Concrete
Overview
Concrete is a composite construction material made by mixing fine aggregate (sand) and coarse aggregate (gravel) into a paste made from cement and water, then hardening it through a hydration reaction. In its liquid state, it can be formed into arbitrary shapes to fit formwork, and after hardening it exhibits high compressive strength, durability, fire resistance, and water resistance. Because it can be mass-produced from cheap and abundant raw materials—water, cement, and aggregate—concrete has become the most common structural material supporting modern cities and infrastructure, and is considered the most consumed substance in the world after water.
Main Content
Definition and Components
Concrete consists largely of three components. First, the cement paste produced by the reaction of cement and water acts as an adhesive binding the aggregates together. Second, fine aggregate (sand, generally up to about 5 mm) and coarse aggregate (gravel and crushed stone) make up about 60–75% of the total volume and form the skeleton. Third, mineral admixtures and chemical admixtures are added as needed to control workability, strength, durability, setting time, and other properties. Increasingly, industrial by-products such as fly ash, ground granulated blast-furnace slag, and silica fume are being used as cement substitutes.
History
Ancient Rome built large structures such as the Pantheon and the Colosseum with pozzolanic cement made by mixing volcanic ash and lime. In 1824, Joseph Aspdin of England patented Portland cement, laying the foundation for modern concrete, and in the mid-to-late 19th century, reinforced concrete, which combines rebar and concrete, emerged to compensate for concrete's weakness in tensile strength. After the 20th century, various technologies such as prestressed concrete, high-strength concrete, and flowing concrete developed, making the construction of ultra-high-rise buildings and long-span bridges possible.
Types
- Normal concrete: The most basic mix, ordinary-strength concrete with a slump of about 8–18 cm.
- High-strength concrete: With a compressive strength of 40 MPa or more, used for vertical members of ultra-high-rise buildings.
- Lightweight concrete: Concrete whose unit weight is reduced by using artificial lightweight aggregate, advantageous for high-rise buildings and bridge decks.
- Fiber-reinforced concrete (FRC): Steel or synthetic fibers are added to increase crack resistance and toughness.
- Pervious concrete: An eco-friendly paving material that artificially creates continuous voids, allowing rainwater to seep into the ground.
- Self-healing concrete: A new material in which encapsulated bacteria or polymers fill cracks by themselves when they occur.
Properties and Mix Design
Concrete is strong in compression but very weak in tension, with tensile strength only about 1/10 of compressive strength. Therefore, for members subjected to bending or tension, reinforced concrete or prestressed concrete with rebar or strands is used. In mix design, the water-cement ratio (W/C) is a key variable; the lower this ratio, the higher the strength and watertightness, but the lower the workability. Recently, the water-binder ratio (W/B) concept and durability-oriented design have been standardized.
Construction and Curing
Concrete goes through the processes of batching → mixing → transport → placing → compaction → curing. After placing, voids are removed with a vibrator, and after surface finishing, curing to maintain a moist state for a certain period is essential. Poor curing causes drying-shrinkage cracks and reduced strength. In winter, cold-weather concrete construction is required to prevent frost damage, and in summer, heat of hydration management is required.
Environmental Impact
Producing one ton of cement emits about 0.6–0.9 tons of carbon dioxide, and about 7–8% of global CO₂ emissions come from the cement industry. Accordingly, decarbonizing concrete has emerged as the biggest challenge for the construction industry.
Latest Trends
The key keywords for the concrete industry in 2024–2025 are low carbon, circularity, and digitalization. First, carbon capture, utilization, and storage (CCUS) technologies are being applied to cement plants, and carbon-curing methods that inject CO₂ into the concrete curing process to mineralize it have entered the commercialization stage. Second, fly ash–slag–limestone blended cement (LC3) and geopolymer concrete are drawing attention as alternatives that greatly reduce the use of cement clinker. Third, 3D-printed concrete is enabling formwork-free freeform construction and leading construction automation. Fourth, AI-based mix optimization, smart curing management using IoT sensors, and structural health monitoring are spreading. Fifth, policies to improve the quality of recycled aggregate from demolished concrete and expand mandatory use of recycled aggregate are being strengthened at home and abroad.
Related Topics
- [[Cement]]
- [[Reinforced concrete]]
- [[Construction industry]]
- [[Carbon neutrality]]
- [[3D printing architecture]]
- [[Aggregate]]