What is Blistering Concrete? Causes of Concrete Blistering | How To Fix Concrete Blisters

 What is Blistering Concrete? Causes of Concrete Blistering | How To Fix Concrete Blisters

 What is Blistering Concrete? | Why Does Concrete Blistering Occur?| How To Fix Concrete Blisters

Blistering Concrete – What Is It?

Blistering happens when air becomes trapped in the concrete and is unable to escape through the seal formed during finishing operations or as a result of a fast-setting surface. Blisters are formed when air gathers in pockets underneath this airtight surface barrier.

Concrete blisters are hollow, unnoticeable lumps on the surface of concrete. Though blisters are typically small, measuring between dime and one inch in diameter, they can grow to three inches in diameter depending on the situation.

Blisters are normally range in diameter from a penny to one inch (25 mm), but can occasionally be as large as two or three inches (50 – 75 mm).

A densely troweled mortar skin approximately 1/8 inch (3 mm) thick conceals an underlying void that moves beneath the surface during troweling.

Blisters may develop immediately after the finishing operation is completed. Small blisters may be difficult to spot while finishing and may not be detected until they rupture under traffic in dimly lit regions.

Why Does Concrete Blistering Occur?

It begins when bleed water or trapped air bubbles pass through the concrete and are unable to escape. Typically, the surface was sealed prematurely during the finishing process, resulting in hidden air and bleed water voids beneath the mortar skin.

While blisters can appear shortly after completion, they might be difficult to detect in dim and/or poorly light environments. In this situation, the blisters are discovered only after the concrete has hardened, typically due to the weight of passing traffic.

What are the Causes of Concrete Blistering?

According to the National Ready-Mix Association, concrete blisters can be caused by a variety of factors, including the following:

  • Blistering occurs when the top surface becomes too hard to allow bleed water and trapped air to rise to the surface and escape. This results in the trapping of water and air beneath a thin dense troweled layer.
  • Thick slabs are particularly susceptible to this, as the surface of the slab sets well before the lower levels.
  • Slabs poured on extremely cold subgrades may experience the same problem, as the cold ground retards the setting of the concrete closest to the ground.
  • Trowel finished air entrained concrete is EXTREMELY sensitive to this occurrence.
  • Blisters can be caused by excessive or inadequate vibration.

Factors Contributing to Blister Formation

Although blisters are a frequent cause of concrete delamination, it is critical to avoid their creation in the first place. After the surface hardens, it is often difficult to remedy this flaw and other forms of concrete problems. Several common factors include the following:

Vibration That Is Either Excessive or Insufficient

Inadequate vibration prevents trapped air from being released, but excessive vibration results in a thick coating of mortar on the surface.

Usage of an Inappropriate Floating Tool

It’s critical to test the surface to ensure you’re using the appropriate instrument, such as a wood or magnesium bull float. Otherwise, an inappropriate floating tool will be unable to seal the surface, resulting in additional concrete difficulties.

Otherwise, an inappropriate floating tool will be unable to seal the surface, resulting in additional concrete difficulties.

Excessive Evaporation of Bleed Water

When this occurs, the surface seems to be ready for final finishing, but the bleed water and trapped air continue to leak from the concrete. It becomes a significant issue when the surrounding environment experiences:

  • Extremely high ambient temperatures;
  • Extremely high wind speeds; and/or
  • Extremely low humidity.

Embedded Air

Whether entrained air is employed or at a higher-than-normal level, it considerably reduces the amount and rate of bleed water within the concrete. As a result, the concrete appears to be ready for flotation and finishing, yet doing so would result in premature finishing.

Subgrade That Is Cooler Than Concrete

Due to the temperature differential, the surface concrete sets faster than the bottom concrete, frequently resulting in an early finish.

Thick Concrete Slab

When the slab thickness is more than the optimal depth, it takes longer for the bleed water and trapped air to rise to the surface. This causes a delay in the entire curing and finishing process.

Concrete That Is Cohesive or Sticky

This condition typically occurs when there is an excess of cementitious elements or particles in the sand, resulting in less bleeding and a slower rate of bleeding. However, while lower content levels allow for faster bleeding over a shorter time period, the concrete suffers from a greater overall volume of bleeding, delaying the finish.

Applying Dry Shake Prematurely

This is a frequent occurrence with air-entrained concrete.

Placing The Slab on A Water-Resistant Base

Placing a concrete slab on top of an impervious base, such as a vapor retarder, prevents the bleed water from being absorbed by the subgrade.

Is It Possible to Prevent Concrete Blisters?

Yes. Caution should be exercised if a concrete surface appears to be ready for finishing ahead of the anticipated completion date.

Any finishing procedures should prioritize the rapid placement, striking off, and bull flotation of the concrete as well as without accumulating a layer of mortar on the surface.

Once these stages are complete, if possible, postpone any further completing. Cover the surface with a covering, such as polyethylene, to prevent evaporation.

Cover a tiny portion of the slab in the presence of high evaporation rates to see if bleeding is evident.

Ensure that the float blades are flat during the first floating to avoid premature densification of the surface.

Using an accelerating additive or warm concrete frequently inhibits blister formation in cool weather.

Additionally, non-air entrained concrete is preferred for interior slabs, although steel troweling any air entrained concrete is not recommended.

Guidelines For Avoiding Concrete Blistering

  • Do not seal the surface until air and/or bleed water have been allowed to escape from the concrete beneath.
  • Do not use dry shakes on air-entrained concrete.
  • Using accelerated or heated concrete facilitates equal settling throughout the slab depth in cooler temperatures.
  • Take precautions to prevent premature drying and/or evaporation of the concrete.
  • Avoid using severe vibration, such as a vibratory screed, on slumps greater than five inches.
  • Avoid using a steel trowel to create air-entrained concrete. If use is required by the project specifications, proceed with utmost caution when scheduling the completing.

How To Fix Concrete Blisters: What to do When Concrete Blisters Occurs?

Generally, there are 2 options. You can flatten the trowel blades or shred the surface with a wood float, then postpone finishing as long as possible. If current conditions result in rapid evaporation, the rate may be delayed between completing processes by using wind breaks, misting the surface with water, adding evaporation retarders, and/or covering the surface with polyethylene film or wet burlap.


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