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Externally bonded reinforcements (EBR) made of ultra high modulus (UHM) carbon fibres seem particularly interesting for shear strengthening. The extremely high modulus can keep shear cracks small to activate aggregate interlock that is an important component of shear load transfer. An experimental study on RC strain specimens with un-deformed low strength steel bars as internal reinforcement has been conducted to provide design values for UHM carbon sheets and information on load interaction. It has been found that there may be combinations of EBR of UHM carbon fibres and internal un-deformed low strength steel reinforcement that show no strengthening effect. It is shown that the load capacity of UHM carbon fibre sheets might be much lower in structural applications compared to values derived from laboratory tests. The reasoning is based on fibre waviness, inhomogeneous delamination, inappropriate high fibre stiffness that result in a very heterogeneous stress distribution and trigger a collapse like failure at very low ultimate loads.