Curing, admixtures, and aggregates Flashcards
What’s curing?
CURING = maintenance of a suitable environment at the surface of the concrete (moisture and temperature) for the first few days after casting.
What does environment need to be for curing?
Environment should be such that relative humidity (RH) within the capillary pores of the concrete does not reduce to less than a critical value until the capillary pores are essentially blocked.
- No significant hydration below 80% RH
- Hydration slow below 90% RH
What are consequences of poor curing?
Poor curing results in higher capillary porosity therefore:
- Reduced strength
- Reduced abrasion resistance
- More easily penetrated by chemicals in the environment
What’s wet curing?
Ensuring that the set concrete surface (when not liable to damage) is continuously in contact with water (= Wet curing) by :
spraying
flooding or ponding
covering with wet materials
Wet curing preferred for low w/c (<0.45) concretes
What methods of curing are there which aren’t wet curing?
Preventing loss of water from concrete surface by:
maintaining formwork in place
covering surface with impermeable material (laid close to surface)
Applying liquid curing membrane to surface
BS EN 197-1 CEM II/A-S 42.5N
What is CEMII, A, S, 42.5 AND N?
CEM II - type of cement,
A - Proportion of PC clinker, A-high to C-low
S - Second main constituent, K=Portland cement klinker
42.5 - strength class
N- Early strength, N for normal R for rapid
What are admixtures?
Materials (other than cement, additions, water and aggregates) that can be used as ingredients of paste, mortar or concrete added before or during mixing to modify the fresh and/or hardened state properties.
How long have admixtures been used, what are they made from and what’s dose rate?
Have been successfully used on a worldwide basis since the early 1930s
Are generally water-based liquids but can be powders
They are blends of proprietary chemicals formulated into consistent and stable products
By European definition the dose rate is ≤ 5% by weight of cement
What do superplasticisers do?
Cement is deflocculated increasing workability
Increase in workability makes concrete easier to compact without reducing strength
More surfaces of the cement grains are exposed to improve hydration
Water/cement may be reduced for the same wet state properties.
What do retarders do (admixture)?
Extended setting time for: long transportation periods delayed placing hot weather concreting elimination of cold joints in large pours
What do accelerators do (admixture)?
Set accelerators
give early setting
Strength (hardening) accelerators
give higher early strength
What are applications of accelerators?
Applications:
To reduce time to formwork removal for precast concrete
For urgent repair work
For cold weather concreting
What are air entraining agents? (AEAs)?
Admixtures entraining air in uniformly dispersed discrete cavities. They:
increase freeze-thaw resistance (provide space for ice expansion)
But:
they also reduce strength
What special purpose admixtures are there?
Special purpose admixtures
- Foaming agents (for trench fill) - Permeability reducers - Pumping aids - Admixtures for sprayed concrete - Admixtures for underwater concrete - Corrosion inhibitors
What are the functions of aggregates?
Cheaper than cement, requiring less energy in their production
Reduce the space to be filled with cement paste
- Should fit together in a concrete mix
- Normally comprise over 75% of concrete
Usually have higher volume stability & durability than cement paste
