Bugholes, pinholes and blowholes

One of the primary influences having an impact on the surface quality of concrete is bugholes or blowholes. Bugholes, pinholes, blowholes, surface voids – they're recognized by various names, but all refer to the same phenomenon. Blowholes are tiny, regular or irregular cavities ( over approx. 5 mm in size and often not exceeding fifteen mm ) found at the surface and in the core of structural concrete.

These surface voids are primarily an aesthetic obstacle for exposed structural concrete. [**] Problems arise if the concrete surface is to be painted or if the voids reach a larger diameter ( typically greater than twenty-five mm )

Bugholes result from the migration of entrapped air ( and to a lesser extent water ) to the fresh concrete-form interface at the time placement and consolidation. During consolidation, the densification and subsequent volume shrinkage of the fresh concrete forces entrapped air and excess water out of the cement matrix. Then the water will have a tendency to migrate upward due to its relatively low density and become bleed water. The air bubbles , however , find the nearest road to reach pressure equilibrium. For a vertical form, the closest distance for the air bubbles' migration is to the interior form surface. Bugholes are , however , found more often in the upper portion of the concrete structure or at angled form surfaces as a consequence of additive accumulation of escaping air voids along the peak of the structure ( PCA, Linder 1992 ).

Blowholes

Parameters influencing blowhole formation

Mix design can be a significant contributor to blowhole formation. A sticky or stiff mix that doesn't reply to consolidation can for instance be without delay linked to increased surface void formation. Workable, flowing mixtures are more easy to place and consolidate and reduce so the risk of blowhole formation. Concrete mixes that are richer in cement tend to show less blowholes than leaner mixes of the same workability. The results of the cement content on a mix made with a well-graded total appears , [**] to be immaterial ( Thomson 1969 ). Silica fume and other pozzolanas such as granulated blastfurnace slag and fly ash have been shown to boost the concrete surface qualities as it reduces bleedwater in the concrete. As a consequence, voids caused by besieged bleed water are absent ( Neville 1995 ).

Concretes made with lightweight aggregates having high water absorption present additional Problems. If the aggregate is fairly dry when mixed, air within the aggregate particles can be displaced by water right after casting, causing more blowholes than would occur in concrete of the same workability made with fully saturated aggregate ( Thomson 1969 ). 4.3.1

Improper vibration is perhaps the most influential cause of blowholes. Consolidation, generally 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 thru the matrix or laterally in a direct route to the form wall. When impermeable forms are used, more vibration is necessary to move the air voids to the free surface of the concrete. The utilization of permeable forms can reduce blowholes seriously by permitting escaping air to move thru the form to the ambient air.

Dense, wet, hard and firm forming panels as well as water-repellent and thickly applied release agents give rise to more porous concrete surfaces than textured, porous, hygroscopic and soft facing materials onto which the emulsifying agents are thinly applied ( Linder 1992 ). Impermeable forms ( i.e. Polymer impregnated wood and steel ) and the employment of form-releasing agents can limit the movement of the air voids between the concrete-form interface that's mandatory for blowhole reduction. It is, urgent a given form-releasing agents is utilized with an acceptable form material. Thomson ( 1969 ) reports moreover that the influence of formwork on the formation of blowholes is secondary provided that the screens are clean and smooth and treated with a reliable form oil. An absorbent shutter lining will obviously reduce the size and number of holes and may absolutely remove them, but such linings can be expensive or unrealistic to use on large contracts. Thomson states, likewise, that the sort of mold oil has little or no influence on the incidence of blowholes in concrete cast at ordinary temperatures. Some shutter materials and release agents may produce what is apparently a better finish by allowing a thin film of grout to hide the blowholes, but this film may be easily broken and removed by weathering.

The temperature of a shutter can cause a marked effect on the concrete finish. The casting of concrete of high or medium workability behind a thin shutter which is exposed to the cold can as an example result in the formation of little water-runs. The lower surface temperature delays the setting of the concrete and increases plastic settlement accompanying segregation of water from solids. The liquid water content increases also by water vapour present in the concrete which condenses in a higher degree on a less warm surface. This effect might be avoided by insulating the screens ( Thomson 1969 ).

Water voids

Water voids,Emerge from bleeding, happen more frequently than air voids and particularly in the edge sections of concrete components. The shapes of small water voids may bear a resemblance to the shapes of air voids. Water voids may also occur as narrow defects of longer length, with chain-like interlinked pores on perpendicular lateral faces. These run often perpendicular in streaks and are so also referred to as water streaks. The amount of water voids increases when release agents containing wetting agents are used, and in conjunction with soft facing materials that give during compaction. Unnecessary compaction, both re duration and power will also increase the amount of formed water voids ( Linder 1992 ).

Delaminations

Delaminations occur when air and bleed water become encircled under a prematurely closed mortar surface. The trapped air and bleed water separate the upper three to 6 mm layer of mortar from the essential concrete. Delaminations are extremely difficult to detect during finishing and become obvious after the concrete surface has dried and the delaminated area is crushed. A smaller and more obvious form of delamination is a blister that forms at the concrete surface from trapped air and bleed water.

The primary cause of delaminations is finishing the surface before bleeding is finished. Delaminations are much more likely to happen when factors that prolong the bleeding time of concrete are combined with factors that accelerate surface setting ( PCA IS536.01 2002 ).

Figure seven : Delaminations and blisters ( PCA IS536.01 2002 )

Minimization of pores

Measures for minimizing pore formation during manufacture of concrete components are given in the following keywords ( Linder 1992 ) :

o Cement with little desire to bleeding ( e.g. Most Portland cements with high fineness of grinding and cements containing pozzolanas, )

o total mix with sufficient ultrafine/sand contents, with a maximum particle size depending on the concrete cover and the kind of beefing up

o Utilization of plasticizers rather than a high water-cement ratio

o Uniform thin application of a suitable form release agent

o in depth compaction, especially near edge sections and corners of parts.

Bugholes, pinholes and blowholes can be minimized through proper mix design and pouring techniques.