CONCRETE

• Concrete


  in construction, structural material consisting of a hard, chemically inert particulate substance, known as aggregate (usually sand and gravel), that is bonded together by cement and water. The lower the water content, all else being equal, the stronger the concrete.
• Concrete


  is characterized by the type of aggregate or cement used, by the specific qualities it manifests, or by the methods used to produce it. In ordinary structural concrete, the character of the concrete is largely determined by a water-to-cement ratio
• Concrete


  high in compression but weak in tension
• Concrete Tensile Strength


  8%-14% of its compressive strength
• Due to low tensile stress flexural cracks develop even at the early stage of the loading
• MATERIAL AND PREPARATION FOR CONCRETE


  Step 1 - Site Work

  Step 2 - Rebar and Form Works

  Step 3 - Mixing

  Step 4-Placement

  Step 5-Early Finishing

  Step 6 - Troweling

  Step 7 - Final Finishing

  Step 8- Curing
• Step 1 - Site Work


  Before concrete can be poured, the site needs to be prepared to reduce the chance of heaving from expansive soils and frost. On small projects, use hand to tools to clear the area of all grass, rocks, trees, shrubs, and old concrete, exposing bare earth. Earth moving equipment speeds up the process, especially for large pours. Then, place and compact a sub base of fill gravel, unless the soil is very compact and stable
• Step 2 - Rebar and Form Works

  Once the sub base is prepared, forms can be set. For residential concrete projects, use wood forms with metal or wood stakes. Attach forms to the stakes with screws or special nails to allow for easy removal after the concrete has cured. Forms should be in good condition, be set to provide the proper slope or grade for drainage, and form clean corners where they meet each other or other structures.
• Step 3 - Mixing


  If you're using bagged concrete purchased at a home center, mix the concrete with water according to the package instructions. For small slabs, you can use a wheel barrow and shovel, but renting a concrete mixer can make the process easier. If your concrete arrives in a ready mix truck, the drum on the back of the truck will be spinning to keep the concrete from settling and getting hard.
• Step 4-Placement


  Pour wet concrete into the forms until they are full to the top edge. While the wet concrete is being poured, use shovels, rakes and "come alongs" (special concrete rake) to move the concrete to make sure there are no voids or air pockets
• Step 5-Early Finishing


  Use a large metal or wood board to screed the top of the concrete. Screeding helps compact and consolidate the concrete, and begins the smoothing and leveling process. Next, use a float to further compact the concrete, even out any high or low areas, and create a smooth finish. Small hand-held floats are good for edges and detail work, large bull floats are best for working large areas. At the same time, work control joints and edges into the concrete with special hand tools.
• Step 6 - Troweling


  If the concrete will receive a rough broom finish, no additional finishing may be needed. If the concrete will be smooth toweled or stamped, a steel trowel finish is needed. Let the concrete rest until the surface begins to firm up. Once firm, use a steel trowel to create a smooth, hard and uniform finish.
• Step 7 - Final Finishing


  Once all the troweling (float or steel) is complete the final finish can be applied to the concrete. The most basic type of finish is known as a "broom finish". A special broom is pulled across the concrete surface creating a rough textured surface. Other types of finishes include stamped, textured, or smooth trowel to name a few.
• Step 8- Curing


  Let the concrete can rest and begin to cure (get hard). The curing process lasts 28 days, with the first 48 hours being the most critical. Apply a liquid chemical curing and sealing compound to help the concrete cure slowly and evenly, which helps reduce cracks, curling, and surface discoloration. You can start to use your concrete for light foot traffic 3 to 4 days after placement, and you can drive and park on your concrete 5 to 7 days after placement.
• CONCRETE ADMIXTURES


  • Various types of admixtures are used in concrete to enhance the performance of concrete.
  • Concrete admixture is defined as the material other than the aggregate, water and cement added to the concrete.
• Water Reducing Admixtures


  they are used to minimize the water demand in a concrete mix.
• Workability


  the important property of concrete which is improved with the addition of water but if water is added more than required the strength and durability properties of concrete gets affected
• Water reducing admixtures


  also called as plasticizers
• Water Reducing Admixtures (3 types)

  • plasticizers
  • mid-range plasticizers
  • super plasticizer
• Normal plasticizer


  reduces the water demand up to 10%
• mid-range plasticizers


  reduce the water demand up to 15%
• super plasticizers


  reduce the water demand up to 30%
• commonly used plasticizers.


  • Calcium
  • sodium
  • ammonium lignosulphonates
• new generation super plasticizers

  • acrylic polymer based
  • poly carboxylate
• Retarding Admixtures


  • slow down the rate of hydration of cement in its initial stage and increase the initial setting time of concrete
  • also called as retarders
  • used especially in high temperature zones where concrete will set quickly
• Accelerating Admixtures


  • reduce the initial setting time of concrete
  • they speed up the process of initial stage of hardening of concrete
  • also called as accelerators
  • these accelerators also improves the strength of concrete in it early stage by increasing the rate of hydration
• Air Entraining Concrete Admixture


  • one of the most important inventions in concrete technology.
  • primary function is to increase the durability of concrete under freezing and thawing conditions
  • When added to concrete mix, these admixtures will form millions of non- coalescing air bubbles throughout the mix and improves the properties of concrete.
• Pozzolanic Admixtures


  • used to prepare dense concrete mix which is bets suitable for water retaining structures like dams, reservoirs etc.
  • They also reduce the heat of hydration and thermal shrinkage.
• Damp-proofing Admixtures


  • water proofing admixtures
  • used to make the concrete structure impermeable against water and to prevent dampness on concrete surface
• water proof property


  • acts like accelerators in early stage of concrete hardening
• Alkali Aggregate Expansion Preventing Admixtures


  • is happened by the reaction of alkali of cement with the silica present in the aggregates
  • It forms a gel like substance
  • cause volumetric expansion of concrete which may lead to cracking and disintegration
• Anti-washout Admixtures


  • used in concrete especially for under water concrete structure.
  • protect the concrete mix from being washed out under water pressure
  • improves the cohesiveness of concrete
  • This type of admixtures are prepared from natural or synthetic rubbers, cellulose based thickeners etc.
• Grouting Admixtures


  • added to grout materials to improve the grout properties according to the requirement of grout
  • there is a need of quick set grout and sometimes there is a need of slow set grout to spread into deep cracks or fissures
• Corrosion Preventing Admixtures


  • is general and it is severe when the structure is exposed to saline water, industrial fumes, chlorides etc.
  • To prevent or to slow down the process of corrosion preventing admixtures are used.
• Bonding Admixtures


  • used to create a bond between old and fresh concrete surfaces.
• In general, if fresh concrete is poured over a hardened concrete surface, there is a chance of failure of fresh concrete surface due to weak bond with old surface.
• Bonding Admixtures


  In general, if fresh concrete is poured over a hardened concrete surface, there is a chance of failure of fresh concrete surface due to weak bond with old surface.
• Fungicidal, Germicidal, Insecticidal Admixtures


  • To prevent the growth of bacteria, germs, fungus on hardened concrete structures, it is recommended that the mix should have fungicidal, germicidal and insecticidal properties.
  • This properties can be developed by adding admixtures like polyhalogenated phenols, copper compounds and dieledren emulsions etc.
• Coloring Admixtures


  • the pigments which produce color in the finished concrete
• Coloring Admixtures


  • used to produce color should not affect the concrete strength
  • Generally coloring admixtures are added to cement in a ball mill, then colored cement can be obtained which can be used for making colored concrete. Some of the coloring admixtures and their resultant colors are tabulated below