Chloe Zhang
Concrete is a mixture of aggregate, water, and cement. When first mixed, concrete is in the plastic stage, where it is soft and malleable, like bread dough. In this state, it can be easily transported, moulded, or poured into any desired shape. This plasticity is due to the workability of the concrete, which is defined as the ease with which it can be mixed, transported, placed, and compacted. A good concrete mix should also be cohesive, ensuring that the ingredients are properly distributed to form a homogeneous mixture. The plastic stage is crucial as it allows concrete to be manoeuvred into the desired position before it hardens and gains strength, transitioning into what is known as the curing and hardened states.
| Characteristics | Values |
|---|---|
| Workability | The ease with which the concrete can be mixed, transported, placed, and compacted |
| Cohesiveness | How well concrete holds together when plastic |
| Hardness | Concrete is hard and durable |
| Durability | Concrete is durable and able to resist changes in temperature, wear and tear from weathering, and shrinkage |
| Density | Concrete is dense and watertight |
| Resistance | Concrete has high resistance |
| Compressive strength | The compressive strength of concrete increases with the ratio of water to cementation materials |
| Bending strength | The bending strength of concrete increases with the ratio of water to cementation materials |
| Initial elastic module | The initial elastic module of concrete decreases with the ratio of water to cementation materials |
| Permeability | Concrete is reasonably watertight, but permeability increases with the ratio of water to cementation materials |
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What You'll Learn
Concrete in the plastic stage is mouldable and transportable
Concrete in the plastic stage is soft and malleable, much like freshly made bread dough. This is the first stage of concrete, when it is freshly mixed and easy to work with and transport. It can be moulded, poured, or placed into any desired shape or formwork. This is an important stage, as it determines how well the concrete will function in its intended, final state.
The workability of concrete is a key property at this plastic stage. Workability refers to the ease with which concrete can be mixed, transported, placed, and compacted. A good concrete mix should be workable without resulting in segregation or bleeding. Segregation occurs when the coarse aggregates separate from the concrete mix, and bleeding is when water appears on the surface of the concrete, along with some particles of cement and fine sand.
To achieve a good level of workability, the worker should adjust the amount of cement paste, use well-graded aggregates, and consider adding admixtures. The water-to-cement ratio is critical, as too much water will reduce the strength and durability of the concrete. Air entrainment can also be used to increase workability, but it will reduce the density and strength of the concrete.
The concrete in the plastic stage is also described as being in a “fresh concrete” state. This is when the concrete is still wet and can be poured into forms for specific projects, such as sidewalks, walls, or trenches. This is the most manoeuvrable state of concrete, which makes it ideal for transportation and placement. Once the concrete is in place, it will begin the curing process, where it develops strength and durability.
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Workability and cohesiveness are key properties
Workability is the most important property of concrete in its plastic stage. It is defined as the ease with which concrete can be mixed, transported, placed, and compacted. A workable concrete mix does not result in bleeding or segregation. Bleeding is when water appears, along with some cement particles and fine sand, on the surface of freshly placed concrete. Segregation is the separation of coarse aggregates from concrete in the plastic stage, creating a non-homogenous mixture. A workable concrete mix should be free from both bleeding and segregation.
To make a more workable mix, more cement paste can be added. Well-graded, smooth, and rounded aggregates can also improve the workability of a mix. However, adding more water to increase workability is not recommended, as this lowers the strength and durability of the concrete.
Cohesiveness is another key property of concrete in its plastic stage. It refers to how well the concrete holds together. Cohesiveness is affected by the grading of aggregates—a mix of aggregate grades, from large rocks to small sands, will result in a more cohesive mix. Conversely, too much coarse aggregate will result in a "boney" mix that lacks cohesiveness.
The cohesiveness of concrete is also influenced by the water-to-cement ratio. If there is too much water and not enough cement, the concrete will be weaker and less durable. It will also be less cohesive and may separate and bleed. Therefore, it is crucial to consider the proportion of ingredients and the grading of aggregates to achieve a cohesive mix.
In summary, workability and cohesiveness are essential properties of concrete in its plastic stage. Workability refers to the ease of handling and transporting concrete, while cohesiveness pertains to how well the concrete holds together. By ensuring proper grading of aggregates, optimal water-to-cement ratios, and the use of well-graded aggregates, concrete can be made more workable and cohesive, resulting in a stronger and more durable final product.
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Concrete should be free from segregation and bleeding
The plastic stage of concrete is when the concrete can be easily transported, moulded, or poured into any place. It is characterised by good workability, which refers to the ease with which the concrete can be mixed, transported, placed, and compacted.
