People typically admire the ultraclean and sleek concrete surface they see in their dream house. There may be fine hairline cracks forming in it and they typically form within the first few hours after pouring, while the concrete is still in its plastic (wet and unset) state.
Though smaller, these structural cracks are a huge red flag over a period. Even tiny cracks can cause devastating structural damage that will put the lives of the occupants at serious risk. This blog will explore deeply this type of hazardous crack in the concrete.
What are Plastic Shrinkage Cracks?
Plastic shrinkage cracks are the structural cracks that appear in the first few hours after the concrete placement. The top reason for cracks for its occurrence is due to excess loss of moisture in the concrete surface, due to high temperatures, low humidity, or wind. These factors lead to shrinkage before the concrete gains enough strength.
Although they may appear minor at first, if left unchecked, they can become pathways for water and chemicals, potentially compromising the durability and long-term performance of the concrete. These cracks are most commonly found in structural elements like beams, foundations, and the tops of walls. These cracks can be hazardous and put the occupants’ lives at risk.
Causes of Plastic Shrinkage Cracks in Concrete
- Plastic shrinkage cracks are primarily created when the rate of moisture evaporation from the surface of the concrete surpasses that with which the concrete can withstand, at which moment the situation turns to become favorable for the development of shrinkage cracks.
- The transition from hot weather to dry weather accelerates the evaporation of moisture from the concrete surface. In an atmosphere of low humidity, the moisture from the concrete surface is drawn away quickly into the atmosphere faster than the concrete can shrink; therefore, cracks may start forming.
- Any amount of wind speeds up the rate of evaporation, and the formation of cracks occurs when moisture evaporates readily off the surface of the concrete.
- Furthermore, the composition of the concrete mix itself influences the resistance of the concrete to cracking by plastic shrinkage. When a high water-cement ratio exists, it incurs higher chances for the very occurrence of plastic shrinkage cracks and vice versa for low water-cement ratios.
Effects of Shrinkage Cracks
Not all cracks in buildings will mean structural problems or even trouble relating to usage. Hence, it is essential to keep the cracks under control and to prevent their development.
- Aesthetic Issues
It should be borne in mind that the size of these cracks can increase since they are mostly not observable and after a certain period, they will become visible, making the surface irregular, and the whole concrete will look bad.
- Reduced Durability
Besides reducing the cross-sectional area for load-bearing surface, cracks also increase the penetration depth of any harmful reactive agents keen on going into the concrete mass. This, in turn, shortens the life cycle of the structure by accelerating the decay of the material.
- Reinforcement Corrosion
Water sluiced into the cracks will corrode the rebar or other steel devices enclosed by the concrete, which will greatly weaken the inner structure of the material. The rusting as it unfolds will push the concrete and make the cracks even bigger.
- Freeze-Thaw Damage
During the frosty period, the water kept in the cracks will expand and contract due to the temperature change and this movement will damage the concrete, especially in the region where the temperature often rises and falls.
- Structural Deterioration and Increased Maintenance Costs
Shrinkage cracks make the concrete’s load-bearing capacity weak and are a crucial contributing factor to the risk of structural failure. Moreover, the cracks will result in a drain on the budget in the long run as they are the main cause of the need for regular repairs, and also, in stubborn cases, retrofitting costs.
Repairing Shrinkage Cracks
1. Identify and Assess the Cracks
First thing first, you need to visually inspect the surface to see whether there are shrinkage cracks on it. You should also measure the width and depth of the cracks. Since this is an indication of the severity of the Structural damage, you will be able to use the most appropriate repair method later.
If the cracks are not more than 1/8 inch in depth (shallow), you are required to make only minor repairs. In case the cracks are deeper, you might have to make the repairs more extensive.
2. Clean the Cracks
Begin the cleaning process either with a wire brush or a pressure washer to clean the crack. It is necessary to get rid of anything unwanted in the crack such as dirt, debris, or even the loose parts of the concrete so that you can repair the crack properly and effectively.
Before moving on to the next step, make sure that the surface of the crack dries out completely.
3. Prepare the Crack
In case of a bigger crack, it is recommended to widen it a little bit using a chisel and a hammer, creating a form of a “V.” The wider crack area makes it easier for the new concrete material or the repair material to stick to the existing concrete.
Sprinkle some water on the crack to make sure that it bonds easily with the filler material.
4. Apply a Paste of Mortar or Sealant
In the event of minor cracks, a concrete crack filler or sealant can be applied and the crack can be closed without leaving any space between the edges. To smooth out the paste of the crack filler, use either a putty knife or a caulk gun.
However, in the case of larger cracks, it may be necessary to use a repair cement mortar or epoxy injection. Refer to the mixing and application guidelines on the product’s label to get the right advice.
5. Smooth and Finish
Smooth the surface with a trowel, so that the patch is at the same level as the surrounding concrete. If need be, go all over the area by simply applying a concrete resurfacer to have an even finish.
6. Cure and Seal
Leave the repair to cure according to the product’s specifications. After the completion of the curing time, it is advisable to seal the surface with a concrete sealer which will ensure that moisture does not penetrate and that there will be no future damages. These methods enable shrinkage cracks to recover, thus, improving the durability and texture of the concrete surfaces.
Tips To Prevent The Amount of Shrinking
- Temporary Windbreaks
In adverse conditions, temporary windbreaks can be erected to lessen the wind action on the surface of the concrete, and, if possible, shade the finished slab to control surface temperature. If the conditions raise critical criteria, the concrete may be placed at the beginning of the later afternoon or dusk. Very hot conditions, however, can allow for better control of the temperatures of the concrete using early morning placement.
- Fog Sprays
In hot and dry weather conditions, fog sprays have to be applied to create fine mists upwind and into the air over the concrete. Reduces the evaporation rates from the concrete surface and must continue until suitable curing materials can be applied.
- Dampening Subgrade
If concrete is to be placed on a dry absorptive subgrade in hot and dry weather, the subgrade needs to be dampened. However, the existence of free-standing water on site before to placement may be unfortunate. Sometimes even forming and reinforcement need dampening with some gravel fill, such as a crusher-run material.
- Covering the Surface
If the concrete surface takes more time to cure, moisture-retaining covers such as wet burlap, polyethylene sheeting, or building paper can be used between finishing operations. In hot dry climates, evaporation retardants can be sprayed on the surface behind the screeding operation immediately, followed by floating and troweling.
- Liquid Curing
Curing of the concrete must take place immediately, by spraying a liquid membrane or by covering it with wet burlap. Maintain that surface and keep it moist continuously for three days at the least, to prevent shrinkage.
- Accelerate Setting Time
Accelerating the setting time of concrete helps minimize temperature differences between the concrete and the surrounding air. This prevents thermal cracking by ensuring even hydration, reduces shrinkage cracks, and stabilizes the setting process, especially in cold weather.
Conclusion
To sum it up, plastic shrinkage cracks may be small, but they say a lot of things beyond that. It can lead to water infiltration, corrosion, and weakening of the structure over time. If left ignored for a long time, it paves the way to costly repairs, and even collapse. It requires immediate attention and repair. Addressing the issue isn’t all about fixing the surface, but also protecting the structural integrity for years to come. Understanding the cause and preventing it at early stages can help prevent this hazardous crack.
Apoorva H P Achar