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Concrete Flashcards

1
Q

Name the positive and negative characteristics of concrete

A
  • Strong in compression
  • Can be formed on-site/in-situ (flexible)
  • Can be poured and cured under water if necessary
  • Good fire resistant properties
  • Durable if constructed and maintained correctly
  • Can have an aesthetic appeal if admixtures are used
  • Weak under tension. Steel reinforcement can be used to improve the tension strength.
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2
Q

What could be considered as concrete?

A
  • Cement paste (cement + water)
  • Mortar (cement + sand + water)
  • Concrete ( cement + sand + gravel + water)
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3
Q

What is fine aggregate?

A

Sand - particles of stone or other material

less than 5mm across

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4
Q

What is coarse aggregate?

A

Gravel - particles between 5mm and 40mm

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5
Q

What is the typical composition of concrete?

A
  • Air 3-6%
  • Cement 10-12%
  • Water 14-18%
  • Fine aggregate 20-27%
  • Coarse aggregate 40-45
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6
Q

Describe the basic forces the beam goes through

A
  • Tension- Pulling. Acts to expand or lengthen the thing it is acting upon
  • Compression- Pushing. Acts to compress or shorten the thing it is acting upon.
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7
Q

Give an example of an item which experiences tension and compression stress?

A

A spring.

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8
Q

Where would the forces take place on a beam?

A
  • Tension impacts the bottom of the beam

* Compression impacts top of the beam

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9
Q

What are the main compounds present in Portland Cement?

A
  • Tricalcium Silicate - C3S - 45-60%
  • Dicalcium Silicate - C2A - 10-25%
  • Tricalcium Aluminate - C3A - 7-12%
  • Tetracalcium Aluminoferrite - C4AF - 5-11%
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10
Q

What do the proportion of the compounds in the finished cement depend on?

A

Depends on the raw materials used and on the production process itself

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11
Q

Which chemicals are included in Portland cement?

A

Manufactured through a closely controlled chemical combination of calcium, silicon, aluminium, iron and other ingredients

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12
Q

What do the properties of concrete depend on?

A

The properties of concrete depend on the quantities and qualities of its components

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13
Q

Why would you use other types of Portland cement?

A

• Because cement is the most active component of concrete and usually has the greatest unit cost, its selection and proper use are important in obtaining most economically the balance of properties desired for any particular concrete mixture

•example, tri-calcium aluminate has poor resistance
to sulphates and as such is used in lower proportions in
sulphate resisting cement

• Variations in the proportions of the compounds affects
the properties of the cement and controlled variations
are exploited in the production of different types of
Portland Cement

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14
Q

Name 5 types of Portland cement

A
  • Ordinary Portland Cement (OPC)
  • Rapid Hardening Portland Cement (RHPC)
  • Sulphate Resisting Portland Cement (SRC)
  • White Portland Cement
  • High Alumina Cement (HAC)
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15
Q

How is ordinary Portland cement manufactured and when is it used?

A

The most common form of cement used for general purposes. It is produced from firing a mixture of clay
or shale, and limestone or chalk. The clinker produced in the kiln is ground to a fine dust

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16
Q

How is rapid hardening portland cement manufactured and when would it be used?

A

Modified (usually by further grinding) to hydrate more rapidly in the first few hours after casting. It is of particular use to manufacturers of pre-cast concrete products because it allows greater productivity from moulds

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17
Q

How is sulphate resisting portland cement manufactured and when would it be used?

A

Modified by the control of constituents to have a greater resistance to sulphate attack than ordinary portland cement. It is often used on buried concrete structures in contact with grounds containing sulphates

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18
Q

When would white portland cement be used?

A

Cement that is white in colour rather than grey. Care is needed in the choice of ingredients and in the manufacturing process to ensure the white colour, hence why it is considerably more expensive than ordinary portland. cement. White cement is used for concretes where colour is important.

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19
Q

When would a high alumina cement be used?

A

Calcium aluminates are used rather than calcium silicates. Its rapid strength development made HAC popular from 1950 to 1970. However, mineralogical ´conversion´ sometimes caused reductions in concrete strength and increased vulnerability to chemical attack

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20
Q

What are the 5 factors of concrete defects?

A
  • Mechanical
  • Chemical
  • Physical
  • Carbonation
  • Corrosive Contaminants
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21
Q

Does the reinforcement in the concrete usually corrode?

A

Normally the reinforcement in the concrete does not
corrode because the concrete provides a protective
alkaline environment due to large quantities of calcium
hydroxide which is produced as portland cement
hydrates and hardens.

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22
Q

What is the common result of the concrete defects?

A

breaking down or spalling

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23
Q

Can defects be visually seen in concrete?

A

Concrete defects are often difficult to diagnose from
a purely visual inspection.

In dense high strength concrete, significant corrosion of the reinforcing bar from chloride attack can take place without the normal visible signs of disruption to the concrete cover, such as spalling.

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24
Q

Explain the hydration process in the concrete?

A

An incorporation of water molecules into a complex with those of another compound

As a result of the hydration reactions of cement, the
pore solution of concrete tends to be alkaline, with pH
values typically in the range 12.5-13.6

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25
Q

What does passivation mean?

