Repair jacket is adapted for repairing or reinforcing elongate structures such as pilings, especially in marine environments. Jacket includes a cylindrical body of fiber-reinforced plastic (FRP) material that is about 0.5 to 3.5 inches greater in diameter than the piling. The body is wrapped around the piling and sealed. Centering/damping springs keep the body centered on the piling and damp the blows of waves so the body is not damaged. A bottom seal is a resilient tube within an abrasion-resistant tube attached to the bottom of the body for sealing the gap. The resilient tube is filled with expanding grout to create a rigid seal at the bottom of the gap. The gap volume between piling and cylindrical body is then filled with grout to permanently attach repair jacket to piling and repair the surface of the piling.