Q: We are considering the use of integrally colored black concrete for a downtown sidewalk. How much more would the colored concrete cost than standard concrete? And should we consider using both integral color and black aggregate? If so, what's the cost of black aggregate vs. standard aggregate?
A: You can color concrete black with metallic-oxide pigments or carbon-based admixtures. The color of carbon-based systems may fade, especially under exterior use. But oxide colors are more expensive.
We can give only rough estimates of price because both the concrete producer and the contractor installing the work will add a percentage to the price they are quoted. As a ballpark estimate, the added cost of integral color for a six-bag mix (564 pounds of cement per cubic yard) should run about $50 to $70 per yard. The producer may also add a washout fee, which, in Chicago, can run $75 per truckload.
Unless the coarse aggregate in the sidewalk will be exposed, the additional cost of black aggregate probably isn't justified. The price of black aggregate primarily depends on transportation costs. We talked with a supplier of colored aggregates who quoted prices for coarse aggregates from two sources--black granite from North Carolina and black traprock from Missouri. Both aggregates are suitable for ready-mixed concrete and are used by local companies in the Chicago area. Traprock costs about $42 per ton delivered in bulk to Chicago, and black granite costs about $75 per ton. We would expect the concrete producer to add a percentage to this. For comparison, normal coarse aggregate for concrete costs about $10 per ton and a cubic yard of concrete can contain nearly a ton of coarse aggregate. So using either of these black aggregates could raise the cost of the concrete $15 to $35 per cubic yard.
As a cost-saving alternative you might consider dry-shake colors, which cost about 40¢ per square foot of concrete. Manufacturers say that dry shakes-mixtures of cement, fine aggregate, and color pigment-increase surface strength to about 8000 psi, making the concrete very wear resistant. Also, the color is much more intense because the oxide materials are concentrated on the surface. Of course, if you intend to use an exposed-aggregate finish, this method won't work.
Whether you use integral or dry-shake color, you should plan to seal the work with a good sealer rated for exterior use-either a silicone-based penetrating sealer or an acrylic sealer. Solvent-based all-acrylic sealers allow moisture vapor to escape, and they won't yellow when exposed to sunlight. Applying sealers soon after concrete is placed will help to manage efflorescence, which can lighten colored concrete or create a mottled appearance.