Concrete professionals have different views on why fresh concrete sometimes develops blisters, especially in cooler weather.
Unlike skin irritations, concrete blisters are small, hollow bumps on the surface. While they are usually small, about the size of a dime or up to an inch wide, they can sometimes grow as large as three inches depending on the circumstances.
Let us explore the causes of blistering and their prevention strategies to help you understand what blistering is all about.
What is Blistering?
Blistering occurs when air gets trapped under the surface. This occurs when the surface seal, created during finish troweling or by a rapidly setting concrete surface, prevents the air from escaping. As a result, the trapped air forms blisters under the sealed surface.
Why it occurs:
Blisters form when bleed water or trapped air bubbles rise through the concrete but cannot escape the surface. This often happens when the surface is sealed too early during finishing, trapping air and water beneath the mortar layer. While blisters can appear shortly after finishing, they may be hard to spot in low light. Often, they become noticeable only after the concrete has cured and the blisters break open under traffic pressure.
Causes of Blistering in Concrete
Even though blisters frequently result in concrete delamination, it’s crucial to prevent the formation in the first place. After the surface hardens, it is typically challenging to fix this flaw as well as other kinds of concrete flaws. Typical elements consist of:
Excessive Evaporation of Bleed Water
When this occurs, the concrete still releases bleed water and trapped air even when the surface looks ready for final finishing. It’s a serious issue when the environment around you goes through:
- Low humidity,
- High wind speeds, and/or
- High ambient temperatures.
Entrained Air
Entrained air, whether intentionally added or present in higher-than-normal amounts, significantly reduces both the amount and speed of bleed water in the concrete. This reduction can make the concrete appear ready for floating and finishing sooner than it is, but proceeding at this stage can lead to premature finishing and potential quality issues.
Cohesive or Sticky Concrete
This circumstance typically occurs when the sand has an excessive amount of particles or cementitious elements, which results in less bleeding and a slower rate. When the air content in the concrete is lower, bleed water rises to the surface more quickly, completing the process in a shorter time. However, if the total amount of bleed water is higher overall, it takes longer for all the water to evaporate, which delays the start of the finishing process.
Prematurely Applying Dry Shake
Premature application of dry shake materials often happens when working with air-entrained concrete. This is because the reduced bleed water caused by the entrained air can make the surface appear ready for dry shake application earlier than it is.
Placing the Slab on an Impervious Base
Placing a concrete slab on an impervious base, such as a vapour retarder, can contribute to blistering because the impervious layer prevents bleed water from escaping downward. The bleed water is pushed upward to the surface instead. If the surface of the concrete sets or hardens too quickly due to finishing or environmental conditions, the trapped bleed water and air below the surface can create pressure. This pressure may cause blisters to form as the surface layer is lifted slightly, resulting in imperfections.
Prevention Strategies for Blistering
Some preventive measures are to be taken to avoid blistering as follows:
Proper Finishing Techniques
Work quickly to place, level, and bull-float the concrete, taking care not to create a thin layer of mortar on the surface. During the first floating, keep the float blades flat to avoid hardening the surface too soon. When using a machine trowel, make sure the blades are properly balanced, and fix any surface flaws with a hand trowel.
Timing of Finishing Procedures
Finish the concrete only when it’s ready. After the first bull floating, wait for the surface to release bleed water naturally before moving to the next step. In cooler weather, you can use accelerating admixtures or heated concrete to speed up the process and prevent blistering. Make sure to leave enough time between troweling passes for the best results.
Use of Appropriate Tools
For the first step, use a magnesium hand float to smooth the concrete. If you have enough time, go over the surface twice with the float to improve the finish. When using a machine trowel, start with flat blades during the first pass to evenly smooth the surface. As you move the machine, ensure each pass overlaps the last one by about half the machine’s width. After using the trowel machine, if you notice any bumps or rough areas, use a hand trowel to smooth them out and achieve a more polished, even finish.
Avoiding Premature Sealing of Surface
Avoid sealing the surface too early, as this traps air and bleed water, leading to blisters. Protect the surface with coverings, such as polyethylene, to prevent evaporation. In high evaporation conditions, test a small section to check for bleeding before proceeding with finishing.
Solutions for Existing Blisters
If you are worrying about your existing blisters, the following solutions will help you to get rid of blisters. These procedures work well to remove existing blisters and enhance the concrete surface’s general condition.
Flattening Trowel Blades
When finishing, use a trowel with flat blades to avoid sealing the concrete surface too soon. This helps reduce blister formation and prevents the surface from becoming too dense too quickly.
Using a Wood Float
During the finishing process, surface sealing can be decreased by using a wood float instead. By improving ventilation for trapped air and bleed water, wood floats help prevent blisters from forming.
Delaying Further Finishing
If there are indications of premature sealing or quick setting on the surface, stop further finishing work. To guarantee adequate curing and prevent air or moisture from being trapped beneath the surface, allow the concrete to stabilise before proceeding.
Protecting Surface from Rapid Evaporation
To keep the concrete sufficiently moist and prevent evaporation, cover it with polyethylene or apply a fog spray. By doing this, the surface is kept from drying too quickly, which could cause scaling and sealing.
Conclusion
It’s crucial to employ the right finishing procedures, ensure there are sufficient curing conditions, and let the concrete adequately bleed and set before finishing in order to avoid blistering. You may reduce the chance of blistering and create a smooth, long-lasting concrete surface by being aware of the causes and taking preventative measures.