CONCRETE CONCRETE –– RELATION BETWEEN STRUCTURE AND RELATION BETWEEN STRUCTURE AND PROPERTIESPROPERTIES

CONCRETE – relation between structure and composition

• Introduction• Cement chemistry – composition, types, application

• Formation of porous structure of concrete

Definition of concrete

Hydration of cement

Concrete structure

Water in hydrated cement microstructure

Pores in hydrated cement microstructurePores in hydrated cement microstructure

External and internal factors effecting porous structure of concrete

• Effect of porosity on concrete propertiesMechanical strength

Permeability

Thermal conductivity

• Effect of external conditions on concrete propertiesEffect of high temperatures

Effect of low temperatures

Concrete destruction due to the effect of agressive substances

Literature

• Chemie ve stavebnictví, O. Henning, V. Lach, SNTL/ALFA, 1983.

• Stavební hmoty, L. Svoboda a kolektiv, JAGA Group s.r.o.,

Bratislava, 2004.

• Czernin, W., Cement Chemistry and Physic for Civil Engineers,

Bauverlag GMBH, Berlin, 1980.

• Powers T. C., The Physical Structure and Engineering Properties of

Concrete, Research and Develop. Bull. Of Portland Cement Ass. Concrete, Research and Develop. Bull. Of Portland Cement Ass.

Skokie, No. 90, 1958.

• Feldman, R. F., Sereda, P. J., A New Model for Hydrated Portland

Cement and its Practical Applications. Engng. Jour. (Canda), 53,

1970, 8-9, 53-59.

• Midness, S., Young, J. F., Concrete, Prantice-Hall, Inc., New Jersey,

1981, 657s..

Introduction I/V• One of the oldest artificial building materials.

• Presently, it is the predominant material used in construction – it

competes directly with all other major construction materials,

because of its versatility in applications

Quantities of Materials Used in U.S. Construction, 2000

Material Volume (106 m3) Weight (106 tones)

Timber 107 -

Concrete 275 640

Cement 33 105

Steel 2 13

Brick and clay

products

- 39

Building stone 0.3 1

Asphalt _ 2

Nonferreous metals _ 29

Introduction II/V• The previous table point clearly to inherent advantages attending the

use of concrete.

Advantages Disadvantages

Ability to be cast Low tensile strength

Economical Low ductility

Durable Volume instability

Fire resistant Low strength-to-weight ration

Energy efficient??

On-site fabrication

Aesthetic properties

Introduction III/V• The first important step within the concrete production was the

understanding to the basis of hydraulic properties of lime binders

containing clay minerals.

Historical overview (development of portland cement):

• James Parker in England took out a patent in 1796 on a natural

hydraulic cement – produced by calcining nodules of impure

limestone containing clay. limestone containing clay.

• A similar process began in France six years later. In 1813 Vicat

(who developed the needles we still use to determine the setting

time of cement) prepared artificial hydraulic lime by calcining

synthetic mixtures of limestone and clay.

• James Frost introduced the same approach in England in 1822.

• Finally, in 1824, Joseph Aspdin, a Leeds builder, took out a patent

on „portlands cement“

Introduction IV/V

• On that account, the other progress in concrete design and

production was mainly focused on the improvement of concrete

binder– cement and also on the improvement of cement production

(mills, kilns, etc.)

• During the concrete development, several technological principles

were formulated that remained in certain modified state until fifties of

the last century.

• The quantitative improvement for the understanding to composite

structure of concrete represents work of T. C. Powers. He has

proved that the strength, durability, freeze ressitance and water

permeability are the functions of the concrete porosity.

Introduction V/V• Contemporary research and development works related to concrete

manufacturing are based especially on the decrease of concrete composite porosity.

• The first successful step represents development of new types of

plasticizers that allow to reduce the amount of batch water and in

the consequence to reduce the number of pores that originate within

the batch water evaporation during hydration.the batch water evaporation during hydration.

• Other development represent addition of fine fillers having latent

hydraulic properties – it improves the mixture homogeneity (better hydration) – decrease of porosity, increase of mechanical

strength – new types of concrete.

