The invention relates to a joint strip in the form of a ribbon, formed by a warp knitted fabric or an open-loop glass-fibre woven fabric, said warp knitted fabric or woven fabric comprising fibres parallel to the length of the ribbon and fibres perpendicular to the length of the ribbon, said strip being characterized by the fact that at least one glass fibre, called the reinforcing fibre, parallel to the length of the ribbon and arranged in the central area thereof, has a linear density at least five times higher than the linear density of the other fibres.