INTRODUCTION TO CONCRETE TECHNOLOGY

• Compressive strength determines a concrete's ability to withstand high pressures without failing or deforming.
• Structural concepts in building design include: skeletal, solid, and surface structures
• Buildings and non-building structures can be classified in various ways: solid, frame, shell, membrane, composite
• A building structure must be designed to resist likely forces/loads it will encounter and ensure stability
• Components of a building system must be stable, structurally sound, and ensure overall strength and stability
• Buildings must carry their own weight, live loads, wind loads, snow loads, hydrostatic pressure, and earthquake forces
• Structural systems in building design refer to the method of assembling and constructing structural elements to support and transmit loads safely
• Structural system design involves analysis, synthesis, appraisal of system performance, and feedback to improve the design
• Primary structural elements include walls, beams, columns, slabs, trusses, domes, membranes, arches, shells, cables, frames
• Structural systems must be able to balance out or neutralize forces like tension, shear, compression, bending, or torsional forces
• Architects and engineers must have clear knowledge of structural systems and how to use them in building design
• Skeletal structural system consists of columns, beams, slabs, and girders to support and resist all forces acting on the building
• Solid structural system relies on solid construction materials with walls acting as both structural and enclosing elements
• Surface structures keep their shape and support loads without a frame or solid mass material inside
• Shell structures may be made of rigid material such as reinforced concrete called rigid shells
• Problems in building shell structures include:

  • A tiny weakness or imperfection on the covering can cause the whole structure to fail
  • Assembly of flexible materials is very precise, so that seams are strong where the pieces are joined
• Surface structures act like cables or arches since they support loads primarily in tension and compression with very little bending
• Surface structures fall into two or three categories:

  • Thin plates of solid materials are given the necessary stiffness by being curved or bent, e.g. RC shell and folded slab structures
  • Walls and roofs may act as both the enclosing and supporting structure but thin-walled, e.g. shell dome
• Folded plates and spatial structures are adopted for construction of large spaces sometimes without columns as the structural components
• RC shell, commonly called thin shell concrete structure, is composed of a relatively thin shell of concrete, usually with no interior columns or exterior buttresses
• Tension structures are used for roofs and are developed from membranes, supported by compressed air or steel cables carrying a thin applied cladding or weatherproof covering
• Heavyweight construction characteristics include:

  • Excellent durability
  • Low maintenance
  • Good thermal mass
  • Suited to climates with a large diurnal temperature range
  • Generally high embodied energy
  • Significant environmental impact
  • Require heavy lifting equipment on site
• Lightweight construction characteristics include:

  • Less durable than heavyweight construction
  • Higher maintenance required
  • Greater responsiveness to outdoor temperature changes
  • Suitable for remote sites with lower embodied energy and environmental impact
  • Can be handled without heavy machinery
• Slabs are structural elements forming horizontal or inclined planes between beams or structural walls/columns, classified into one-way and two-way slabs based on their way of structural support in transmitting loads
• Columns are structural members designed to carry compressive loads, with different shapes in cross-section such as square, rectangular, circular, tied columns, spirally reinforced columns, composite columns, short columns, and slender columns
• Beams are load-bearing units that carry horizontal and vertical loads, classified as simple beams, continuous beams, semi-continuous beams, and cantilever beams
• 1. beams are produced when floor slabs and beams are poured simultaneously, creating a monolithic structure where the portion of the slab at both sides of the beam serves as flanges of the T-beam
• Concrete is a mixture of fine & coarse aggregates and/or other admixtures (additives), bound together by a water-based binder (cement) and water
• Concrete has no form of its own and no useful tensile strength
• Properties of Concrete:

  • Good compressive strength
  • Weak tensile strength
  • Good resistance to weather and chemical attacks
  • Resistance to fire (does not burn)
  • Generally heavy and dense
  • Does not rot and lasts
• Aggregates used in construction include sand, gravel, crushed stone, slag, recycled concrete, and geo-synthetic aggregates
• Aggregates serve as reinforcement to add strength to the overall composite material
• Properties or characteristics of aggregate that influence the properties of resulting concrete mix include:

  • Composition
  • Size & Shape
  • Surface Texture
  • Specific Gravity
  • Bulk Density
  • Voids
  • Porosity & Absorption
  • Bulking of Sand
  • Fineness Modulus of Aggregate
  • Surface Index of Aggregate
  • Deleterious Material
  • Crushing Value of Aggregate
  • Impact Value of Aggregate
  • Abrasion Value of Aggregate
• Two main aggregates used in concrete and masonry construction are Fine Aggregates (Sand) and Coarse Aggregates
• Fine Aggregates (Sand):

  • Inert or chemically inactive material
  • Most passes through a 4.75 mm IS sieve
  • Contains not more than 5% coarser material
  • Types include natural sand, crushed stone sand, and crushed gravel sand
  • Purpose is to fill open spaces between coarse particles
• Sources of sand for masonry and concreting in Ghana:

  • Pit sand
  • River sand
  • Quarry Dust/sand
  • Sea sand (not ideal due to chloride content and damage to coastline)
• Nature and quality of sand used in a mix can affect compressive strength

  • Sand must be free of deleterious materials, silt, clay, and organic substances
  • Procedure to determine silt content in sand
• Coarse Aggregates (Stones) consist of naturally occurring materials such as gravel or resulting from the crushing of parent rock
• Preferred characteristics of coarse aggregates:

  • Clean, hard, strong, and durable
  • Free from absorbed chemical and clay coating
  • Free from porous and soft materials
  • Aggregates capable of splitting are not desirable
• Two broad classifications of aggregates are Heavy and Lightweight aggregates

  • Heavy Aggregates have a unit weight greater than 2100 kg/m3
  • Lightweight Aggregates have a unit weight less than 1120 kg/m3