Control Blocks. Control blocks come in various sizes and are similar to standard concrete
construction blocks, except that protrusions in the block ends provide a tongue-and-groove interlock
between units (Figure 26). Designed to be hand-placed on a filter cloth with the cells vertical, the blocks
can be aligned with their long axes parallel to shore, but optimum performance probably results from
placement with their long axes perpendicular to the water's edge.
Concrete Masonry Blocks. Standard construction masonry blocks should be hand-placed on a
filter cloth with their long axes perpendicular to the shoreline and the hollows vertical. Their general
availability is a primary advantage, but their wide use also makes them susceptible to theft. They form a
deep, tightly fitting section which is stable provided the toe and flanks are adequately protected. Their
primary disadvantage is that standard concrete for building construction is not sufficiently durable to
provide more than a few years service in a marine environment.
Shiplap Blocks. Shiplap blocks can be fabricated by joining standard concrete patio blocks with
an epoxy adhesive. At 100 pounds or more per unit, they are designed to be hand-placed on a filter
(Figure 27). The precautions about concrete deterioration apply here as well.