From a sructural viewpoint, there is absolutely no need to add the lower ring of the arch; in fact, it will not add any strength at all and would be purely cosmetic.
You will effectively be building the arch from below, which is very difficult. When the arch was originally built, the bricklayer obviously did it from above, and when the weight gradually went on the bricks as the breast went up, it would cause the mortar joints to be squeezed up and thereby tighten up on the bricks.
Doing it from below, this won't happen. In fact, you will find that as the mortar sets and shrinks very slightly, a hairline crack will open up above the lower ring and your arch will be only supporting itself.
If you must do it, you will first need to make a timber form, The mortar joints to the first 3 bricks either side will accomodate the arch shape, but the middle brick will have to be cut. If you are using old solid commons, you could try the simple way of using a hammer and bolster chisel, though you'll probably go through a few bricks before you get it right. If you have a disc cutter, that would be more accurate.
You will have to push the top brick in sideways due to its shape; because of this, the mortar will tend to push out.
You have set yourself a difficult task!
You will effectively be building the arch from below, which is very difficult. When the arch was originally built, the bricklayer obviously did it from above, and when the weight gradually went on the bricks as the breast went up, it would cause the mortar joints to be squeezed up and thereby tighten up on the bricks.
Doing it from below, this won't happen. In fact, you will find that as the mortar sets and shrinks very slightly, a hairline crack will open up above the lower ring and your arch will be only supporting itself.
If you must do it, you will first need to make a timber form, The mortar joints to the first 3 bricks either side will accomodate the arch shape, but the middle brick will have to be cut. If you are using old solid commons, you could try the simple way of using a hammer and bolster chisel, though you'll probably go through a few bricks before you get it right. If you have a disc cutter, that would be more accurate.
You will have to push the top brick in sideways due to its shape; because of this, the mortar will tend to push out.
You have set yourself a difficult task!
