Concrete: A Common Material
Written by GJ Stott
Concrete, the most common man-made material in construction, consists of approximately 10-15% cement. Cement is a combination of oxides created by heating calcium-rich limestone with silicon sand to form a substance known as clinker. Clinker is then ground with one of various types of sulfate. The result is a cement powder that, when hydrated, becomes concrete, a durable solid (Portland Cement Association) that can be used in the making of cement fences.
Concrete is formed when cement powder is mixed with aggregates, water and optional admixtures. Concrete aggregates can range from fine sand to coarse crushed stone, depending on budget and desired finish. The water added for concrete is carefully measured to ensure a proper ratio, which determines final concrete integrity. The more water added, the more workable the concrete is; however, the inverse relationship between concrete workability and strength means that as more water is added, the cured concrete has less strength.
Organic and artificial admixtures can be integrated during concrete batching for strengthening, air-entraining, coloring, accelerating, retarding and plasticizing. Such additions produce desired characteristics in the finalized concrete. Silica fume, a reactive pozzolan, increases concrete durability. Air bubbles disperse in concrete to protect against freezing and thawing conditions. Iron ore oxide pigments tint the color of the concrete for an aesthetically matching hue. Accelerators increase the rate at which the concrete hardens while retardants decrease that rate. Plasticizers improve concrete workability during placement. Substances, such as fly ash, which is ground granulated blast-furnace slag, can be used to replace portions of the cement powder.
Concrete Hydration Process
Contrary to today’s commonly used terminology, concrete does not dry in order to harden. Made primarily of cement (discussed above), concrete hardens by gluing the aggregates together. The cement, when mixed with water, forms a paste used to bind this synthetic conglomerate.
The water within concrete does not dry; instead, it is used in the hydration process. Fundamentally, hydration is a chemical procedure during which reactionary compounds form chemical bonds with water molecules. As the bonds increase and strengthen, the concrete hardens.
In its hydrated state, the cement within concrete retains moisture. This retention ensures that the concrete remains cured or hardened. Eventually, concrete can dry, and when it does, it no longer increases in strength. The reaction of the water with the cement in the concrete is extremely important to the product’s properties.
Concrete ready-mix is sent via mixer trucks to be utilized in concrete projects and construction. Concrete fence can be poured in place or precast. Poured concrete is used for footings and foundations while precast is created facades, sound barriers or other types of walls, floor systems and bridges, and even concrete hurricane fences. The number of concrete applications is continuously growing, as concrete is the most widely used artificial substance on the earth today. Last year alone, six billion tons of concrete were produced, and that number will continue to rise exponentially.