Understanding Concrete Curling
Concrete curling is a condition that causes a corner or edge of a concrete slab to lift or bend, forming a curled or warped appearance. While this may not sound like a big deal, curling ultimately can cause cracks, instability, and additional maintenance costs.
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Concrete curling occurs when there is a differential moisture or temperature condition from the upper to the lower surface of the slab.
When these conditions exist, one side of the slab can contract or expand at a different rate than the other, causing a curled condition. In some cases, the center lifts while the edges stay down or vice versa.
Depending on the applied loads in these areas, damaging cracks may form, and excessive stress from vehicles and equipment can cause edges to chip away.
While curling is often observed in the early curing days of the new slab, it can also become evident as long as a number of weeks or even months later.
Why is Curling an Issue in Concrete Floors?
Curling creates serious problems in situations where flatness and stability are crucial. For example, in a warehouse or industrial plant, even small amounts of unevenness in the floor slab can cause problems for dollies and forklifts.
Indoor concrete slabs curl due to moisture differential across the slab’s.
The outer layer will dry above the lower level of the slab. But because the outer layer dries faster than the inside or bottom layers, and shrinks at a faster rate, curling will occur, commonly on the edges of the slab.
Repeated traffic from heavier vehicles will continue to pose a risk for cracking in the curled, uneven edges. Even if no cracks are visible, the surface will still be uneven, which will cause issues with machine movement and efficiency.
Additionally, thinner slabs and a larger joint gap will make curling increasingly likely. For this reason, most thin overlay areas are also supplied with closely spaced joints, which is not a problem in itself, but an excess of joints can lead to problems, such as wear and repair over time.
What happens when Concrete curls?
Let’s take a look at the main reasons why curling happens when working with Concrete slabs.
Moisture Variation
Moisture variation is one of the biggest issues leading to curling. The surface of the slab tends to lose moisture faster than the underside. Because of the shrinkage difference, the top is drying and shrinking, pulling the edges of the slab upward.
Rapid Surface Drying
If curing is poor or too rapid, the surfaces of the concrete slab will dry too quickly and resulting in shrinkage. Poor or rapid curing will simply dry the top of the slab too fast and create curling.
Bleeding Water
Concrete that has a high water content in the mix or surface water will bleed. Bleeding forces water to the surface and can evaporate too fast, leading to the top drying and shrinking more than the base. This is even worse when concrete is poured directly on a vapor retarder, like plastic sheeting.
Temperature Differences
Sunlight warms the surface, which results in the surface expanding more than the cooler bottom and then curves downwards. In the reverse, on cooler nights, the surface is contracting while the bottom of the slab is still warm from the ground, which will cause curling upwards. These thermal imbalances compound the problem.
Practical Ways to Prevent Curling
Preventing curling begins in the planning stage and ends with a little construction know-how. There are several methods that can be employed that will substantially reduce the chance of curling.
1. Control Drying Shrinkage
As concrete loses moisture, it experiences shrinkage. Using the lowest water content in the mix that still meets workability requirements will aid. In addition, large coarse aggregates shrink less than fine aggregates, thus lowering overall shrinkage.
2. Control Bleeding
When concrete is placed on a wet or absorptive base, it will bleed less than concrete placed on a dry, non-absorptive base because it takes longer for the water to move to the surface.
For these reasons, avoid placing concrete directly on plastic or other moisture-preventing surfaces, unless absolutely necessary.
3. Lower Cement Content
More cement would generally mean more shrinkage, so when you are worried about high cement content (such as when using high cement content is unavoidable), use pozzolanic materials like fly ash or silica fumes to minimize risk. These pozzolanic materials also help to increase concrete durability and sustainability.
4. Cure Properly
Curing is very important in minimizing curling. Proper curing helps retain moisture in the slab (wetting will happen above the Modulus of Rupture) and reduces the chance of the top layer drying out prematurely.
If you are using membrane curing, do it twice, once in each direction to ensure even coverage, especially at the edges.
5. Cast Thicker Slabs
Thicker slabs have shown less curling. If increasing the slab’s thickness is not an option, you can still impact curling by adding additional thickness at the edge. Additionally, ensure that the spacing of your joints does not exceed 24 times the slab’s thickness.
6. Reinforcement Placement
Steel reinforcements that are strategically placed can assist in holding the slab and reducing upward or downward movement. If load-transfer devices are being implemented, it is best to have them across construction joints to minimize vertical displacement between slab sections.
7. Moisture-Control Sealers
There are specialty sealers and coatings available that let the concrete breathe while controlling the internal moisture balance. With moisture-control sealers, the drying process is better equalized overall, reducing the risks of curling.
Advanced Solutions for High-Risk Applications
If curling is totally unacceptable, as in key industrial floors or automated storage facilities-there are improvement possibilities. These methods tend to take additional resources, but they might be worth the investment:
Shrinkage-Reducing Admixtures: These chemicals lessen the ultimate amount of shrinkage during the curing phase.
Shrinkage-Compensating Concrete: The shrinkage-compensating concrete includes materials that expand during early hydration to counter any later shrinkage.
Post-Tensioning: Post-tensioning applies tension to the steel tendons in the slab, keeping it under compression and minimizing movement.
Vacuum Dewatering: Vacuum dewatering removes extra water during the early setting period, squeezing water out of the mat and improving concrete density and reducing shrinkage.
In most cases, there is a need to plan ahead in order to get these improvements; adding them mid-construction could be unmanageable and costly.
Dealing with Existing Curling
When a slab has already curled, there are remedies that can help get the slab back to usable service.
Ponding: Simply flooding the surfaces will balance the moisture content in the slab and level it, although impermanently.
Saw Additional Joints: By cutting additional joints, this may help relieve some stress and assist with future curling.
Grind High Areas: Mechanical grinding will help remove trip hazards and help the transitions of the lifted areas.
Inject Grout: If the slab is voided underneath, the injection of grout will restore support of the base and eliminate the danger of slab edge failure.
The Use of Retarders
Retarders can be used to slow the set times of concrete, especially in hot weather. Retarders are not specifically used to prevent curling, but they provide workers with enough time to properly place, finish, and cure the slab, reducing the impact of drying irregularity. By indirectly limiting curling risk, retarders have an added benefit when used in a comprehensive plan.
Conclusion
Concrete curling is more than a surface defect, it can lead to structural issues, safety issues, and long-term maintenance issues. Fortunately for the industry, concrete curling can be completely prevented. By focusing on the proper mix design, placement, and curing, the risk can be drastically reduced.
And if curling does occur, don’t panic. Solutions like grinding, grout injection, or moisture ponding can bring the slab back to acceptable performance. With the right knowledge and preparation, concrete curling doesn’t have to be a costly problem.
Sandip Agrawal, Polymer Engineer and MD of Sakshi Chem Sciences Pvt. Ltd., leads innovation in construction chemicals, shuttering oils, and industrial lubricants. With expertise in polymer science and eco-friendly solutions, he drives R&D and sustainable advancements, ensuring high-performance products for India and global markets.