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Steelworks

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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For column reinforcements, why is helical reinforcement sometimes designed instead of normal links?

The use of links for column design in Britain is very popular. However, in U.S.A. engineers tend to use helical reinforcement instead of normal links because helical reinforcement has the potential advantage of protecting columns/piles against seismic loads.

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Why does the presence of tension reinforcement lead to increasing deflection in concrete structures?

In BS8110 a modification factor is applied to span/depth ratio to take into account the effect of tension reinforcement. In fact, deflection of concrete structure is affected by the stress and the amount of tension reinforcement.

To illustrate their relationship, let’s consider the following equation relating to beam curvature:

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What is the purpose of setting minimum amount of longitudinal steel areas for columns?

In some design codes it specifies that the area of longitudinal steel reinforcement should be not less than a certain percentage of the sectional area of column. Firstly, the limitation of steel ratio for columns helps to guard against potential failure in tension. Tension may be induced in columns during the design life of the concrete structures. For instance, tension is induced in columns in case there is uneven settlement of the building foundation, or upper floors above the column are totally unloaded while the floors below the column are severely loaded. Secondly, owing to the effect of creep and shrinkage, there will be a redistribution of loads between concrete and steel reinforcement. Consequently, the steel reinforcement may yield easily in case a lower limit of steel area is not established.

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Why are longer tension lap lengths designed at the corners and at the top of concrete structures?

In BS8110 for reinforced concrete design, it states that longer tension lap lengths have to be provided at the top of concrete members. The reason behind this is that the amount of compaction of the top of concrete members during concrete placing is more likely to be less than the remaining concrete sections. Moreover, owing to the possible effect of segregation and bleeding, the upper layer of concrete section tends to be of lower strength when compared with other locations.

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