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Concrete Casting News from the Hill and Griffith Company

Avoiding Surface Imperfections in Concrete: bugholes, crazing, dusting, flaking, honeycombing and popouts

Posted by Hill and Griffith Company on Jan 30, 2020 12:01:12 PM

Excerpt from the July 2008 issue of Cement, Concrete and Aggregates Australia

Bugholes

What are bugholes

Bug holes are individually rounded or irregular cavities that are formed against the formwork and become visible when it is stripped. Small bugholes (less than, say, 10 mm) tend to be approximately hemispherical, while larger ones are irregular and often expose coarse aggregate particles. They tend to be more prevalent towards the top of a concrete placement than at the bottom, due to the increased compaction and static head at the bottom layer of the pour. Generally, they are regarded as an appearance problem though a concentration of large bugholes may lead to loss of durability. Under AS 3610 Formwork for Concrete, the size/extent of bugholes is, therefore, one of the criteria by which an off-form surface finish can be evaluated. This Standard incorporates full-size photographs, which enable a particular surface to be assessed for compliance with the specified class of finish. When using normal (i.e., impermeable) forms, it is impossible to achieve a bughole-free surface. However, the use of permeable forms may significantly reduce, if not eliminate, the incidence of bugholes.

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What Causes Bugholes?

Bugholes are caused by the entrapment of air against the inside face of the formwork. The extent to which they occur is dependent on:

  • the texture and stickiness of the formwork surface,
  • the inclination of the surface (the incidence of blowholes is increased where the formwork surface slopes inwards), 
  • the use of a poorly proportioned or sticky concrete mix, and
  • the amount of vibration.

Practices to Minimize the Occurrence of Bugholes

To minimize the incidence of bugholes:

  • Use rigid well-braced formwork.
  • Avoid the use of inwardly-sloping forms where possible.
  • Apply a thin coat of a form-release agent that spreads evenly and is not sticky.
  • Where appropriate use permeable formwork.
  • Avoid "sticky" concrete mixes, e.g., ones that may be over-sanded or have a high percentage of air-entrainment, and mixes that are too lean.
  • Place concrete at a rate such that its rise up the form is not less than 2 m/h vertically.
  • Ensure that the member is adequately compacted (see Compaction of Concrete data sheet for guidance on size of vibrator, spacing of insertion points and technique).
  • Pull vibrator up slowly through the concrete layer allowing time for the entrapped air to rise to the surface. 
  • Ensure the concrete against the surface is properly compacted.
  • Re-vibrate the top placement layer at about the same time as if a further layer was being placed on top.

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Crazing

What is Crazing?

Crazing or craze cracking (sometimes referred to as map cracking) is a network of fine random surface cracks spaced from 10 to 70 mm apart, dividing the surface up into irregular hexagonal areas. They are always most prominent when the surface has been wet and then dries off, leaving the damp cracks outlined against the dry surface. They are a surface feature and though unsightly, are unlikely to lead to structural or serviceability problems. There is no repair method; thus it is best to take precautions, as outlined below, to avoid them.

What causes Crazing?

Crazing is caused by the shrinkage of the surface layer relative to the base concrete. Usually, it occurs because one or more poor concrete practices are adopted, for example: 

  • Using too wet a mix
  • Finishing of the surface too early, i.e., while bleed water is present
  • Overworking the surface, thus bringing too many fines to the surface
  • Adding driers to the surface to try and remove bleed water
  • Not commencing curing early enough (three hours after completion of finishing is too late) or using inadequate curing procedures (such as intermittent wetting and drying).
On formed surfaces, it usually occurs where shiny, impermeable formwork is used and this is coupled with inadequate curing.

 

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Flaking

What is Flaking?

Flaking is where discrete pieces of the surface become detached, leaving a rough indentation behind. The pieces are usually flat, hence the name "flakes." Scaling should not be confused with flaking. Scaling is delamination of the concrete surface when exposed to freeze-thaw cycles, and although the appearance is similar, the mechanism is different.

What Causes Flaking Floors?

Flaking is caused by inappropriate finishing techniques that seal the surface and trap the water, which would otherwise have risen to the surface as bleed water. This water accumulates below the surface, forming a plane of weakness and resulting in delamination of the surface layer. Premature sealing of the surface can be caused by:

  • Commencing finishing too early because the ambient conditions dry the bleed water from the surface and the lack of sheen suggests that bleeding has finished. Note that some finishing tools more than others tend to seal the surface, e.g., a hand strike-off with a magnesium straightedge tends to seal the surface while a strike-off with a wood or magnesium bull-float pass leaves the surface open5.
  • The use of driers on the surface to absorb bleed water

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What is Dusting

A dusting floor surface is marked by an accumulation of fine material requiring to be swept up after the floor has been used. Also, a hand rubbed over the surface of a dusting floor will be coated with a fine powder

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What is Honeycombing?

Honeycombing refers to voids in concrete caused by the mortar not filling the spaces between the coarse aggregate particles. It usually becomes apparent when the formwork is stripped, revealing a rough and 'stony' concrete surface with air voids between the coarse aggregate. Sometimes, however, a surface skin of mortar masks the extent of the defect. Honeycombing may extend some depth into the member. Honeycombing is always an aesthetic problem, and depending on the depth and extent may reduce both the durability performance and the structural strength of the member.

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Popouts

Popouts are roughly conical depressions in the concrete surface created by localized pressure within the concrete, usually occurring after the concrete has been in place for some time. They can be categorized as small, medium or large depending on whether the diameter of the cavity is 10 mm or less, 10 to 50 mm, or greater than 50 mm respectively.

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