Dense Concrete Blocks
Dense concrete blocks are usually manufactured from cement, fine aggregate and coarse aggregate moulded under pressure, and are produced in a range of crushing strengths (measured in Newtons/mm ) and widths up to 300mm (‘trench blocks’). Being dense (in the region of 2200kg to 2400kg/m3), they have poor thermal insulation qualities, so they tend to get used for load-bearing
partitions, structural walls, sub-structure work (below ground) and party walls. When used externally above
ground, dense concrete blocks are usually rendered due to their tendency to absorb a lot of water. Any walls built in dense blocks that will be rendered or plastered should be allowed to dry out thoroughly first, as they will shrink on drying and could crack any plaster or render that has been put on top prematurely.
Their high density also means they are very heavy as constructional units (up to nearly 21kg in some cases for a 100mm block), which makes them difficult and tiresome to handle on site. Dense concrete blocks should be laid in
mortars of average strength – 1:1:6 or 1:2:9 (cement:lime:sand) – although slightly stronger mixes are recommended below ground.