DURABILITY OF CONCRETE

LECTURE NOTES

PROF. DR. KAMBZ RAMYAR

Ege University, Civil Engineering Department, IZMIR

1. THE STRUCTURE OF CONCRETE

Structure property relationship is at the heart of modern material science. Concrete

has a highly heterogeneous and complex structure. Therefore, it is very difficult to

constitute exact models of the concrete structure from which the behavior of the material

can be reliably predicted. However, knowledge of the structure and properties of the

individual components of concrete and their relationship to each other is useful for

exercising some control on the properties of the material.

In this chapter, three components of concrete structure - the hydrated cement paste

(hcp), the aggregate, and the transition zone between the cement paste and the aggregate

- will be described. The structure-property relationships are discussed from the standpoint

of durability.

The composition, amount, size and distribution of phases in a solid constitute its

structure. The gross elements of the structure of a material can readily be seen by unaided

human eye. The term macrostructure is generally used for the gross structure, visible to

the human eye (limit of resolution of the unaided human eye: 1/5 mm or 200 m). The finer elements are usually resolved with the help of microscope. The term microstructure is

used for the microscopically magnified portion of a macrostructure. The magnification

capability of the modern electron optical microscopes is of the order of 105 times. Progress

in the field of materials has resulted primarily from recognition of the principle that the

properties of a material originate from its internal structure; in the other words the

properties can be modified by making suitable changes in the structure of material. The

structure of concrete is heterogeneous and highly complex. The structure property

relationships in concrete are not yet well developed. However, an understanding of some

of the elements of the concrete structure is essential for discussing the engineering

properties of concrete such as strength, elasticity, shrinkage, creep and cracking as well

as durability.

Durability is the resistance of concrete to environmental (weathering) conditions,

chemical effects, abrasion and other harmful facts during its service life. Some of the

aspects involved in concrete durability are;

- Freezing thawing resistance

- Wetting drying resistance

- Heating cooling resistance

- Abrasion resistance

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- Fire resistance

- Acid resistance

- Resistance to chemical reactions causing volume expansion such as alkali-

aggregate reaction, sulfate attack, etc

1.1. Complexities of Concrete Structure

At the macroscopic level, concrete may be considered to be a two-phase material,

consisting of aggregate particles of varying sizes and shapes dispersed in a matrix of the

hydrated cement paste.

At the microscopic level, the complexities of the concrete structure becomes visible;

i.e. the two phases of the structure are neither homogeneously distributed with respect to

each other, nor are they themselves homogeneous. For instance, in some areas the hcp

mass appears to be as dense as the aggregate while in others it is highly porous. It is also

known that the volume of capillary voids in the hcp would decrease with decreasing W/C

ratio or with increasing degree of hydration.

For a well-hydrated cement paste, the inhomogeneous distribution of solids and

voids alone can perhaps be ignored when modeling the behavior of the material. However,

micro-structural studies have shown that this cannot be done for the hcp present in

concrete. In the presence of aggregate particles is usually very different from the structure

of the bulk paste or mortar in the system. In fact, many aspects of concrete behavior under

stress can be explained only when the cement paste-aggregate interface is treated as a

third phase of the concrete structure. Thus, the structure of concrete can be summarized

as follows:

1. There is a third phase, the transition zone, which represents the interfacial region

between coarse aggregate particles and the hcp. Existing as a thin shell typically 10 to 50

m thick around large aggregate, the transition zone is generally weaker than the other two phases of concrete. Therefore, it has a great influence on the mechanical properties of

concrete than is reflected by its size.

2. Each of the three phases is itself multiphase in nature. For instance, each

aggregate particle may contain several minerals, in addition to micro-cracks and voids.

Similarly, both the bulk hcp and the transition zone generally contain a heterogeneous

distribution of various types and amounts of solid phases, pores and micro-cracks.

3. Unlike other engineering materials, the structure of concrete does not remain

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stable; i.e., it is not an intrinsic characteristic of the material. This is due to the fact that hcp

and the transition zone are subject to change with time, humidity and temperature.

The highly heterogeneous and dynamic nature of the structure of concrete are the

primary reasons why the theoretical structure property relationship models, generally so

helpful for predicting the behavior of engineering materials, are of little use in the case of

concrete. A broad knowledge of the important feature of the structure of individual

components of concrete is nevertheless essential for understanding and controlling the

properties of the composite materials.

1.2. Structure of the Aggregate Phase

The aggregate phase is essentially responsible for the unit weight, elastic modulus

and dimensional stability of concrete. These properties of concrete depend to a large

extend on the bulk density and strength of the aggregate, which, in turn, are determined by

the physical rather than chemical characteristics of the aggregate structure. In other

words, the chemical or mineralogical composition of the solid phase in aggregate is usually

less important than the physical characteristics such as the volume, size and distribution of

pores.

Obviously, the chemical and mineralogical character of aggregate in some cases, i.e.

alkali aggregate reaction, become of great importance affecting the durability of

concrete.

In addition to porosity, the shape and surface texture of the coarse aggregate also

affect the concrete properties. Generally, natural gravel has a rounded shape and a

smooth surface texture. Crushed rocks have a rough texture and usually angular shape;

however, depending on the rock type and choice of crushing equipment, the crushed

aggregate may contain a considerable proportion of flat and elongated particles, which

adversely affect many properties of concrete.

Lightweight aggregate particles from pumice, which are highly cellular, are also

angular and have a rough surface texture, but those from expanded clay or shale are

generally rounded and smooth.

Being generally stronger and more durable than the other 2 phases of concrete, the

aggregate phase has no direct influence on the properties of concrete except in the case

of some highly porous, or weak or alkali-reactive aggregates.

The size and shape of coarse aggregate may, however, affect the strength of

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concrete in an indirect way. It is known that increasing the maximum size of the aggregate

increases the proportion of elongated and flat particles. Besides, in such a case (higher

Dmax) the tendency for water films to accumulate next to the aggregate surface (internal

bleeding) increases. Internal bleeding, in turn, weakens the cement paste aggregate

zone.

1.3. Structure of Hydrated Cement Paste

The details of the composition and properties of the hydrated cement paste are out of

the scope of this course. However, a summary of the composition will be beneficial in

understanding some aspects of concrete durability.

Anhydrous Portland cement is a gray or brownish gray powder that consists of

angular particles typically in the size range of 1 to 50 m. It is produced by pulverizing a clinker with ~2-5% gypsum. The clinker is a heterogeneous mixture of various minerals

(mainly calcium silicate and calcium aluminates) produced by high-temperature reactions

between calcium oxide (CaO), Silica (SiO2), alumina (Al2O3) and iron oxide (Fe2O3). The

chemical composition of the principal clinker minerals (major compounds) corresponds

approximately to C3S, C2S, C3A and C4AF.

In ordinary portland cement the amounts of the major compounds varies in the

following ranges;

C3S 45 to 60%

C2S 15 to 30%

C3A 6 to 12%

C4AF 6 to 8%

When portland cement is dispersed in water, the calcium sulfate and the high-

temperature compounds of calcium tend to go into solution, and the liquid phase gets

rapidly saturated with various ionic species. As a result of combinations between calcium,

sulfate, aluminate and hydroxyl ions within a few minutes of cement hydration, first the

needle-shaped crystals of a calcium sulfoaluminate hydrate called ettrengite make their

appearance; a few hours later large prismatic crystals of calcium silicate hydrates begin to

fill the empty space formerly occupied by water and the dissolving cement particles. After

some days, depending on the alumina-to-sulfate ratio of the Portland cement, ettringite

may become unstable and decompose to form the monosulfate hydrate, which has

hexagonal-plate morphology. Hexagonal-plate morphology is also characteristics of

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calcium aluminate hydrates, which are formed in the hydrated pastes of either

undersulfated or high-C3A portland cements. The relevant chemical reactions may be

expressed as:

323644 HSAC ag. Ca6 SO3 AlO 22 ..Ettringite

18644 HSAC ag. Ca4 SO AlO 22 ..Monosulfate

CH H - S - C H SC or SC 32

After depletion of sulfate ions in the solution, when the aluminate concentration goes

up again due to renewed hydration of C3A (and C4AF), ettringite becomes unstable and is

gradually converted into monosulfate, which is the final product of hydration of Portland

cements containing more than 5% C3A:

1843 3236 HSA3C A 2C HSAC

A model of the essential phases present in the microstructure of a well hydrated

Portland cement paste is shown in Fig 1.1.

Fig 1.1 Model of a well-hydrated Portland cement paste.