Cement chemistry

• Portland cement is mixture of silicates, aluminates and alumino-silicates, their solid solutions and gypsum.

• The chemical composition of cement is usually expressed in the form oxides.

Range of chemical composition of portland cement:

• Except the basic oxides (CaO, SiO2, Al2O3, Fe2O3) PC contains also other substances that in certain way effect its properties.

• Types of cement produced from PC– there is specified the value of strength class and appropriate requirement on high initial strength (letter R, effect of temperatureT).

Oxide CaO SiO2 Al2O3 Fe2O3 MgO Na2O K2O TiO2 P2O5 SO3

% 60-69 20-25 4-7 1-5 <6 0,2-1 0-0,1 0,1-0,5 0,1-0,4 3-3,5

Cement manufacturing

� Cement kiln

- used for the pyroprocessing stage of manufacture of Portland and

other types of hydraulic cement

- calcium carbonate reacts with silica-bearing minerals to form a

mixture of calcium silicates

- as the main energy-consuming and greenhouse-gas–emitting

stage of cement manufacture → improvement of their efficiency has stage of cement manufacture → improvement of their efficiency has

been the central concern of cement manufacturing technology

� The manufacture of cement clinkerThe manufacture of cement clinker

1. grinding a mixture of limestone and clay to make a fine "rawmix"

2. heating the rawmix to sintering temperature in a cement kiln:

70 to 110 °C free water is evaporated

110 to 450 °C adsorbed water evaporated

450 to 600 °C clay-like minerals decomposition

600 to 900 °C calcium carbonate reacts with SiO2 to form belite (C2S). 600 to 900 °C calcium carbonate reacts with SiO2 to form belite (C2S).

900 to 1050 °C calcium carbonate decomposes to calcium oxide and CO2.

1050 to 1300 °C formation of belite (C2S), C4AF and C3A

1300 to 1450 °C partial (20–30%) melting takes place, and belite reacts

with calcium oxide to form alite (C3S).

3. grinding the resulting cooled clinker to make cement

� The manufacture of cement clinkerThe manufacture of cement clinker

� Methods of raw mixture preparationMethods of raw mixture preparation

Dry-ground to form a flour-like powder

It is very difficult to keep the fine powder rawmix in the kiln, because the

fast flowing combustion gases tend to blow it back out again. It became a

practice to spray water into dry kilns in order to "damp down" the dry mix,

and thus, for many years there was little difference in efficiency between

the two processes, and the overwhelming majority of kilns used the wet

process.process.

Wet-ground with added water to produce a fine slurry with the consistency of

paint, and with a typical water content of 40–45%

1. obvious disadvantage that, when the slurry was introduced into the kiln, a

large amount of extra fuel was used in evaporating the water

2. a larger kiln was needed for a given clinker output, because much of the

kiln's length was used up for the drying process.

3. wet grinding of hard minerals is usually much more efficient than dry

grinding. When slurry is dried in the kiln, it forms a granular crumble that is

ideal for subsequent heating in the kiln

� CementCement

� 1400-1450 °C - required to complete the reaction

� hot clinker falls into a cooler which recovers most of its heat, and cools the

clinker to around 100 °C

� cement kiln system is designed to accomplish these processes efficiently

� clinker is mixed with gypsum to retard the initial setting of cement and then

ground to a very fine powder, partical size of 5 to 50 mm

� interground with other active ingredients to produce:

blastfurnace slag cement

pozzolanic cement

silica fume cement

� if stored in dry conditions - can be kept for several months without

appreciable loss of quality

Rotary kiln in Permanente v Los Altos, Kalifornia.Wet process, 163 m length, diameter 4m

• Typical composition of ordinary portland cement

• Ze slínkových minerálů jsou prakticky významné:

3CaO.SiO2 C3S trikalciumsilikát (alit)

2CaO.SiO2 C2S dikalciumsilikát (belit)

3CaO-Al2O3 C3A trikalciumaluminát (amorfní fáze)

4CaO.Al2O3.Fe2O3 C4AF tetrakalciumaluminoferrit (celit)

