Fresh concrete properties Flashcards
What is consistence in fresh concrete?
A measure of the stiffness/sloppiness or fluidity of a concrete mix, typically assessed using the slump test.
How is consistence tested?
Using the slump test, where the distance a concrete sample slumps after removal of a slump cone is measured.
What are the types of slump results?
True slump: Even subsidence (ideal).
Shear slump: One side slides down (partial collapse).
Collapse slump: Mix is too wet or lacks cohesion.
How can low slump be increased?
By adding water or plasticizers, but avoid excessive water to prevent segregation and strength loss.
What is cohesiveness?
The tendency of concrete to resist segregation and bleeding, ensuring uniform composition.
Define workability.
The ease with which concrete can be placed, compacted, and finished without segregation.
What factors affect workability?
Mix proportions (water, paste, cement, sand).
Aggregate characteristics (shape, size, grading).
Binder type (GGBS/FA improve; CSF reduces).
Admixtures (plasticizers, air-entrainers).
How does water content affect workability?
Too low: Harsh, unworkable mix.
Too high: Risk of segregation and bleeding.
What is plastic shrinkage?
Volume reduction due to water loss from fresh concrete before setting, leading to cracks if restrained.
How to control plastic shrinkage?
Pre-wet soil/subgrade.
Cure concrete (keep surface moist).
Use windbreaks/sunshades to reduce evaporation.
What causes settlement and bleeding?
Density differences: water rises (bleeding), while aggregates/cement sink (settlement).
What are the effects of bleeding?
Positive: Reduces plastic shrinkage cracks.
Negative: Weakens surface (higher w/c), creates voids.
How to control bleeding?
Increase fines (e.g., CSF, finer sand).
Reduce water content (use plasticizers).
Use air-entraining agents.
What is slump loss?
Reduction in consistence over time due to hydration/evaporation, making concrete stiff.
Should you add water to fix slump loss?
No! It alters water-cement ratio, weakening the concrete. Use plasticizers instead.
What determines water requirement in concrete?
Material properties, mix proportions, and desired consistence (slump).
