Composite reinforced concrete

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.

Effects of (1) cement alkalinity (low, normal and high), (2) exposure conditions (RH and temperature), (3) rebar surface condition (as-received versus cleaned) and (4) density and distribution of air voids at the steel-concrete interface on the chloride threshold and time-to-corrosion for reinforcing steel in concrete have been studied. Also, experiments were performed to evaluate effects of RH and temperature on the diffusion of chloride in concrete and develop a method for ex-situ pH measurement of concrete pore water. Once specimens were fabricated and exposed to a corrosive chloride solution, various experimental techniques were employed to determine time-to-corrosion, chloride threshold, diffusion coefficient and void density along the rebar trace as well as pore water pH. Based upon the resultant data, several findings related to the above parameters have been obtained as summarized below. First, time for the corrosion initiation was longest for G109 concrete specimens with high alkalinity cement (HA). Also, chloride threshold increased with increasing time-to-corrosion and cement alkalinity. Consequently, the HA specimens exhibited the highest chloride threshold compared to low and normal alkalinity ones. Second, high temperature and temperature variations reduced time-to-corrosion of reinforcing steel in concrete since chloride diffusion was accelerated at higher temperature and possibly by temperature variations. The lowest chloride threshold values were found for outdoor exposed specimens suggesting that variation of RH or temperature (or both) facilitated rapid chloride diffusion. Third, an elevated time-to-corrosion and chloride threshold values were found for the wire brushed steel specimens compared to as-received ones. The higher ratio of [OH-]/[Fe n+] on the wire brushed steel surface compared to that of as-received case can be the possible cause because the higher ratio of this parameter enables the formation of a more protective passive film on the rebar. Fourth, voids at the steel-concrete interface facilitated passive film breakdown and onset of localized corrosion. This tendency for corrosion initiation increased in proportion to void size irrespective of specimen type. Also, [Cl -]th decreased with increasing void diameter. In addition, new ex-situ leaching method for determining concrete pore water alkalinity was developed.

The aim of this study was to investigate the diffusion of chloride ions into concrete samples that were exposed in scenarios that simulate the splash, tidal, atmospheric, and immersed portions of a marine structure. To study the atmospheric deposition, the project also investigated the relationship between chloride ion deposition on the wet candle and its accumulation into concrete samples. Results from the wet candle experiment indicated that between 2% and 45% of the chlorides deposited per square meter of exposed area could be found within the concrete samples. After 6 months, slag G1a blocks showed the most resistance to chloride penetration in the tidal and splash simulations. After 10 months of exposure, fly ash samples had the slowest rates of diffusion in the tidal simulation while the fly ash + silica fume samples and the slag samples measured similar rates of diffusion within the tidal zone. After 90 days of curing, cylinders composed of 20% fly ash & 8% silica fume measured the highest average resistivity values and were found to be less vulnerable to chloride ion penetration than the 20% fly ash and the 50% slag concrete through rapid migration tests.

With the need to improve corrosion resistance in columns and piles, the innovative idea of Centrally Prestressed Fiber Reinforced Concrete (CPFRC) columns is a promising solution. The first step is to compare if the compressive strength of any mix is affected by the size, geometry, or even the inclusion of polyolefin fibers in a specimen. The results showed that the cylinder size of 4 in. x 8 in., which is the most common size used by the testing labs, has the highest compressive strength. There was no sign on compressive strength improvement with the use of polyolefin fibers, except for reduction in cracking size and concrete spalling. The second step compared the ultimate strength, ductility characteristics and failure mode of CPFRC columns to conventional columns. CPFRC showed adequate axial and flexural resistance, in addition to ductile behavior similar to regular reinforced concrete columns.