Concrete mixing

1
Concrete mixing

• Stationary mixers
• Stationary mixers should be equipped with timing
  interlocks to prevent under or over mixing.
• Add cement with aggregates at the point when
  about 10 of the aggregates are in the mixer.
• Add water first and let it continue to flow while
  other ingredients are entering the mixer.
• Add liquid admixtures with the water.
• Maintain uniformity in batches either by visual
  observation or by consistency meters that measure
  the amperage draw of the motor drivers.

2
Pan Mixer

3
Tilting Drum Mixers

4
Truck Mixer

5
Transporting concrete

• Concrete is usually transported to its point of
  placement in agitating or non agitating trucks
• Concrete should be transported without
  significantly altering its desired properties
  such as slump, w/c ratio, air content and
  homogeneity.
• Loss of workability during warm weather may be
  minimized by
• expediting delivery and placement
• adding crushed ice to mixing water or cooling the
  water by refrigeration.
• injecting liquid nitrogen into the mixer or
  agitator truck
• using a retarding admixture.

6
Placing concrete

• Methods of placing
• Buckets, hoppers, manual or motor propelled
  buggies, conveyor belts, pumps, tremie, paving
  equipment
• keep formwork, reinforcement and subgrades cool
• provide shade
• spray forms with water prior to placing concrete
• In very hot areas pour concrete during the night
• Concrete should be placed so that it is kept
  plastic and free of cold joints.
• Each concrete layer should be placed while the
  underlying layer is still responsive to
  vibration.

7
Concrete conveyor

8
Concrete Pump

9
Pump booms

10
Formwork

11
Compacting concrete

• Compaction expels entrapped air and packs the
  aggregates to increase the density of fresh
  concrete
• Immersion (poker) vibrators are the most
  efficient since they compact concrete directly.
• Vibrators should not be used to move concrete
  laterally.
• Shutter vibrators are installed externally
  outside of forms and impart oscillation to the
  forms and are suitable for small members or
  narrow heavily reinforced members.
• Surface vibrators are used in the construction of
  floors and road slabs and other thin sections
  with large surface areas.

12
Immersion vibrator

13
Screed vibrator

14
Formwork vibrators

15
Curing concrete

• Curing prevents the loss of moisture from
  concrete until the hydration of cement takes
  place.
• Methods of curing
• Leave formwork in place
• Cover with plastic sheeting
• Apply a curing compound
• apply soon after the sheen has disappeared while
  the concrete is still damp
• not effective if application is delayed
• Ponding water
• Sprinkling or fog curing
• Wet coverings
• High early strengths can be obtained by steam
  curing.

16
Curing