Batching, Mixing, and Handling. Ordering or Specifying Concrete Alternative (1) Common: When the...
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Transcript of Batching, Mixing, and Handling. Ordering or Specifying Concrete Alternative (1) Common: When the...
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Batching, Mixing, and Batching, Mixing, and HandlingHandling
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Ordering or Specifying Concrete Ordering or Specifying Concrete
• Alternative (1) Common: When the owner requires the concrete supplier to assume responsibility for the concrete mix proportions.
• Alternative (2) Prescription: When the owner assumes responsibility for the mix proportions and properties of the concrete.
• Alternative (3) Performance: When the owner requires the concrete supplier to assume responsibility for the concrete "as delivered".
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Batching of ConcreteBatching of Concrete
• The process of measuring concrete mix ingredients by either mass or volume and introducing them into the mixer.
• To produce concrete of uniform quality, the ingredients must be measured accurately for each batch.
• Most concrete today is batched and mixed by ready mixed concrete plants
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Nonagitating trucks are used with central batch plants
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Truck Agitators
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Continuous Mixer
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Transporting ConcreteTransporting Concrete
Good planning and handling to avoid:
- Delays.
- Early Stiffening and drying out.
- Segregation.
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Direct Chute Discharge
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Front-discharge Truck
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Bucket and Crane
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Truck-mounted Pump and Boom
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Discharging from Flexible Hose
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Placing and FinishingPlacing and FinishingLecture 16Lecture 16
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Preparation Before PlacingPreparation Before Placing
Preparation prior to placing concrete includes the following:
• Compacting, trimming, and moistening the subgrade (see Figs. 11-1, 11-2, and 11-3);
• Erecting the forms;
• Setting the reinforcing steel and other embedded items securely in place.
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Depositing the ConcreteDepositing the Concrete
• Concrete should be deposited continuously as near as possible to its final position without objectionable segregation.
• concrete should be placed in walls, thick slabs, or foundations in horizontal layers of uniform thickness; each layer should be thoroughly consolidated before the next is placed.
• To avoid segregation, concrete should not be moved horizontally over too long a distance.
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Consolidating ConcreteConsolidating Concrete
• the process of compacting fresh concrete; to mold it within the forms and around embedded items and reinforcement; and to eliminate stone pockets, honey-comb, and entrapped air.
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Consolidation is accomplished byConsolidation is accomplished by
– hand rodding, that is, thrusting a tamping rod or other suitable tool repeatedly into the concrete.
– or by mechanical methods.
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VibrationVibration
• The most widely used method for consolidating concrete.– Internal or immersion-type vibrators.– External vibrators.
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Consequences of UndervibrationConsequences of Undervibration
• (1) honeycomb: spaces between coarse aggregate particles do not become filled with mortar
• (2) excessive amount of entrapped air voids;• (3) cold joints: discontinuity resulting from a
delay in placement that allowed one layer to harden before the adjacent concrete was placed.
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Consequences of UndervibrationConsequences of Undervibration
• (4) placement lines: dark lines between adjacent placements of concrete batches
• (5) subsidence cracking: as concrete settles over reinforcing steel in relatively deep elements
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Defects from overvibration Defects from overvibration includeinclude
• (1) segregation as vibration and gravity causes heavier aggregates to settle while lighter aggregates rise;
• (2) loss of entrained air in air-entrained concrete; • (3) excessive form deflections or form damage; • (4) form failure caused by excessive pressure
from vibrating the same location too long.
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Concrete FinishingConcrete Finishing
• Screeding (Strikeoff)
Screeding or strike off is the process of cutting off excess concrete to bring the top surface of a slab to proper grade
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Concrete FinishingConcrete Finishing
• Bullfloating or Darbying
To eliminate high and low spots and to embed large aggregate particles, a bullfloat or darby should be used immediately after strikeoff.
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Concrete FinishingConcrete Finishing
• Brooming Finish
Brooming should be performed before the concrete has thoroughly hardened, but it should be sufficiently hard to retain the scoring impression to produce a slip-resistant surface
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Concrete FinishingConcrete Finishing
• Highway pavements are textured by "tining" the surface with stiff wires; this improves traction and provides vehicles with a surface that significantly reduces the chance of hydroplaning.
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Curing and ProtectionCuring and Protection
• A newly placed and finished concrete should be cured and protected from drying, extreme changes in temperature, and damage.
• The curing should begin immediately after finishing.