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What is the difference in bonding performance to concrete between epoxy-coated bars and galvanized bars?

Based on the findings of CEB Bulletin 211, the bonding of galvanized bars to concrete is lower in early age owing to hydrogen release when zinc reacts with calcium hydroxide in concrete and the presence of hydrogen tend to reduce the bond strength between galvanized bars and concrete. However, bonding will increase with time until the full bond strength of ungalvanized bars is attained.

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Does the presence of rust have adverse impact to the bond performance of bar reinforcement?

In fact, the presence of rust in bars may not have adverse impact to the bond performance and it depends on the types of bar reinforcement under consideration.

For plain round bars, the rust on bars improves the bond performance by the formation of rough surfaces which increases the friction between steel and concrete.

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Which type of bar reinforcement is more corrosion resistant, epoxy-coated bars, stainless steel bars or galvanized bars?

Based on the experiment conducted by the Building Research Establishment, it was shown that the corrosion resistance of galvanized steel was the worst among the three types of bar reinforcement. For galvanized steel bars, corrosion started to occur when a certain chloride content in concrete (i.e. 0.4% by cement weight) was exceeded. However, for epoxy-coated bars, they extended the time taken for cracking to occur when compared with galvanized steel bars.

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Tying wires for reinforcement adjacent to and above Class F4 and F5 finishes should be stainless steel wires. Why?

If plain steel tying wires are used for reinforcement adjacent to Class F4 and F5 finishes, it poses the problem of rust staining which may impair the appearance of exposed concrete surfaces. The rate of corrosion of plain steel tying wires is similar to normal steel reinforcement.

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What is the effect of rusting on steel reinforcement?

The corrosion of steel reinforcement inside a concrete structure is undesirable in the following ways:

(i) The presence of rust impairs the bond strength of deformed reinforcement because corrosion occurs at the raised ribs and fills the gap between ribs, thus evening out the original deformed shape. In essence, the bond between concrete and deformed bars originates from the mechanical lock between the raised ribs and concrete. The reduction of mechanical locks by corrosion results in the decline in bond strength with concrete.

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