Effects of De-icing and Anti-icing Chemicals on the Durability of Reinforcing Steel in Concrete
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Concrete is strong in compression; however, it is quite fragile in tension. To overcome this flaw, concrete is frequently reinforced with bars typically made of low grade, low carbon steel. The environment inside of concrete is favorable for steel; unfortunately when passive steel is exposed to chlorides, active corrosion can initiate, resulting in damage to the structure. One source of chloride contamination is through anti-icing agents which are used to inhibit the formation of ice on roadways, ensuring safe driving conditions. This represents a serious concern from both the cost associated with rehabilitation (Canadian infrastructure deficit in 2003 was $125 billion ) and as a safety concern to the public. In Canada, 5 million tonnes of road salts are used each year , of which Ontario uses 500 to 600 thousand tonnes . As a result, the Ministry of Transportation Ontario (MTO) has requested a study of four frequently used anti-icing agents: 25.5% NaCl, 31.5% MgCl2, 37.9% CaCl2 and 32.6% multi Cl- (12% NaCl, 4% MgCl2 and 16% CaCl2). The objective of the study is two-fold, the first is comparing the effects of the solutions on steel embedded in concrete (high pH environment) and the second is to compare the effects of the anti-icing agents to a variety of construction steels in atmospheric conditions (neutral pH). Macro-cell and micro-cell corrosion in concrete were tested using both modified ASTM G109 prisms and concrete beams with 6 embedded black steel bars. Unfortunately, these tests proved inconclusive; all of the steel remained passive. This was a result of casting a high quality concrete in laboratory conditions which ultimately lead to minimal diffusion of the anti-icing solutions. Therefore, it is recommended that for short term corrosion testing (<2 years), poor quality concrete or cement paste should be used. Micro-cell testing in synthetic concrete pore solution contaminated with the anti-icing solutions was conducted in order to obtain results in the period of the M.A.Sc. program and to directly observe the corrosion. The initial concentration of Cl- in each solution was 0.00% Cl-; this was incrementally increased by 0.005% Cl-/week. Potentiostatic linear polarization to resistance measurements and pH measurements were used to monitor the corrosion on a weekly basis. The results of this test showed that MgCl2 has the most detrimental effects due to the drop in pH (from 13.5 to 9.1) caused by Mg replacing Ca in Ca(OH)2 to form the less soluble Mg(OH)2. The transition from passive to active corrosion initiated at 0.7, 0.4-0.9, 0.6 and 0.6% Cl- for NaCl, MgCl2, CaCl2 and multi Cl-, respectively. The active corrosion current densities were 11mA/m2 for NaCl, CaCl2 and multi Cl-, whereas MgCl2 had active corrosion rates of ~100 mA/m2. One bar exposed to CaCl2 showed corrosion rates as high as 600 mA/m2. This was a result of crevice corrosion between the shrink fitting and the rebar. Once the expansive corrosion products broke through the shrink fitting and ample supply of oxygen became available, allowing the corrosion rates to spike dramatically. The following steels were tested directly in the diluted solution in a cyclic corrosion chamber: stainless steels: 304L, 316LM, 2101, 2205, 2304, XM28; corrosion resistant steel reinforcing bars (rebar): galvanized rebar, guard rail (galvanized plate steel) and MMFX; carbon steels: black steel rebar, box girder, drain, weathering steel. The reinforcing bars were virgin steels whereas the remaining steels were components from the field. The testing regime followed SAE J2334 using the anti-icing solutions diluted to 3% by wt. Cl- as the immersion liquid. Unfortunately, the mutli Cl- solution was not tested due to time constraints. The mass change per unit area was measured every five cycles. All stainless steels exposed to all anti-icing solutions exhibited similar changes in mass per unit area, less than 10 g/m2. All plain carbon steels including weathering steel exhibited mass changes per unit area of more than 1000 g/m2 with some variability between the various anti-icing solutions and steel types, although the black steel rebar typically outperformed the other carbon steels. The corrosion products of MMFX were non-adherent, resulting in inconclusive results. The galvanized layer on the guard rail, which had been exposed to the environment in service, proved to be more protective than the fresh zinc coating on the galvanized rebar. When exposed to the MgCl2 solution, the mass change of both new and used galvanized steels was comparable to that found in the stainless steels. When exposed to NaCl solutions, the galvanized guard rail also exhibited this trend, whereas the new galvanic coating did not, suggesting that with exposure to the atmosphere a galvanic coating will protect the steel against NaCl. In all cases galvanized steel exposed to CaCl2 solutions exhibited mass changes per unit area of less than 100 g/m2 this is considered moderate, as this value is one order of magnitude higher than the stainless steels and one order of magnitude lower than the carbon steels exposed to the same test. It is recommended that galvanic coatings be utilized in areas heavily exposed to anti-icing solutions. The weathering steel offers no advantages over carbon steels when directly exposed to anti-icing solutions. Furthermore, in areas with high amounts of exposed galvanized steel, CaCl2 should be avoided. Between the four solutions tested, NaCl solutions are recommended as the anti-icing agents that, overall, causes the least amount of damage to both the reinforcing steel in concrete and to exposed metallic components. NaCl is followed by multi Cl- and CaCl2. Even though MgCl2 causes less damage when directly exposed to carbon steels and galvanized steels than CaCl2, it is much easier to repair external components than internal components. Therefore, MgCl2 is not recommended.