Concrete and steel behave as complementary materials in reinforced construction. Concrete mixes are proportioned around the water-cement ratio, the weight of mixing water divided by the weight of cement. Lower ratios produce denser, stronger, more durable concrete, but at the cost of workability, so admixtures are often used to maintain placement quality. The resulting design strength is characterized by the 28-day compressive strength, denoted f'c, determined by crushing standard-cured cylinders or cubes; this value is the cornerstone for specifying structural concrete and forms the basis of nearly every strength equation used in reinforced concrete design.
Because plain concrete is strong in compression but weak in tension, steel reinforcement is embedded in members to carry tensile stresses, particularly along the tension face of beams and in slabs and walls. The two materials act together through strain compatibility, and analysis is simplified using the modular ratio n = E_s / E_c, the ratio of steel to concrete elastic moduli. The modular ratio allows the steel area to be transformed into an equivalent concrete area for purposes of computing section properties, cracking moments, and stresses.
Reinforced concrete members are detailed to resist both flexure and shear. Stirrups, typically closed loops of steel bar, are provided as shear reinforcement along the beam span, but especially near supports, where shear forces are highest and diagonal tension cracks tend to form. Beyond strength under initial loading, concrete exhibits time-dependent behavior. Creep is the continued strain under sustained constant stress, causing long-term deflections and prestress losses that exceed the initial elastic deflection. Shrinkage is the volume reduction that occurs as concrete loses moisture during curing and drying; when restrained by reinforcement or adjoining elements, shrinkage generates tensile stresses that can produce cracking. Both effects are considered in design for serviceability and durability.
For structural steel, design hinges on two key stress thresholds. The yield strength is the stress at which steel begins to deform plastically and no longer returns to its original shape after unloading, generally around 250 MPa for mild structural steel and 350 MPa for higher-strength grades. The ultimate tensile strength, always greater than the yield strength, marks the maximum stress the steel can sustain before necking and fracture. Engineers design members so that working stresses stay well below yield, while anticipating reserve capacity up to the ultimate level. In the United States, design and construction of structural concrete is governed by ACI 318, which prescribes minimum requirements for strength, serviceability, and durability, integrating the material behaviors described above into a coherent framework for beams, columns, slabs, and walls.