08 Miscellaneous CT Questions

Question 1

What makes concrete a good construction material?

Answer 1

1. Strong in compression so can sustain large loads, but weak in tension, so steel reinforcement is often used to provide tensile resistance
2. Extremely flexible - can be poured into many forms and sizes (in situ on site in its wet form or pre-cast in a factory and delivered to site as a complete component)
3. Good fire resistant properties
4. Durable if constructed correctly and maintained well

Question 2

How is concrete made?

Answer 2

Mixture of:

1. Cement
2. Fine aggregate
3. Coarse aggregate
4. Water

Question 3

What is the purpose of the cement in a concrete mix?

Answer 3

Acts as the binder of the concrete mix

Question 4

How must cement be stored?

Answer 4

Must be kept air-tight and dry

Question 5

What is the purpose of the aggregate in a concrete mix?

Answer 5

Forms the bulk of the mixture

Question 6

Why should aggregate in a concrete mix be graded?

Answer 6

To ensure a reasonable balance of small and large particles so voids in the mix are minimised

Question 7

What is the difference between fine and coarse aggregate in a concrete mix?

Answer 7

• Fine aggregate is generally that which can pass through a 5mm sieve (e.g. sand)
• Coarse aggregate has particles ranging from 5-40mm (e.g. stone or chippings)

Question 8

What is the purpose of the water in a concrete mix?

Answer 8

Added to start the chemical reaction (hydration) with cement and to give the mix workability

Question 9

How much water should be added to a concrete mix?

Answer 9

Water/cement ratio is usually about 0.4 to 0.5

Question 10

Why is the amount of water added to a concrete mix critical?

Answer 10

• If too much is added, the excess will evaporate and leave tiny voids in the concrete, which seriously reduces its strength
• If too little is added, sufficient hydration will not take place, making the concrete difficult to place and compact

Question 11

What sort of quality should water in a concrete mix be?

Answer 11

Must be of a quality fit for drinking

Question 12

How is a concrete mix expressed in terms of its component parts?

Answer 12

Expressed as a ratio (e.g. 1:3:6 / 20mm):

• 1 part cement
• 3 parts fine aggregate
• 6 parts coarse aggregate
• 20mm max. size of coarse aggregate

Question 13

What does C40 mean in relation to concrete and what does the 40 relate to specifically?

Answer 13

Grade mixes are given depending on the concrete’s components, quality and manufacturing control (e.g. C40, with the number relating to its minimum strength - i.e. 40 N/mm²)

Question 14

What guidance is available for specifying concrete?

Answer 14

BS 8500:2015 ‘Method of specifying and guidance for the specifier’

Question 15

What is the minimum depth of cover for reinforcement in concrete?

Answer 15

BS EN 1992 - usually between 25-50mm but depends on a number of factors including diameter of the steel, concrete strength class, environmental conditions, use of additional protection and an allowance for deviation on site

Question 16

What is concrete curing?

Answer 16

The process in which concrete is protected from loss of moisture and kept within a reasonable temperature range in order to increase its strength and decrease its permeability

Question 17

What is ‘slip forming’ and when is it used?

Answer 17

• Method in which concrete is poured into a continuously moving form enabling a non-interrupted (i.e. no joints) structure to be formed
• Used for tall structures (e.g. towers, bridges etc.) as well as horizontal structures (e.g. roads)

Question 18

Where would you expect to find the main reinforcement in the following structural concrete components and why: beam, column, staircase, suspended slab, cantilevered slab?

Answer 18

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Question 19

How do fire regulations affect the structure and finishes of a building?

Answer 19

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Question 20

How are steel framed buildings fire-proofed?

Answer 20

1. Intumescent coatings - paint like material that expands at high temperatures to form a protective layer around the steel
2. Sprayed coatings - cementitious based material that forms a protective layer around the steel
3. Encasement - fire-rated boards can be used to encase the steel, offering protection against fire as a dry trade alternative
4. Flexible blanket systems - fire protection material that can easily encase steel of a complex shape where application must be completed as a dry trade
5. Concrete encasement - used commonly until the late 1970s when boards and coatings were introduced

Question 21

Explain how suspended ceiling systems work.

Answer 21

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Question 22

How are suspended ceiling voids compartmentalised for fire protection?

Answer 22

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Question 23

How can buildings be protected against flooding?

Answer 23

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Question 24

What is a concrete slump test?

Answer 24

• Means of assessing the consistency of fresh concrete and that the correct amount of water has been added to the mix
• Three equal layers of the mix are poured into a steel slump cone (rodding each layer to ensure compaction), the cone is then removed and if the concrete collapses or a layer shears off, it would mean too much water is in the mix