Masonry Systems
Masonry is one of the oldest, most durable, and most common construction
methods globally. It involves building structures by laying individual units
(like brick, stone, or concrete blocks) and binding them together with mortar.
When reinforced with steel and grout, masonry systems can withstand
significant axial, shear, and flexural loads.
Concrete Hollow Blocks (CHB)
Concrete Masonry Units (CMUs), commonly known in the Philippines as Concrete
Hollow Blocks (CHB), are the standard building blocks for most low-to-mid-rise
structural and non-structural walls.
Anatomy of a CHB
A standard block consists of two solid outer 'Face Shells' and internal
partitions called 'Webs'. The empty spaces formed between them are called
'Cells', which reduce the block's weight, improve thermal insulation, and
provide a conduit for vertical steel reinforcement and grout.
Checklist
- Standard Nominal Dimensions: Typically 400 mm long 200 mm high.
- Standard Thicknesses: Commonly manufactured in 100 mm (4"), 150 mm (6"), and 200 mm (8") thicknesses. 150 mm and 200 mm are typically used for load-bearing exterior walls, while 100 mm is restricted to non-load-bearing interior partitions.
- Compressive Strength (ASTM C90): Standard load-bearing CMUs must have a minimum net area compressive strength of 13.1 MPa (1900 psi) averaged over three units.
Masonry System Strength Simulator
Adjust the block and mortar properties to see how they affect the composite masonry compressive strength ($f'_m$).
System Output
Composite Strength ($f'_m$):12.96 MPa
*The composite strength is heavily influenced by the block strength, but weak mortar limits the overall assembly.
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Compressive Strength of Masonry ()
The specified compressive strength of the masonry assemblage (the combination
of blocks, mortar, and grout). It is the critical design parameter for masonry
walls, tested via masonry prisms or calculated empirically based on unit
strength and mortar type.
Binders: Mortar and Grout
While blocks provide the primary compressive mass, mortar and grout are
essential for bonding the system together, transferring stresses, and
anchoring reinforcement.
Mortar (ASTM C270)
Mortar
A workable paste composed of cement, lime, sand, and water used to bind
masonry units together, fill the irregular gaps between them, and seal the
wall against moisture penetration.
Mortar is classified by its compressive strength and intended application. A
common mnemonic for remembering mortar types in decreasing order of strength
is: MaSoN wOrK (M, S, N, O, K).
Checklist
- Type M (17.2 MPa / 2500 psi): High compressive strength. Used for heavily loaded foundations, retaining walls, and below-grade applications where maximum strength and durability against freeze-thaw or groundwater are required.
- Type S (12.4 MPa / 1800 psi): Moderate-to-high strength. The standard choice for reinforced masonry, exterior walls subject to high wind or seismic lateral loads, and load-bearing walls.
- Type N (5.2 MPa / 750 psi): Medium strength. A general-purpose mortar for above-grade, non-load-bearing exterior walls, veneers, or moderately loaded interior partitions.
- Type O (2.4 MPa / 350 psi): Low strength. Used exclusively for non-load-bearing interior walls, historic restorations, or pointing (repairing mortar joints) where a softer mortar is required to prevent damaging historic, soft bricks.
Grout (ASTM C476)
Grout
A highly fluid, high-slump mixture of cement, sand, fine gravel, and water
used to fill the hollow cells in masonry units. Its primary purpose is to bond
the masonry directly to the steel reinforcement, creating a composite
structural element.
Note
Unlike concrete, masonry grout requires an exceptionally high slump (200 mm to
250 mm or 8-10 inches) so it can flow easily into small, restricted cells
without honeycombing. The excess water is rapidly absorbed by the porous
masonry blocks, lowering the effective water-cement ratio and resulting in
high final strength.
Structural Design and Estimation
Estimating material quantities is a fundamental task for construction
engineers to ensure accurate cost forecasting and logistics.
Brick and Clay Masonry
While Concrete Hollow Blocks are ubiquitous, traditional fired clay bricks
remain a fundamental masonry material, particularly for architectural veneers
and load-bearing walls in specific regions.
Checklist
- Building Brick (ASTM C62): Solid clay units used for structural purposes where appearance is not a primary concern (e.g., backup walls).
- Facing Brick (ASTM C216): Solid clay units used where appearance is critical. They are manufactured with strict tolerances for color, texture, and size.
- Hollow Brick (ASTM C652): Clay units with a core area greater than 25% of the gross cross-sectional area, designed to accommodate reinforcement and grout.
Absorption and Initial Rate of Absorption (IRA)
Clay bricks are highly porous. IRA measures how quickly a brick absorbs water.
If a brick has a high IRA, it will rapidly suck the moisture out of the fresh
mortar, leading to a weak, powdery bond. To prevent this, high-IRA bricks must
be pre-wetted before laying.
Reinforcement in Masonry
Unreinforced masonry (URM) is brittle and highly susceptible to failure under
tensile stresses caused by wind or seismic activity. Modern structural masonry
heavily relies on reinforcement.
Checklist
- Vertical Reinforcement: Steel reinforcing bars (rebar) placed in the hollow cells of CMUs or hollow bricks and subsequently grouted solid. This provides the primary resistance to flexural and tensile loads.
- Horizontal Joint Reinforcement: Prefabricated wire assemblies (ladder or truss type) embedded within the horizontal mortar joints. They help control shrinkage cracking and tie wythes (layers) of a multi-wythe wall together.
- Bond Beams: Special U-shaped masonry units laid horizontally, containing horizontal rebar and filled with grout. They act as continuous horizontal lintels over openings, distribute concentrated loads, and tie the top of a masonry wall together.
Testing of Masonry Assemblages
Because masonry is a composite of block, mortar, and grout, testing individual
components often overestimates the strength of the actual wall.
Checklist
- Prism Test (ASTM C1314): The most accurate representation of wall strength. A small "prism" (typically two blocks stacked with mortar) is built and crushed to determine the specified compressive strength of masonry ().
- Mortar Cube Test (ASTM C109): Tests the compressive strength of 50 mm (2-inch) standard mortar cubes cast in brass molds.
- Grout Prism Test (ASTM C1019): Because block absorption lowers the ratio of grout in the field, testing grout in non-absorbent plastic or steel cylinder molds yields falsely low strengths. Grout is therefore poured into a square mold formed by actual masonry blocks lined with paper, perfectly simulating field curing conditions.
Key Takeaways
- CMU/CHB Structure: The face shells provide exterior bearing surfaces, while the internal webs and hollow cells reduce weight and allow for vertical reinforcement.
- Mortar Types (M, S, N, O): Dictate the compressive strength and application, with Type M being the strongest for foundations and Type S being standard for structural walls.
- Grout: Must have a high slump (highly fluid) to fill cells without honeycombing, bonding the block matrix to the steel rebar.
- Composite Testing: The true structural strength of a masonry wall () is best determined by a composite Prism Test (ASTM C1314) rather than testing blocks or mortar individually.