4A

• Concrete is an intimate mixture of binding material, fine aggregate, coarse aggregate, and water. This can be easily molded to desired shape and size before it loses plasticity and hardens.
• Plain concrete is strong in compression but very weak in tension.
• Functions of Ingredients:

  • Cement - the binding material
  • Coarse Aggregate - consists of crushed stones, which give mass to the concrete and prevent shrinkage of cement
  • Fine Aggregate - enters the voids in coarse aggregates and adds density to the concrete
  • Water - gives workability to concrete
• Proportions of Ingredients (Cement - Fine - Coarse)

  1 : 1 : 2 - For machine foundation, footings for steel columns, and concreting under water

  1 : 1.5 : 3 - Water tanks, shells and folded plates, for other water retaining structures

  1 : 3 : 6 - Piers, abutments, sill of windows, concrete walls, and floors

  1 : 4 : 8 - Mass concretes like dam, and foundation course for walls

  1 : 2 : 4 - Used for reinforced concrete works like beams, slabs, tunnel lining, bridges
• Preparing and Placing Concrete

  1. BATCHING
  2. MIXING
  3. TRANSPORTING AND PLACING
  4. COMPACTING
• BATCHING is the measurement of materials for making concrete.

  The following two methods of batching is practiced:

  (a) Volume batching (b) Weight batching
• In Volume Batching, sand and concrete are batched by volume.
• Wet sand has higher volume for the same weight of dry sand. It is called bulking of sand.
• Weight Batching is the recommended method of batching.
• MIXING is necessary to produce uniform and good concrete.

  The following methods are practiced:

  (a) Hand Mixing (b) Machine Mixing
• The method used depends on the project size:

  Small projects:

  • Concrete is passed in iron pans from hand to hand of a set of workers in a chain.
  • Wheelbarrows and hand carts are also used for short distances.

  Large projects:

  • Chutes are used to direct concrete flow downward.
  • Belt conveyors move concrete horizontally.
  • Pumps and pipes can transport concrete over long distances or to higher placements
• Compaction simply means packing the concrete particles closer together to squeeze out the air. There are two main ways to compact concrete:

  (a) by hand or (b) using vibrators.
• Hand Compaction involves using tools to physically push the concrete around and force out the air.
• Compaction by Vibrators are much faster and more efficient than hand compaction. Vibrators are machines that send high-frequency rumbles through the concrete.
• The following types of vibrators are commonly used in concreting:

  (a) Needle or immersion vibrators

  (b) Surface vibrators

  (c) Form or shutter vibrators

  (d) Vibrating tables.
• Needle vibrators are used in concreting beams and columns. Surface vibrators and form vibrators are useful in concreting slabs. Vibrating tables are useful in preparing precast concrete elements.
• Curing may be defined as the process of maintaining satisfactory moisture.
• Curing for 14 days is very important. Better to continue it for 7 to 14 days more.
• CURING METHODS

  • SPRAYING OF WATER
  • COVERING THE SURFACE WITH WET GUNNY BAGS, STRAW ETC.
  • PONDING
  • STREAM CURING
  • APPLICATION OF CURING COMPOUNDS
• In SPRAYING OF WATER, walls, columns, plastered surfaces are cured by sprinkling water.
• Spray at least 4–5 times daily, more frequently in hot or windy conditions.
• Columns and other vertical surfaces may be cured by covering the surfaces with gunny bags or straw. And, horizontal concrete members are covered with sawdust, sand, or earth to keep them wet.
• In PONDING, the horizontal surface like slab and floors are cured by stagnating the water to a height of 25 to 50 mm by providing temporary small hunds with mortar.
• STEAM CURING accelarates curing process, resulting into the reduction of curing period
• APPLICATION OF CURING COMPOUNDS such as calcium chloride, keeps the concrete surface wet for a long time.
• Types of Concrete Curing Compounds:

  • Synthetic resin compound
  • Acrylic compound
  • Wax compound
  • Chlorinated rubber compound
• IMMERSION is the best water curing method. In this method, finished concrete is immersed in the curing tank or curing pond.
• Concrete in the plastic stage is also known as Green Concrete. It refers to the state of concrete when it is freshly mixed and can be molded or shaped. It’s called “plastic” because it is soft and malleable.
• Hardened concrete refers to a form of concrete that has experienced a chemical change within a blend of cement, water, and aggregates.
• Workability defined as the ease with which concrete can be compacted fully without segregating and bleeding.
• Segregation is the separation of coarse particles from the green concrete. It may happen due to lack of sufficient quantity of finer particles in concrete or due to throwing of the concrete from greater heights at the time of placing the concrete. It can cause the concrete to lose its cohesiveness and honey combing results.
• Bleeding refers to the appearance of the water along with cement particles on the surface of the freshly laid concrete. This happens when there is excessive quantity of water in the mix or due to excessive compaction. This causes the formation of pores and renders the concrete weak.
• Harshness is the resistance offered by concrete to its surface finish. It is due to presence of lesser quantity of fine aggregates, lesser cement mortar and due to use of poorly graded aggregates. It may result due to insufficient water as well. With harsh concrete, it is difficult to get a smooth surface finish and concrete becomes porous
• Properties of Green Concrete:

  • Workability
  • Segregation
  • Bleeding
  • Harshness
• Properties of Hardened Concrete:

  • Strength
  • Dimensional Change
  • Durability
  • Impermeability
• Strength defined as the compressive strength of 150 mm size cubes after 28 days of curing below which not more than 5 percent of the test results are expected to fail. It is gained in the initial stages. In 7 days, the strength gained is as much as 60% to 65% of 28 days strength.
• Durability is the period of existence of concrete without getting adversely affected by these forces.
• Impermeability is the resistance of concrete to the flow of water through its pores.