Concrete Delamination Repair | What Causes Concrete Delamination? | Spalling Vs Delamination
Concrete Delamination Repair | Concrete Spalling And Delamination | Preventing Concrete Spalling and Delamination?
What is Delamination of Concrete?
Delamination of concrete occurs when the paste layer at the surface separates, forming an unbonded concrete layer between it and the slab body. This problem occurs when troweled concrete is placed on a chilly subgrade, most frequently in the early spring and late fall.
A delamination is a hollow, horizontal plane beneath the surface of the concrete that formed when a finish is put before the water and air can escape.
When a delaminated piece of the slab is crushed by traffic or a strong load, it separates from the slab, exposing aggregate.
While the separation is usually caused by rising daytime temperatures, it can occur at any moment depending on the concrete’s qualities and the finishing procedures applied.
Which Components of the Concrete Are Affected by Delamination?
Typically, just the upper 1/8- to 1/4-inch densifies. This is primarily due to early and incorrect finishing, as well as the presence of a thin layer of air or water.
Affected areas can range in size from several square inches to square feet, depending on the degree of the delamination.
Cracking and color variations on the surface are frequently observed when the thin surface dries too quickly during the curing process.
On the other hand, freezing and/or driving may result in the disintegration of substantial sections of the surface.
What Factors Contribute to Delaminated Concrete?
Densifying or sealing a freshly poured concrete surface using a trowel when the underlying concrete is still flexible and bleeding or leaking air.
While Concrete delaminations typically occur after floating and the initial troweling pass, they can occur during the floating stage if the freshly worked and densified surface is overworked and densified.
When conditions favor rapid drying of the surface, such as low humidity, sun, and wind, the likelihood of delaminated concrete increases. This is because the conditions provide the appearance of a trowel-ready surface when the concrete beneath is still flexible.
What Causes Concrete Delamination?
Delamination in concrete is commonly caused by a high air content that becomes trapped beneath the surface due to a tight power-troweled finish. The tight finish traps microscopic air bubbles. They merge to form larger bubbles when they become stuck.
As air attempts to leave the top of the concrete slab, these bubbles push the closed surface upward. Later, these elevated blisters harden and eventually rupture as a result of traffic.
This sort of delamination is most frequently observed in nickel- or quarter-sized spots, but it can occur in bigger areas as well. I’ve seen areas of widespread delamination caused by air that are around two feet square.
Trapped bleed water is the second most common cause of delamination. As concrete cures, it extracts water from the mixture. When a vapor barrier is positioned beneath a slab, water escapes through the top.
Occasionally, particularly in cold weather or when the slab is covered with a roof and walls are in place, the concrete set takes a lengthy period of time. When the troweling operation is completed without allowing the bleed water to exit the slab completely.
Similar to the air issue, this results in the bleed water becoming trapped behind the tightly troweled surface paste.
Small pockets of water congregate to form larger areas where the water presses up against the surface paste, forming blisters.
Water evaporates or is absorbed into the slab structure over time, leaving empty blisters. These blisters eventually rupture due to traffic or maintenance, revealing a delamination.
Factors That Contribute to Delamination
When the concrete’s initial setting is delayed and the rate of bleeding is reduced, the likelihood of delamination increases. While they can occur in a variety of situations, they are more prevalent when the following nine elements are present:
- A cold subgrade retards the setting of the underlying concrete;
- The temperature of the concrete or the composition of the mixture impedes the setting of the concrete;
- There is entrained air in the concrete, or the overall air content is greater than desired for the application;
- A greater proportion of cementitious material or sand-fines has resulted in a sticky concrete mixture;
- Environmental circumstances have accelerated the drying process, resulting in a crust-like surface and the impression of completion;
- Excessive vibration attracts an excessive amount of mortar to the surface;
- Using a dry shake, which is particularly effective with air-entrained concrete;
- A substantial slab of concrete; and the slab is directly above a vapor retarder.
Delamination Caused by Corrosion
This occurs when the reinforcing steel in the higher layer rusts, causing the steel to separate from the surrounding concrete. It is more likely to occur when the concrete cover is thin and the concrete is permeable, allowing chloride to penetrate to the steel layer.
Spalling Vs. Delamination
Delamination happens when the newly troweled concrete surface is sealed or densified while the underlying concrete remains plastic and continues to bleed and/or release air. Delaminations occur very late in the finishing process, following flotation and the initial troweling pass.
Spalling on the other hand refers to areas of concrete that have cracked and delaminated from the substrate. Spalling can occur for a variety of reasons, including freeze thaw cycles, the expanding effects of the Alkali Silica Reaction, or exposure to fire.
How Can I Prevent Concrete Spalling and Delamination?
- In cool conditions, accelerators or warm concrete frequently used to prevent delamination.
- Be keen of a concrete surface that appears to be ready to trowel ahead of schedule. The finishing process should focus on screeding, straight-edging, and floating the concrete as quickly as possible – without building up an excessive layer of mortar or sealing the surface layer. When initially floating, the float blades should be flat to avoid premature densification of the surface.
- Final finishing procedures to create a smooth surface should be postponed as long as feasible, and the surface should be protected from evaporation using polyethylene or another vapor-proof material.
- During finishing procedures, delamination may be difficult to detect. If you notice delamination, rip the surface with a wood float and postpone finishing as long as possible. Any measures that can be taken to retard evaporation should be beneficial.
- If a vapor retarder is necessary, at least four inches (100 mm) of a trimmable, compactable granular filler should be used (not sand). Concrete should not be placed directly atop a vapor retarder. If a moisture-sensitive floor covering is to be installed on interior slabs, the concrete will often be laid immediately on a vapor retarder; however, additional steps may be required.
- Air-entrained concrete should not be used for interior floor slabs with a hard troweled surface that are not subjected to freeze-thaw cycles or deicing salt application. If entrained air is required to protect inner slabs from freezing and thawing cycles, air levels greater than 3% should be avoided.
Concrete Delamination Repair
Delaminated surfaces can be repaired by patching after the surface layer is removed and the underlying concrete is properly cleaned.
Extensive delamination may need to be repaired by grinding and overlaying a new surface.
Delaminated surfaces due to steel corrosion will additionally require sandblasting to remove rust from the steel.
Is It Possible to Restore a Delaminated Concrete Surface?
Yes. After removing the surface layer and thoroughly cleaning the underlying concrete, patching is conducted.
For severe delamination, grinding and resurfacing may be essential. When steel corrosion is the cause, proper rust removal requires sandblasting the steel.
Observe These Guidelines to Avoid Delamination
- Avoid premature sealing of the concrete surface. Below, the air and bleed water must have adequate time to escape to the surface.
- Avoid applying dry shakes on concrete that contains air.
- Utilize accelerated or heated concrete to ensure uniform setting throughout the slab.
- Never install concrete on top of a vapor retarder if possible.
- If a trowel finish is required on an internal slab, avoid using air-entrained concrete.
- If the temperature of the substrate is less than 40 degrees Fahrenheit, the concrete should not be placed.