Abstract: Two slab bolster elements having rails supported on legs for carrying rebar during casting are connected end to end by a blade for sliding longitudinally of the rail into a female receptacle and a releasable locking mechanism which acts to lock the blade in the female receptacle. The locking mechanism comprises a notch on one side surface of the blade and a projecting portion of the female receptacle engaged into the notch. The projecting portion is carried on the female receptacle by a portion which can flex to cause the projecting portion to move away from the side surface of the blade. The female receptacle includes a manually depressible member on a side of the female receptacle opposite the projecting portion which when depressed provides motion toward the female receptacle which causes the projecting portion to move out of the notch.