Saturday, 31 January 2009

i never love him be4 and will never after

i know it
but how come i keep dreaming about him these days
i said i dont like him
i never really fall in love with him
why i keep dreaming about him
why why?

Thursday, 29 January 2009

still talkin 'bout title

i dont want to just look them down
but hey hey guys face the truth
the truth is that arsenal is in 5th place right now
if the season end up like this
we certainly can not qualified 4 the Champion league
and the champion
i have to say now
they should be the champion

Tuesday, 27 January 2009

so the truth is that i didnt really love him

he's not someone the i have really really loved
though i've cried 4 him
being heart broken
but the feeling is not real
everything is not real
i was never in love with him

Sunday, 18 January 2009


hull city...
that shame day in emirates
but today ~
we beat them 3-1
we cant bear to lose any game again
currently at 5 in league table means that the ticket to next season's champions league is not in our hand
Aston Villa is strong this season~
the consequence to in UEFA cup is fatal
so so so keep going~
always support
i feel good finally see the smile on Wenger's face

Thursday, 15 January 2009


Timber is a natural material which is not subjected to pre-use factory processing. As timber cannot be manufactured to specification, remember to check availability before specifying certain type or grade of timber.

Timber is converted mainly from the trunk.

The periphery of the trunk

Medullary ray
Groups of cells running radially

The older timber at the heart
Composed of dead tissue
Usually darker in color
Drier and harder than the outer layer of sapwood

The living outer layer
Contain more moisture than the heartwood
Not as strong in the green state as heartwood, but after seasoning, may be as dense and strong as heartwood
Sapwood is inferior to heartwood in respect of durability, containing starches which may attract insects and provide food for fungal growth. Sapwood, however, is very permeable and more easily impregnated with preservative and, where the conditions under which the timber must serve are such that treatment with a preservative is essential, it may be beneficial to use sapwood as a deliberate choice.

Tree classification:
Angiosperms or dicotyledons which have broad leaves shed in the autumn
Slower growth, more expansive, not grown in some countries
Mainly used for furniture and interior finish
Gymnosperms or conifers which have needle-like leaves, broadly evergreen
Faster growth, normally cheaper
Used mainly for construction

Conversion of timber:
Conversion is the process whereby the felled truck is converted into marketable sizes of timber.
Different strength characteristics are possible with varying methods of sawing the trunk.

There are two main methods of converting timber:
Through and through (or Plain or Crown sawn) which produces tangential boards and
Quarter Sawn which produces radial boards.
The Quarter sawn is far more expensive because of the need to double (or more) handle the log. There is also more wastage. It is however more decorative and less prone to cup or distort. Note also there are two ways of sawing the quarter.
Through and through produces mostly tangentially sawn timber and some quarter sawn stuff. (see diagram) Tangential timber is the most economical to produce because of the relatively less repetitive production methods. It is used extensively in the building industry.
There are other ways but they are all variations of tangential and radial cuts to obtain the best or most economical boards for the use it is to be put. These basic cuts are not always able or need to be, on the exact tangent or radius of the trunk. The cuts, that fall between, crown and quarter are called 'rift' and between 'rift' and 'quarter' are identified as 'figured' - see below for explanation. Boxing the heart refers to eliminating the heartwood from the boards that would otherwise produce shakes, juvenile wood or may even be rotten.

Tangential boards (crown, plain or flat sawn) are used extensively for beams and joists. They are stronger when placed correctly edge up with the load in the tangential axis. These type of boards suffer from 'cupping' if not carefully converted, seasoned, and stored properly. Annual growth rings form an angle less than 45 degrees.

Radial boards (radial, figured or quarter sawn) are typically cut on 'the quarter' and produce a pattern of the medullary rays especially in quartered oak. Such timber is expensive due to the multiple cuts required to convert this board. The radial face of the board is slightly stronger and stiffer than the tangentially face but the cross section and condition of the timber has more effect on strength. Annual growth rings form an angle greater than 45 degrees.
Crown sawn is obtained by sawing tangentially to the annual rings. It is also referred to as 'Plain Sawn' or 'through and through'.
Rift sawn is the cut which falls between crown and true quarter sawn. It is straight grained and in oak, does not reveal any 'silver ribbon' grain features. Quality floor boards are prepared from rift sawn timber because it wears well and shrinks less. Annual growth rings form an angle between 30 and 60 degrees.
Quarter sawn boards are radial cut from the centre of the tree. It produces the distinctive silver ribbon effect (in oak) across the whole board. Annual growth rings form an angle greater than 45 degrees. True quartered boards producing the best features will have the angle on or very much closer to 90 degrees.
'Figured' - is the cut between 'rift' and 'true quartered'. It has varying degrees of 'silver ribbon' (in oak) showing through but not the full figured effect found in true quarter sawn boards.
Different species have their best features enhanced by choosing the best cut appropriate to their species.

Billet sawing

Commercial quarter sawing

True radial sawing

Strength of timber:
The strength of timber is affected by:
Variability within a species (density, rate of growth, position in trunk etc.)
Defects (knots, decays, insect attack etc.)
Moisture content
The strength of timber increases with decrease of moisture content, and vice versa
Duration of loading
Strength decrease with duration of loading, may be up to 35%
The tensile & compressive strength in the direction parallel to the grain are much higher than in the direction perpendicular to the grain; the shearing strength along the grain is much lower than that across the grain

Moisture content:
moisture content= (wt.of moisture contained)/(dry wt.of timber)
The condition when all the cells are dry but the walls are still saturated is referred to as the fibre saturation point, usually between 23 to 27% moisture content. Below the fibre saturation point, all properties of timber improve with decease in moisture content except toughness or the shock resistance.

