CIV
CIV209 Durability 2017.pdf

7 5 o c non air entrained o c mechanism of frost

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Saturation > 91.7% - 5 o C Non-air-entrained 0 o C Mechanism of Frost Damage in Concrete Saturation > 91.7% - 5 o C
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7 Non-air entrained retaining wall on Bloor St. West ~1920 premature freezing How to protect/mitigate Air entrainment greatly increases resistance to freezing and thawing Entrained voids act as empty chambers where water can move to and freeze ~5-8% air content for RC exposed to F/T Size and spacing of air voids are important factors Spacing factor < 230 μm Air-entrained 0 o C Mechanism of Protection by Air-Voids Saturation > 91.7% - 5 o C M. Thomas Air-entrained 0 o C Mechanism of Protection by Air-Voids Saturation > 91.7% - 5 o C
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8 Air-entrained 0 o C Mechanism of Protection by Air-Voids Saturation > 91.7% - 5 o C Air-entrained 0 o C Mechanism of Protection by Air-Voids Saturation > 91.7% - 5 o C Air void before and after ice formation D. Corr How to Prevent F/T Damage 1. Low w/cm concrete 1. Higher tensile strength at early ages and keeps water out 2. Adequate curing before exposure to freezing Lower permeability – takes up less water in wet weather Low porosity – reduces amount of “freezable” water in capillary pores Higher tensile strength 3. Avoid aggregates susceptible to freeze/thaw damage Effect of W/C on Frost Resistance of Non-Air-Entrained Concrete CAC Chapter 8 Effect of Air Content on De-Icer Salt Scaling CAC Chapter 11 Cyclic Freezing and Thawing of slabs with 3% NaCl solution ponded on top
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9 Insulation on Forms and Heated Enclosure for Winter Concrete Corrosion Steel reinforcement in normally immune to corrosion inside concrete. The high pH pore water keeps it in a passive state. However, the passivity can be broken down (resulting in corrosion=rust) if: a) the concrete carbonates (pH drops) or b) chlorides from marine or de-icing salts penetrate to the steel. Corrosion Concrete is a high pH environment (>13.5) Provides protection for steel reinforcement (passivation at pH >11.2) Depassivation (corrosion) can occur from: 1. Carbonation – ingress of CO 2 lowers the pH ~10 CO 2 + H 2 O + Ca(OH) 2 CaCO 3 + 2H 2 O (consumes calcium hydroxide) 2. Ingress of Cl - ions from deicing salts, sea water, etc. [Cl - ] / [OH-] 0.6 (at high pH need more Cl - to depassivate steel) Corrosion Corrosion of steel is an electro-chemical process. If one area gets de-passivated, it becomes an anode, and the passive area becomes a cathode. (ie a voltage gradient is set up) Electrons flow though the steel bars to the cathode. The circuit is completed by OH ions moving through the pore water to the anode. Corrosion Reactions Electrons are produced at anode and flow to cathode Electrons at cathode result in release of OH- Microcell Corrosion (anode and cathode on same bar)
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10 Macrocell Corrosion Top steel = Anode Bottom steel = Cathode Tie wires = electric connection (allows electron flow) Ions in pore water = electrolyte Corrosion is an electrochemical process Fe 2+ Fe(OH) 3 ·nH 2 O (>6 times volume increase) Can cause tensile stresses leading to cracking and delamination.
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  • Fall '13
  • MarikoUda
  • Cathode

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