Properties of Fresh Concrete Flashcards
What are the principles of placement of concrete?
–We cannot obtain the full value of well-designed concrete without using proper placing procedures.
–Good concrete placing and compacting techniques produce a tight bond between the paste and aggregates and fills the gaps completely.
–Both of these factors contribute to the full strength and best appearance of concrete.
What are the principles of compacting of concrete?
–Compaction tends to expel the unintended air bubbles which ensures the optimum density of the mix is achieved, increasing the compressive strength.
–Permeability of the concrete decreases, decreasing honeycombing entrapped air and segregation.
What is curing?
Curing is applying a protective layer over concrete to maintain satisfactory moisture content and temperature so that it may develop the desired strength and hardness.
Proper curing greatly increases the durability of concrete.
Correctly cured, concrete shrinks, cracks and dusts less, while gaining strength, durability and wear resistance.
What if inadequate curing?
Without an adequate supply of moisture, the Portland Cement in the concrete cannot chemically react to form a quality product.
Drying may remove the water needed for hydration, and the concrete will be weak (Up to 50%).
For outdoor concrete, temperature, relative humidity, and wind conditions are also important; they contribute to the rate of hydration process and moisture evaporation from the concrete
What are the methods of curing?
Wet curing: Curing concrete with water
Membrane curing: Curing concrete with membrane or plastic sheeting
Membranes that have UV protection are recommended as some membranes give concrete a yellow tint.
Wet curing is usually provided once concrete has set.
What is workability? What are the factors affecting it?
It is the property to be properly compacted, transported, placed and finished sufficiently easy without segregation.
The primary characteristics of workability are consistency (or fluidity) and cohesiveness. Consistency measures the ease of flow of fresh concrete, and cohesiveness describes the ability of fresh concrete to hold all ingredients together without segregation and bleeding.
Factors that affect workability are
water content
aggregate mix proportion
maximum aggregate size
aggregate properties
cement
admixtures
ambient temperature and time
What is segregation and bleeding? How to avoid bleeding?
Segregation is the separation of the constituents.
Bleeding is a form of segregation where some water in the mix tends to rise to the surface of freshly placed concrete.
It’s caused by the inability of the solid constituents to hold all the mixing water when they settle downwards.
So the top layer may become too wet and a porous and weak layer of non-durable concrete would form.
Increasing the percentage of fine materials in concrete mix can avoid bleeding to occur.
Learn briefly about why the factors of workability affect it.
Done?
What are the tests for workability?
Conventional methods commonly available:
Slump test, compacting factor test, Vebe test, flow table test, ball penetration test
Advanced methods:
Slump flow test, V-funnel test, U-box, L-box, J-ring, Fill box test and segregation test
In slump tests, what are the ranges for certain workability?
Very low: 0-25 mm
Low: 25 - 50 mm
Medium: 50 - 100 mm
High: 100 - 175 mm
What tests should be used for certain workability?
Very low - Vebe time
Low - Vebe time, compacting factor
Medium - Compacting factor, slump test
High - Compacting factor, slump test, flow table
Very high - Flow table
What is the process of fresh concrete production and placement?
Weighing
Dry mixing
Wet mixing
Delivery
Conformance checking
Placing in formwork
Compaction
Finishing
Curing
Removal of formwork
