A device for filling or creating a void within a construction medium. The device has a body having at least one inflatable body portion. Each inflatable body portion has one or more inflatable cavities. There is at least one valve member fluidly connected to the one or more inflatable cavity to control flow of fluid into and inflate said cavity. In-use, the one or more cavities are shaped to substantially fill or create the void in a construction medium when in an inflated state.