cracking of concrete

7/28/2019 Cracking of Concrete

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Abul Kalam AzadCivil Engineering DepartmentProf.,

Presented by


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Whenevertensile stress in concrete, caused by various

effects, exceeds the tensile strength of concrete,

concrete will crack.

In the early stage of concrete formation, concrete strength

is relatively weak, and this makes it vulnerable tocracking.

Cracking


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Broadly divided into:

(a) Non-structural cracks

(due to environmental factors andconstruction process)

(b) Structural cracks

(due to applied loadings)


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Cracking phenomena begin with the plastic

state, right after casting, and continues through

the hardened state and service life. The

volumetric changes in concrete during hardening

is a major factor for cracking.

Concrete is subjected to cracking both at the

plastic and hardened state.


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Cracking in Plastic State

Plastic shrinkage

Plastic settlement

Construction movement


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Plastic Shrinkage

Caused by rapid loss of moisture due to wind,

low relative humidity and high differential temperature

Occurs within few hours of casting

Typically few cm deep; wider at top

Slab, wall-type members vulnerable


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Typical Plastic Shrinkage Cracking


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Plastic Settlement

Takes place after casting and finishing

Upward movement of water downward

movement of solids

during consolidation Results in cracks or void underrebaror

large aggregate or obstacles

Cracks usually run along the rebar

Happens within 10 mins 3 hrs.


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Plastic Settlement Cracking


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Construction Movement

Cracking due to accidental movement of

formwork within a few hours of casting.


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Hardened Concrete

Cracks can develop due to various

causes. They can be grouped into:

volumetric changes,

environmental factors,

chemical attack, and

load effects.


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Drying Shrinkage

Shrinkage from continual loss of moisture

Support restraints produce tensile stress

If tensile stress > tensile strength, cracks occur

Several weeks to months

Rapid drying at top results in deeper cracks


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Evolution of Tensile Strength and Influence

of Shrinkage and Creep on Cracking.


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Chemical Effects

Sulphate attack

Salt-weathering

Carbonation

Alkali-aggregate reaction

Corrosion of reinforcement


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Sulphate Attack

sulphates react with tricalcium aluminate

and forms calcium sulphoaluminate

results in volume expansion

leads to cracking / spalling

moisture is a key accessory


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Salt-Weathering

Evaporation of salt water absorbed by concrete

leaves salt crystals

Can cause rupture of aggregates or matrix

Attack proceeds from the surface to inward

Vulnerable areas: above ground level and splash zones

Takes few years to develop cracks


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Carbonation

Alkalinity reduced by acidic gases

(CO2, SO2) that penetrates into concrete,

by forming carbonates and sulphates. The Ph value greatly reduced.

If the carbonated front advances to rebar,

passivity of rebar is destroyed.

Rebar becomes vulnerable to corrosion.


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Cracking due to Carbonation

(a)

(b)

(c)


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Thermal Cracking

Three possibilities:

Freeze-thaw

Early thermal contraction

Temperature variation


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Early Thermal Contraction

Heat of hydration increases temp. within concrete

This causes a small expansion in volume

As concrete cools, it contracts

Any restraint to contraction causes tensile stress

Can occur within one day to 2 weeks from casting


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Freeze-Thaw

In low temp., moisture within concrete and

pore water freezes.

The expansion due to freezing causes

internal pressure.

Repeated freeze-thaw causes damage.


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Temperature Variation

Two effects:

(a) Expansion / contraction

(b) Thermal gradients

Another effect: Thermal incompatibility of aggregates

T2

T1T1

T2


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Alkali-Aggregate Reaction

Reaction of some aggregates with alkali

to form expansive products in moist environment Common form of reaction alkali-silica,

though alkali-carbonate also may occur.

Moisture is necessary for such reaction


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Alkali-aggregate reaction


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Corrosion of Rebars

Formation of anodic and cathodic cells

Transformation of metallic ion into rust causes

large volume expansion

Volumetric expansion leads to cracking

Corrosion cracks runs parallel to bars


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Corrosion Cracks


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Chloride-induced Corrosion

Predominate cause of corrosion

Chloride destroys the passivity

Concrete has the ability to chemically bind some

chloride

Excess chloride triggers corrosion

Needs moisture and oxygen for electrochemical

corrosion


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Sources of Chlorides

contaminated aggregates (internal)

mixing / curing water

external

salt-laden environment(sabkha, coastal belts, etc.)

several mechanisms for chloride penetration

into concrete chloride diffusion and absorption are common

mechanisms


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Poor Construction

Improper practice can help the cracking

phenomena and magnify cracks due to

various causes:

Impropermix design,construction methods,

design &detailing


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Structural Cracks

Stress-induced cracking from external loads and forces

Usually fine for well-designed members

Under excessive tensile stress (tension, bending

and torsion), cracks can be wide and well-formed.


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Cracks in Beams


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Thank you( Questions and discussion)

cracking of concrete

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