Concrete is an exceptionally versatile building material with malleable or solid properties, depending on its stage of curing. It consists of aggregates and rocks mixed with fluid cement. After a certain amount of time, concrete hardens into a rock-like mass due to a chemical reaction known as hydration. Once hardened, it becomes exceptionally strong and durable at a strength of 3000–20,000 psi—enough to hold up bridges, skyscrapers, and dams.
Concrete is produced at a plant or on a job site. Equipment can vary from hand tools to large industrial machinery. Whatever the scale of production, concrete components must be thoroughly mixed, molded, and shaped within specific time constraints. Any disruptions can affect the integrity and appearance of the final product.
Large-scale concrete production
Large-scale concrete production takes place in two types of concrete plants: ready-mix plants and central mix plants. Ready-mix plants mix all concrete components except for water. Central mix plants mix all concrete components including water and are best for accurate control. Typically, concrete is a viscous fluid that is poured into molds to give a desired shape. However, concrete can also be in non-fluid form. This dryer version is preferred for manufacturing precast concrete products.
Small-scale concrete production
Smaller amounts of concrete are made at a job site using a volumetric mixer or mobile batch mixer. These mixers serve as mini concrete plants, capable of producing various types of concrete. They are perfect for sites that only require minimal concrete for installing smaller applications. For example, an installer uses a volumetric mixer to make concrete for embedding bike racks, or for installing bollards and steel pipes as traffic safety devices.
Types of concrete
There are dozens of concrete types available on the market for construction and building needs. Listed below are a few common types of concrete used in modern infrastructure.
Ordinary concrete is one of the most popular forms of concrete. A typical mix includes cement, sand, and coarse aggregates mixed with specific quantities of water. It has a setting time of approximately 30–90 minutes with strength values of 1450–5800 psi. At 28 days of curing, 75–80% of total strength is achieved, and after 90 days, 95% is achieved.
High-performance concrete has higher strength, workability, and durability compared to ordinary concrete. It has long-term mechanical properties and early age strength. It can withstand harsh environments and is resistant to creep and shrinkage which minimizes cracking. Strength ranges from 10000–15000 psi.
Reinforced concrete uses various forms of steel as reinforcement. The combination of concrete (with its high compressive strength) and steel (with its high tensile strength) gives reinforced concrete its unique strength properties. Reinforced concrete has the capacity to endure many forms of stress in any type of construction.
Precast concrete casts concrete into molds in a controlled environment. Once completely set and hardened, they are transported to the construction site. The curing stage happens under controlled conditions where temperature and humidity are monitored. Steam curing is sometimes used to produce precast products with high strength with less curing time.
Lightweight concrete is any concrete with a density less than 240 kg/m³. It includes lightweight aggregate concrete, foamed concrete, and autoclaved aerated concrete. Lightweight concrete typically improves thermal abilities and fire resistance. However, it is more susceptible to creep and shrinkage.
Pervious concrete allows air or water to circulate through a series of holes or voids created within the concrete. Water can drain naturally, allowing for the drainage of surface water and replenishment of groundwater. It is a low-impact development material that protects water quality and used in sustainable construction.
Before starting on any concrete application, the appropriate concrete type must first be determined. For example, reinforced concrete is suitable for building materials requiring high tensile strength, such as columns and beams. Lightweight concrete is best for building light concrete blocks for home construction.
Concrete is a versatile material and its applications are plentiful. Concrete’s malleable, yet tough characteristics make it ideal base materials for constructing buildings, urban infrastructure, and various precast products.
Concrete is widely used to build homes and commercial buildings, as well as related fixtures such as driveways and columns. Insulating concrete keeps heat inside the walls, reducing energy usage by over 40%. Smog-eating concrete can help to reduce nitric oxides in surrounding polluted environments by over 60%. Concrete buildings require less maintenance and last longer, while providing better indoor air quality. Other benefits include resistance to fire, wind, and hurricanes.
Roads and highways
Concrete roads and highways are very durable and require minimal maintenance. They are not susceptible to rutting or spring thaw load restrictions. They are tough enough to withstand consistent traffic of heavily loaded vehicles.
Concrete is one of the most economical and quickest materials for bridge construction; it is commonly used for bridge superstructures (the upper portions of bridges). These include decks, curbs, sidewalks, and side traffic barrier walls. With proper planning and by using precast concrete products, the process can be completed efficiently and competitively. Visual impacts and decorative features may also be added.
Precast concrete is used for traffic guidance devices such as concrete bollards. These concrete bollards can complement surrounding architecture while offering impact protection from vehicles. They are typically reinforced with epoxy-coated steel rebars to add to their durability. Concrete bollards are ideal for high-traffic areas such as building entrances and parking lots to help protect pedestrians and furnishings.
Dams are one of the most demanding and substantial structures made from concrete. The Grand Dixence Dam in Switzerland took 6 million cubic meters of concrete to construct. It also uses steel reinforcement to add maximum strength to concrete properties. To limit any cracking, water is circulated through pipes within the concrete. Dam designs require careful selection of the concrete mix with the right type of aggregates.
Even with its resilient properties, concrete still requires proper maintenance to extend its life span. Concrete should be cleaned a minimum of once a year to remove grime and dirt, as well as rust or other stains. Cracks must also be routinely repaired for a structurally sound surface. Failure to repair cracks can lead to water intrusion, causing problems with the sub grade. Periodic resealing of the surface reduces moisture infiltration and staining. It will also prevent dirt and weed from forming through those joints.
The construction industry is part of a zero-waste movement with 140 million tons of concrete recycled annually in the US. When concrete structures are demolished, leftover concrete is recycled for new construction projects, gravel, landscaping stones, and mulch. Concrete aggregates are collected and run through a crushing machine. Concrete with steel rebars undergo the same process, and the metal pieces are removed later with magnets and other sorting devices.
The objective of recycling is not only to eliminate concrete from landfills, but to conserve energy during reprocessing. Crushing old concrete on-site eliminates the need for transporting, thereby reducing costs and emissions. Large road-portable plants can crush concrete at 600 tons per hour. Smaller crushers can crush 150 tons per hour, with the added benefit of being able to fit into tighter areas.
Self-healing concrete aims to tackle one of the greatest flaws of concrete: cracking. Once concrete cracks, it can result in corrosion of the steel reinforcements used in the structure. Cracking also poses immediate hazards—concrete containers for toxic waste can have disastrous results from cracking, not to mention risk to maintenance workers. Engineers at Ghent University have pioneered research in developing self-repairing concrete. It is filled with super-absorbent polymers that swell and block concrete cracks for self-healing. Concrete research and development continues for a future in construction that advances efficiency, safety, and sustainability.