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We know that in Jointed Reinforced Concrete pavements we use dowels and tie bars. Why are dowel bars Smooth and tie bars are deformed? Dowels are used to transfer about 40% of the load from one slab to another slab and to limit the differential deflection. Can't a normal deformed bar provide the same effect? if not how does the smoothness affect this case?

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It depends on the type of the joint:

The smooth dowel is used in an "expansion/contraction joint" to allow for free (lateral) movement due to thermal expansion/contraction of the concrete on either side of the joint. A smooth dowel has less surface for bonding, and no rib (small deformation around the bar) to restrict the movement. Sometimes stainless steel dowel or debonding agent is specified to further strengthen this feature.

The deformed bar is used in a "control joint", which function in weakening the concrete flexural strength and continuity along the joint to reduce the amount and size of the shrinkage cracks (due to drying) that may occur in concrete between the joints. Sometimes, instead of the dowel, half of the rebars that passing the joint are cut to achieve the intended function.

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