Thursday 3 June 2010

Reinforced Concrete Structure

What is RC Concrete?

Concrete that needs to be strengthened in tension

Concrete good in compression poor in tension

How?

Steel bars embedded into the concrete

Cut to a variety of shapes

Cut & bent on site or delivered cut & bent

Shape Codes

Formwork

Before a concrete structure can be cast a mould has to be made

This called formwork

Formwork can be made from timber, metal, GRP and Polystyrene

Formwork can be made with patterns in it and/or motifs

Often called ribbed formwork

Labour intensive, therefore expensive

Formwork can be proprietary

Fixing and supporting formwork is crucial.

Accurate calculations are required if the formwork is not to burst open.

Formwork can be massive and supported/reinforced by steel strongbacks and walings.

Formwork design is serious business.

Accurate setting out is vital

Re-use?

Falsework

Falsework is the part of formwork that supports the forms.

Bridge decks, bridge abutments, multi-storey (table forms)

It is important to keep the reinforcement in the formwork in position

Reinft. must not move when concrete is poured.

Concrete

Designed mixture of cement, sand, aggregate and water

Water/cement ratio crucial

Cement is most expensive

Manufactured to a BS 810

Aggregates must be clean and chemically inert

Course aggregates can be pit gravels, beach gravels, crushed angular stone and/or crushed blastfurnace slag

Air entraining

Testing

Specification will describe tests

Reinforcement

Hot rolled mild steel bars, BS 4449

Most common, 6mm to 40mm, max length generally 12m

Hot rolled & processed high tensile alloy bars, BS 4486

Less common

Steel wire, BS 4482

Steel fabric, BS 4483

200mm x 200mm, 1.54kg to 6.16kg per square metre

Reinforcement cutting, bending and placing is a skilled job

Accuracy is crucial

Reinforcement must not move during pouring

Can be constructed in or out of mould

Needs careful handling/lifting to install to ensure no deformation

Placing Concrete

By hand

By ready-mix lorry

By skip

By cableway

By pumping

By conveyor

Which ever way discharge needs careful control to prevent segregation

Not less than 5C

Within 30min of discharge

Needs compacting – vibrating

Needs curing

Joints

Contraction joints

Expansion joints

Joggle joints

Sliding joints

Temporary joints

Waterbars

Used if shrinkage of concrete would cause leaks in water retaining structures

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