CONFINEMENT OF REINFORCED CONCRETE

IN COLUMNS

BY ABRAHA TESFAY

Introduction Concrete

Concrete is a mixture of different material which have very small tensile strength, it is usually reinforced with reinforcement whenever used in structures.knowledge of the behavior of concrete structures under

conditions of large deformation is necessary(under large loads).

the strength and ductility would depend greatly on the

effectiveness of the confinement provided to the concrete .

confinement. The concrete which is affected by a favorable

action of axial compression stress and hydrostatic compression stress of the transverse reinforcement is called confined concrete.

Concrete is confined by transverse reinforcement in the form of circular spirals or rectangular square hoops with different types of configuration. And it is generally accepted that the transverse reinforcement results in an increase of strength and ductility of the confined concrete.

Contd……There are two types of confinement. Active and passive confinement. Active confinement is when transverse stress is from some externally applied action, This can be achieved by encasing a reinforced concrete member with a jacket that is slightly larger than the cross section. The void between the jacket and the concrete surface is then filled with pressure grout .passive confinement, is achieved by adding transverse reinforcement to the concrete cross section that engages as a result of lateral expansion of the concrete section under axial loading. Since passive confinement can be as effective as active confinement and does not require the additional effort of pre-stressing.

The main objectives of confinement are

( i ) to prevent the concrete cover from spalling.

(ii) to provide lateral support to the longitudinal reinforcement.

(iii) to enhance concrete strength and deformation capacities.

Advantages of confinement

1. It increases strength of concrete and , which compensates for possible losses caused by spalling.

2. It reduces the slope of the descending branch of the stress-strain curve, therefore it increases the maximum usable strain cu. In other words the ductility of concrete is increased by confinement

Stress strain diagram of confined and unconfined concrete

confinement is provided in two ways

These are, using circular spiral or square/rectangular hoops.confinement by circular spirals in general more effective than those by square/rectangular hoops. This is because in the case of rectangular/square hoops, confining reaction is only applied in the corner regions of the hoops. While in circular spiral uninterrupted confinement pressure is produced along the whole circumference.

Parameters affecting the confinement of RC In column

1.The ratio of Transverse Reinforcement

2.The Yield Strength of Transverse Steel

3.The Spacing of Hoops

4.The Longitudinal Reinforcement

5.The Compressive Strength of Concrete

6.The Hoop Pattern

The ratio of Transverse Reinforcement

• This is expressed as the volumetric ratio defined as the

ratio of the volume of hoops to the volume of confined

core of the member. it is clear that with increasing the

ratio of transverse reinforcement, both the strength

and the ductility of confined concrete also increased.

Deficiency of transverse reinforcement rectangular cross section

Well distribution of transverse reinforcement bars

2) The Yield Strength of Transverse

• It is understood that the higher the strength of stirrups, the higher the confining pressure they can exert. 3) The Spacing of HoopsFor a given volumetric ratio of hoops, the efficiency of confinement increases as the spacing becomes closer, since the regions of the members which remain without confinement becomes smaller. This will assist in preventing premature buckling of the longitudinal bars after the spalling of concrete cover.

4)The Longitudinal Reinforcement

The content and size of longitudinal reinforcement increase confinement effect in preventing the lateral expansion of the core. The larger the diameter of the bars and their ratio, the larger their contribution to confinement.

5) The Compressive Strength of Concrete

Higher strength concrete is less ductile than lower strength concrete. Therefore, it is anticipated that confinement will be more efficient in this case, since the hoops will be stressed more than in a high strength concrete member.

6) The Hoop Pattern When multiple hoop patters are used in member, the region of effectively unconfined concrete becomes smaller resulting in strength and ductility enhancement. poorly confined improved confinement well confined

• Lap splices are not allowed in potential plastic hinge regions

• This fig shows a failure at the base of a column that had splices in the hinge region. (Building C, Adapazari, Turkey, 1999 Izmit earthquake

Summary • The amount of lateral reinforcement had significant

effect on the behavior of the confined concrete core. Ductility of concrete is improved as the lateral steel content is increased.

• Lateral steel that is widely spaced or improperly detailed will not provide adequate confinement, irrespective of the lateral steel amount, leading to premature failure.

• Yield strength of transverse reinforcement has a direct correlation with the cross Section’s confinement effectiveness. High yield strength increases a cross section’s confinement effectiveness.

• Hoop Configuration - Column hoops should be configured with at least three hoop or crosstie legs restraining longitudinal bars along each face.

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