Question

What is prestressing?

• A. Internal tensile stresses are introduced in concrete to counteract external compressive stresses
• B. It reduces elasticity of concrete to take large loads
• C. It helps keep concrete in tension, to reduce cracking and improve durability
• D. Internal compressive stresses are introduced in concrete to counteract external tensile stresses

Hint:

Think of prestressing like gripping a horizontal row of books between your hands. By squeezing the books tightly from both ends (applying internal compressive stress), you can lift the entire row into the air without the middle books slipping out or sagging under gravity (external loads).

Answer 1

The Correct Option is D

Concept: Concrete is an inherently brittle composite material. It possesses exceptionally high strength under compression but is notoriously weak when subjected to tension, with its tensile capacity typically being only about 10% of its compressive strength. In standard structural configurations, such as a flexural beam under gravity loads, the bottom fibers undergo tension and crack easily. Prestressed concrete is an innovative structural approach designed to artificially eliminate or reduce these weak tensile zones by pre-loading the structural element before it handles its actual working load.

Step 1: Understanding the mechanism of prestressing.
High-strength steel tendons or wires are inserted into the structural element and stretched using hydraulic jacks. When these tensioned steel tendons are anchored back onto the concrete structure, they attempt to snap back to their original length. This elastic recoil transfers a powerful, deliberate internal compressive force across the concrete cross-section. \[ \sigma_{\text{net}} = \sigma_{\text{prestressing}} \pm \sigma_{\text{bending}} \] Where:
• \(\sigma_{\text{prestressing}}\) is the initial internal compressive stress induced intentionally.
• \(\sigma_{\text{bending}}\) is the stress caused by external service loads (producing tension at the bottom fibers).

Step 2: Evaluating the interaction of counteracting stresses.
When external working live and dead loads are subsequently applied to the structural member, they produce bending action that tries to stretch the bottom fibers (inducing tensile stress). However, because the concrete was already pre-compressed by the tendons, the incoming external tensile stress must first overcome this pre-existing internal compressive barrier. As a result, the net tensile stress is either completely neutralized or reduced safely below the modulus of rupture of the concrete, preventing structural cracking and mitigating excessive deflections.

Step 3: Checking the validity of the wrong options.

• Option (A) states that internal tensile stresses are introduced. This is incorrect because introducing tension would crack the concrete immediately before it even receives structural service loads.
• Option (B) implies it alters the material's elastic modulus property. Prestressing is a method of loading and balancing stresses; it does not materially alter the intrinsic modulus of elasticity of the concrete matrix.
• Option (C) mentions keeping concrete in tension, which directly contradicts the fundamental structural goal of maintaining concrete within its strong compressive domain. Consequently, prestressing is accurately defined as the process where internal compressive stresses are introduced to counteract external tensile stresses.