• Do slínku je přidáván také sádrovec (CS – CaSO4) ve formě přírodního sádrovce, sádrovcových střepů z použitých sádrových forem, průmyslového odpadního sádrovce, energosádrovce

• Množství sádrovce 2 – 6% - regulátor rychlosti tuhnutí

• Typical composition of ordinary portland cement – clinker minerals

• Množství sádrovce 2 – 6% - regulátor rychlosti tuhnutíChemical name Chemical formula Shorthand

notationWeight percent

Tricalcium silicate

(alitte)

3CaO.SiO2 C3S 55

Dicalcium silicate

(belitte)

2CaO.SiO2 C2S 18

• Addition of gypsum (natural gypsum, waste gypsum, energy gypsum)acts as regulator of the hydration velocity

Tricalcium aluminate

(amorphous phase)

3CaO-Al2O3 C3A 10

Tetracalcium

aluminoferrite

(celitte)

4CaO.Al2O3.Fe2O3 C4AF 8

Calcium sulphate

dihydrate (gypsum)

CaSO4.2H2O CSH2 2 - 6

• Comparison of the rate of hydration of clinker minerals

Time (days)

Silicate concretes

- Portland cement and other types of silicate cements are hydraulic powder binders that are produced by gridning the clinker with gypsum – after their mixing with batch water, the setting and hardening of concrete is started

• Types of PC – high performance cement (contains higher amount of C3S, finely milled), blended cements, expansive cements, self-stressing cement, masonry cement, oil-well cements, etc.

Blended cements• Latent hydraulic substances – blast furnace slag, active pozzolans

• The latent hydraulicity is evoked by the presence of Ca(OH)2

• Their hardening time is longer than of ordinary PC, their hydration heat is lower – proper for massive casting, hydraulic engineering and for foundation engineering, the blended cements can also enhanced the durability of concrete – lower porosity, higher compressive and bending strength in time

Blended cementsBlended cements

- In the United states, the blended cements are covered by a separate specification according to the ASTM C 595.

- However, they are in U.S. relatively uncommon, since mineral admixtures are usually added at the concrete mixture.

- In contrast, almost all European Portland cements are in fact blended cements.blended cements.

TypesTypes ofof PC in PC in EuropeEurope

� There are different standards for classification of Portland cement. The two major standards are the ASTM C150 used primarily in the U.S. and European EN-197.

� EN 197 cement types CEM I, II, III, IV, and V do not correspond to the similarly-named cement types in ASTM C 150.

� EN 197-1 defines 5 classes of common cement that comprise Portland cement as a main constituent:Portland cement as a main constituent:

� I Portland cement - comprising Portland cement and up to 5% of minor additional constituents

� II Portland-composite cement - Portland cement and up to 35% of other single constituents

� III Blastfurnace cement - Portland cement and higher percentages of blastfurnace slag

� IV Pozzolanic cement - Portland cement and up to 55% of pozzolanic constituents

� V Composite cement - Portland cement, blastfurnace slag and pozzolana or fly ash

Expansive cementsExpansive cements

� One of the major disadvantages of Portland cement concrete is the volume contraction that takes place on drying (shrinkage) and its susceptibility to tensile cracking if this contraction is wholly or partially restrained.

� Since random cracking is unsightly and compromise the integrity of structure, control of shrinkage cracking must be allowed for in design and construction.allowed for in design and construction.

� Cracking is particularly critical in water-retaining structures, or when entry of water must be prevented.

� Volume expansion during early hydration and hardening could be used to offset shrinkage - see figure.

� Although the ordinary Portland cements have very small expansion during moist curing, they can be modified to enhance early expansion – shrinkage control

- Shrinkage compensate cements

Drying shrinkage of concretes made with (a) portland cement and (b) expansive cement

Drying shrinkage of concretes made with (a) portland cement and (b) expansive cement

Composition of expansive cementsComposition of expansive cements

� all cements under this specification are based on formation of ettringite in considerably quantities during the first week of hydration

� three variants, K, M and S, are produced, depending on the nature of the aluminate compound used to generate ettringite

Calcium aluminate + S_ + H ettringite

� Reacted calcium aluminate replaces the C3A in the cement and the calcium silicates still control lon-term properties.

