What makes concrete a good construction material?
- Strong in compression so can sustain large loads, but weak in tension, so steel reinforcement is often used to provide tensile resistance
- 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)
- Good fire resistant properties
- Durable if constructed correctly and maintained well
How is concrete made?
- Fine aggregate
- Coarse aggregate
What is the purpose of the cement in a concrete mix?
Acts as the binder of the concrete mix
How must cement be stored?
Must be kept air-tight and dry
What is the purpose of the aggregate in a concrete mix?
Forms the bulk of the mixture
Why should aggregate in a concrete mix be graded?
To ensure a reasonable balance of small and large particles so voids in the mix are minimised
What is the difference between fine and coarse aggregate in a concrete mix?
- 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)
What is the purpose of the water in a concrete mix?
Added to start the chemical reaction (hydration) with cement and to give the mix workability
How much water should be added to a concrete mix?
Water/cement ratio is usually about 0.4 to 0.5
Why is the amount of water added to a concrete mix critical?
- 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
What sort of quality should water in a concrete mix be?
Must be of a quality fit for drinking
How is a concrete mix expressed in terms of its component parts?
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
What does C40 mean in relation to concrete and what does the 40 relate to specifically?
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)
What guidance is available for specifying concrete?
BS 8500:2015 'Method of specifying and guidance for the specifier'
What is the minimum depth of cover for reinforcement in concrete?
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
What is concrete curing?
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
What is 'slip forming' and when is it used?
- 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)
Where would you expect to find the main reinforcement in the following structural concrete components and why: beam, column, staircase, suspended slab, cantilevered slab?
How do fire regulations affect the structure and finishes of a building?
How are steel framed buildings fire-proofed?
- Intumescent coatings - paint like material that expands at high temperatures to form a protective layer around the steel
- Sprayed coatings - cementitious based material that forms a protective layer around the steel
- Encasement - fire-rated boards can be used to encase the steel, offering protection against fire as a dry trade alternative
- Flexible blanket systems - fire protection material that can easily encase steel of a complex shape where application must be completed as a dry trade
- Concrete encasement - used commonly until the late 1970s when boards and coatings were introduced
Explain how suspended ceiling systems work.
How are suspended ceiling voids compartmentalised for fire protection?
How can buildings be protected against flooding?
What is a concrete slump test?