Lightweight aggregate concrete is not a new invention of modern concrete technology, but dates back even to before the Christian era. Natural aggregates like scoria or pumice were utilised in masterpieces such as Babylon of the Sumerians, Hagia Sophia in Istanbul or the Pantheon of the Romans. The demand for lightweight aggregate concrete increased over time because of its advantages, specifically properties such as its thermal insulating properties and low density. It has also become an important structural material in off-shore construction during recent years. A comprehensive study was carried out in METU Mechanics of Materials Laboratory in order to investigate some characteristics of fibre-reinforced semi-lightweight concrete for seismic strengthening purposes of reinforced concrete framed structures. Semi-lightweight concrete containing unexpanded perlite, both as lightweight aggregate and as a supplementary cementing material, was reinforced by polypropylene and steel fibres, separately. Compressive strength, split tensile strength and modulus of elasticity measurements were carried out on cylinder specimens. Steel-mesh-reinforced semi-lightweight concrete plates were also tested as reference specimens for the toughness test and the results were compared with those for fibre-reinforced semi-lightweight concrete plates. Cylinder test results indicated a considerable increase in 28-day compressive strength in the case of unexpanded perlite powder replacement; while providing lower tensile strength and modulus of elasticity. Toughness test results indicated the superiority of polypropylene fibre-reinforced semi-lightweight concrete for seismic strengthening purposes in the case of fibre utilisation.