What factors could cause bugholes? How does aggregate gradation affect bugholes?
NPCA Technical Services engineers answered:
There are many different variables that could affect the surface finish of hardened concrete and cause bugholes. For conventional wet-cast concrete, the consensus is that the primary culprit is improper vibration, but that’s true even with self-consolidating concrete. The rapid placement of SCC could lead to entrapped air and, if procedures to remove the air aren’t in place, bugholes could form.
Try to slow down the placement of fresh SCC into the form to see if this provides beneficial results. Also, you may need to reevaluate where the concrete is being placed in the form. The concrete may be getting pushed too far and trapping air on tricky corner sections or in areas with heavy reinforcement congestion. Other factors, including not properly applying form release agent, could also promote bughole development.
Your suggestion of aggregate gradations certainly can contribute to surface defects, including bugholes. The Portland Cement Association states, "Mix design can also be considered a significant contributor to bughole formation. Mix designs vary widely in their use of aggregate type, size, and grading and their use of admixtures and air-entrainment."
PCA also states, "Workable, flowing mixtures are easier to place and consolidate and therefore reduce the risk of bughole formation. Concrete with an optimally graded aggregate that avoids excessive quantities of fine aggregate, properly proportioned cement content, and any admixture that provides increased flow, workability, or ease of consolidation contributes to bughole reduction."
Another item to look into is the gradation of the fines. Very fine sands tend to hold more water, increase bleed water and add to the potential for trapped water, all of which could lead to bugholes. Also, sands that are very coarse without uniform gradation have been known to trap more air and, consequently, form bugholes. The best solution is to set up trial mixes to optimize the best results.
If you have a technical question, contact us by calling (800) 366-7731 or visit precast.org/technical-services.
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