Seasoning of timber:
The purpose of seasoning is to expel or dry up the sap and water in the wood to improve its strength and other properties.
Air of natural seasoning
Air or natural seasoning is affected by stacking the timber in such a way that air can circulate freely around all surfaces. Air seasoning may take weeks or even months to complete.
Kiln or artificial seasoning
Kiln or artificial seasoning is carried out by subjecting the timber in a kiln or chamber to a current of hot air. Kiln seasoning may take a few hours to a few days.

Structural grading rules
It is not economically justified to use defect free or clear timber for structural purposes. Instead of specifying that structural timber “shall be free from defects”, it is much more practical to accept defects, but to place definite limits on the size and number that will be permitted and to base working stresses on the strength of the piece of minimum acceptable quality.
Select grade
Weakest piece shall be 75% as strong as defect-free timber
Standard grade
Common grade
40 to 50%

Defects in timber
Natural defects:
Lengthwise across annual rings
Lengthwise between annual rings
Part of a branch which becomes enclosed in a growing tree
Seasoning defects
Various defects in timber:
a = twist; b = cupping;
c = honeycomb checks;
d = bowing; e = checks;
f = end split;
g = compression failure;
h = behaviour of test sample from a case-hardended board;
i = spring.

Seasoning defects are due to distortion of timber during seasoning which may arise from excessive or uneven drying, exposure towind and rain, and poor stacking.

(1) A knot which is a portion of a branch and which has become incorporated in the body of a tree. All provisions of this chapter relating to the presence of knots apply only to the surface of the piece on which the knot appears, and all such provisions limiting the size of knots apply to the mean or average diameter as measured on the surface.
(2) Cross-grained wood in which the fibres are not parallel with the axis or longitudinal edge of the piece. It is expressed in this chapter as the slope of the grain with respect to the edges of the piece. For instance, one in 12 means that in a distance of 12 inches the grain deviates one inch from the edge. The presence in any surface of local discontinuity of grain or local deviations in the straightness of grain because of knots permitted in the piece shall be disregarded in applying the provisions of this chapter.
(3) A shake which is a separation along the grain, the greater part of which occurs between the rings of annual growth.
(4) A check which is a separation along the grain, the greater part of which occurs across the rings of annual growth.
(5) A pitch pocket which is an opening between the grain of the wood, containing more or less pitch or bark.
(6) Decay which is the destruction of the wood substance due to the action of wood destroying fungi.
(7) A cross-break which is a separation of the wood cells across the grain of the wood.
(8) A compression failure which is a deformation of the fibres due to excessive compression along the grain. This deformation takes the form of a buckling of the fibres.
(9) Compression wood, or proudwood, which is an abnormal growth occurring in conifers (softwood) and is characterized by relatively wide annual rings, usually eccentric and a comparatively large proportion of summerwood, usually 50% or more, which merges into the springwood without exhibiting a marked contrast in color.

When timber is seasoning and it's moisture content (MC) is reduced below the Fibre Saturated Point (FSP) continued drying will cause dramatic change such as increase in strength but also distortion and shrinkage.
Shrinkage is the greatest tangentially over the radial direction with little loss along the length of the board, etc.

Because of this varying shrinkage rates tangential boards tend to cup because of the geometry of the annual rings shown on the end grain. It can be seen that some rings are much longer than the others close to the heart. Therefore they will be more shrinkage at these parts than the others ~ cupping is the result.
In square section timber cut from the same place, diamonding is the result.

Knots are the result of the trees attempt to make branches in the early growth of the tree. They are the residue of a small twig, shoot, etc. that died or was broken off by man or an animal in the wood or forest. The tree subsequently continued its growth over this wood.
The knot may be live, sound, or tight or if it has become separated and is contained in residue of bark, dead.
Dead knots become loose and downgrade the appearance and stability of the board. Most grading systems uses the amount of knot area as an indication of its quality. The more knots the less the quality.
Dead loose knots are extremely dangerous to machinists. The cutters may pick these up and eject them rapidly towards the operator.

A separation of the wood fibres along the grain forming a fissure that extends through the board from one side to the other.
It is usual in end grain and is remedied by cutting away the defected area. All boards should have an allowance so that some end grain may be cut away because of possible shakes or splits.

Checks and end checking
A separation of the fibres along the grain forming a fissure which shows up on one face or at the end grain but does not continue through to the other side.

