Concrete (construction material)
Concrete is a composite building material of cement, aggregates and water. This article covers composition, types, properties, reinforcement, production, uses, curing, durability and environmental considerations.
Concrete is a manufactured material widely used to form permanent buildings and civil structures. In its simplest form it combines Portland cement, sand, gravel or other aggregates, and water. When mixed the ingredients make a workable paste, often compared to dough, that can be cast into forms. As the cement hydrates the paste stiffens and binds the aggregates into a stone-like mass. Because of its durability and mouldability, concrete is the most-used man-made material globally, with production measured in billions of cubic metres each year.
Image gallery
10 Images
Composition and characteristics
The essential components of concrete each contribute specific properties:
- Cement: a binder that reacts chemically with water; Portland cement is the most common type and may be blended with supplementary cementitious materials.
- Aggregates: sand and gravel or crushed stone provide bulk, reduce shrinkage and influence strength and durability.
- Water: water triggers the setting reaction; the water-to-cement ratio is a key control on strength and porosity.
- Admixtures and additives: chemical or mineral ingredients alter workability, setting time, frost resistance and other performance traits.
Hydration is the chemical process by which cement and water form interlocking crystals that harden the mix. This chemical reaction, commonly called hydration, continues for months; proper curing—maintaining moisture and temperature—is essential for attaining design strength and reducing cracking.
History and development
Concrete-like materials have a long history. Ancient builders mixed lime binders and volcanic ash to make durable mortars; the Romans used volcanic pozzolans to produce hydraulic concretes that could set under water. Modern Portland cement and industrial mixing advanced use in the 19th and 20th centuries, enabling large-scale infrastructure and high-rise construction.
Types and specialised concretes
Concrete is formulated for different tasks. Common variants include ordinary reinforced concrete, reinforced concrete, precast units, and mixes designed for high strength or high durability. Other types are lightweight concrete for reduced structural weight, roller-compacted concrete for pavements, shotcrete for sprayed repairs, and proprietary high-performance mixes that resist chemicals or extreme weather.
Mixing, placing and curing
Quality depends on correct proportioning, mixing, placing and curing. Ready-mix concrete is delivered by truck and placed promptly; site batching is used where logistics require. Workability is assessed by tests such as the slump test, while vibration and consolidation remove trapped air. Curing—keeping the surface moist and at controlled temperature—promotes continuous hydration and reduces surface cracking.
Strength, reinforcement and structural use
Concrete is strong in compression but relatively weak in tension. To resist bending and tensile forces it is commonly combined with steel reinforcement. Reinforced concrete enables beams, slabs and columns to act together, tying foundations, walls and floors into a unified system. For long spans or specialised designs, prestressing or post-tensioning can improve performance.
Durability, maintenance and testing
Durability depends on mix quality, cover to reinforcement, exposure conditions and maintenance. Common deterioration mechanisms include chloride penetration, freeze–thaw damage, alkali–silica reaction and corrosion of embedded steel. Engineers use laboratory and field tests—compressive-strength testing, permeability assessments and condition surveys—to plan maintenance and repair.
Uses and common forms
Concrete appears in many forms because it can be cast, precast, pumped, sprayed or placed in situ. Typical uses include pavements and floors, foundations, architectural elements and decorative panels, bridges and tunnels, multistorey parking and parking structures, walls, footings for gates, fences and poles, and specialised items such as pipes and boats.
Environmental considerations and innovation
Cement manufacture is a significant source of carbon dioxide, so research and industry practice increasingly favour measures to reduce the environmental footprint: use of supplementary cementitious materials (fly ash, slag, silica fume), optimisation of mix design, recycling of demolished concrete as aggregate, and innovations such as low-carbon binders and self-healing or fibre-reinforced concretes. Codes and standards govern materials and testing; designers consult industry guidance and technical literature (material guides, paving, architecture).
Practical notes
Good practice includes specifying appropriate exposure classes, ensuring adequate cover to reinforcement, selecting suitable aggregates and admixtures, controlling mixing water, and providing proper curing. While concrete provides excellent compressive capacity and long service life when well designed and constructed, achieving safe performance in seismic regions or aggressive environments requires specialised design, detailing and construction quality control (earthquake resistant design principles).
Basic characteristics and use
Normal concrete usually has a compressive strength of at least 20 Newtons per square millimetre (N/mm²). Concrete with lower strength is used for the production of clean layers, backfill and in gardening and landscaping. High performance concrete reaches strengths of over 150 N/mm².
