Civ 285 - Midterm 3 (My Notes)

Air Entraining Admixtures for Concrete

- Improve durability and workability in concrete exposed to freeze thaw

- ASTM C260

- Mechanical Reaction - works like washing your hands and creating a soap sud

How do air entraining admixtures help w freeze and thaw?

- Entrained bubbles act as relief valve reliving pressure developed b y the water expansion to ice

- The air bubbles give the concrete a place to expand into

Entrapped Vs Entrained Air

Entrapped - one large bubble - not durable and easily popped

Entrained - lots of well spaced tiny bubbles

How does entrained air air affect slump value?

For every 1% air you entrain you can see 25 mm of slump increase

How does air content affect compressive strength of concrete?

Increase in air content = decrease in strength

- balance between freeze thaw expansion vs. Strength

Testing of air entrained concrete (Air Content)

1. Pressure Method (Used in Lab)

2. Volumetric Method (accurate but involved)

3. Scale

- if you use 2 methods that line up you should be accurate

Water-Reducing Admixtures

- Reduce mixing water required to produce a certain slump (increases quality of concrete)

- Reduce water cement ratio (increasing compressive strength)

- Reduce cement content (Saving concrete producer money and lowering CO2 emissions)

- Increase Slump ( Making concrete more workable and easier to place)

- indicates that a higher strength concrete can be produced without increasing the amount of cement.

Is W/C or Slump a greater indicator of strength?

W/C - given the high range water reducers, slump is only an indicator of workability

Are Superplastacisers water reducers?

Yes - high range water reducers

Reduced W/C ratio produced concrete with:

- Increased early strength

- f'c > 70 MPa

How water reducers work (think diagram)

Release trapped water, and the individual "flocs" of concrete separate into grains which can easily spil past each other like ball bearings which improves the workability (electrostatic repulsion)

Viscosity Modifying Admixtures (VMA)

Think slump tower example (high viscosity slumps faster)

Admixtures which are used to:

- compensate for poor agg. Grading (specifically lack on fines in the sand)

- reduce bleeding in concrete

- used in conjunction with plasticizers to optimize yield point (which is the energy needed to get concrete to move)

controlled-density fill (CDF)

Compacted earth has low strength, Use CDF to avoid having to do multiple layers of compaction (time consuming)

retarders vs. Hydration controllers

- Retarders act by controlling the rate at which the cement hydrates (once the chemical wears off, it goes like a rocket):

- Pro: When it wears off concrete resumes setting like it was just poured

- Con: Heat rises exponentially so not good for mass pours

- hydration is still taking place

- used for mass pours where heat of hydration needs to be considered

- Hydration controllers coat cement particles preventing interaction with water molecules

- concrete maintains it's fresh qualities once the chemical wears off

Hydration Stabilization

- Gives total control over hydration process allowing concrete to be held without setting for an extended period of time

- Allows extended transport of concrete to distant jobs

- controls and slows down the hydration of concrete by coating cement particles and prevent hydration with water molecules.

How do you reactivate stabilized concrete (3 ways)

1. Add more cement

2. Increase internal heat

3. Addition of an accelerator

Accelerators

- Allows concrete to be used (sustain load) faster

- finishing faster, high early strength, can strip forms faster

- can place concrete in the cold (freeze protection - concrete can set before the water can freeze)

- CSA requires that concrete be delivered at a min. Of 10 C

Anti-washout admixtures

Liquid cellulose admixture developed for underwater applications

- A class of VMAs

- concrete will naturally set under water - trying to treat the internal viscosity of the material so it doesn't wash out (clouding)

Workability Retention Admixture

- Used when slumping retention is required without retardation

4 basic steps for cement production

1. Raw Materials (Ca + Si)

2. Blend and Heat in Kiln (150 C)

3. Grind Clinkers and add Gypsum

4. Cement storage and Transport

What are clinkers?

Gray pellets produced when raw materials are burnt in the kiln at high temperatures

Stages of Hydration

1. Rapid run stage - 1st 5 min (C3A)

2. Dormant Stage - 5 min - 2 hr (Can manipulate concrete)

3. Setting stage - 2-7 hr

4. Crystallization stage

W/C

- Unitless

- Values range from 0.3-0.65 usually 0.5

- hydration requires 0.4 W/C

W/C < 0.4

Excess cement which will not hydrate

W/C > 0.4

Excess water evaporates and leaves pores (weak)

How does cement fineness affect hydration rate

Dinner = more SA = higher hydration rate

- More SA = faster hydration

Setting Time of Cement

IST (Initial Setting Time) - 2 h

FST - 4 hr (cannot manipulateconcrete at this time)

Why do we add gypsum to cement

Added in small quantity but controls the setting time

Composition of Portland Cement