Uses of timber
Marine work
Wharves, piers, sheet piling, and cofferdams
Requires high density, close grain, high strength timber
Timber has good resistance to impact
Problems with fungal and marine growth attack
Nowadays concrete or steel more popular for the main structure but timber is still used for denders

Heavy constructional work
Timber piles, bridges, gantries etc.
Once popular, nowadays concrete or steel used instead

Medium/light constructional work
Houses, roof trusses, partitions, floors etc.
Often prefabricated to reduce labor cost
With adequate maintenance, can last>30 yrs

Plywood is an assembled product made up of piles glued together. The plies are “peeled” off a pre-boiled log by rotating it against a knife.
Plywood is manufactured in the form of flat sheets of various sizes. Thickness ranges from 3 to 25mm. Plywood is always built up of an odd number of plies, i.e. 3, 5 or 7 plies. In general, the plywood built up of a large number of plies is stronger and more stable, but it is more expensive to make.
The direction of the grain of each ply runs at right angles to that of the ply on each side so that strength properties are more uniform in the length and width of the boards than in solid timber.
The tendency of any ply to shrink transverse to the grain under change of moisture content is resisted on account of the direction of the grain of the adjacent plies.
Plies are not always of equal thickness and the inner ply or plies can be of lower strength than those on each side, provided the strength and moisture movement properties on each side of the centre are “balanced”

Fire resistance
Timber is easily ignited at 220 to 300℃. It easily becomes charred at the surface, but the unburnt interior of a timber member does not lose strength with a serious rise in temperature and there is no significant increase in length. The god thermal insulation of timber prevents a marked rise in the temperature of members on the side remote from the fire.


Structural steel

Steel-composed of iron, alloyed with various other materials (mainly carbon, sulphur & phosphorus).
The physical properties of steel depend on the nature and relative amounts of these special ingredients.
An increase in carbon content results in an increase in strength, but this is accompanied by a marked decrease in ductility.
Based on the carbon content, steel may be divided into the following categories:
Low carbon steel (<0.15%) ksi =" kilopound" 1000="ksi" 012="Kg/mm2" 895="N/m.m2" 145="ksi" 895="Mpa" 145="ksi" 703="kg/m.m2" 422="ksi" 1="N/m.m2" 807="Mpa" e="210,000N/mm2" m="mass" l=" length">32mmφ 1 from each 55 ton 1 from each 110 ton
Prestressing steels
Prestressing steels have very high yield strength in tension. The steel must be able to withstand the prestresses without suffering significant creep relaxation.
0.6-0.9% carbon, 0.5-0.9% manganese steel is suitable for pre-tensioning. It has better tensile properties than mild steel owing to the high carbon content and can be further enhanced by cold working to give
Characteristic strength = 1470-1720MPa
Min, 0.2% proof stress = 1250-1460MPa

Cold drawn steel wire ropes
0.6-0.9% carbon, 0.5-0.9% manganese
Tensile strength of 1735-1850MPa
0.1% proof stress > 70% of tensile strength
Cold worked steel rods
0.5-0.6% carbon, 0.7-1.0% manganese, 1.5-2.0% silicon
Tensile strength of 1030-1110MPa
0.2% proof stress of 870-95-MPa

mechanical properies and material testing

The mechanical properties of a material are used to determine its suitability for a particular application. A range of material tests is available to measure the properties.

The tensile test
The most comprehensive of all mechanical tests is the “tensile test”, which determines the strength of the material when subjected to a simple stretching operation. Typically, standard dimension test samples are pulled slowly and at uniform rate in a testing machine while the strain (elongation) and the engineering stress (applied force divided by the original cross-sectional area) are measured and plotted. A typical tensile test for a metal first stretches the material and then causes it to neck and fail.

The engineering stress-strain curve of a ductile material usually drops past the tensile strength point. This is because the cross-sectional area of the material decreases (necking). Because of the decreased area, a smaller amount of force is required to continue the material’s deformation. A plot of the true local stress vs. true strain, based on the changing specimen dimensions rather than the original dimensions, would continue to rise when necking occurs. However, these are rarely used because they are difficult to measure.
The stress-strain diagram
Plotting the applied stress versus the strain or elongation of the specimen shows the initial elastic response of the material, followed by yielding, plastic deformation and finally necking and failure. Several measurements are taken from the plot, called the engineering stress-strain diagram. These include:

Modulus of elasticity
The initial slope of the curve up to the elastic limit
A measure of the stiffness of the material
Related directly to the strength of the atomic bonds.

Yield strength, YS
The stress at which the specimen shows a consistent and measurable amount of permanent deformation.
For materials without a clear yield point, the YS is determined by constructing a line parallel to the initial portion of the stress-strain curve but offset by a strain of 0.002 from the origin. The intersection of this line and the measured stress-strain line is used as an approximation of the material’s yield strength. This is called the 0.02% offset yield.
Some materials do show an abrupt yield point. In plain carbon steels, the carbon steels, the carbon atoms may diffuse the dislocations in the material and pin them so that they cannot easily move. When the applied stress causes the dislocations to jump free of these points, they can move more easily. In many polymers a similar effect is produced when bonds between molecules break and they begin to move.

Tensile strength, TS(ultimate tensile strength, UTS)
The maximum stress applied to the specimen
YS or TS is a measure of the loading ability (strength) of the material.

The maximum amount of plastic deformation that the specimen can undergo without breaking. Ductile vs. brittle
Measured by the % elongation at failure (or the % Reduction in area)
%EL=(l_final-l_0)/l_0 ×100

Other properties
Modulus of resilience
The area under the linear part of the curve, measuring the stored elastic energy

Failure stress
The stress applied to the specimen at failure (usually less than the maximum tensile strength because necking reduces the cross-sectional area)

The total area under the curve, which measures the energy absorbed by the specimen in the process of breaking

Mechanical properties of some common meals:
Metal/alloy YS (MPa) TS(MPa) Ductility,(%EL)
Aluminum 35 90 40
Brass(Cu-Zn) 75 300 68
iron 130 262 45
steel 180 380 25
titanium 450 520 25

The hardness test
The hardness test offers the engineer a quick, inexpensive and nondestructive way to estimate the tensile strength of a specimen. Hardness tests all make a small (sometimes microscopic) indentation into the surface of a specimen, and then use the force applied and the size of the indentation to calculate a “hardness number”. The correlation between this value and the tensile strength allows this to be used as a quality control parameter. However, the relationship between hardness and tensile properties is unique for each class of materials. Two of the commonly used hardness tests are:

The Brinell hardness test
The Brinell hardness test utilizes a steel sphere which is usually 10mm in diameter. The sphere is forced into the surface of a material. Then, the diameter of the resulting impression is measured. The corresponding “Brinell Hardness number” is then calculated.

