Chapter 7 Portland Cement Concrete

1. Chapter 7 Portland Cement Concrete

2. 7-1 Portland Cement • Main minerals • Lime (CaO) • Silica (SiO2) • Alumina(Al2O3) • Iron Oxide (Fe2O3) • Main component is lime (60-65%)

3. 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

4. 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.

5. 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

6. 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

7. Portland Cement Concrete • Paste • Portland cement • Water • Air • Aggregate • Fine aggregate • Course aggregate

8. 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)

9. 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

10. 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)

11. 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

12. 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

13. 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

14. 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

15. 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)

16. 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

17. 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

18. 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

19. 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

20. 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

21. 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

22. Mix Design Problems • Page 311 • 7-6 • 7-7 • 7-7.3 • 7-7.5 • 7-8

23. 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

24. 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

25. 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

26. 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

27. 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

28. 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.

29. 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

30. 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

31. 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