A

The changing of the chemically active surface of a metal to a much less reactive state

Under alkaline conditions, reinforcing steel tends
to passivate. However, due to the porous nature of concrete, corrosive species and chemical species influencing corrosion reactions can enter the concrete and lead to corrosion problems

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26
Q

What is carbonation?

A

A mildly acidic solution is formed from a mixture of carbon dioxide and water which. The acid mixture forms within the pores of the concrete. The mixture reacts with alkaline calcium hydroxide forming insoluble
calcium carbonate. The passive layer decays when the pH value drops below 10.5. The steel is then exposed to moisture and oxygen.

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27
Q

How does a mixture of carbon dioxide and water affect the pH value and reinforced concrete?

A

When insoluble calcium carbonate is produced, the pH value can drop from more than 12.5 to about 8.5.

When the pH value drops below 10.5. The steel is then exposed to moisture and oxygen.

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28
Q

How would you test carbonation?

A

Undertaken by applying a phenolphthalein solution to a freshly fractured or freshly cut surface of concrete.

Noncarbonated areas turn red or purple while carbonated areas remain colourless. The hardened paste changes colour at a pH of 9.0 to 9.5.

The pH of good quality non-carbonated concrete without admixtures is usually greater than 12.5

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29
Q

How does chloride effect concrete?

A

Chloride ions penetrate into the concrete from de-icing
salts or marine environments. The calcium chloride into the concrete mix acts as an accelerator.

Salt causes corrosion. Salt mixed with water sodium chloride forms a versatile, highly corrosive solution of sodium ions (Na+) and chloride ions (Cl-)

The chloride ions disperse through concrete pores in solution and where they come into contact with the reinforcement after they pass the passive layer

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30
Q

How does steel corrode?

A

Steel corrodes in the presence of air and water to form rust which has a volume of up to 10 times that of the steel consumed

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31
Q

How does chloride attack visually effect concrete?

A

Significant corrosion of the reinforcing bar can take place without the normal visible signs of disruption to the concrete cover, such as spalling.

Cracks will occur when as little as a tenth of a millimetre of steel has corroded.

Horizontal cracks form causing corners to ‘spall’ and
surfaces to ‘delaminate’. The concrete cover becomes detached and falls away in sheets.

Can be seen on the underside of road bridges and many buildings and structures beside the sea

32
Q

How does alkaline silica reaction occur?

A

The alkaline chemicals in the cement reacts with silica.

silica is a substance occurring naturally in certain types of aggregate or stone used to make concrete.

33
Q

What is silica gel?

A

A material used to absorb moisture.

34
Q

How is silica gel formed?

A

Some silicaceous minerals,react with water in a high alkaline environment to form silica gel. Silica gel swells when it absorbs moisture.

35
Q

How can alkaline silica impact the visual appearance of concrete?

A

The material can cause concrete to crack, and white, weeping deposits of silica appear.

Very few structures show signs of significant alkaline silica reaction damage, as the reactive aggregate components which causes the problem are consumed in the process

36
Q

How significant is alkaline silica reaction?

A

Alkaline silica reaction is superficial and harmless, but it is unattractive and difficult to treat however, the ‘jelly like’ substance forms around the stones and expands. If water gets in and freezes, the cracks become wider, which may expose the reinforcement.

The most effective remedy is to dry out the structure.

37
Q

How could the risk of alkaline silica reaction be lowered?

A

Use a concrete product which intercepts to alkaline silica reaction via the materials used.

38
Q

How does external suphate attack typically occur?

A

Occurs where water containing dissolved sulphate penetrates the concrete

39
Q

How does sulphate attack effect concrete?

A
  • Extensive Cracking
  • Expansion
  • Loss of bond between the cement paste and aggregate
  • The effect of these changes is an overall loss of concrete strength
40
Q

Describe sulphate attack

A
  • The surface of the concrete is normal, or near normal
  • Within the concrete, the composition and microstructure of the concrete will have changed
  • This is what causes cracking
41
Q

Name other sources of sulphates which can form an attack

A 
• Seawater
• Oxidation of sulphide minerals in clay (such as 
  copper) adjacent to the concrete
• Bacterial action in sewers
• Bricks
42
Q

How does the oxidation of sulphide minerals in clay

adjacent to the concrete create sulphate acid?

A

The oxidation of sulphide minerals can produce sulphuric acid which reacts with the concrete

43
Q

How does bacterial action in sewers create sulphuric acid?

A

Anaerobic bacterial produces sulphur dioxide which dissolves in water and then oxidises to form sulphuric acid

44
Q

What is the source of the sulphate which effects mortar?

A

In masonry, sulphates are present in bricks and can be gradually released over a long period of time, causing sulphate attack of mortar

45
Q

Name 3 types of sulphates?

A
  • Sodium sulphate
  • Potassium sulphate
  • Magnesium sulphate
46
Q

Which one of the 3 sulphates has the most impact?

A

Solutions containing magnesium sulphate are generally more aggressive, for the same concentration as the other types of sulphates.

47
Q

What are sulphates?

