Use LEFT and RIGHT arrow keys to navigate between flashcards;
Use UP and DOWN arrow keys to flip the card;
H to show hint;
A reads text to speech;
19 Cards in this Set
- Front
- Back
|
What are the important properties of fresh concrete |
• Workability • Consistency • Cohesiveness • Segregation and bleeding • Time of set and hardening |
|
|
What is workability and why is it important |
Is the property of concrete to be properly compacted, transported, placed and finished sufficiently without segregation and minimal voids
· Long term properties are significantly affected by its degree of compaction therefore workability of concrete must be such that is able to be properly compacted |
|
|
What factors affect workability of concrete |
• Water content • Aggregate type and grading • Aggregate/cement ratio • Presence of admixture • Fineness of cement |
|
|
Effect of W/C on workability |
Increasing water content increases workability as the interparticle lubrication is enhanced, compressive strength decreases and increases shrinkage cacking |
|
|
Abraham’s law, what effect does adding more water have on the compressive strength |
Water to cement ratio x axis, compressive strength y axis. Shows the relationship that higher water to cement ratio results. in a lower compressive strength. |
|
|
Aggregate type and grading effect on workability |
- Particle size increases = greater workability. Finer particles require more water due to larger surface area - Better graded aggregates = greater workability. Well graded aggregates tend to fill up voids and easily get workability - Rounded/smooth aggregates = greater workability, Less friction resistance |
|
|
Aggregate/cement ratio effect on workability |
For a constant water to cement ratio: - Greater workability when aggregate to cement ratio is reduced - Amount of water to total surface of solids increased |
|
|
Presence of admixture effect on workability
|
- Water reducing admixture such as superplasticizer can increase the workability even in a low water/cement ratio - Air entraining admixtures are also used to increase the workability by reducing the friction between aggregates; this phenomenon is achieved by the presence of air bubbles in the mixture which acts as ball bearing between the aggregates. |
|
|
Fineness of cement effect on workability |
Finer cement particles require more water for same workability than the comparatively les fine cemen |
|
|
Consistency |
Defined as the ease with which concrete will flow or the degree of concrete wetness Higher slump value = more wetness = greater workability = more chance of segregation |
|
|
Cohesiveness |
The ability of fresh concrete to hold all ingredients together without segregation and excessive bleeding during handling, placing and compacting |
|
|
Types of segregation |
· Separation of coarse aggregates from cement paste (Honeycomb defects) · Migration of water to the surface (concrete bleeding) |
|
|
What does it mean by initial and final setting of concrete |
Initial setting: · Refers to loss of consistency and at this point concrete is unworkable · Indicates approximate time limit for handing and placing the concrete Final setting · Concrete completely loses its plasticity |
|
|
Compressive strength |
Ability to resist crushing loads |
|
|
Factors affecting compressive strength |
• Water to cement ratio • Void content • Curing condition • Cement type |
|
|
Effect of voids on compressive strength |
Higher amount of air voids decreases the strength of concrete |
|
|
Effect of curing on compressive strength |
Curing has a significant effect on degree of cement hydration. Moisture must be present for a sufficient time to allow the reaction to proceed and full strength to develop |
|
|
Cement type effect on compressive strength |
GB (General blended cement) has lower strength than GP (General purpose cement at early stages, but it has higher strength for longer term. |
|
|
What is durability and the factors effecting it |
The ability of concrete to achieve the designed service life without excess deterioration. · Permeability Concrete resistance to the passage of gasses or liquids through it · Absorptivity · Measure of the amount of water the concrete will ‘soak up’ when immersed in water |
