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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.