Abstract: A device for forming surface joints in hardenable slab type structural materials such as freshly poured concrete is disclosed. The elongated joint former includes a pliable section having a depth which may be approximately one-fourth the thickness of the slab, with outwardly extending fins near its bottom and with a split upper end formed by a pair of separable walls. For implanting the pliable section in a fresh slab prior to its setting, a relatively rigid T-shaped implanting member is inserted into the split and the joint former is driven or vibrated down into the slab until the upper end of the pliable section is approximately even with or slightly below the surface of the slab.