The Rockwell hardness test
The Rockwell hardness test utilizes two kinds of indentors. A small steel ball is used for soft materials and a diamond-shaped cone is used for hard materials. To perform the test, the indentor is pushed into the surface of the material being tested. The test machine measures the depth of penetration and automatically converts this data into a “Rockwell Hardness number”.
Empirical relationship between hardness and Ts for most steels:
TS (MPa)=3.45Xbrinell hardness number

Impact testing
It is important to examine a material’s reaction to short yet intense loads, because under such conditions the material may behave in a more brittle manner than is indicated from a simple tensile test. The charpy impact test is commonly used for this purpose. A notched bar is placed in the test machine, and then the hammer is allowed to fall and break it. The energy absorbed in fracturing the specimen is measured by the height to which the hammer rises.
This is a direct relationship between the energy absorbed in impact and the toughness measured as the area under the stress stain curve. Materials with a high toughness value absorb more energy in fracture, which may provide a margin of safety in real structures in the event of failure. Some newer cars use plastic body parts and bumpers which have a large range of elastic deformation so that fracture does not occur at all.

The mechanical properties of a material are used to determine its suitability for a particular application. It is convenient to break the properties, and the tests that measure them, into several types

Slow application of stress, as in the tensile test, allows dislocations time to move.
Rapid stress application, as in an impact test, measures the ability of the material to absorb energy as it fails
The materials response to the presence of cracks and flaws that act as stress concentrators is measured by fracture toughness
Repeated application of stresses below the failure stress determined in a tensile test can cause fatigue failure.
At high temperatures, materials deform continuously under an applied stress, as measured by the creep test.


Stone masonry

Igneous rock, sedimentary rock, metamorphic rock

Cement mortar or cement-lime mortar
Except for extremely weak stones, the compressive strength of masonry structures is not dependent on the stones but the mortar bonding the stones.

Solid masonry walls
Load bearing walls consisting of stone blocks bonded to a rubble pattern
Composite masonry walls
Load bearing walls consisting of stone blocks bonded to a brick or concrete backing
Stone facing
Non-load bearing facing wall consisting of thin pieces of stone secured by metal cramps to the load bearing wall

Rubble patterns
Random rubble
This consists of stones of all sizes roughly rectangular but not dressed. The stones are not laid to horizontal courses. The small stones are used to fill up the voids between the larger stones.

This pattern is made from stones of different shapes and sizes without much order. The pieces fit together fairly well.

Coursed random rubble
The stone work is laid roughly in courses. It is leveled at certain heights to an approximately horizontal surface.

This pattern is made from uncut stones placed in rows called courses.

This consists of stones accurately worked to plane surface with sharp right angles. The stones are laid truly horizontal. Ashler is used mainly for facing. Very expensive.

This pattern is made of cut stones that fit together, but are not in rows.

This is a pattern of cut stones that fit together and are placed in rows.

Like the mosaics of ancient Rome, this is a pattern of different stones that fit together very well.

Brickwork & blockwork
A walling unit not exceeding 337.5mm in length, 225mm in width, or 112.5mm in height
A walling unit exceeding in length, width or height the dimensions specified for bricks.

Standard format of bricks
Designation Length Width Height
225X112.5X75 215mm 102.5mm 65mm

The nominal dimensions include the thickness of a mortar joint equal to 10mm.
4 courses to 300mm (12’’).

Manufacture of bricks
Clay (silica and alumina) mixed with water, moulded and pressed to shape, dried and then fired to form brick units.

Varieties of bricks
Common bricks
For non-structural uses, such as partitions
Facing bricks
Architectural finish
Engineering bricks
Dense bricks conforming to defined limits of strength, for structural uses, such as load bearing walls

Compressive strength of bricks
Engineering bricks
Class A 69.0N/mm2
Class B 48.5N/mm2
≅5N/mm^2, strength unimportant

Mortar for bonding brickwork or masonry is composed of cement and sand, with or without lime. The addition of lime improves the workability and bonding properties. It also reduces the tendency to crack.

Cement mortars
Note: 1:3 means 1 part of cement to 3 parts of sand by volume. A small quantity of lime may be added.

Cement-lime mortars
1:3:12 used for internal non-load bearing walls
Note: the proportion are cement: lime: sand by volume

Cement mortar with plasticizer
(Plasticizer for improving workability)
1:6 used for non-load bearing external walls
1:8 used for non-load bearing internal walls

Bonding of brickwork:
Header - A brick which is laid in a way that only the short end is visible in the wall

Stretcher - A brick which is laid in a way that allows only the longer side of the brick to be exposed.

