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 Main minerals  Lime (CaO)  Silica (SiO2)  Alumina(Al2O3)  Iron Oxide (Fe2O3)  Main component is lime (60-65%)

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Presentation on theme: " Main minerals  Lime (CaO)  Silica (SiO2)  Alumina(Al2O3)  Iron Oxide (Fe2O3)  Main component is lime (60-65%)"— Presentation transcript:

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2  Main minerals  Lime (CaO)  Silica (SiO2)  Alumina(Al2O3)  Iron Oxide (Fe2O3)  Main component is lime (60-65%)

3  Raw materials ground up, mixed and burned in kiln  Kiln reaches 1500 degrees C  Produces particles called clinker  Clinker is add to 5% gypsum

4  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.

5  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

6  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 for portland cement  Soundness  Ability of the paste to retain volume after setting  Air content of the mortar

7  Paste  Portland cement  Water  Air  Aggregate  Fine aggregate  Course aggregate

8  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)

9  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

10  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)

11  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

12  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

13  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

14  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

15  Cone 300mm high –three levels tamp at each level 25 times cone removed slump measured  Ordinary structural concrete is usually mm (2-4in)  High slump concrete – mm(4-6 in)  Zero slump – 0-30mm (0-1 in)

16  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

17  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

18  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

19  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

20  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

21  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

22  Page 311  7-6  7-7    7-8

23  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

24  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 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

25  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

26  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

27  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

28  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.

29  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

30  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 ft  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

31  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


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