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8-2. A concrete block is a masonry unit that either contains single or multiple hollows, or is solid. It is made from conventional cement mixes and various types of aggregate, including sand, gravel, crushed stone, air-cooled slag, coal cinders, expanded shale or clay, expanded slag, volcanic cinders (pozzolan), pumice, and scoria (refuse obtained from metal ore reduction and smelting). The term concrete blocks was formerly limited to only hollow masonry units made with such aggregates as sand, gravel, and crushed stone. But today the term covers all types of concrete blocks (both hollow and solid) made with any kind of aggregate. Concrete blocks are also available with applied glazed surfaces, various pierced designs, and a wide variety of surface textures. Although a concrete block is made in many sizes and shapes (see Figure 8-1 below) and in both modular and nonmodular dimensions, its most common unit size is 7 5/8 by 7 5/8 by 15 5/8, known as 8- by 8- by 16-inch block nominal size. All concrete blocks must meet certain specifications covering size, type, weight, moisture content, compressive strength, and certain other special characteristics. Concrete masonry is an increasingly important type of construction due to technological developments in both the manufacture and the use of concrete blocks. Properly designed and constructed concrete masonry walls satisfy many building requirements, including fire prevention, safety, durability, economy, appearance, utility, comfort, and acoustics.


Figure 8-1. Typical unit sizes and shapes of concrete masonry units


8-3. Concrete blocks are used in all types of masonry construction, such as:

  • Exterior load-bearing walls (both below and above grade).
  • Interior load-bearing walls.
  • Fire walls and curtain walls.
  • Partitions and panel walls.
  • Backing for bricks, stones, and other facings.
  • Fireproofing over structural members.
  • Fire-safe walls around stairwells, elevators, and enclosures.
  • Piers and columns.
  • Retaining walls.
  • Chimneys.
  • Concrete floor units.


8-4. The main types of concrete masonry units are:

  • Hollow, load-bearing concrete block.
  • Solid, load-bearing concrete block.
  • Hollow, nonload-bearing concrete block.
  • Concrete building tile.
  • Concrete brick.

8-5. The load-bearing types of blocks have two grades. Grade N is for general use, such as exterior walls both above and below grade that may or may not be exposed to moisture penetration or weather, and for back-up and interior walls. Grade S is for above-grade exterior walls with weather-protective coating and for interior walls. The grades are further subdivided into two types: type I moisture-controlled units (for use in arid climates) N-I and S-I, and type II nonmoisture-controlled units N-II and S-II.


8-6. The concrete masonry units made with either heavyweight or lightweight aggregates are referred to as such. A hollow, load-bearing concrete block is 8 by 8 by 16 inches with nominal-size weight from 40 to 50 pounds. These types of blocks are normally made with heavyweight aggregate such as sand, gravel, crushed stone, or air-cooled slag. The same type and nominal-size block weighs only from 25 to 35 pounds when made with coal cinders, expanded shale, clay, slag, volcanic cinders, or pumice. Your choice of masonry units depends on both availability and the requirements of the intended structure.


8-7. ASTM specifications defines a solid concrete block as having a core area not more than 25 percent of the gross cross-sectional area. Most concrete bricks are solid and sometimes have a recessed surface like the frogged brick shown in Figure 8-1 above. In contrast, a hollow concrete block has a core area greater than 25 percent of its gross cross-sectional area--generally 40 to 50 percent.


8-8. Concrete masonry units are available in many sizes and shapes to fit different construction needs. Both full- and half-length sizes are shown in Figure 8-1. Because concrete block sizes usually refer to nominal dimensions, a unit actually measuring 7 5/8 by 7 5/8 by 15 5/8 inches is called an 8- by 8- by 16-inch block. When laid with 3/8-inch mortar joints, the unit will then occupy a space exactly 8 by 8 by 16 inches. Before designing a structure, contact local manufacturers for a schedule of their available unit sizes and shapes.


8-9. To precast concrete blocks, use a power-tamping machine available from several manufacturers. Tamp the concrete into the mold, then immediately strip off the mold. This way you can make many blocks rapidly using a single mold. The mix should be dry enough for the block to retain its shape.


8-10. To precast blocks by hand, pour concrete of fluid consistency into sets of iron molds, then strip off the molds when the concrete hardens. This procedure makes dense block with little labor; however, it requires a large number of molds.


8-11. Make blocks subject to weathering with a concrete mix of at least six sacks of cement per cubic yards of mix. When using lightweight, porous aggregate, premix it with water for 2 minutes before adding the cement.


8-12. Steam is the best way to cure concrete blocks because it takes less time. Concrete blocks cured in wet steam at 125o F for 15 hours have 70 percent of their 28-day strength. If steam is not available, cure the blocks by protecting them from the sun and keeping them damp for 7 days.

David L. Heiserman, Editor

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Revised: June 06, 2015