week 7 concrete quality control & admixtures

7/30/2019 Week 7 Concrete Quality Control & Admixtures

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Concrete Quality Control

& Admixtures

Week 7


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Concrete Mix Design & BS EN 206-1

Principles requires the selection of the correctproportions of cement, fine & coarse aggregate & water,to meet specified properties

Many properties can be specified but the most usualare:

Workability/Consistence

Compressive strength - Cylinder/Cube (atspecified age)

Durability (min. cement, max. w/c or type ofmaterial)


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Workability/Consistence - Slump

Typical results:

S1 - 10 to 40mm

S2 - 50 to 90mm

S3 - 100 to 150mm

S4 - 160 to 210mm

S5 - 220mm


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Compressive Strength - Cylinders

BS EN 12390

British/European

standards for samplemanufacture, storage

and testing.

http://en.wikipedia.org/wiki/File:Concrete_Compression_Testing.jpg
http://upload.wikimedia.org/wikipedia/commons/2/29/Concrete_Compression_Testing.jpg


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Compressive Strength - Cubes

Part 1 Moulds

Part 2 Making & curing

Part 3 Compressivestrength cylinders &

cubes

Standards also cover

reporting of results


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Durability

Linked to min cement content

Max free water/cement ratio

Compaction

Curing


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Contd

From restricted data available - mix proportions

are derived which attempt to match the required

workability/consistence and strengthA trial mix is made - from the results and

information available adjustments of the mix

proportions is possibleA revised trial mix is prepared


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Normal Distribution of Concrete

Strengths

25 30 35 40 45 50 55

Mean strength

Specified

characteristic

Strength

5%

defectives 1.64s

Margin

Compressive strength (N/mm2)

(%) (K)

10 1.28

5 1.64

2.5 1.96

1 2.35

Fig 1


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Standard Deviation

This normal distribution curve is symmetrical

about its mean, has a precise mathematical

equation and is completely specified by twoparameters, its mean, m and its standard

deviation, s


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Equation for Standard Deviation

The standard deviation is a measure of the

variability calculated from the equation:

1

2

n

mx

s

Where, x = an individual result

n = the number of results

m = the mean of the n results


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Standard Deviation

& Characteristic StrengthIt is now generally accepted that at a given level

of control the standard deviation increases as

the specified characteristic strength increases upto a particular level and is independent of the

specified strength above this level


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Basic Concepts of

Mix Design - Strength MarginDue to the workability of concrete, a mix must be

designed to have a considerably higher mean

strength than that specifiedThe characteristic strength specified in BS

8110 replaces the old concept ofminimum

strength and the difference between this andthe target mean strength is called the margin


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Target Mean Strength

ksff cm Target mean strength Characteristic strength Margin


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BS EN 206 1 Compressive

strength classesWhere concrete is classified with respect to its

compressive strength, Table 7 for normal-weight

and heavy-weight concrete or Table 8 for light-weight concrete apply

The characteristic compressive strength at 28

days of 150 mm diameter by 300 mm cylinders(fck,cyl) or the characteristic compressive

strength at 28 days of 150 mm cubes (fck,cube)

may be used for classification


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Contd

NOTE In special cases intermediate strength

levels between those in Table 7 or 8 may be

used if this is permitted by the relevant designstandard


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Table 7 of the Standard

Compressive Minimum characteristic Minimum characteristic

strength class cylinder strength cube strength

fck,cyl (N/mm ) fck,cube (N/mm )

C8/10 8 10

C12/15 12 15

C16/20 16 20C20/25 20 25

C25/30 25 30

C30/37 30 37

C35/45 35 45

C40/50 40 50

C45/55 45 55

C50/60 50 60

C55/67 55 67

C60/75 60 75

C70/85 70 85

C80/95 80 95

C90/105 90 105

C100/115 100 115


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Table 8 of the Standard

Compressive Minimum characteristic Minimum characteristic

strength class cylinder strength cube strength

fck,cyl (N/mm ) fck,cube (N/mm )

LC8/9 8 9

LC12/13 12 13

LC16/18 16 18

LC20/22 20 22

LC25/28 25 28

LC30/33 30 33

LC35/38 35 38

LC40/44 40 44LC45/50 45 50

LC50/55 50 55

LC55/60 55 60

LC60/66 60 66

LC70/77 70 77

LC80/88 80 88


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Quality Control Techniques

Based on statistical schemes level of

defectives

Simple calendar charts

Moving average charts

Shewhart charts

Cussum charts

Operating characteristics curves (O-C curves)


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Shewhart Charts

Calendar charts can be turned into Shewhart

charts for control purposes. The objective is to

detect significant variations in the mean orstandard deviation.


