Concretes containing silica fume help prevent corrosion of steel reinforcement because of their high resistance to chloride-ion penetration, but finishers often have trouble striking off and troweling these concretes, which are sticky and bleed very little, especially at silica-fume dosages higher than 10% by mass of the cement. A recently developed blended cement offers another way to keep out the chlorides, but with a less pronounced effect on finishability.
Fordyce Concrete Co., Overland Park, Kan., a subsidiary of Ash Grove Materials Corp., has developed an IP cement made with a calcined shale and clay. They reason that it should reduce chloride permeability. Trial batches for a parking garage at the Kansas City, Mo., International Airport proved that assumption to be correct. Based on field tests, chloride permeability of the resulting concrete ranged from 600 to 800 coulombs, and the contractor reported no problems with finishing the modified mixture.
On another project, a Pocatello, Idaho, medical-center parking garage, Pocatello Ready Mix replaced a mixture that contained 11% silica fume by cement mass with a mixture that included 3% silica fume and the same blended cement used for the Kansas City project. The revised mixture reduced concrete cost by about $8 per cubic yard, yielded an average strength of 6600 psi and met the chloride-permeability specification. But most important of all, the customer was satisfied.
Keywords: calcined, shale, clay, silica fume, chloride, ion, permeability, blended cement, pozzolan, Ash Grove, IP cement, Fordyce, Pocatello, Duracem