Stretcher bond-stretchers in every course
Stretcher bond is the most widespread pattern found in modern buildings. It has become the popular choice for cavity walls with half-brick thick outer leaves because it makes the maximum use of whole bricks, minimising the requirement for snapheaders or for cutting bricks on site. Stretcher bond is thus quick to lay and, overall, is the most economic bond pattern.
Aesthetically, however, stretcher bond has come to be regarded as somewhat bland. To relieve this, there has been a resurgence of interest both in using the more traditional types of bond patterns and in enlivening the appearance of stretcher bond by introducing designs in bricks of contrasting colors. A typical example of the latter is the highlighting of a diamond pattern in stretcher bond. This reflects an effect achieved historically by incorporating flared headers - bricks which had been accidentally overfired, producing a different colour - and is particularly successful in the modern context when blue bricks feature as the contrast. The opportunities available by using polychromatic brickwork to enhance the appearance of a building are the subject of ARCHIFACTS Decorative Brickwork. It is also possible to achieve some degree of variation in stretcher bond by using quarter- or third-lap instead of the more common half-lap.

Each joint is above the middle of the lower brick. It has a low strength in the transverse direction and is used for thin walls only.

Header joint-headers in every course
It provides excellent strength in the transverse direction but longitudinally it is not so good. It is primarily used for walls curved in plan.

English bond
It consists of alternate courses of stretchers and headers.

English Bonding - A brickwork pattern in which the headers and stretchers form alternate rows.

Flemish bond
It consists of alternate headers and stretchers in every course, each header being centrally placed between the stretchers above and below.

Flemish Bonding - A brickwork pattern in which the headers and stretchers alternate in every row.

Fire resistance
Clay bricks withstand temperatures in the region of 1000℃ or more without damage, but under very severe and prolonged heating, the surface of a brick may fuse. Spalling can occur with some types of brick particularly of the perforated type.
A loadbearing wall exposed to fire will suffer a progressive reduction in strength due to deterioration of the mortar in the same manner as concrete. Severe damage is more likely to be caused by the expansion or collapse of other members.

material for construction

Materials for construction

Required properties of construction materials:
1. strength
The strength of a material refers to its load-carrying capacity in tension, compression, shear and torsion.

The strength may vary appreciably with
-the rate and frequency of loading
-In non-homogenous materials, with the direction of loading.
-Moisture content (timber)
-temperature (rubber)

the own weight of the material usually ‘use up’ a considerable proportion of its strength.

Permissible stress= characteristic strength/factor of safety

2. ductility
ductility generally decreases with strength in tension. Materials that are linearly elastic up to failure are highly unsuitable for structural purposes. They give no warning of approaching failure: moreover, they often shatter under impact.

3. durability
All materials deteriorate with time, but at different rates. Direct or indirect causes of deterioration include the following:
(a) Corrosion of materials- corrosion usually occurs in moist conditions and in the presence of atmospheric pollution or flue gases.
(b) Biological agencies- insects attack organic materials, mainly timber.
(c) Water- some materials, such as limestone, slowly dissolve in water; timber becomes weaker when it is wet; water also promote fungal growth; water tapped in pores or interstices causes cracking and bursting when it expands during freezing.
(d) Sulphate attack- ground waters and industrial wastes often contain soluble sulphates which attack cement products(such as concrete) and metals.

4. fire resistance
In fires, materials may
-melt, loose strength
-expand, and crack

-timber, plastics
-bricks, stones, concrete and metal

Steel may expand and cause collapse.
Asbestos cement sheets may spall, shrink and allow the passage of flame through cracks.

Fire resistance is a property of a structural element, e.g. walls, columns, floors, beams, and not of individual materials. It is expressed as the period of time (in hours) during which a structural element survives the standard fire test while continuing to perform its normal structural function.
Vary according to the use of the structure, its height, floor area and cubic capacity.

Selection of structural materials:

Factors to be considered:
1. structural form
2. foundation
3. site condition
4. construction method
5. availability of the materials and the relative cost
6. time available for the construction

failure fatigue creep

A material under loading will deform and if the load increases, the material will eventually fail by fracturing. Fracture may be either brittle or ductile. In a ductile fracture, failure occurs only after a considerable amount of plastic deformation “necking” of the material will occur. The stress level in the necked portion builds up and cracks begin to form. Finally, fracture occurs by shear failure at an angle of about 45° to the loading direction. This is because shear stress is at a maximum at 45° to the line of direct stress. In a brittle failure, there is little or no plastic deformation prior to fracture.
The type of failure depends largely on the mechanical properties (ductile or brittle) of the material but is also affected by other factors such as the rate of application of load (slow or impact), the type of load (tensile or lateral), the temperature and the environmental conditions.
The failure behavior of a material is affected by the geometry and surface conditions of the component. A sudden change of section or a surface notch will act as a point of stress concentration where cracks form mare easily.

Fatigue failure
Fatigue refers to the failure of a material after a large number of repeated or periodic loading cycles. The material fails even though the maximum stress in any one cycle is considerably less than the failure stress of the material as determined by “static” loading test (e.g. tensile test). In practice, many engineering components are subjected to periodic or fluctuating loading cycles (e.g. shafts, structures under wind loads). In the loading cycles, the stress may be alternating in sign(tensile and compression). In a fatigue test of a material to determine its fatigue strength, periodic alternating stress cycles with different maximum stress levels but with a mean stress value of zero is usually applied to a specimen. The number of cycles, N, before the specimen fails is noted and plotted against the maximum stress level, S, in the cycle.
The S-N curves of most steels show a clear fatigue limit and the strength at the fatigue limit is about one-half of the tensile strength. Non-ferrous metals usually show no definite fatigue limit. It is then only possible to design for a limited life of the material and a life of 106 or 107 cycles is usually used.
Fatigue is due to crack initiation at a low stress level and the propagation of cracks. The fatigue strength of a material is affected by many factors. These include surface condition, component design and the environmental conditions. Specimens for fatigue testing are usually prepared with a polished surface and this condition will give the best fatigue performance. If there is a scratch or notch on the surface or the surface is roughly grounded, the fatigue strength will be reduced. A sharp change in section provides points for stress concentration and crack initiation and therefore leads to a poor fatigue performance.

