Untitled Quiz

Batching

• Batching refers to preparing or measuring quantities of concrete constituents.
• There are two methods:
  • Batching by weight
  • Batching by volume
• Proportions for different concrete classes are given in tabular format.

Mixing

• Mixing aims to coat aggregate particles with cement paste and blend ingredients into a uniform mass.
• It can be done manually or using equipment.

Manual Mixing

• Spread aggregate on a clean, non-porous base.
• Spread cement over the aggregate.
• Mix dry materials thrice with a shovel until uniform.
• Gradually add water and mix until uniform.

Machine Mixing

• Tilting Drum Mixer:
  • Can be one-bagger or two-bagger.
  • Drum tilts for discharge, even during mixing.
  • Suitable for mixes with low workability and large aggregates.
• Non-Tilting Drum Mixer (Reversing Drum):
  • Drum axis is always horizontal.
  • Uses a chute for discharge.
  • Rotation can be reversed for discharge.
  • Slow discharge rate can lead to segregation.
  • Requires checking the mix at the start and end.
• Pan-Type Mixer:
  • Efficient for stiff and cohesive mixes.
  • Uses forced mixing action, like an electric cake mixer.
  • Scraping action at sides ensures uniform mix.

Machine Mixing Terminologies

• Charging the mixer: Putting ingredients in the mixer.
• Buttering: Coating the mixer sides with an initial amount of mortar.

Mixing Time

• Mixing time depends on the mixer size, type, and rotation speed.
• Minimum mixing times are provided for different mixer sizes.
• Prolonged mixing can lead to:
  • Evaporation of water
  • Grinding of aggregates

Mixing: Intermittent Mixing

• Does not affect strength and durability.
• Workability decreases with time unless moisture loss is prevented.
• Retempering: Adding water to restore workability.

Transporting

• Dump trucks: Used for short distances.
• Agitator trucks: Water is added at the plant, mixing occurs during transportation.
• Transit mixers: Water is added during transportation.

Handling

• Concrete can be handled using various methods like buckets, wheelbarrows, concrete buggies, belt conveyors, portalifts, pumps, cranes, and buckets.

Pumpcrete

• Concrete pumped under pressure.
• Types of pumps:
  • Direct-acting concrete pump
  • Squeeze-type concrete pump
• Pipe diameters should be greater than three times the maximum aggregate size.
• Recommended slump: 40-100mm.

Pumpcrete Advantages

• Delivers concrete to inaccessible areas.

Pumpcrete Disadvantages

• Too dry mix can block the pump.
• Too wet mix can lead to segregation.

Shotcrete (Gunite)

• Pneumatically-applied mortar.
• Mortar is pumped under pressure at high velocity onto a backup surface.

Placing

• Deposit concrete as close as possible to its final position to minimize segregation and ensure full compaction.
• Methods:
  • Chute
  • Tremie
  • Flexible drop chute

Compacting and Vibrating

• Vibrating removes entrapped air, fills forms, and ensures a uniform mix.
• Methods:
  • Manual
  • Mechanical

Vibrating: By Hand

• Rodding or ramming.

Vibrating: Internal Vibrators (Poker or Immersion)

• Vibrate at 12,000 cycles per minute.
• Apply for 5-30 seconds, depending on consistency, at 0.50-1m on center.

Vibrating: External Vibrators

• Used for precast or thin in-situ sections.
• Clamped to formwork, vibrating both form and concrete.
• Operate at 3,000-6,000 cycles per minute.

Vibrating: Vibrating Tables

• Formwork is clamped to a vibrator, used for precast concrete or testing.
• Operate at 1,500-7,000 cycles per minute.

Vibrating: Revibration

• Used for concrete placed in layers to:
  • Reduce settlement cracks
  • Reduce internal effects of bleeding

Curing

• Procedures promoting cement hydration.
• Involves:
  • Temperature control
  • Moisture movement control

Curing Purpose

• Enhances hydration.
• Prevents water evaporation.
• Replaces evaporated water.

Curing Methods

• Ponding
• Wet sand, earth, sawdust, straw
• Waterproof paper
• Wet burlaps
• Spraying
• Steam curing

Ready-Mixed Concrete

• Two types:
  • Central-mixed: Batched and mixed at the central plant.
  • Transit-mixed: Batched at the central plant, mixed in mixer trucks.

Ready-Mixed Concrete Advantages

• Close quality control during batching.
• Convenient for congested sites.
• Agitator trucks ensure proper transportation.
• Convenient for small quantities or intermittent mixing.

Ready-Mixed Concrete Disadvantages

• Maintaining workability until placing.
• Higher cost.