To　address　the　excessive　requirements　of　lifting　and　transportation　equipment　for　integral　reinforced　concrete　(RC)　precast　cap　beams,　as　well　as　water　seepage　at　the　joints　and　a　discontinuous　force　transmission　of　longitudinal　reinforcement　at　the　interface　for　a　segmental　precast　cap　beam,　a　semi-precast　composite　cap　beam　with　cast-in-place　RC　in　an　ultra-high-performance　concrete　(UHPC)　formwork　shell　was　proposed.　Static　tests　were　performed　on　UHPC　shell-RC　(URC)　composite　cap　beams　with　and　without　shear　keys　and　on　an　RC　cap　beam,　and　the　influence　of　the　degree　of　bonding　of　the　inter-surface　on　the　mechanical　properties　and　failure　modes　of　the　URC　composite　cap　beams　were　also　analyzed　using　a　finite　element　model.　The　results　show　that,　just　as　for　the　cast-in-place　RC　cap　beam,　the　URC　composite　cap　beam　was　subjected　to　shearing　compression　failure,　but　interface　separation,　core　concrete　arch　and　vertical　cracking　of　the　UHPC　shell　were　observed　in　the　URC　composite　cap　beams.　However,　the　cracking　load　and　ultimate　bearing　capacity　of　the　URC　composite　cap　beam　without　shear　keys　were　42.1%　and　13.8%　larger,　respectively,　than　those　of　the　cast-in-place　RC　cap　beam.　A　shear　key　used　in　an　URC　composite　cap　beam　can　increase　the　bonding　degree　of　the　interface,　and　also　lead　to　a　decrease　in　the　propagation　speed　of　the　maximum　crack　and　crack　width　at　the　interface.　The　interface　cracking　load　and　ultimate　load　capacity　of　the　URC　composite　cap　beam　with　shear　keys　are　50.0%　and　12.1%　larger,　respectively,　than　those　of　the　URC　composite　cap　without　shear　keys.　Under　the　ideal　interface　bonding　state,　the　UHPC　formwork　can　achieve　ultimate　compressive　strain,　indicating　that　the　material　properties　can　be　fully　utilized　and　the　UHPC　formwork　can　fully　participate　in　the　overall　stress.　However,　the　ultimate　load　capacity　is　only　8.8%　higher　than　that　of　the　URC　composite　cap　with　shear　keys.　All　these　results　indicate　that　the　UHPC　formwork-RC　composite　cap　with　shear　keys　has　good　section　bonding　strength　and　overall　mechanical　performance,　and　can　be　recommended　for　practical　purposes.　©　2021,　Editorial　Department　of　China　Journal　of　Highway　and　Transport.　All　right　reserved.