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DURABILITY of CONCRETE

STRUCTURES

Assist. Prof. Dr. Mert Yücel YARDIMCI

Part- 3 Concrete Cracks

This presentation covers the subjects in CEB Durable Concrete Structures Guideline and has been prepared by the graduate students under the supervision of Prof.Dr.Bülent BARADAN in Dokuz Eylul University.

Causes of cracks

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Cracking will occur whenever the tensile strain to which concrete is subjected exceeds the tensile strain capacity of the concrete. The tensile strain capacity of concrete varies with age and with rate of application of strain.

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TYPES & FORMATION of CRACKS EARLY FROST DAMAGE

FRESH STATE PLASTIC SHRINKAGE

SHRINKAGE

SETTLEMENT CONSTRUCTURAL

MOVEMENTS FORMWORK MOVEMENT

BASE SETTLEMENT

HARDENED STATE

PHYSICAL

SHRINKABLE AGGREGATES

DRYING SHRINKAGE

BLEEDING

CHEMICAL

BIOLOGICAL

CORROSION of REINFORCEMENT

ALKALI-SILICA REACTION

ACID ATTACK

SULFATE ATTACK

THERMAL

FREEZING & THAWING

TEMPERATURE DIFFERENCES

EARLY TERMAL SHRINKAGE EXT.

INT.

CONSTRUCTURAL

(MECHANICAL)

EXCESSIVE LOADING

CREEP

INAPPROPRIATE DESIGN

SETTLEMENT OF SUPPORT

CARBONATION

DELAYED ETTRINGATE FORMATION (DEF)

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Bending Cracks

A

A J

I I I

C

K

C

E

F

Rust stains

B

L

M

N

N I

I

D

D

I

H

H

G

G

Cold dilations

C

O

O

O

O

O

Relative Shrinkage craks

Relative shrinkage

craks

Late, ineffective dilations

Cold dilations

Shear crack

Bonding cracks

K

B

Bending crack

C

G

See Table 3.1 in CEB

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FORMATION of CRACKS & TIMES

Loading, service

conditions

Alkali-Aggregate

Reaction

Corrosion

Drying

Shrinkage

Early thermal

Shrinkage

Plastic

Shrinkage

Plastic

Settlement

1 hour 1 day 1 week 1 month 1 year 50 years

1 hour 1 day 1 week 1 month 1 year 50 years

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13.3 CRACK CONTROL

Cracks that may cause corrosion or may influence the apperance of structures should not be permitted.

TS 500 (February 2000) (Requirements for design and construction of reinforced concrete structures)

TYPES & FORMATION of CRACKS

Max crack width

(max)

0.4 mm Normal environment and indoors

0.3 mm Normal environment and humid indoors

0.2 mm Humid environment and outdoors

0.1 mm Aggressive environment, indoors & outdoors

Shrinkage

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Shrinkage is the reduction in volume of concrete by different mechanisms.

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DISADVANTAGES of SHRINKAGE:

2. FORMATION of EXTRA STRESSES IN REINFORCEMENT

SHRINKAGE CRACKS

1. FORMATION of CRACKS

HAZARDS of CRACKS • DECREASE TENSILE STRENGTH

• WITH THE EASIER INGRESS of WATER

• FREEZE-THAW RESISTANCE

• DURABILITY AGAINST CHEMICAL ATTACKS

RESTRAINMENT of SHRINKAGE DEFORMATIONS CAUSE CRACKS.

DECREASES!

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•DRYING SHRINKAGE (Hydraulic shrinkage)

TYPES OF SHRINKAGE

•THERMAL SHRINKAGE

LOSS of PORE WATER

DIFFERENCE of TEMPERATURE – IMPORTANT in MASS CONCRETE

•AUTOGENOUS SHRINKAGE REDUCTION in VOLUME DUE to HYDRATION of CEMENT

•PLASTIC SHRINKAGE BLEEDING RATE < EVAPORATION RATE

•CARBONATION SHRINKAGE EVAPORATION of WATER in CHEMICAL REACTION

3Ca(OH)2+CO2 CaCO3+H2O

Drying shrinkage

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The moisture content of hardened concrete has a potentially significant influence on the volume that it occupies, with an increase in the level of moisture producing an increase in volume.

Moisture content is controlled by the relative humidity of the surrounding environment

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Drying shrinkage

The reason of drying shinkage is associated with; the development of capillary pressure the drying of calcium silicate hydrate (CSH)

gel, a reaction product formed during the setting and hardening of cement.

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Drying shrinkage

In a concrete pore, the two fluids are water and air. A pressure difference develops when two immiscible fluids are present in a capillary and only one is capable of wetting the surface. This attractive capillary pressure (pc) is described by the Young–Laplace equation.

As the relative humidity in concrete drops (i.e. as it dries), water in progressively smaller pores will evaporate, causing the value of r in the equation to decrease. This increases the capillary pressure and causes shrinkage.

Development of capillary pressure

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Drying shrinkage

The main reaction product of Portland cement is CSH gel, which typically comprises 50% to 60% by volume of a mature Portland cement (PC) paste.

