Material Properties & Classification

Before studying individual materials, you need a common vocabulary for describing how materials behave under load, how they respond to temperature and moisture, and how they are classified. This lesson establishes that vocabulary — the mechanical, thermal, and moisture-related properties that define every construction material.

Mechanical properties describe how a material responds to applied forces.

Strength

Strength is a material's ability to resist force without failure. There are several types of strength, each corresponding to a different type of loading:

• Compressive strength: Resistance to being crushed or shortened. Concrete and masonry are valued primarily for their compressive strength. Concrete's compressive strength is typically specified as f'c — the 28-day compressive strength of a standard test cylinder, expressed in pounds per square inch (psi). Common values range from 3,000 to 6,000 psi for conventional concrete, with high-performance concrete reaching 10,000 to 20,000 psi.
• Tensile strength: Resistance to being pulled apart. Steel excels in tension — mild structural steel (A36) has a tensile strength of approximately 58,000 psi. Concrete, by contrast, has a tensile strength of only about 300-700 psi (roughly 10% of its compressive strength), which is why reinforcing steel is embedded in concrete to handle tensile forces.
• Shear strength: Resistance to forces that cause one part of a material to slide past another. Shear is critical in connections (bolts, nails, welds) and in beams near their supports.
• Flexural strength (Modulus of Rupture): Resistance to bending. This is a combination of tension and compression — when a beam bends, the top is in compression and the bottom is in tension. Flexural strength is important for beams, slabs, and any member subjected to bending loads.
• Bearing strength: Resistance to concentrated compressive forces applied over a small area, such as a beam sitting on a wall or a bolt pressing against the side of a hole.

Elasticity and Plasticity

When a force is applied to a material, it deforms. The nature of that deformation is critical:

• Elastic deformation: The material returns to its original shape when the load is removed, like a rubber band. All materials exhibit elastic behavior at low stress levels.
• Plastic deformation: The material deforms permanently — it does not return to its original shape. Steel, for example, becomes plastic (yields) when stressed beyond its yield point.
• Modulus of Elasticity (E): Also called Young's Modulus, this measures a material's stiffness — its resistance to elastic deformation. A higher E means a stiffer material.

Steel is roughly 8 times stiffer than concrete and 17 times stiffer than wood.

• Yield strength (Fy): The stress at which a material transitions from elastic to plastic behavior. For structural steel A992 (the most common wide-flange steel), Fy = 50,000 psi (50 ksi). Yield strength is a primary design parameter — structural engineers design so that stresses in steel members stay below Fy under normal loading.

Ductility and Brittleness

• Ductility: The ability of a material to undergo significant plastic deformation before fracture. Steel is highly ductile — it can stretch and bend extensively before breaking. This is a safety feature: a ductile structure gives warning (visible deformation) before failure.
• Brittleness: The opposite of ductility — the material fractures with little or no plastic deformation. Concrete, glass, cast iron, and masonry are brittle materials. They crack and break suddenly without warning. This is why brittle materials are reinforced with ductile materials (rebar in concrete, steel lintels in masonry).

Hardness and Toughness

• Hardness: Resistance to surface indentation or abrasion. Important for flooring, countertops, and wear surfaces. Measured by Mohs scale (minerals), Brinell, or Rockwell tests (metals).
• Toughness: The total energy a material can absorb before fracture — the area under the stress-strain curve. A tough material is both strong and ductile. Steel is tough; glass is strong but not tough (it shatters).

Fatigue

Fatigue is the weakening of a material caused by repeated loading and unloading (cyclic loading), even when the stress is well below the material's ultimate strength. Fatigue is a concern in:

• Bridge members subjected to repeated traffic loading
• Crane runways
• Connections in structures subjected to vibration
• Wind-induced vibrations in tall buildings

• Thermal conductivity: The rate at which heat flows through a material. Metals have high thermal conductivity (they feel cold to the touch); insulation materials have low thermal conductivity.
• Thermal expansion: All materials expand when heated and contract when cooled. The coefficient of thermal expansion (CTE) measures this behavior. Concrete and steel have nearly identical CTEs (~6 × 10⁻⁶ in/in/°F), which is why they work well together in reinforced concrete — they expand and contract at the same rate.
• R-value: The measure of thermal resistance. Higher R-value = better insulation. R-values are additive — the total R-value of a wall is the sum of the R-values of all its layers (sheathing, insulation, drywall, air films).
• U-factor: The overall heat transfer coefficient — the inverse of R-value (U = 1/R). Lower U-factor = better insulation. U-factors are used for windows and entire wall assemblies.

• Permeability: The ability of a material to allow water vapor to pass through it. Measured in perms. Materials with high permeance (>10 perms) are considered vapor-permeable (e.g., house wrap, latex paint). Materials with low permeance (<0.1 perms) are vapor impermeable (e.g., polyethylene sheet, glass, metal).
• Absorption: The amount of water a material can take in. Important for masonry, concrete, and wood. High absorption can lead to freeze-thaw damage, staining, and efflorescence.
• Moisture content (MC): The amount of water in a material expressed as a percentage of its dry weight. Critical for wood — lumber should be below 19% MC for framing and below 12% for finish work.

Construction materials are classified in several ways:

By Composition

• Natural: Stone, wood, sand, clay, aggregates
• Processed: Concrete, steel, glass, brick, lumber
• Synthetic: Plastics, polymers, composites, sealants

By Structural Function

• Structural materials: Materials that carry loads (steel, concrete, wood, masonry)
• Envelope materials: Materials that enclose the building (cladding, roofing, waterproofing)
• Finish materials: Materials that provide the final appearance (drywall, flooring, paint, tile)
• Mechanical/systems materials: Materials for building systems (pipe, conduit, ductwork)

By Fire Resistance

• Noncombustible: Will not ignite, burn, or release flammable vapors (steel, concrete, masonry, glass)
• Combustible: Can ignite and sustain combustion (wood, plastics, many insulation types)
• Fire-resistant: Materials that can withstand fire exposure for a rated duration (fire-rated assemblies achieve 1-hour, 2-hour, 3-hour, or 4-hour ratings)

By CSI MasterFormat Division

The Construction Specifications Institute (CSI) organizes construction materials and work into 50 divisions. Key material-related divisions include:

• Division 03: Concrete
• Division 04: Masonry
• Division 05: Metals (structural steel, miscellaneous metals)
• Division 06: Wood, Plastics, and Composites
• Division 07: Thermal and Moisture Protection
• Division 08: Openings (doors, windows, glazing)
• Division 09: Finishes (drywall, flooring, paint, tile)