The modulus of elasticity of concrete refers to the ratio of normal stress to the corresponding strain, and this applies to both compressive and tensile stresses below the proportional limit of concrete, as per the guidelines set by ACI 318-14. However, it is important to note that the values of elastic modulus of concrete obtained through the use of formulas provided by various codes may not always be accurate.
To obtain a more precise value of the elastic modulus of concrete, various tests can be conducted. One such method is through the use of ASTM C469, which is a standard test method that can accurately determine the static modulus of elasticity and Poisson’s ratio of concrete when subjected to compression. This test can provide a more accurate estimate of the elastic modulus of concrete, which is essential in various engineering applications.
ASTM C469 Test Method
Apparatus
- Testing Machine, Fig. 1.
- Compressometer, Fig. 2.
Test Specimen
It is recommended to conduct the testing of moulded cylindrical specimens within an hour of their removal from the curing or storage room. This means that the testing process should be initiated promptly to obtain accurate results.
Alternatively, the modulus of elasticity can be determined using a drilled core specimen. However, it is essential to adhere to the same storage and ambient conditions for the drilled core specimen as for the moulded cylindrical specimens immediately prior to the test. This ensures that the test results are reliable and consistent.
Overall, it is important to prioritize prompt testing and consistent storage conditions to obtain accurate results for both moulded cylindrical and drilled core specimens.
Test Procedur
To ensure accurate results, it is important to maintain a consistent ambient temperature throughout the testing process. Additionally, two specimens should be used to determine the compressive strength before proceeding with the modulus of elasticity test.
Next, the strain-measuring equipment must be properly attached to the specimen. Once this is done, the specimen should be placed onto the lower platen or bearing block of the testing machine. It is crucial to align the axis of the specimen with the center of thrust of the spherically-seated upper bearing block.
The specimen should be loaded carefully to seat the gauge, and any abnormal responses should be promptly corrected. Once the gauge is properly seated, the load should be applied continuously. During this process, it is important to record the applied load and longitudinal strain when the longitudinal strain reaches 50 millionth and the applied load is at 40% of the ultimate load.
Calculation of Modulus of Elasticity
The modulus of elasticity of concrete is an important property that relates the stress applied to it with the resulting strain produced. This relationship can be described by the equation provided. The equation includes several variables, such as E, S1, S2, and e2.
The modulus of elasticity, denoted by E, is measured in psi (pounds per square inch). In order to compute E, we need to know the values of S1, S2, and e2. S1 represents the stress corresponding to a longitudinal strain of 50 millionths, measured in psi. S2, on the other hand, corresponds to a stress that produces 40% of the ultimate load that the concrete can withstand. Finally, e2 represents the longitudinal strain produced by the stress S2.
When computing the modulus of elasticity of concrete using this equation, we need to take into account that the value obtained will be rounded to the nearest 50000 psi, which is equivalent to 344.74 MPa. This rounding is likely done to simplify the calculations and make the results easier to interpret.