Your choices affect the surface aesthetics of self-consolidating concrete.



Self-consolidating concrete (SCC) has been used to produce many aesthetically critical projects since its inception into the American market 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:



1. Forming Materials - The most popular are steel and plywood.

2. 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.



Self-consolidating concrete with an appropriate release agent yields defect-free surfaces.



Self-Consolidating Concrete Research Study



In 2003, Sika launched a study to evaluate the effects of form conditions on the finished surface of self-consolidating concrete. Two SCC mixes were developed; both could produce an equally defect-free formed surface. One design was a “high fines” SCC, 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 roughly five percent 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.



Results From the SCC Study



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 self-consolidating concrete (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. 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. 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.



Types of Release Agents



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 sulphates 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.



Any defect in a forming system will become extremely visible in well-developed self-consolidating concrete. 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.



When a reactive release agent has been chosen, test specimens should be cast to ensure that the material performs well with the self-consolidating concrete 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.



For more information on Sika’s self-consolidating concrete (SCC), contact a specialist today!