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Non-structural

Concre

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Letter Type of Cracking SubdivisionMost Common

Location

Primary Cause

(excludingrestraint)

Secondary

Causes/Factors

Time of

Appearance

A Over reinforcement Deep sections

B Arching Top of columns

C Change of depthTrough and waffle

slab

D Diagonal Roads and slabs

E RandomReinforced concrete

slabs

F Over reinforcementReinforced concrete

slabs

Ditto plus steel near

surface

G External restraint Thick wallsExcess heat

Excess bleedingRapid early drying

conditions

Plastic shrinkage

Plastic settlement

Rapid early drying

Low rate of bleeding

Ten minutes to

three hours

Thirty minutes to six

hours

H Internal restraint Thick slabsExcess temperature

gradients

I Long-term drying shrinkageThin slabs (and

walls)Inefficient joints

Excessive shrinkage

inefficient curing

Several weeks or

months

J Against formwork"Fair faced"

concrete

Impermeable

formworkRich mixes

K Floated concrete Slabs Over troweling Poor curing

L NaturalColumns and

beamsLack of cover

M Calcium chloride Precast concreteExcess calcium

chloride

I Alkali-aggregate reaction Damp locations

Reactive aggregate

plus high-alkali

cement

More than five years

Crazing

Corrosion of reinforcement

Early thermal contraction Rapid cooling

Poor quality

concreteMore than two years

One to seven days,

sometimes much

later

One day or two or

three weeks

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Why does concrete shrink?

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Non-structural Cracks

Fresh

Settlement

Plastic shrinkage

Drying shrinkage

Thermal dilation

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Before Hardening

Early frost damage

Plastic

Shrinkage

Construction movement

Formwork

Sub-grade

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PLASTIC SHRINKAGE CRACKING

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Typical Plastic ShrinkageCracking

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Evaporation of SurfaceMoisture from Concrete

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Plastic Settlement

Concre

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Subsidence Cracking

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Resistance to subsidence bytop reinforcement

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Cracks usually appear alongreinforcement bars

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..but can also form fromdifferential settlement

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Resistance to subsidence byvoid tubes in hollow core

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After Hardening Physical

Shrinkable aggregates

Drying shrinkage

Chemical

Corrosion of rebar

Alkali aggregate reaction

Carbonation

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After Hardening Thermal

Freeze thaw cycles

External seasonal temperature variations

External restraint

Internal temperature gradients

Structural

Accidental overload

Creep

Design loads

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Drying Shrinkage

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Why cracks form

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Shrinkage and Cracking

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Drying Shrinkage Cracks Occur: In thin sections (low V/S ratio)

When: Several weeks after casting

Causes:

High paste content

Inefficient joints

Poor curing

Remedies:

Reduce water content of mix

Improve curing

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Cracking tendency dependson many factors

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Shrinkage is Size Dependent

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Theoretical ShrinkageStresses

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Mitigation of DryingShrinkage Cracks Aggregate

Content Size

Workability

Curing

Eliminate externalrestraints by allowing

joint movement

Ease of placement

Consolidation

Admixtures

Chemical

Mineral

Prov e crac contro

steel distribution

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The role of fibers Fibers will:

Reduce plastic shrinkage cracking

Reduce bleeding

Fibers can:

Bridge cracks

Distribute stresses and limit crack widths

Volume is critical!

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Tensile DeformationPlain Concrete

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Tensile DeformationSteel Reinforced Concrete

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Tensile Deformation

Fiber Reinforced Concrete

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Thermal Cracking

ConcreteTechn

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Internal Thermal Restraint

HOT CENTERHOT CENTER

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Temperature Rise and

Cement Type

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ACI 224

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ACI 224

Tolerable Crack Widths

Exposure conditionTolerable crack

width, in.

Dry air or protective membrane

Humidity, moist air, soil

Deicing chemicals

Seawater and seawater spray;

wetting and drying

Water-retaining structures

0.016

0.012

0.007

0.006

0.004

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Crack Control

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Control of Cracking Joints

Isolation Contraction

Reinforcement

Cover

Size of crack

Frequency (numerous tight cracks)

Whats allowable (crack width)

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Take an

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Take an

example Sidewalk set

above pavement 500 ft long

avement stri s

= 6x10-6 in/in/F Approximately

0.7/100/100F

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Result Omission of full

depth isolation/expansion joint

Ex ansion of

adjacentpavement results

in cracking and

buckling of

concrete sidewalk

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Mid-Panel Cracking

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Crack Repair Fix large cracks prior to exposure

Use: MMA

Sealers

Routing and sealing

Effect of cover

Importance of curing on cracking

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Summary Many forms of cracking

Concrete is weak in tension Shrinkage!

o n ng

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Codes

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Questions?

Concr

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