BUGHOLES IN CONCRETE

One of the primary influences affecting the surface aesthetics of concrete are bugholes. ACI 347-04, “Guide to Formwork for Concrete,” defines bugholes, or surface voids (blowholes), as small regular or irregular cavities, usually not exceeding about 5/8 inch (15 or 16 mm) in diameter, resulting from entrapment of air bubbles in the surface of formed concrete during placement and consolidation. These surface defects manifest themselves mostly in vertical surfaces. To a lesser extent, accumulation of extra water or bleed water between the form face and concrete can also cause such blowholes.

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 difference 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 1 inch).

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. The degree or amount of vibration required is inversely proportional to the consistency of the mix. For too wet mix minimal vibration is required but for concrete with low slump more vibration is required. It is important to note the area of concrete vibrated during the vibration process. Usually, the area of influence of a vibrator head is four times the width of the vibrator head, as shown in the sketch. Hence for thicker sections the number of insertions must be increased.

Surface condition of the forms plays an important role in the formation of bugholes. The geometry, type of material, and condition of forms all play a role in reducing bug holes. The number of visible bug holes can be reduced by using smooth forms, free of dents, holes, or build-up, along with the proper form release agent. The clean form surface gives air voids at formed surfaces the opportunity to move upward more freely as it is displaced by the concrete. In addition, check that the forms are tight. Otherwise, concrete can seep out of the edges, causing voids or honeycombing to form.

Another factor that promotes bughole formation is the form material itself. Non-permeable 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. Wipe away excess form release agent from each form, as additional release agent on the form surface can bubble up and give air an opportunity to consolidate. 

 The type of release agent has also an important role to play. There are mainly two types of release agents used generally at site.

 (1) Chemically reactive agents: When a chemically reactive form release agent is used, a nonviolent chemical reaction takes place when fatty acids react with free lime on the surface of fresh concrete. This reaction results in the formation of a metallic soap, a slippery material that allows air bubbles to rise along the vertical surface. This soapy film prevents the hardened concrete from adhering to the forms during stripping.

 (2) Barrier release agents: Thicker coatings on forms are typical of the older barrier type materials, like heavyweight used motor oil, vegetable oil, diesel fuel and kerosene. Barrier type release agents are less expensive than chemically reactive agents, but they are not recommended for reducing bugholes.

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. As mentioned earlier, a stiff and sticky mix needs more vibration than usual. The air content of the mix also can be checked to assess the entrapped air content in the mix.

 Specifications

The permissible number and size of bug holes in a concrete surface is not defined in either ACI 301-05 or ACI 301-10, “Specifications for Structural Concrete.” In fact, ACI 301-05 does not limit either the number or size of surface voids in as-cast surface finishes.

 Specifications for Structural Concrete (ACI 301-10) was the first document to establish in mandatory language, and mostly measurable and objective requirements, three different as-cast formed-finishes. They were not changed in ACI 301-16 and are described, as follows:

 Surface finish-1.0 (SF-1.0):

(a) No formwork facing material is specified

(b) Patch voids larger than 1-1/2 in. wide or 1/2 in. deep

(c) Remove projections larger than 1 in.

(d) Tie holes need not be patched

(e) Surface tolerance Class D as specified in ACI 117

(f) Mockup not required

 Surface finish-2.0 (SF-2.0):

(a) Patch voids larger than 3/4 in. wide or 1/2 in. deep

(b) Remove projections larger than 1/4 in.

(c) Patch tie holes

(d) Surface tolerance Class B as specified in ACI 117

(e) Unless otherwise specified, provide mockup of concrete surface appearance and texture

Surface finish-3.0 (SF-3.0):

(a) Patch voids larger than 3/4 in. wide or 1/2 in. deep

(b) Remove projections larger than 1/8 in.

(c) Patch tie holes

(d) Surface tolerance Class A as specified in ACI 117

(e) Provide mockup of concrete surface appearance and texture

Cure

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. The vibrator influences the area around its head at about four times its width of the head.

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.

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.

Pour rate. While most contractors want to get the pour done as quickly as possible, if it is too fast, the air does not have a chance to escape. Filling forms at a smooth, slightly slower rate allows for the right balance, so that forms can be filled, while giving air the chance to get out. Place concrete at a rate such that its rise in the form is not less than 2 m/h vertically.

Placement method. During placement, minimize the vertical distance between the concrete discharge point and the form to minimize the free-fall distance. Having too much free-fall distance during concrete placement can increase the amount of air to be entrapped in the concrete.

Bugholes as surface defects are not detrimental to structural concrete from strength and 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.

Repair

1-    Chip off the weak areas and loose edges around the bughole.

2-    Clean it with brush or air to remove and the loose debris.

3-    Wet the area with water.

4-    Apply a scrub coat made with 1:1 cement- sand and sufficient water.

5-    Prepare repair mortar using the same material as the concrete but without any coarse aggregate. Use only enough water to create a mortar with stiff consistency.

6-    When the scrub coat begins to lose the water sheen, apply the repair mortar, and thoroughly consolidate into place.

7-    Strike off mortar, leaving the patch slightly higher than the surrounding surface to compensate for shrinkage.

8-    Leave the patch undisturbed for 1 hour, then finish to match the texture of the surrounding concrete.

9-    Continue wet curing for minimum 7 days for better result.

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Postscript: I am not the author of the above article. None of the sentence is mine. I have compiled the information available in books and on websites just for information purpose only.