What Is Concrete?

Concrete is a composite construction material formed by combining a cementitious binder, typically Portland cement, with aggregates such as sand and crushed stone, and water in proportions that produce a plastic mixture capable of being cast into any form before hardening into a durable solid. When water reacts with cement particles through a process called hydration, calcium silicate hydrate gel forms and binds the aggregate matrix into a dense, load-bearing solid. Concrete is the most widely used construction material by volume in the world, and its engineering properties, particularly high compressive strength and fire resistance paired with relatively low tensile strength, have shaped structural design practice for more than a century. Research programs at the National Institute of Standards and Technology address measurement science challenges associated with innovative concrete formulations and quality assurance for advanced applications.

Concrete engineering draws from materials science, structural mechanics, and chemistry. The discipline encompasses mix design, fresh-concrete workability, hydration kinetics, hardened-concrete characterization, and long-term durability under environmental exposure. Standards from the American Concrete Institute (ACI) and ASTM International define test methods, mix proportioning procedures, and acceptance criteria for concrete in structural applications.

Composition and Microstructure

The microstructure of hardened concrete is a heterogeneous assemblage of unreacted cement grains, calcium silicate hydrate gel, calcium hydroxide crystals, pores, and aggregate particles. The water-to-cement ratio is the single most influential mix parameter: decreasing it reduces porosity, increases compressive strength, and improves durability, but also reduces workability and increases the difficulty of placement. Supplementary cementitious materials, including fly ash, slag cement, and silica fume, replace a portion of Portland cement; they react with calcium hydroxide to form additional binding gel, reduce permeability, and lower the carbon footprint of the mixture.

The 28-day compressive strength, measured in megapascals on cylindrical specimens per ASTM C39, is the standard acceptance criterion in most construction specifications. Normal-weight structural concrete typically achieves 20 to 40 MPa; high-performance mixes reach 80 to 120 MPa; and ultra-high-performance concrete (UHPC) formulations incorporating densely packed fine particles and steel microfibers exceed 150 MPa.

Reinforced and Prestressed Concrete

Plain concrete is brittle in tension, fracturing at roughly one-tenth of its compressive strength. Reinforced concrete addresses this limitation by embedding deformed steel bars within the concrete cross-section; the steel carries tensile and shear forces while the concrete carries compression. The composite action relies on bond between the deformed bar surface and the surrounding concrete matrix. Prestressed concrete further improves flexural performance by imposing a pre-compression on the concrete through high-tensile-strength steel tendons or wires that are tensioned either before casting (pretensioning) or after hardening (post-tensioning), counteracting in-service tensile stresses and permitting longer spans and thinner sections than conventional reinforcement alone can achieve.

The NIST Virtual Cement and Concrete Testing Laboratory provides computational tools for simulating hydration and microstructure development, supporting mix design optimization without the resource demands of extensive physical testing programs.

Durability and Deterioration

Concrete durability refers to its resistance to environmental agents that degrade the material over time. Carbonation, which converts calcium hydroxide to calcium carbonate and lowers the pH of the pore solution, reduces the passive film protecting embedded steel from corrosion. Chloride ingress from seawater or deicing salts accelerates steel corrosion when chloride concentrations at the rebar depth exceed a threshold value. Alkali-silica reaction between reactive aggregate minerals and alkaline pore fluid produces expansive gel that cracks the concrete from within. NIST technical notes on aggregate characteristics document how aggregate selection significantly influences long-term compressive strength development and durability in both ordinary and blended-binder concretes.

Applications

Concrete has applications across a wide range of construction and infrastructure contexts, including:

• Building foundations, frames, floors, and shear walls
• Bridges, overpasses, and highway pavements
• Pressure vessels and containment structures for nuclear power facilities
• Dams, retaining walls, and water treatment infrastructure
• Tunnel linings and underground structures
• Marine and offshore structures exposed to seawater