Concrete Quality Control & Admixtures Week 7 Cont d NOTE In

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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
  correct proportions of cement, fine coarse
  aggregate water, to meet specified properties
• Many properties can be specified but the most
  usual are
• Workability/Consistence
• Compressive strength - Cylinder/Cube (at
  specified age)
• Durability (min. cement, max. w/c or type of
  material)

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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 sample
  manufacture, storage and testing.

http//en.wikipedia.org/wiki/FileConcrete_Compres
sion_Testing.jpg
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Compressive Strength - Cubes

• Part 1 Moulds
• Part 2 Making curing
• Part 3 Compressive strength 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 strength
• A trial mix is made - from the results and
  information available adjustments of the mix
  proportions is possible
• A revised trial mix is prepared

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Normal Distribution of Concrete Strengths
Mean strength

• () (K)
• 1.28
• 1.64
• 2.5 1.96
• 1 2.35

Specified characteristic Strength
5 defectives
1.64s
Margin
25 30 35 40 45 50
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Compressive strength (N/mm2)
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 two
  parameters, 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

Where, x an individual result n the number
of results m the mean of the n results
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Standard Deviation Characteristic Strength

• It is now generally accepted that at a given
  level of control the standard deviation increases
  as the specified characteristic strength
  increases up to a particular level and is
  independent of the specified strength above this
  level

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Basic Concepts ofMix Design - Strength Margin

• Due to the workability of concrete, a mix must be
  designed to have a considerably higher mean
  strength than that specified
• The characteristic strength specified in BS
  8110 replaces the old concept of minimum
  strength and the difference between this and the
  target mean strength is called the margin

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Target Mean Strength
Target mean strength Characteristic strength
Margin
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BS EN 206 1 Compressive strength classes

• Where 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 design
  standard

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Table 7 of the Standard
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Table 8 of the Standard
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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 or
  standard deviation.

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Changes in the Mean
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Changes in the 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 (by design)
• Probably the most important factor in the
  development use is the need to produce economic
  concrete

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Usage

• Country of all concrete with admixture
• France 70
• Italy 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 or
  during 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
  particular reason 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 of
  any 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 admixtures
• Part 2 - Air-entraining admixtures
• Part 3 Super-plasticisers
• BS EN 934 - 2 is the new standard covering
  admixtures 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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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
  durability against freeze/thaw effect
• Entrains finely dispersed bubbles normally
  lt0.5mm, typically in the range 0.01 0.02mm
• Increases workability, pressure relief, breaks up
  continuous capillaries, provides barrier to water
  movement by capillary action
• Dosage 3-9 in normal dense concrete, optimum is
  13 in cement paste basically a surfactant

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Superplasticisers

• Two main types
• 1.) Sulphonated melamine formaldehyde
  condensates
• 2.) Sulphonated naphthalene formaldehyde
  condensates

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Common uses

• Uses flowing concretes high strength water
  reduced concretes
• Act as dispersion agents preventing cement
  flocculation

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Mineral Admixtures

• Used to increase workability improve other
  properties
• Basically powders some act as pore fillers
  whilst 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 insulation materials
  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