Bugholes in Concrete - Causes and Prevention

Bugholes are one of the surface defects that are observed in a finished hardened concrete structure. These are formed when entrapped air within the fresh concrete migrates and reach the surface of the concrete. They are observed as small bug holes or surface voids or small cavities not exceeding 15 mm in diameter (As per ACI 347-04).

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This mainly occurs during the consolidation and densification process of concrete. During this process, the volume of fresh concrete decrease , shrinks the fresh concrete letting the entrapped air voids and excess water out of the cementitious matrix. 

Bugholes are mainly observed in vertical surfaces and on the upper portion of the concrete structure or at the angled form surfaces due to excessive escaping of air voids along the height of the structure. These surface defects is a primary concern when it comes to aesthetics for exposed structural concrete. If the voids reach in a larger diameter on the surface ( when greater than 1 inch), problem arises during painting.

Formation of Bugholes in Concrete

During the consolidation process, the concrete is subjected to forces that force the entrapped air voids and excess water to migrate to top of the surface and forms bleed water. These air bubbles takes the easiest path for migration i.e. in vertical direction and closer to formwork. It also accumulate between the concrete-form interface which increase bughole formation.

What Causes Bugholes in Concrete?

1. As mentioned above, bugholes tend to occur during the consolidation process. It a more precise manner, it is due to improper vibration of concrete. Vibration in a proper rate and amount would help to send these entrapped air and excess water to the free surface of the concrete either in a vertically or laterally. Improper vibration would liberate these voids in an insufficient manner that also result in segregation  and bleeding.

Note* Details and recommendations for vibration of concrete is given in ACI 309 

2. Another major cause of bughole formation in concrete is due to the form material. Use of nonpermeable forms and application of form-releasing agents can restrict the movement of air voids between the concrete-form interface which can efficiently reduce bughole reduction.

3. The third important factor that result in bughole formation is improper mix design of concrete. A sticky or a stiff mix that do not cooperate with compaction or consolidation can increase surface void formation.

How to Cure Bugholes in Concrete?

  1. Proper vibration of concrete by having lift after every concrete placed. With each successive lifts, the vibrator must penetrate the previous lift area. This would let the entrapped air to moved vertically up and towards form.
  2. Use permeable forms to make the air voids move to the free surface of the concrete. Use of proper releasing agent is also recommended. Nonpermeable forms like polymer impregnated wood and steel are good forms.
  3. A concrete mix that is workable, flowing and easy to consolidate can reduce the risk of bughole formation. Use of well graded aggregates, proportioned cement and admixture usage contributes to good mix for consolidation.
Bugholes are surface defects as explained above and hence they do not cause any harm to the structural behaviour and performance of concrete in terms of durability. The use of good quality concrete mix, good workmanship and supervision, a structure concrete with less surface voids and high surface quality free of bugholes are obtained.

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