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Pretensioning is a method used in civil engineering, particularly in the construction of concrete structures, to introduce internal stresses that counteract the stresses that will be imposed by external loads. This pre-stressing of the concrete and reinforcing steel is achieved before or during the application of the service loads. The primary goal is to improve the structural performance of the concrete element.

There are two main types of pretensioning: pre-tensioned and post-tensioned. In pre-tensioned concrete, the reinforcing tendons (typically steel wires or strands) are stretched and anchored to formwork before the concrete is poured. Once the concrete has cured sufficiently and gained enough strength, the tendons are released, transferring their tensile force to the concrete as compressive force. This creates a pre-compressed concrete member.

In post-tensioned concrete, the tendons are placed within ducts or sleeves that are cast into the concrete. After the concrete has cured, the tendons are stressed by hydraulic jacks and then anchored at the ends of the member. The space within the ducts can then be grouted to provide bond between the tendons and the concrete, or left ungrouted in some applications.

Pretensioning is commonly used in precast concrete elements such as beams, girders, and bridge decks, as well as in prestressed concrete tanks and poles. It allows for longer spans, thinner sections, and reduced deflection compared to conventionally reinforced concrete. This technique also enhances durability by reducing cracking under service loads.