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Gives concepts of Concrete processes for proper field practices
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CE 308
Plain and Reinforced Concrete I
Mixing, Handling and Placing of Concrete
Dr. Wasim Khaliq
Sequence
• Processes
– Storage– Batching–Mixing– Transporting– Placing– Compaction– Finishing– Curing
• Methods, Techniques, Critical Steps, Tips
Ref: ACI 304R-89
Storage Aggregate
• Coarse and Fine Aggregate–Moisture Control is very important– In case of washed/wet aggregate, drain the excess
water before movement to bins/storage areas– Stockpile various sizes at various dumps– Drop aggregate vertically if piling mechanically
Storage Cement
• Cement/Pozzolans
• Water Tight and Properly Ventilated Areas
• Bulk
– Silos• Bags
– Piles/Layers– Temporary < 60 days (Max = 14 layers)– Permanent > 60 days (Max = 7 layers)
Admixtures Treat the powder types as cement For liquid ones, avoid evaporation and freezing
Batching
• Batching involves preparing the constituents of the concrete in the correct proportions according to the mix design.
• Two types– Volume Batching–Weigh Batching
Weight of 1 bag of cement = ?• 50 kg
Volume of 1 bag of Cement = ?• 1.27 ft3 (0.036 m3)
0.3X0.3X0.4 m
Mixing
• Mixing Types– Hand (10% extra cement)– Mechanical
• 1. Mobile (Drum-Type)– Titling– Non-Tilting– Horizontal (reversing)
• 2. Transit (Truck Type)• 3. Fixed & Mobile
– Pan Types
– Mixing/Feeding Sequence– Mixing Time Small and Big Jobs
Small Jobs
Mixing
• Mobile Transit (Trucks)
Two sets of opposed mixing spirals
Capacity = 4-12m3
Mixing
• Feeding Sequence
– 25% water– 50% coarse aggregate– 50% fine aggregate– 100% cement– 50% coarse aggregate– 50% fine aggregate– 75% water Start of Mixing Time
Mixing Time - Concept
Minimum Time
1 minute for 3/4m3 plus 1/4 minute for each additional m3
Transportation
• Loading Concrete
– Pans– Wheel Barrows– Buckets– Skip and Hoist– Trucks
Transportation
• Pans
– More surface area is exposed, more evaporation– easy placement above or below ground without
segregation– very small jobs only– slow and expensive method
Wheel Barrows
– Chances of segregation: pneumatic wheels, wooden planks pathway
– Ground level only– small jobs, small hauls, and over muddy ground
Transportation
• Buckets
– Cable operated Upper floors
– Crane-assisted for Mass Concreting
Trucks– Long Hauls– Agitation
Placing
• Pans
• Wheel Barrows
• Chutes
• Conveyor Belts
• Buckets
• Pumps
Placing
• Preparations
– Surface should not be Wet nor Dry– Damp ?
Placing - Horizontal Members
Direction of Placement
Placing - Vertical Members
Placing
• Sloping Surface
Placing - Pumping
400 m Horizontal80 m Height15-150 m3/hour
Placing
• Concreting Underwater
– Buckets– Tremie Pipe
Placing
• Rules
– Drop Height < 1.5m– Throw Concrete on already placed fresh Concrete– Start at the far end and work to the near end.– On a slope use stiffer concrete and work up the slope– In vertical members, use differential slump– Use Tremie method for underwater concreting
Compaction
• Hand - Sequence
– Spread the concrete using a short-handled, square ended spades or shovels; never use vibrating poker to spread the concrete
– Use Rods and Spades to compact (consolidate) the concrete along edges of the formwork
– Use Screed (wood or metal straightedge) to strike off and level the concrete
Compaction
• Mechanical-Sequence
– Spread the concrete using square ended spades or shovels; never use vibrating poker to spread the concrete
– Use vibratory pokers for compaction / consolidation
• Types
– Internal– External
Compaction
• Sequence
• Spacing=0.5-1.0m
• 5-30secs
• 2min for stiffer ones
Compaction
• External Vibrators
– For very complex formwork and reinforcement– For Precast plants– Compaction of Thin Layers only–Watch for over vibration
Compaction
• Rules
– How do you know that compaction is complete??– “No more Air bubbles rising (<1% voids)”– “No Honeycombing and No mortar collection”– Vertical - not more than 10° Angle– Compaction around rebars is very important, any air or
water left there will induce early corrosion– Don’t touch rebars and formwork– Gradual Withdrawal to close the holes– Re-vibration ?
Improperly consolidated Concrete
Finishing
• Sequence
– Correction of Defects!– Float–Wait for Bleeding– Edge– Joint– Trowel– Texture
Finishing
• Floating
– Start as soon as struck off–Wood or Magnesium– One or two passes– Never seal the surface– Never use steel trowel– Finish before bleeding starts
Bull Float
Finishing
• Wait for Bleeding!
– All the sheen (shine) is gone– Dusting, Crazing, Scaling, and Blisters–Watch for high evaporation rate– Delamination
General Rule
– Never Sprinkle Cement or Water while Finishing!!