|
Bugholes
Concrete Technology
Home > Concrete Design & Construction >Bugholes
Bugholes: Causes and Prevention of a Pesky
Problem
 The
ever-increasing use of structural concrete as an architectural building
material has catapulted concrete’s measure of quality in surface
appearance to a prominent position within the concrete construction
industry. One of the primary influences affecting the surface quality
of concrete is bugholes. Bugholes are surface voids that result
from the migration of entrapped air (and to a lesser extent water)
to the fresh concrete-form interface. These surface defects manifest
themselves mostly in vertically formed surfaces.
During consolidation, the densification and subsequent volume shrinkage
of the fresh concrete forces entrapped air voids and excess water
out of the cementitious matrix. The water tends to migrate upward
due to a density differential and become bleed water. The air bubbles,
however, seek the nearest route to reach pressure equilibrium. When
in a vertical form, the closest distance for the air bubbles’
migration is to the interior form surface. If these bubbles are
not directed vertically to the free surface of the setting concrete,
after form removal, bugholes will be present if not abundant. Bugholes
are found more frequently in the upper portion of the concrete structure
or at angled form surfaces as a result of additive accumulation
from escaping air voids along the height of the structure. These
surface voids are primarily an aesthetic problem for exposed structural
concrete. However, problems do arise if the concrete surface is
to be painted or if the voids reach a larger diameter (typically
greater than 25mm [1 in.]).
Causes
Perhaps the most influential cause of bugholes is improper vibration.
Consolidation, usually through vibration, sets the air and water
bubbles into motion. A proper amount of vibration sends both entrapped
air and excess water to the free surface of the concrete –
either vertically winding through the matrix or laterally in a direct
route to the form wall. Improper vibration will either insufficiently
liberate the voids or over-consolidate the concrete resulting in
segregation and bleeding. (See ACI 309 or PCA’s Design
and Control of Concrete Mixtures for a full description
of consolidation using vibration.)
Another factor that promotes bughole formation is the form material
itself. Nonpermeable forms (i.e. polymer impregnated wood and steel)
and the use of form-releasing agents can restrict the movement of
the air voids between the concrete-form interface that is necessary
for bughole reduction. It is imperative that when form-releasing
agents are used, they are used according to the manufacturer’s
recommendations and used only with specified form material.
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.
A sticky or stiff mixture that does not respond to consolidation
can be directly linked to increased surface void formation.
Prevention
(1) Proper consolidation. Vibration should be completed
with each lift of concrete placed. As successive lifts are placed,
the vibrator should penetrate the previous lift, working the entrapped
air towards the form and then vertically up the sides.
(2) Permeable Forms. When impermeable
forms are used, more vibration is necessary to move the air voids
to the free surface of the concrete. The use of permeable forms
has been shown through research to reduce bugholes significantly
by allowing escaping air to move through the form to the ambient
air. Choosing the proper form releasing agent in the proper amount
can also improve the surface quality.
(3) Mix design. 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.
Self-Consolidating Concrete (SCC) is becoming increasing popular
for industries (precast especially) to improve surface quality.
 Bugholes
as surface defects are not detrimental to structural concrete from
a durability standpoint. Nevertheless, with the increased use of
structural concrete in finished construction, surface quality is
increasingly important. Through careful selection of materials,
quality workmanship, and dutiful supervision, surface voids can
be minimized.
Related Publications:
Bugholes in
Concrete Surfaces: Annotated Bibliography (LB14)
Guide for Surface
Finish of Formed Concrete (LT233)
Painting Concrete
(IS134)
Concrete Slab
Surface Defects: Causes, Prevention, Repair (IS177)
References:
Berger, Dean M., “Preparing concrete surfaces for painting,”
Concrete Construction, v. 22, no. 9, Sept. 1977, p. 481-484,
526-527. (ftp://209.143.248.167/woc/C770481.pdf)
Identification and Control of Visible Effects of Consolidation
on Formed Concrete Surfaces (ACI309.2R-98) Reported by ACI
Committee 309. American Concrete Institute, 2003, p. 6, 11, 18-19.
Price, W.F.; Widdows, S.J., “The effects of permeable formwork
on the surface properties of concrete,” Magazine of Concrete
Research, v. 43, no. 155, 1991, p. 93-104.
Samuelsson, Paul, “Voids in concrete surfaces,” ACI
Journal, Proceedings, v. 67, no. 11, Nov. 1970, p. 868-874.
|
|
|