The Structure of Concrete -DEFINITIONS -SIGNIFICANCE -COMPLEXITIES -STRUCTURE OF THE AGGREGATE PHASE -STRUCTURE OF HYDRATED CEMENT PASTE -TRANSITION ZONE.

The Structure of Concrete

-DEFINITIONS

-SIGNIFICANCE

-COMPLEXITIES

-STRUCTURE OF THE AGGREGATE PHASE

-STRUCTURE OF HYDRATED CEMENT PASTE

-TRANSITION ZONE IN CONCRETE

-Siddharth shankar

22

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 ndash Aggregate ndash Hardened cement paste (hcp) ndash Transition (interface) zone1048711

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

Setting ndash Solidification of the plastic cement paste

bullInitial set ndash beginning of solidification ndash Paste become unworkable ndash loss in consistency - not lt 45 min

bullFinal set ndash Time taken to solidify completely ndash Not gt 375min

Hardening ndash Strength gain with time ndash after final set

Hydration process

Crystal formation of Cement

Calcium silicate hydrate

The calcium silicate hydrate phase abbreviated C-S-

H 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 CS

ratio varies between 15 to 20 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-S-H 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 Powers-Brunauer 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 hellip 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 structure-

property relationships

It has already been stated that during the early

stages of hydration the sulfatealumina 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 gt C3S gt C4AF gtC2S

Heat of hydration (Calg)

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 around large aggregate during mixing

Characteristic Larger CH crystals more porous framework relatively weak1048711 1048711

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 ndash It lower the strength ndash It increase the permeability ndash It prompt non-linear behavior ndash It favorites crack formation

Microstructural improvement

Use of silica fume

ndash reduce the porosity of the ITZ geometrical effect

(no space) reduces the amount of CH due to

pozzolanic reaction