Concrete Uniformity and Quality Control

Uniformity

• Uniformity refers to the similarity of quality of concrete samples from the same batch (within batch uniformity) or different batches (batch-to-batch uniformity)
• It can be checked by slump test, air content test, unit weight test, and compressive strength test

Obtaining Good Concrete

• Desired properties of hardened concrete can be obtained by:
  • Proper mix-design
  • Proper production of concrete
  • Proper transportation of fresh concrete
  • Proper placing of fresh concrete
  • Proper consolidation of fresh concrete
  • Proper finishing of fresh concrete
  • Proper curing

Proper Mix-Design

• Materials must satisfy their standard specifications
• Materials must be selected considering job-site conditions
• Proportions of materials must be calculated considering desired performance (workability, strength, and durability)

Proper Production

• Proper storage of materials
• Proper weighing of materials
• Proper mixing

Proper Transportation

• Selection of transportation methods considering:
  • Structure and concrete type
  • Job-site conditions (geography, temperature, etc.)
  • Distance of transportation
  • Cost

Proper Placing

• Selection of placing methods considering workability
• High consistency is necessary for workability
• Low consistency is necessary for workability

Consistency

• Most widely used test methods:
  • Slump test (conventional concrete)
  • Ve-Be test (for low consistency)
  • Slump flow test (for high consistency)
• Slump test:
  • Fill the slump cone in 3 layers
  • Every layer is evenly rodded 25 times
  • The cone is removed vertically
  • Slump value: the vertical distance between the top of the mold and the displaced original center of the top surface of the specimen
• Ve-Be test:
  • Fill the slump cone as described previously
  • Remove cone and place the plastic disk on the concrete
  • Start the vibrator and stop-watch simultaneously
  • Stop the watch when the bottom of the disk is covered with paste
  • The time is called Ve-Be time
• Consistency comparison:
  • Slump (cm) vs Ve-Be time

Segregation

• Segregation refers to a separation of the components of fresh concrete, resulting in a non-uniform mix
• Causes of segregation:
  • Differences in specific gravity and size of constituents of concrete
  • Improper mixing, improper placing, and improper consolidation
  • Large Dmax (>25mm) and high amount of large particles
  • Low amount of fine particles
  • Very low or very high amount of water

Bleeding

• Bleeding is a form of segregation in which some of the water in the mix tends to rise to the surface of freshly placed concrete
• Undesirable effects of bleeding:
  • The top portion becomes weak and porous
  • Water rising to the surface carries fine particles of cement, weakening the top portion and forming laitance
  • Water may accumulate under the coarse aggregate and reinforcement
• Bleeding can be decreased by:
  • Increasing the fineness of cement
  • Increasing the rate of hydration (C3S, C3A)
  • Adding pozzolans
  • Reducing water content

Setting Time

• Setting time is the duration starting from the mixing of the materials until the moment that the concrete gains rigidity
• Importance of setting time:
  • Fresh concrete has to be placed and consolidated before setting time
• Test method:
  • Sieve the concrete through No.4 sieve
  • Penetrate the needle to the mortar by 2.5 cm and determine the necessary force
  • Calculate the penetration strength (PS) by dividing the force to the needle area
  • Draw a PS vs. time curve
• Setting time:
  • Initial setting time occurs when PS=35 kgf/cm2 (3.5 MPa)
  • Final setting time occurs when PS=280 kgf/cm2 (28 MPa)

Unit Weight

• Unit weight is the weight of fresh concrete per unit volume
• The unit weight of conventional concrete is around 2400 kg/m3
• It is generally used to check the uniformity of fresh concrete
• Test method:
  • Fill a container with concrete
  • Determine the weight of the concrete
  • Weight of concrete divided by the volume of the container is the unit weight