26. PRESTRESSED CONCRETE

Beams with longer spans are architecturally more appealing than those with short ones. However, for a reinforced concrete beam to span long distances, it would have to have to be relatively deep (and at some point the self weight may become too large relative to the live load), or higher grade steel and concrete must be used. However, if we were to use a steel with f much higher than 60 ksi in reinforced concrete (R/C), then to take full advantage of this higher yield stress while maintaining full bond between concrete and steel, will result in unacceptably wide crack widths. Large crack widths will in turn result in corrosion of the rebars and poor protection against fire. One way to control the concrete cracking and reduce the tensile stresses in a beam is to prestress the beam by applying an initial state of stress which is opposite to the one which will be induced by the load.

In this chunk, we learn that long-span beams are more visually appealing, but they face challenges due to increased self-weight. To counteract these challenges, engineers consider using higher grades of steel and concrete. Prestressing helps mitigate the cracking and tensile stress issues by creating an initial compression in the concrete, which opposes the stresses caused by the load.

Imagine a bow and arrow: when you pull the string back (prestress), the bow is under tension. This tension helps the bow hold its shape and prepares it to shoot stronger arrows. Similarly, prestressing a concrete beam helps it resist the pulls of weight and use its materials more effectively.

For a simply supported beam, we would then seek to apply an initial tensile stress at the top and compressive stress at the bottom. In prestressed concrete (P/C) this can be achieved through prestressing of a tendon placed below the elastic neutral axis. Main advantages of P/C: Economy, deflection & crack control, durability, fatigue strength, longer spans. There are two types of Prestressed Concrete beams: Pretensioning: Steel is first stressed, concrete is then poured around the stressed bars. When enough concrete strength has been reached the steel restraints are released. Postensioning: Concrete is first poured, then when enough strength has been reached a steel cable is passed thru a hollow core inside and stressed.

This chunk explains the application of stresses in a prestressed concrete beam. By applying initial tensile stress at the top and compressive stress at the bottom, we can achieve improved performance of the beam. It introduces two methods of prestressing: pretensioning, where stresses are applied before the concrete hardens, and postensioning, where stresses are added after the concrete has set. The benefits of prestressed concrete include economic advantages, crack control, durability, resistance to fatigue, and the ability to span longer distances.

Think of a trampoline: before using it (like pouring concrete), the springs (prestressing tendons) are already stretched (pretensioned). When you jump on it, it supports your weight without sagging too much. In construction, this means we can safely build longer bridges or tall buildings.

P/C beams usually have higher compressive strength than R/C. Prestressed beams can have f c' as high as 8,000 psi. The importance of high yield stress for the steel is illustrated by the following simple example. If we consider the following: 1. An unstressed steel cable of length l s 2. A concrete beam of length l c 3. Prestress the beam with the cable, resulting in a stressed length of concrete and steel equal to l = l = l s0 c0 4. Due to shrinkage and creep, there will be a change in length Δl = (εsh + εcr)l c. We want to make sure that this amount of deformation is substantially smaller than the stretch of the steel (for prestressing to be effective).

In this section, we explore the materials used in prestressed concrete. These beams are stronger due to a higher allowable compressive strength compared to regular reinforced concrete. The section emphasizes the significance of using high-yield-strength steel, explaining that as the steel is prestressed, any changes in length due to shrinkage or creep must be kept smaller than the elongation of the steel. This allows for effective prestressing.

Consider a high-quality rubber band that can stretch but also returns to its original form: if we stretch it too much, it won't be effective at holding anything. Similarly, the materials used in prestressed concrete must be carefully managed to ensure they can handle the stresses without failing.

Having shown that losses would be too high for low strength steel, we will use strands usually composed of 7 wires. Grade 250 or 270 ksi. Note that yield stress is not well defined for steel used in prestressed concrete; usually we take 1% strain as effective yield. Steel relaxation is the reduction in stress at constant strain (as opposed to creep which is reduction of strain at constant stress). Relaxation occurs indefinitely and produces.

This chunk discusses the potential loss of prestress in concrete beams due to various factors, particularly focusing on the importance of using strong steel strands. It explains the concepts of yield stress and steel relaxation, emphasizing that flexibility in steel performance can cause stress loss over time, making it critical to choose the appropriate materials and methods.

Think of a stretchable toy that eventually loses its bounce as you play with it. Over time, the material may not return to its original state as effectively. In construction, understanding and mitigating prestress loss is just as crucial for ensuring long-term performance.

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