MAB 1033 Structural Assessment and Repair · composite material because – Similar coefficients of thermal expansion – Good bond between rebars and concrete MAB 1033 Structural

MAB 1033Structural Assessment and Repair

Professor Dr. Mohammad bin IsmailC09-313

6. CONCRETE BEHAVIOUR

LOAD EFFECTS

Learning Outcome

At the end of the course students should be able

to understand:to understand:

• effect of load on reinforced concrete structure

Introduction

• Concrete structures and individual members

carry loads (self weight or applied load)

• Concrete change volume when subject to

stress

MAB 1033 Structural Assessment & Repair 4

stress

– Tensile stress-concrete stretches

– Compressive stress-shortens

• Reinforced concrete work effectively as composite material because

– Similar coefficients of thermal expansion

– Good bond between rebars and concrete


– Good bond between rebars and concrete

– Quality concrete protects reinforcement

• Concrete problems, such as deflection, cracking, or spalling may be caused by volume change – associated with load

s s

Moving Loads

Live LoadsLive Loads

T

T

T

C

C

C

Compressive Stress (C )


Dead Loads

s s

C

Impact Loads

Tensile Stress (T)

Shear Stress (S)

Basic Engineering Principles

When reinforcing bars are subjected to tension, they

stretch. The concrete around the reinforcing bars is

consequently subject to tension and stretches

When tension in excess of tensile strength of concrete


is reached, transverse cracks may appear near the

reinforcing bars

Reinforcing Bar

Neutral Axis

1. Uniform Loads Applied.

Simply Supported Reinforced Concrete Beam.


Reinforcing Bar

2. Beam deflects under load.

3. Flexuralcrack forms.

Tension is greatest at midspan.

Cracking Modes: Continuous Spans

Load Load Load Load

Continuous Concrete Beam


Flexural Cracks

Combination Shear and Flexural Crack (diagonal tension crack)

Path and location of Tension

Shear Crack

Slab/Beam-to-Column Shear

• Column connections to slabs and beams

experience considerable shear stress

• Excessive stress produces cracks in the beams

and in the surrounding slab


and in the surrounding slab

Slab and beam are

inclined to fall from

supporting column

Applied Loads


Diagonal

Shear

Cracks


Horizontal Forces


Cantilevered Members

• For cantilevered members, tension forces are acting in the member’s top portion

• Critical factors - cantilevered members

1. Reinforcement must be in the correct position


1. Reinforcement must be in the correct position (top)

2. Tension cracks develop over the moment are natural canyons for moisture and aggressive substance



Continuous Structures

• Continuous span transfer load to adjacent

spans. Concrete in tensile zone area subject

to tension cracking

• These cracks provide direct access for


• These cracks provide direct access for

moisture and other corrosive substance

Negative Moment Area

Possible Tension

Cracks


Column

• When concrete is compressed, the member shortens (vertical strain) and bulges (horizontal strain)

• The bulge generates tension forces that are


• The bulge generates tension forces that are restrained by steel reinforcement

• Shortening of columns consists of elastic shortening, creep shortening and drying shrinkage

1. Elastic shortening: occurs as loads are applied = stress/E

2. Creep shortening: occurs over time and affected by constant stress and long term


affected by constant stress and long term loss of moisture

3. Drying shrinkage:occurs over time with loss of moisture (time dependent process)


Bulge

▲-▲-Elastic

ShorteningCreep

Shortenin

g

Applied Load

Column

ars


Tension Cracks

Working Example: Reinforced concrete column in 500’(153m) tall building under substained service

stress of 1500 psi(10.3Mpa) will shorten 8”(204mm) (2.5”(64mm) elastic, 2.5”(64mm) drying shrinkage,0.5” (12mm) other).

Note: Bulge shown is exaggerated. Crack is shown to demonstrate tensile stress. Column ties resist tension stress.


Post-Tensioned Members

• The stretching of the strands compresses the concrete to offset any tension stress from future service loads

• Upon stressing, the concrete shortens (elastic


• Upon stressing, the concrete shortens (elastic shortening)

• After stressing creep will take place.ultimate creep reach after 1500 days

∆ ∆Elastic

shortening

takes place

after

stressing


∆-- ∆--Additional

shortening

occurs as

a result of

creep and

drying

shrinkage.

Post-Tensioned Members-

Restrained Volume Change

• Lack of design consideration of volume

changes in members caused by elastic and

plastic (creep) shortening

• Column design for vertical loads and subject


• Column design for vertical loads and subject

to horizontal pulling in opposite directions can

cause shear cracking

Direction of PullShort Column

Short Column

Direction of PullShear Cracking


Cylindrical Structures-

Buried pipe

• Buried pipes loaded with surrounding backfill and

overburden may result in deformation of the pipe

• The pipe is compressed in the vertical axis and bulges

in along the horizontal axis


• Cracks may develop, forming hinges at three possible

locations: the crown and at two spring line location

Overburden Loads Crack in Crown of

Pipe


Crack in Spring Line of Pipe

Pipe Deforms under Load

• The loads imposed on tank are proportional to

the material’s density and the height of liquid

in the tank

• Internal pressure pushes against the tank wall,

Cylindrical Structures-

Tanks


• Internal pressure pushes against the tank wall,

creating tension

Fluid Loading

Tension is created by loading.


Vertical cracks form if horizontal reinforcement has excessive stress levels.

Connections

• Precast structures are comprised of many

components, each interacting with others

• Point loading of contact points is quite common,

often resulting in excessive tension and shear


• Extremities and edges of members subject to point

loading are free to crack and spall when tension

stresses exceed the tensile capacities of the concrete

Slab Construction Joint with Keyway


Cracking of Unreinforced Contact Points

• Precast double-T stems resting on ledger

beams often point load the front edge of the

non reinforced portion of the ledger beam

• Point loading can be a result of rotation or


• Point loading can be a result of rotation or

length change (seasonal thermal changes)

Rotation of T at

bearing can

cause point

loading.

Double T


Ledger Beam

Spall from

Unreinforced

Front Edge.

• Slab cast on grade are separated by construction

joints.

• Shear transfer between slabs at these joints are

location where point loading can occur


• Rolling loads place the joint edges into contact with

one another, often creating stresses that spall and

crack the non-reinforced portions

• Filling of open joint with non –compressible

debris, preventing the joint from undergoing

free thermal expansion

• Restrained volume change can induce very


• Restrained volume change can induce very

high shear, compression and tension stresses.

Load induced crack

Pure flexure

Torsion (Helical cracking)

Pure tension

Shear

Bond

Concentrated load

Thank You

Documents

MAB 1033 Structural Assessment and Repair · composite material because – Similar coefficients of thermal expansion – Good bond between rebars and concrete MAB 1033 Structural