A represents aggregation of poorly crystalline C-S-H particles which have at least one colloidal dimension (1 to 100 nm). Inter particle spacing within an aggregation is 0.5 to

3 nm (av. 1.5 nm).

H presents hexagonal crystalline products such as CH, 184 HSAC and C4AH19.

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They form large crystals, typically 1 m wide. C represents capillary voids originally occupied with water and is not filled

completely with the hydration products. Size of capillary voids ranges from 10 nm to 1 m. In low W/C ratio and well hcp they are < 100 nm.

From the microstructure model of the hcp shown in Fig. 1.1, it is obvious that the

various phases are neither uniformly distributed nor they are uniform in size and

morphology. In solids micro-structural heterogeneities may lead to serious affects on

mechanical properties because these properties are controlled by the micro-structural

extremes, not by the average microstructure. Thus, in addition to the generation of the

microstructure as a result of cement hydration, attention has to be paid to certain

rheological properties of freshly mixed cement paste which are also influential in

determining the microstructure of the hcp. For example, the unhydrated cement particles

have a tendency to attract each other and form flocks, which entrap large quantities of

mixing water. Obviously, local variations in W/C ratio would be the primary source of

evolution of the heterogeneous pore structure. With highly flocculated cement paste

systems not only the size and shape of pores but also the crystalline products of hydration

are known to be different when compared to well dispersed systems. The types, amounts

and characteristics of the four principle solid phases generally present in a hcp, that can

be resolved by an electron microscope, are as follows:

1.3.1. Calcium silicate hydrate:

C-S-H phase makes up 50 to 60% of the volume of solids in a completely hydrated

Portland cement paste and is, therefore, the most important phase in determining the

properties of the paste. C-S-H is not a well defined compound; the C/S ratio varies

between 1.5 and 2.0 and the structural water content varies even more. The morphology

of C-S-H also varies from poorly crystalline fibers to reticular network. Due to their colloidal

dimensions and tendency to cluster, C-S-H crystals could only be resolved by electron

optical microscopy.

The internal crystals structure of C-S-H also remains unresolved. Previously it was

assumed to resemble the natural mineral tobermorite, this is why C-S-H was sometimes

called tobermorite gel.

Although the exact structure of C-S-H is not known, several models have been

proposed to explain its properties. According to Powers Brunauer model, the material has

a layer structure with a very high surface area. Depending on the measurement technique,

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surface areas on the order of 100 to 700 m2/g have been proposed for C-S-H. The

strength of the material is attributed mainly to the Van der Waals forces, the size of gel

pores or solid to solid distance being about 18 A. According to Feldman-Sereda model the C-S-H structure is composed of an irregular or kinked array of layers which are

randomly arranged to creat interlayer spaces of different shapes and sizes (5 to 25 A). Models of C-S-H structure will be discussed more in detail later. Two forms of C-S-H can

be identified in the microstructure.

Early Product C-S-H: During early hydration, C-S-H grows out from the particle surface into the surrounding water-filled space in the form of a low-density arrangement of

thin sheets. This form of outer product C-S-H (early product) has a higher microporosity

and on drying rearranges to a variety of morphological forms and coarser porosity. This C-

S-H also contains a high level of impurities (aluminum, sulfate, alkalis) and probably

admixed with monosulfate at the nanometer level. This rather variable component of C-S-

H also has been called both the groundmass and undesignated product (UDP).

Late Product C-S-H: Once hydration has become diffusion controlled, C-S-H forms primarily as a denser coating around the hydrating cement grains, referred to as either late

or inner product. These coatings form the diffusion barrier during later hydration and

thicken with time, growing inwards as well as outwards. The coatings maintain the shape

of the original grains by surrounding unhydrated residues. These prominent features have

been called phenograins although the term has been used to describe any significant

feature that is distinct from the groundmass regardless of their composition. This late

product C-S-H is denser, has less impurities, and is more resistant to physical change on

drying. The proportion of late product C-S-H increases as hydration increases or the W/C

ratio decreases.

Model of C-S-H Structure

Because of

- its amorphous character,

- compositional variation,

- and poorly resolved morphology,

C-S-H is a difficult material to study. Various conceptual models have been proposed that

emphasize different aspects of the structure to explain observed experimental results. No

one model can be considered to be correct description in any absolute sense, but a good

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model will provide additional insights into the behavior of a material and predict hitherto

unrecognized properties.

Models must be modified as new data are obtained. The following simplified description is

based on several models.

C-S-H can be considered to have a degenerated clay structure by which it means

that it is based on a layer structure. A well x-tallized clay mineral has the structure shown

in Fig.a. it can be thought of as being composed of layers of bread and filling to make a

sandwich. The bread is composed of silico aluminate sheets that are stacked in a

specific orientation the filling is made up of metal ions that hold the sheets together with

comparatively weak electrostatic attractions between positive changes on the metal ions &

residual negative charges on the sheets. Water also present between the layers. In some

clays, the layers can be expanded to accommodate additional water, thereby expanding

the x-tal. Loss of interlayer water on drying allows the layers to collapse again and the x-tal

to contract. Thus, clays show large volume changes on wetting and drying, & C-S-H

behaves similarly.

Fig a) Well x-tallized clay mineral Fig b) poorly x-tallized C-S-H

In C-S-H the bread is calcium silicate sheet and the filling is additional calcium

ions and water molecules. Unlike a well x-tallized clay mineral, however, the sheets are

distorted and randomly arranged. Thus, they do not fit together neatly (Fig.b).

As a result, the spaces between the sheets are irregular and vary considerably in

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their dimensions. (Clay minerals may be visualized as a stack of sheets of copy paper. If

these sheets are crumpled up one by one, smoothed out, and restacked, they will not lie

perfectly flat and will be more randomly arranged with respect to each other. This is the C-

S-H structure).

The space between the calcium silicate sheets is the intrinsic porosity of C-S-H.

Three kinds of pores may be distinguished.

- Interlayer pores (I),

- Micro-pores (M),

- Isolated capillary pores (P).

Capillary pores are spaces in which water can behave as bulk water and menisci are

created as the pores are filled or empitied. In microscopes, the adjoining surfaces are so

close together that water can not form menisci and consequently has a different behavior

from bulk water. Water in micro-pores acts to keep the layers apart by exerting a disjoining

pressure. The disjoining pressure depends on the relative humidity and disappears below

50%RH. When the sheets forming the micro-pores approach closely in a specific

orientation, they may form clay like interlayer spaces (I) that bond the sheets together at

this point. Interlayer bonding can be regarded as a special case of Van Der Waals

bonding. In addition, sheets will from time to time be bonded directly by strong ionic-

covalent bonds, which do not involve the weaker interlayer bonding.

Calcium-hydroxide Portlandite or calcium hydroxide crystals constitute 20 to 25% of

the volume of solids in the hydrated paste. In contrast to C-S-H, calcium hydroxide is a

crystalline material with a fixed composition. It tends to form large crystals with a distinctive

hexagonal prism morphology. The morphology is affected by the available space

temperature of hydration and impurities present in the system. Due to its considerably

lower surface area, the strength contributing potential of CH is very low because of limited

van der waals forces. Besides, the presence of a considerable amount of CH in hcp has

an adverse effect on chemical durability to acid solutions as well as sulfates. The former is

because of higher solubility and the latter is due to higher reactivity of CH compared to that

of C-S-H.

1.3.2. Calcium sulfa-aluminates

These occupy 15 to 20% of the solids volume in the hydrated paste and therefore,

play only a minor role in the structure-property relationships. It was already mentioned that

during the early stages of hydration the sulfate/alumina ionic ratio of the solution phase

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generally favors the formation of ettringite ( 3236 HSAC ), which forms needle-shaped

prismatic crystals. In pastes of ordinary Portland cement, ettringite eventually transforms to

he monosulfate hydrate, 184 HSAC , which forms hexagonal-plate crystals. The presence of

monosulfate in Portland cement concrete makes the concrete vulnerable to sulfate attack

upon secondary ettringite formation. It is known that both ettringite and monosulfate

contain small amounts of ironoxide, substituting for some alumina in the crystal structure.

1.3.3. Unhydrated Cement :

Depending on the particle size distribution and degree of hydration of the Portland

cement, some of its particles may remain unhydrated in the microstructure of hcp , even

long after hydration. As stated earlier, the particles of modern cements range in size from

1 to 50 m. with the progress of hydration reactions, the smaller particles get dissolved and than the larger particles appear to grow smaller. Since the available space between

particles is limited, the hydration products tend to crystallize in vicinity or even on the

surface of unhydrated clinker particles. This gives the appearance of a coating formation

around them. At later ages, due to lack of available space, the hydration of unhydrated

particles results in the formation of a very dense hydration product, which at times

resembles the original clinker particle in morphology.

