Self Consolidating Concrete and Bug Holes

Self-consolidating concrete (SCC) has been used to produce many aesthetically critical projects since its development nearly 20 years ago. Although mixes of various levels of strength and durability can be designed to generate smooth, defect-free surfaces, this does not ensure that the finished structure will be unflawed.

Two outside influences can greatly affect the appearance of concrete and bug holes. The first is forming materials—the most popular are steel and plywood. The second factor is release agents. Different types and brands of release agents (form oils) give varying degrees of surface defect. The method of application of these agents also plays a part in the final product appearance.

Five agents were analyzed in this study.

Release agents fall into two primary types, barrier and reactive. Barrier release agents create a physical barrier between the form and the concrete. The barrier agent used here was a plain, low viscosity petroleum oil containing paraffin that acted like a wax, aiding the release of form materials.

Reactive release agents contain weak acids derived from vegetable oils or animal fats. They may also include ligno-sulfates and tall oils, byproducts from paper manufacture. Reactive agents fall into two primary categories: vegetable oils and petroleum-based. Most reactive release agents on the market today have petroleum-based carrying agents. In this study one vegetable-based and three petroleum-based oils were considered.

Vegetable-oil-based agents had two disadvantages. After five to eight castings on a form, a buildup of flaky residue from the agent was noticed. Repeated form cleaning was necessary to maintain a good finish. Also, vegetable oils normally turn rancid when exposed to air and heat. The limited temperature range for product storage poses a problem.

The vegetable- and petroleum-based reactive agents were applied to the form in a thin mist, as prescribed by the manufacturers. The barrier type agent allowed the forming material to release from the concrete only when applied heavily. Heavy applications increased the presence of bug holes. The barrier agent also required five times more clean-up time to return the form surface to an acceptable casting condition after each pour. The vegetable-based agent provided the best results when used on new plywood, though only a minor improvement over two other reactive agents in the study. The petroleum-based reactive agents produced the best average product appearance when used with steel forms. Not all of the reactive agents gave an acceptable appearance; even in certain reactive form oils various carrying agents can cause flaws in the concrete appearance. The barrier type agent consistently produced a poor finish, even when more labor than usual was put into release agent application.

Conclusion

Any defect in a forming system will become extremely visible in well-developed SCC. An overall smooth surface will exaggerate the appearance of marks left on the concrete from scratches in formwork, rust pits, concrete paste buildup, or other defects.

Barrier type release agents should not be used with SCC when the appearance of the formed finish is important. When barrier agents are applied thinly, the concrete does not release well from the form, and the surface of the concrete "peels." When applied heavily, the barrier agent traps large numbers of air pockets or bug holes.

When a reactive release agent has been chosen, test specimens should be cast to ensure that the material performs well with the SCC being used. Not all reactive agents perform equally well with any concrete, though they generally do give a better finish than barrier agents. Reactive agents should always be applied in a thin layer, as prescribed by the manufacturer.

In 2003 we launched a study to evaluate the effects of form conditions on the finished surface of SCC. Two SCC mixes were developed that could produce a defect-free formed surface. One design was a "high fines" SCC (Mix 1 in the table), and the other used a stabilizer, or viscosity modifying admixture (VMA).

Both were well-designed, stable mixes, verified by casting and testing samples. Both mixes also attained 5% ±1.5% entrained air content that met the industry accepted criteria for specific surface and spacing factors, exhibiting a very stable air matrix.

Note that entrained air content does not affect the presence of bug holes; entrapped air—air bubbles too large to benefit the concrete—is what clings to the formwork. Entrapped air can be generated during the casting process (most bug holes appear near where a form is filled), or large air bubbles can be generated and trapped in the concrete because of the superplasticizer. The new-generation polycarboxylate-based high-range water-reducing (HRWR) admixtures often contain significant amounts of defoamer to reduce air entrapment, but this can wreak havoc on the entrained air matrix.

Forms

Wood forms and metal forms will show significant differences in surface defects. Wood forms tend to produce fewer bug holes than metal because wood forms soak up excess release agent that has been hastily applied. Any small amount of extra oil on a steel form will react with the concrete mix and create small bug holes, perhaps better termed "pinholes." Therefore, proper application is absolutely necessary. Steel forms require more attention to ensure a clean, smooth surface. Any defect on the form will create a blemish on the concrete surface.

A form's cleanliness and smoothness greatly affect the appearance of the concrete surface. This simple, logical truth cannot be overstated when dealing with SCC.

Forms should be as smooth as possible to allow entrapped air to move easily upward along the form system; they must be kept free of paste buildup and laitance, which prevent air and water pockets from traveling to the concrete surface. In our study, as paste built up on each form with subsequent castings, the concrete surface appeared worse. Scratches or gouges will hold air against the surface of the concrete. Any steel forms pitted with rust will cause blemishes, which at times produce more bug holes than are noticeable with vibrated conventional concrete. We also noticed that when the form skin had a lower temperature than the SCC, air voids smaller than usual were present. That occurred at approximately a 25° F temperature difference.

Whenever you grind a "seasoned" steel form, you remove the protective barrier previously produced by the reactive form release agent. Rusted forms have negated the barrier that was in place. Once the form is ground, raw metal is exposed. The reactive portion of the form release agent, typically a fatty acid, has a natural affinity for metal. The fatty acid attacks the raw metal and forms metallic oleate, which acts as a protective coating. Subsequent applications of reactive form release agents are prevented from getting to the metal by the protective layer of metallic oleate, allowing the reactive portion of the form release to be available to react with the free lime on the surface of the concrete. This reaction forms a chemically inert metallic soap, which gives good release and allows free air to rise more easily to the surface on vertical walls. Until the form is seasoned, or the protective barrier is formed, the reactive portion combines with the metal, leaving nothing to react with the free lime. The steel forms used in this study were seasoned after cleaning and before further castings took place. That aided the finish somewhat but the pits left in the forming material by the rusting process trapped air voids, creating bug holes.

Paul Ramsburg