Early age and mechanical properties Flashcards
What is meant by the term ‘early age’ of concrete?
The time between batching and formwork removal when curing has completed.
It includes; mixing, transporting, placing, compacting, finishing, curing
Approximately how long does the ‘early age’ last?
~12 hrs for high early age strength concrete
~24 hrs for normal strength concrete
~48 hrs for concretes with high PFA (pulverised fly ash) or GGBS (ground granulated blast furnace slag)
Why is the early age so important?
The operations performed withing this time period can have a huge impact on the long term properties - these can have an economic impact
What is the definition of workability?
The ability to be easily mixed, transported, placed, compacted and finsihed using available equipment without seggregation and bleeding
What is batching?
The operation of measuring ingredients an putting them into the mixer
How are the materials involved in batching ususally measured?
weight (volume is used very occasionally)
What is mixing?
Rotating and stirring using a drum, pan or central batching plant
What is the objective of mixing?
To coat all the aggregates with a uniform homogeneous cement paste
What is important to check when batching?
Aggregate moisture content, on and off site quality
How long after mixing is completed should you place the concrete?
withing 1.5 hrs
How long does mixing usually take?
between 1 and 3 minuts dependent on the mixer, and longer if specialist equipment is available
Where should cocnrete be placed?
As near as possible to its final mixing position (to prevent segregation)
When placing a deep section how should you place the concrete?
In layers, each layer should be placed while the lower level is still plastic.
How is segregation controlled?
Using a down pipe, flexible chute or tremie
How do we compact concrete?
Using mechanical concrete
Why do we compact concrete?
To remove entrapped air. It decreases the internal particle friction, making the concrete more fluid. This helps with the packing of coarse aggregate
N.B. the difference between entrapped and entrained air
Why do we finish concrete?
To produce a flat dense surface. This helps to increase resistance to impact and abrasion. It als improves aesthetics and can better other properties such as skid resistance.
What are the key factors of rheology of fresh concrete?
- Consistency: resistance to flow, ability to flow into formwork and around rebars
- stability; behaviour when flowng, ability to remain homogeneous without bleeding or segregation
- compactibility
- finsihability
How to we test workability (rheology)?
Slump test (mainly), flow table test etc.
What is the main equipment for a slump test?
Tamping rod and slump cone
What are the different types of slump?
True slump, slump remains fairly in shape
Shear slump, slump deforms and test needs to be redone
How do you measure the consistency of the concrete in a slump test?
The heigh difference between the slump cone and the highest point of the slumped concrete in mm
What are the weaknesses of the slump test?
It is more sensitive for mixes with medium and high workability and is heavily operator dependent.
It also does not measure compactability
Why do we use the flow table test?
For flowing concrete that contains superplasticizer, also works for concrete with very high workability
What material is involved in the flow table test?
Slump cone, hinged steel plate board
How do you measure the workability?
flow = (A+B)/2 where A is the diammeter of the slump in one direction and B in the other
What flow (mm) shows high workability?
400-500 mm
What flow (mm) shows very high workability?
500-65mm
What are a couple advantages of the flow table test?
It indicates mix cohesivness and segregation
What are some innovations from rheology?
Roller compacted (0 slump) concrete, pumped concrete, self-compacting concrete, sprayed/shotcrete, underwater concrete and 3D printed concrete.
Why is concrete cured?
To keep concrete as nearly saturated as possible
What is the benefit of curing concrete?
- Promote continuous hydration
- Reduce porosity
- Achieve good strength development and durability
- Minimise plastic shrinkage
- Protect from adverse weather
- Prevent high temperature gradient
What type of water should be used in curing?
Any water that is suitable for mixing is suitable for curing (drinking water)
When should curing start and how long should it last?
As early as possible and should be continuous and last as long as is economically viable (slide 33 lecture 8)
What are the main methods for curing?
- Impermeable Sheets
- Curing compounds
- Steam Curing/ Autoclaving (factory conditions)
In order for the concrete to be impermeable and durable, what needs to happen to the capillay pores?
They need to become discontinuous
What are some common problems during the early ages of concrete?
- Poor compaction
- Honeycombing
- too dry a mix leads to cracking and not enough concrete around rebar
- too wet a mix leads to slumps
- Segregation
- Bleeding
- Plastic settlement cracks
- Plastic shrinkage cracks
What does poor compaction lead to?
- Strength loss
- Weakened bond between concrete and rebar
- Increased transport of aggresive agents
- visual blemishes
What is the strength reduction for every 1% vol. of air?
5-6%
What is segregation of concrete?
