Lec 8 FINAL Flashcards
Tests for Other Important Properties of Fresh Concrete
- Setting Time Test
- Air Content Test
- Unit Weight Test
Setting Time Test
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- The coarse aggregate should be scooped out
- Press the metal probe into the mortar (required 25,4 mm deep), convert to Penetration Resistance
- The initial and Final setting times -> load bearing 3,5 MPa to 27,6 MPa or 500 psi to 4000 psi
- 3,5 MPa -> ability to recover its original plasticity under no vibration 27,6 Mpa -> concrete reached compression resistance about 700 kPa strength
Air Content Test
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- Using the Pressure Method
- Principle: Boyles Law (PV = nRT)
- most commonly used
Unit Weight Test
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- Almost the same as slump test
- Filling concrete into a container then measuring its weight
- Can estimate air content and the correctness of the ratio
Main Concrete Construction
- Concrete materials
- Concrete mix ratio
Concrete Mixing (ideal time, mixing time estimation)
- Ideal Time depends: type and performance of the mixer (volume and rotation speed)
- Less paste, dry or coarse mixes require longer mixing time
- Mixing volume of less than 1 m^3 -> at least 1 minute, each additional 0,8 m^3 -> added 15 seconds (CNS 3090)
- High performance concrete with low water ratio -> more than 2 minutes
Placing (ready-mixed concrete, segregation minimization, concrete mixture layers)
- the ready-mixed concrete should be placed as near as possible to its final position
- To minimize segregation: concrete should not be moved a too long distance
- Concrete mixture is placed in horizontal layers of uniform thickness and each layer is compacted.
Placing (common things about placing)
- there must be no interruption of concrete process
- Placing: pouring concrete, compacting, and levelling.
- the area of the process take must be clean
Placing (new and old concrete)
- before placing concrete, there must be a layer of cement mortar about 1,5 to 2,5 cm at the bottom (with the same w/c as concrete) and slump 15 to 20 cm
- using high pressure flushing or sand blasting to treat the contact surface, then pour the new concrete.
Curing is protection (what does this mean?)
Protection from moisture loss from soon placing, and for the first few days of hardening
Eliminate plastic shrinkage cracking and ensure adequate supply of water for hydration and strength gain
Curing (what does it provide? for how long? what to do at the last step?)
- Provide proper humidity and temperature during strength growth to ensure continuous cement hydration
- at 90 days the concrete strength is 100 % with 90% complete the hydration reaction
- After placing, concrete must be maintained at more than 10 C in humid for more than 7 days
Curing is to develop desired properties
compressive strength, water permeability, durability
https://ibb.co/dkvqWLS
compressive strength vs age concrete (curing)
1 and 2 Moisture
3 and 4 Air Dry
Curing methods
- spraying the surface
- use windbreaks and sunshades
- covering surface with wet hessian or polythene sheets
- applying curing membrane to exposed surface and prevent moisture loss
Types of curing
- Moisture curing
- Membrane Curing
- Steam curing
- Sprinkling Curing
Moist curing (3 methods)
- ponding method -> effectively control the plastic shrinkage and dry shrinkage cracks, water can control the cracking and continue to deteriorate, good for strengthening hydration and promoting strength development. BUT for high-demand labor
- watering method -> provides moisture, reducing temperature of hydration, prevents rapid evaporation (loss of water). BUT the surface must be kept wet and someone has to keep go.
- covering method -> to continuously deteriorate plastic and dry shrinkage, control the cracks, easy to interfere with subsequent levelling. BUT concrete surface will be rough and easy to contaminate.
Membrane curing
- use chemical liquid spraying in the surface to prevent evaporation
-> easy to spray, good adhesion, suitable for structures that are difficult to wet. BUT, can only prevent water loss and not provide water, too early or late spraying will result to poor end, many brand of curing agents, spend more time for large project
Membrane curing
- use chemical liquid spraying in the surface to prevent evaporation
-> easy to spray, good adhesion, suitable for structures that are difficult to wet. BUT, can only prevent water loss and not provide water, too early or late spraying will result to poor end, many brand of curing agents, spend more time for large project
Steam curing
- for concrete which needs early strength to promote the hydration
-> one day curing = 28 days curing of other methods, dry shrinkage is 1/2 of moist curing methods. BUT, reducing the bonding of concrete and steel, must control the temperature steam, high cost, high temp. stimulates the initial hydration reaction and the hydration distribution is poor, large pores give no conductive to strength and durablity
Sprinkling curing
- general curing methods
- has lost best time to eliminate plastic skhrinkage, cannot effectively suppress cracks
- if curing carried out during puring process -> inconvenient and surface will be rough
- if the sprinkling curing used to distribute the spray over the entire working plane then it will better to suppress cracks.
Water evaporation from concrete surface
- depends on: temperature, humidity, wind speed, atmospheric temperature
- 16 to 32 C gives evaporation rate accelerated by nearly 300%
- not controlled evaporation gives less suppressible cracks of plastic shrinkage
What are the requirements of fresh concrete
- easy to mix and transport, good plasticity
- good fluidity, easy-to-fill template
- no bleeding and segregation
- proper setting time
- no excessive volume change
