Silica fume produces concrete that is stronger and more durable than conventional concrete. Field strengths of 14,000 psi have been achieved with this highly reactive pozzolan. Also, rebar corrosion is reduced because the reaction products of the extremely fine silica fume particles plug internal pores. This slows carbonation and helps keep chlorides out of the concrete. Because of these benefits, many engineers are now specifying silica-fume concrete for high-strength structural applications, abrasion-resistant surfaces, and structures exposed to deicing agents or salt water. Silica fume is available commercially in several forms: dry, densified dry, and slurried, each with or without chemical admixtures. Silica fume or products containing silica fume are available in bulk, drums, and bags, depending on the supplier. PRODUCING THE CONCRETE Three critical tasks must be performed with care when producing concrete containing silica fume: measuring, adding, and mixing. Measure silica fume with the same degree of accuracy as other concrete ingredients. Typically, accuracies of plus or minus one percent by mass or volume are specified. The type of material being used determines when to add the silica fume product. Make a few trial batches to find the best batching sequence for a particular plant. The silica fume must be uniformly distributed throughout the concrete. Within a single load, compressive strength variations of 3000 psi can result from poor mixing. PLACING THE CONCRETE Depending on the amount of silica fume, the fresh concrete can be more cohesive and less prone to segregation than conventional concrete. Because of the increased cohesiveness, use a slump that is 1 to 2 inches higher than normal for the same type of placement. FINISHING The biggest difference between conventional concrete and silica-fume concrete shows up during finishing. Adding higher amounts of silica fume reduces bleeding and may eliminate it. This makes silica-fume concrete more susceptible than conventional concrete to rapid surface drying and plastic shrinkage cracking. There are two general recommendations regarding finishing. First, under-finish the concrete. Second, conduct a trial placement so the finishing crews can perfect their approach to the silica-fume concrete.