ANS:A - one diameter

The minimum spacing between horizontal parallel reinforcement bars of the same size should not be less than one diameter. This guideline is a fundamental requirement in reinforced concrete design to ensure proper concrete cover, adequate bond between the reinforcement and the surrounding concrete, and effective transfer of stresses between the bars. Here's why:

1. Concrete Cover: Reinforcement bars need to be adequately covered by concrete to protect them from corrosion, fire, and other environmental factors. A minimum spacing of one diameter ensures that there is sufficient concrete cover over each reinforcement bar.
2. Bond and Anchorage: Adequate spacing between parallel bars allows for proper development length, which is crucial for achieving bond between the reinforcement and the surrounding concrete. Insufficient spacing could lead to inadequate anchorage and reduced bond strength, compromising the structural integrity of the member.
3. Concrete Compaction: During concrete placement and compaction, adequate spacing between reinforcement bars facilitates proper compaction of the concrete around each bar. Insufficient spacing can result in inadequate compaction and the formation of voids, which may lead to reduced concrete quality and durability.
4. Structural Performance: Maintaining a minimum spacing of one diameter between reinforcement bars helps ensure that the bars can effectively resist applied loads and distribute stresses within the concrete member. Congestion of reinforcement due to inadequate spacing may lead to localized stress concentrations and potential structural failure.