2. Shrinkage of Concrete

Shrinkage is the time-dependent volume reduction of concrete occurring without the application of external load. It mainly results from loss of moisture or chemical reactions in the paste. Shrinkage is an unrestrained contraction, but in real structures, restraints lead to tensile stresses, which may cause cracking.

Shrinkage refers to the gradual reduction in size of concrete over time, which occurs without any external forces being applied. This phenomenon mainly happens due to the evaporation of water and chemical reactions within the concrete's composition. When concrete shrinks, it contracts evenly if unrestrained, but in practice, when there are constraints (like walls or other structural elements), it can create internal stresses. These stresses can exceed the concrete’s tensile strength, leading to cracks.

Consider a sponge that dries out. When the sponge is wet, it is big and full; however, as the water evaporates, it shrinks and can even crack if it is pressed into a container that restricts its movement. Similarly, concrete shrinks as it loses moisture, and if there are barriers around it, it might crack instead of just shrinking uniformly.

1. Plastic Shrinkage
   • Occurs within few hours after placing concrete.
   • Caused by rapid evaporation of surface water.
   • Results in cracking due to differential shrinkage.
   • Preventable by curing, windbreaks, fogging.
2. Drying Shrinkage
   • Occurs when hardened concrete loses water from capillary pores to the environment.
   • Most significant type.
   • Can continue for months to years.
   • Heavily influenced by humidity, aggregate type, and curing.
3. Autogenous Shrinkage
   • Volume reduction due to chemical reactions (hydration) in low water-cement ratio concretes.
   • Prominent in high-strength concrete (w/c < 0.4).
   • Can cause early-age cracking.
4. Carbonation Shrinkage
   • Caused by reaction of CO₂ with calcium hydroxide in concrete to form calcium carbonate.
   • Leads to minor long-term shrinkage.
   • Prominent in exposed concrete surfaces.

Shrinkage can be classified into several types based on the conditions under which it occurs. Plastic shrinkage happens quickly after concrete placement, usually due to fast water evaporation, while drying shrinkage occurs over a prolonged period as the concrete releases moisture to the environment. Autogenous shrinkage happens even in sealed conditions, particularly in mixes with low water-cement ratios. Finally, carbonation shrinkage relates to chemical reactions that take place between carbon dioxide in the air and components of concrete, leading to minor volume changes.

Think about putting a wet towel in the sun. Initially, it is thick and heavy (like freshly placed concrete). After a short period, the surface water evaporates quickly (plastic shrinkage), and then as time passes, the towel continues to lose moisture and becomes thin (drying shrinkage). If we take a look at a towel that's made from a different material that shrinks only when wet (autogenous shrinkage), we notice that it can shrink even when enclosed. Lastly, if we were to leave a towel out in an area with a lot of smoke or pollution (carbonation), it might undergo changes and shrink due to those reactions.

• Water-Cement Ratio: Higher w/c leads to more drying shrinkage. • Aggregate Volume: More aggregates reduce shrinkage by restraining the paste. • Curing Duration: Inadequate curing leads to higher shrinkage. • Humidity: Low relative humidity (below 50%) significantly increases drying shrinkage. • Size and Shape: Slabs and thin sections have more surface area, hence higher shrinkage. • Type of Cement: Rapid-hardening cements can increase early-age shrinkage.

Various factors influence the shrinkage of concrete. One of the most significant is the water-cement ratio; more water in the mix can lead to greater amounts of drying shrinkage. The amount of aggregate in the concrete also matters, as a higher volume can help limit shrinkage by physically restraining the concrete paste. The duration of curing plays a critical role: if concrete isn't cured properly, it will shrink more. Environmental factors like humidity also have an impact; lower humidity usually accelerates drying shrinkage. Additionally, the size and shape of concrete elements affect shrinkage; thinner structures typically exhibit more shrinkage due to higher surface area exposure. Lastly, the type of cement used, especially rapid-hardening varieties, can influence how quickly concrete shrinks.

Imagine a thick, wet sponge held down by some weights (high aggregate volume) versus a thin sponge lying flat. If both were left in a dry room, the thin sponge would dry out and shrink much faster than the thicker one. If it wasn’t allowed adequate time to dry evenly (like improper curing), or was in a low-humidity space (like below 50%), it would shrink unevenly and possibly crack. So, the sponge’s weight and how it's sealed play a big role in how it changes.

Standard test specimens (e.g., 75mm × 75mm × 285mm prisms) are used. Shrinkage strain is measured using: • Length comparator • Mechanical or digital dial gauges The shrinkage is expressed as microstrain (µε):

ΔL Shrinkage Strain= ×106 (με) L Typical drying shrinkage for ordinary concrete: • 400 to 800 µε at 1 year.

To quantify the shrinkage of concrete, standard specimens are prepared, typically shaped like small prisms. The shrinkage strain is measured using specialized equipment like length comparators and dial gauges. The results of these measurements are expressed in units called microstrain, where a small length change is scaled up for clarity. For example, typical drying shrinkage for regular concrete can range significantly after one year, highlighting how much volume loss occurs over time.

Think of measuring your height with a certified measuring tape. The shrinkage strain is like measuring how much you've 'shrunk' over a year due to gradual changes. If you measured every day with a tape marked for tiny changes, you’d find that, just like concrete, small adjustments add up to a significant difference over time. The small increments (like microstrain) help show the full picture of your growth or shrinkage—just as engineers look at microstrain to understand concrete behavior.

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