The typical diameter range used is between 9.5mm and 37.5mm. The most frequent aggregate size used in construction is 20mm. In mass concrete, a greater size, 40mm, is more frequent. Coarse aggregates are usually larger than 37.5mm. The density of dry gravel is about 80% of that of water. The percentage will be lower if it contains any clay or silt. The higher the proportion of rock, the lower the density will be. For example, sand has a density of 2kg/m3 while granite has a density of 25kg/m3. Concrete has a density of 1150kg/m3. This means that, for a given volume, you need much more coarse material than fine. Bricks and blocks have a density of 900-1100kg/m3. As far as fire resistance is concerned, the key factor is the thickness of the material. The thicker the wall, the better it will resist heat and flame. Thicker walls are also more resistant to mechanical damage from being hit by vehicles or objects dropped from above. Thickness is also important in relation to insulation.