Here are various photos of some of the middle arch stones being fit between voussoirs assembled already positioned across the form on a couple of older bridge projects.
On a segmented arch these middle stones which complete the arch don't have to be special keystones, in that they don't have to be necessarily bigger or different than the other voussoirs. As long as they are properly shaped to fit snug in the arch they will work.
As you can see, this middle voussoir on the Monarch Bridge we built is going to slip in very tightly. Care needed to be taken so that it fit enough to be snug, but not so tight as to force the other stones out of alignment. Some of the thickness of this stone needed to be chiseled away before it went in properly. If it had been too thick, and we had forced it into the opening, a bulge would have been created somewhere along the rest of the curve of the arch, and the integrity of the whole bridge structure would have been compromised.
Here in this photo of the MacDougal Bridge in Russel Ontario,
when Evan felt confident it was going to fit, the chunky middle voussoir was hammered down into place with a heavy stone.
It was pounded down until the bottom touched the wooden form, and thus completed the continuous look of the curve created by the other arch stones.
The rest of the voussoirs needed for filling in the middle space that extends across the form are all carefully fit and pounded in the same way. Some of these last interior voussoirs can be difficult to wedge all the way to the bottom. You may not be able to see if they are flush the intrados, but you can tell when these stones go down far enough because of the sound of stone hitting against wood.
On another occasion at the Hubb Creek Bridge project a special 'keystone' did have to be made. Prior to the arch being built a suitable stone had been chosen and the letter K (for Karlo Estates) sandblasted onto the face.
When the time came to fitting this pre-made keystone into the space left between the other voussoirs that we had already set in place, I was disappointed to see how loosely it fit.
In order to correct this and not have to take apart the other arch stones that were fitting so well we decided to cut out a portion of the form, to allow the keystone to fit down lower and so fit more snugly between the voussoirs to the left and right. This turned out to be a perfect solution and ended up being a signature visual feature of the bridge.
As you can see, we were only able to do this 'recessing' of the keystone because the 2x4s that made up the supporting curved surface of the form, extended beyond the outer rib. This extension had been done in order to build Hubb Creek Bridge two feet wider than the 6 foot wide bridge form it had previously been designed for.