2.4. Measurement of Shrinkage

Standard test specimens (e.g., 75mm × 75mm × 285mm prisms) are used.

The measurement of shrinkage in concrete is performed using specific standardized specimens. The standard size for these specimens is typically 75mm × 75mm × 285mm prisms, which are shaped for uniformity during testing. This consistency ensures reliable and comparable results across different tests and experiments.

Think of these specimens as a specific type of cookie cutter. Just like using a standard cookie cutter ensures all cookies are the same shape and size, using standardized specimens in concrete testing ensures all the results are consistent and comparable.

Shrinkage strain is measured using:

• Length comparator
• Mechanical or digital dial gauges

To measure shrinkage strain accurately, specific tools are utilized. A length comparator is used to detect changes in length due to shrinkage, while mechanical or digital dial gauges provide precise readings of these changes. Together, these tools allow for a detailed understanding of how much the concrete has shrunk over time.

If you've ever used a ruler to measure the length of something, think of the length comparator as a more advanced version of that ruler, designed to detect much smaller changes. The dial gauges are like a digital thermometer, giving you an exact reading rather than just an approximation.

The shrinkage is expressed as microstrain (µε): Shrinkage Strain = ΔL / L × 10^6 (με)

Shrinkage strain is quantified in microstrain, which measures very small changes in length relative to the original length. To calculate this, you take the change in length (ΔL) due to shrinkage and divide it by the original length (L), then multiply by one million (10^6) to express it in microstrain. This method emphasizes the slight but significant shrinkage that occurs in concrete over time.

Imagine stretching a rubber band and then letting it go. The tiny amount it returns to its original length can be viewed similarly to shrinkage in concrete. Just as we might measure the rubber band's change with a ruler, we use the formula for shrinkage strain to get an accurate reading of how much concrete has shrunk over time.

Typical drying shrinkage for ordinary concrete:

• 400 to 800 µε at 1 year

On average, ordinary concrete will experience drying shrinkage strains ranging from 400 to 800 microstrain after one year of curing and exposure. These values give engineers and builders a benchmark to anticipate how much shrinkage they might expect in their concrete structures, helping them to design appropriately.

Consider a sponge that shrinks slightly as it dries out after being soaked in water. Just like we can expect the sponge to become smaller over time with less water, concrete too shrinks as it loses moisture; the values given help us understand how significant this reduction may be.

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