Bugholes in Precast

Are you bugged with bugholes?  Bugholes are air bubbles that show up on the surface of the concrete after it is stripped.  They are a problem for both precast and pour-in-place concrete.  In this blog, I will provide some tips for evaluating the root cause of the bugholes and ultimately preventing their reappearance.

Often, these tiny, visible air bubbles are only a cosmetic issue, and they are not detrimental to the performance of the concrete.  When they become larger and cover more area, it is something that should be evaluated.  Occasionally these voids are large enough, or so numerous that it renders the product defective, requiring immediate corrective action.

Bugholes are the result of a void in the concrete paste.  A very common cause of bugholes is poor consolidation practices.  Either the concrete is insufficiently vibrated, or it is actually over vibrated, and the result is a void, or series of voids in the concrete.  The general rule for vibration is to vibrate until the breaking bubbles stop.  These are the smaller bubbles that pop up to the top.  Over vibration will cause bubbling of the concrete at the exposed surface, and this may be creating entrapped air.

Stinger vibrators must be used properly.  When I visit a precast plant, it is rare that I see the best practices being used.  These consolidation tools are inserted vertically into the concrete at a rate of 1’ per second.  The withdraw should be half that speed at 1’ every two seconds.  The vibration produces a radius of action.  This is the area of consolidation at a certain distance from the head of the vibrator.  Each insertion of the vibrator should be separated enough that the radius of action will overlap by about 2”.  A stinger vibrator must never be used to move concrete laterally.  This will cause segregation.

Another cause of bugholes is form release agent; either the lack of it, the wrong type, or over application.  There are two basic types of release agents: barrier and reactive.  A barrier release agent is typically oil based and it creates a physical barrier with between the paste and the form surface.  A reactive type will chemically react with the concrete paste to form a soap that creates a slippery surface, preventing the concrete from sticking to the forming surface.  The manufacturers of these products can provide information on the best product for your application.

Something else that can cause bugholes is a poorly designed concrete mix.  The best concrete will have well graded aggregates with various sizes from very large to very small that naturally consolidate and fill the voids.  The paste will be sufficient to fill the void space and glue every aggregate particle together.  The amount of water is kept at a minimum to reduce bleeding.  Bleeding is the transport of the excess water from the concrete to the surface.  Contact a mix design specialist for advice on your mix design.  Often, your cement or admixture supplier will have a specialist who can provide the advice you need.

Self Consolidating Concrete (SCC) has improved the aesthetic qualities of concrete since its introduction in the 1980’s.  Today, many precast concrete plants are able to place a very strong, high quality concrete with very little or no vibration.  The flowability of SCC produces a surface that is virtually bughole free.  The added cost of the mix is offset by the saving realized in labor and the improved customer satisfaction.  For more information on SCC, contact the National Precast Concrete Association (www.precast.org), and ask for the SCC White Paper.