A Study on the Construction Process

Precast ConcreteIn-situ cast ConcreteShoring and Underpinning

Introduction

Precast concrete

In-situ cast concrete

Shoring

Underpinning

Precast Concrete

What is Precast Concrete?

• Cast into specific shapes at another location

• Developed in the late 1960s

• First use: stair treads, coping, lintels, and window sills

• Known for its high quality architectural products

Characteristics

• Great accuracy and high quality

• Additional features: reinforcing, moles, anchor bolts, etc

• Characteristics controlled by different materials

• Reinforcement: Conventional reinforcing or prestressed steel

Manufacture

• Casting beds used

• High strength steel strands pretensioned

• Transverese bulkhead seperators inserted

• Additional reinforcement added

• Concrete poured and vibrated

• Cured with live steam or radiant heat

Slabs

• The most standardised precast elements

• Includes solid flat slab, hollow core slab, double tee slab, and single tee slab

Solid flat slab

Hollow core slabs

Double tee slabs

Single tee slabs

Beams

• Made in different sizes

• Rectangular beams, L shaped beams, inverted T beams and AASHTO beams

Wall panels

• Prestressed or conventional reinforcement

• Used as load bearing walls

• Includes solid wall panels, hollow core wall panels and tee wall panels

Solid wall panels

Hollow core wall panels

Tee wall panels

How are precast buildings put together?

Double tee slabs supported on frame of columns and girders

Hollow core slabs supported on load bearing walls

Double tee slabs supported on load bearing walls, columns and inverted tee beams

In-situ cast concrete

What is in-situ cast concrete?

• Concrete that is cast into forms on the building site

• Any shape that can be formed can be cast

• Certain types of concrete elements cannot be precast, and can only be cast in-situ

Selecting an in-situ cast concrete framing system

• Are the bays of the building square or nearly square?

• How long are the spans?

• How heavy are the loads?

In-situ Concrete mix

• Appropriate concrete properties designed

• Appropriate mix design developed

• Mixing, transporting and handling coordinated with placing and finishing

In-situ concrete placement

• Should not be placed more rapidly than it can be spread

• Deposit as near as possible to the final position

• The concrete is placed in forms and consolidated

Curing

• Satisfactory moisture content and temperature required

• Concrete in formwork is protected from drying out

• Curing compounds and treatments prevent loss of moisture

• Large surface areas problematic during curing

Advantages

• Easy transportation of wet concrete

• Flexible when it comes to geometric shapes

• Relatively easy to do late changes to structure

• Structure becomes monolithic

Disadvantages

• Produced in an unprotected environment

• Additional time required for drying out process

• Requires more temporary work

• Complex process with many inputs and flows

Shoring

What is shoring?

• Construction of a temporary structure to support an unsafe structure

When do we need shoring?

• When walls bulge out

• When walls crack

• When an adjacent structure needs to be pulled down

• When openings are to be newly made or enlarged in a wall

• During construction

Shoring during construction

Raking shores

• Rest upon the ground and support the wall by being inclined against it

Using raking shores

• Size of the raker decided on anticipated thrust from the wall

• Center of the raker and wall should not meet at floor level

• Shoring may be spaced at 3 - 4.5m

• Wedges should not be used on sole plates

• For tall buildings, rider raker can be used

Dead Shores

Flying Shores

Scaffold type shoring

• Scaffolding was first designed to support loads imposed by workers

• Often used as support for formwork

• Provides rapid utilisation for formwork support

Underpinning

What is underpinning?

• Strengthening and stabilising the foundation of an existing building

• Installation of temporary or permanent support

• Provides additional depth or increase bearing capacity

When do we need underpinning?

• Construction of a new building adjacent to the old one

• Settlement of an existing building

• Change in how a structure is used

• Need to add a basement below an existing building

Temporary support with maintenance jacking

• Light structures may be supported with timber or concrete mats

• If settlement occurs, mechanical or hydraulic jacks keep the structure level

Bracket pile underpinning

• Steel bracket piles driven adjacent to the structure

• Load is transferred from the structure into the pile through a steel bracket

• Backfilled with lean sand cement mix

Minipiles

• Minipiles inserted through the existing foundation

• Generally do not require temporary support of the building

New foundation wall and footing constructed underneath the existing foundation

• The area is excavated• The new footing is

constructed• The excavated area is

backfilled

Support during underpinning

• Digging trenches at intervals• This leaves most of the

foundation supported by the soil

Support during underpinning

• Using needle beams• Beams are threaded through

holes cut in the wall• Entire wall can be exposed at

once