1.4. Voids in Hydrated Cement Paste

In addition to solid phase, hcp contains several types of voids which have an

important effect on its properties. The typical types of voids in hcp as well as the size of

solid phases are shown in Fig. 1.2.

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Fig.1.2. Typical size ranges of solids and pores

Interlayer spacing in C-S-H Powers suggested that the width of the interlayer space

within the C-S-H structure to be 18 A and determined that it accounts for 28 percent porosity in solid C-S-H. However, Feldman and Sereda found that the space may vary

from 5 to 25 A. This pore size is too small to have an adverse effect on strength and permeability of the hcp. However, water in these small voids can be held by hydrogen

bonding, and its removal under certain conditions may contribute to drying shrinkage and

creep.

Capillary pores: Capillary pores are originally water filled spaces which are not filled by the solid

hydration products of cement. During the hydration process the total volume of cement-

water mixture remains essentially unchanged. Since the interlayer space within the C-S-H

phase is considered as a part of the solids in hcp, the average bulk density of the

hydration products is considerably lower than the density of unhydrated cement. It is

estimated that 1 cm3 of cement, upon complete hydration, occupies roughly 2.05 cm3.

Thus, upon progress of hydration process the space originally filled by cement and water

is gradually replaced by hydration products. The space not taken up by the unhydrated

cement or hydration products consists of capillary voids, the volume and size of which

depends on W/C ratio (determining the original distance between the anhydrous cement

particles in the freshly mixed cement paste) and the degree of hydration.

In well-hydrated, low W/C ratio pastes, the capillary pores may range from 10 to 50

nm; in high W/C ratio, at early stages of hydration the capillary voids may be as large as 3

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to 5 m. It is shown that the pore size distribution controls the strength, permeability and volume changes in a hcp rather than the total porosity capillary pores larger than 50 nm,

referred to as macropores, are assumed to be detrimental to strength and impermeability,

while voids smaller than 50 nm, referred to as micropores, are assumed to be more

effective upon drying shrinkage and creep of the hcp.

Air Voids; In spite of capillary pores which are irregular in shape, the air voids are generally

spherical. In order to improve the freezing-thawing resistance or workability or for

economical considerations, air entraining admixtures may be added to concrete to entrain

very small air voids (in the range of 50 to 200 m) in the cement paste. Besides, during mixing and placing operations, entrapped air voids as large as 3 mm (3000 m) may usually be formed in the fresh cement paste. Thus, both the entrapped and entrained air

voids in the hcp are much larger than the capillary voids, and are capable to affect the

strength and impermeability adversely.

1.5. Water in Hydrated Cement Paste

Electron microscopic examination reveals that the voids in the hcp are empty. This is

because the specimen preparation techniques require drying the specimen under high

vacuum. Actually, depending on the relative humidity and porosity of the paste, the

untreated cement paste is capable of holding a large amount of water. Water can exist in

the hcp in many forms. The classification of water into several types is based on the

degree of difficulty or ease with which it can be removed from the hcp. Since there is a

continuous loss of water from a saturated cement paste as the relative humidity is

reduced, the dividing line between various states of water is not rigid. In spite of this,

classification is useful for understanding the properties of hcp. In addition to vapor in

empty or partially water-filled pores, water exists in hcp in four different states.

1.5.1. Capillary Water;

This is the bulk water free from the influence of the attractive forces exerted by the

solid surface and existing in pores greater than 50 A. Actually depending on the behavior of capillary water in the hcp, it may be divided into two categories. (1 nm = 10 A = 10-10 m)

a. The water held by capillary tension in small capillaries (5 to 50 nm) which or

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removal may cause shrinkage.

b. The free water held in large voids of the order of > 50 nm, the removal of which

does not cause any volume change.

1.5.2. Absorbed Water;

This is the water close to the solid surface due to the influence of attractive forces

causing water molecules physically absorbed on to the solid surface in the hcp. It has

been suggested that up to six moleculer layers of water (~15A) can be physically held by hydrogen bonding. Since the bond energies of the individual water molecules decrease

with distance from the solid surface, a major portion of the absorbed water can be lost by

drying the hcp to 30% relative humidity. The loss of absorbed water is the main cause of

the shrinkage of the hcp on drying.

1.5.3. Interlayer Water;

This is the water associated with the C-S-H structure. It has been suggested that a

monomolecular water layer between the layers of C-S-H is strongly held by hydrogen

bonding. This water is lost only on strong drying (i.e. below ~10% relative humidity). The

C-S-H structure shrinks considerably upon interlayer water loss.

1.5.4. Chemically Combined Water;

This water is an integral part of the structure of cement hydration products. It is not

lost by drying, but it is evolved when the hydrates decompose on heating. The various

form of water classified according to the ease with which it may be removed from the hcp

are shown in Fig. 1.3.

Fig. 1.3. Types of water in hcp

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HOMEWORK-1: Calculate the volume of hydration products and capillary porosity and show them on bar

charts for

a) Hydration of 100 cm3 of cement with a water/cement ratio of 0.63 for 0, 50, 75 and

100% hydration degrees.

b) 100% hydration of 100 cm3 of cement for water/cement ratios of 0.3, 0.4, 0.5, 0.6

and 0.7

c) Calculate the compressive strength of the paste for each case

Assumptions:

- Specific gravity of cement : 3.14,

- 1 cm3 of cement produces ~2cm3 of the hydration products upon full hydration,

- The original volume of the paste remains unchanged during hydration,

- Volume of entrapped air is negligible,

- The strength solid to space (gel/space) ratio relationship is given as

S=23,5x3 (MPa) . x = gel/space ratio

HOMEWORK-2: Using 315 g cement, a paste with W/C ratio of 0.63 is prepared. The specific gravity

of cement is 3.15. Assuming that 1 ml of cement upon hydration produces ~2ml hydration

products,

a) Determine the volume of hydration products and capillary pores for 50, 75 and

100% degrees of hydration.

b) Repeat the same problem for W/C ratios of 0.7, 0.6, 0.5, 0.4 and 0.3 for 100%

degree of hydration,

c) Show the results of the both cases on bar chart diagrams.

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1.6. Structure Property Relationships in hcp

The required engineering properties of hardened concrete, i.e., strength, dimensional

stability and durability, are influenced not only by the proportion but also by the properties

of the hcp, which, in turn, depend on the micro-structural features such as the type,

amount and distribution of solids and voids. The structure property relationships of the hcp

are discussed briefly below.

Strength: The major source of strength in the solid products of hcp is the existence of

Van der Waals forces of attraction. Adhesion between two solid surfaces can be attributed

to these physical forces, the degree of adhesive action being dependent on the extent and

nature of the surfaces involved. The small crystals of C-S-H, H-A - S-C and hexagonal C-A-H posses large surface areas and adhesive capability. These hydration products tend

to adhere strongly to each other as well as to low- surface- area solids such as CH,

anhydrous clinker grains and fine or coarse aggregates particles.

Since strength resides in the solid part of the material, voids are detrimental to

strength; resultantly there is an inverse relationship between porosity and strength.

In the hcp, the interlayer space within the C-S-H structure and the small voids which

are within the influence of the Van der Waals forces of attraction can not be considered

detrimental to strength, because stress concentration and subsequent rupture on

application of load begin at large capillary pores and microcracks. It was mentioned earlier

that capillary porosity of the paste depends on W/C ratio of the mix and degree of

hydration of cement.

It is shown that there is a process of progressive reduction in the capillary porosity

either with increasing degree of hydration or with decreasing water/cement ratio. The

degree of hydration of the cement depends on the curing conditions (duration of hydration

or age, temperature and humidity).

For normally hydrated Portland cement mortar, Powers proposed the following

exponential equation;

S = k.x3 Where

S: compressive strength (MPa)

x: Solids to space ratio (gel/space ratio)

k: a constant equal to 235 MPa

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The effect of solid/space ratio (opposite of porosity) on the strength and permeability

and the combined effect of W/C ratio and degree of hydration on the porosity are shown in

Fig. 1.4.

Shaded area shows Typical capillary porosity range in hcps.

Fig 1.4 a,b . Effect of W/C ratio and degree of hydration on porosity, strength and permeability of hcp.

Dimensional Stability: Saturated hcp is stable at a relative humidity of 100%.

