Page 249 - Challenges in Corrosion Costs Causes Consequences and Control(2015)

CORROSION CONTROL OF BRIDGES 227

combination of both. High-performance concretes were developed to impede the
ingress of chloride to the rebar by reducing concrete permeability. This is obtained
by using lower water-to-cement ratio concrete and adding mineral admixtures
such as silica fume and fly ash (pozzolanic materials) to the concrete mix. Low
water-to-cement ratios are achieved using high-range water reducers.
In addition to low chloride permeability, mineral admixtures impart other proper-
ties to the concrete depending on the admixture selected such as:
1. Higher corrosion resistance (higher chloride threshold for corrosion and low
corrosion rate following initiation).
2. Greater cumulative corrosion prior to cracking.
3. Higher resistivity to minimize macrocell corrosion.
An FHWA study by Thompson and Lankard (19, 20) reviewed the effect on the
corrosion of steel in the concrete of several variables, including cement type, mineral
admixtures, water-to-cement ratio, and aggregate type. This study showed silica fume
to be the most effective mineral admixture in the mitigation of corrosion of steel rebar.
It also suggested that careful selection of the concrete mix components could extend
the life of a concrete bridge member. It is estimated that use of a silica fume admixture
provides an increase of expected life of 10 years beyond that provided by black steel
rebar in conventional concrete.

4.9.1.4 Corrosion-Inhibiting Concrete Admixtures In the past two decades, the
use of corrosion-inhibiting concrete admixtures has become a promising method for
delaying the onset of corrosion of prestressing and conventional reinforcing steel
(21). Inhibitors are generally used with permeability-reducing pozzolanic additives
such as fly ash or silica fume. As the concrete has low permeability, and the corro-
sion inhibitor essentially increases the chloride concentration required for corrosion
initiation. The inhibitor may also reduce the corrosion rate during the postinitiation
period, leading to less corrosion-induced concrete deterioration.
Corrosion inhibitors can be either inorganic or organic compounds and reduce
the corrosion rates to acceptable levels when present at low concentrations. Organic
inhibitors function by the formation of a protective film, metal–inhibitor complex, on
the metal surface. Inorganic inhibitors function by either the oxidation or reduction
reactions at the steel surface.
There is extensive literature on the corrosion inhibition behavior of calcium nitrite
as an inhibitor. Calcium nitrite can function as an oxidizing inhibitor in relatively high
concentrations of chloride. The suitable organic inhibitors are amides and esters. A
National Cooperative Highway Research Program (NCHRP) project reviewed the
performance of corrosion inhibitors used in concrete and assessed the performance
of commercial inhibitors (22).
The economics of the use of calcium nitrite in corrosion-inhibiting admixtures
with and without the addition of silica fume has been documented (23). The cost of a
2
calcium nitrite protection system was estimated to be $5.40/m , and the cost increase
to construct the deck using calcium nitrite inhibitor is 1.1%. It is estimated that the