GROUP-4

• Plain concrete, also known as concrete, is an intimate mixture of binding material, fine aggregate, coarse aggregate and water.
• Depending upon the proportion of ingredient, strength of concrete varies.
• After addition of water it hydrates and binds aggregates and the surrounding surfaces like stone and bricks.
• setting time starts after 30 minutes and ends after 6 hours.
• coarse aggregate consists of crushed stones.
• coarse aggregates should be well graded and stones should be igneous origin.
• coarse aggregates should be clean, sharp, angular and hard.
• fine aggregate consists of river sand. it prevents shrinkage of cement.
• fine aggregate adds density to concrete, since it fills the voids.
• water activates hydration of cement and form s plastic mass.
• water gives workability to concrete which means water makes it possible to mix concrete with ease and place it in final position.
• for hand mixing the water cement ratio should be 0.5 to 0.6
• for machine mixing the water cement ration should be 0.4 to 0.45
• steps in concreting: batching, mixing, transporting and placing, compacting
• the measurement of materials for making concrete is known as batching.
• the following methods of batching is practiced: volume batching, weight batching.
• in this method, cement, sand and concrete are batched by volume.
• volume batching is not ideal method of batching.
• Weight batching - this is the recommended method of batching.
• a weighting platform is used in the field to pick up correct proportion of sand and coarse aggregates.
• large weigh batching plants have automatic weighing equipments.
• Hand mixing - required amount of coarse aggregate for a batch is weighed and is spread on an impervious platform.
• The process of mixing in hand mixing is 6-8 minutes of adding water.
• in large and important works machine mixing is preferred.
• 4 to 5 rotations are made for dry mixing and then required quantity of cement is added and dry mixing is made with another 4 to 5 rotations.
• water is gradually added and drum is rotated for 2 to 3 minutes during which period it makes about 50 rotations.
• in the process of placing concrete, air is entrapped.
• in compaction of concrete. the entrapped air reduces the strength of concrete up to 30%
• hand compaction - in this method concrete is compacted by ramming, tamping, spading or by slicing with tools.
• compaction by vibrations - compacted by using high frequency vibrators.
• vibration reduces the friction between the particles and set the motion of particles.
• the use of vibrators reduces the compaction time.
• The following types of vibrators are commonly used in concreting: needle or immersion vibrators, surface vibrators, form or shutter vibrators, 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 and temperature conditions for freshly placed concrete for some specified time for proper hardening of concrete.
• the following curing methods are employed: spraying of water, covering the surface with wet gunny bags etc., ponding, steam curing and, application of curing compounds.
• spraying of water: walls, columns, plastered surfaces are cured by sprinkling water.
• wet covering the surface: columns and other vertical surfaces may be cured by covering the surfaces with wet gunny bags or straw.