How to prevent concrete from blistering

Concrete blisters are hollow, low profile bumps that form on flat surfaces, such as concrete floors and pillars. These bumps can range in size from a few millimetres up to five centimetres. Blisters mostly form just after the concrete surface has been troweled and finished. 

There are a few reasons as to why blisters may form in the concrete. The main cause is trapped air bubbles which leave voids in the concrete as they travel to the surface. However, the troweled and compact surface may trap these bubbles. The surface of the concrete usually dries and cures faster than the wet interior. This is why contractors vibrate concrete after pouring.

Why do blisters form in concrete?

Blisters are more likely to form for the following reasons:

  1. The concrete was not compacted enough after pouring. This insufficient vibration will not allow trapped air to escape through the surface of the wet concrete.
  2. The concrete was vibrated too much. This causes a thick layer of mortar to rise to the surface, which traps air bubbles and prevents them from escaping.
  3. The wrong tools are used for floating and troweling the surface of the concrete. The surface should be tested before deciding whether to use a wooden or a metal bull float. The floating tool should not seal the surface too early and it should be kept flat during the floating process.
  4. Excessive bleeding occurs whereby water is released to the surface of the concrete, leaving voids as it travels through the concrete. This problem is made worse when the concrete surface is finished and sealed too early (before the bleeding stops).
  5. The underlying soil is cooler than the concrete. This causes the top surface of the concrete to set faster than the interior and the bottom layer. 
  6. A sticky concrete mix, caused by too much cement and not enough aggregate, can trap air bubbles before they can escape through the surface. 
  7. The concrete is poured directly on top of a plastic sheet or impervious base. This prevents bleed water from being absorbed by the underlying soil.

How to prevent concrete from blistering

Contractors need to be careful of concrete that looks ready to be finished and sealed before expected. If it seems too soon for the final finishing, then it probably is. When contractors start the finishing process, they should be careful not to work up a thick layer of mortar at the surface of the concrete. This will trap any remaining air bubbles underneath.

After finishing the surface for the first time, any further processes should be delayed for as long as possible and the surface covered with a plastic sheet to protect it from excessive evaporation. If high evaporation rates exist (high temperatures and low humidity), then contractors can cover a portion of the slab to see if bleeding is still taking place.

It is recommended that contractors avoid using air-entrained concrete mixes for interior slabs and floors. They should also avoid using a steel trowel when working with air-entrained concrete mixes outdoors. If blisters do start forming, use a wooden float to tear the surface and release the air bubbles. Then, delay the rest of the finishing process as long as possible.

Blisters can cause a number of problems for concrete surfaces. They weaken the top layer and are prone to collapsing over time. This can leave large spalls and chips in the concrete surface. The trapped air can also lead to cracks within the concrete and are likely to affect and layers of paint or sealer that are applied on top of the concrete.


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Written by Joshua Oates


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