MAHA Tips for Good Construction Practices

1. Guidance on materials

Cement is used for variety of applications in construction and manufacture of building materials. It is always used with other materials like sand, aggregate, water etc., Therefore its performance in a particular application depends not only on its own quality but also on its correct usage and quality of other materials used along with it.

Therefore we hope the following guidelines or tips on its usage will greatly help you build long lasting homes.

To begin with, let us see how to check whether the cement is genuine or not.

  • Purchase only genuine recognized brand.
  • Ensure that the bag has ISI mark with license no.
  • Ensure that the bags are stitched properly and no lumps are present.
  • You may check the Week No and Month and Year of manufacture.

Commonly available types of cement in the market are:

  • OPC 43 Grade
  • OPC 53 Grade
  • Portland Pozzolana Cement
  • Portland Slag Cement

Good cement alone will not ensure strong and durable structure. Other materials used in the concrete play equally important role. The other materials used in concrete are sand, coarse aggregates, water and sometimes chemical admixtures. Now let us see how to select good raw materials for concrete

Sand is used in concrete to fill the space between coarse aggregate or Kankara to make concrete dense. River sand is preferable but manufactured sand can also be used if it is of good quality with less fines. Following precautions should be taken while selecting sand.

  • Sand used should be free from impurities like clay lumps, mud, silt etc. Simple field test to identify good sand is: if you take handful of sand, rub between your palms and release, then mud should not stick to your palm.
  • It should be free from organic matters and chemicals
  • Fine sand is better for plastering. Coarse and medium sand should be used for concrete.
  • Sea sand is not good for RCC work.

There are three types of coarse aggregates with maximum sizes as 40 mm, 20mm and 10 mm respectively. 40 mm size coarse aggregatesare generally used in PCC or Road. Well graded coarse aggregates for general purpose concrete should contain both 20 mm and 10 mm sizes in the ratio of 60:40.

  • Coarse aggregates should be chemically inert.
  • Coarse aggregates used should be round or cubical in shape. Avoid angular aggregates.
  • Coarse aggregates should be free from organic matters.
  • Generally we should avoid flaky and elongated aggregates

Water is a necessary evil for cement. Without water, cement is of no use. To get the strength from cement, we need to add water. But if you add water more than required, it will reduce the strength of the concrete. The optimum quantity of water varies from work to work. Following guidelines may be used.

  • For concrete, use about 25 to 30 liters per bag
  • Generally potable water is better for concreting
  • Generally avoid sea water for RCC works

2. Guidance on processes

 a.      Concrete Proportioning

Concrete of different strength is made by different proportions. In large projects, the concrete mix proportions are decided by qualified engineers in a laboratory. However for small works we may use nominal mixes as per the guidelines of IS 456. The general mix proportions for small works are given below:

Type of Work Proportion of Cement: Sand: Coarse Aggregates (20 mm)
High Strength Concrete for Arches & High Load Column 1: 1: 2
Normal RCC work like Column, Beam, Slab, Stair case etc. 1: 1.5: 3 or 1: 2: 3
Foundations, Mass concrete, etc. 1: 3: 6 or1: 4: 8

To obtain good concrete, not only do we need to use good quality materials in proper proportion but also need to maintain optimum water content for better results. Normally one bag cement requires 25 to 30 liters water for good mix. Too much water or too less water either reduces the strength of concrete or makes it less workable. 

b.      Batching

Generally we can mix concrete in two ways: 1 Volume batching and 2. Weigh batching. Weigh batching is preferable over volume batching. If it is not possible to arrange for weigh batching, volume batching is adopted. It is better to use farmas to measure the quantities. 

c.       Mixing

We can mix concrete in two methods; mixing by machine and mixing by hand (machine mixing is advisable). It is better to add 10% extra cement for concrete if it is hand mixed.

  • Use concrete mixers for better concrete. The material should be mixed till uniform color of the concrete is obtained.
  • Mixing time is for about 2 minutes.
  • Add sand and cement first and then coarse aggregate. Water should be added last.
  • If materials are hand mixed, mix it on a hard surface. 

d.      Transportation and Placing

  • Transport the concrete to its final placement at the earliest. Avoid drying out and segregation of materials.
  • Do not use dried concrete by adding extra water.
  • Do not pour the concrete from a height of more than 1.5 meters to avoid segregation.
  • Pour concrete in layers without any break to avoid cold joints. 

e.       Compaction and Finishing

Proper compaction of concrete is very important for strength and durability.

  • Use proper vibrators for compaction.
  • Avoid under or over vibration. Vibrate the concrete till the all air in the concrete expelled and sheen of water appear on the surface.
  • Finish the concrete with wooden floats when the concrete slightly stiffens.
  • If shrinkage cracks appear after finishing, close them by re-toweling or re-vibrating. 

f.        Curing

  • Start the curing after concrete hardens sufficiently to withstand nail marks i.e., after final setting.
  • Keep concrete moist continuously till at least 14 days as recommended by IS code.
  • Alternate drying and wetting makes concrete crack. Continuous curing is necessary for strong and durable concrete. 

g.      Brick Work

Generally bricks used should be well burnt and corners should be sharp. Well burnt bricks should give metallic ringing sound when we strike two bricks with each other. A good brick should not break when we drop it from about 1 meter height.

  • Good brick should not soak water more than 15% of its weight when kept under water for 24 hours.
  • Bricks should be well-watered before use otherwise absorb water from cement mortar.
  • Both vertical and horizontal joints in brick work should not more than 10 mm thickness.
  • Generally we use 1:6 cement mortar mix for brick works.

 

h.      Plastering

Plastering is the final stage where cement is used in construction.