� Example: type E-1(K) – used only in U.S., composed of calcium sulfoaluminate (C4A3S_) with anhydrite (CS_) –CaSO4, together with free lime to enhance the rate of ettringite formation, and hence expansion.

White White portlandportland cementcement� white Portland cement differs physically from the gray form

only in its color, and as such can fall into many of the above categories

� its manufacture is significantly different from that of the gray product, and is treated separately

In combination

� with white aggregates to produce white concrete for prestige construction projects and decorative work

� with pigments to produce brightly colored concretes and � with pigments to produce brightly colored concretes and mortars

� the white color is achieved by eliminating iron form the cement – thus it is cement with a high C3A content and no C4AF

� Iron free clay (kaolinite or china clay) must be used, and bauxite (aluminum oxide) is often needed to achieve the required alumina content

� Special ball mills must be used to prevent iron contamination during grinding.

� The higher cost of raw materials and changes in manufacturing procedures make white cement expensive.

High alumina cement I/III

- HAC is hydraulic binder for production of refractory concretes (can survive until 1600°C) and high temperature resistant concretes (can be exposed to temperatures higher than 20°C)

- The raw mixture is formed by poor limestone and bauxite

- Its production is expensive

� Electrical melting in electrical arc furnace at temperature 1500-1600°C (so-called melted corundum), the melt is slowly cool down and tavenina se pomalu ochlazuje tak, aby vznikl krystalický CA, and tavenina se pomalu ochlazuje tak, aby vznikl krystalický CA, který se následně mele na prášek

- The clinker consists of 45% of Al2O3 (refractory concretes even 81%), 40% CaO, the residue involves oxides of iron, silica and other residual admixtures.

- The final properties of refractory concretes affects also the used type of aggregates.

Clinker minerals in alumina concrete:

- CA (monocalcium aluminate)

C2A (calcium dialuminate)

C3A5, C3A2, C2AS, C4AF, C5A3

High alumina cement II/III

Clinker after mixing with batch water quickly hydrates to CaO. Al2O3.10H2O, this process is accompanied by high hydration heat 550-650 J/g (PC 270-400 J/g) and high initial strength 20-60 MPa/24 hrs.

Type and composition of calcium aluminate hydrates depends on ambient temperature of hydration

22°C CA+10H→CAH10

22-30°C 2CA+11H→C2AH6+2AH3

30°C 3CA+12H→C3AH6+2AH3

above 30°C 3CA+10H→C3AH6+2AH3+18H

Metastable - increase of porosity, cracks formation →strength decrease, there is necessary to decrease water/cement ration

High alumina cement III/III

- within insufficient curing (wetting immediately after setting), the danger of formation of low strength calcium aluminate hydrate C3AH6 is very actual

- it can caused reduction of the strength of material in time

- several breakdowns of structures built from high alumina cement in the past – since 1985, the HAC can not be used in Czech republic for load bearing strctures

Refractory concrete - aggregates

- for manufacturing of refractory concrete having bulk density higher than 1500 kg/m3 exposed to temperature lower than 700°C - natural aggregates can be used

- for high density refractory concretes that should find use at temperatures higher than 700°C, artificial aggregates must be used

- the natural aggregates can not change its machanical properties at higher temperatures, and also its temperature shrinkage must be excluded – the most suitable natural stones for refractory concrete excluded – the most suitable natural stones for refractory concrete are basalt, diabase or andesite

- unsuitable materials are silicious aggregates and granite

- the silicious aggregates due to the high temperature crack and the granite is shrinked

- for high density concretes for temperature exposition between 800°C - 1000°C the natural aggregates can not be used – usage of grinded ceramics, grinded blast furnace slag

- for temperature higher than 1000°C, grinded corundum, bauxite, and fire clay can be used

Formation of porous structure of concrete

Definition of concrete:

From the point of view of materials engineering, the concrete can be defined as heterogeneous system of aggregate interconnected by cementitious gel with dispersed pores.

