03 Concrete & Steel

Building Construction3. Concrete and Steel Construction

CE 2330Civil Engineering Materials and Construction

Structural Elements

• Foundation – isolated and combined footings, rafts, piles etc.

• Columns• Beams• Slabs

Construction with concrete

• Concrete – weak in tension, good in compression

• Steel used for reinforcing – provides load carrying capacity to tensile zones

• Why steel?• Other modern reinforcement options –

fibre reinforced plastic (FRP)

Concrete construction in India

• Reinforced cement concrete (RCC) most popular construction material (Why??)

• Concrete can be site-mixed or supplied from central mixing plants by ready mix concrete trucks

• In India, most concrete is still site mixed• Labour intensive construction…• Volume batching still prevalent…

Site mixing

RMC

Can you state some advantages and disadvantages of RMC over site-mixed concrete??

For large projects…

Batching plants are set up and concrete is conveyed using conveyor belts / hoppers / trucks…

Critical aspects of concrete construction

• Reinforcement detailing• Formwork design• Concrete mixture design• Conveyance of concrete, placement,

compaction, finishing and curing

Reinforcement detailing

- Most critical aspect from design and performance perspective- Detailed structural analysis - followed by design- Rebar diameter and spacing- Provision of cover (use of cover blocks)

Formwork design

- Formwork should be able to support the pressure from concrete before it hardens

- Should be easy to assemble and dissemble- Popular materials – steel, wood, aluminium, plastic etc. (some combined

systems also available)

Concrete mixture design

• Concrete is designed to get a required (compressive) strength and workability

• Strength – specified by structural designer• Workability – specified by construction

(site) engineer• Volume batching vs. weigh batching…• Durability of concrete

Concreting operations

..contd

..contd

Building with steel

• Excellent tensile strength and ductility make steel a top building material

• Can carry enormous compressive loads too…

• Steel elements are factory made – so quality is good

• Expensive!• Requires skilled manpower

Steel construction

• In India, steel construction is only favoured for industrial buildings (factories / workshops etc.) – Why is it so?

• Preformed segments are assembled at the construction site

• Most critical factor – connections!• Welds, rivets, bolts etc.

Steel sections

Conventional steel construction

Curved steel

Steel systems for skyscrapers

Connections

Steel concrete composites

• Can combine the response of steel and concrete in compression – hollow steel tubes filled with concrete

• Presence of concrete increases resistance against buckling

• Steel confinement around concrete ‘increases’ load carrying capacity

• ‘Cheating’ concrete to believe that it is not experiencing any tension!

• Pre-compression applied to concrete overcomes tensile stresses due to loading

Prestressed concrete

Types of prestressing

• Transfer of compressive stress to concrete is by steel (high strength strands)

• If strands are stressed before pouring concrete – pre-tensioning

• If strands are stressed after concrete has gained sufficient strength – post-tensioning (in this case, concrete is cast with a duct)

Pre-tensioning

Post-tensioning

References

• Various web sources