress level, which is also equal to the ratio of stress and strain at that point in the deformation process.
Finally, the slope of the line joining the initial tangent point and the point of working stress represents the secant modulus, which is also equal to the ratio of stress and strain but over a range of deformation between the initial point and the working stress point.
Test Report
A report needs to be prepared, and it must include specific information such as the identification mark, the date of the test, the age of the specimen, and the shape and nominal dimensions of the specimen. The identification mark is a unique identifier that helps to distinguish the specimen from others. The date of the test is essential because it provides a reference point for when the specimen was examined. The age of the specimen refers to how long it has been since it was collected, and this information can be useful in determining its properties and behavior. Finally, the shape and nominal dimensions of the specimen must be included to provide a clear description of its physical characteristics. This information is critical in ensuring that the specimen is properly identified and that accurate conclusions can be drawn from the test results.
Result
The given information provides three different modulus of elasticity values for a specific type of concrete. The initial tangent modulus of the concrete is not specified in the context. The tangent modulus at working stress and the secant modulus, also known as the modulus of elasticity, are provided in the context.
The tangent modulus at working stress refers to the slope of the stress-strain curve at a specific point where the concrete is experiencing stress during its intended use. This value is given in units of N/mm², which represents the stress per unit area that the concrete can withstand without experiencing permanent deformation.
The secant modulus, on the other hand, is an average modulus of elasticity value that is calculated by measuring the stress and strain at two points on the stress-strain curve. This value is given in units of N/mm² and represents the stiffness of the concrete material.
The initial tangent modulus of the concrete, which is not given in the context, refers to the slope of the stress-strain curve at the point where the concrete material is first subjected to stress. This value is typically higher than the tangent modulus at working stress and can be used to estimate the deformation of the concrete at the early stages of loading.
Overall, the given information provides useful parameters for analyzing the behavior of a specific type of concrete under different types of loading conditions.
Precautions
To ensure accurate results, it is essential to maintain a continuous reading when conducting tests. Any interruption in the reading process can lead to inconsistencies in the data, which can compromise the validity of the results. Therefore, it is necessary to take readings without any delay.
Moreover, if the readings obtained from different trials show a significant difference of more than 5%, then it is imperative to repeat the test. This criterion is crucial to ensure that the results obtained are reliable and consistent. If the test is not repeated, there is a high probability that the results will be inaccurate, which can have significant implications, especially in scientific research and experimental studies. Therefore, it is necessary to maintain strict quality control measures when conducting tests to ensure that the results are valid and reliable.