The construction industry regularly faces the need to strengthen existing reinforced concrete (RC) members to maintain or extend service life. Shear failure in beams is brittle in nature and may occur without warning leading to catastrophic failure and loss of life. This thesis investigates the strengthening of RC T-beams in shear with ultra high performance concrete (UHPC) U-shaped jackets. Studies of RC beams strengthened with UHPC jackets have mainly focused on bending with only limited investigation of shear failure for which there is no agreed design method. This research studies the effectiveness of UHPC U-shaped jackets at increasing the shear strength of RC T-beams through an experimental program, nonlinear finite element analysis (NLFEA) and analytical work. Two groups of four beams were tested, with three of the beams in each group having UHPC U-shaped jackets that varied in geometry and embedded longitudinal reinforcement. Tests were monitored using digital image correlation (DIC) to determine the kinematics of the critical shear cracks. The contributions to shear resistance of aggregate interlock, fibre bridging, dowel action, stirrups and the uncracked compression zone, were determined from the crack kinematics. Fibre bridging was found to be the dominant shear resisting mechanism in the jacketed specimens, but its contribution became less dominant towards failure due to widening of the critical shear cracks. The measured strengths of the tested beams are compared with design code based predictions and numerical analysis. Direct superposition of individually calculated design strengths of the RC beam and UHPC jacket is shown to overestimate shear strength particularly for thicker jackets...