Building Design and Construction Handbook - Merritt - Ventech!

4.13.5 Mineral Admixtures
BUILDING MATERIALS 4.17
Fly ashes, pozzolans, and microsilicates are included in the mineral admixture classification
(Arts. 4.9 and 4.10). Natural cement (Art. 4.4) is sometimes used as an
admixture.
4.13.6 Corrosion Inhibitors
Reinforcing steel in concrete usually is protected against corrosion by the high
alkalinity of the concrete, which creates a passivating layer at the steel surface.
This layer is composed of ferric oxide, a stable compound. Within and at the surface
of the ferric oxide, however, are ferrous-oxide compounds, which are more reactive.
When the ferrous-oxide compounds come into contact with aggressive substances,
such as chloride ions, they react with oxygen to form solid, iron-oxide corrosion
products. These produce a fourfold increase in volume and create an expansion
force greater than the concrete tensile strength. The result is deterioration of the
concrete.
For corrosion to occur, chloride in the range of 1.0 to 1.5 lb/yd 3 must be present.
If there is a possibility that chlorides may be introduced from outside the concrete
matrix, for example, by deicing salts, the concrete can be protected by lowering
the water-cement ratio, or increasing the amount of cover over the reinforcing steel,
or entraining air in the concrete, or adding a calcium-nitrate admixture, or adding
an internal-barrier admixture, or cathodic protection, or a combination of these
methods.
To inhibit corrosion, calcium-nitrate admixtures are added to the concrete at the
time of batching. They do not create a physical barrier to chloride ion ingress.
Rather, they modify the concrete chemistry near the steel surface. The nitrite ions
oxidize ferrous oxide present, converting it to ferric oxide. The nitrite is also absorbed
at the steel surface and fortifies the ferric-oxide passivating layer. For a
calcium-nitrite admixture to be effective, the dosage should be adjusted in accordance
with the exposure condition of the concrete to corrosive agents. The greater
the exposure, the larger should be the dosage. The correct dosage can only be
determined on a project-by-project basis with data for the specific admixture proposed.
Internal-barrier admixtures come in two groups. One comprises waterproofing
and dampproofing compounds (Art. 4.15). The second consists of agents that create
an organic film around the reinforcing steel, supplementing the passivating layer.
This type of admixture is promoted for addition at a fixed rate regardless of expected
chloride exposure.
4.13.7 Coloring Admixtures
Colors are added to concrete for architectural reasons. They may be mineral oxides
or manufactured pigments. Raw carbon black, a commonly used material for black
color, greatly reduces the amount of entrained air in a mix. Therefore, if black
concrete is desired for concrete requiring air-entrainment (for freeze-thaw or aggressive
chemical exposure), either the carbon black should be modified to entrain
air or an additional air-entraining agent may be incorporated in the mix. The mix
design should be tested under field conditions prior to its use in construction. Use