<P> Concrete masonry units may be formulated with special aggregates to produce specific colors or textures for finish use . Special textures may be produced by splitting a ribbed or solid two - block unit; such factory - produced units are called "split - rib" or "split - face" blocks . Blocks may be scored by grooves the width of a mortar joint to simulate different block modules . For example, an 8 - by - 16 - inch (200 mm × 410 mm) block may be scored in the middle to simulate 8 - by - 8 - inch (200 mm × 200 mm) masonry, with the grooves filled with mortar and struck to match the true joints . </P> <P> Concrete block, when built in tandem with concrete columns and tie beams and reinforced with rebar, is a very common building material for the load - bearing walls of buildings, in what is termed concrete block structure (CBS) construction . American suburban houses typically employ a concrete foundation and slab with a concrete block wall on the perimeter . Large buildings typically use copious amounts of concrete block; for even larger buildings, concrete blocks supplement steel I - beams . </P> <P> Concrete masonry can be used as a structural element in addition to being used as an architectural element . Ungrouted, partially grouted, and fully grouted walls are the different types of walls allowed . Reinforcement bars can be used both vertically and horizontally inside the CMU to strengthen the wall and results in better structural performance . The cells in which the rebar is placed must be grouted for the bars to bond to the wall . For this reason, high seismic zones typically only allow fully grouted walls in their building codes . The American design code that guides design engineers in using CMU as a structural system is the Masonry Standards Joint Committee's Building Code Requirements & Specification for Masonry Structures (TMS 402 / ACI 530 / ASCE 5). The compressive strength of concrete masonry units and masonry walls varies from approximately 1,000 psi (7 MPa) to 5,000 psi (34 MPa) based on the type of concrete used to manufacture the unit, stacking orientation, the type of mortar used to build the wall, and other factors . </P>

How big are the holes in a cinder block