However, when exposed to environmental humidity (< 100%), the material begins to lose

water and shrink. The relationships between loss of water and relative humidity as well as

shrinkage and loss of water are shown in Fig. 1.5.

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Fig.1.5 a) Loss of water as a function of relative humidity

b) Shrinkage of cement mortar as a function of the water loss

As soon as the relative humidity drops below 100%, the free water held in large

cavities (e.g.,> 50 nm) begins to escape to the environment. Since the free water is not

attached to the structure of the hydration products by any physico-chemical bonds, its loss

would not be accompanied by shrinkage (Curve A-B in Fig.1.5). Thus, a saturated hcp

exposed to relative humidities close to 100% may lose a considerable amount of total

evaporable water before undergoing any shrinkage.

When most of the free water has been lost, on continued drying it is found that further

loss of water begins to result in considerable shrinkage. This phenomenon shown by curve

B-C in Fig. 1.5, is attributed mainly to the loss of absorbed water and the water held in

small capillaries. It has been suggested that when confined to narrow spaces between two

solid surfaces, he absorbed water causes a disjoining pressure. The removal of the

absorbed water reduces the disjoining pressure causing shrinkage of the system. The

interlayer water, present as a mono-molecular water film within C-S-H layer structure, can

also be removed by severe drying conditions. This is because the closer contact of the

interlayer water with the solid surface and the tortuosity of the transport path through the

capillary network, call for a stronger driving force. Since the water in small capillaries (5 to

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50nm) exerts hydrostatic tension, its removal tends to induce a compressive stress on the

solid walls of the capillary pore, thus also causing contraction of the system.

It is known that the mechanisms which are responsible for drying shrinkage are also

responsible for creep of the hcp. In the case of creep, a sustained external stress moves

the physically absorbed water and the water held in small capillaries. Thus, creep strain

can occur even at 100% relative humidity.

Durability: The service life of concrete may be markedly reduced by the disintegrating

effects of:

1- Weathering, including the disruptive action of freezing and thawing; the differential

length changes due to temperature variations, and alternative wetting and drying,

2- Reactive aggregates,

3- Aggressive waters in alkali regions,

4- Leaching in hydraulic structures,

5- Chemical corrosion, and

6- Mechanical wear or abrasion.

The term durability of a material relates to its service life under given environmental

conditions. Exposure to acidic solutions is detrimental to hcp due to its alkaline character.

Under these conditions, impermeability or water tightness becomes a primary factor in

determining durability. It is known that the methods of preparing and subsequent treatment

(curing) of concrete are among the major factors influencing the water tightness. The

impermeability of concrete is also of great significance because it is assumed that an

impermeable hcp would result in an impermeable concrete (regarding the aggregate being

impermeable). Permeability is defined as the ease with which a fluid can flow through a

solid. Obviously, the pore structure (size and continuity of the pores) determines the

permeability of the material. Strength and permeability are interrelated in the sense that

both are closely related to the capillary porosity or the solid/space (gel/space) ratio as

shown in Fig. 1.4. (a).

The exponential relationship between permeability and porosity are shown in Fig 1.4.

can be understood from the influence that various pore types exert on permeability. As

hydration proceeds, the void space between the originally discrete cement particles

gradually is filled up with the hydration products. It was mentioned that the total capillary

porosity decreases with the reducing W/C ratio and/or increasing degree of hydration.

Mercury-intrusion porosimetric studies on the cement pastes hydrated with different

W/C ratios (Fig 1.6) and to various ages have shown that the total capillary porosity was

1-19

associated with reduction of large pores in the hcp.

Fig 1.6. Pore size distribution of pastes having different W/C ratios.

When the data of Fig. 1.6 are plotted after omitting the large pores

(i.e.>1320 A), it is found that a single curve could fit the pore size distributions in the 28-

day-old pastes made with four different W/C ratios (Fig.1.7). This indicates that in hcps,

the increase in total porosity resulting from increasing W/C ratio manifests itself in the form

of large pores only. This observation has great significance from the stand point of the

effect of W/C ratio on strength and permeability, which are controlled by large pores.

Fig. 1.7. Distribution plots of small pores in cement pastes of varying W/C ratios

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From the data in Fig.1.4 it is obvious that the coefficient of permeability shows an

exponential drop when the fractional volume of capillary porosity reduces from 0.4 to 0.3.

This range of capillary porosity, therefore, seems to correspond to the point when both the

volume and size of capillary pores in the hcp are so reduced that the interconnections

between them have become difficult. As a result, upon the progress of the hydration of a

young paste, its permeability may show reductions in the order of 106 times. It is shown

that a cement paste with even a W/C ratio of 0.6 upon full hydration is as impermeable as

a dense rock such as basalt or marble.

It should be noted that the porosities due to the C-S-H interlayer space and small

capillaries do not contribute to permeability of hcp. On the contrary, with increasing degree

of hydration, there is a considerable increase in the volume of these pores, but the

permeability is greatly reduced. Mehta and Manmohan noted a direct relationship between

the permeability of hcp and the volume of the pores larger than about 100nm. This is

probably because the pore systems, comprised mainly of small pores, tend to become

discontinuous.

1.7. Transition Zone in Concrete

The following features of concrete are interesting,

It is brittle in tension but relatively tough in compression. The paste matrix and aggregate (two main components of concrete) when tested

separately in a uniaxial compression remain elastic until fracture, whereas

concrete itself shows inelastic behavior.

The compressive strength of concrete is higher than its tensile strength by an order of magnitude.

At a given cement content, W/C ratio and age of hydration cement mortar will always be stronger than the corresponding concrete. Also the strength of

concrete goes down as the coarse aggregate size is increased (at least from a

definite size on).

The permeability of concrete made from a very dense aggregate will be higher by an order of magnitude than the permeability of the corresponding cement paste.

On exposure to fine, the elastic modulus of a concrete drops more rapidly than its compressive strength

Regardless of the strength of concrete, at later ages the elastic modulus

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increases at a faster rate than the compressive strength

The reasons for the above behavior of concrete lie in the transition zone existing

between large aggregate particles and the hcp. Although composed of the same elements

as the hcp, the structure and properties of the transition zone are different from the bulk

hcp. Therefore, it is desirable to treat it as a separate phase of the concrete structure.

1.7.1. Structure of the Transition Zone

Due to experimental difficulties, relatively loss information is available about the

transition zone of concrete. However, some understanding of its structural characteristics

may be obtained by following the sequence of its development from the time concrete is

placed.

First, in freshly placed and compacted concrete, water films accumulate around the

large aggregate particles due to the internal bleeding. Thus, there will be a higher W/C

ratio in areas closer to the larger aggregate particles than in the bulk mortar.

Next, similar to the bulk paste, Ca2+, SO42-, OH- and aluminate ions, produced by the

dissolution of calcium sulfate and calcium aluminate compounds, combine to form

ettrengite and calcium hydroxide. Owing to the high W/C ratio, these crystalline products in

the vicinity of the coarse aggregate consist of relatively larger crystals, and therefore, form

a more porous framework than in the bulk cement paste or mortar matrix. The plate like

CH crystals tends to form in oriented layers perpendicular to the aggregate surface.

Finally, with the progress of hydration, poorly crystalline C-S-H and a second

generation of smaller crystals of ettrengite and CH start filling the empty space existing

between the framework created by the large ettrengite and CH crystals. This improves the

density and hence the strength of the transition zone.

A diagrammatic representation of the transition zone in concrete is shown in Fig 1.8.

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Fig.1.8. Diagrammatic representation of the transition zone and bulk cement paste in

concrete.

The volume and size of voids in transition zone are larger than in the bulk cement

paste or mortar. The size and concentration of 3236 HSAC and CH crystalline compounds are also larger in the transition zone. The cracks are formed easily in the direction to the C

axis, which account for lower strength of the transition zone.

1.7.2. Strength of the Transition Zone (Bond Strength)

As in the case of the hcp, the cause of bonding hydration products and aggregate

particles is the Van der Waals forces of attraction; therefore, the strength of the transition

zone at any point depends on the volume and size of the voids presents. Even for low W/C

ratio concrete, at early ages the volume and the size of voids in the transition zone will be

larger than in the bulk mortar; consequently, the former is weaker in strength. However,

with increasing age the strength of the transition zone may become equal to or even

greater than the strength of the bulk mortar. This could happen as a result of crystallization

of new products in the voids of the transition zone by slow chemical reactions between the

cement paste constituents and aggregate (probably formation of calcium silicate hydrates

bin the case of siliceous aggregates, or formation of carboaluminate hydrates in the case

of limestone.

