Q.: We manufacture wood shapes for chamfers, moldings, rustications and various special uses. On a single day recently we had 3 separate inquiries for half-round keyways including a total of four different diameters: 1 3/8, 1 5/16, 2 1/4 and 4 inches. It is not only costly to tool up to produce any given diameter (a knife for a 1 9/16-inch half round currently costs $325) but it requires 4 to 5 weeks for delivery. This is frequently an impossible delay for the contractor. If some agency could standardize on a limited number of diameters, the cost of the keyways could be greatly reduced. It seems obvious that there would be little or no difference in performance between a 1 3/8-inch and 1 5/16-inch half-round keyway, which are only 1/16 inch different in diameter. But surely incremental differences in diameter could be considerably greater than that. Why not a joint design that makes use of only five standard sizes--1-,1 1/4-, 1 1/2-, 1 3/4- and 2-inch diameters? If the designer were to specify only the total cross section that goes into shear the contractor could have his choice whether to use, for example, one 2-inch half round or two 1-inch half rounds.
A.: The use of half-round keyways is, as far as we know, fairly new and not widespread. The details shown here are taken from "Concrete Streets: Typical Pavement Sections and Jointing Details," Concrete Information IS211.01P, published by the Portland Cement Association, 5420 Old Orchard Road, Skokie, Illinois 60077.
This 4-page publication is available at 45 cents a copy (minimum order $3.00). As noted in the lower detail, taken from this publication, the half round has a diameter of 0.2d, where d is the pavement thickness. According to this requirement, every inch of difference in pavement thickness leads to a difference of 1/5 inch (approximately 3/16 inch) in the diameter of the half-round keyway.
Gordon Ray, of the Portland Cement Association, explains that the intention of this requirement, however, was to simplify the choice of keyway stock, not complicate it. It was anticipated that the contractor could choose the size of available half round that comes closest to 0.2d, which has been found to be the optimum size. A good rule, he says, would be to choose a stock size that falls within the range 0.2d to 0.25d. According to this rule, the diameters of half round to use for various pavement thicknesses are shown in the following table. The two center columns show the values for 0.2d and 0.25d.
Pavement thickness, inches
Diameter of half round to use, inches
1 or 1 1/4