Concrete is completely fluid before the cement sets, then progressively hardens. The cement and water mixture that has crystallized in this way encloses the aggregate particles and produces a dense material. The concrete continues to harden over several months. Heat speeds up the setting and hardening of cement, and cold slows it down and can even completely stop the processes.
Floating produces a relatively even, but slightly rough, texture that has good slip resistance and is frequently used as a final finish for exterior slabs. If a smooth, hard, dense surface is required, floating is followed by steel troweling. Curing begins after the exposed surfaces of the concrete have hardened sufficiently to resist marring.
Curing ensures the continued hydration of the cement so that the concrete continues to gain strength. Concrete surfaces are cured by sprinkling with water fog, or by using moisture-retaining fabrics such as burlap or cotton mats. Other curing methods prevent evaporation of the water by sealing the surface with plastic or special sprays called curing compounds. Special techniques are used for curing concrete during extremely cold or hot weather to protect the concrete. The longer the concrete is kept moist, the stronger and more durable it will become.
The rate of hardening depends upon the composition and fineness of the cement, the mix proportions, and the moisture and temperature conditions. Concrete continues to get stronger as it gets older. Most of the hydration and strength gain take place within the first month of concrete's life cycle, but hydration continues at a slower rate for many years.
Joshua Gilman, P. Cement Industry Annual Yearbook U. When both are present, the result is concrete. However other aggregates might be used depending on the required characteristics of the cured mix. For example, vermiculite or perlite aggregates produce a lightweight concrete that has good insulating properties and can be easily sawn.
Improving Performance Concrete suppliers often use additives, called admixtures, to alter or improve the qualities of the mix for a specific application. When it's important to have a workable concrete that pours easily without adding extra water, a mineral additive such as fly ash is added.
Alternatively, superplasticizers are used to improve workability while increasing strength because less water is required. Retarding and accelerating admixtures are used to alter curing time as necessitated by climatic conditions. To help remedy this, air-entraining agents are added. These admixtures create a dispersion of very fine air bubbles that cushion the concrete against the effects of freezing water.
Buying Concrete The form in which you buy concrete depends on the size and nature of your job. Concrete is normally measured in cubic yards. To determine how much you'll need, figure the volume inside your forms in cubic feet and divide by 27 the number of cubic feet in a cubic yard. For example, a 4-in. Projects using up to about a cubic yard can be handled with a portable cement mixer that you can rent.
The proportions of cement, sand, gravel and water can vary depending on the use of the concrete. For example, thin work—between 2 in. An average mix contains one part cement, two parts sand and three parts gravel. The amount of water you use depends on how wet the sand is. For smaller projects, you can buy premixed bags that contain cement and aggregate—you just add water.
For bigger jobs, the best route to take is ready-mix concrete. Return New search Print e-mail a friend. The basic hardening process When the constituents of concrete cement, aggregate and water are brought together, the cement and water chemically react and eventually set to form a hard mass which adheres to and binds together the aggregates to form concrete.
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