CE 308 Lec 7 Mixing, Handling and Placing of Concrete

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!!