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, a 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 3 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 mm). 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

-          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 can not 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 mm 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 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 two 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 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 mm. 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 .

In ordinary portland cement the amounts of the major compounds varies in the following ranges;

                           

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 calcium aluminate hydrates, which are formed in the hydrated pastes of either undersulfated or high-C₃A portlandcements. The relevant chemical reactions may be expressed as:

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:

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.

 Arepresents 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,and C₄AH₁₉. They form large crystals, typically 1 mm wide.

 Crepresents 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 hm. 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, 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 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 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 generally favors the formation of ettringite (), which forms needle-shaped prismatic crystals. In pastes of ordinary Portlandcement, ettringite eventually transforms to he monosulfate hydrate,, 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 hm. 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.

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 several 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 to 5 mm. 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 mm) in the cement paste. Besides, during mixing and placing operations, entrapped air voids as large as 3 mm (3000 mm) 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 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

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, 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 23.5 MPa

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.

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 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 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

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 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⁺, SO4^2-, 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.

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   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 forces of attraction, but also because they surve as preferred cleavage (kayma sayfaları) 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 σu, 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 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.