Manufacturing process of Concrete blocks

A concrete block is one among several precast concrete products used in construction. They are primarily used in the construction of walls.

Mixing, molding, curing and cubing  are the four basic processes involved in the manufacture of cement

Stage 1 - Mixing:

  • Sand and gravel are kept outside in piles and are transferred into storage bins in the plant by a conveyor belt as it is necessary. The portland cement is stored and kept outside in large vertical silos to protect it from moisture.
  • The production starts and the required amounts of sand, gravel, and cement are transferred by gravity or by mechanical means to a weigh batcher that measures the proper quantity of each material.
  • For several minutes, dry materials flowing into a stationary mixer are blended together. Two types of mixers are frequently used. Planetary or pan mixer resembling a shallow pan with a lid is one type and Horizontal drum mixer resembling a coffee can be turned on its side, having mixing blades attached to a horizontal rotating shaft inside the mixer is the other type.
  • A small amount of water is added to the mixer when the dry materials are blended. Colouring pigments and admixture chemicals may also be added for this time. The concrete is then mixed for almost ten minutes.

Stage 2 - Molding:

  • Once the mixing is done, it is dumped into an inclined bucket conveyor and transported to an elevated hopper. Again the mixing cycle begins for the next load.
  • On top of the block machine, the concrete is conveyed to another hopper at a measured flow rate. The concrete in the block machine is forced downward into molds. The molds contain an outer mold box consisting of several mold liners. The outer shape and inner shape of the block cavities is determined by the liners. Fifteen blocks may be molded at once.
  • The concrete is compacted by the weight of the upper mold head coming down on the mold cavities only when the molds are full. Hydraulic pressure cylinders may be supplemented by air whereas acting on the mold head. For further aid compaction, most block machines use a short burst of mechanical vibration.
  • The compacted blocks are moved down and out of the molds onto a flat steel pallet. The blocks and pallet are pushed out of the machine into a chain conveyor. The blocks in some operations pass under a rotating brush which removes loose material from the top of the blocks.

Stage 3 - Curing:

  • The pallet of blocks is conveyed to a loader which keeps them in a curing rack. Several hundred books are there in each rack. And it is rolled into a set of rails and moved into a curing kiln only when the rack is full.
  • The kiln is an enclosed room that has the capacity to hold several racks of blocks at once. The two basic types of curing kilns are low-pressure steam kiln and high-pressure steam kiln. In, low-pressure steam kiln, the blocks are held for one to three hours at room temperature to allow them to harden slightly in the kiln. At a controlled rate of 60°F per hour (16°C per hour). The steam is shut off and the blocks are allowed to soak in the hot moist air for 12-18 hours when the curing temperature has been reached. Then once the soaking is done the blocks are dried by exhausting the moist air and raising the temperature in the kiln. The whole curing cycle takes about 24 hours.
  • In the high-pressure steam kiln that is also called as Autoclave. In this type, the temperature is raised to 300-375°F (149-191°C), and the pressure is raised to 80-185 psi (5.5-12.8 bar). The blocks are soaked for five to ten hours. The pressure is then quickly vented, that causes the blocks to quickly release their trapped moisture. The autoclave curing process needs more energy and a high expensive kiln, that can produce blocks in less time.

Stage 4 - Cubing:

  • The pallet of blocks are unstacked and the racks of cured blocks are rolled out of the kiln that is placed on a chain conveyor. The empty pallets are fed back into the block machine when the blocks are pushed off the steel pallets to receive a new set of molded blocks.
  • When the blocks are to be made into split -face blocks, they are first molded as two blocks joined together. If these double blocks are cured, they pass through a splitter, that strikes them with the help of a heavy blade across the section between the two halves. This makes the double block to fracture and form a rough stone-like texture on one side of each piece.
  • The blocks pass between a cuber that aligns every block and that stacks them into a cube three blocks along by six blocks deep by three or four blocks that are high. Thus, cubes are moved outside with a forklift and placed in storage.

Advantages of concrete blocks:

  • The building’s carpet area will be increased due to the small width of concrete block as compared to the brick masonry wall.
  • It gives better thermal insulation, improved fire resistance and sound absorption.
  • Saving of precious agricultural land will be its outcome that is used for manufacturing bricks.
  • Compared to masonry brick the construction of concrete block masonry is easier, faster and stronger.
  • Concrete block has perfect shape and size that makes the work of a mason much easier.
  • The blocks can be prepared by making the vertical joints staggered automatically by reducing skilled supervision.
  • The Concrete block masonry method of construction can be adopted and can be reached widely.

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