Unreinforced concrete, on the other hand, can only absorb low tensile stresses without cracking, as its tensile strength is only about one tenth of its compressive strength. Tensile stresses are therefore usually absorbed by inserted bars or meshes of reinforcing steel that have a tensile strength of over 400 N/mm². This combination has proven to be advantageous for several reasons:
- Concrete and steel have similar coefficients of thermal expansion, so that no temperature-induced stresses occur in the composite material,
- the basic pH value of the concrete prevents corrosion of the steel,
- In the event of a fire, concrete prevents the rapid loss of strength of unprotected steel due to temperature.
Typical applications of reinforced concrete:
- Foundations, (basement) walls, slabs, columns and ring beams in general building construction,
- Skeletal supporting structures of high-rise buildings and commercial buildings,
- Transport structures such as tunnels, bridges and retaining walls.
Unreinforced concrete is used for gravity walls, curved gravity dams and other compact, massive structural components that are loaded predominantly in compression. Larger tensile stresses must either be avoided by design or there must be no risk from fracture of the material. This is the case, for example, with smaller prefabricated elements such as blocks for masonry construction or (exposed aggregate) concrete slabs in horticulture. Due to its low cost, arbitrary formability and comparatively high density of about 2400 kg/m³, concrete is also used for counterweights on cranes and for breakwaters.
The shrinkage of the component volume during drying and due to chemical processes must be taken into account. The degree of shrinkage depends on the composition of the starting material. A certain amount of creep occurs in all loaded components and describes the increasing deformation under load over time.
Distinguishing Features
See also: List of common concretes
Concrete can be distinguished on the basis of various characteristics. Common distinctions are according to
- the dry bulk density in light concrete, normal concrete and heavy concrete,
- the strength, with the compressive strength taking the most important role,
- the place of production in site or ready-mixed concrete,
- the intended use in, for example, waterproof concrete, underwater concrete,
- the consistency in classes from stiff to (very) flowable,
- the type of compaction in vibrated concrete, tamped concrete, rolled concrete, flowing concrete, poured concrete, shotcrete, ...
- the type of aggregate in sand concrete, gravel concrete, crushed concrete, ...
- the hardening state into the fresh concrete that can still be processed, the green concrete that has already been placed and compacted, the young concrete whose hardening has already begun and finally the hardened hardened concrete,
- the requirements for quality assurance in formulated concrete (production class R according to ÖNORM 4200 or class B I according to DIN 1045) and concrete after suitability testing (production class E or class B II according to DIN).
Like concrete, mortar is a mixture of a binder, aggregate and additives or admixtures. The difference is in the size of the aggregate, which in the case of mortar must not exceed 4 mm in diameter. There is an overlap in the case of spray plasters and masonry mortars, which in special cases may contain a maximum aggregate size of up to 16 mm, and in the case of screed, which is usually mixed with 8 mm aggregate size.
Questions and answers
Q: What is concrete made of?
A: Concrete is a composite material made of Portland cement, sand, gravel or aggregate, and water in varying proportions depending on the task.
Q: How much concrete is produced each year?
A: As of 2006, about 7.5 billion cubic meters of concrete are made each year—more than one cubic meter for every person on Earth.
Q: What does mixing the ingredients together create?
A: Mixing the ingredients together creates a paste, similar to making dough for bread.
Q: How does concrete solidify?
A: Concrete solidifies due to a chemical reaction known as hydration. The water reacts with the cement, which bonds the other components together, eventually creating a strong stone-like material.
Q: What are some common uses for concrete?
A: Common uses for concrete include pavements, pipes, architectural structures, foundations, motorways, bridges and multi-story parking structures. It can also be used to make walls, footings for gates and fences and even boats.
Q: What is one advantage of using concrete over other materials?
A: One advantage of using concrete over other materials is that it bonds bricks and stones better than any other method known to mankind.
Q: When was concrete first used?
A: Concrete was first used as early as 5600 BC.
Related articles
Author
AlegsaOnline.com Concrete (construction material) Leandro Alegsa
URL: https://en.alegsaonline.com/art/22415
Sources
- romanconcrete.com : The Roman Pantheon: the triumph of concrete
- minerals.usgs.gov : "Minerals commodity summary – cement – 2007"
- djc.com : Brief history of concrete
- inventors.about.com : "The history of concrete and cement"