4B

• Tests on Concrete:

  1. Slump Test
  2. Compaction Factor Test
  3. Crushing Strength Test
• Slump Test is conducted to determine the workability of concrete.
• Slump Cone is a vessel in the shape of a frustum of a cone with diameter at bottom 200mm and 50mm at top and 300mm high. This cone is kept over an impervious platform and is filled with concrete in four layers. Each layer is tamped with a 16mm pointed rod for 25times.
• Slump - The decrease in the height of the concrete
• Compaction Factor Test is conducted on a concrete mix to check the ratio of its partially compacted weight to the fully compacted weight. The test equipment consists of two hoppers and a cylinder fixed to a stand.
• The ratioW1/W2, is termed as the compaction factor.
• Crushing Strength Test Also known as compressive strength test, is a common laboratory procedure used to determine the ability of a material to withstand axial compressive loads without failure. Tests on Concrete Metallic molds of size 150mm x 150mm x 150mm are used for casting concrete cubes
• The crushing load is noted and crushing strength is found as load divided by surface area.
• After 28 days of curing, cubes are tested in the compression testing machine
• Desirable Properties of Concrete:

  • Desired Workability
  • No Segregation in Transporting and Placing
  • No Bleeding
  • No harshness
• Hardened Concrete should have:

  • Required characteristic strength
  • Minimum dimensional changes
  • Good durability Impermeable
  • Good resistance to wear and tear
• Uses of Concrete:

  • As bed concrete below column footings, wall footings, on wall at supports to beams.
  • As Sill Concrete
  • Over the parapet walls as coping concrete
  • For flagging the area around buildings
  • For Pavements
  • For Making Building Blocks
• Compaction of the concrete is the process to get rid of the entrapped air and voids, elimination of segregation occurred and to form a homogeneous dense mass.
• Hand Compaction is used for small and unimportant jobs. However, this method if extremely useful for thin elements. It is achieved by rodding, ramming, or tamping.
• Rodding is done with the help of 16 mm diameter, 2 m long steel rod to pack the concrete between the reinforcement, sharp corners and edges.
• Ramming is permitted only for unreinforced concrete constructions. The roof and floor slabs are usually tamped for achieving compaction. The tampers are 100 x 100 mm in section and about 1 m long.
• Tamping bars not only compact the concrete but also level the top surface. The limitation of this method is that a large water-cement ratio is required for full compaction.
• COMPACTION BY VIBRATION is the most common method of compacting concrete for any structural element.
• Vibrations do not affect the strength of the concrete but concrete of higher strength and better quality can be made with less water and given cement content.
• Various Types of Vibrators:

  • Needle Vibrator
  • Formwork Vibrator
  • Surface Vibrator
• Needle Vibrator can be used for any type of concrete. Consists of a steel tube (poker), a flexible shaft, and a power unit. Needle diameter: 20-75 mm Length: 250-900 mm.
• Formwork Vibrator is used for concreting columns, thin walls, and precast units.
• Vibrating Table is a special case of formwork vibrator.
• Surface Vibrator is used for floor and roof slabs and pavement surfaces
• Compaction by Spinning also known as centrifugation, is used for producing circular elements such as pipes.
• Compaction by Jolting is used in precast units such as hollow blocks, cavity blocks, etc.
• Jolting is a vibrating action of low frequency and high amplitude.
• Compaction by Rolling is used to manufacture precast concrete products such as tiles.