Construction materials: Masonry

Definition

the term masonry has been widened from its traditional meaning of structures built of natural stone to encompass all structures produced by stacking, piling or bonding together discrete chunks of rock, fired clay, concrete, etc to form the whole

generally made from 2 components

strength is hard to determine because there are hundreds of potential combinations of mortar and brick/ block configurations

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Structural forms

although the wall is the most useful and effetive masonry structure there are other forms:

  • columns
  • piers
  • arches
  • tunnels
  • floors
  • pavements

MASONRY IS NOT GOOD IN TENSION

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Types of masonry units

  • Clay bricks- BS EN 771-1
    • clay or other argillaceous material
    • fired at sufficiently high temp to achieve ceramic bond
  • Calcium silicate BS EN 771-2
    • lime and hardened by high pressure steam
  • Aggregate concrete blocks BS EN 771-3
    • lightweight or normal weight concrete
  • Autoclaved aerated concrete blocks BS EN 771-4
    • low density blocks
    • low strength but good thermal resistance
  • Manufactured stone BS EN 771-5
    • concrete units made to resemble natural stone
  • Natural stone BS EN 771-6
    • natural stone cut to required shape
    • properties determined by test
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Manufacture of clay bricks

brick forming process:

  • clay is dug, crushed and ground then blended with water at 25-30% using mixers to make a sloppy mud
  • mud is formed into lumps the size of a brick and then dipped in sand to reduce the stickiness of the surface 
  • the lump is thrown by hand/ machine into a mould and excess is cut off with wire
  • machine made bricks are formed into a continuous column of rectangular section by extrusion and then cut into bricks using wire
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Manufacture of calcium silicate units

  • manufactured from mixtures of lime, silica sand and water,(+ additives and pigments)
  • mixture is pressed to a high pressure in a die in a static press
  • ejected
  • set on cars
  • placed in autoclaves
    • prevents air bubbles forming
  • cured in high pressure steam for several hours
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Manufacture of aggregate concrete blocks

  • composed of hydraulic binder, aggregate and water
  • vibro-compacted
  • quickly extruded from mould
  • very dry: low water content
  • high fine aggregate content
  • leaner mixes used: ~100kg/m3 cement content 
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Manufacture of aerated concrete blocks

  • mixing slurry of fine siiceous base material, a binder, some lime, raising agent and aluminium powder 
  • aluminium powder reacts with alkalis to produce fine bubble of hydrogen
  • mixture is poured int a mould maintained in warm surroundings
  • hydrogen makes slurry rise (like dough)
  • 'cake' is then cured for several hours at elevated temperatures, demoulded, trimmed to a set height and cut with two orthogonal sets of oscillating parallel wires
  • cut units are placed on cars run on rails into large autoclaves
  • calcium silicate binder forms by reaction under the influence of high pressure steam
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Material properties

  • Density: 2250-2800 kg/m3
  • Modulus of elasticity: 5-30 GPa
  • Normalised compressive strength, fb shape factor x actual mean compressive strength
  • Modulus of rupture: 5-10% of fb
    • brick ceramic is relatively weak in tension
  • Compressive strength  depends on the combination of brick unit and mortar used
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Efflorescence

  • caused by dissolution of soluble salts due to evaporation of water from within brickwork
  • the salts crystallise on the the surface as white powder or encrustation
  • lime staining is caused by calcium hydroxide leaching from mortar and being carbonated at the surface to form white deposit of calcium carbonate crystals
  • usually temporary and can be removed using a weak acidic wash
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Sulphate attack

  • Reaction between sulphate ions in water solution and the components of set OPC in mortar to form ettringite
  • the resulting expansion causes both local disruption of mortar beds and stress in the brickwork
  • only occurs in wet/ saturated conditions with source of dissolved sulphate compounds
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Frost attack

  • erosion
  • due to stresses created by expansion of water on freezing in the the pore systems of units and mortars
  • only occurs in water saturated or near saturated masonry
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Thermal conductivity

  • affects amount of heat loss from building through wall
  • controlled by proportion of crystalline constituents and porosity
  • decreases with decrease in density
  • increases with increase in moisture content
  • coefficeint of thermal expansion: 5-7 x10-6 K-1
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Fire resistance

very effective in resisting and preventing spread of fire:

  • relatively low thermal conductivity
  • relatively high heat capacity
  • zero flammability and surface spread of flame

refractory properties: retains strength and integrity up to very high temperatures (~1000 C)

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