Under a constant continuous load, a material will show additional deflection slowly with time. This is called “creep”. For most metals at normal temperatures, creep is negligible. At raised temperatures, creep becomes more significant. A plot of strain against time shows three phases of creep: primary, secondary and tertiary creep. In secondary creep, strain increases at a steady rate with time. Tertiary creep will eventually lead to failure.

defects in crystalline and pastic deformation

Imperfections in crystal
Metals are crystalline in solid state. In a piece of real metal, the vast majority of atoms line up evenly and follow the perfect arrangement in the ideal crystalline lattice. However, a very tiny fraction of the atom sites, often less than one in a million, are not perfect. Defects in crystals may be points (a vacant site), lines (a dislocation) or surfaces (a grain boundary).
Even though crystal imperfections account for few of the atom sites, they usually dominate the properties of the microscopic specimen. Vacancies enable diffusion in solid metals, and dislocations produce plastic deformation. A materials engineer must know how to introduce defects, control their number and arrangement so as to produce a metal with desirable combinations of properties. This is usually achieved through composition, fabrication processing and heat treatment.

Block slip
An early theory to explain plastic deformation was the “block slip” theory. Large blocks of atoms were thought to slide over one another across the slip planes under the action of a shear force. However, it was found that the yield stress calculated from the block slip was far much higher than the actually observed yield stress. It means some other mechanisms involving much lower energy occurs first. There is still something correct about block slip. Plastic deformation occurs through slip across slip planes and along slip directions. It is because this requires the least energy for sliding of a layer of atoms over another.

Dislocations are line imperfections where the three-dimensional lattice is offset. One common type is edge dislocation. An edge dislocation can be most easily visualized as the insertion of an extra plane of atoms into the lattice, so that where it ends, the orderly rows of atoms are offset. The edge of the half-plane of atoms is called the dislocation line. Atoms near the dislocation are not in their regular alignment and can move relatively easily from one side of the dislocation to the other.

Slip by dislocation movement
The presence of dislocations enables slip to occur in a progressive manner instead of the block slip manner. Not all the metallic bonds involved need to be broken at the same time (which requires a very high level of stress). Now bonds are broken and formed again between atoms along the dislocation line only. This can occur at a much lower stress level.
Effectively, a dislocation allows atom movement and thus deformation. Atoms near the dislocation are not in their regular alignment and can move relatively easily from one side of the dislocation to the other. This represents the displacement of the entire plane of atoms, and effectively shifts the dislocation. Repeating the process until the dislocation reaches the surface produces a deformation step of one atom spacing.
Slip still occurs under a shear force. The direction of dislocation movement is still preferably the slip direction. Slip occurs most easily on slip planes. In most real metals, there are many grains with different orientations, so there are many slip planes. Some of them will be inclined to the direction of the applied force and slip can always occur when the load is high enough to star dislocation movement. Slip is responsible for the necking of specimens pulled in tension.

Point defects
A point defect is an imperfection at an atom site. Examples are vacancies, interstitial atoms, or substitutional atoms (large or smaller than the parent atoms). When an atom is missing from the lattice, there is a vacancy. If another atom manages to squeeze in between the normal sites, it is called an interstitial atom. Other atoms may also take the place of the regular atom.
The properties of a metal may be controlled by intentionally creating and controlling point defects through heat treatment, strain hardening, solid solution strengthening, and other related processes. Point defects hamper the movement of dislocations by disrupting the atom positions and making the slip planes bent or bumpy, and therefore strengthen the material. Either larger or smaller substitution atoms produce strengthening, which depend only on the amount of the size difference. Steel is a good example of how interstitial atoms strengthen a metal. Carbon atoms are quite small (a radius of 0.071nm) and can occupy interstitial positions in the lattice of iron.
The presence of vacancies enables diffusion in solid metals. Diffusion means the migration of atoms from one atomic site to another in the crystal lattice. Effectively, the diffusing atom breaks the bonds with the originally neighboring atoms and causes some lattice distortion on its migration to the next site when new bonds are formed with the new neighbors. High energy is required and this is usually vibrational in nature (at elevated temperature). The presence of a vacancy lowers this energy and makes diffusion easy to occur.

Surface defects and grain boundaries
Most metals consist of many small crystalline regions, called grains, that form independently with different orientations. The surfaces of these grains are the boundaries that form when the growing crystals run into each other. The atomic arrangement is imperfect along these internal surfaces. A large angle boundary may have local stresses because of the atom mismatch. In any case, it might seem that such boundaries would weaken material, but in fact the opposite is true. In a material containing dislocations, the grain boundaries represent places that the dislocations can begin or end, and can be consumed or created. A small grain size produces many boundaries. Reducing the grain size normally produces an increased yield strength.

