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Concrete Structures Study Notes - CVEN3304 - UNSW
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CVEN3304 Study NotesTrimester 2, 2019
INTRODUCTION TO CONCRETE
• In its simplest form, concrete consists of a binder (cement) and filler
(aggregate)
• Around 33 billion tonnes of concrete is used globally per year, this is the
highest volume of any material used by humans other than water
• An accelerating trend of 3D printing of concrete has emerged in recent years
• The four key areas of concrete performance as a material:
o Environmental Concern
o Energy Consumption
o Cost
o Physical Performance
• Concrete has a Green Degree of around 1.042, which indicates that the
theoretical usage of natural resources is actually higher than that of the actual
usage for concrete (eco-friendly)
• General advantages of concrete include:
o Durability
o Low cost
o Formed on site
o Energy efficient
o Non-flammable
o Thermal resistance
o Water resistance
o Aesthetics
• General disadvantages of concrete include:
o Brittle
o Slow strength development
o Low strength/mass ratio
o Volume instability
o Low strength in tension
HISTORY OF CONCRETE
• Egyptians used an ancient form of concrete in the Great Pyramids, using
gypsum and straw as a binding agent
• Ancient Romans used mortar combining volcanic ash and lime to build
structures (Pozzolana cement)
• Joseph Aspdin developed Portland cement in 1824, which is still the most
commonly used recipe used in concrete structures today
• The main areas of research for improvement of concrete is as follows:
o Reducing CO2 emissions during production
o Increasing energy efficiency during production
o Controlling physical properties on a nanoscale
,CONSTITUENTS OF CONCRETE
• The main components of concrete are:
o Cement (10% volume)
o Fine Aggregate (32% volume)
o Coarse Aggregate (40% volume)
o Water (14% volume)
o Chemical additives
• Mortar is composed of cement, water and fine aggregate – which combines to
form a binding property for the concrete
• Whilst cement only makes up around 10% of the concrete volume, it often
accounts for up to 2/3rds of the cost of production
CEMENT
• Originally, a stationary kiln was used to produce Portland cement
• The far superior rotary kiln is a rotating device which burns raw materials to
create cement
• The key oxides used to produce cement include:
o Lime (CaO) – 65% by weight
o Iron (Fe2O3) – 5% by weight
o Silica (SiO2) – 20% by weight
o Alumina (Al2O3) – 5% by weight
• In the study of cement, an abbreviated notation system is typically used to
indicate various oxides (known as cement notation)
o C = CaO
o S = SiO2
o A = Al2O3
o F = Fe2O3
o S = SO3
o H = H2O
• Before entering the kiln, all raw limestone and clay is grinded and
homogenized
• A wet-process kiln is large and provides inefficient transfer of heat
o The raw cement materials were blended into a slurry consisting of
around 40% water
• A dry-process kiln is much smaller and provides greater transfer of heat to
the raw materials
o For centuries, it was considered more difficult to get dry powders to
blend adequately to produce clinker
o In the dry-process kiln, the blended raw materials are processed
beforehand in a pre-heating tower
o Excess heat energy from the kiln is funnelled into this tower, which
sends ‘cyclones’ of air through the raw materials to evaporate moisture
• Clinker is the output material from a kiln, which is usually one of the key
cement oxides with a bonded metal ion
o Before entering the kiln, the dominant compounds include limestone,
quartz, clay and iron oxides
, o After heating up to around 1450 degrees Celsius, the dominant
compounds include Alite and Bellite
o After being in the kiln, the alite is stabilised through rapid cooling
• Gypsum (CSH2) is hydrated calcium sulfate, which is added to clinker in
order to prevent flash set of concrete (rapid forming of rigidity and heat)
• The clinker is then grinded to increase surface area, where the normal surface
area is around 350m2/kg
• There are 7 standard recipes for cement given in Australia by the
AS3972-2010 standards, separated into two categories:
o General Purpose
o Special Purpose
• There are several thermal zones within the kiln (from front to back):
o Dehydration zone – the entrance of the kiln where all remaining
moisture is removed from the material
o Calcination zone – free lime, bellite and aluminates are formed
through the decarbonation reaction (CaCO2 > CaO + CO2)
o Transition zone – gradual increase in bellite (C2S) proportion as the
heat increases rapidly
o Clinkering zone – allows for production of allite (C3S)
o Cooling zone – temperature drops rapidly once the clinker has moved
past the bottom of the kiln, forming C3A (aluminate) and C4AF
(ferrite)
HYDRATION OF CEMENT
• Hydration is a loose term describing the reactions that occur between
Portland cement and water particles
• The two chemical processes occurring during the hydration of cement:
o Dissolution
o Precipitation
• During dissolution, the Portland cement grains dissolve and separate within
the water
• Through precipitation, the cement grains react with the water to form a layer
of hydration products
• The extent to which the hydration process has occurred can be monitored by
measuring the heat energy given off, since this is an exothermic reaction (this
usually plateaus after around 3 days)
• The four stages of hydration of cement in terms of heat energy:
o Rapid heat evolution – less than 15 mins
▪ CH and ettringite are formed
o Dormant period (very low heat energy) - 1-3 hours
▪ CH, ettringite and CSH products form
o Accelerating stage – 3-8 hours
▪ CSH products rapidly form
o Decelerating stage – 12-24 hours
▪ Full range of products are slowly produced
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