30/04/2014 - Revision

Today I will be revising Concrete and Concrete structures.

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Lecture one

concrete is a mixture of cement, water, fine aggregate, coarse aggregate.
Cement and water makes up paste at 1:0.5 ratio. Add in fine aggregate to make mortar at two ratio and concrete coarse aggregate at three ratio.

Making Cement:Water:Fine Aggregate: Course Aggregate = 1:0.5:2:3
add in 2% steel Rebar

Cost: £28-£48 per tonne, £70-100 per metre cubed
Usual density: 25kNm^-3
Usual strength: 20 to 80 Nmm^-2


Aggregates:

• Fine
• Natural Sand
• Crushed Limestone
• Course
• Gravel
• Crushed Limestone
• Crusted Granite

All 300-400Nmm^-2

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Lecture two: Manufacture, Chemistry & Properties

Manufacture

1. Minging
1. Explosives are used to collect limestones
2. Higher up you get more impure wiht silica, iron and aluminium oxide
2. Transport
1. Transported in 50t trucks to the processing plant
3. Primary crushing
1. Softball size
2. Water is sprayed to stop dust leaving
4. Secondary crushing
1. Golfball size
5. Storage and mixing of high and low purity
1. Tripper makes piles of correct proportion
2. These piles are raw mix.
6. Raw mix grinding
1. Ground in a roller mill
7. Blending and storage
1. Minerals addrd
1. Silica
2. Iron
3. Aluminium oxide
2. Called raw meal.
8. Pre heater
1. Bonds minerals for hardening
2. uses heat from rotery kiln via heat exchanger
3. Removes carbon via flash calcinator
1. CaCO_3 --> CaO + CO_2
4. 800 degrees C
9. Rotary Kiln
1. 49 -80 meters long
2. 1500-1700 degrees C
3. Powder fuses into marble size
4. Clinker
5. Cooled quickly to 80 degrees C with large fans
10. Finish Grinding
1. Gypsum added here
1. Used so that the cement retains workability for up to 2 hours before hardening
2. Ball mills 
1. Uses little iron balls of different size to grind

Electrostatic Precipitator used to take dust away (fires charged electrons at the dust which contact then move to positvely charged plates which get banged to let the dust fall)

Packed into 25kg bags.

The kiln fuel is a mixture of coal and tyres

200kg of coal makes 1 tonne of cement
100kg of rubber tyres makes 1 tonne of cement

Chemsitry

C₃A    - 10.8
C₃S    - 54.1
C₂S    - 16.6
C₄AF  - 9.1

This makes up 93-95% and the rest are minor compounds like magnesium,, potassium and sodium.

C = CaO
S = SiO₂
A = Al₂O₃

F = Fe₂O₃

Properties

Can be changedto suit wants and needs

OPC - Ordinary portland cement                                  C₃S+C₂S>67%

RHPC - Rapid hardening portland cement                     C₃S is greated  but still <70% (C₂S is lower)

SRPC - Sulphate resistant portland cement                    C₃A+SO₄₊H₂O      and C₃A <3.5%

White - White portland cement                                      C₄AF<1%

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Lecture 3: Aggregates

Why?

• Paste has 
• Low modulus of elasticity
• High creep and shrinkage

This means that it has a high dimentional changes

The maximum aggregate size is preferablefor strength.

Source?

• Primary
• Made for concrete
• Secondary
• By-product from another process (e.g. metal mining)
• Recycled
• Recycled from demolition sites etc.

Extraction

• Crushed Rock
• Drilling/blasting
• Crushing
• Screening
• Blending
• Stockpiling
• Sea dredged
• 15% of uk sand and gravels
• Good quality
• Smooth
• Fine aggregates
• Size <4mm
• Lighweight aggregates
• obtained from air voids
• density < 200kgm^-3
• f_c>15MPa
• Pumis, Ligtag, Perlite

Grading

• Use a seive
• Find the % passing
• Draw grading curve
• compare to EN12620 table 2,

Particle Size

• Take account of shape unlike sieve & grading
• influences void size (smooth packs down better
• Tyoes
• Flaky, elongated, round, angular, irregular
• Angularity Index

A=67-(100G)/(V_wγ)=67-100(V_agg/V_w)

V_w = Volume of water to fill the voids in a control vessell
γ = Specific gravity of aggregate
67 = % volume occupied by perfect spheres
G= weight of aggregate in vessel

Strength

• Concrete strength < Crushed aggregate strength
• Stength of parent rock not necessarily the strength of the crushed aggregate

Graph of Parent rock strength vs. 10% fine crushed value

One tested in cube test, the other tested by crushing in a vessell and stop when 10% of the sample is <2mm in size.

