Graduation Year

2015

Document Type

Thesis

Degree

M.S.E.S.

Degree Name

MS in Engineering Science (M.S.E.S.)

Degree Granting Department

Civil and Environmental Engineering

Major Professor

Alberto Sagüés, Ph.D.

Committee Member

Sarina Ergas, Ph.D.

Committee Member

Qiong Zhang, Ph.D.

Keywords

durability, novel cement, carbon dioxide sequestration, greenhouse gas emissions, sustainability

Abstract

Reinforced concrete structures are expected to have a long service life with minimal maintenance. Corrosion of reinforcing steel is a major factor in reducing concrete structure lifespan, as corrosion products occupy a larger volume than that of the consumed steel and generate tensile stresses that crack the concrete cover. Procedures to control corrosion in traditional concrete, which is made with Portland-cement (PC), have been well established. However, in recent years novel concrete materials based on alternatives to normally cured PC have been developed in response to global needs to reduce greenhouse gases emissions. In particular, a promising new cement has emerged that cures into concrete in a CO2 / water environment resulting in a silica and calcium carbonate matrix. This material has the potential to lower the release of greenhouse gases into the environment at the time that provides a more efficient use of resources. This new material poses a reinforcement corrosion control challenge because unlike PC based materials, the pore water of the carbonated material has a significantly lower pore water pH value, thus making it more difficult to retain a stable passive film. On the other hand, other properties of the new material, including high electric resistivity, may contribute in mitigating the corrosion process. The corrosion behavior of steel reinforcement in these circumstances needs to be investigated.

To evaluate the corrosion behavior of steel in concrete, plain steel rebars were tested with various surface finishes including: as received (mill scale), sandblasted and epoxy coated with intentional coating breaks. The specimens were exposed to environments including fresh distilled water, mildly corrosive water, salt water and an atmospheric environment with 85% relative humidity. The open circuit potential was monitored to determine the time when the activation of the steel surface likely occurred. The corrosion rate of the specimens was determined by means of electrochemical impedance spectroscopy (EIS), the applicability of this technique to corrosion evaluation in novel cementitious media is discussed as well.

Specimens exposed to air at 85% RH appeared to retain a passive steel condition during the entire exposure period. Apparent corrosion rates (ACR) were extremely low. Pending confirmation by prolonged testing, this result is encouraging for applications such as indoor hollow slabs and similar structural components not subject to direct wetting.

Specimens exposed to immersion regimes in Fresh, Mild and Salt water media showed signs of activation after a very short time (days-week) of exposure. Corrosion assessment thus occurred predominantly in the corrosion propagation stage. Early activation is thought to reflect a combination of rapid water absorption and only moderately alkaline pH (~8.8) of the concrete assessed.

The ACRs for plain and sandblasted rebar in the Fresh and Mild water regimes were very low, e.g, < 2 µm/y. If confirmed by further evaluation, applications for contact with fresh natural water with moderate service life needs might in principle be achievable. The SC concrete assessed here exhibited much higher resistivity values than conventional PC concrete under similar conditions. This higher resistivity may have been an important factor in limiting corrosion rate.

ACRs or plain and sandblasted rebar in the salt water exposure were initially significantly greater that those indicated above, as expected, given the highly aggressive nature of the medium. Those ACR values, if sustained, would cause concrete cracking in an OPC application in a short time, on the order of only a few years. ACRs however decreased with exposure time; those results need careful follow up evaluation for possible artifacts such as excessive water pickup from salt deliquescence, which could have transport-limited the rate of the cathodic reaction.

ACRs of epoxy-coated rebar specimens were extremely small in all test conditions. However, this result may stem from limitations of the EIS tests to detect undercoating corrosion and interpretation of this finding needs to be postponed until future autopsy of test specimens.

The investigation illustrated an array of methodologies that can be deployed to evaluate the corrosion performance of reinforcement in new media and provide a framework for extended investigations.

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