2
The Structure of Concrete
-DEFINITIONS
-SIGNIFICANCE
-COMPLEXITIES
-STRUCTURE OF THE AGGREGATE PHASE
-STRUCTURE OF HYDRATED CEMENT PASTE
-TRANSITION ZONE IN CONCRETE
-Siddharth shankar
DEFINITIONS
 The type, amount, size, shape, and distribution of phases
present in a solid constitute its structure.
 The gross elements of the structure of a material can readily be
seen, whereas the finer elements are usually resolved with the
help of a microscope.
 The term macrostructure is generally used for the gross
structure, visible to the human eye.
 The limit of resolution of the unaided human eye is
approximately one-fifth of a millimiter (200 μm).
 The term microstructure is used for the microscopically
magnified portion of a macrostructure.
Modern electron microscopes
 The magnification capability of modern
electron microscopes is of the order of 105
times; thus the application of transmission
and
scanning
electron
microscopy
techniques has made it possible to resolve
the structure of materials to a fraction of a
micrometer.
SIGNIFICANCE
 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.
 The properties can be modified by making
suitable changes in the structure of a
material.
COMPLEXITIES
 From examination of a cross section of concrete, the
two phases that can easily be distinguished are
aggregate particles of varying size and shape and
the binding medium, composed of an incoherent
mass of the hydrated cement paste (henceforth
abbreviated hcp).
Macroscopic level
 At the macroscopic level, therefore, concrete may be
considered to be a two-phase material, consisting of
aggregate particles dispersed in a matrix of the
cement paste.
Microscopic level
 At the microscopic level, the complexities of the
concrete structure begin to show up. It becomes
obvious that 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.
(a) 結構之示意圖
(b)各組成所佔有體積之比例關係
混凝土之微結構
9
Three-phase theory
 Three phases:
– Aggregate
– Hardened cement paste (hcp)
– Transition (interface) zone
THREE PHASES OF CONCRETE
STRUCTURE OF THE AGGREGATE
PHASE
AGGREGATE PHASE
 The aggregate phase is predominantly responsible
for the unit weight, elastic modulus, and dimensional
stability of concrete.
 These properties of concrete depend to a large
extent 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 phases in aggregate is
usually
less
important
than
the
physical
characteristics such as the volume, size, and
distribution of pores.
Aggregate particles
 Natural gravel has a rounded shape and a smooth
surface texture.
 Crushed rocks have a rough texture; depending on
the rock type and the choice of crushing equipment,
the crushed aggregate may contain a considerable
proportion of fault or elongated particles, which
adversely affect many properties of concrete.
 Lightweight aggregate particles from pumice, which
is highly cellular, are also angular and have a rough
texture, but those from expanded clay or shale are
generally rounded and smooth.
STRUCTURE OF HYDRATED CEMENT
PASTE
 Anhydrous portland cement is a gray powder
that consists of angular particles typically in
the size range 1 to 50 μm.
 It is produced by pulverizing a clinker with a
small amount of calcium sulfate, the clinker
being a heterogeneous mixture of several
minerals produced by high temperature
reactions between calcium oxide and silica,
alumina, and iron oxide.
STRUCTURE OF HYDRATED CEMENT
PASTE
 The chemical composition of the principal
clinker minerals coresponds approximately
to C3S, C2S, C3A, and C4AF; in ordinary
portland cement their respective amounts
usually range between 45 and 60, 15 and 30,
6 and 12, and 6 and 8 percent.
Hydration process
Setting – Solidification of the plastic cement paste
•Initial set – beginning of solidification – Paste become
unworkable – loss in consistency - not < 45 min.
•Final set – Time taken to solidify completely – Not > 375min.
Hardening – Strength gain with time – after final
set
Crystal formation of Cement
Calcium silicate hydrate
 The calcium silicate hydrate phase, abbreviated C-SH, makes up 50 to 60 percent of the volume of solids
in a completely hydrated portland cement paste and
is, therefore, the most important in determining the
properties of the paste.
 The fact that the term C-S-H is hyphenated signifies
that C-S-H is not a well-defined compound; the C/S
ratio varies between 1.5 to 2.0 and the structural
water content varies even more.
Calcium silicate hydrate
 The morphology of C-S-H also varies from poorly
crystalline fibers to reticular network. Due to their
colloidal dimensions and a tendency to cluster, C-SH crystals could only be resolved with the advent of
electron microscopy.
 Although the exact structure of C-S-H is not known,
several models have been proposed to explain the
properties of the materials. According to the PowersBrunauer model, the material has a layer structure
with a very high surface area.
Calcium hydroxide
 Calcium hydroxide crystals (also called portlandite)
constitute 20 to 25 percent of the volume of solids in
the hydrated paste. In contrast to the C-S-H, the
calcium hydroxide is a compound with a definite
stoichiometry化學計量學, Ca(OH)2.
 It tends to form large crystals with a distinctive
hexagonal-prism morphology. The morphology
usually varies from nondescript to stacks of large
plates, and is affected by the available space,
temperature of hydration, and impurities present in
the system.
Calcium hydroxide
 Compared with C-S-H, the strength-contributing
potential of calcium hydroxide due to van der Waals
forces is limited as a result of a considerably lower
surface area.
 Also, the presence of a considerable amount of
calcium hydroxide in hydrated portland cement has
an adverse effect on chemical durability to acidic
solutions because of the higher solubility of calcium
hydroxide than C-S-H.
Mono-Sulfoaluminate … Ettringite
Ettringite
Calcium sulfoaluminates
 Calcium sulfoaluminate compounds occupy 15 to 20
percent of the solids volume in the hydrated paste
and therefore play only a minor role in the structureproperty relationships.
 It has already been stated that during the early
stages of hydration the sulfate/alumina ionic ratio of
the solution phase generally favors the formation of
trisulfate hydrate, C6AS3H32, also called ettringite,
which forms needle-shaped prismatic crystals.
Reaction rate: C3A > C3S > C4AF >C2S
Heat of hydration (Cal/g)
Compound
3 days
90 days
13 years
C3S
58
104
122
C2S
12
42
59
C3A
212
311
324
C4AF
69
98
102
Three-phase theory
-Transition zone
Three-phase theory
-Transition zone
 A thin shell layer (10-50 μm thick) around
large aggregate.
 Formation: Water films
aggregate during mixing.
around
large
 Characteristic: Larger CH crystals; more
porous framework; relatively weak.
Transition Zone
Miexhta and Monteiro: Concrete
Transition zone
-Influence on concrete properties
 Fraction of transition zone in size is much
smaller than other two phases, its influence
on concrete properties is far greater.
–
–
–
–
It lower the strength
It increase the permeability
It prompt non-linear behavior
It favorites crack formation
Microstructural improvement
 Use of silica fume
– reduce the porosity of the ITZ geometrical effect
(no space) reduces the amount of CH due to
pozzolanic reaction