Such interactions are strength contributing because they also tend to reduce the

concentration of CH in the transition zone. The large CH crystals posses less adhesion

capacity, not only because of lower surface area and correspondingly weak Van der Waals

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forces of attraction, but also because they surve as preferred cleavage (kayma sayfalar)

sites owing to their oriented structure.

In addition to the large volume of capillary pores and oriented CH crystals, a major

factor responsible for the lower strength of the transition zone in concrete is the presence

of microcracks. The amount of microcracks depends on numerous parameters, including

aggregate size and grading, cement content, W/C ratio, degree of compaction of fresh mix,

curing conditions, environmental humidity and thermal history of concrete. For example, a

concrete mixture containing poorly graded aggregate is more prone to segregation in

compacting; thus thick water films can form beneath the coarse aggregate particles. Under

identical conditions, the larger the aggregate size the thicker would be the water film. The

transition zone formed under these conditions will be susceptible to cracking when

subjected to the tensile stresses induced by differential movements between the

aggregate and the hcp.

Such differential movements commonly arise either on drying or on cooling of

concrete. Thus, concrete has microcracks in the transition zone even before loading.

Obviously, short-term impact loads, drying shrinkage, and sustained loads at high stress

levels will have the effect of increasing the size and number of microcracks.

1.7.3. Influence of Transition Zone on Properties of Concrete

The transition zone being the weakest link of the chain is considered the strength -

controlling phase in concrete. The presence of the transition zone causes the concrete to

fail at a considerably lower stress level than the strength of either of the two main

components of concrete (i.e., aggregate and hcp or mortar). Since it does not take very

high energy levels to extend the cracks already existing in the transition zone, even at 40

to 70% of the ultimate strength, higher incremental strains are obtained per unit of applied

stress. This explains the phenomenon that the concrete components usually remain elastic

until fracture in a uniaxial compression test, whereas concrete itself shows inelastic

behavior.

At stress levels higher than 70% of the u, the stress concentrations at large voids in the mortar matrix become sufficient to initiate cracking there. With increasing stress, the

matrix cracks gradually spread until they join the transition zone cracks. The crack system

then becomes continuous and the material ruptures. Considerable energy is needed for

the formation and extension of matrix cracks under a compressive load. However, under

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tensile loading cracks propagate rapidly and at a much lower stress level. This is why

concrete fails in a brittle manner in tension but is relatively tough in compression. This is

also the reason why the tensile strength is much lower than the compressive strength of a

concrete.

The structure of the transition zone, especially the volume of voids and microcracks

present, have a great influence on the elastic modulus of concrete. In the composite

material, the transition zone serves as a bridge between the mortar matrix and the coarse

aggregate particles. Even when the individual components are of high stiffness, the

stiffness of the composite may be low due to the broken bridges (i.e., voids and

microcracks in the transition zone ) which prevents stress transfer. Thus, due to the

microcracking on exposure to fire, the elastic modulus of concrete drops faster than the

compressive strength.

The characteristics of the transition zone also influence the durability of concrete.

Prestressed and reinforced concrete elements often fail due to corrosion of the

reinforcement. The rate of corrosion of steel is greatly affected by the permeability of

concrete. The presence of microcracks at the steel coarse aggregate interface is the

primary reason that concrete is more permeable than the corresponding hcp or mortar. It

should be noted that the existence of air and water is a necessary prerequisite to corrosion

of the steel in concrete.

The effect of W/C ratio on permeability and strength of concrete is generally

attributed to its influence on the porosity of the hcp in concrete. However, regarding the

effect of structure and properties of the transition zone on concrete, it is more appropriate

to consider the effect of W/C ratio on the concrete mixture as a whole. This is because

depending on aggregate characteristics, such as Dmax and grading, it is possible to have

large differences in the W/C ratio between the mortar matrix and the transition zone. In

general, everything else remaining the same, the larger the aggregate the higher will be

the local W/C ratio in the transition zone and, consequently, the weaker and more

permeable will be the concrete.

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2. DURABILITY OF CONCRETE

A properly designed, produced and cured concrete is inherently durable to the

environments it will be exposed. Besides, a carefully produced concrete with good quality

control is capable of maintenance-free performance for decades without the need for

protective coatings, except in highly corrosive environments. However, concrete is

potentially susceptible to attack in variety of different exposures unless certain precautions

are taken. Deterioration of concrete can be caused by the adverse performance of any one

of the three major components: aggregate, paste or reinforcement, and can be due to

either chemical or physical causes (Table 2.1). n most of the cases, an individual

environment factor initiates distress, then other factors may contribute and aggravate the

situation.

A major difficulty in studying durability is predicting concrete behavior several

decades in the future on the basis of short-term tests. Most of the knowledge of the

durability has been accumulated through a direct study of actual field problems. The

prediction of concrete durability under a variety of service conditions, is still a major

problem.

Table 2.1. Durability of Concrete Chemical Attack Physical Attack

Leaching and efflorescence (P) Freezing and Thawing (P, A)

Sulfate Attack (P) Wetting and Drying (P)

Alkali Aggregate Reaction (A) Temperature Changes (P, A)

Acids and Alkalis (P) Wear and Abrasion (P, A)

Re-bar Corrosion (R)

Letters in parenthesis indicates the concrete component most affected, in order of

importance: AAggregate ; P Paste ; R Reinforcement Water is generally involved in every form of concrete deterioration, and in porous

solids permeability of the material to water usually determines the rate of deterioration.

Therefore, in this chapter the structure and properties of water are described with special

reference to its destructive effect on porous materials; then the permeability of cement

paste, aggregates and concrete as well as the factors controlling their permeability are

discussed.

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Physical effects that adversely influence the durability of concrete include surface

wear, cracking due to crystallization pressure of salts in pores, and exposure to extreme

temperatures. Deleterious chemical effects include leaching of the cement paste by acidic

solutions, and expansive reactions involving sulfate attack, alkali-aggregate reaction and

rebar corrosion in concrete. The significance, physical manifestations, mechanism, and

control of various causes of concrete deterioration are discussed in detail.

Definition: durability is generally considered synonymous with long service life.

Since durability under one set of conditions does not necessarily mean durability under

another, it is customary to include a general reference to the environment when defining

durability. According to ACI Committee 210, durability of Portland cement concrete is

defined as its ability to resist weathering action, chemical attack, abrasion, or any other

process of deterioration that is, durable concrete will retain its original form, quality and

serviceability when exposed to its environment.

Generally, as a result of environmental interactions the microstructure and

consequently, the properties of materials change with time. A material is assumed to reach

the end of service life when its properties under given conditions of use have deteriorated

to an extent that the continuing use of the material is ruled either unsafe or uneconomical.

Significance: it is generally accepted now that in designing structures the durability

characteristics of the materials under consideration should be evaluated as carefully as

other aspects such as mechanical properties and initial cost.

Mostly a substantial portion of the total construction budget is used for the repair and

replacement of existing structures arising from material failures. For example, it is

estimated that in industrially developed countries, over 40% of the total resources of the

building industry are applied to repair and maintenance of existing structures, and less

than 60% to new installations. The escalation in replacement cost of structures and the

growing demand on life-cycle cost rather than first cost are forcing engineers to become

durability conscious. Furthermore, a close relationship exists between durability of

materials and ecology. Conservation of natural resources by making materials having

longer service life is, after all, an ecological step. Besides, the uses of concrete are being

extended to new applications, such as offshore platforms, containers for handling liquefied

gases at cryogenic temperatures and high pressure reaction vessels in the nuclear

industry.

General Observations: before a discussion of important aspects of durability of

concrete, a few general remarks on the subject will be helpful.

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1. Water, the primary cause of both creation and destruction of many natural

materials, is also control to most important durability problems in concrete. In porous

solids, water is the case of many types of physical process of degradation. As a carrier of

aggressive ions, water can also be a source of chemical process of degradation.

2. The physico-chemical phenomena associated with water movement in porous

solids are controlled by the permeability of the solid. For example, the rate of chemical

deterioration would depend on whether the chemical attack is limited to the surface of

concrete or whether it is also work inside the material.

3. The rate of deterioration is also affected by the concentration level of ions in water

and by the composition of solids. Due to the presence of alkali calcium compounds in

hydration products of Portland cement, unlike many natural rocks and minerals, concrete

is a basic material. Therefore, acidic waters are expected to be harmful to it.

