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A short class with Sarah and her students. The other ways are under More.
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Today let's talk about plastic shrinkage, which happens within a few hours after we place concrete. Can anyone tell me what causes it?
Is it because of evaporation of water from the surface?
Exactly! Rapid evaporation can lead to differential shrinkage and cracking. Remember, preventive measures like curing can help mitigate this. What are some methods for curing?
Using wet blankets or fogging can help keep moisture in.
Great! Always remember: 'Prevent, Protect, and Cure'.
Now let's discuss drying shrinkage. Can someone explain what it is?
It happens when the concrete hardens and starts losing water from its pores, right?
Exactly! It's the most significant type of shrinkage and can continue for months. How might humidity affect it?
Lower humidity increases drying shrinkage.
Correct! Remember the phrase: 'Moisture Matters!' to help you recall this concept.
Let’s examine autogenous shrinkage and carbonation shrinkage. Who can explain autogenous shrinkage?
That occurs in high-strength concrete without external moisture loss, right?
Yes! It's caused by internal chemical shrinkage. How about carbonation shrinkage?
That’s when CO₂ reacts with the concrete to form calcium carbonate, causing minor shrinkage.
Great job! Always remember: 'Autogenous is internal, carbonation is external.' Let's wrap up today’s discussion.
Initial surface volume reduction caused by evaporation after placement.
Volume reduction over time as concrete loses water from its pores.
Internal volume reduction due to chemical reactions in low water-cement ratio mixes.
Reaction with CO₂, leading to minor long-term shrinkage.
Plastic shrinkage can usually be seen as cracks forming on the surface of fresh concrete on a hot day.
Drying shrinkage can be observed in an old concrete slab that has cracked over time due to moisture loss.
Autogenous shrinkage can occur in a high-performance concrete structure where a very low water-cement ratio was used.
Carbonation shrinkage is often observed in concrete exposed to the atmosphere, where CO₂ is present.
Plastic cracks when it's hot, drying shrinks a lot!
Imagine a fresh layer of concrete on a sunny day—it's hot! Water evaporates quickly, causing cracks—this reminds us to protect our concrete during curing.
To remember types of shrinkage, think P, D, A, C: Plastic, Drying, Autogenous, Carbonation.
Memory aid 'P-DAC' helps to recall the types of shrinkage in concrete.
Term
Plastic Shrinkage
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Term
Drying Shrinkage
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Term
Autogenous Shrinkage
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Term
Carbonation Shrinkage
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The volume reduction that occurs shortly after the placement of concrete, caused by rapid evaporation of surface moisture.
The time-dependent volume reduction occurring as hardened concrete loses moisture to the environment.
Volume reduction in concrete due to internal chemical reactions, particularly in low water-cement ratio mixes.
Shrinkage caused by the reaction of carbon dioxide with concrete, leading to a reduction in volume.
This section outlines the various types of shrinkage that concrete can undergo, including plastic, drying, autogenous, and carbonation shrinkage.
Concrete shrinkage is a time-dependent volume reduction that can occur without external loads, which can adversely impact structural integrity. The main types of shrinkage discussed are plastic, drying, autogenous, and carbonation shrinkage, each influenced by factors such as moisture loss, curing, and aggregate type.
Concrete undergoes different types of shrinkage, which are critical to consider during structural design to mitigate potential negative effects like cracking. Here are the main types:
Plastic shrinkage occurs shortly after the concrete is placed and is caused by rapid evaporation of surface water. This can lead to cracking, which is preventable through measures like curing and using windbreaks.
This type of shrinkage occurs when hardened concrete continuously loses moisture over a long period. It is heavily influenced by environmental factors such as humidity and curing practices and can lead to significant volume reduction.
This occurs due to internal chemical reactions in low water-cement ratios, particularly in high-strength concrete. It leads to volume reductions without external moisture loss and can cause early-age cracking.
This type results from the reaction between CO₂ and calcium hydroxide in concrete, forming calcium carbonate, which leads to minor long-term volume reduction.
Understanding these types of shrinkage is crucial for civil engineers, as they impact the structural integrity and durability of concrete structures.