Concrete Block Masonry Units
- Sizes and shapes:
- Standard block: most common with nominal size of 8" x 8" x 16" (specified in width x height
x length)
- Actual dimensions are 3/8" less than nominal to allow for standard mortar joint thickness in a modular planning grid.
- Solid or hollow units:
- Solid = core holes £ 25% of gross area.
- Hollow = core holes > 25%
- Most commonly used because of:
- Reduced weight
- Ease of handling
- Ease of reinforcing
- Generally » 50% solid.
- Most commonly used because of:
- Standard block: most common with nominal size of 8" x 8" x 16" (specified in width x height
x length)
- Raw materials and manufacture:
- Raw materials:
- Portland cement
- Usually Type I, all purpose
- Aggregate
- Account for as much as 90% of the block's composition.
- 2 types:
- Heavy-weight (normal-weight):
- Well graded sand, gravel or crushed stone.
- Light-weight:
- Pumice, cinders, expanded slag.
- Reduce block weight by 20 - 45% with little reduction in strength.
- Heavy-weight (normal-weight):
- Water
- Additives:
- Pozzolons (reduce expansion characteristics, adds sulfate resistance)
- Air entrainments (increase workability, lowers compressive strength)
- Color pigments
- Water repellants (control water absorption of unit, can disrupt the mortar bond if mortar not compatible)
- Portland cement
- Modern mass production:
- Made from a relatively dry mix.
- Molded and cured under controlled conditions.
- The manufacturing process is depicted in the following figure which was adapted from A Set of Class Notes in Masonry Structures by Daniel Abrams, 2nd ed, September, 1993.
- Raw materials:
- Grades, types, and density.
- Grades assigned to load-bearing blocks:
- N: general use for both above and below grade. Higher strength and better resistance to moisture penetration and frost action.
- S: limited to above grade walls with weather protection. Moderate strength and moderate resistance to frost action and moisture penentration.
- Two types exist for each grade.
- Type I: moisture content of unit is controlled to limit shrinkage of block due to moisture loss.
- Type II: non-moisture controlled unit.
- Controlling unit moisture is important:
- If moist units are built into a wall
- Shrinkage is restrained
- Excessive cracking can occur
- When Type I block is not available or cannot protect Type I from weather, pay attention to control joints to minimize cracking.
- 3 weight classifications for hollow CMU controlled by using light weight aggregate or foaming agents.
Class Oven-dry weight (pcf) Average weight of 8X8X16 (lb) Light 85 - 110 22 - 28 Medium 110 - 125 35 Normal > 125 40 - 44
- Grades assigned to load-bearing blocks:
- Unit Strength and Absorption Properties
- Higher compressive block strengths associated with:
- Wetter mixes.
- Extra compaction to reduce voids.
- Slower curing.
- Estimate other unit properties from the unit's compressive stresses.
- Tensile » 3 to 5%
- Flexural » 7 to 10%
- E » 100 to 600 times.
- Water absorption Þ related to durability and resistance to freeze-thaw.
- Highly absorptive: permeated water can fracture units upon freezing.
- Higher compressive block strengths associated with:
