We’ve been asked to bid on an upcoming concrete bridge deck. The project requires a hardened concrete that will resist corrosion. Normally on jobs like this we recommend micro-silica. But in this case, there’s a catch.
The bridge is part of a high-profile project that features decorative concrete sidewalks and architectural precast panels. The designer wants a brighter surface on the bridge deck. Is there a way to lighten the concrete?
From what we’ve been told, many producers have been successful in providing a mix using metakaolin to satisfy both the anti-corrosion and architectural requirements of the contract specification. Metakaolin produces high-performance concretes meeting or exceeding the performance of silica fume.
High-reactivity metakaolin is a pozzolan that reacts with calcium hydroxide (lime) formed as a byproduct of portland cement hydration. The admixture causes an additional hydration reaction produces more calcium silicate hydrate (CSH). As these CSH interlocking crystals grow and solidify, concrete transforms from a plastic material to its hardened state.
At the proper proportions, the extra CSH provided makes concrete stronger and denser, less permeability, and more durable. According to Charles Cleary, a testing technician at Engelhard Corp., of Iselin, N.J., reacice metakaolin will add strength to a mix. The admixtures can also provide higher early-strength that will allow contractors to remove forms earlier.
The added crystals can also affect the concrete density. “Using proper design methods, you will get a denser surface with less porosity and greater resistance to corrosion,” says Cleary. “Plus, you will be able to produce a more uniform appearance.”
Unlike more common pozzolans such as fly ash and silica fume, metakaolin is a bright white material. This feature will help with your need for an architectural look.
Concrete using the metakaolin admixture has an additional feature. The metakaolin addition often yields a creamy fresh concrete that contractors find easier to handle.
Switching to this admixture shouldn’t be cost-prohibitive. In most locations, high-reactivity metakaolin is priced comparably to silica fume. As a rule of thumb, material costs range from about 45 to 50 cents per pound and less in truckload quantities.
Since MetaMax replaces 5% to 20% of the portland cement in a mixture, you have to deduct the cost of cement to determine the true cost of metakaolin. Doing the math, metakaolin adds about $10 per cubic yard of concrete. If you are using white portland cement, you may be able to use metakaolin without additional cost.
Keep in mind that the overall cost of using MetaMax also includes the ease of workability and the reduced requirement for high-range water reducer compared to silica fume.
Engelhard has posted an extensive library of case studies, technical reports, DOT approvals, a continuing education program, and other resources at its Web site, www.engelhard.com/metamax. There’s also an interesting animated film that explains cement hydration, the causes of common problems, and how metakaolin improves concrete.