Reinforced Concrete Fundamentals: Materials and Design Principles

Chapter I: Introduction to Reinforced Concrete

Basic Properties of Concrete

• Concrete is a mixture of cement, inert aggregates (sand and gravel), and water (often with additives).
• It has high resistance to compressive forces.
  • Typical compressive strengths (f'c): 180, 210, 240, 280, 300 kg/cm².
  • Concrete has low tensile strength, approximately 1/10 of its compressive strength (f'c).
• Due to concrete's low tensile strength, steel reinforcement is used in civil engineering works to resist tensile or traction forces.

  Figure 1: Concrete and Steel Interaction

• In its fluid state, concrete can be cast into molds of any shape.
• Plain concrete is considered a homogeneous material; however, with the addition of steel reinforcement, it becomes a heterogeneous composite.
• When properly proportioned and mixed, the component materials yield a strong and durable finished concrete product.

The reaction between water and cement is called hydration, a crucial process for the setting and hardening of concrete. The resulting gel acts as an adhesive, binding the inert aggregate materials that constitute the concrete.

Analysis and Design of Concrete Sections

1. Determination of Loads:

   • Dead loads (self-weight)
   • Live loads
   • Special or environmental loads

2. Material Stresses:

   • Cement quantity
   • Quality of steel reinforcement

3. Applicable Codes and Standards:

   • ACI 318 (e.g., ACI 2002)
   • Ecuadorian Building Code

4. Deformation:

   • Linear and rotational deformation

5. Calculation and Design Methods:

Chapter II: Concrete Components and Quality Control

Concrete Components and Methods for Achieving Different Qualities

Portland Cement

Manufacture:

• Components:
  1. Calcium oxide (lime)
  2. Silica
  3. Aluminum oxide (iron oxide is also added)

Fabrication Process:

• Components are mixed and heated in large kilns.

Resistance:

• Compressive strength is a key property.

Average Composition Percentages:

Influence of Fineness on Strength Development:

• Finer grinding of cement particles generally increases the concrete's strength.

Influence on Concrete Durability:

• The presence of tiny, evenly distributed air voids can increase concrete's resistance to disintegration. For this reason, air-entraining additives are often incorporated into the concrete mix.

Heat Generation During Initial Set:

• The chemical reaction between cement and water (hydration) generates heat.

Water and Air in Concrete

Water:

• Water is essential for cement hydration. Potable water (suitable for human consumption) is generally recommended for concrete mixing.

Air Entrainment and Bleeding:

• The gradual evaporation of excess water within the concrete mix creates tiny gaps. If these gaps are evenly distributed (air entrainment), concrete strength and durability can increase. However, if excess water rises to the surface, forming channels and weakening the concrete, this phenomenon is called bleeding.

  Figure 2: Water and Air in Concrete

Water-Cement Ratio:

• An incorrect water-cement ratio (either in excess or deficiency) will result in low-quality concrete.

Aggregates

Inert aggregates constitute 60% to 80% of the concrete's total volume.

Coarse Aggregate:

• Minimum diameter: 6 mm. The maximum diameter is determined by the specific requirements of the civil engineering work.
• Must be hard, derived from rocks such as andesite, basalt, quartz, and various metamorphic rocks.
• Must be clean and free of organic and inorganic contaminants.

Fine Aggregate:

• Maximum diameter: 5 mm; Minimum diameter: 1 mm.
• Must be clean for use and sourced from hard rock.

Additives

• Additives are ingredients incorporated into the concrete mixture either before or during mixing. Their function is to modify the properties of concrete to meet specific project requirements.