Shrinkage Concrete in Construction .pptx

Study of Shrinkage Concrete

1. SHRINKAGE OF CONCRETE Concrete is subjected to changes in volume either autogenous or induced. Volume change is one of the most detrimental properties of concrete, whichaffects the long –term strength and durability. it causes unsightly cracks in concrete and called concrete shrinkage. Types of shrinkage Plastic shrinkage in concrete Drying shrinkage in concrete Autogenous shrinkage in concrete Carbonation shrinkage in concrete

2. SHRINKAGE OF CONCRETE

3. PLASTIC SHRINKAGE Cracks appear in the surface of fresh concrete soon after concrete soon after it is placed and while it is plastic. T hese cracks appear mostly on horizontal surfaces. Plastic shrinkage cracking is more likely to occur when high evaporastion rates causes the concrete surface to dry out before it has set

4. PLASTIC SHIRNKAGE

5. DRYING SHRINKAGE Shrinkage is defined as the contracting of a hardened concrete mixture due to the loss of capillary water. This shrinkage causes an increases in tensile stress, which may lead to cracking, internal warping, and external deflection, before the concrete is subjected to any kind of loading.

6. DRYING SHRINKAGE

7. Plastic settlement cracks Cracks caused when the settlement of fresh concrete is restrained by reinforcement or formwork. Plastic settlement cracks can form in young concrete, within the first few hours after placing. As water moves upward through the mixture, the denser constituents move downward. This downward movement may be obstructed by the top layer of reinforcement or by the shuttering. The plastic concrete may arch over the top of individual reinforcing bars, bringing the surface into tension. Cracks may develop at regular spacing and usually follow the line of the uppermost bars, giving a series of parallel cracks; there may also be shorter cracks at right angles over the bars running in the opposite direction.

8. It is sometimes possible for plastic settlement cracks to form on a vertical face where reinforcement has restricted the free flow of concrete within the formwork. In such cases it is possible that the cracks are formed between the lines of the reinforcement. The concrete can also be supported by the shuttering, causing restraint to the concrete in connected members. This typically happens at mushroom-heads on columns but can also occur at other locations, such as under spacer blocks. Cracks at mushroom heads of columns are generally horizontal. They are also typically 1 mm wide and can cross the full section.