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Chapter 7 Portland Cement Concrete. 7-1 Portland Cement. Main minerals Lime ( CaO ) Silica (SiO2) Alumina(Al2O3) Iron Oxide (Fe2O3) Main component is lime (60-65%). Manufacture of Portland Cement. Raw materials ground up, mixed and burned in kiln Kiln reaches 1500 degrees C - PowerPoint PPT Presentation
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CHAPTER 7 PORTLAND CEMENT CONCRETE
7-1 Portland Cement Main minerals
Lime (CaO) Silica (SiO2) Alumina(Al2O3) Iron Oxide (Fe2O3)
Main component is lime (60-65%)
Manufacture of Portland Cement
Raw materials ground up, mixed and burned in kiln
Kiln reaches 1500 degrees C Produces particles called clinker Clinker is add to 5% gypsum
Portland Cement Consider a hydraulic cement
Sets or hardens with the addition of water Chemical process occurs This process is called hydration
Total amount of water to hydrate cement is about 25% of the mass of cement
Page 278 book for types of cement compounds Hydration produces heat called heat of hydration Massive structures causes problems About 50% of the total heat is released in first 3
days.
Portland Cement Types Type 1 – Normal Portland Cement
Most common (90-95%) Type III – High early strength
Cost 10-20% more 90% stronger one day Same strength after 90 days Contains more C3S Cement is also ground finer so water can reach cement particles faster
Type IV – Low Heat Smaller amounts of C3S and C3A
Type V – Sulfate Resisting Used when groundwater contains sulphate C3A is about 1/3 that of Type I
Type II –Moderate Used when moderate resistance to sulphate is present
Properties of Portland Cement Fineness
Controls hydration – smaller particles absorb water faster
Setting Time required for cement to turn from paste to solid
state Compressive strength Tensile strength Relative density -3.15 for portland cement Soundness
Ability of the paste to retain volume after setting Air content of the mortar
Portland Cement Concrete Paste
Portland cement Water Air
Aggregate Fine aggregate Course aggregate
Water/Cement Ratio Ratio of water mass to cement mass Example 190kg of water and 340g of cement
= .56 w/c ratio is usually between .4 and .7 w/c ratio of .5 is 5.64 us gallons (8.33 lb/gallon)
per 94 Ib sack of cement Water is required for
React chemically with cement to harden Make the mix plastic to work
1kg of water is required for 4kg of cement for hydration (w/c of .25) but this would not give necessary workability (see chart page 287)
Air Entrain Protects against freeze thaw cycles Been used since 1940’s Small bubbles of air are form in concrete by special
chemicals called air-entraining agents These air bubbles relieves the pressure developed by freezing
of water in pores 9% air provides(paste plus fine aggregate) adequate
protection –except concrete subject to deicing chemicals
Concrete made with small size coarse aggregates requires more mortar to fill spaces between the coarse particles Proportion of whole mix increases as the size of the largest
particles decrease
Compressive strength Expressed in MPa (psi)
Obtain by dividing total failure load by cross sectional area
Normal concrete strength at 3,7,14 days is 40%,60,75% of total strength
Strength will vary based on w/c ratio Air entrain reduces strength of concrete,
however less w/c is necessary with air-entrain and as a result strength is very similar (see page 289)
Other Properties Tensile strength
Very low about 10% of compression strength Flexural strength or modulus of rupture
Strength of pavement concrete Tensile stress at bottom of beam Usually about 15% of compressive strength
Durability Reactive aggregates Cycles of freeze thaw Deicing chemicals create hydraulic pressure Ground water with high sulphates levels can cause disintegration Seawater as well
Permeability High w/c ratio will have more air voids and be less water tight
Abrasion resistance Depends on aggregate choice Concrete strength
Plastic Properties Workability
Consistency or plasticity of placing and molding concrete without segregation
Increase water content increase workability Air entrainment also increases workability To much w/c can cause bleeding and segregation Bleeding – movement of water to the surface
Causes week layer Segregation –coarse aggregates separate from cement paste
Dropping concrete from heights and excess vibration Workability is measured by slump test
Harshness Finishing quality of concrete Harsh mix will have too much coarse aggregate and will not
finish well
Volume Changes Temperature change
Varies with type of aggregate Average value for coefficient of expansion is 10um/m per degree
C Example problem in book page 294
Shrinkage During curing moisture escapes Range is 400 to 800 u/m Example problem in book page 294 About 1/3 shrinkage occurs first 30 days – 90% first year Reinforce concrete rate drops to 200 to 300um/m
Concrete creep Change in volume due to continuously applied load
Only important in prestressed concrete
Basic Tests Problem page 295 Problem page 295 To find 28 day results sooner
Submerge cylinder in boiling water for period of time Cure cylinder in autogenous curing box Both methods cylinder can be tested at 2 days to
give 28 day strength Concrete subject to bending loads
Concrete bean 150mm x 150mm and 900 mm long is cast
Load beam at three points to find flexural strength Problem page 296
Slump Test Cone 300mm high –three levels tamp at
each level 25 times cone removed slump measured
Ordinary structural concrete is usually 50-100mm (2-4in)
High slump concrete – 100-150mm(4-6 in)
Zero slump – 0-30mm (0-1 in)
Air Content Volumetric method or pressure method
Known volume is filled Top part of apparatus is clamped on Standpipe is filled with water apparatus is
inverted Drop of water level is calibrated to give air
content as percentage
Admixtures 80% concrete produced in North America has
chemical additives Used since 1900’s Small quantities up to 1% to 2% of mass of cement ASTM Standard