Drying of CSH gel

The gel is an aggregation of colloidal particles (particles having at least one dimension between 1 nm and 10 μm). Each gel particle is composed of layers of calcium silicate sheets, which are distorted and arranged on top of each other in a disordered manner. The nature of this configuration means that there is much space between the layers, which can be occupied by ‘interlayer’ water.

The absorption of this “interlayer water” by the gel leads to swelling, and evaporation of water leads to shrinkage.

Water

CSH layers

Effects of aggregates on drying shrinkage

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Most aggregate materials shrink to a lesser extent relative to cement paste, and although the cement fraction comprises a much smaller volume fraction than the aggregate, it normally has the largest influence on shrinkage.

The higher modulus of elasticity of most aggregate relative to cement paste has a restraining effect that limits shrinkage.

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Effects of aggregates on drying shrinkage There is usually a good correlation between the water absorption capacity of aggregates and the magnitude of drying shrinkage observed in concrete containing it

The reason for the correlation between water absorption and shrinkage is the result of the strong relationship between the porosity of aggregate and its stiffness.

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Effects of w/c ratio on drying shrinkage

As the w/c ratio reduces, the modulus of elasticity of the hardened cement paste increases, presenting a greater resistance to shrinkage

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Effects of different mineral admixtures on drying shrinkage

Shrinkage curves obtained from mortars containing a combination of Portland cement and various size fractions of FA is shown below. In all cases, FA has the effect of reducing shrinkage, with the finer fractions producing the least amount of length change. The cause of this effect is that the finer FA fractions permit greater reductions in water content to achieve a given consistence, which means that the mortars with finer ash fractions have lower W/C ratios. However, there may be other mechanisms effective because even the coarsest ash, which is coarser than the PC and requires a higher W/C ratio, produces lower shrinkage than the control mortar.

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Silica fume typically has the effect of very slightly increasing shrinkage at early ages. Later, as ultimate shrinkage is approached, shrinkage is lower at higher silica fume levels.

Effects of different mineral admixtures on drying shrinkage

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•INTERNAL EFFECTS

FACTORS THAT INFLUENCE SHRINKAGE

CHEMICAL COMPOSITION (CaO, MgO & SO3)

CEMENT DOSAGE

HEAT of HYDRATION

QUANTITY of MIXING WATER

MODULUS of ELASTICITY of AGGREGATE

•EXTERNAL EFFECTS

LOW HUMIDITY

SPEED of WIND

HIGH TEMPERATURES

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•DESIGN DETAILS

FACTORS THAT INFLUENCE SHRINKAGE

AREA/VOLUME RATIO of STRUCTURAL ELEMENTS

V1 = V2

Eva1 < Eva2

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•DESIGN DETAILS

FACTORS THAT INFLUENCE SHRINKAGE

AREA/VOLUME RATIO of STRUCTURAL ELEMENTS

PERCENTAGE of REINFORCEMENT

UNIFORMITY of REINFORCEMENT PLACEMENT

UNPROPER CONSTRUCTION JOINTS

F1 = F2

RIGHT WRONG

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•MIX DESIGN

PREVENTIVE MEASURES for SHRINKAGE CRACKS

•OPTIMUM AMOUNT of CEMENT

•MINIMUM AMOUNT of MIXING WATER

•MAXIMUM AMOUNT of COARSE AGGREGATE (GOOD QUALITY)

•CEMENT MINIMUM SHRINKAGE, LOW HEAT of HYDRATION, NOT TOO FINE.

•GOOD CURING

•DESIGN DETAILS SUFFICIENT AMOUNT of REINFORCEMENT,

UNIFORM PLACING

•INCORPORATION of MICRO FIBERS

Effect of aggregate content on drying shrinkage

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24 Back

What is the form of crack formation in plain and reinforced concrete sections due to drying shrinkage?

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Restraint is important!

The crack spacing can be reduced by reducing the bar diameter, improving the bond, increasing the quantity of steel. Reducing the depth of the cover will also reduce the spacing.

Strain development due to drying (Eurocode 2 approach)

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The tensile strength of concrete is substantially less than its compressive strength, usually between 5% and 15%. It develops with time in a similar manner with compressive strength, as shown in Figure 2.18.

If the concrete element is restraint, the tensile stress due to shrinkage may exceed the tensile strength capacity of concrete. If this happens concrete cracks!

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Strain development due to drying (Eurocode 2 approach)

Typical plot of drying shrinkage against time

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No instantaneous deformation due to drying

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Autogenous shrinkage

As Portland cement undergoes hydration reactions, free water will be converted to chemically combined or chemisorbed water on hydrate surfaces. This loss of free water can also lead to shrinkage. This is called autogenous shrinkage.

This means that, even if evaporation from concrete is entirely prevented, there will be a considerable reduction in the volume of water present, and shrinkage will occur via the same mechanisms as for drying shrinkage.

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Reducing the problem of cracking resulting from shrinkage

Inclusion of movement joints Using reinforcement for crack control Using steel fibres or macrosynthetic fibres Reducing the cement content The minimum cement content which is required for strength and durability requirements.

