The main concern people who own concrete structures have when it comes to concrete repairs is the appearance of cracks or potential ingress of water, but there are other, more unusual phenomena to be wary of.
One of these is bubbling or blistering concrete, most recently seen at St Saviour’s Reservoir as the result of years of environmental factors. However, it can also be seen with newer concrete projects and can be a cause for concern.
Bubbling concrete is caused by the movement of air or water through the concrete during the finishing process. If it cannot escape the surface it remains under a thin skin or mortar, developing a hollow nodule.
This can be caused by a wide variety of reasons, including concrete that is finished and sealed too far ahead of scale, a cooler subgrade than the concrete that leads to fluid and air displacement, strong winds that reduce the moisture on the surface of the concrete, or either too much or too little vibration.
Most of the time these blisters are not an immediate sign of structural issues, although since they can be complex to repair, the best step to avoiding the problem is to take steps to ensure that they don’t appear in the first place.
The biggest step is to take your time with the concrete curing process; make sure all of the air and bleed water has escaped before sealing the surface.
During cooler weather, use heated concrete or accelerator additives to ensure that the slab sets evenly.
Take full advantage of windbreaks if you are constructing or repairing a concrete surface that is inevitably going to face strong winds such as a tunnel, as this can cause premature drying that can exacerbate blistering.
Ultimately, most advice in this regard boils down to the typical construction principle of “measure twice, cut once”, the more time you take to prepare the materials, the better the chance you will get the application right the first time.