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Questions and Answers
What is the primary objective of mixing concrete?
What is the primary objective of mixing concrete?
Which method of batching involves preparing quantities by measuring weight?
Which method of batching involves preparing quantities by measuring weight?
Which type of mixer is characterized by a chamber that tilts for discharging?
Which type of mixer is characterized by a chamber that tilts for discharging?
What is the process of coating the sides of the mixer with an initial amount of mortar called?
What is the process of coating the sides of the mixer with an initial amount of mortar called?
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Which factor does NOT affect mixing time?
Which factor does NOT affect mixing time?
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What is a characteristic of the pan-type mixer?
What is a characteristic of the pan-type mixer?
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Which of the following mixes requires lower workability and is suitable for a tilting drum mixer?
Which of the following mixes requires lower workability and is suitable for a tilting drum mixer?
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What is the proportion of cement to sand to gravel in Class C concrete mix batch?
What is the proportion of cement to sand to gravel in Class C concrete mix batch?
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What is the main drawback of using a non-tilting drum mixer?
What is the main drawback of using a non-tilting drum mixer?
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Which statement correctly describes the process of manual mixing?
Which statement correctly describes the process of manual mixing?
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What is the minimum mixing time for a one-bagger mixer with a volume of 0.09 m3?
What is the minimum mixing time for a one-bagger mixer with a volume of 0.09 m3?
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What is a potential consequence of prolonged mixing of concrete?
What is a potential consequence of prolonged mixing of concrete?
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Which method is used to restore workability in concrete if it decreases over time?
Which method is used to restore workability in concrete if it decreases over time?
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What type of truck is used to mix water during transportation of concrete?
What type of truck is used to mix water during transportation of concrete?
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What is the minimum recommended slump for Pumpcrete to ensure medium workability?
What is the minimum recommended slump for Pumpcrete to ensure medium workability?
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What is the primary function of vibrating in concrete placement?
What is the primary function of vibrating in concrete placement?
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Which type of vibrators operates at a frequency of 12,000 cycles per minute?
Which type of vibrators operates at a frequency of 12,000 cycles per minute?
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What is the purpose of revibration in concrete work?
What is the purpose of revibration in concrete work?
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What is one disadvantage of ready-mixed concrete?
What is one disadvantage of ready-mixed concrete?
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Which curing method involves keeping concrete submerged in water?
Which curing method involves keeping concrete submerged in water?
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In the context of Pumpcrete, what issue arises when the mix is too dry?
In the context of Pumpcrete, what issue arises when the mix is too dry?
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What is an advantage of shotcrete technology?
What is an advantage of shotcrete technology?
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What could happen if concrete is placed too wet?
What could happen if concrete is placed too wet?
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Which method is NOT considered a type of handling for concrete?
Which method is NOT considered a type of handling for concrete?
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Study Notes
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.
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