Most of our knowledge of physico-chemical processes responsible for concrete

deterioration comes from case histories of structures in the field, because it is difficult in

the laboratory to simulate the combination of long-term conditions normally present in real

life. However, in practice, deterioration of concrete is seldom due to a single cause;

usually, at advanced stages of material degradation more than one deleterious

phenomena are found at work. In general, various causes of deterioration are so closely

intertwined and an interacting so that even separation of the cause from the effect often

becomes impossible. Therefore, a classification of concrete deterioration processes into

neat categories should be treated with some care. Since the purpose of such

classifications is to explain, systematically and individually, the various phenomena

involved, there is a tendency to overlook the interactions when several phenomena are

present simultaneously.

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2.1. Water as an Agent of Deterioration

Water is the most aboundant fluid in nature in the form of seawater, groundwater,

rivers, lakes, rain, snow and vapor. Being small, water molecules are capable of

penetrating extremely fine pores or cavities. As a solvent, water is able to dissolve more

substances than any other liquid. This is due to the presence of many ions and gases in

some waters, which in turn, become instrumental in causing chemical decomposition of

solid materials.

It may also be noted that eater has the highest heat of vaporization among the

common liquids, therefore, at ordinary temperatures it has the tendency to remain in a

material in the liquid state, rather than to vaporize and the material dry.

In porous solids, internal movements and changes of structure of water are known to

cause disruptive volume changes. For example, freezing water into ice, formation of

ordered structure of water inside fine pores, development of osmotic pressure due to

differences in ionic concentration, and hydrostatic pressure build up by differential vapor

pressures can lead to high internal stresses within a moist solid. A brief review of the water

structure will be useful for understanding these phenomena.

2.1.1. Structure of Water

The H-O-H molecule is covalently bonded. Due to asymmetric character of water

molecule, the charge centers of hydrogen and oxygen are different. Thus the porosity

charged proton of the hydrogen ion belonging to a water molecule attracts the negatively

charged electrons of the neighboring water molecules. This relatively weak force of

attraction, called the hydrogen bond is responsible for the ordered structure of water.

The highest manifestation of the long-range order in the structure of water due to

hydrogen bonding is seen in ice (Fig 2.1.a). Each molecule of water in ice is surrounded by

other four molecules, one molecule at the center and four molecules at the corners of

tetrahedron. In all three directions the molecule and groups of molecules are held together

by hydrogen bonds. When ice melts at 0C ~15% of the hydrogen bonds breakdown in directionality of the tetrahedral bond, thus, each water molecule can acquire more than

four nearest neighbors, which causes the density to rise from 0.917 to 1. Upon

solidification of liquid water, reverse process occurs, thus expansion forms rather than

contraction.

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Fig. 2.1. a) Structure of ice ; b) structure of oriented water molecules in micro pore. (The structure and properties of water affected by temperature and by the size of pores in a solid).

Compared to the structure of ice, water at room temperature has about 50% of the

hydrogen bonds broken. Materials in the broken-bond state have unsatisfied surface

charges, which give rise to surface energy. The surface energy in liquids causes surface

tension, which accounts for the tendency of a large number of molecules to adhere

together. It is the high surface tension of water (defined as the force required to pull the

water molecules apart) which prevents it from acting as an efficient plasticizing agent in

concrete mixes until suitable admixtures are added.

Formation of oriented structure of water by hydration bonding in micropores causes

expansion. In solids the surface energy is more when numerous fine pores are present. If

water is able to permeat such micropores, and if the forces of attraction at the surface of

pores are strong enough to break down the surface tension of bulk water and orient the

molecules to an ordered structure (analogous to the structure of ice), this oriented or

ordered water, being less dense than the bulk water, will require more space and will

therefore tend to cause expansion (Fig 2.1-b)

2.2. Permeability

Water as a necessary ingredient for the cement hydration and as a plasticizing agent

for concrete components is found in the structure of concrete from beginning. Gradually,

depending on the ambient conditions and the thickness of a concrete element, most of the

evaporable water in concrete, will be lost leaving the pores unsaturated or empty. Since it

is the evaporable water which is freezable and also free for internal movement, the

(a) (b)

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concrete will not be vulnerable to water-related destructive phenomena, provided that

there is a little or no evaporable water left drying, and provided that the subsequent

exposure of the concrete to environment does not lead to resaturation of the pores. The

resaturation of the pores to a large extent depends on the hydraulic conductivity, formed

as (coefficient of) permeability (K).

Many attempts have been made to relate the microstructural parameters of cement

hydration products with either diffusivity (the rate of diffusion of ions through water-filled

pores) or permeability. (The rate of viscous flow of fluids through the pore structure).

According to Garboezi as cited by Mehta, for a variety of reasons the diffusivity

predictions need more development and validation before their practical usefulness can be

proven therefore, in this course only permeability is discussed, implying that this property

covers the overall fluid transport characteristic of the material.

Permeability is defined as the property that governs the rate of flow of a fluid into a

porous solid. For steady-state flow, the coefficient of permeability (K) is determined from

Darcys expression:

LHAK

dtdq

Where dtdq

: rate of fluid flow

: viscosity of the fluid

H : pressure gradient A : the surface area

L : thickness of the solid

The coefficient of permeability of a concrete to gas or water vapor is much lower than

the coefficient for liquid water, therefore, tests for measurements of permeability are

generally carried out using water that has no dissolved air. Besides, due to their

interactions with cement paste the permeabilities of solutions containing ions would be

different from the water (pure water) permeability.

2.2.1. Permeability of Cement Paste

In a hcp the size and continuity of the pores at any stage during the hydration

process would control the coefficient of permeability. The mixing water is indirectly

responsible for permeability of the hcp, because its content determines first the total space

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available for hydration products and subsequently the unfilled space after the water is

consumed by either cement hydration reactions or evaporation to the environment.

Generally the permeability of hcp is controlled by W/C ratio and degree of hydration as

shown below:

***Lets consider the hydration process of 1cc cement, accepting that 1 cc cement

upon full hydration produces 2.1 cc hydration products.

For W/C = 0,40 by weight

Weight of cement Wc = 1 x 3,15 = 3,15 g Weight of water Ww = 0,40 x 3,15 = 1,26 g Volume of water Vw = 1,26 cc

For 50% hydration

Volume of hydrated cement Vhc = 0.5 cc

Vhp = 0.5 x 2.1 = 1.05 ml volume of hydration products

Vcp = Total volume - Volume of hydration products - Volume of unhydrated cement

Vcp = 2.26 - 0.50 - 1.05 = 0.71 cc

For 100% hydration

Volume of hydrated cement Vhc = 1 cc

Vhp = 1 x 2.1 = 2.1 ml volume of hydration products

Vcp = Total volume - Volume of hydration products - Volume of unhydrated cement

Vcp = 2.26 - 2.1 = 0.16 cc

1,26 cc 0.16cc ,Vcp

Vcp

1,26 cc Water

1.26 cc 2.1 cc Vhp Vhp 1 cc Cement

0.5 cc Cement

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0% hydration 50% hydration 100% hydration

*** Lets repeat the same problem for W/C = 0.80

Vc = 1 cc

Wc = 1 x 3,15 = 3,15 g

Ww = 0,80 x 3,15 = 2.52 g

Vw = 2.52 cc

For 50 % hydration Vhp = 0,5.(2,1)=1,05 ; Vcp = 3,52 - 0,5 - 1,05 = 1,97 cc

For 100 % hydration Vhp = 1,0 . (2,1) = 2,10 ; Vcp = 3,52 - 2,10 = 1,42 ml

1,97 cc 1.42cc Vcp

Vcp

2,52 cc Water

1 cc 2.1 cc Vhp Vhp

1 cc Cement

0.5 cc

0% hydration 50% hydration 100% hydration C

The coefficient of permeability of freshly mixed cement paste is of the order of 10-4 to

10-5 cm/s with the progress of hydration both the capillary porosity and coefficient of

permeability decrease. However there is no direct proportionality between the two. This is

because, in the beginning, as the cement hydration process progresses even a small

decrease in the total capillary porosity is associated with considerable segmentation of

large pores, thus greatly reducing the size and number of channels of flow in the cement

paste. Typically, 30% capillary porosity represents a point when the interconnections

between the pores have already become so tortuous that a further decrease in porosity of

the paste is not accompanied by a substantial decrease in the permeability coefficient.

