Reinforced concrete construction--Corrosion

Effects of reinforcement and coarse aggregate on chloride ingression into
concrete and reinforcement corrosion initiation have been studied with experimental
and modeling (finite element method) analyses. Once specimens were fabricated
and exposed to a chloride solution, various experimental techniques were employed
to determine the effect of reinforcement and coarse aggregate on time-to-corrosion
and chloride ingress and concentration at corrosion locations. Model analyses were
performed to verify and explain the experimental results. Based upon the results, it
was determined that unexpectedly higher chloride concentrations were present on
the top of the rebar trace than that to the side at the same depth and an inverse
concentration gradient (increasing [Cl-] with increasing depth) occurred near the top
of rebars. Also, coarse aggregate volume profile in close proximity to the rebar and
spatial distribution of these aggregates, in conjunction with the physical obstruction
afforded by reinforcement to chloride flow, complicates concrete sampling for Cl- intended to define the critical concentration of this species to initiate corrosion.
Modeling analyses that considered cover thickness, chloride threshold concentration,
reinforcement size and shape, and coarse aggregate type and percolation confirmed
the experimental findings. The results, at least in part, account for the relatively
wide spread in chloride corrosion threshold values reported in the literature and
illustrate that more consistent chloride threshold concentrations can be acquired
from mortar or paste specimens than from concrete ones.

This report describes the results obtained from reinforced concrete slabs having different fly ash and silica fume content. The specimens are submitted to periodic seawater ponding. PH measurements as well as free and total chloride analyses were achieved at 1213 days in order to study the alkalinity, resistance to chloride ingress and binding properties afforded by each mix design. Water absorption experiments were also conducted at different relative humidities and in water. Pore water pH was found to decrease with increasing admixture content and increasing relative humidity. The permeability to chloride ions was significantly reduced in the fly ash blends whereas most of the silica fume blends exhibited effective diffusion coefficients and chloride concentrations at the depth of steel marginally better than the controls. Both the fly ash and silica fume blends showed similar binding capacity at a given replacement ratio, the percentage of bound chloride increasing with increasing admixture content.

A study was performed of the effect of reinforcing bars on chloride diffusion into concrete. These bars act as obstacles to chloride movement, and this obstruction allows the chloride concentration to build-up faster along the top of the reinforcing bar than elsewhere. As a consequence, the critical chloride threshold to initiate corrosion of the steel is reached sooner than otherwise expected. This research was performed using two different methods. First, chloride analyses were performed on chloride exposed concrete blocks by taking cores in the concrete and drillings along the trace of the top bars. Second, concrete blocks models were analyzed by finite element analysis and the effect of the obstruction by the bar quantified. The role of the reinforcing bar in the chloride diffusional flow is discussed considering these two methods.