class 8 - early age and mechanical properties of concrete Flashcards
What is meant by the ‘early age’ of concrete?
The period from batching and mixing until formwork removal and completion of curing
Why is the early age of concrete important despite being short?
Several key processes (workability, curing, compaction, etc.) occur that significantly affect long-term performance.
Define workability in concrete.
The ability of fresh concrete to be mixed, transported, placed, compacted, and finished without excessive segregation or bleeding.
What factors influence concrete workability?
Cement content, water content, aggregate shape/size, admixtures, environmental conditions, mixing method, placement, etc.
What is batching?
The process of measuring and adding ingredients to the mixer, usually by weight.
What are common mixing methods for concrete?
Drum mixers, pan mixers, and central batching plants
Why must concrete be placed near its final position?
To prevent segregation and maintain uniformity.
How is compaction achieved, and why is it important?
Using mechanical vibration to remove entrapped air, improve density, and enhance strength
What are common surface finishing methods for concrete?
Troweling, broom finishing, power floating, etc., depending on application and durability requirements
Why are workability tests needed?
Workability is subjective; tests help assess consistency, compactability, and flowability.
What are the two most common site tests for workability?
Slump test and flow table test.
What does a high slump indicate?
A more workable, flowable concrete mix.
What are limitations of the slump test?
It does not capture all workability aspects like cohesion or segregation resistance.
What is the purpose of the flow table test?
A flow table test machine is a device used to determine the flow characteristics of cementitious materials, such as concrete or mortar
What is the slump test used for?
The concrete slump test measures the consistency of fresh concrete before it sets. It is performed to check the workability of freshly made concrete, and therefore the ease with which concrete flows. It can also be used as an indicator of an improperly mixed batch.
What are some examples of innovative concrete types?
Self-compacting concrete, roller-compacted concrete, sprayed concrete, 3D-printed concrete, underwater concrete.
How is self-compacting concrete different
It flows under its own weight without the need for vibration.
Define curing and explain its importance.
Curing is maintaining moisture and temperature to promote hydration and strength development.
What happens if concrete is not cured properly?
Reduced strength, durability, increased cracking risk.
Name common early-age problems in concrete.
Poor compaction, segregation, bleeding, plastic settlement, plastic shrinkage cracking.
What causes plastic shrinkage cracks?
Rapid moisture loss from the surface before setting
What is the most commonly tested mechanical property of concrete?
Compressive strength.
How is compressive strength typically measured?
Using cube or cylindrical samples tested at 28 days.
Why is cylinder compressive strength lower than cube strength?
Due to end restraint and size effects (~20% lower).
