A mold part (6) for manufacturing vertical joints of keystones (5) comprising a foam piece (7); and a silicone piece (11) covering the foam piece (11), wherein the foam piece (7) and the silicone piece comprise a plurality of protrusions (17) and recesses (16), thus providing a toothed surface in a concrete surface of the vertical joint. The mold-part can comprise stiffeners (20) and handles (19, 21) for demolding manually after curing the concrete keystones, leaving the inner surface of the keystone with a toothed appearance.

A mold part (6) for manufacturing vertical joints of keystones (5) comprising a foam piece (7); and a silicone piece (11) covering the foam piece (11), wherein the foam piece (7) and the silicone piece comprise a plurality of protrusions (17) and recesses (16), thus providing a toothed surface in a concrete surface of the vertical joint. The mold-part can comprise stiffeners (20) and handles (19, 21) for demolding manually after curing the concrete keystones, leaving the inner surface of the keystone with a toothed appearance.