To ensure that concrete is free from segregation and bleeding, it is important to understand these phenomena and implement appropriate measures. Segregation occurs when the constituents of concrete, such as aggregates and cement paste, separate from each other, leading to an uneven distribution of materials. This separation can be influenced by the disparity in specific gravity among the concrete materials, with heavier particles settling at the bottom and lighter cement-sand paste rising to the top. Excessive vibration during consolidation, especially with needle vibrators, can exacerbate segregation.
To prevent segregation, it is crucial to ensure proper mix design and handling practices. This includes using a well-graded aggregate mix and maintaining a suitable water-cement ratio. Avoiding excessive vibration or rough handling is also essential. Proper placement techniques are necessary to ensure even distribution and compaction of the concrete.
Bleeding, on the other hand, refers to the migration of water to the surface of freshly placed concrete. It occurs when the solid components of the concrete cannot hold all the mixing water, allowing it to rise to the surface due to its low specific gravity. Bleeding is more likely to occur in mixes with high water-cement ratios or excessive fine aggregates. While moderate bleeding can enhance workability, excessive bleeding can compromise the strength of the concrete.
To control bleeding, proper proportioning, air entrainment, the use of finer cement, and controlled vibration during consolidation are recommended. Additionally, delaying finishing operations until the bleed water has evaporated can help prevent surface scaling and ensure a smooth finish.
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Concrete curing increases durability
Concrete in the plastic stage is easily transportable and can be moulded or poured into any place. Workability is the most important property of concrete at this stage. Workability refers to the ease with which the concrete can be mixed, transported, placed, and compacted.
Concrete curing is a crucial process that significantly impacts the durability of concrete structures. Curing involves maintaining the proper temperature and dampness to allow the concrete to gain strength through a chemical process. While concrete is dry enough to walk on after 24 to 48 hours, full curing typically takes about 28 days.
There are three primary methods of curing concrete: adding water to the surface to replace evaporating water, sealing the concrete to prevent water evaporation, or a combination of both. Proper curing is essential to prevent the concrete from drying too quickly, which can lead to cracking and reduced durability.
The effects of curing on concrete durability are evident in its ability to prevent the ingress of harmful substances. Inadequate curing can result in a weak and porous surface, making the concrete vulnerable to environmental damage. Curing compounds are widely used to enhance the curing process and improve the durability of the concrete.
The efficiency of curing methods can be assessed through techniques such as monitoring water evaporation, oxygen permeability tests, and measuring total porosity and hydration rates. These tests help ensure that the curing process results in high-quality concrete with improved strength and durability.
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Air entrainment improves workability and cohesion
Concrete in its plastic stage is easy to transport, mould, and pour. Workability is a crucial property of concrete, referring to how easily it can be mixed, transported, placed, and compacted.
Air entrainment is a process that involves the deliberate creation of tiny air bubbles in a concrete batch by adding an air-entraining agent during the mixing stage. This process improves workability and cohesion in several ways:
Firstly, it makes concrete more workable during placement. The air bubbles act as internal cushioning, absorbing energy during impact and increasing resistance to physical forces such as shock or vibration. This improves the concrete's overall durability and makes it easier to handle and pump.
Secondly, air entrainment helps prevent "bleeding" and segregation, which are unwanted processes that can occur during mixing. Bleeding refers to the appearance of water and fine particles on the surface of freshly placed concrete. Segregation is the separation of coarse aggregates from concrete in the plastic stage. By reducing bleeding and segregation, air entrainment creates a more cohesive and uniform concrete mix.
Thirdly, air entrainment improves the durability of concrete, especially in climates with freeze-thaw cycles. The air bubbles provide small pockets for water to expand into during freezing, alleviating internal pressure and preventing cracking or breaking. This is particularly important in exterior concrete exposed to frost damage, vehicle traffic, and de-icing salts.
Lastly, air entrainment can enhance the concrete's resistance to fire damage and improve its overall strength. The presence of air bubbles increases the concrete's porosity, reducing water and liquid permeability, which further prevents cracking and fissures.
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Frequently asked questions
The plastic stage of concrete refers to the stage when it is wet and malleable. It is called the plastic stage because it is similar to plasticine or bread dough.
Concrete is made of aggregate, water, and cement. These ingredients are mixed to create a building material that can be manoeuvred into position.
The most important property of concrete in the plastic stage is workability, which refers to the ease with which the concrete can be mixed, transported, placed, and compacted. Other important properties include cohesiveness, which is how well concrete holds together, and freedom from segregation and bleeding.
Segregation is the separation of the constituent materials of concrete, particularly the separation of coarse aggregates from the cement paste. A good concrete is one where all the ingredients are properly distributed to form a homogeneous mixture.
Bleeding is when water, along with some particles of cement and fine sand, appears on the surface of freshly placed concrete. It is not completely harmful if the rate of evaporation of water is equal to the rate of bleeding. However, delayed bleeding, when the concrete has lost its plasticity, can cause harm to the concrete.
