Good Practice

TESTING OF MATERIALS FOR CONCRETE:

CEMENT:

Quality:

• Obtain cement Test Certificate
• Check Compressive strength against requirement
  • 3 day
  • 7 day
  • 28 day
  • Check Setting time
  • Initial Setting time
  • Final Setting time

Storage :

• Weather proof building or shed.
• Few windows which are tightly shut.
• Flooring should be atleast 150 mm thick concrete
• Plinth should be fairly high, so that lorry backing to the door will ensure that chassis and building floor are almost at the same level, thus making loading and unloading of bags easy.
• Stacking of bags should not be against the wall.
• Pile of bags should not be more than 15 bags.
• Passage should be provided between the piles.
• Cement should not be stored indefinitely since there may be a reduction in the strength of concrete made with such a cement .

AGGREGATES:

• Check the size of aggregates(Size of coarse aggregate – 4.75 to 20 mm and that of fine aggregate or sand is less than 4.75 mm).
• Check the source of coarse aggregate.
• Check the source of sand or fine aggregate.
• Check the shape of aggregates(Rounded, Cubical, Flaky, Angular, Elongated. Preference to be given for rounded and cubical shapes).
• Check the distribution of the size of aggregates using sieve analysis.
• Check water absorption of aggregates.
• Check bulking of sand(volume change in sand due to moisture).
• Check silt and clay content in sand(silt and clay can interfere with setting) .
• Check the percentage of fines in sand. Very fine sand belonging to Zone IV should not be used .
• Aggregates of different sizes to be piled and stacked separately.

WATER:

• Water used for concreting should be free from oil, acids, alkalis, salts, sugars, organic materials.Water should be of potable quality.
• Ph value of water shall be not less than 6.

CHEMICAL ADMIXTURES :

• Check quality of admixture as per IS 9103 with emphasis on the following parameters: ?
• Dry Material content
• Ash content
• Relative density
• Chloride content
• pH

CEMENT ADMIXTURE COMPATIBILITY:

• Check cement admixture compatibility at site by conducting trials for every grade of concrete. (All cements are not compatible with every admixture)
• Check quantity of admixture to be added to concrete by caliberating the dispenser. ?

CONCRETE MIX PROPORTIONING:

• Specify the required compressive strength of concrete
• Specify the required workability. Workability measured as slump would be different for site mixed concrete and for pumpable concrete.
• Determine water cement ratio(w/c) based on workability , compressive strength and durability criteria.
• Conduct trial mixes with various proportions of cement, sand, water and aggregates.
• Based on the requirement of workability, compressive strength and durability, determine the quantity of sand, 20 mm aggregates, water cement and admixture .

BATCHING AND MIXING:

• Check whether batching of ingredients are by weight or by volume.Weigh batching has reduced errors)
• Check whether mixing is by machine or hand.Machine-mixing is preferable and gives good quality concrete.
• Check whether mixers are run for sufficient time to ensure proper mixing .
• Check whether the entire contents of concrete mixer are emptied as nothing should be left inside the mixer.
• Check whether the mixer is clean as any concrete that is stuck to the blades of the mixer may affect quality of concrete.
• Ensure that water cement ratio is controlled.

TRANSPORTING CONCRETE :

• Check the distance between concrete mixing and delivery.
• For transportation over small distance, concrete can be moved in wheel barrows and for larger distances, the transportation can be through transit mixers.
• Check for segregation of concrete during transportation. If segregation occurs, take steps to make the concrete cohesive.

PLACING AND COMPACTING CONCRETE :

• Ensure concrete is placed before setting.
• Adopt measures to compact concrete. Select the right type of vibrator.
• Care should be taken to avoid displacement of reinforcement and movement of form work.
• Care should be taken to avoid under-vibration or over-vibration of concrete
• Concrete should be placed in horizontal layers of uniform thickness with each layer thoroughly compacted before the next is placed.

CURING:

• Ensure satisfactory temperature regime is maintained in concrete by preventing loss of moisture.
• Maintain enough water through ponding, immersion, spraying or saturated wet coverings.
• Ensure curing of concrete for sufficient period so that concrete attains the required compressive strength and is durable.A minimum of 14 days is recommended for best results. The longer the curing period, the better the quality of concrete.