February.20.2016, By Mohammed Zaid, CEng.M.ASCE

Precast concrete consists of concrete (a mixture of cement, water, aggregates and admixtures) that is cast into a specific shape at a location other than its in service position. The concrete is placed into a form, typically wood or steel, and cured before being stripped from the form, usually the following day.

These components are then transported to the construction site for erection into place. Precast concrete can be plant-cast or site-cast, but this book deals specifically with plant-cast concrete. Precast concrete components are reinforced with either conventional reinforcing bars, strands with high tensile strength, or a combination of both. The strands are pre-tensioned in the form before the concrete is poured. Once the concrete has cured to a specific strength, the strands are cut (detensioned). As the strands, having bonded to the concrete, attempt to regain their original untensioned length, they bond to the concrete and apply a compressive force. This “precompression” increases load-carrying capacity to the components and helps control cracking to specified limits allowed by building codes

Precast components are used in various applications and projects of all types. Key components includes but not limited to the followings

• Wall panels, which can include an inner layer of insulation and be load supporting if desired
• Spandrels, which generally span between columns and are used with window systems in office buildings or in parking structures.
• Double tees, so named due to the two extending “stems” perpendicular to the flat horizontal deck. These tees are often used for parking structures and buildings where long open spans are desired.
• Hollow-core slabs, which are long panels in which voids run the length of the pieces, reducing weight while maintaining structural strength.
• Columns and beams, including columns and a variety of beam shapes.
• Bridge components for both substructure and superstructure designs, including girders in a variety of shapes, box beams, and deck panels.
• Piers, piles, caps and other supporting components for bridges.