METHODS TO PREVENT PLASTIC SHRINKAGE CRACKING OF CONCRETE AND EFFECT OF SHRINKAGE ON CONCRETE STRUCTURE
10 METHODS TO PREVENT PLASTIC SHRINKAGE CRACKING OF CONCRETE
Plastic shrinkage cracking (as shown below) is caused by rapid drying of the concrete surface. They can occur at any time of the year, from cooler low-humidity weather conditions to, in particular, hot weather conditions involving any one or a combination of high temperature, low humidity and wind.
Plastic Shrinkage Cracking of Concrete
These types of cracks occur while the concrete is bleeding and during the finishing operation. Fresh concrete may be exposed to the elements for a considerable time during these stages, causing the concrete at the surface to dry and shrink before it has any strength to resist the shrinkage forces.
Concrete cracks in much the same way as clay soils.
METHODS OF PREVENTING PLASTIC SHRINKAGE CRACKING OF CONCRETE
1. ERECTING TEMPORARY WIND BREAKS
Higher wind velocities can greatly affect the rate of evaporation of water from the concrete being placed. This in turn will result is plastic shrinkage cracking of concrete. Therefore erect temporary wind breaks to reduce the wind velocity over the surface of the concrete.
2. PROVIDING SUNSHADES
In case of construction of concrete slabs, it is very effective to provide sunshades over the concrete slabs and thereby controlling the surface temperature of the slab.
3. PROPER PLACEMENT TIMING
In very hot weather conditions, concrete should not be placed in the afternoon, because it can lead to plastic shrinkage cracks on concrete due to loss of water. Therefore it is a good practice to schedule concrete placement in early morning or early evening. This way we can have better control over concrete temperature.
4. USING FOG SPRAYS
In very hot and dry periods the wind is very hot. Therefore use fog sprays to spray fog into the air over the concrete surface and in the direction opposite to the direction the wind is blowing. Using of fog sprays reduce the rate of evaporation from the concrete surface.
5. DAMPENING THE SUBGRADE
During construction of concrete pavement in hot and dry weather, dampen the subgrade which is liable to water absorption from concrete. But remember the subgrade should not be over dampened. The formwork and the reinforcement should also be dampened.
6. USING EVAPORATION RETARDER
Spray an evaporation retarder such as aliphatic alcohol over the freshly screeded concrete surface. Surfaces can dry prematurely when the rate of evaporation is greater than the rate of bleeding. These products form a film on the surface that reduces the rate of evaporation of water, thus allowing time for all the bleed water to rise to the surface. In extreme conditions, re-application of the evaporation retarder may be necessary both before and during trowelling. These products do not affect the strength or performance of the concrete and can act as a finishing aid.
7. START CURING IMMEDIATELY
Commence curing as soon as possible after finishing. Spray the surface with liquid membrane curing compound or cover the surface with wet burlap or start water curing after concrete placement.
8. ACCELERATING SETTING TIME OF CONCRETE
Use some form of chemical admixture in concrete to accelerate the setting time of concrete and avoid large temperature differences between concrete and air temperature.
9. ADDING POLYPROPYLENE FIBRES IN CONCRETE MIX
Polypropylene fibres can be added to the concrete mix to reduce the incidence of plastic shrinkage cracking. Fibres tend to bind the surface of the concrete together and provide some strength to the plastic concrete to resist the shrinkage forces.
10. BE READY
Have proper manpower, equipment and supplies on hand so that the concrete can be placed and finished promptly.
When concrete structures are not permanently water-saturated, it starts to lose moisture and starts to shrink from the time when the process of curing ceases.
The structural effects of shrinkage area as follow.
1. JOINTS
Shrinkage of concrete between movement joints causes joints to open or makes it wider. Therefore joints must be designed to accommodate the widening caused by shrinkage.
2. OTHER MATERIALS
Where other materials, such as ceramic tiles, are fixed on top of concrete surface, shrinkage of the concrete causes relative movement between the different materials. The resulting stresses can cause failure at the interface.
3. CRACKING
If shrinkage is restrained, the concrete is put into tension and when tensile stress becomes equal to tensile strength, the concrete cracks.
shrinkage cracks in concrete
Structural designers can design structures to minimize restraint, prestress the concrete to prevent tensile stress, or use reinforcement to control cracking.
4. LOSS OF PRESTRESSING FORCE
Shrinkage causes a reduction in prestressing force. When calculating prestresing forces, designers take into account to ensure that residual stress is structurally adequate.
5. REPAIRS
If concrete is used to fill a cavity in old concrete, shrinkage of the new concrete is restrained by the surrounding old concrete. Repair concretes and mortars must be specially formulated (by incorporating a polymer material) to prevent cracking caused by this restraint.
6. BOND STRENGTH
Shrinkage of the concrete causes the concrete to grip reinforcing bars more tightly. This increases friction between concrete and steel and so improves bond strength, especially for plain bars.
7. DEFLECTIONS
The deflection of flexural members is increased by shrinkage. This is because the lightly reinforced compression zone is free to shrink more than heavily reinforced tension zone
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