Concrete is material which properties are not constant – in concrete structure the time dependent changes can be observed – in binder (hydrated cement mortar) and in the interphace – in binder (hydrated cement mortar) and in the interphace transition zone between thes solid binder and aggregates –crystaliaztion of hydrated products, water evaporation etc.

Concrete = Filler + Binder

Portland cement

concrete =

Aggregate (fine

and coarse) +

Portland cement

paste

Mortar = Fine aggregate + Paste

Paste = Cement + Water

Theory of concrete hardening

� Le Chatelier’s crystal theory (1882):

1st phase – gradual dissolving of cement in water (hydrolysis, hydration), the results is oversaturated solution (oversaturated by hydrates)

2nd phase – crystalization from solution, formation of needle crystals that are interconected

�Michaelis’ colloid theory (1892):

1st phase – partial dissolution, formation of colloid substance from CS-, CA- and CF-hydrates, C-S-H gels are formed

2nd phase – shrinkage of hydrogel due to the effect of „inner water exhaustion“ by not hydrated cement grains

• Gel-crystalization theory by Bajkov (1923)

• theory of microstructure formation by Rebinděr and Polak (1960)

• theory of gel strcture by Powerse (1961)

• etc.

Portland cement hydration- Cement hydration is realised within three induction periods:

1st period: (10 – 15 minutes)

- almost directly reacts substantial part of C3S and C-S-H gel is formed as well as crystal portlandite

2(3CaO.SiO2) + 6H2O 3CaO.SiO2.3H2O + 3Ca(OH)2

- simulaneously reacts C3A at the gypsum presence

- hexagonal crystal ettringite is formed, that is transformed on monosulphate forming boards

3CaO.Al2O3 + 3CaSO4.2H2O + 26H2O 3CaO.Al2O3.3CaSO4.32H2O

3CaO.Al2O3.3CaSO4.32H2O + 2(3CaO.Al2O3) + 4H2O

3(3CaO.Al2O3.CaSO4.12H2O) monosulphate

Needle crystals of ettringite in hydrated cement binder(magnification 5000x)

Cement hydration II2nd period: ( it ends after 12 – 24 hrs)

- It is characteristic by transformation of cement paste to solid phase

- The basis hydration reaction of C3S is developed and lon-fibre C-S-H

gels and enhanced portlanditte crystals are formed

- rising of specific surface area of the C-S-H system (cca 100x)

- The cement grains are coming near to each other by inter-growing of - The cement grains are coming near to each other by inter-growing of

crystals of hydrated products

- also the hydration of ferritte phase takes place within this period

4CaO.Al2O3.Fe203 + 4CaO(OH)2 + 22H2O 4CaO.Al2O3.13H2O

+ 4CaO. Fe203.13H2O

Cement hydration III3rd period:

- time unlimited period of concrete hydration including the hydration of

C2S

- maturing of concrete, hydration of non-hydrated cement grains,

recrystalization of hydrated products due to the effect of water

diffusion from ambient environment

2(2CaO.SiO ) + 4H O 3CaO.2SiO .3H O + Ca(OH)2(2CaO.SiO2) + 4H2O 3CaO.2SiO2.3H2O + Ca(OH)2

The amount of hydration heat is dependet on mineralogical

composition of concrete, fineness of grinding, temperature of

hydration, special additives and admixtures and water/cement ratio-

With rising temperature, the hydration is accelerated.

Time (days)

Structure of concrete I

Macrostructure – it is evaluated according to cross-section of concrete

element, it shows concrete like material formed from two basic

componenets – aggregates of different dimensions and shapes and

binder – discontinuous layer of hydrated cement interconnecting

stone mineral filler

Concrete

macrostructure

Concrete structure II

Microstructure – microscopic observation e.g. By SEM

– binder structure differs in specific places, apparently homogeneous

binder has porous structure having different dimension and shape of

pores

� interconnection by pores is dependent above all on water/cement

ratio, concrete and cement composition and concrete curing

ošetřování během hydratačního procesu

Concrete

microstructure

Structure of concrete III

Electron microscopy enabled identification of four basic constituents

of solid phase of hydrated cement paste:

