Compressive and Tensile Strength of Concrete: Anisotropic Properties vs Plywood

Understanding the Distinction: Concrete vs Plywood

In the realm of construction materials, concrete and plywood exhibit distinct properties concerning their strength characteristics. Unlike plywood, which displays isotropic properties with consistent strength in all directions, concrete is anisotropic, meaning its strength varies based on the direction of the applied load.

Anisotropic Nature and Concrete Strength Characteristics

Concrete's compressive strength is inherently high, making it well-suited for applications where forces act inward. This property is uniform in all directions when the concrete is properly mixed and cured. However, concrete's tensile strength, which is much lower than its compressive strength, can be enhanced through the use of reinforcement. When reinforced with steel bars (rebar) or fibers, concrete becomes better equipped to resist tensile forces and cracking.

Tensile Strength: Significantly Lower and Directionally Variable

Concrete's tensile strength is typically around 10-15% of its compressive strength. This weaker tensile capability can be influenced by various factors, including the orientation of rebar, the quality of the mix, and the curing process. In contrast, plywood exhibits similar tensile strength in all directions due to its layered construction and isotropic properties. The layered structure of plywood ensures that its strength remains consistent across all directions, making it an excellent material for applications where uniform strength is required.

Compressive Strength and Its Application

Compressive strength, the ability of concrete to withstand inward forces, is one of its most valuable properties. The most common form of concrete, Class 3000 concrete, achieves a compressive strength around 3000 psi after 30 days of aging. The actual strength is determined by test cylinders cast from the concrete shortly after placement. These cylinders are then aged and tested to estimate the concrete's strength at various time points. Class 3000 concrete usually tests 15 to 25% higher than its nominal 3000 psi strength due to variations in test conditions.

Tension and Reinforcement: A Structural Solution

Given the low tensile strength of concrete, it is often reinforced with steel bars (rebar) to enhance its ability to resist tension. Rebar, typically manufactured to a class of 60000 psi, provides the necessary tensile strength to counteract tensile forces. By placing rebar within the concrete structure, it forms a composite that combines the strengths of both materials. This creates a system where concrete resists compression and steel resists tension, each sized appropriately to handle the forces acting on the beam, column, or other structural member.

Pre-Positioning of Reinforcing Steel

In cases where precise placement of the rebar is crucial, the rebar is placed at the specific stress location and wired into position within the formwork. This ensures that the rebar is accurately positioned and bonded to the concrete, maximizing its effectiveness in resisting tensile forces.