High-early strength concrete, also known as fast-setting concrete, is a mixture of ordinary concrete constituents and special admixtures designed to achieve the specified compressive strength at an early age than normal concrete. The time required to reach this strength varies from a few hours to several days.
Uses of high-early-strength concrete vary from rapid form stripping to high-speed cast-in-place construction, cold weather construction, quick repair of pavements to reduce traffic downtime, and fast-track paving.
Compressive strength varies widely, depending on the application and design. For example, commercial structures that need to withstand freeze/thaw cycles require higher psi than residential projects. Typically, footings and slabs on grade need 3,500 to 4,000 psi. Structural elements such as beams, columns, and girders often need a higher psi.
The use of supplementary cementitious materials, such as calcium chloride, can increase the early-strength, but it is important to use only minimal amounts of CaCl. A study of over 169 concretes showed that using more than 2% CaCl can cause flash set, premature curing, and reduced durability.
Other methods of increasing early-strength include adding alkalies, varying the cement fineness, and modifying the water-to-cementitious material ratio. All of these methods, however, decrease the long-term durability of the concrete, especially if they are used in combination with other techniques.
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