Strengthening mechanisms in mechanisms in metals
If dislocation movements are made more difficult, a high stress is required to move the dislocations and produce plastic deformation. The yield stress is thus increased and the metal becomes “stronger” or harder. Strengthening a material can be accomplished by placing obstacles in the path of the moving dislocation. Three commonest mechanisms are:
Grain size strengthening
Solid solution strengthening
Work hardening, also known as strain hardening, cold work strengthening

Refining the grain sizes is a mechanism of strengthening because grain boundaries are barriers to dislocation movements. Interstitial or substitutional atoms hinder the movement of dislocations by disrupting the atom positions and making the slip planes bent or bumpy, and therefore strengthen the material. Other dislocations present in the material can also act to hinder dislocation movement due to the interactions among dislocations. This is the basis of “work hardening” in cold work.

Iron is an allotropic metal, meaning that its crystal structure changes with temperature. Above910℃, the structure is FCC. The largest site between the iron atoms has a radius of 0.052nm, somewhat smaller than the carbon atom. A maximum of about 2.1% carbon by weight can dissolve in FCC iron, limited by distortion of the lattice due to the presence of the interstitial atoms. Below 910℃ the iron structure changes to BCC. Even though this is a less close-packed structure, the largest available interstitial site is smaller than in FCC, only about 0.026nm. Because of the greater lattice distortion the atoms cause, less carbon can be dissolved in the BCC iron (a maximum of about 0.02%). Heat treating steels often involves heating the alloy so that it transforms to FCC and dissolves all the carbon, then cooling it so that the carbon atoms are either trapped in the BCC structure (causing distortion) or forcing the atoms out of solution to form iron carbides.

Plastic deformation
Plastic deformation of metals takes place most commonly by the slip process. Slip usually takes place on the closest-packed planes and in the closest-packed directions. A slip plane and a slip direction make a slip system. In general, metals with a high number of slip systems are more ductile.
The “block slip” theory assumes sliding of one plane of atoms over an adjacent plane at the same time. The stress required is over one order of magnitude larger than the observed stress.
Plastic deformation actually occurs through the slip process which takes place progressively with the movement of a dislocation.
The easier dislocations can move, the easier the metal can deform plastically. On the other hand, the metal can become stronger and harder if dislocation movement is hindered.
Grain boundaries (at lower temperatures) usually strengthen metals by providing barriers to dislocation movement.
Some strengthening (hardening) processes are work hardening, solution hardening and precipitation hardening.

Solute atoms in a solid solution can serve as obstacles to dislocation movement. The alloy becomes stronger and harder by this strengthening process known as “solution hardening”
“Cold working”, “work hardening” or “strain hardening” is a common strengthening process for metals ad alloys. Plastic deformation is imposed on the metal by cold work. During cold working, the number of dislocations increases. As the dislocation density increases with deformation, it becomes more and more difficult for the dislocations to move through the existing “forest of dislocations”. (A dislocation hinders the movement of another dislocation.) Thus, the metal strain hardens with increased cold work.
Cold worked metals can be made soft again by a process known as “annealing”. Annealing is a reheating treatment and the temperature at which atomic mobility is sufficient to affect mechanical properties is about 1/3 to 1/2 the absolute melting point.
During annealing, the metal will go through a series of changes called
Grain growth

Alloys are slid solutions of pure metals (and some other elements). Solid solutions can be substitutional or interstitial.
In a substitutional solid solution, solute atoms substitute for some parent solvent atoms in the crystal lattice.
In an interstitial solution, solute atoms fit into the spaces or holes in the solvent-atom lattice.
Solid solutions can also be formed by the diffusion process.

crystalline nature of metals

Crystalline nature of metals
In the solid state, atoms in a metal are bonded together by metallic bond. Outer (valence) electrons are given up and form an “electron sea” that glues the positive ion cores (nclei and inner electrons) together. The metallic bond is non-directional, so atoms pack around each other densely. The metallic bond is a strong bond and the bond strength accounts for the melting point and the Young’s modulus relating to elastic deformation.

Under the metal bonding, atoms in a solid metal are arranged in an orderly pattern with a constant equilibrium distance between adjacent atoms. The so-called “crystalline lattice” is three dimensional inspace and can have different arrangements.

Three commonest “lattices” found in most metals are:
1. BCC- body centre cubic

2. HCP-hexagonal close-packed

3. FCC-face centred cubic

They give the shape and arrangement of a “unit cell” which is the repeating basic element in the lattice.

Those 3 lattices can be built by stacking two-dimensional layers of close-packed atoms one over the other
1. Stacking square packing over one another in ABAB… manner –BCC
2. Stacking diamond packing over one another in ABAB… manner-HCP
3. Stacking diamond packing over one another in ABCABC… manner-FCC

