Steel in Concrete
Newly mixed concrete consists
primarily of aggregates, cement powder, and
water, the latter two components forming
cement paste, which soon harden by a process
called “hydration.” As the cement paste
hardens it binds the aggregate as a solid
matrix which gives concrete its load
carrying ability and durability. The
consumption of the mix water in the
hydration-hardening reaction leaves
capillary, and “gel” pores in the concrete
matrix through which atmospheric gasses,
pollutants, and water can penetrate when the
concrete is “wetted” by rain, condensation,
or spray. The retained water within the
pores of the concrete matrix becomes
saturated with the chemical components of
the cement and forms a highly alkaline
solution, with a nominal pH of ~12.5,
depending on the specific cement powder
used.
Bare reinforcing steel is
normally passivated in the initial pH of the
contained-water in newly hardened concrete,
however, wetting and drying cycles allow for
atmospheric gasses including carbon dioxide
and sulfur dioxide to dissolve in the pore
water, and their acidity in solution begins
to lower the pH of the pore water. This
process is called carbonation.
Bare reinforcing steel
begins to loose its “passivation” or
dormancy as the pH surrounding the steel
passes below about pH 11.5, and rusting of
the steel begins and progresses, the
resulting corrosion products take up more
space than the steel consumed and this
volume expansion within the constraining,
rigid concrete matrix, is that substantial
stresses are exerted on the surrounding
concrete.
In addition to the lowering
of the pH of pore space water below the threshold
of passivation of imbedded bare steel, chloride
ions from the structure surroundings, also
are dissolved in the pore water, and once
permeation down to the steel surface, further
act to destroy the passivation of the embedded
bare steel. The time elapsed before the combination
of acidic atmospheric components and chlorides
permeate to the embedded steel surface is
a function of the environment, wetting cycles,
porosity and composition of the concrete,
and the length and difficulty of traversing
the labyrinthine path to the bar surface.
This latter factor is partly related to the
“depth of cover” of the embedded bar.
Galvanizing
in Concrete
