Concrete block is cast in forms using a high-density mixture of sand, cement, and aggregate. Although building blocks are often generically called cinder blocks, a true concrete block differs from a cinder block. Cinder blocks are made with aggregates of clay or pumice. They weigh less but fracture easily. If you're looking for a worry-free, long-lasting wall, use concrete blocks.
Most blocks have webs that separate two or three cavities called cores. This type of construction reduces the weight of the unit without compromising its strength. Rebar is often placed in the cores, which are then filled with concrete to strengthen the wall.
The basic building unit is the stretcher block, which has flanges on both ends. The blocks are butted together with mortar applied to the flanges. End blocks and corner blocks have one or more flange faces so they present a smooth, outside finished face. You will also find cap blocks, thinner, solid, flangeless blocks used to cap the top of a wall.
Most walls are built from a block measuring 7-5/8 x 7-5/8 x 15-5/8 inches. When laid with a standard 3/8-inch mortar joint between them, the block dimensions become 8x8x16 inches. These dimensions are the most common, but concrete block comes in a wide variety of sizes and shapes. Before you make plans for a wall, visit local building-supply dealers and research the available blocks. The sizes and shapes you find might make you decide to reconsider the wall design or dimensions.Designing with concrete block
Normally structures built of standard concrete blocks are finished with a facing material such as stucco, brick, stone, or veneer facing. For some uses, the facing is left unfinished; other times, it's painted.
Block is available in a variety of shapes, colors, and textures. Such architectural blocks need no additional facing material or finishing. Many are cast to look like cleft stone. Others are manufactured with fluted or scored surfaces, ribs, and faces recessed with geometric patterns. Homeowners use blocks with open designs to make screens. You'll even find some that look like wood -- use them to build a structure that looks more like a fence than a wall. If your budget allows, you may opt for prefaced blocks in a rainbow of colors and colored glazes.
A mortared concrete block wall must be built on a solid concrete footing with dimensions and reinforcement that conform to local building codes.Interlocking concrete block
You can also buy concrete blocks that don't require mortar. Interlocking concrete blocks rely on different methods to hold them together. Some are cast with flanges on one side that hook one course to the preceding one. Others use pins engaged in holes. Still others use a system of the blocks' own concave/convex ridges or depressions. Another style provides a low-labor way to build a curved wall without cutting a large number of blocks. Most interlocking blocks are available in various surface textures and colors. Such blocks can save much time and effort in do-it-yourself projects.The only block you need to mow
Turf block isn't technically a concrete block in the same sense as the block used in building walls. It's a precast concrete paver formed from pressurized concrete. Its compressive strength is such that it can withstand the weight of automobiles and trucks. Thus, it makes an unusual driveway surface that lets grass grow up through the recesses. It's also suitable for paths and walks, although the recesses make a rough ride for wheeled garden equipment.Pro Tip: Buying concrete block
You'll find concrete block at all major home improvement centers and some lumberyards.
To estimate how many standard 8x16-inch blocks you'll need, figure that 100 square feet of wall will require roughly 113 blocks.
You can get a more precise estimate with the following calculations:
- Multiply the length of the wall in feet by .75 to get the number of blocks in each course.
- Multiply the height of the wall in feet by 1.5 to get the number of courses.
- Multiply the two results together to get the total number of blocks.
- Subtract the number of corner or end blocks you'll need and order 10 percent more to allow for cutting, mistakes, and breakage.