In general, when W/C ratio is high and the degree of hydration is low, the cement

paste will have high capillary porosity; it will contain a relatively large number of big and

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well-connected pores and, therefore, its coefficient of permeability will be high. As

hydration progresses, most of the pores will be reduced to small size (100 nm or less) and

will also lose their interconnections; thus the permeability drops. The coefficient of

permeability of hcp when most of the capillary pores are small and disconnected is of the

order of 10-12 cm/s. It is observed that in normal cement pastes the discontinuity in the

capillary network is generally reached when the capillary porosity is about 30%. With 0,40 ,

0,50 , 0,60 and 0,70 W/C ratio pastes this generally happens in 3, 14, 180 and 365 days of

moist curing, respectively. Since the W/C ratio in most concrete mixtures seldom exceeds

0,70, it should be obvious that in well-cured concrete the cement paste is not the principal

contributing factor to the coefficient of permeability.

2.2.2. Permeability of Aggregates

Compared to 30 to 40% capillary porosity of typical hcps, the volume of pores in most

natural aggregates is usually under 8% and rarely exceeds 10%. Thus, it is expected that

permeability of aggregate would be much lower than that of typical cement paste. This

may not necessarily be the case. It is shown that the coefficient of permeability of

aggregates are as variable as those of hcp of W/C ratios in the range of ~0,4 to 0,7. The

reason some aggregates, with as low as 10% porosity, may have much higher

permeability than cement paste is because the size of capillary pores in aggregate is much

larger than cement paste. (Most of capillary porosity in nature hcp is 10-100nm; average

pores in aggregate > 10 m ; 100 1000 times greater than pores in hcp).

2.2.3. Permeability of Concrete

Theoretically, the interaction of aggregate particles of low permeability into a cement

paste is expected to reduce the permeability of the system, because the aggregate

particles should intercept the channels of flow within the cement paste matrix. Therefore,

compared to neat cement paste, mortar or concrete with the same W/C ratio and degree of

maturity, should give a lower coefficient of permeability. However, this is not the case, and

the addition of aggregates to cement paste or mortar increases the permeability

considerably. The permeability of concrete depends mainly on the W/C ratio and degree of

hydration (Which controls the size, volume and continuity of capillary pores) as well as

maximum aggregate size (which determines the microcracking in the transition zone

between coarse aggregate and cement paste); in fact, the larger the max aggregate size,

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the greater the coefficient of permeability.

Owing to the significance of the permeability to physical and chemical processes of

deterioration of concrete, a brief review of the factors controlling permeability of concrete

should be useful. Since strength and permeability are related to each other through the

capillary porosity, as a first approximation the factors that influence the strength also affect

the permeability (Fig.2.2).

Fig 2.2 Influence of capillary porosity on comp. strength and coefficient of permeability

A reduction in the volume of a large capillary pores (>100 nm pores) in the hcp

matrix would reduce the permeability. This would be possible by using low W/C ratio,

adequate cement content, proper compaction and curing. Similarly proper attention to

aggregate maximum size & grading, thermal & drying shrinkage strains, and avoiding

premature or excessive loading are necessary steps to limit the transition zone

microcracking which appear to be the major cause of high permeability in concrete. Finally,

the thickness of the concrete element is of importance since it determines the tortuosity of

the path of fluid flow, which in turn, determines permeability.

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3. CLASSIFICATION OF CAUSES OF CONCRETE DETERIORATION

As it was mentioned earlier, the causes of concrete deterioration may be physical or

chemical. The typical causes of concrete deterioration may be grouped into two

categories; surface wear and cracking as follows:

Fig 3.1. Physical causes of concrete deterioration

Similarly, the chemical causes of deterioration may be grouped into three as follows;

Fig 3.2. Chemical causes of concrete deterioration

It should be emphasized that the distinction between the physical and chemical

causes of deterioration is purely arbitrary; in practice the two are frequently superimposed

3-36

on each other. For example, higher permeability of concrete increases the risk of rebar

corrosion and corrosion of reinforcement causes cracking and further increase in

permeability chemical causes of deterioration will be discussed later. Cracking of concrete

due to normal temperature and humidity gradients are out of scope of this course. Here,

the physical causes of concrete deterioration will be explained.

3.1. Deterioration by Surface Wear (Abrasion)

Under many conditions, such as abrasion, erosion and cavitation, progressives loss

of mass from concrete surface may occur.

Abrasion: refers to dry attrition (e.g. wear on pavements and industrial floors by vehicular traffic)

Erosion: occurs mainly in hydraulic structures, refers to wear by the abrasive action of fluids containing solid particles in suspension. Erosion takes place in spillways, pipes,

sewage system.

Cavitation : refers to loss of mass by formation of vapor or bubbles and their subsequent collapse due to sudden charge of direction in rapidly flowing water.

Hardened cement paste has a low resistance to attrition. Service life of concrete can

be greatly reduced under repeated cycles of attrition, especially when paste matrix of

concrete is of high porosity or low strength, and is inadequately protected by an aggregate

which lacks wear resistance. There is a good correlation between the W/C ratio and

abrasion resistance of concrete. Thus, ACI Committee 201 recommends a minimum 28

day compressive strength of ~30 MPa. Suitable strengths may be attained by a low W/C

ratio, proper grading of fine and coarse aggregate (limiting Dmax to ~25mm), lowest

consistency practicable for proper placing and compacting (max. slump 75 mm; for

toppings 25mm), and minimum air content consistent with exposure conditions.

When a fluid containing suspended and rolling particles is in contact with concrete,

the impinging, sliding and rolling action of particles may cause surface wear. The rate of

erosion depends on the quantity, shape, size and hardness of the particles being

transported, on the velocity of the moving particles as well as on the porosity or strength of

concrete.

For silt-size particles (2-150 m), erosion will be negligible at bottom velocities up to 1.8 m/s (min. velocity to transport particles).

When severe erosion or abrasion conditions exist, it is recommended that in addition

3-37

to the use of hard aggregates, the min. 28 day compressive strength of concrete should be

~40 MPa, and before exposure to aggressive conditions concrete should be adequately

cured. European Standard ENV206 (1992) recommends a period of curing twice as long

as normal in order to achieve good resistance to surface wear.

For a proper attrition resistance, at least the surface of concrete should be of high

quality. The properties of concrete surface zone are strongly affected by the finishing

operations; vacuum dewatering is beneficial. The presence of laitance should be avoided

by delaying floating and troweling until the concrete has lost its surface bleed water.

Moreover, the bleeding capacity should be reduced by taking suitable measures such as

using mineral admixtures. Industrial floors or pavements may be designed to have a 25 to

75 mm thick topping, consisting of a low W/C ratio concrete containing hard aggregate of

~12 mm Dmax. Due to very low W/C ratio, concrete toppings containing Latex admixtures

are becoming increasingly popular for abrasion resistance.

Resistance to abrasion can also be achieved by application of surface hardening

solutions to well-cured new floors or abraded old floors. Solutions most commonly used

are magnesium or zinc fluosilicates or sodium silicate which react with CH present in hcp

to form insoluble reaction products, thus, sealing the capillary pores at or near the surface

and hence to improve the resistance to ingress of fluids or dusting due to abrasions.

As far as aggregate is concerned, strong and hard aggregate is preferred with

inclusion of some crushed sand. However, the abrasion resistance of aggregate, as

determined by the Los Angeles test does not seem to be a good indicator of the abrasion

resistance of concrete made with a given aggregate.

From cement content point of view, rich mixes are undesirable, a max. cement

content of 350 kg/m3 is necessary to allow coarse aggregate particles present just below

the surface of concrete.

Shrinkage compensating concrete has a significantly increased abrasion resistance

probably due to the absence of fine cracks which would encourage the progress of

abrasion.

The non linear flow at velocities exceeding 12 m/s (7 m/s in closed conduits) may

cause severe erosion of concrete through cavitation. In flowing water, vapor bubble form

when the local absolute pressure at a given point in the water is reduced to ambient vapor

pressure of water corresponding to the ambient temperature. As the vapor bubbles flowing

downstream with water enter a region of higher pressure, they implode with great impact

because of the entry of high-velocity water in to the previously vapor occupied space, thus

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causing severe local pitting. Therefore, the concrete surface affected by cavitation is

irregular or pitted, in contrast to the smoothly worn surface by erosion from suspended

solids. The cavitation damage does not progress steadily: usually, after a period of initial

small damage, rapid deterioration occurs, followed by damage at a slower rate.

Best resistance to cavitation damage is obtained by the use of high strength

concrete. The Dmax near the surface should not exceed 20 mm, because cavitation tends

to remove large particles. Hardness of aggregate is not important (unlike the case of

erosion resistance) but a strong bond between aggregate and mortar is vital.

