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. 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.

SCC with an appropriate release agent yields defect-free surfaces.
SCC with an appropriate release agent yields defect-free surfaces.

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.