What is Concrete?
Concrete is a composite material comprised of a number of coarse aggregates (particulate materials such as sand, gravel, crushed stone, and slag) bonded with cement. Cement is a substance that is used to bind materials, such as aggregate, by adhering to said materials, then hardening over time.
While there are many types of cement, Portland cement is the most commonly used cement, and is the primary binding ingredient in concrete, mortar, and plasters.
Purchasing and Mixing Concrete
Concrete can be purchased in multiple forms, including in pre-mixed 60-pound or 80-pound bags (ideal for DIY projects), or delivered in large amounts by specialized concrete mixer trucks (measured in cubic yards) for larger jobs like foundations and driveways.
Proper mixing is essential for the production of strong, uniform concrete. It involves mixing water, aggregate, cement, and any desired additives in specific ratios. Production of concrete is time-sensitive, and the concrete must be placed before it hardens since it is usually prepared as a viscous fluid. Some concretes are designed to harden more quickly for applications that require rapid set time. Alternatively, in some factory settings, concrete is mixed into dryer forms to manufacture precast concrete products such as concrete walls.
The Curing Process
The process of concrete hardening once it has been placed is called curing, and it is a slow chemical process, not simply "drying out." It typically takes concrete around four weeks to reach over 90% of its final strength, and the strengthening can continue for up to three years.
Ensuring that the concrete is kept damp can increase the strength of the concrete during the early stages of curing. This is achieved through techniques such as spraying concrete slabs with curing compounds that create a film over the concrete that retains water, as well as ponding, where concrete is submerged in water and wrapped in plastic.