7 Tips to Prevent Concrete Curling

7 Tips to Prevent Concrete Curling

Prevent Concrete Curling: Curling of concrete is a flaw that occurs in concrete slab elements. Curling refers to upward or downward bending of the edges of a slab. Curling of concrete slabs is primarily caused by temperature differences between the slab’s upper and bottom surfaces or by variations in the slab’s moisture content.

This post discusses the critical characteristics of concrete slab distortion caused by curling and some critical steps to avoid it.

Curling in Concrete Slabs

Variation in the temperature or moisture content of the top and bottom surfaces of the concrete slab results in the raising of the slab’s edges or middle section. In both cases, the curling effect results in a section that is unsupported. When a load greater than the allowable weight is added to this bent slab, the surface cracks, and the slab’s edges chip off or spall as a result of the cracks. This is most prevalent in high-traffic areas. According to case studies, the majority of slab curling happens during the early stages. However, this does not preclude curling from occurring in the slab’s later stages.

Prevent Concrete Curling
Curling due to Differential Drying Shrinkage

Reasons for Curling in Concrete Slabs

Curling in concrete is mostly caused by drying shrinkage and temperature variations. Both of these variables result in a shift in the dimension of the slab relative to the other—this dimension change leading to the warping of slab edges in the direction of length shortening.

Curling as a result of Drying Shrinkage

Drying shrinkage occurs as a result of changes in the moisture content. Curling occurs as drying occurs to the upper surface of slab and shrinks in relation to the bottom surface. Drying shrinkage manifests itself as upward curling of the concrete slab. Drying due to shrinkage occurs as a result of the following:

  1. Inappropriate Curing
  2. Rapid drying of the surface
  3. Significant bleeding
Curling of the Concrete Slab
Curling of the Concrete Slab

Curling as a result of Temperature Differences

Curling of the slab is often caused by temperature variations between the upper and bottom surfaces of the concrete slab. The top cover, which is more open to sunshine, expands more than the slab’s cooler bottom. This results in downward curling of the edges (a). When the temperature drops, the reverse occurs, resulting in upward curling of concrete slabs.

Apart from shrinkage and temperature variations, the subgrade state (wet or dry), temperature cycles, and construction practices all have an impact on the concrete slab curling.

Preventing Concrete Slab Curling

Concrete curling is primarily caused by shrinkage, temperature fluctuations, soil moisture content, and construction efficiency. Several vital practices that has to be followed to avoid the causes of concrete slab curling are described below:

1. Decrease Drying Shrinkage

To a certain degree, drying shrinkage can be avoided by using the least amount of water available in the concrete mix and the maximum coarse aggregate possible.

2. Decrease Bleeding

During building, all preventative measures to avoid accidental bleeding must be followed. To some degree, bleeding can be prevented by putting the concrete on a damp or absorptive surface. This assists in preventing bleed water from reaching the surface. This method is recommended for interior slab construction.

3. Decrease the amount of cement in mix

Always use the minimum amount of cement possible. When a high amount of cement is needed, supplementary cementitious materials such as fly ash may be used instead.

4. Proper curing

Curing of concrete should be done correctly, ensuring that the joints and edges of the slab are properly sealed. When using the membrane curing process, the membrane is applied twice. The second application is rendered perpendicular to the first.

5. Depth of concrete slab

To avoid curling, it is recommended to build a thicker slab. If not, add additional thickness to the edges.

6. Adequate Steel RFT

Steel reinforcement that is properly built and installed will help avoid curling to a certain degree. Any load-transfer devices used to avoid vertical movement are positioned perpendicular to the direction of the building joints.

7. Application of Sealers/Coatings

There are special breathable coatings that help to mitigate moisture differences, thus preventing curling.

References: ACI 302.1R-15: Guide to Concrete Floor and Slab Construction

Read Also: 8 Common Defects in Concrete Structures & How to Prevent Them

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