3.4 Materials
Key Takeaways
- Concrete is strong in compression but weak in tension; design compressive strength f′c is measured on 28-day cylinders per ASTM C39.
- Structural steel ASTM A992 (wide-flange) has Fy = 50 ksi and Fu = 65 ksi; ASTM A36 has Fy = 36 ksi.
- Modulus of elasticity: structural steel E ≈ 29,000 ksi (200 GPa); normal-weight concrete Ec ≈ 57,000√f′c psi.
- Steel is ductile with a defined yield plateau; concrete and aggregates are brittle and fail without significant yielding.
- Standard tests: concrete slump (ASTM C143) for workability, cylinder compression (ASTM C39), and steel tension (ASTM E8) for Fy/Fu.
What the Materials Area Tests
The Materials area is worth 4–6 FE Civil questions and supports the Structural and Construction domains. Expect questions on characteristic strengths, stress–strain behavior, standard tests, and durability mechanisms for concrete, steel, aggregates, and asphalt.
Concrete
Concrete is strong in compression and weak in tension (tensile strength ≈ 10–15% of compressive). Its design compressive strength f′c is the 28-day strength of standard cylinders tested per ASTM C39. Typical structural f′c is 3,000–5,000 psi; ACI 318 sets a minimum of 2,500 psi for structural concrete.
Key concrete relations from the NCEES FE Reference Handbook:
| Property | Relation / value |
|---|---|
| Modulus of elasticity, normal weight | Ec ≈ 57,000√f′c (psi) |
| Modulus of rupture | fr ≈ 7.5√f′c (psi) |
| Water–cement ratio effect | lower w/c → higher strength, lower permeability |
| Workability test | slump, ASTM C143 |
A lower water–cement (w/c) ratio raises strength and reduces permeability but lowers workability. Durability problems include sulfate attack, alkali–silica reaction (ASR), corrosion of reinforcement, and freeze–thaw damage (mitigated by air entrainment).
Structural Steel
Structural steel is ductile, with a clear elastic region, a yield plateau, strain hardening, and necking before fracture. Common grades:
| ASTM grade | Use | Fy | Fu |
|---|---|---|---|
| A36 | plates, angles | 36 ksi | 58 ksi |
| A992 | W-shapes (wide flange) | 50 ksi | 65 ksi |
| A572 Gr. 50 | HSS, plates | 50 ksi | 65 ksi |
For all structural steel, E ≈ 29,000 ksi (200 GPa) regardless of grade — higher-strength steel is not stiffer, only stronger. Steel tension properties (Fy, Fu, elongation) come from a standard tension test, ASTM E8/E8M. Durability concern: corrosion, controlled by coatings, galvanizing, or weathering (A588) steel.
Stress–Strain Behavior
- Ductile (steel): linear elastic → yield plateau → strain hardening → fracture; large deformation warns of failure.
- Brittle (concrete, aggregates, cast iron): nearly linear then sudden fracture with little warning; much stronger in compression than tension.
The toughness is the area under the full stress–strain curve; the modulus of resilience is the elastic-region area (energy absorbed without permanent set).
Aggregates
Aggregates occupy 60–75% of concrete volume and most of an asphalt mix. They are graded by sieve analysis (ASTM C136); a well-graded blend minimizes voids and cement/binder demand. Abrasion resistance is measured by the Los Angeles abrasion test (ASTM C131). Clean, hard, properly graded aggregate improves strength and durability.
Asphalt
Asphalt concrete (hot-mix asphalt, HMA) is a viscoelastic mix of aggregate and asphalt binder. Behavior is temperature- and rate-dependent: stiff at low temperature (thermal cracking risk) and soft at high temperature (rutting risk). The Superpave performance-grade (PG) system (e.g., PG 64-22) selects binder by climate. Optimum binder content is set by mix design (Marshall or Superpave gyratory) to balance stability, durability, and resistance to moisture damage. Confirm specific material values in the Materials and Structural sections of the NCEES FE Reference Handbook rather than memorizing every constant.
A normal-weight concrete has a specified compressive strength f′c = 4,000 psi. Using the NCEES handbook relation, what is its approximate modulus of elasticity Ec?
Which statement best describes the difference in stress–strain behavior between structural steel and plain concrete?