� Calcium silicate hydrates (C-S-H)

� Calcium hydroxide (C-H)

� Calcium sulpho-aluminates (C-S-A-H)

� Non-hydrated cement grains

Structure of concrete IV

� Calcium silicate hydrate (C-S-H), C-S-H gel- it fills 50-60% of hydrated cement volume and represents

determining factor of cement gel properties

- it has variable morphological structure and is characteristic by the

existence of crystal fibres and crystal reticular forms

- formation of C-S-H gel begins with rising of fibrous compounds on

cement grains after reaction with batch water

- in time, the thickness of hydrated part of cement grain is rising and

restricts the moisture transport to grains – the velocity of hydration is

slowing down

Structure of concrete V

� Calcium hydroxide (C-H), portlanditte- it fills 20 – 25% of hydrated binder volume

- forms large hexagonal crystals

- it has negative influence on chemical resistivity and stability,

especially in acid environments

� Calcium sulpho-aluminates (C-S-A-H)� Calcium sulpho-aluminates (C-S-A-H)- fill 15 – 25% of hydrated products volume

- in the initiation period of cement hydration and setting, ettringite is

formed that after it transform into form of monosulphate hydrate

C4ASH18 that forms hexagonal crystals

- it decrease the resistivity of concrete agains sulphates

�Non-hydrated cement grains – their presence and quantity are

dependent on water/cement ratio of concrete mixture, size of cement

grains and aggregates, rate of hydration etc.

Water in hydrated cement binder IWater is pernament constituent of hydrated cement paste

microstructure (cement gel).

� Capillary water - free water in macropores (>0,05 mm) and technological cavities

dependent above all on exterior environment of concrete, changes of

its amount have not significant effect on mechanical parameters of

concrete

- on the other hand, water present in small capilaries is tightly bonded - on the other hand, water present in small capilaries is tightly bonded

and its decrement is acompanied by shrinkage

- evaporation of physically adsorbed water on the surface of hydration

products is also related to shrinkage and cracks formation

� Gel water

- in C-S-H structure there is present monomolecular water layer, that

is tightly bonded by hydrogen bounds (bridges) – at relative humidity

lower than 11% it leads to high volume changes of hardened cement

paste

Water in hydrated cement binder II

�Chemically bonded water- it is a part of crystal structure of hydration products

- its removal is possible only at high temperatures and leads to

decomposition of concrete it self

model of Feldman, Seredamodel of Feldman, Seredamodel of Feldman, Seredamodel of Feldman, Sereda

Adsorbed water

Water in capillarypores

Water betweenlayers of C-S-H gel

Pores in hydrated cement paste

� pores in concrete structure is necessary to divide according to their

origin within the production process (transformation from

heterogeneous viscous suspension into solid substance)

� gel pores, capillary pores, technological pores (their rise within

mixing by ambient air, artificial closed spherical pores formed by

special additives, pores of aggregates)

� pores of aggregates are typically within the range 1 – 5%,

limestone e.g. 24%

� there were several views on arrangement of porous structure of C-

S-H gels based on quality of experimental apparatuses

� on the other hand, the definition of gel pores is the same – they are

defined as a part of inner structure of C-S-H gel

Clasification of pores in hydrated cement paste

Name Dimension Characterization Relation to concrete properties

Technological

pores

1000-15 µm big, aproximately

spherical pores

effect on strength and permeability

Capillary pores 15-0.05 µm

50-10 nm

wider capillaries

midle size

capillaries, capillary

effect on permeability, strength and

shrinkage within the begining of

setting

effect on strength, permeability and

shrinkage at higher relative humidity

cavities

Gel pores 10-2.5 nm

2.5-0.5 nm

< 0.05 nm

narrow capillaries

pores between

forms of hydrated

gel

pores between gel

layers

effect on shrinkage at relative

humidity higher than 50%

affect shrinkage and creep

affect shrinkage and creep

Arrangement of porous structure of C-S-H gel according

Powerse and Brownyarda

Capillary pore

Model struktury C-S-H gelu podle Feldmana a Seredy

External pore

Spherical capillary pore

Inner pore

Internal and external factors affecting porous structure of concrete

� chemical process of concrete hardening is affected by number of of

internal and external factors that define its final properties

� pore dimension and their distribution are time dependent parameters

� chemical composition of clinker

� fineness of grinding

� water/cement ratio (w/c)

