Blisters in Concrete - Identification and Causes

Blisters in concrete are hollow and unnoticeable nodules observed on the concrete surface. Their size range between one and three inches in diameter based on the extremity of the defect.

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Fig.1. Blisters Formed on Concrete Surface

All those construction practices and situations that result in the entrapment of air or water bubbles below the concrete surface layer are the major causes of blisters formed in concrete. 

In general, the main cause of entrapped air on the concrete surface is the improper finishing process. Prevention of blisters in concrete is achieved by putting forward measures to reduce air bubbles and excess water accumulation on the concrete surface.

How to Identify Blisters in Concrete?

Blisters in concrete are observed as dense troweled skin of mortar having a thickness of 3mm with voids underlying it. These air or water bubbles move around under the surface during the troweling process. 

If you would have noticed, these blisters generally occur just after the completion of the finishing operation. In areas where there is no light, small blisters are tough to identify. These are identified only when they crack under heavy traffic loads. 

Fig.2. Formation of Blisters on Concrete

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What Causes Blisters in Concrete?

Some of the major causes of blisters in concrete are:

1. Improper Vibration of Concrete

Over-vibration can result in bleeding water or a layer of mortar forming over the concrete surface and under-vibration cannot completely throw out excessive air in concrete efficiently. Both cases are undesirable.

2. Improper Finishing Tool for Concrete

The finishing or the surface tool is chosen based on the concrete surface. The floating tool used must be kept as flat as possible to avoid improper usage.

3. High Evaporation of Moisture

This is caused by the high-temperature conditions of the site. Excessive vibration of the bleed water brings a look of premature finishing that would result in the starting of finishing operations. But in reality, bleeding water and air bubbles are still underlying within the concrete layer.

4. Improper Subgrade

If the subgrade is cool, the top layer of the concrete surface dries fast and the bottom layer is wet. This gives a wrong interpretation and performs finishing operations.

5. Structural Features of Structure

With the increase in thickness of the structure, the setting time also increases giving time for the entrapped air and the bleed water to rise on the top surface.

6. Improper Concrete Mix

Concrete with less cementitious material bleeds rapidly and tends to delay the finishing process.

7. Improper use of air entrainment in concrete

Giving air entrainment insufficiently to concrete reduces the bleeding chances and gives an appearance that is concrete is ready to finish. But it results in premature finishing.

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