CHAPTER 7 PORTLAND CEMENT CONCRETE 7 1 Portland

CHAPTER 7 PORTLAND CEMENT CONCRETE

7 -1 Portland Cement � Main minerals � Lime (Ca. O) � Silica (Si. O 2) � Alumina(Al 2 O 3) � Iron Oxide (Fe 2 O 3) � 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 � � Type III – High early strength � � � Smaller amounts of C 3 S and C 3 A Type V – Sulfate Resisting � � � Cost 10 -20% more 90% stronger one day Same strength after 90 days Contains more C 3 S Cement is also ground finer so water can reach cement particles faster Type IV – Low Heat � � Most common (90 -95%) Used when groundwater contains sulphate C 3 A is about 1/3 that of Type II –Moderate � Used when moderate resistance to sulphate is present

Properties of Portland Cement � Fineness � Controls � 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 � hydration – smaller particles absorb water faster 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 190 kg of water and 340 g 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 � 1 kg of water is required for 4 kg 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 � � Flexural strength or modulus of rupture � � Reactive aggregates Cycles of freeze thaw Deicing chemicals create hydraulic pressure Ground water with high sulphates levels can cause disintegration Seawater as well Permeability � � Strength of pavement concrete Tensile stress at bottom of beam Usually about 15% of compressive strength Durability � � � Very low about 10% of compression strength 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 10 um/m per degree C � � Shrinkage � � � Example problem in book page 294 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 300 um/m Concrete creep � Change in volume due to continuously applied load Only important in prestressed concrete

Basic Tests � � � 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 150 mm x 150 mm and 900 mm long is cast � Load beam at three points to find flexural strength � Problem page 296

Slump Test � � Cone 300 mm high –three levels tamp at each level 25 times cone removed slump measured Ordinary structural concrete is usually 50100 mm (2 -4 in) High slump concrete – 100 -150 mm(4 -6 in) Zero slump – 0 -30 mm (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) � � � 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 � � � Fly ash Granulated slag Silica fume 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. 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 -6 rpm) 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 18 in apart in forms and be used less then 15 s

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 (14 f ) 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 (41 f) 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 -23 ft apart) � Dowelled pavements – smooth steel dowel under sawed joint Joints place (13 -23 ft apart) Better load transfer between slabs then plain � Reinforced pavements – uses heavy reinforcing steel bars Joints place 40 -100 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

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/in 2 below specified strength � Require one strength test for each 150 yd 3