Concrete is cement, water, sand and stone; its engineering is everything that follows from the fact that it gets strong slowly, shrinks as it dries, creeps under sustained load and cracks at a tensile stress around a tenth of its compressive strength.

Bridge concrete is specified by 28-day compressive strength: 28–35 MPa (4–5 ksi) for decks and substructure, 40–70 MPa (6–10 ksi) for prestressed girders, where high strength lets more strand be used. Modulus of elasticity scales roughly with the square root of strength and matters for deflection and prestress loss. Creep can double the long-term deflection of a girder and transfers stress from concrete to steel; shrinkage restrained by the girder cracks the deck. Both are predicted by the AASHTO 5.4.2.3 models and both depend on humidity, member size and age at loading.

Durability, not strength, is what ends a concrete bridge's life. Chloride from de-icing salt or the sea reaches the reinforcement, the steel corrodes, the concrete spalls. The defences are cover (50–75 mm on exposed faces), low permeability (low water–cement ratio, supplementary cementitious materials such as fly ash, slag and silica fume), and keeping water off the structure in the first place.

Reinforcing bars carry the tension concrete cannot. Standard bars are deformed for bond, Grade 420 MPa (60 ksi) yield in most codes, with Grade 550 (80 ksi) permitted in AASHTO for many applications. The design questions are how much steel, where, and how it is anchored: development length is the embedment needed for a bar to reach its yield strength through bond, and it scales with bar diameter, bar stress, concrete strength, cover and coating.

plain deformedepoxy-coatedstainlessGFRP

Corrosion protection has produced a family of alternatives: epoxy-coated bars (cheap, but the coating is damaged in handling and the debate about their long-term value continues), galvanised bars, stainless steel (expensive, used at the worst exposures such as deck edges and splash zones), and glass-fibre-reinforced polymer (GFRP) bars, which cannot corrode at all but are elastic to failure and need their own design rules.

Prestressing works because the steel is so strong that the losses do not use it up. Standard seven-wire strand is 12.7 or 15.2 mm in diameter (0.5 or 0.6 in), Grade 1860 MPa (270 ksi), low-relaxation, stressed to about 75% of its ultimate strength at jacking. Bars (Grade 1035) and wires are used in special cases.

Pretensioning stresses the strand against abutments in a casting bed before the concrete is placed, and is the factory method for precast girders. Post-tensioning threads tendons through ducts in hardened concrete, jacks them from the ends, and fills the ducts with grout; it is how boxes, segmental bridges and long girders get their prestress on site.

The prestress you jack is not the prestress you keep. Losses come from elastic shortening of the concrete as it is compressed, from creep and shrinkage over years, from relaxation of the strand, and, in post-tensioning, from friction along the duct and from anchorage set as the wedges seat. Together they take 15–25% of the jacking force. Grouting protects the strand from corrosion, and when it is done badly the consequences are hidden: the Ynys-y-Gwas bridge (Wales, 1985) collapsed with no warning because water had reached ungrouted tendons at the segment joints.

Bridge steel is specified for strength, toughness and weldability. In North America, ASTM A709 covers Grade 36, 50, 50W (weathering) and the high-performance HPS 50W and 70W (yield 345 and 485 MPa) whose toughness and weldability let designers use them in fracture-critical members. Europe uses S355 and S460 to EN 10025.

Weathering steel forms a tight rust patina that protects it, saving paint for the life of the bridge, though only where it can dry: not in salt spray, not under a leaking joint, not in a permanently damp tunnel. Toughness is specified by Charpy V-notch energy at a temperature zone, because steel that is tough at room temperature can be brittle at −30 °C. Fatigue is governed by the detail, not the steel grade: welded attachments, cover plates and stiffener ends are assigned categories A to E' with sharply different allowable stress ranges.

Connections are welded in the shop and bolted in the field with high-strength bolts (ASTM F3125 Grades A325 and A490) designed as slip-critical where movement would matter. Composite action with the concrete deck through shear studs is what makes modern steel girders efficient.

Steel–concrete composite construction is the most important "composite" in bridges: the deck takes compression, the steel takes tension, and studs make them one section. It halves the steel in a positive-moment region.

shear studsUHPC jointCFRP strip

Fibre-reinforced polymers arrived in the 1990s: GFRP decks and bars, carbon-fibre (CFRP) strips and wraps for strengthening, and CFRP stay cables (Stork Bridge, Winterthur, 1996) that weigh a fifth of steel and cannot corrode but are hard to anchor and to inspect.

Ultra-high-performance concrete (UHPC), with compressive strengths of 150–200 MPa and steel fibres that give it tensile ductility, is used for joints between precast deck panels, for thin shell girders (Sherbrooke, 1997, was the first), and for overlays. Its cost keeps it in small volumes at critical places.

Stainless steel reinforcement, stainless-clad strand, and duplex stainless for cable components are the durability upgrades for marine environments, including the Gulf, where the combination of heat, humidity and chloride is as severe as anywhere in the world.

Masonry bridges are the largest stock of very old structures still in service in Europe, and assessing them is its own discipline: the arch is checked by thrust-line or limit analysis rather than by stress, and fill, spandrel walls and ring separation matter more than stone strength. They fail by scour of the piers far more often than by overload.

masonry arch, thrust line dashedglue-laminated timber

Timber is back, as glulam and cross-laminated panels, for footbridges and low-volume road bridges where a light, quick, low-carbon structure is wanted. Its rules are about moisture: keep it dry, detail the ends, and expect the connections, not the timber, to govern.