For concrete columns, drilled shafts, or caissons, in addition to the reinforcing, we provide a shear key in the concrete, by forming a pocket about 2" deep in the top of the shaft, and with square plan dimensions approximately 40% of the diameter. So for a 36" circular shaft, the formed keyway would be 2" deep and 15" by 15".
Rod Smith, P.E., The artist formerly known as HotRod10