Plastic armour (also known as plastic protection) was a type of vehicle armour originally developed for merchant ships by Edward Terrell of the British Admiralty in 1940. It consisted of small, evenly sized aggregate in a matrix of bitumen, similar to asphalt concrete. It was typically applied as a casting in situ in a layer about 2 in (51 mm) thick on to existing ship structures made from 1⁄4 in-thick (6.4 mm) mild steel or formed in equally thick sections on a 1⁄2 in-thick (13 mm) steel plate for mounting as gun shields and the like. Plastic armour replaced the use of concrete slabs, which although expected to provide protection, were prone to cracking and breaking up when struck by armour-piercing bullets. Plastic armour was effective because the very hard particles would deflect bullet