An apparatus for forming a joint between concrete floor slab panels, the apparatus comprising: a divider plate for bounding a side of a concrete floor slab panel; and first and second top strips (3,4), the first top strip being connectable to the divider plate, and the second top strip being frangibly connectable to the divider plate and/or the first top strip; wherein each of the first and second top strips (3,4) comprises: an elongate concrete engagement member, wherein the longitudinal axis of the elongate concrete engagement member is, in use, positioned parallel to the longitudinal axis of the divider plate; and a top section comprising an elongate top face presenting an elongate edge configurable to lie at the level of the top surface of a concrete floor slab panel; and an elongate concrete abutment face extending from the elongate edge of the top face, which abutment face is a continuous face adapted for concrete to be cast against and which extends along the length of the relevant top strip (3,4); wherein: the arrangement of the first and second top strips (3,4) is such that in use after casting concrete, the elongate concrete abutment face extends downward from the elongate edge of the top face at an angle such that the angle between the abutment face and the top surface of an abutting concrete floor slab panel is from 65° to 135°; and characterised in that each of the first and second top strips (3,4) is unitary such that the respective elongate concrete engagement member and the respective top section are integral in a single component. a divider plate for bounding a side of a concrete floor slab panel; and first and second top strips (3,4), the first top strip being connectable to the divider plate, and the second top strip being frangibly connectable to the divider plate and/or the first top strip; wherein each of the first and second top strips (3,4) comprises: an elongate concrete engagement member, wherein the longitudinal axis of the elongate concrete engagement member is, in use, positioned parallel to the longitudinal axis of the divider plate; and a top section comprising an elongate top face presenting an elongate edge configurable to lie at the level of the top surface of a concrete floor slab panel; and an elongate concrete abutment face extending from the elongate edge of the top face, which abutment face is a continuous face adapted for concrete to be cast against and which extends along the length of the relevant top strip (3,4); an elongate concrete engagement member, wherein the longitudinal axis of the elongate concrete engagement member is, in use, positioned parallel to the longitudinal axis of the divider plate; and a top section comprising an elongate top face presenting an elongate edge configurable to lie at the level of the top surface of a concrete floor slab panel; and an elongate concrete abutment face extending from the elongate edge of the top face, which abutment face is a continuous face adapted for concrete to be cast against and which extends along the length of the relevant top strip (3,4); an elongate top face presenting an elongate edge configurable to lie at the level of the top surface of a concrete floor slab panel; and an elongate concrete abutment face extending from the elongate edge of the top face, which abutment face is a continuous face adapted for concrete to be cast against and which extends along the length of the relevant top strip (3,4); wherein: the arrangement of the first and second top strips (3,4) is such that in use after casting concrete, the elongate concrete abutment face extends downward from the elongate edge of the top face at an angle such that the angle between the abutment face and the top surface of an abutting concrete floor slab panel is from 65° to 135°; and characterised in that each of the first and second top strips (3,4) is unitary such that the respective elongate concrete engagement member and the respective top section are integral in a single component. the arrangement of the first and second top strips (3,4) is such that in use after casting concrete, the elongate concrete abutment face extends downward from the elongate edge of the top face at an angle such that the angle between the abutment face and the top surface of an abutting concrete floor slab panel is from 65° to 135°; and characterised in that each of the first and second top strips (3,4) is unitary such that the respective elongate concrete engagement member and the respective top section are integral in a single component.