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Lecture 4: Properties of Fresh Concrete

Mixing

• 50-80 seconds - Drum Mixer
• 40-70 seconds - paddle/blade mixer
• 30-50 seconds - Superplasticiser

All running at 15-20rpm

Delivery

• 12m^3 (50% water added on site)
• or add all water on site
• Take consistency sample 15 mins after mixing

Placement

• Use a laser screed to level
• bleed water on the surface must be vacuumed off

Properties

• Fresh
• Workability - Enables mixing/transport/compacting without segregation of aggregate and paste and without bleed water on surface
• Hardened
• Strength
• Durability

Test

• Slump
• Pour cement into a container
• poker to make sure air is out
• Lift container
• Measure from top of container to top of concrete
• This is the slump
• High workability (100-175)
• Low workability (0-25)
• Compacting factor
• w_1 = Weight of uncompacted concrete
• w_2 = Weight of compacted concrete

C.F.=w_1/w_2

• RC mix is usually 0.8 - 0.9
• V-B time
• Vebe consistometer
• make slump test in a circular container
• time to get a level surface in larger container after slump container removed and vibrator switched on.

Compaction

• Poker vibrator
• Stop when water/cement slurry rises to surface
• Better than hand compaction
• Hand compactor
• Pour concrete in layers 
• Look at graph in notes showing concrete strength with compaction vs. water/cement ratio

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Lecture 5: Properties of Hardened Concrete

For sutrctures

• Compressive strength
• f_ck=20-50 Nmm^-2
• Tensile strength in flexure
• f_ctm=2.5-4.5 Nmm^-2
• Direct Tensile Strength
• f_ctd=1.5-3.0 Nmm^-2
• Young's modulus
• E_cm=30-38 kNmm^-2
• Poisson's ratio
• 0.15-0.25
• Zero when cracked

For building services

• Thermal movement
• α=11x10^-4 
• Shrinkage
• 300x10^-6 strain (0.3%)
• Curing can be used to stop this, continually supply water to the concrete for the first few days. 
• Vibration
• Acoustic
• Fire

Tests

• Compressive Strength
• Cylinder test
• L/d=2
• f_ck=P_max/area
• Cube test
• f_cu=P_max/area
• f_ck=f_cux1.2
• Tensile Strength due to Flexure
• Flexure Bending Test



f_ctm,fl=M/Z
M=Pa/2
Z=a^3/6
f_ctd=(0.7xf_ctm,fl)/1.5
• Splitting tensile Strength
  

f_ctm,sp=(2P)/(pi x LD)

f_ctd=(0.9xf_ctm,fl)/1.5

• Young's Modulus

E_cm=22[(f_ck+8)/10]^0.3 

(x by 0.9 for limestone)

• PUNDIT
• Time for ultrasonic pulse to pass through prism
• Velocity calculated from this
• long equation here (can't be bothered to type out!
• ERUDITE
• Finds the first natural frequency of the concrete prism

E_c(t)=4 x 10^-15 x n^2 x L^2 x density

n=frequency

E_cm=E_c(t)/1.05

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Going for a run now, then I will try and get to the end of at least Lecture 7 before bed.

Lecture 6: Concrete mix design

 A short lecture, it goes through the basic steps for mix design, this is not necessary for the moment, I will go over it before the exam.

Statistics are used - this is because of a lot of samples.

Aim to make 30Nmm^-2
Look for 95% characteristic strength.

This margin is = ks where k is a constant and s is the standard deviation.

f_m=f_k+ks
where f_m is the mean strength and f_k is the 95% characteristic strength

The rest of the lecture is the mix design.

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Lecture 7: RC Intro

Limit states design

• Limit state design makes sure the structure is safe under worst loads
• It also makes sure there are no extreme deformations whilst under working loads that can effect:
• appearance
• durability
• performance

Partial Safety Factors

• γ_f
• Partial Safety Factor for loads.
• Dead = 1.35
• Live = 1.5
• γ_m
• Partial Safety Factor for materials
• Concrete = 1.5
• Rebar = 1.15
• Design Strength f_cd = [Characteristic Strength (fck)] / [PSF γ_m]
• Design Load = [Characteristic Load] x [γ_f]

On your beam, find the largest possible bending moments: 

Load our beam with different cases combine choose greatest from each section (hogging & sagging) This generates the Bending Moment Envelope.

Typical Cross Section

Usually h / b = 1.5 - 3.0

A_s is the area of steel, usually between 1-4% for main bars and 0.13-1% secondary bars

Generally the distance between rebar pieces is > the size of aggregate + 5mm or 20mm. whichever is bigger.

All rebar must have links turning 90^o around them at some point.

That's it for tonight, I will continue on Dissertation all tomorrow and probably geotechnics the day after.