FAQ'S - Cement | Concrete | Construction Stages In Building House

FAQs ON CEMENT

Cement has a shelf life of 3 months after manufacturing; after which it should not be used. If someone wants to still use it then, relevant tests should be first conducted before using that cement.

Colour of cement does not play any role in its performance. Appearance of colour purely depends upon the percentage of addition of a particular raw material which goes in the manufacturing of cement.

A ball of cement paste should be made and kept in a shaded environment for 24 hours. If it hardens then cement is good.

Blended cements (PPC) perform better than OPC in Indian climate and weather.

Colour of cement has got no bearing on its strength and durability.

Any cement is governed by initial setting time and final setting time. Indian standards have specified certain timings for these. Initial setting time of cement should not be less than 30 minutes while final setting time should not be more than 600 minutes.

Strong, durable, right setting time and uniform size distribution.

FAQs ON CONCRETE

Concrete is a mixture of cement, sand, stone aggregates and water.

A concrete member in which reinforcement is also placed is known as a Reinforced Cement Concrete member (R.C.C.)

Mortar is a mix of cement, sand and water, to be used for brickworks/block works and plaster.

Normally, the amount of water that is required per bag of cement is 21 – 25 litres only.

Quality of cement has nothing to do with its colour.

1:1.5:3, where 1 part of cement is to be mixed with 1.5 parts of sand and 3 parts of coarse aggregates. Ideal water requirement for the mix should be 20 litres per bag of cement.

The ways in which concrete may be spoilt are many, most common of them being:

  • • Use of too much or too little water for mixing or water carelessly added during mixing
  • • Improper mixing of aggregate with cement
  • • Improper grading of aggregates resulting in segregation or bleeding of concrete
  • • Improper compaction of concrete
  • • Using concrete which has already begun to set
  • • Leaving the finished concrete surface exposed to sun and wind during the first ten days after placing without protecting or keeping it damp by proper methods of curing

Freshly prepared mortar at site has to be consumed within 15 to 30 minutes. Same goes with concrete consumption, if concrete is prepared at site. However, concrete prepared in ready mix plants can be used after 2 to 3 hours also because concrete in ready mix plants is manufactured as per specific requirements and controlled conditions.

Compaction is essential in order to properly compress the concrete by removing entrapped air inside it. If compaction is not done properly then the strength of concrete will be compromised.

The concrete is liable to become weaker and more prone to seepage.

Sand volume should be increased and the water volume should be reduced with some approximation.

Steel bars are placed because concrete is weak in tension

FAQs ON CURING

Curing of Concrete is a method by which the concrete is protected against loss of moisture required for hydration and kept within the recommended temperature range. Curing also increases the strength and decreases the permeability of hardened concrete.

Walls: Water should be sprinkled on masonry walls twice a day for 7 days

  • Slab: Ponding should be done on the slab by constructing bunds of mortar of approximately 1m x 1m and water should be stored for a period of 7 days.
  • Beams and columns: Columns and beams can be cured by wrapping gunny bags around them and periodically sprinkling water on it for a period of 7 days

Water Ponding, periodical sprinkling of water, covering R.C.C. member with a wet hessian cloth, sprinkling of curing compound which hardens to form a thin protective membrane, are some of the methods by which concrete can be cured.

Curing should be started just after the R.C.C. surfaces begin to harden. Curing should be done for a minimum of 7 days.

Ideal curing period in case of concrete as well as mortar is 7 days

Best method for curing sloped slab is covering it with wet hessian cloth and periodically sprinkling water on it for a period of 7 days.

FAQs ON FORMWORK

Formwork is the term used for the process of creating a temporary mould into which concrete is poured and formed. Traditional formwork is fabricated using 18mm thick plywood, but it can also be constructed from steel, timber and other materials.

Camber in the upward direction is provided in the formwork for horizontal members to counteract the effect of downward deflection which will be caused due to the weight of reinforcement and concrete placed over that.

Shuttering boards/Planks should be properly oiled or greased with Deshuttering oil before the placement of reinforcement. The oil film sandwiched between concrete and formwork surface not only helps in easy removal of shuttering but also prevents loss of moisture from the concrete through absorption and evaporation.

