The loadings and level of stress in the structure can be exceptionally high for large scale infrastructure. Designers of concrete structures sometimes resort to the ‘brute force’ of large diameter reinforcement (32mm or larger) to make it work. Whilst this may be acceptable, the designer must bear in mind the downside of large diameter reinforcement, which is explained in this blog post.
Large diameter reinforcement is harder and less safe to handle manually on site. 20-25kg is considered to be the maximum safe weight lifting limit for a fit person of normal strength. The practical limit would reduce if the physical situation on site is confined and constricted, forcing workers to extend out or lift in an awkward posture. Straight reinforcement are normally manufactured to a maximum length of 12m to 15m, which means a 25mm diameter bar close to maximum length weighs 40 to 60 kg. This would normally need to be handled by two people sharing the total weight, making it 20-30kg per person. This is pretty much the practical limit on site even with strong and experienced workers. Handling bars with a diameter of 32mm or larger would be increasingly less easy and safe and should be avoided by the designer in the first place if possible.
Large diameter reinforcement is less efficient in controlling crack width. Each reinforcement bar has a ‘zone of influence’ for its grip of the surrounding concrete, which contributes to the control of crack in the concrete. The ‘zone of influence’ is largely proportionate to the surface area (or the circumference on a cross-sectional view) of the reinforcement. For a given weight/volume of reinforcement, the surface area increases as the bar diameter decreases. In other words, a group of smaller bars has a larger surface area compared to a group of larger bars of the same weight. As best practice, a surface mesh reinforcement over the main reinforcement is best provided to cracking of the concrete cover, and prevent spalling thus improving performance in fire scenario. This previous post talks more about crack control: https://www.si-eng.org/post/the-calculation-of-crack-width
The figure below is based on Eurocode 2.
Mechanical continuity of large diameter bars is structurally complicated and costly to achieve. To maximize their structural capacity, large diameter reinforcement is best mechanically coupled rather than lapped, especially in the Ultimate Limit State where the stress in reinforcement can be close to its yield stress. If couplers were to avoided (for their cost, perhaps), large diameter reinforcement needs exceptionally long lap lengths and also with help of transverse reinforcement, and maybe even shear links for confinement (preventing bursting cracks). Secondary transverse reinforcement should be provided in general for large diameter reinforcement, especially in anchorage zones.
Spacing of large diameter bars needs careful thoughts – not too big nor small. Large diameter reinforcement is specified by the designer normally for the large capacity they can bring, and for this reason the specified spacing for them normally isn’t very big in order to achieve the number of bars required. But bear in mind that the spacing shouldn’t be too small either, in order to 1) allow sufficient gap between bars so that a vibrator can be inserted to compact the concrete. The vibrator is normally 50-75mm diameter and needs a bout another 10-30mm room for manoeuvre. 2) have sufficient surrounding concrete to develop the bond between the steel and the concrete. For a 32mm diameter bar, I would typically start with the minimum spacing of 150mm (125mm is possible with more careful detailing). In a more complex situation, if more than one layer of large diameter reinforcement is specified, spacers in between layers should be provided to ensure clearance for developing sufficient bonding.
Other detailing and practical issues: Large diameter bars do not make sharp turns very well, which means they can fit in less geometry situations than their slimmer counterparts. Large diameter bars normally have to be pre-bent in factories, as they are too stiff to be bent on site with the limited tools available.
In a nutshell: as a designer, think twice before specifying large diameter reinforcement!
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