To determine the unit weight of fresh concrete.

Object: To determine the unit weight of fresh concrete.

Code:

ACI ASTM C138.

Apparatus:

• Concrete,
  
• Cylindrical mold/ container,
  
• Plate,
  
• tray and ramming rod,
  
• weight machine,
  
• bowls.
  

Theory:

Unit weight is the weight per unit volume of a material. It is usually measured in Unit weight of concrete is used to determine bulk density values used in selecting proportions for concrete mixes. Itis influenced by specific gravity, water content, air content, proportions of ingredients, w/c ratio.Change in density of concrete effect pumpability, placeability, finishability and strength of all types of concrete.

The unit weight is a useful tool in determining the concrete batch yield and air content. Since air adds no weight to the concrete and only occupies a volume, the unit weight of a concrete gives a very good indication of the air content of the concrete.

Unit weight =

Unit weight =

W1= weight of fresh concrete in container (mold).

W2 = weight of empty container /mold.

V = volume of concrete / container.

Volume of cylinder = (cft)

V= (3.14)*((1)

V= 0.196 cft

Procedure:

• Find the mass of the empty container ‘W2’.
  
• Place the container/mold on a flat, level and sturdy surface / tray.
  
• Rodding: Fill the container with 3 layers of concrete of an equal volume and rod each layer with 25 blows if the container is less than 0.5 in volume.
  
• When you are done consolidating the concrete, make sure that the container does not have an excess or deficiency of concrete. An excess of concrete sticking out about of an inch above the top of the mold is optimum.
  
• Strike off the top surface of the plate over the top of the container. Move it side to side while pulling it backwards off of the container.
  
• Clean the excess concrete off of the sides of the container with a rag or sponge and then weigh the container full of concrete.
  
• Now you can calculate the unit weight by dividing the weight of concrete by its volume.
  

Calculation:

Result:

The unit weight of fresh concrete is found to be