BT

• Concrete


  An artificial, stone-like building material made by mixing cement and various mineral aggregates with sufficient water to cause the cement to set and bind the entire mass
• ANCIENT ROMANS
• Discovered mortar which contained all the essential ingredients of modern Portland cement and that the Romans were the inventors of concrete construction
• Joseph Aspdin patented an artificial cement named portland cement in 1824 after English Portland limestone
• Reinforced concrete, in which steel bars are embedded to resist tensile forces, was developed in the 1850s
• French gardener, Joseph Monier, obtained a patent for reinforced concrete flower pots in 1867 and went on to build concrete water tanks and bridges of the new material
• Engineering design methods had been developed for structures of reinforced concrete and a number of major structures had been built
• General Characteristics of Concrete

  • Concrete is the universal material of construction
  • Concrete is relatively low in cost
  • Concrete can be used for every building purpose
• Specific Characteristics of Concrete

  • Concrete does not rot or burn
  • Concrete is relatively low in cost
  • Concrete can be used for every building purpose
• It is important to know the characteristics of concrete to understand its weaknesses and strengths as a material in construction
• Physical Characteristics of Concrete

  1. Concrete has no form of its own

  2. Concrete is inherently strong in compression but weak in tension

  3. Concrete is capable of being formed into any shape with a variety of surfaces
• Qualities of concrete

  • Provision for forming and molding is required before placing for setting and curing
  • Inherently strong in compression but weak in tension (steel reinforcement is required to handle tensile and shear stresses)
  • Capable of being formed into any shape with a variety of surface finishes, textures, and patterns
  • Relatively low-cost and inherently fire-resistant
  • Liability is its own weight (150 lbs/cu.ft)
• Workability of concrete

  • The ease with which concrete can be placed in the formwork and finished. The addition of water makes concrete more workable but decreases its strength
• Weight of concrete


  • Depends largely on the display of the coarse aggregate: lightweight aggregates reduce the overall density of the concrete but can also adversely affect its strength
• It is generally believed that the specified strength of concrete will be reached after 28 days for ordinary concrete
• The strength of concrete increases almost indefinitely with time if adequate moisture is present. After the initial setting period, the concrete will harden at a rate depending on the cement type, the water-cement ratio, and the curing environment
• Long duration stress produces stress over time and consequently causes permanent deformation. This phenomenon can be reduced through the use of reinforcement in the compression zone
• Concrete is incombustible and somewhat insulative, but long exposure to fire can be damaging. Density of the concrete and type of aggregate affect fire resistance
• Ordinary concrete shrinks during the curing process. This may account for potential movement over a 100ºF temperature differential causing cracks. This must be controlled by providing expansion joints and steel reinforcements
• Hardness of concrete is related to durability as well as appearance. A slab can be troweled or floated to draw more paste to the surface, but the harder superficial layer has a tendency to spall due to the different coefficient of expansion from the core material
• Porosity of concrete affects the watertightness. Sizes of pores left during the hydration process or created by excessive evaporation and shrinkage cracks affect the watertightness of concrete. Apply compaction techniques
• Concrete is not waterproof, and consequently will absorb water and potentially allow it to reach the reinforcing—the primary component that can be harmed by exposure to moisture. When this happens, the reinforcing will oxidize (rust) and expand, causing cracks in the concrete at the location of the bar. This reduces or destroys the concrete bond to the steel and allows even more water to enter the system
• Types of Cement

  • Type I: Normal - Portland cement for general construction
  • Type II: Moderate - sulfate resistant, where heat build-up can damage the structure
  • Type III: High Early Strength (HES) - cures faster (3 - 7 days) and gains earlier strength
  • Type IV: Low Heat - generates less heat hydration where heat build-up can significantly damage the structure and used for massive concrete structures
  • Type V: Sulfate Resistant - used where resistance to severe sulfate action is required
• Type III: High Early Strength (HES)


  • Cures faster (3 - 7 days) and gains earlier strength
• Type IV: Low Heat


  • Generates less heat during hydration, used for massive concrete structures
• Type V: Sulfate Resistant


  • Used where resistance to severe sulfate action is required
• Other Uses of Cement

  • Mortar = cement + sand + water
  • Plaster = cement + lime + sand + water
  • Grout = cement + sand + considerable amount of water
  • Paste = cement + water
• Aggregates are any of various hard, inert, mineral materials, such as sand and gravel, added to cement or mortar
• Aggregates represent 60% to 80% of the concrete volume and are important to the strength, weight, and fire-resistance of the hardened concrete
• Aggregate properties

  Hard, dimensionally stable, free of clay, silt, and organic matter to prevent the cementing matrix from binding the particles together
• The strength of a concrete mixture is highly dependent on its composition of aggregates
• Water used in concrete or mortar mix

  Exclusive of any absorbed by the aggregate and free of harmful substances like organic material, clay, and salts
• Water Cement Ratio


  Controls the strength, durability, and water tightness of hardened concrete
• Abram’s Law (D.A. Abrams, 1919): '“the compressive strength of concrete is inversely proportional to the ratio of water to cement”'
• Effects of water to the Concrete Mixture: Too much water weakens concrete, little water is dense but causes difficulty in placement and workability, excessive water causes bleeding and laitance
• Types of Admixtures and its Effects to the Concrete Mixtures

  • Pozzolanic Admixture
  • Water Reducing Admixture
  • Coloring Agent
  • Surface Sealing Agents
  • Set Inhibiting Agent
  • Dispersal Agents
  • Bonding Agent
• Concrete additives

  • Dispersal Agents
  • Bonding Agent
  • Air-Entraining Agent
  • Gas Forming Agent
  • Non-Skid Surfaces
  • Hardener
  • Accelerator
  • Retarder
• Dispersal Agents


  • Prevents bleeding of concrete
• Bonding Agent


  • Improves the bond between old and new concrete using metallic aggregate or synthetic latex emulsion
• Air-Entraining Agent


  • Enables the concrete to be highly resistant against corrosion