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Changes in the Mean

Gradual increase in mean Sudden increase in mean


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Changes in the Standard Deviation

Gradual increase in standard

deviation

Sudden increase in standard

deviation


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Variability of Concrete in Practice

Variations have been attributed to differences in

random structure

No two pieces of concrete can be identical &therefore they cannot have the same properties

Variation is seen to follow a Normal Distribution


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Practical Differences

Differences in practice have other origins:

Materials Placing

Batching Curing

Mixing Age

Transportation Testing


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Admixtures BS EN 934 - 2


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Admixtures

Usually added to effect a change in the

properties of the concrete (or mortar) which

would not be possible by normal means (bydesign)

Probably the most important factor in the

development & use is the need to produceeconomic concrete


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Usage

Country % of all concrete with admixture

France 70Italy 70

U.K. 70

Germany 80

Australia 90

Japan 100

U.S.A. 100


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What Are Admixtures?

Admixtures are chemical substances (other than

fine and coarse aggregates, cement, or water),

which are added in small amounts just before orduring the mixing stage to concrete products.


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When Should They be Used?

They should normally only be used where it is

desirable to modify the properties of either fresh

or hardened concrete (or both) for particularreason when such a change cannot be effected

by changes in the composition or properties of

the normal mix.


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How Much to Use?

Dosages are critical, but basically a

fundamentally poor quality concrete cannot be

converted to a good concrete by the addition ofany type of admixture.


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Main Types

Retarders

Accelerators

Water reducing / plasticising

Air-entraining

Superplasticisers


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British & Euro Standards

BS 5075 Admixtures for concrete:

Part 1 - Accelerating admixtures, retarding

admixture & water reducing admixturesPart 2 - Air-entraining admixtures

Part 3 Super-plasticisers

BS EN 934 - 2 is the new standard coveringadmixtures for concrete


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Retarders

Delay the beginning of setting & hardening

Useful in hot countries for slip forming &

transportation

Set retarded from 1 to 4 hours

Lignosulphates, carbohydrates, CaSO4


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Accelerators

Traditionally CaCl2 used in flake or granular form

Dosage about 2%

Now use calcium formate

Used when more rapid set & hardening required


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Heat of Hydration

20.0

30.0

40.0

50.0

60.0

70.0

80.0

90.0

100.0

110.0

0 100 200 300 400 500 600 700 800 900

Time (minutes)

TemperatureC

opc rhpc srpc H A


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Heat of Hydration

20.0

25.0

30.0

35.0

40.0

45.0

50.0

55.0

0 100 200 300 400 500 600 700 800 900Time (minutes)

Tempe

ratureC

opc opc + 5%Ca opc + pfa rhpc


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Water Reducing / Plasticising

Reduces water required for a given workability

Causes better dispersion of cement particles

Results in higher workability


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Air-entraining

Discovered by accident & used to increase durabilityagainst freeze/thaw effect

Entrains finely dispersed bubbles normally


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Superplasticisers

Two main types:

1.) Sulphonated

melamine formaldehydecondensates

2.) Sulphonated

naphthalene

formaldehydecondensates


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Common uses

Uses flowing concretes & high strength water

reduced concretes

Act as dispersion agents preventing cementflocculation


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Mineral Admixtures

Used to increase workability & improve other

properties

Basically powders & some act as pore fillerswhilst others act react with the cement

Lime, bentonite, PFA, slag, clay


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Water Proofers

Used to prevent absorption

No use against water under pressure

Usually metallic soap, vegetable or mineral oils

Gradually leached and lose effect act as pore

blockers


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Expansion Admixtures

Shrink compensating or self-stressing

Used to minimise cracking due to dry shrinkage

Magnesia, finely granulated iron and chloride,

sulphoaluminates


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Surface Hardening Agents

Used to reduce surface abrasion

Physical and chemical types in use

Metallic carborundum, fused alumina, iron

particles

Chemical silicofluorides, SiF4


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Gas Formers

Used to form lightweight concrete (80% of

buildings in Sweden)

Used to produce high thermal insulationmaterials that can be sawn and nailed

Low density reduces dead load

Air whipped, foamers, gas formers (Al, H2O2,CaC2)


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Bonding Admixtures

Assist in bonding of fresh to hardened concrete

Several types

Metallic - Iron Filings

Latex - Synthetic resin polymer as an

emulsion


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Others

Corrosion inhibitors - sodium benzoate

Fungicidal - sodium nitrite

Germicidal and - halogenated phenols

Insecticidal admixtures- copper compounds

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