When the distribution of agregate particles is no longer uniform
What causes segregation?
- Difference in density and size of particles (excessive amount of large particles)
- poor handling of fresh concrete
How do we detect segregation?
Flow table test
How do you control segregation?
- Improve the grading of aggregates
- Increase the fines content
- Care in transporting, placing and compacting fresh concrete
- Air entrainment - helps keep particles in suspension
What are fines?
Aggregate smaller than 4 mm - another term for fine aggregate
What is bleeding?
- when mix water rises to the surface of freshly compacted concrete
What causes bleeding?
The inability of solids to hold mix water when heavy particles settle .
Some water gets trapped under large aggregate and rebars
What are the effects of bleeding?
- a w/c ratio gradient, resulting in a weak top surface
- a dusting of cement on the top that allows the paste to be easily abraded
- reduces the bond strenght between rebar and concrete
- reduces plastic shrinkage cracking
What can be done to mitigate bleeding?
- Increase cement fineness and content, reduce w/c
- Air entrainment
- Use accelerator or a rapid hardening cement
- reduce evaporation rate (early curing)
What increases bleeding?
- Retarders
- Deep sections
- low ambient temperature
What causes plastic settlement cracks?
Excessive settlement and bleeding, restrained by large obstructions
How do you control Plastic settlement?
Reduce bleeding and segregation
- air entraining (admixture)
- adopt revibration
Where is plastic settlement common?
- Concrete with rebar, large aggregate and changes in depth
- In deep sections
What causes plastic shrinkage cracks?
Rapid loss of water (evaporation or absorption)
Where does platic shrinkage occur?
At the surface (top is under tension) - concrete dries and contracts
What helps reduce plastic shrinkage?
Bleeding (one of the times where it is useful)
Where is plastic shrinkage common?
Thin sections where bleeding is less common
How do you control Plastic shrinkage?
Control rate of surface evaporation to less than 1 kg m^-2 per hour but ideally below 0.25
This is done by curing and cooling aggregated and mixing water
Why is compressive strength the most common test for hardened concrete?
- It is used in structural design
- For compliance and quality control
- For formwork removal
- Transfer of pre-stress
- easy to measuer
- correlates to many properties
How do we measure compressive strength?
Standard uniaxial compression test on cubes or cylinders
How long after casting do we measure compression?
28 days
What size of cube correlates to aggregates <20mm ?
100mm
What size of cube corelates to aggregates >20mm
150mm cube
What size of cylinders are used in compression testing?
150x300mm
What influences the test results
- Ingredients and mix proportions
- Early age conditions
- Test Paramters (size, humidity, loading rate)
What is the ration of cube strength to cylinder strenght and why is it difference?
cylinder strength ~0.8 * cube strength
This is because of friction between the platens of the testing machine and the cube - restraining effect
Why is tensile strenght important?
- It is resistant to cracking
- Useful in shear desing
- Assesment of fatigue performance
How do we measure tensile strength?
- Direct tension (axial)
- splitting tension
- flexure
Why arent Direct tensile tests usually done on concrete
It is difficult to avoid secondary forces
How does splitting tension test work?
- Loaded in compression
- Transverse tensile stress causes splitting along the vertial diammeter
- overestimates axial tensile stress by ~10-15%
- usually on cylinders
What are the equations needed for the splitting tension test?
fsp = 2F/piLD
axial tensile strenghth = 0.9*fsp
How does a flexural test work?
Beam is loaded at 2 points until failure
Single crack between two points
Not valid if crack is not in middle third of span
What is the equation for the flexural test?
Modulus of rupture = FL/bd^2
What are the equations for the relationship between compressive and tensile strength (empirical)?
For C50/60
fctm = 2.12*ln(1+(fck+8)/10)
Where fctm is the 28-day mean tensile strength and fck is the 29-day characteristic cylinder strength
How is modulus of elasticity measured?
Slope of stress strain curve
How is the secant modulus calculated
Slope of line drawn from origin to a point corresponidng to 0.4*ultimate stress
How is the Chord modulus calculateed
slope of the line corresponding to 50uE from the point 0.4*ultimate stress
What is the strenght of concrete a function of?
Porosity
What is the relationship between strenght and porosity?
As porosity goes down, strenght goes up
How much does dryin increase measured strength by?
10-25% - C-S-H boundaries
Why is tensile strenghth < compressive strength?
- Flaws propogate under tensile strength
- Cracks orthogonal to the direction of load
- Stress concentrations increase rapidly
- Cracks in compression form in direction of load
- cracking does not change the area under load
