Materials

Concrete

Make sure your Contractor/Rajmistry carefully selects the building materials such as sand and stone chips. He must wash them thoroughly 24 hours prior to dhalai, then

• lets them dry.
  
  
• keeps the shuttering totally watertight.
  
  
• checks for the cover blocks of 15 mm x 15 mm x 15 mm under the reinforcing bars.
  
  
• uses wooden boxes of a specific size, for convenience of batching. He must have at least 4 to 6 boxes made, of 1.5 ft x 1.5 ft x 1 ft or slightly larger size, as convenient for the labourers. This ensures uniform proportioning.
  
  
• uses a mixer machine for best results. If hand mixing is necessary, he must use 10% additional cement and a pan large enough to provide uniform mixing.
  
  
• uses only enough water to obtain the minimum workability. Additional water weakens the concrete and greatly enhances cracking potential. For a 1:2:4 mix, when the aggregates are dry, addition of water must be restricted to 28 to 30 litres per 50 kg bag of cement. For a 1:1.5:3 mix, addition of water must be restricted to 25 to 26 litres per 50 kg bag of cement. If a vibrator is used, water must be reduced by 5 to 10%.
  
  
• uses a vibrator vertically for proper compaction. Concrete must not be over vibrated or moved with the vibrator.
  
  

• does not add water while spreading or compacting.

• plans in advance to have all materials present at the site when concreting for a roof slab. This is because interruptions or breaks in concrete operation like that of a concrete roof slab can give rise to joints and thereby weaken the structure and this can lead to seepage problems at a later stage. He must also make sure that the mixer machine along with the vibrator and fuel are readily available in place. Concreting should start early in the morning. No interruptions must be allowed until the entire roof slab has been concreted. Concreting operations must be avoided during peak hours of high temperature in summer. Concreted portions must be covered to avoid direct sunlight which reduces evaporation losses
  
  

• starts curing by mildly sprinkling water after concrete has set. This can happen around 12 to 15 hours after casting at a temperature around 27°C For floors and roof slabs - 3" high bunds of 2 ft x 2 ft must be made with cement, sand, mortar (1:6). After 24 hours, the bunds must be filled with 2" deep water. Till the entire curing period is over, generally between 14 to 21 days, the bunds must always be full of water.
  
  For columns - after the shuttering is removed, they must be wrapped with used jute bags which must be kept wet by intermittently sprinkling water at least 3 to 4 times a day.
  
  For beams - after the side shuttering is removed, these must be wrapped with used jute bags which should be kept wet as in the case of the columns. Or else, water must be sprayed at least 5 to 6 times a day.
  
  
• The concrete work must be stopped in case it starts raining between the operation, and the concreted portion well covered with a plastic sheet in such a way that water drains off easily. Ideally, concreting must be avoided during rain or if rain is imminent.
  
  
  
• Ask your engineer to follow the standard code for removal of shuttering. Removing shuttering early is not good.

Stone Chips

Technically known as coarse aggregates, stone chips are a major ingredient of concrete, giving it strength and solidity. The quality of concrete depends very much on the characteristics of aggregates used.

Stone chips should be angular or round, not flat or flaky
They should not contain marks or layers of any other colour
They should be free from mud and other impurities, which are harmful for concreting. It is advisable to wash the stone chips before mixing to make it free from dust, dirt and mud.

Stone chips are generally sold or supplied in multiples of 100 cubic feet (cft). A standard truck generally carries 300 cft of aggregates. This is calculated by multiplying the carrying area of the truck by the average depth of the load, measured by inserting a rod at four to five points to calculate the mean depth. A shrinkage of up to 2% per truck is allowed.

Price is a function of ex-works (quarry) price + transportation cost + handling + involvement of intermediaries. One should get aggregates from the source to get the best price. Always insist on graded aggregates and not on 'pure' ones.