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When Stress Is Good: Pre- and Post-Tensioned Concrete

written by: John Moehring • edited by: Lamar Stonecypher • updated: 6/20/2011

Portland cement concrete is a strong, dependable, and relatively inexpensive construction material. Until it is stressed under tension, that is. Then it typically exhibits only about 15% of its equivalent compressive strength, which can spell catastrophe for structural applications.

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    Portland cement concrete, usually just referred to as concrete, is an impressive building material. The first use of material most similar to modern concrete was by Roman engineers and was known as opus caementicium. Instead of the clay mud binders used previously, they discovered that using a special volcanic dust called pozzolona in the binder formulation created a better bond with the aggregates. Regions of the empire that did not have access to pozzolona used local sources of lime or gypsum instead. The engineers soon discovered that compressive structural elements such as arches, domes, and vaults were ideal applications of this “poured stone." The Pantheon still stands today as the largest un-reinforced concrete dome in the world. Presumably they also discovered very quickly that the material failed catastrophically in attempts to use it in tensile structural applications. There is no evidence to suggest they used reinforcement techniques commonly incorporated today to counteract tensile failures, though there are indications they used additives such as horse hair and blood to reduce cracking and frost damage.

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    Structural Concrete Renaissance

    The methods, materials, and formulations used to create the first structural concrete elements were apparently lost for centuries after the collapse of the Roman empire. It was not until the 17th century that this construction material was again in evidence. And it was not until the 20th century that pre-stressed concrete elements were successfully used in structures. Before this the concept of pre-stressed construction was well known, other methods were used that achieved the same effect- such as incorporating a metal rim band around a wooden wheel. The metal band was heated before being applied to the outside circumference of the assembled components of the wheel, and as it cooled it shrank and compressed the wooden rim, spokes, and hub. The metal rim was consequently held in place by tensile stress around its circumference, while the wooden elements were held together by compressive stresses. Extending this principle to concrete materials led to the development of pre-tensioned, bonded, and unbonded post-tensioned structural elements.

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    In pre-tensioned concrete tensile elements such as cables, ribbons, or rods are clamped under calculated tensile stress. Concrete is cast around these elements and allowed to cure. When fully hardened the clamps are released and the stress is transferred within the rigid concrete. Only in this case the tensile elements are not holding the structure together, rather they act to place the concrete into a state of compressive stress prior to load application. When a load is applied within the design limit, the concrete structural piece will never see tensile stress of sufficient magnitude to cause failure.

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    Unbonded post-tensioning is accomplished by coating the tensile elements with lubricant and covering them with extruded polymer sleeves. These are then arranged within the concrete molding form with the ends of the tensile elements attached to anchors placed at the perimeter of the casting. Since the tensile elements are free to move within the casting, when curing is complete they can be pulled into tension and clamped to the embedded anchors at the edges of the piece. Once again the concrete is placed into a preloaded compressive state of stress by the tensioned elements. Unlike pre-tensioning, however, the post-tensioned elements can catastrophically pull out of the casting if the anchors fail or unskilled repairs are attempted. However they can also be adjusted to compensate for changes in loading or field conditions. Finally, bonded post-tensioning utilizes conduits placed into the concrete molding form. The tensioning elements are threaded through the conduits before the concrete is cast. Once curing is complete, the elements are pulled into tension and clamped or wedged at the conduit openings to hold the applied tensile stress. These elements are then grouted into place at the openings to permanently bond them with the structural casting.

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    Rise of Precast Concrete

    First used in 1928, prestressed concrete has led to an explosion of applications in buildings, bridges, slabs, foundations, and roadways, and has driven development of the precast concrete industry. Although the cost is typically greater that standard reinforced concrete the increase in structural performance can more than offset the additional expense. Lighter, stronger, longer lasting, and requiring fewer joints, prestressed concrete construction materials have shown how good stress can be.