Concrete Block Classification

Density

• Hollow blocks are the lightest classification of concrete blocks. Lightweight blocks have a cubic foot weight of less than 105 pounds, while medium-weight blocks have a cubic foot weight of less than 125 pounds. Blocks must have a cubic foot weight of more than 125 pounds in order to be classified as normal.

Strength

• The density of a block does not always directly correlate with its strength. While normal-weight blocks are often the strongest, builders who need extra strong blocks check their compressive strength. All blocks, despite weight, must meet the specified 1900-pounds-per-square-inch compression strength, but builders can request blocks that are stronger.

Sound Absorption

• Concrete blocks are very effective in eliminating or reducing noise transmission. Because they are less dense and therefore can absorb more, it is the lightweight blocks that are most effective at blocking sound. Builders with a particular interest in using the blocks to absorb sound typically choose lighter weight blocks unless they need the higher density or higher compressive strength rating. Sound absorption ratings are measured in decibels; concrete blocks that achieve a 60 decibel reduction in sound are rated as 60-decibel blocks.

Fire Rating

• The fire rating is particularly important in concrete blocks used for building homes. The fire rating is determined by the aggregates in the block as well as its overall thickness. Sand, gravel and slag is added to hollow blocks to increase their fire rating. Cavity barriers are also added to reduce the spread of smoke and flames through cavities in the blocks; these barriers are made of asbestos-free board, metal and even timber.