Indian standard IS: 456 has given a timeframe for stripping of formwork for different R.C.C. members when sufficient curing is done and ambient temperature is more than 15°C. Details are given below:-

  • • Vertical formwork of columns, walls & beams – 16 to 24 hours
  • • Bottom formwork of slabs (Props to be refixed immediately after formwork removal) – 3 days
  • • Bottom formwork of beams (Props to be refixed immediately after formwork removal) – 7 days
  • • Props to slabs
    • o Spanning upto 4.5 m – 7 days
    • o Spanning over 4.5 m – 14 days
  • • Props to beams & arches
    • o Spanning upto 6.0 m – 14 days
    • o Spanning over 6.0 m – 21 days

Column casting should be done upto beam bottom (i.e. 8 feet) in one lift under normal circumstances when floor to floor height is 10 feet.

Procedure of deshuttering a cantilever slab remains same as regular slab. But props will have to be kept for a minimum of 14 days for span less than 4.5m and 21 days for span more than 4.5m.

FAQs ON REINFORCEMENT

Steel reinforcement also known as rebars are made of different diameters and come in 6m and 12m lengths. They are provided in conjunction with concrete to make the member strong. Any member will be subjected to compressive as well as tensile forces. Concrete helps in resisting compressive forces whereas steel reinforcement helps in resisting tensile forces.

Bar-bending-schedule is a format used to calculate exact requirement of reinforcement in a particular project.

  • A detailed tabular format is made which consists of the following:-
  • • Type of member with its identification
  • • Type and shape of bars
  • • Diameter of bars
  • • Length of bars
  • • Number of bars
  • • Total length of bars
  • • Unit weight of bars
  • • Total weight of bars
  • • Summary

Steel reinforcement is available in various sizes and grades (yield strength). There are 4 different grades of reinforcement that can be used in construction. They are Fe250, Fe415, Fe500 and Fe550. Number denotes the yield strength of reinforcement in N/mm². Fe500 is most widely used nowadays because it gives high yield strength and design becomes economical by the use of Fe500 reinforcement. Bars of various diameters are available in markets from 8mm to 40mm. Diameter of the bar used at site should be as per structural design requirements.

Cover blocks are provided between the formwork and reinforcement. Main purpose of providing cover block is to protect reinforcement from getting exposed to atmosphere and in turn protect it from getting corroded. Indian Standard IS:456 suggests use of different size of cover for different elements and different hours of fire resistance.

  • Values of minimum cover for different R.C.C. elements to resist ½ an hour of fire is as given below:-
  • a) Footing – 50mm
  • b) Column – 40mm
  • c) Beams – 20mm
  • c) Slabs – 20mm

Yes. You can receive the test certificate of the batch of materials supplied at our site. The certificate will confirm the compliance of the quality of supplied materials as per the requirement of relevant Indian Standard Code.

FAQs ON BRICKWORK

Normally, the bricks are tested for compressive strength, water absorption, dimensional tolerances and efflorescence. However, at small construction sites, the quality of bricks can be assessed based on the following, which is prevalent in many sites.

  • • Visual check – Bricks should be well burnt and should be of uniform size and colour
  • • Striking two bricks together should produce a metallic ringing sound
  • • The surface should be so hard that it can’t be scratched by the fingernails
  • • It should not break if dropped in standing position from one metre above the ground level
  • • It shouldn’t absorb the moisture of more than 15-20% by weight when soaked in water. For example, a good brick of 2 kg shouldn’t weigh more than 2.3 to 2.4 kg if immersed in water for 24 hours

The following precautions should be taken:

  • • Bricks should be soaked in water for an adequate period so that the water penetrates to its full thickness. Normally, 6 to 8 hours of wetting is sufficient
  • • A systematic bond must be maintained throughout the brickwork. Vertical joints shouldn’t be continuous but staggered
  • • The joint thickness shouldn’t exceed 1 cm. It should be thoroughly filled with the cement mortar 1:4 to 1:6 (Cement:Sand by volume)
  • • All bricks should be placed on their bed with frogs on top (depression on top of the brick for providing bond with mortar)
  • • Thread, plumb bob and spirit level should be used for alignment, verticality and horizontality of construction
  • • Joints should be raked and properly finished with trowel or float, to provide a good bond
  • • A maximum of one-metre wall height should be constructed in a day
  • • Brickwork should be properly cured for at least 10 days

The brickwork should be cured with water and kept moist for a minimum 7 to 10 days period for proper development of strength.

