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Bug Holes are voids formed
during placement. Small pockets of air or water are
trapped against the form and revealed when forms are removed.
Honeycombing is the formation of pockets of
coarse aggregate during placement. Both problems occur in
vertical forming. The problem can increase with the height of the lift.
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Common
Causes
Both
problems are primarily caused by the way concrete is placed and
compacted:
- Vibration
may not be adequate or well spaced.
- The
mix may be sticky.
- Entrapped
air not removed by vibration, air bubbles move to the form surface.
- Improper
application of Form Release agent or wrong type
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Prevention:
(a)
Avoiding
Bug Holes:
- Work
the voids at the form face up and out of each lift.
- Let
the vibrator drop through the lift, then vibrate upward.
- Don't
overvibrate at the center of the wall.
- Move
the vibrator as close to the form as possible.
- Add
upward external vibration if necessary.
- Reduce
the height of each lift to make void removal easier.
- Consult
ready mix producer and review aggregate size and shape options.
- Reduce
sand content.
- Use
low slump concrete.
(b)
Avoiding
Honeycombing:
- Do
not place concrete over 4" slump-for a higher slump, use a high range
water reducer admixture.
- Improve
compaction during concrete placement.
- Deposit
concrete as uniformly and vertically as possible. A Tremie may be
required to reduce free fall.
- Decrease
the lift height for more efficient vibration.
- Insert
the vibrator more frequently.
- Vibrate
as close to the form face as possible.
- Be
sure to vibrate through each lift.
- Minimize
delays between lifts.
- Use
a super-plasticized concrete if the reinforcing or the section's design
restricts vibration.
- Adjust
the concrete mix proportions by decreasing coarse aggregate and
increase sand content.
- Consider
air-entrained concrete.
- Check
the tightness of joints in the forms. Be sure to secure all form joints.
- Look
for grout leakage through openings in the formwork.
"A good scientist is a
person with original ideas.
A
good engineer is a person who makes a design that
works with as few original ideas as possible"
Freeman
Dyson
Be
a one-step-ahead Civil Engineer
by being aware of
& making use of latest
ideas/developments in fields of construction material, equipment,
machinery
& methodology
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Concrete-Techgroup
thanks Theron Tobolski
for permitting the use
of this article from their website (http://www.prairie.com)
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