� concrete curing

� ambient temperature of hydration

Pore size distribution in hardening cement paste for different

time of hydration

3 days3 days

Pore diameter (Å)

320 days

Pore diameter (Å)

Effect of hydration temperature on porous strcture of concrete

� increased temperature accelerates hydration and its decrease

slows down the velocity of hydration reaction – interruption of

hydration

� hydration starting at lower temperature leads to formation of crystal

of tobermoritte (Ca5[Si3O8(OH)]2·2-5H2O – zvýšení celkové pevnosti

� fast initial hydration at higher temperature form thicker zone on

cement grain surface that is less permeable for external water and

hydration is decelerated

� effect on propagation and rate of shrinkage (tensile stress,

narrowing of capillaries – the velocity of water evaporation is changed)

Effect of porous space on concrete properties I

The porous structure is characterised by its porosity, specific volume

of pores, specific surface area of pores and by their distribution

function.

� effect on concrete strength

0

kpS S e−= ⋅

S strength of material having specific porosity

p porosity

S0 strength of material having porosity equal to 0

k constant (material characteristic)

For concrete, the relation between porosity and mechanical strength

complicates also problem of microcracks that originates during

concrete ageing and hardening by shrinkage, especially in the

transition layer between hardened cement paste and aggregates.

Compressive strength of concrete as a function of porous space

fro three different concrete mixtures after 28 days of hardening

Compressive strength as a function of porosity

str

en

gth

(MP

a)

porosity

co

mp

res

siv

es

tre

ng

th

Effect of porous space on concrete properties II

� effect on thermal conductivity- thermal conductivity is dependent on porosity and water content

present in

- the degree of saturation affects thermal conductivity more than

porosity

Cement paste Thermal conductivity [W/mK]

w/c = 0.4 1.3

w/c = 0.5 1.2

w/c = 0.6 1.0

Water 0.5

Air 0.026

Effect of porous space on concrete properties III

� permeability K- it defines permeability of concrete for liquids transport

- it has clear relation to durability with regard to resistivity against

cyclic freezing (mechanical loading of porous structure)

- defined by Darcy’s law

-dq/dt velocity of liquid flux

- m liquid viscosity

- ∆H pressure gradient

- A surface area

- L thickness of materials

dq HK

dt Lµ

∆= ⋅

Changes of cement paste permeability in dependence on time of

hydration (w/c = 0.7)

Hardening time [days] Permeability [10-11 cm/s]

Fresh mixture 20 000 000

5 4 000

6

8

1 000

4008 400

13 50

24 10

Hardened mixture 6

Effect of external environment on hardened porous strcture of cement binder I

� impact of high temperatures - negative influence of ambient high temperatures on concrete is

related to water content decrement in hydrated cement binder

accompanied by crucial porosity changes

- free and capillary water are gradually evaporated– volume changes, shrinkage

Time (days)

Sh

rin

kag

e (%

)

Effect of external environment on hardened porous strcture of cement binder II

� action of high temperatures- at temperature 150°C the gel water and crystalline water of sulpho-

aluminate is evaporated

- cca from 500°C starts decomposition of portlanditte (fracture of

cement binder microstructure)

- concretes containing silicious aggregates change at cca 570°C beta

form of SiO2 to alpha form

- total decomposition of cement binder starts at temperatures higher

than 800°C – entire destruction of CaCO3

- measurement using thermal analysis

Thermal analysis of cement binder samples

Temperature (°C)

Silica– polymorphous material- several forms of silica can be distinguishedModifications: presently, 22 forms is known