Dry bricks will absorb water from the cement mortar and make the joints of brickwork weak.

FAQs ON PLASTERING

Plaster protects the structure from temperature variations, external attacks of sulphates, chlorides, etc. Plaster also provides smooth & aesthetic surface on RCC & Brickwork surface.

The following precautions should be taken:

  • • Preferably use cement which releases low heat of hydration – A blended cement is a good choice
  • • Use optimum water at the time of mixing
  • • Do not use dry cement on the plaster surface
  • • At the junction of Brickwork & RCC, chicken mesh or fibre mesh may be used
  • • Wet the surface before plastering
  • • Cure the surface for at least 10 to 12 days

For inside wall plaster it is 1 cement : 6 medium sand by volume and the thickness should not be more than 12 mm. For ceiling plaster it is 1 cement : 3 medium sand by volume and the thickness should not be more than 6 mm.

  • • External Plaster – 20mm thickness in 2 layers (12mm for 1st layer & 8mm for 2nd layer)
  • • Internal Plaster – 12mm thickness in 1 layer
  • • Ceiling Plaster – 6mm thickness

FAQs ON WATER SUPPLY

  • A) The lines should be laid in a way that there is no risk of contamination of water supply. For this, the following three things are necessary:
  • • No cross-connection anywhere between pipes carrying potable water and wastewater
  • • No backflow from any appliance towards the source of supply
  • • Water supply pipes and wastewater pipes (drainage pipes) should not be laid very close to each other
  • B) Pipelines should be properly protected against any damage – Underground pipeline should be enclosed in cement mortar and have sufficient earth cover.
  • C) Pipe network should be simple and straight as far as possible

There are many options like copper/brass pipes, galvanized iron (GI) pipes, plastic pipes, etc. However, GI pipes are more commonly used in India. But PVC pipes should be used as they can be easily replaced during repairs.

Only small-diameter pipes used for the supply of water for domestic purpose can be concealed. During the laying of pipes, care should be taken to see that the nodes or the joints are properly fixed without any chance of leakage. But it is always recommended to conceal pipes only in unavoidable situations and limit it to small lengths.

FAQs ON PAINTING

Following precautions would be necessary before painting the internal surface of the house:

  • • Check & rectify water leakage/dampness from any place
  • • Fill the cracks in the external and internal wall with a suitable ready-made sealant after widening the crack up to 4mm to ensure proper filling
  • • Newly plastered surface should be allowed to dry. Remove loose particles by rubbing with sandpaper & ensure that the surface is dry and free from dust, dirt, etc.
  • • Apply a coat of primer
  • • Apply acrylic-based polymer putty or Plaster of Paris (POP) to level undulations
  • • Apply one more coat of primer. Allow it to dry for 10-12 hours. Apply the first coat of paint after addition of thinners as per manufacturer’s specification. Keep an interval of 4-8 hours between any two coats. Generally 2-3 coats of finish paint give satisfactory results.

FAQs ON DOORS AND WINDOWS

Cracks will appear at the junction of door frame and joints if lintel is also not provided above the opening.

The following precautions should be taken for safety and quality purposes:

  • • Always buy electrical items with ISI mark
  • • Identify locations for switchboards in advance so that they would not obstruct while decorating rooms
  • • Proper earthing should be done for the entire house
  • • Proper class of wiring should be done
  • • Each power board should have a separate fuse
  • • Registered electrical contractors should be engaged for carrying out electrical works

Use of RCCB/ELCB (Residual Current Circuit Breaker / Earth Leakage Circuit Breaker) prevents accidental shocks and also keeps a check on the earthing system of the house. In case of leakage of very small current in milliamperes, the circuit would trip and prevent the mishap.

Earthing is required to bypass the overrated current caused due to fluctuation in circuit parameters such as phase-frequency, line voltage, etc., to save human life and electrical assets from short circuits.