Differentiate between plastic shrinkage cracking and plastic settlement cracking

1.1 Concrete is made by mixing cement, water and aggregate together to form a paste like slurry. It is molded or placed as desired and left to consolidate and harden. The cement in the concrete forms a chemical reaction with water called hydration. This hydration process is directly responsible for concrete to harden. The properties of concrete in its fresh state directly  influence the quality of hardened concrete. Concrete continues to harden/cure over time.

1.2 Plastic shrinkage cracks:

we use the term plastic shrinkage to describe shrinkage which takes whilst concrete is still in a workable/plastic state. The extent of plastic shrinkage are usually restricted by internal friction, formwork and reinforcement. Plastic shrinkage cracks usually form on concrete surfaces exposed to high surface temperatures, strong winds or low humidity. These environmental conditions tends to increase the rate of evaporation of bleed water. Cracks forms when the rate of evaporation is greater than the concrete bleeding rate.Plastic shrinkage cracks has a typical and non continuous crack pattern which are not usually a concern from a structural aspect.

Plastic settlement cracking:

After concrete has been compacted but before  it sets, the solid phase of the mix settles and displaces water to the surface. This process is called bleeding. Plastic settlement cracks form when the settlement of the solid phase is hampered by formwork, reinforcement or void formers. As a result of these restraints the concrete settles away from these obstructions and forms a weakened bond around it, this is undesirable and has to be fixed. Plastic settlement cracks can be prevented by re-vibration or by selecting a more cohesive mix design.

2.1) Calcium Sulfoaluminate is the mineral name for Etrignite, which is normally found in portland cement concretes. Gypsum is a calcium sulphate source which is added to Portland cement to regulate early hydration reactions to improve strength development, decrease flash setting and reduce drying shrinkage. Gypsum and other Calcium sulphate compounds react with calcium aluminate in the cement to form Ettringite, within a few hours of mixing with water. Majority of the sulfate in the cement is normally consumed to form ettignite in the fresh, plastic concrete is the mechanism that controls stiffening.

2.2)  the expanding reaction of cement  between cement paste and the reactive non-crystalline silica , given sufficient moisture, is known as Alkali silica reaction. The reaction causes the expansion of the altered aggregate by the formation of a soluble and viscous gel of sodium silicate.  
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Wessel Peens

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