0708 Solutions Concrete

8/2/2019 0708 Solutions Concrete

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Higher temp means quicker hydrationj

Assume 100g of cement for 40g of water

Vol of cement = 31.8 ml (Specific gravity of cement = 3.15)

Vol of water = 42ml

0% Hydration 100% HydrationWater = 40 ml Capillary pores

Gel poresSolid products of hydration

Cement = 31.8


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PFA Slaaaaaaag! Silica fume Metakaolin

Physics Data

Surface AreaBulk density

Specific gravityChemical Data

SiO2Fe2O3

Al2O3

CaO

MgONa2O + K2O

Table 1


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PFA

y Increase in long term strength,y lower water demandy improve workabilityy increase durabilityy lower early strength.

GGBFS

y Higher long term strengthy increased resistance to sulphatesy reduced early strength

Silica fume

y to development of strengthy increased workabilityy better long term strengthy improved chemical resistance

Metakaolin

y twice the reactivity of most other pozzolansy the filler effect, the acceleration of OPC hydration, and the pozzolanic reaction. The filler

effect is immediate, while the effect of pozzolanic reaction occurs between 7 to 14 days.[4]#


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y --------------

Hydration is required to


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Environmental Impact of OPC

y Quarrying of materials impacts environment visually and also energy is requiredy Energy is used to heat the materials to 1500degC and to grind the resulting clinkery Fossil fuel is burned to produce the energy resulting in greenhouse gas emissionsy As a result of the calcining (burning) process, calcium carbonate breaks down releasing

carbon dioxide.

y For 1ton of cement, 1 ton ofCO2 is releasedAlternatives

y Blended cements (composite cements)o Partial replacement of OPC with cementitious or pozzolanic materialso Most benefit when they are by products of other industries(e.g. slag (40-70%

replacement) and PFA (20-50%))

o Other replacements such as silica fume and metakolin but less benefit as they havelower levels of replacement and metakolin is manufactured rather than a by product

o Binding by CSHo When used properly, cement has superior quality to straight OPC

y Low energy cemento Lower temperature of manufacture (1200C -1300C) c.f Portland (1450C -1500C )o Belite cement (based on C2S)

Requires less calcium carbonate for its production (c.fC3S) Lower manufacturing temperature, lower emissions Low rate of strength gain so undesirable for normal construction Cement is quenched at end of manufactuing stage then blended with C3S or

OPC or with calcium sulfoaluminate which is used in easern Europe.

y Calcium sulfoaluminate cement (CSA)o

Save energy 25%, reduce CO2 by 40%o Can be used with cement replacement materials (PFA, slag etc)o Based on Kliens compound and pure CSA is C4A3 ( is SO3)o Binding material ettringiteo High early strength (most in 7days and is double that of OPC)o Pore fluid low in pH so could be useful in systems sensitive to alkalis (e.g glass fibre

reinforcement)

o Fire stability lower than OPC


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o Used in Chinao Inherent sulphate resistanceo When used with OPC, can produce either high early strength or shrinkage

compensating effect reducing cracking

y Alkali activation of Slago

Slag can be used as sole binder (without any OPC) when activated by an alkalisolution (sodium silicate)

o High strength and may be suitable for structural applicationso Sensitive to poor curingo Binding through CSH

y Geopolymerso Alkali activation of aluminosilicate materials containing no calciumo Early dayso Potential route for activating many wastes and by products materials containing

alumuinosilicates that havent been exploited yet

o High temperature (60C ) curing required with strengths of 50MPa at day 1 recordedo Binding through chains of alumina, silica and oxygen

Where

= flow rate (cm3/s)

a= cross sectional area of specimen (m2)

L = length of specimen (m)

p =fluid pressure across the head of specimen (N/m2)

= viscosity of the fluid (Ns/m2)

K = intrinsic permeability (m2)

Flow rate considered at the avg pressure across the specimen.

Gas flow rate, R (cm3/s), is measured at down stream pressure P1 bar (atmospheric pressure). If

input pressure (upstream) is P2 bar (absolute) then:

barAverage pressure, Flow rate at avg pressure 1 bar = 10

5N/m2


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With

R in cm3/s

P1 and P2 in barL in m

A in m2

Avg pressure = 1 bar so P1=P2=Pavg

y Gas permeability values are influenced by the test pressure.o Low pressure, gas molecules have fewer collisions with pore walls so the gas can

slip through.o At high pressure, the flow is impeded by large number of molecular collisions due to

shorter mean free path (the distance between gas molecules)

o At extreme pressure, the gas may resemble a liquid and may give similarpermeability values.


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Indentify method of degradation

Concrete could deteriorate in the following ways:

Physical Chemical

Abrasion Sulphate attack

Salt crystalisation Corrosion of steeleinforcement

Ice formationAlkal-aggregate reaction

Other than abrasion, degradation caused by internal expansion leading to cracking (when the tensile

strength of the concrete is exceeded). Then most reactions involve the ingress of either

y Liquids (water)y Gases (oxygen, CO2)y Ions (chlorides, sulphates)


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y Sulphate attacko Locations

Foundations and bellow ground works in locations where soil orgroundwater is naturally high in sulphates

Re development of old industrial sites (gas works especially) where theground may have been contaiminated with sulphate salts)

Maritime works (immersed in water or subject to sea spray) Tunnel works (Pyrites) Sewage works (sulphur oxidising bacteria)

o Types Classic sulphate attack


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Remedial measures

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0708 Solutions Concrete