Type A-water reducing Type B- retarding Type C –accelerating Type D – water reducing and retarding Type E –water reducing and accelerating Type F – high range water reducing (HRWR) Type G- high range water reducing and retarding
Admixtures Type A – can reduce amount of water by 20% -30%
Type A – also known as superplasticizers Increase slump and workability Better flow through pumping
Type B – delay the time required for setting and hardening
Type C – retard setting and hardening – used below 5 degree C (41 degrees F)
Other Admixtures Corrosion inhibitors Pumping additives Microsilica
Supplementary Cementing Materials
Materials suitable to replace portion of portland cement – reduce cost Main types are supplementary cementing materials (SCM)
Fly ash Granulated slag Silica fume
Also referred to as mineral admixtures Fly ash is lighter then cement
Improves placing and workability Easier to pump Resistance to sulphate attack
Slag by product of blast furnaces Similar to fly ash benefits Segregation or bleeding are more of a problem
Silica fume –fills spaces between cement particles Creates denser mixes with fewer air and water voids Improves pumping and reduces bleeding
Aggregate Should be clean, hard, strong and durable
Hardness or resistance to wear Important for pavement
Soundness or resistance to freeze thaw Ability to withstand weathering Water expands 9% when it freezes
Chemical stability Particle shape and texture
Long thin aggregate should be avoid Relative density and absorption Deleterious substance
Maximum size Limit coarse aggregate to 1/5 width of forms, ¾ of space
between reinforcing, 1/3 depth of slab
Mix Design Designed for strength and resist deterioration Owner or agency specifies proportions required in a mix
Most cases only required strength, exposure conditions and placing conditions specified
Items to be determined according to standards Relative density and absorption of the aggregates Dry rodded density of coarse aggregates Fineness modulus of fine aggregates Slump w/c ratios for various strengths Overdesign factors Harshness or finishing potential Maximum size of aggregates Air-entrainment requirements Use of SCM’s or special admixtures
Mix Design Problems Page 311
7-6 7-7 7-7.3 7-7.5 7-8
Trial Mixes 7-8.1 page 314
Choose slump Choose maximum size of the aggregate Estimate the amount of mixing water Select the w/c Calculate the cement Estimate the proportion of coarse aggregate Estimate the mass of fine aggregate using the
estimated Calculate the adjustments required for aggregate
moisture 7-9 page 317
Mixing, Placing, and Curing prepared a batch at a time Aggregates and cement weigh into a stationary mixer 10% of the water place in mixer initially
The rest with the admixtures and aggregates Three types of mixing can follow
Central mixed – stationary mixer at plant – delivered to site in rotating drum
Shrink – mixed – partially mixed at plant- complete mixing in truck Truck mix – concrete is mixed in truck
Mixing requires 70-100 revolutions of drum at 6-18 rpm Followed by agitating until concrete is placed (2-6rpm) Mixing time is 1 min for 1 yd/cu plus 15 sec for each additional yard
Placement of concrete needs to take place within 2 hours
Concrete Placement Use of buckets, chutes, pumps and belt
conveyors In forms place in 8-20 in thick layers Vibration is used to consolidate and
remove voids Vibrators place every 18in apart in forms and
be used less then 15s
Curing Proper curing requires
Water Good temperature
Hydration stops when water is no longer present Methods of curing
Ponding – fogging action – expensive Wet covering – special types of burlap used kept
damp layed over concrete Wet hay straw – may discolor concrete Waterproof paper – consisting of two sheets of paper
with an asphalt adhesive or plastic sheets Curing compounds – sprayed on surface
Acceleration Hydration can be accelerated Methods
Steam curing High early strength cement Accelerating admixtures
Steam applied about 4-5 hours after pouring Turned off in about 24 hours 80% of design strength in three days
Joints Cracks happen from volume change in concrete
Drying shrinkage Temperature changes
Control joints used to allow for drying shrinkage Place no more then 30 times slab’s thickness – both
directions Construction joints – located at the end of one
days pour – allow load to be transfer from one slab to next
Isolation joints – used to separate slabs from structure pour – filler matter used to absorb expansion of the two concrete units.
Temperatures Hot weather – danger of low slump , quicker setting,
poor finishing conditions, variable air content Concrete should not be placed if mix is more then
90 degrees f (ASTM) Below 5 degrees c (41 F) slow the rate of hydration Below -10 degrees c (14f ) hydration stops ASTM requires placing concrete mix at above 55
degrees c (13 c) If air temp within or after 24 hours of pour is below
5 degrees c (41f) precautions need to be taken
Concrete Pavements 5% of N. America roads use concrete Volume changes major problem
Slab shrinks as it cures Expansion and contraction due to temp. changes
Allow for these changes Plain pavements – sawed or formed joints
Cracks form beneath joint Load transfer between slabs Joints place (13-23ft apart)
Dowelled pavements – smooth steel dowel under sawed joint Joints place (13-23ft apart) Better load transfer between slabs then plain
Reinforced pavements – uses heavy reinforcing steel bars Joints place 40-100ft
Continuously reinforced pavement – heavy reinforcement No joint built in
Saw joints need to be done within 24 hours after set up Saw ¼ depth of slab
Inspection 2 slump test made on first load each day
Sample taken at about 15% and 85% of truck Consistency must fall with in ½ for low slump and 1
in for medium slump 2 compressive strength test are required
Cylinders not disturbed and protected on site for 24 hours
Then moved to lab Strength acceptable if the average of 3 tests is equal
to or greater then specified and no individual test is more then 500 lb/in2 below specified strength
Require one strength test for each 150yd3