1. Introduction

Concrete is a mixture made of aggregates, water, cement, and different additives. The "lightweight" term can be added to different types of concrete which are all common in one specification, and that is "lower density" than normal weight concrete (NWC). This reduction in density is achievable by different methods such as using lightweight aggregate (LWA) in concrete, foamed concrete (FC), and autoclaved aerated concrete (AAC) or by any other techniques that reduce the final specific weight of the product, and thus the achieved weight is less than what we have in NWC mixtures. Whereas NWC weighs from 2240 to 2450 kg/m3 , lightweight concrete weighs <sup>300</sup>–2000 kg/m<sup>3</sup> , but the practical range of density for lightweight concrete is 500–1850 kg/m<sup>3</sup> . Before talking about the background of LWC, we prefer to explain a little more about the different types of LWC and their mechanical properties.
