Determining the Elastic Modulus of Concrete According to ASTM C469

Understanding how concrete responds to stress is crucial in construction. One essential property is the Modulus of Elasticity, which relates normal stress to strain within specific limits. While various codes provide formulas, accuracy can be a concern. To address this, ASTM C469 offers a reliable method for precisely determining the modulus of elasticity of concrete.

The ASTM C469 Test Method

Apparatus

Two main tools are employed in the ASTM C469 test:

Testing Machine:

A sturdy machine designed for assessing the modulus of elasticity.

Compressometer:

An apparatus attached to the concrete specimen for measuring strain accurately.

Test Specimen

Concrete specimens, either molded cylindrical or drilled core, must adhere to specific conditions:

Molded Cylindrical Specimen:

Test within an hour of removal from curing/storage.

Drilled Core Specimen:

Follow the same storage and ambient conditions as molded specimens.

Test Procedure

To ensure accurate results, follow these steps:

Steady Ambient Temperature:

Maintain a constant temperature throughout the test.

Compressive Strength Determination:

Prioritize the determination of compressive strength using two specimens.

Strain-Measuring Equipment:

Attach strain-measuring equipment securely to the specimen.

Machine Setup:

Place the specimen on the lower platen, aligning its axis with the upper bearing block.

Load Application:

Apply the load carefully, observing gauge performance. Correct any abnormal responses.

Continuous Loading:

Apply continuous load and record applied load and longitudinal strain at 50 millionths strain and 40% of the ultimate load.

Modulus of Elasticity Calculation

Using the obtained data, the modulus of elasticity (E) can be calculated using the equation:

$E = e 2 S 1$

Where:

$E$ : Modulus of elasticity (in psi)
$S 2$ : Stress corresponding to 40% of ultimate load
$S 1$ : Stress corresponding to a longitudinal strain of 50 millionths (in psi)
$e 2$ : Longitudinal strain produced by stress $S 2$

The modulus of elasticity is computed with an accuracy to the nearest 50000 psi (344.74 MPa), providing a reliable measure for concrete properties.