A

Sulphates are salts in which the negatively charged ion forms a compound with a metal positively charged ion

48
Q

When was sulphate attack common?

A

• The use of fill material in house construction was
became common in the 1940’s (post war period)
• Construction materials were in short supply.
• Fill material was used as hardcore which was used underneath the concrete slab
• The Government promoted fill material and waste materials such as burnt colliery shale, blast fu rnace slag
•However this type of fill contained high sulphate levels and resulted in major problems and expensive remedial works to replace those affected floors

49
Q

Are there still properties with solid floors and fill material?

A

Domestic dwellings constructed through the 1940’s
and 1970’s may have solid floors that include fill
material with high levels of sulphates

50
Q

Why were solid floors used in and around the war period?

A

Timber was required during war therefore suspended flooring may have been very expensive due to its scarcity.

51
Q

Why didn’t polythene sheets (DPM) prevent sulphate attack?

A

Polythene sheets were not common until the 1960’s.

52
Q

What was designed to resist sulphate attacks?

A

High Alumina Cement but it soon became used as a very rapid-hardening cement in the 1960’s

53
Q

What is HAC manufactured from?

A

limestone or chalk and bauxite

54
Q

What is bauxite?

A
  • Bauxite is a rock.

* It consists of hydrated alumina, oxides of iron and titanium, with small amounts of silica

55
Q

How does HAC differ from normal Portland cement?

A

HAC is composed of calcium aluminates rather than calcium silicates

56
Q

Why was HAC stopped?

A

Mineralogical ‘conversion’ of the concrete caused serious reductions in strength and increased vulnerability to chemical degradation

57
Q

Give an example of where HAC has failed?

A

Three UK roof collapses in the mid-1970s led to widespread inspection and monitoring of HAC concrete units, exhaustive research and curtailment of HAC use for structural purposes.

58
Q

Are there still properties with HAC ?

A

• A large stock of UK buildings containing HAC concrete remains, in which the HAC is now usually highly converted

• The probability of sudden collapse is now perhaps remote, there is continuing concern over long-term durability, particularly where carbonation has occurred to the depth of steel reinforcement or pre-stressing
wires

59
Q

Explain the ‘conversion’ process of HAC

A

The conversion process results in conversion of the cement from one crystalline form into another weaker form

60
Q

What are hydrated calcium aluminates?

A

A chemical reaction involving the addition of

water to a compound

61
Q

What effect does normal temperature have on HAC?

A
  • At normal temperatures, the hydration of HAC results in the formation of hydrated calcium monoaluminate
  • Smaller amounts of calcium aluminate and hydrous alumina are also formed
  • At normal temperatures, conversion may take many years but at temperatures in excess of 40° C a considerable amount of conversion can occur within a few months
62
Q

What effect does high temperature and high moisture have on HAC?

A
  • Hydrated calcium aluminates at higher temperatures and in the presence of moisture, change to give the unstable hydrated calcium aluminate.
  • Special attention should be paid to any areas which have been subjected to high moisture penetration such as roof leaks or excessive condensation or damp
63
Q

How does conversion effect HAC?

A
  • Strength Loss
  • Increased porosity
  • Reduced resistance to chemical attack
64
Q

How is the effect of carbonated HAC concrete different compared to other cement concretes?

A

Carbonation of HAC concrete may enhance the rate of corrossion due to HAC concrete becoming more porous after conversion.

65
Q

How could you visually inspect HAC concrete?

A
  • The colour of the concrete often changes from grey to much darker grey
  • Defects may reveal signs of failure such as excessive deflection, lateral bowing and cracking
66
Q

How would the building surveyor inspect HAC concrete?

A

‘intrusive’ (samples) testing is necessary for significant proof

67
Q

Name 6 other concrete defects

A
  • Poor expansion joints
  • Poor workmanship
  • Poorly installed construction joints
  • Plastic shrinkage
  • Insufficient cover
  • Crazing
68
Q

What is crazing?

A

Crazing is the development of a network of fine random cracks or fissures on the surface of concrete.

69
Q

What is crazing caused by?

A

shrinkage of the surface layer such as plaster

70
Q

How serious is crazing?

A

Crazing does not affect the structural integrity of
concrete and rarely do they affect durability or wear
resistance.

71
Q

How deep are crazing cracks?

A

The cracks are rarely more than 3 mm deep and are more noticeable on steel troweled surfaces.

72
Q

When is sulphate resistant cement used?

A

It is often used on buried concrete structures in contact with grounds containing sulphates

73
Q

How does the manufacturer benefit from rapid hardening cement?

A

It is of particular use to manufacturers of pre-cast concrete products because it allows greater
productivity from moulds

74
Q

How long does it take for original cement to cure?

A

Concrete which is moist cured for 7 days is about 50% stronger than uncured concrete.
It takes about 30 days for concrete to reach nearly 80% of its strength.

75
Q

Can cement be contaminated?

A

corrosive species can enter the mix if ‘contaminated’

mix ingredients are used (water, aggregates, additives)

76
Q

Can the corrosion of reinforcement itself force cracking?

A

Corrosion damage to the reinforcing steel results in the build-up of voluminous corrosion products. This generates internal stresses

Concrete (1 decks)

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