Special high strength cables used in concrete post-tensioned slabs consist of a greased, _____ strand, _____ inch diameter wire enclosed in a plastic sheath that is cast into the concrete. Before or after the concrete is hardened (depending on whether the structural member is pre or post tensioned), the cable is tensioned to about 25000 lbs.

• A. Five-strand and three-quarter inch
• B. Seven-strand and half-inch
• C. Nine-strand and one-half inch
• D. Seven-strand and one-quarter inch

Answer: (B)

In post-tensioned concrete, the tendon is a high-strength cable that is greased and enclosed in a plastic duct so it can be run through the formwork and later tensioned to compress the concrete. The standard tendon configuration used for many slabs is seven strands of wire with a diameter of about half an inch. This combination provides enough cross-sectional area to develop the target prestress force—around 25,000 pounds—while remaining flexible enough to install and tension through the duct. The other options would imply configurations that aren’t the typical balance of strength and practicality for this application. The seven-strand, 1/2 inch diameter arrangement is the common, proven choice for achieving the specified tension in these systems.