Lesson 7 - Cement & Concrete Flashcards
What is batching of concrete?
- Measuring correct proportions of components and placing in the mixer
- By weight is more accurate because air voids do not matter
Points on mixing of concrete?
- Until uniform appearance
- Usually batch mixers (one at a time) but sometimes continuous (conveyors automatically feed components into mixer)
- Start with 10% of water in mixer, then add solids with 80% of the water, and then remaining water
Types of ready-mixed concrete?
- Central-mixed: In a central plant and delivered in
agitator truck - Shrink-mixed: Partially mixed in plant and delivered
in mixer truck - Truck-mixed: Mixed completely in a mixer truck
Process of testing and sampling?
- Pull samples at the job site
- Test on site e.g slump test, air content test.
- Prepare samples for later testing e.g cylinders + beams.
What does C40/30 mean?
C: normal weight concrete (LC = lightweight)
40: Cylinder strength
30: cube strength
Cube strength is roughly 1.25x bigger than cylinder strength.
When is a slip form paver used?
- Used to consolidate, form into geometric shape and finish by pulling the forms continuously through.
- Most appropriate for larger jobs that require high production rates
What is screeding?
A form of finishing of concrete; when you strike concrete off to desired level.
What is bullfloating?
- It eliminates high and low spots and embeds large aggregate particles immediately after strikeoff
Why does concrete need to be cured?
- Hydration stops as internal relative humidity drops below 80%. Hydration resumes if curing stopped and resumed.
- To develop its optimum properties: Increased maturity, Reduced capillary porosity, Increased strength and durability
- To minimise volume changes in concrete due to shrinkage
- Maintain moisture and temperature in the concrete to promote continued hydration and strength gain.
What are the parameters affecting curing? and what are the effects?
Interrupted Curing:
- Re-saturated concrete will resume its interrupted hydration.
- Strength developed is not as high as it would have been if moist curing had not been interrupted.
Relative humidity
- When internal RH falls below 80%, hydration will cease altogether.
Temperature
- Higher temperature = increased early strength, but lower ultimate strength.
Points on the duration of moist curing
- 7 days of moist curing for most structural concrete is necessary to attain 70% of the specified strength.
- For unreinforced mass concrete, minimum curing times should be longer: Two weeks, or three weeks if a pozzolan is used
- Longer times allow for slower strength development of a low heat meets and of the cement-pozzolan reaction.
What are the three curing approaches from best to worst?
Water curing -> Sealed curing -> Heat & additional moisture
When should water curing start? And what are teh 3 methods of water curing?
- Should scary about 1 hour preceding the time of initial set
Three methods: - Ponding: Smaller jobs, flat work and lab work
- Spraying/fogging: Expensive and lots of water
- Wet coverings: Burlap, cotton, rugs etc.
What does sealed curing do? And how does it work
- Prevents moisture loss only
- Uses impervious paper or plastic sheets
- Membrane forming compounds
- Leave forms in place
Points on heat curing
Insulate ->
Steam (good for early strength gain and in freezing weather) ->
Heating (heating coils, electrical heated forms or pads) (usually in precast plants only)
What is the maturity concept and how to calculate it?
- Samples of a concrete mixture of the same maturity will have similar strength, regardless of the combination of time & temperature yielding the maturity.
- Maturity = T x (change in time)
Where T = temperature above the datum (-10C)
What is steam curing and what are its advantages?
- Curing in live steam at atmospheric pressure
- Increases rate of strength development
- Used primarily in precast concrete.
Advantages:- Rapid turnover of moulds and framework
- Shorter curing periods before shipment
- Less damage during handling
What are the properties of concrete by steam curing?
- Chemistry of hydration does not change
- Increased early strength due to rapid hydration in early stage
- Higher rate of hydration will lead to lower ultimate strength
- Decrease in ultimate strength due to a less uniform distribution of products
What is autoclaving?
- High pressure steam curing
- Only used for precast concrete
- Pros: Products ready for use within 24h with equivalent 28 day strength under normal curing
- Curing cycle similar to low pressure steam curing
What considerations need to be taken into account when deciding on method of curing?
- Availability of curing materials
- Size and shape of structure
- Production facilities (in-place or precast)
- Aesthetic appearance
- Economics
What problems do hot weather make for concrete?
- Increases the water requirement to maintain a given slump
- Accelerated setting decreases the time to place, compact and finish
- Increases the danger of plastic shrinkage
- Lowers the ultimate strength
What is the optimum and maximum range of concrete temps?
Optimum: 10 - 15C
Maximum: 30 - 33C