Use of steel fibers may improve the cavitation resistance. However, in contrast to

erosion or abrasion, a suitable and strong concrete may not necessarily be effective in

preventing damage due to cavitation for an indefinite time. The best solution lies in

removal of the causes of cavitation, such as surface misalignments, or abrupt changes of

slope or curvature that tend to pull the flow away from the surface. If possible, local

increase in velocity of water should be avoided as damage is proportional to the sixth or

seventh power of velocity.

Test methods for the evaluation of wear resistance of concrete are not always

satisfactory, because the damaging action varies depending on the exact cause of wear,

and none of the test procedures may satisfactorily simulate the field conditions of wear.

Therefore, laboratory methods are not intended to provide a quantitative measurement of

the length of service that may be expected from a given concrete surface; they can be

used to evaluate the effects of concrete ingredients and curing or finishing procedures on

the abrasion resistance of concrete.

ASTM C 418-90 prescribes the procedure for wear determination by sandblasting;

the loss of volume of concrete serves as a basis for judgment. ASTM C 779-89 describes

three optional methods for testing the relative abrasion resistance of horizontal concrete

surfaces. In the steel-ball abrasion test, load is applied to a rotating head containing steel

balls while the abraded material is removed by water circulation; in the dressing wheel

test, load is applied through rotating dressing wheels of steel; and in the revolving-disk

test, revolving disks of steel are used in conjunction with a silicon carbide abrasive. In

each of the tests, the degree of wear can be measured in terms of weight loss after a

definite time.

Proneness to erosion by solids in flowing water can be measured by means of a

shot-blast test. Here, 2000 pieces of broken steel shot (850 m [#20 ASTM sieve size]) are

ejected under air pressure of 0.62 MPa against a concrete specimen 102 mm away.

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Besides, due to direct relationship between the abrasion and erosion resistance, the

abrasion resistance data can be used as a guide for erosion resistance.

3.2. Cracking by Crystallization of Salts in Pores

A purely physical action of crystallization of the (sulfate) salts in the pores may

account for considerable damage in concrete. For instance, when one side of a slab or

retaining wall of a permeable concrete is in contact with a salt solution and the other sides

are subjected to evaporation, the material can deteriorate by stresses resulting from the

pressure of salt crystallization in pores.

In many porous materials, the crystallization of salts from super saturated salt

solutions can occur only when the concentration of the solute (C) exceeds the saturation

concentration (Cs) at a given temperature. As a rule the higher the (C/Cs) ratio (or degree

of super saturation) the greater the crystallization pressure. The crystallization pressures

for salts that are commonly found in pores of concrete, calculated from the density,

molecular weight, and molecular volume for a C/Cs ratio of 2 are shown in Table 3.1. At a

degree of super position of 10, the calculated crystallization pressure of NaCl is 1835 atm

at 0C and 2190 atm at 50C.

Table 3.1. Crystallization Pressures for Some Salts Pressure (atm)

C/Cs = 2 Salt Chemical Formula Density (g/cm3)

Molecular Weight (g/mol)

Molar Volume (cm3/g) 0C 50C

Anhydrite CaSO4 2,96 136 46 335 398 Epsomite MgSO4.7H2O 1,68 246 147 105 125 Gypsum CaSO4.2H2O 2,32 127 55 282 334 Halite NaCl 2,17 59 28 554 654 Hepta hydrite Na2CO3.7H2O 1,51 232 154 100 119 Hexa hydrite MgSO4.6H2O 1,75 228 130 118 141 Kieserite MgSO4.H2O 2,45 138 57 272 324 Mirabilite Na2SO4.10H2O 1,46 322 220 72 83 Natron Na2CO3.10H20 1,44 286 199 78 92

Tach hydrite 2MgCl2.CaCl2. 12H2O 1,66 514 310 50 59

Thenardite Na2SO4 2,68 142 53 292 345

Thermonatrite Na2CO3.H2O 2,25 124 55 280 333

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4. DETERIORATION BY FROST ACTION

The frost action is one of the major problems in concrete structures, such as

pavements, retaining walls, bridge decks and railings, requiring heavy expenditures for

repair and replacement. The deterioration of hardened concrete by frost action is attributed

to the complex microstructure of the material as well as to the specific environmental

conditions. Thus, a concrete that is frost resistance under a given freeze-thaw condition

may be damaged under a different condition. The frost action in concrete can take several

forms. The most common is cracking and spalling of concrete causing by progressive

expansion of cement paste matrix from repeated freeze-thaw cycles. Concrete slabs

exposed to freezing and thawing in the presence of moisture and deicing chemicals are

susceptible to scaling. (i.e. finished surface or skin flake or peels off). The following

theories, each may contribute partially to an explanation of the ultimate failure of concrete

surfaces, are proposed to explain scaling:

- Pressure developed by expulsion of water from saturated aggregate particles.

- Hydraulic pressure developed in capillaries just below the concrete surface

- Acceleration of moisture to ice crystals in capillaries below the surface

- Osmotic pressure caused by concentration of salt in capillaries immediately

beneath the concrete surface

- Finishing operation which may create a surface condition where the finished

surface is dissimilar to the underlying concrete. Improper finishing may also

density the surface and destroy the effectiveness of air entrainment.

- Additional freezing of subsurface ice crystals caused by melting snow and ice with

deicing salts,

- Replenishment of surface moisture during freezing by melting snow and ice with

deicing salts.

Certain coarse aggregates, which contain a relatively high pore volume confined to a

narrow pore size range (0.1 to 1 m), in slabs are known to cause cracking, usually parallel to joints and edges, which eventually acquires a pattern resembling a large capital

letter D (cracks curving is described as D-line (deterioration line) cracking or simply D-

cracking.

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Fig. 4.1. D-cracks run approximately parallel to joints or edges of concrete surface. As deterioration progresses these parallel cracks occur further away from the joint. The deterioration is further advanced to cause disintegration and spalling of concrete near the joint.

4.1. Frost Action on Hardened Cement Paste

According to Powers a saturated cement paste containing no entrained air expands

on freezing due to generation of hydraulic pressure. With increasing air entrainment, the

tendency to expand decreases because the entrained air voids provide escape boundaries

for the hydraulic pressure. A diagrammatic representation of Powers hypothesis is shown

below.

Fig.4.2. Response of saturated cement paste to freezing and thawing both without entrained air.

Powers also proposed that, in addition to hydraulic pressure caused by water

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freezing in large cavities, the osmotic pressure resulting from partial freezing of solutions in

capillaries may be another source of destructive expansion in hcp. Water in capillaries is

not pure and solutions freeze at lower temperatures than pure water. (The higher the

concentration of a salt solution, the lower the freezing point). The existence of local salt

concentration gradients between capillaries is envisaged as the source of osmotic

pressure.

The hydraulic pressure due to an increase in the specific volume of water on freezing

in large cavities and osmotic pressure due to salt concentration differences in the pore

fluid are not the only causes of expansion of hcp exposed to frost action. This became

clear by observing the expansion of cement even when benzene was used as pore fluid

instead of water (Benzene contracts on freezing).

A capillary effect involving large-scale migration of water from small pores to large

cavities is believed to be the main cause of expansion in porous hcp body. Accordingly,

the rigidly held water by C-S-H (both interlayer and absorbed in gel pores) can not

rearrange itself to from ice at normal freezing point of water because the mobility of water

existing in an ordered state is rather limited.

It is estimated that water in gel pores does not freeze above -78 C. Therefore, in a

saturated cement paste subjected to freezing the water in large capillaries turns into ice,

while the gel water continuous to exist as liquid water in a super-cooled state. This creates

a thermodynamic disequilibrium between the frozen water in capillaries, which acquire a

low-energy state, and the super cooled gel water which is in a high-energy state. This

causes the gel water to migrate to lower-sites (large cavities where it can freeze. This

fresh supply of water from the gel pores to the capillary pores increases the volume of ice

in the capillary pores steadily until there is no room to accommodate more ice. Any

subsequent tendency for the super-cooled water to flow toward the ice bearing regions

would obviously cause internal pressure and expansion of the system.

It may be noted that during frost action on cement paste, the tendency for certain

regions to expand is balanced by other regions that undergo contraction (e.g., loss of

absorbed water from C-S-H). The net effect on the specimen is obviously the result of two

opposite tendencies. This satisfactorily explains why cement paste containing air

entrainment shows contraction during freezing.

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4.2. Frost Action on Aggregate

Depending on the response of aggregate on frost action, an air entrained concrete

may still be damaged. The mechanism involved in the development of internal pressure on

freezing a saturated cement paste is also applicable to porous aggregates. However, not

all porous aggregates are susceptib