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Concrete curling is a defect observed in concrete slab structures. Curling is the bending of slab edges either upwards or downwards. Curling of concrete slabs is mainly caused due to the temperature variation between the top and bottom surfaces of the slab or due to variation in the moisture content.
The article explains the critical features of distortion in concrete slab due to curling and some essential measures to prevent it.
Contents:
Curling in Concrete Slabs
Temperature or moisture content variation on the concrete slab's top and bottom surfaces results in the lifting of the slab edges or the middle portion of the slab, as shown in figure-1 below. In both the cases, the curling effect leaves an unsupported portion. When a higher load, more than the permissible weight is applied over this distorted slab, the surface cracks, and as an aftereffect of cracks, the edges of the slab may either chip off or spall. This is mainly observed in high traffic areas. As per the case studies, most of the curling of slab occurs at the early stages. But it doesn't mean that curling cannot occur in the later stages of the slab.
Causes of Curling in Concrete Slabs
The significant causes of curling in concrete are drying shrinkage and temperature differences. Both the factors bring a change in the slab dimension relative to the other- This change in dimension results in the warping of edges in the direction of relative shortening.
Curling in Concrete Slabs due to Drying Shrinkage
Drying shrinkage is caused due to moisture content variation. When the top surface of the concrete slab dries and undergoes shrinkage with respect to the bottom surface, it results in curling. As shown in figure-1, the drying shrinkage occurs in the upward curling of the concrete slab. Drying shrinkage is caused due to:
- Improper Curing
- Rapid surface drying
- High bleeding
Curling in Concrete Slabs due to Temperature Differences
The temperature differences on the top and bottom surfaces of the concrete slab also result in the curling of the slab. The top cover, which is more exposed to the sunlight, undergoes more expansion when compared to the cooler bottom of the slab. It results in downward curling of the edges, as shown in figure-2(a). During cold temperature, the opposite happens, which results in upward curling in concrete slabs.
Other than the shrinkage issues and temperature variation, the condition of subgrade (wet or dry), the temperature cycles, and the construction practice influence the concrete slab curling.
How to Prevent Curling in Concrete Slabs?
The primary factors that result in concrete curling are shrinkage, temperature variations, moisture content of the soil, and the quality of construction. Some of the critical practice that must be followed to prevent the causes in curling in concrete slabs are enlisted below:
1. Prevent Drying Shrinkage
Drying shrinkage, to some extent, can be prevented by using the lowest effective water content for the concrete mix and by using the largest possible sized coarse aggregate.
2. Prevent Bleeding
All preventive measures to avoid excessive bleeding must be practiced in the construction. Bleeding, to some extent, can be avoided by placing the concrete on a surface that is damp or absorptive. This helps to prevent the bleed water from coming on the top. This method is recommended for the construction of interior slabs.
3. Reduce cement content
Always employ the necessary cement content. When high cement content is essential, using pozzolana like fly ash, silica fume, etc. is an alternative.
4. Quality Curing of Concrete Slab
Cure the concrete properly, including the joints and edges of the slab. When the membrane curing method is used, the application of the membrane is performed twice. The second application is done at right angles to the first application.
5. Thickness of the concrete slab
It is recommended to design a thicker slab to prevent curling. If not, provide an increased thickness at the edges.
6. Proper Steel Reinforcement
Properly designed and placed steel reinforcement can help prevent the curling to some extent. Any load transferring devices used to prevent the vertical movement is placed across the direction of construction joints.
7. Use of Sealers and Coatings
There are unique breathable sealers that help to minimize the difference in moisture, which hence prevents curling.
Frequently Asked Questions
What is Concrete Slab Curling? Curling is the process of bending of slab edges either upwards or downwards. When a higher load, more than the permissible weight is applied over this distorted slab, the surface cracks. As an aftereffect of cracks, the edges of the slab can either chip off or spall. This is mainly observed in high traffic areas. From different cases studied, most of the curling of slab occurs at the early stages. But curling can also occur at later stages of the slab.
What Causes Concrete Slab Curling? The significant causes of curling in concrete are drying shrinkage and temperature differences.
Drying shrinkage is caused due to moisture content variation. When the top surface of the concrete slab dries and undergoes shrinkage with respect to the bottom surface, it results in curling.
The temperature differences on the top and bottom surfaces of the concrete slab also result in the curling of the slab. The top cover, which is more exposed to the sunlight, undergoes more expansion when compared to the cooler bottom of the slab.
Other than the shrinkage issues and temperature variation, the condition of subgrade (wet or dry), the temperature cycles, and the construction practice influence the concrete slab curling.
How temperature change cause concrete slab curling? The temperature differences on the top and bottom surfaces of the concrete slab also result in the curling of the slab. The top cover, which is more exposed to the sunlight, undergoes more expansion when compared to the cooler bottom of the slab. It results in downward curling of the edges, as shown in figure-2(a). During cold temperature, the opposite happens, which results in upward curling in concrete slabs.
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