Fresh concrete is defined as plastic concrete when in the plastic state its components are fully mixed but its strength has not yet developed. When the concrete has not completely stiffened, means it may reduce volume due to loss of water then it is said to be in the plastic state.
Types of Cracks in Concrete at Plastic State:
In the plastic state, two types of cracks in concrete may occur which is known as Plastic Shrinkage cracks and Plastic Settlement cracks.
Both plastic shrinkage cracks and plastic settlement cracks occur within a few minutes of hours of concrete casting and finishing.
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They occur when concrete is still in plastic state and has not fully hardened. Their appearance is sometimes as early as in 10 minutes from the time of placing and finishing to as late as 3 hrs after placing.
What Causes these Types of Cracks in Concrete at Plastic State?
Concrete with a desirable quality of its physical and chemical properties is prone to plastic cracking. Most of the concrete structures develop cracks. Cracks cannot be eliminated totally but can be restricted or reduced to a great extent.
Fortunately, most of cracking is on a microscopic scale and does not appear visibly as a fault. The cracking can be macroscopic and can result in loss of strength, stability, and durability. It can also cause decrease in sound insulation and overall efficiency and also affects aesthetics to a great extent.
The main causes of these types of cracks in concrete at plastic state can be attributed to the following:
- Poor workmanship and negligence
- Defects and errors in construction practices
- Poor quality of concrete materials and improper concrete mix
- Movement of concrete arising from physical properties
- Over trowelling and impermeable formwork
- Reduced continuity of the structural member
- Deficiencies in design
- Ageing – carbonation
- Weathering action
- Foundation problems
- Improper or modified use of the structure
- Progressive loading
- Poor maintenance
- Improper structural repairs or modification
- Chemical attacks by chlorides & sulphates
- Differential thermal stress – heat of hydration of cement
- Alkali aggregate reaction