Concrete Uniformity and Quality Control

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Questions and Answers

What is the minimum slump value required for a concrete to be considered dry-plastic?

10 cm

5 cm

2.5 cm (correct)

7.5 cm

Which of the following is NOT a reason for segregation in concrete?

Large Dmax (>25mm)

High amount of fine particles (correct)

Differences in specific gravity and size of constituents

Improper mixing

What is the purpose of the Ve-Be test in concrete?

To determine the setting time of concrete

To determine the consistency of concrete with very low workability (correct)

To determine the bleeding characteristics of concrete

To determine the compressive strength of concrete

What is the effect of segregation on the quality of concrete in a structure?

It decreases the quality of the concrete in different locations Signup and view all the answers

What is the range of slump values for a flowing concrete?

15-17.5 cm Signup and view all the answers

What is the term for the separation of the components of fresh concrete?

Segregation Signup and view all the answers

What is the purpose of the slump cone in the slump test?

To measure the consistency of concrete Signup and view all the answers

What is the range of Ve-Be time for a very dry concrete?

18-32 seconds Signup and view all the answers

What is the primary objective of obtaining uniformity in concrete?

To ensure similar quality of concrete samples Signup and view all the answers

Which of the following tests is NOT used to check the uniformity of concrete?

Setting time test Signup and view all the answers

What is the primary purpose of proper curing in concrete?

To start immediately after finishing concrete Signup and view all the answers

What is the primary consideration for selecting materials in concrete production?

Desired performance of concrete Signup and view all the answers

What is the primary purpose of proper mix-design in concrete?

To ensure the desired performance of concrete Signup and view all the answers

What is the primary consideration for selecting transportation methods in concrete production?

Structure and concrete type Signup and view all the answers

What is the primary purpose of proper consolidation in concrete?

To prevent segregation in concrete Signup and view all the answers

What is the primary purpose of proper finishing in concrete?

To provide a smooth surface finish Signup and view all the answers

What is the primary reason for the top portion of concrete becoming weak and porous due to bleeding?

Water rising to the surface carries fine particles of cement Signup and view all the answers

How can the rate of hydration be increased to decrease bleeding in concrete?

By increasing the fineness of cement Signup and view all the answers

What is the primary purpose of determining the setting time of concrete?

To place and consolidate fresh concrete before it gains rigidity Signup and view all the answers

What is the unit weight of conventional concrete?

2400 kg/m³ Signup and view all the answers

What is the primary purpose of determining the unit weight of fresh concrete?

To check the uniformity of fresh concrete Signup and view all the answers

At what penetration strength (PS) does the initial setting time of concrete occur?

35 kgf/cm² Signup and view all the answers

What is the primary effect of bleeding on the top portion of concrete?

It becomes weaker and more porous Signup and view all the answers

What is the final setting time of concrete determined by?

PS = 280 kgf/cm² Signup and view all the answers

Study Notes

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

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Description

This quiz covers the concept of uniformity in concrete, including within batch and batch-to-batch uniformity, and the importance of proper operations for obtaining desired concrete properties.