Reducing the W/C ratio Since the water content of the concrete will be fixed to provide the required consistence to the fresh concrete mix, a reduction in the W/C ratio will lead to an increase in the cement content. For this reason, the use of water-reducing admixtures or superplasticisers is likely to be necessary to allow a reduction in the water content. NOTE: Remember that reduction of the W/C ratio to very low levels, while reducing drying shrinkage significantly, may lead to higher levels of autogenous shrinkage.

Selection of aggregate with a high stiffness Using shrinkage-reducing admixtures can be used to modify

the surface tension of the pore water.

Go to fiber pictures

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SETTLEMENT CRACKS

Main Causes:

Poor grading, too much mixing water,

unsufficient compaction,

Water & cement mortar

Before Settlement

Reinforcement

After Settlement

Cracks

Coarse aggregate

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A

PLASTIC SETTLEMENT CRACKS

Subdivision : Over Reinforcement – Deep Sections

Causes: Excess Bleeding – Rapid early drying conditions

Remedy : Reduce bleeding (air entrainment) or revibrate

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A

B

B

PLASTIC SETTLEMENT CRACKS

Subdivision : Arching – Top of columns

Causes: Excess Bleeding – Rapid early drying conditions

Remedy : Reduce bleeding (air entrainment) or revibrate

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A

B

B

C

PLASTIC SETTLEMENT CRACKS

Subdivision : Change of depth – Trough & waffle slabs

Causes: Excess Bleeding – Rapid early drying conditions

Remedy : Reduce bleeding (air entrainment) or revibrate

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SETTLEMENT CRACKS

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PLASTIC SETTLEMENT CRACKS

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CAUSE:

EVAPORATION > BLEEDING RATE

DRYING OF TOP SURFACES

Evaporation

Bleeding

PLASTIC SHRINKAGE CRACKS

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PLASTIC SHRINKAGE CRACKS

D

Subdivision : Diagonal – Roads & Slabs

Causes: Rapid early drying – Low rate of bleeding

Remedy : Improve early curing

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PLASTIC SHRINKAGE CRACKS

D

E

Subdivision : Random – Reinforced concrete slaps

Causes: Rapid early drying – Low rate of bleeding

Remedy : Improve early curing

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D

E

F

PLASTIC SHRINKAGE CRACKS

Subdivision : Over reinforcement – Reinforced concrete slaps

Causes: Rapid early drying – Steel near surface

Remedy : Improve early curing

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PLASTIC SHRINKAGE CRACKS

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PLASTIC SHRINKAGE CRACKS

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PLASTIC SHRINKAGE CRACKS

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EARLY THERMAL CONTRACTION

G

Subdivision : External restraint – Thick walls

Causes: Excess heat generation – Rapid cooling

Remedy : Reduce heat &/or insulate

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EARLY THERMAL CONTRACTION

G

H

Subdivision : Internal restraint – Thick slabs

Causes: Excess temperature – Rapid cooling

Remedy : Reduce heat &/or insulate

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LONG TERM DRYING SHRINKAGE

I

I

Subdivision : Thin Slaps (and walls)

Causes: Insufficient joints – Excess shrinkage, inefficient curing

Remedy : Reduce water content, improve curing

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LONG TERM DRYING SHRINKAGE

Insufficient joints

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SPALLING (crazing)

J

Subdivision : Against formwork – “fair-faced” concrete

Causes: Impermeable formwork- Rich mixes, poor curing

Remedy : Improve curing & Finishing

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SPALLING (crazing)

J

K

Subdivision : Floated concrete - Slabs

Causes: Over-trowelling - Rich mixes, poor curing

Remedy : Improve curing & Finishing

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CORROSION of REINFORCEMENT

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CORROSION of REINFORCEMENT

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L

Rust

Stains

CORROSION of REINFORCEMENT

Subdivision : Natural – Columns & Beams

Causes: Lack of cover – Poor quality concrete

Remedy : Eliminate causes listed

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L

M

CORROSION of REINFORCEMENT

Subdivision : Calcium chloride – Precast concrete

Causes: Excess calcium chloride – Poor quality concrete

Remedy : Eliminate causes listed

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CORROSION of REINFORCEMENT

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CORROSION of REINFORCEMENT

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CORROSION of REINFORCEMENT

Electrical pillars with corrosion damage in İZMİR

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ALKALI-AGGREGATE REACTION

N

Subdivision : Damp locations

Causes: Reactive aggregate plus high-alkali cement

Remedy : Eliminate causes listed

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ALKALI-AGGREGATE REACTION

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ASR

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ASR

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DEF (DELAYED ETTRINGITE FORMATION)

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THAUMASITE FORMATION

Swiss tunnel structures: concrete damage by formation of thaumasite (Romer vd. 2003)

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LOADING CRACKS

PURE BENDING

PURE TENSION

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LOADING CRACKS

TORSION

Bending

Bond Cracks

CONCENTRATED

LOAD

SHEAR

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LOADING CRACKS

SHEAR

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LOADING CRACKS

SETTLEMENT CRACKS

Settlement of Support

Cracks

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Influence of various concrete constituent characteristics on bleeding and plastic shrinkage cracking

Configurations in concrete structural elements that may cause plastic settlement cracking.