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