ββββ-silica (low-temperature for)rhombic crystal lattice ρρρρ=2,65 g cm-3

γγγγ-tridymite rhombic c. l. - 2,26 g cm-3

ββββ-cristobalite tetragonal c. l. 2,32 g cm-3

Phase diagram of SiO2

� Effect of low temperatures on concrete properties

- deterioration of porous structure of cement paste is caused by the

phase change of liquid water in macropores and capillaries

- the phase change is accompanied by increase in volume (cca 9% -

inner tension)

- on this account, the water/cement ratio has significant influence on

concrete resistivity against freezing cycles

- also the shape and dimension of pores are very important factors

that limits the freezing resistivity of concrete

- it is beneficial to produce concretes with addition of aerators that

forms spherical pores – they are not fully filled by water (reserve for

volume changes of ice)

� Destructive chemical reactionsIn principle, all the substances having pH lower than 12.5 decline the

alkalinity of liquid that fills the pores and forms the equilibrium among

main components of hydrated cement paste (C-S-H, C-H).

Effectiveness and velocity of harmful reactions are function of

aggressiveness of specific substance and porosity of concrete based

material.

There are several substances aggressive for concrete – typically in

the ambient air and underground water are present e.g. CO , SO , the ambient air and underground water are present e.g. CO2, SO2,

SO3, SO4, NOx a Cl-

Reaction of airy CO2 with Ca(OH)2 leads to the formation of CaCO3 in

surface layer of concrete, where as a consequence decreases pH

value under 9.0, which is limiting value for passivation of embeded

steel reinforcement– the volume increase of steel bars induces

tension in concrete, spalling and destruction of concrete structures.

Effect of substances containing ions of NO3, SO4 and Cl is given by

rising of crystal substances that were formed during reactions with

hydrated products of concrete – efflorescence, crystallic pressures,

recrystallization.

Classification of concrete – ČSN EN 206-1

� according to the bulk densityStandard concrete (ordinary) 2 000-2 600 kg m-3

Light weight concrete < 2 000 kg m-3

Heavy concrete > 2 600 kg m-3

� according to the strength� according to the strengthe.g. in accordance with compressive (Mpa) measured on cylindres

having diameter 150 mm and high 300 mm (number ahead of slash)

in accordance with compressive (Mpa) measured on cubes having

dimensions 150 mm, the measurements are done after 28 days from

casting(number behind the slash)

C8/10, C12/15, C 16/20, C 20/25, C 100/115

LC 8/9, LC 12/13, LC 80/88

� According to the consistence of fresh concrete → rank in accordance with specific testing methodsSlump test S1-S5

VeBe V0-V4

In accordance with compactibility C0-C3

In accordance with classification of flow table test F1-F6

� According to the biggest size of aggregate� According to the production technologyDirectly on building site

Transportconcrete

� According to the reinforcementMass concrete (unreinforced concrete)Mass concrete (unreinforced concrete)

Reinforced concrete (steel bars and nets)

Prestressed concrete(steel reinformcement is prestressed)

Fibre concrete (contains fibres of several types of proper materials)

� According to the purposes of usage and function:Structural

Concrete filler

� According to the additional function :Concrete for water structures

Structural and insulative concrete (AAC)

Pavement concrete

Massive concrete

Decorative concrete (architectural and face concrete)

Concrete specification

= summary of all requirements on properties or composition of fresh

as well as of hardened concrete for its production, transport,

compaction, curing etc.

- integral part of project of concrete structure

- necessary information for concrete producers

Concrete specification involves:Concrete specification involves:The way of usage of fresh and hardened concrete

Conditions of curing

Information on structure dimension (regarding to the development of

hydration heat)

Information on ambient conditions

Requirements on surface finish

Requirements on maximal dimension of aggregate

Concrete must be specified as type or specific, for given application

and structure.

Specification of typified concrete

Fundamental requirements from orderer:

� Strength class

� Degree of environment effects

� Maximal upper limit of aggregate fraction� Maximal upper limit of aggregate fraction

� Category of chloride content

� Degree of consistence or measured value of consistence

In detail in „Stavební hmoty“, L. Svoboda a kol., JAGA, Bratislava

2004.