Aggregate

Aggregate

1. why needed?

   • cheap filler
   • helps hardened structure resist load
   • MAIN: adds stability
   • provides desired properties of concrete

2. how to classify

   • shape
     • flakiness , shape index
   • texture
     • smoother = better, helps to bond
   • grading(size)
     • ideally want a range, irregular grading = packs well + less voids
   • physical properties
     • strength
     • deformation
     • toughness
     • hardness
     • volume change
     • porosity
       • =weight of water absorbed by dry aggy to saturated aggy
     • relative density
       • is density of solid to water (if pores are saturated, but surface dry)
       • will affect w/c ratio
       • bulk density includes pores
     • durability
   • chemical properties
   • rock type /source

3. undesirable constituents

   • effects:
     • chemical interference
     • prevents bond between surface + cem.paste
     • low durability of aggregate
     • changes fresh concrete props
     • causes fresh cement expansion
   • cause
     • clay
     • coal
     • weak particle coating
     • mica(flaky surface)
     • organic(acid)

4. how it affects concrete

   • shape : affects consistence(workability)
   • texture: affects porosity + consistence
   • physical props: strength reduced (if agg comp.strength <= concrete)
   • deformation charac. affects creep + shrinkage
   • toughness(resistance to failure by impact) + hardness(resistance to local wear - BS 812 part 113)
   • soundness(resistance to disintegration/cracking)
     • test = repeated immersion in sodium /magnesium sulfate
       • salt crystals form in pores
         • after approx. 5 cycles - wash + measure mass loss
           • determines degree of soundness
   • durability
     • should resist:
       • freeze/thaw epxand+contract
       • heat/cool - disintegration from change in temp
       • abrasion -wear over time
       • chem. reaction - silica+alkali expansion

5. environment

   • quarrying = high embodied energy
   • gerenal disruption from quarring - noise
   • aesthetics destroyed
   • quarry changes geolog. equilibrium

6. considerations w/marginal aggs/substitutes

   • satisfactory performance against standards
   • financial effectives/associated costs
   • environmental costs/benefits
   • examples
     • glass cullet
     • sewage sludge inciner. bot.ash = increased strength by 5-10% if fine
     • RCA - recycled concrete agg. ---coarse + fine crushing + screening
       • for
     • specialist
       • lightweight (sintered shale/foam slag)
         • effect: highly porous + high absorption
           • effect: bulk density; 350-850kg/m3 coarse , 750-1100 fine
             • good fire resistance
       • heavy(magnetite/haemitite)
         • props: high density, low porosity ; bulk dense <= 2500