Reinforced Concrete: Properties and Uses
Reinforced Concrete
Reinforced concrete is a composite material formed by combining cement, water, fine aggregate (sand), coarse aggregate (crushed stone or gravel), and air. Admixtures, also known as additives, can be included to modify the properties of the fresh or hardened concrete. The concrete mixture is placed in formwork and then cured to facilitate the hydration reaction, a chemical process that produces a hard, rock-like material.
Aggregates
- Fine Aggregates: Typically natural or manufactured sand.
- Coarse Aggregates:
- Crushed natural stone of volcanic, sedimentary, or metamorphic type.
- Natural gravel from weathering and the action of flowing water in rivers.
- Artificial coarse aggregates like expanded shale and slag, used for lightweight concrete.
- Heavy aggregates (steel punches, barite, magnetite, and limonite) used for nuclear safeguards.
Characteristics of Good Concrete
- Density: The concrete should be dense, with the space fully occupied by solid aggregates and cement gel.
- Strength: The concrete must possess sufficient internal strength to resist various types of failure.
- Water-Cement Ratio (w/c): This ratio must be carefully controlled to achieve the required design strength.
- Texture: A dense, hard texture is crucial for resisting adverse weather conditions.
Parameters Affecting Concrete Quality
- Appropriate Cement Type: Low C3A, MgO, free lime, and low in Na2O and K2O.
- Resistance to Deterioration by Wear: Low w/c ratio, proper curing, dense and homogenous concrete, high strength, added resistance to wear, and good surface texture.
- Economy: Large maximum aggregate size, efficient gradation, minimum slump, minimum cement content, optimal automated plant operation, and inclusion of air and other additives.
- Strength: Good quality paste, low w/c ratio, optimum cement content, solid aggregates, proper gradation and vibration, and low air content.
- Resistance to Weathering and Chemicals: Appropriate cement type, proper curing, alkali-resistant aggregates, suitable additives, use of superplasticizers or polymers, and air entrainment.
Additives
- Accelerators: Reduce setting time and accelerate early-age strength development (e.g., calcium chloride, bromides, carbonates, silicates, triethanolamine).
- Air-Entraining Agents: Increase workability during placement and improve freeze-thaw resistance.
- Water Reducers and Set-Controlling Agents: Increase concrete strength and allow for a reduction in cement content proportional to the reduction in water content.
- Finely Divided Mineral Admixtures: Used to correct deficiencies in the concrete mixture, such as a lack of fine aggregates.
- Admixtures for No-Slump Concrete: No-slump concrete is defined as concrete with a slump of 1 inch or less immediately after mixing.
- Polymers: New types of additives that can produce very high compressive strength concrete (up to 15,000 psi or more). The principle is to replace part of the water with a polymer to achieve high compressive strength.
- Superplasticizers: Chemical additives that provide high-level water reduction.
Concrete Structural Systems
- Floor Systems: The main horizontal elements that transfer live loads and dead loads to vertical supports. Examples include slabs on beams, ribbed slabs, flat slabs (without beams), and composite slabs on beams.
- Beams: Structural elements that transmit loads from floor slabs to vertical columns.
- Columns: Vertical elements that support the floor system. They are compression members subject, in most cases, to axial load and bending, and are of great importance in the safety considerations of any structure.
- Walls: The vertical cladding of building frames.
- Foundations: Structural concrete elements that transmit the weight of the superstructure to the soil (e.g., footings, piles driven into rock, combined footings, and mat foundations).