Slip in a single crystal
Solid metals have always been formed from solidification of molten metals. As such, there are a large number of small crystals in a piece of metals. Metals are poly-crystalline in nature. The crystals are called grains and have different orientations.
Consider a piece of single metal crystal (which can only be formed under highly controllable condition) under loading. When the load is low, it just stretches the distances between adjacent atoms to produce some elastic deformation. Upon release of the load, the original metallic bond will pull the atoms back to the same equilibrium distance as before.
When the load is high enough, it will stretch the distances between the atoms to the extent that the metal bond is broken. The atoms then separate and the crystal break. Another possibility arises if the “slip” planes of the crystal lie at an oblique direction to the applied load. In that case, slip occurs leading to plastic deformation.
In a crystal, there exist planes on which the atoms are closely packed (the original stacking planes in BCC, FCC and HCP). They are called slip planes. On a slip plane, the directions along which the atoms are arranged in the highest density is called slip direction.
When the slip planes are inclined to the applied load the component of the load along the slip planes and along the slip direction will have a shear effect on the layer of close-packed atoms. When the load component is high enough, it will drag one layer of atoms (and the part of crystal above it) ober the lower layer. The metal bonds among the original neighboring atoms are broken and then now bonds are formed among the new neighbors. This mechanism is called “slip” which leads to plastic defomation.
In a large real metal crystal, large groups of atoms do not slide over other in a simultaneous manner (“block slip”). This is because the process requires too high a load to break many bonds simultaneously. Another mechanism requiring less energy is possible and this is due to the presence of imperfections in metal crystals.

In the Simple Cubic (SC) unit cell there is one lattice point at each of the eight corners of a cube. Unit cells in which there are lattice points only at the eight corners are called primitive. In general, the number of lattice points is denoted by the letter "Z"; thus, for SC, Z = 1.

Let a host atom of radius r occupy each lattice point, and assume that each atom touches as many adjacent atoms as possible (in this case, there are six such contacts). Then each of the three unit cell edges is equal to the sum of two atomic radii: a = b = c = 2r. The volume of the cell is thus
Vc = 8r3
In a simple cubic cell, there is one host atom wholly inside the cube, because each of the eight corner atoms contributes one eighth of an atom to the cell interior. In general, the total volume of the cell which is occupied by the host atoms is
Vs = 4/3r3.Z.
The packing efficiency of a lattice is defined as the ratio Vs:Vc. Thus, for SC, the packing efficiency is about 52%.

In the Body Centered Cubic (BCC) unit cell there is one host atom (lattice point) at each corner of the cube and one host atom in the center of the cube: Z = 2. Each corner atom touches the central atom along the body diagonal of the cube, and it is easy to show by that the unit cell edge, an irrational number, is about 2.3r. Thus, the corner atoms do not touch one another.

The packing efficiency in this lattice can be shown to be about 68%, much higher than the packing efficiency in a simple cubic lattice.

In the Face Centered Cubic (FCC) unit cell there is one host atom at each corner and one host atom in each face. Since each corner atom contributes one eighth of its volume to the cell interior, and each face atom contributes one half of its volume to the cell interior (and there are six faces), then Z = 1/8.8 + 1/2.6 = 4.

The corner and face atoms touch along the face diagonal, and it is easy to show that the cube edge (a) is about 2.8r. Thus, the corner atoms do not touch one another.
The packing efficiency is about 74%. This is the maximum packing efficiency for spheres of equal radius and is call closest packing. Thus a face centered lattice of atoms is also called Cubic Closest Packing (CCP).

In any lattice it is always possible to choose a primitive (Z = 1) unit cell. In fact, there is an infinite number of such choices, and it can be shown that all of the primitive unit cells on a lattice have the same volume. However, only one of these primitive unit cells has the three shortest cell edges (a,b,c), and this unit cell is called the standard reduced cell.
In a FCC lattice, the standard reduced cell is a rhombohedron, with a = b = c = 2r. The three interior angles formed between unit cell edges are called:
(alpha, between edges b & c)
(beta, between edges a & c)
(gamma, between edges a & b)
In the FCC rhombohedral standard reduced cell, it can be shown that  =  =  = 60o. Note that a cube is a just a special rhombohedron, with  =  =  = 90o.

In a plane, spheres of equal size are most densely packed (with the least amount of empty space) when each sphere touches six other spheres arranged in the form of a regular hexagon.

When two such hexagonally closest packed planes are stacked directly on top of one another, a primitive hexagonal array results.

The primitive unit cell, outlined in black in the crossed stereo pair above, has cell edges
a = b = 2r and c = 2r. Thus, the ratio c:a = 1, and it can be shown that the packing efficiency of this three dimensional lattice is only about 60% (compared to 74% for closest packing), even though the atoms are closest packed in two dimensions.

Instead of stacking hexagonal closest packed planes directly above one another, they can be stacked such that atoms in successive planes nestle in the triangular "grooves" of the adjacent plane. (note that there are six of these "grooves" surrounding each atom in the hexagonal plane, but only three of them can be covered by atoms in the adjacent plane).

Let the first plane (at the bottom) be labeled "A" and the next plane above it be labeled "B". If a third hexagonal closest packed plane is stacked above B but in the "A" orientation, and succeeding planes are stacked in the repeating pattern ABABA... = (AB), a hexagonal unit cell can be chosen (using the nine atoms labeled "h"), with Z = 2.

For the stack of hexagonally closest packed spheres of equal radius (r) described above, the interplanar spacing between adjacent planes is proportional to r. The proportionality constant is an irrational number called the Closest Packed Interlayer Spacing, CPIS, and its value is about 1.6. Thus, the interlayer spacing is about 1.6.r (compared to 2.r for simple hexagonal stacking).
Thus, the "c" unit cell edge (in the stacking direction) has a length c = 2.CPIS.r, the ratio
c:a = CPIS, and it can be shown that this lattice has a packing efficiency which is identical to the FCC lattice. Thus, the name Hexagonal Closest Packing (HCP) for this array is justified.
Each host atom in an HCP lattice is surrounded by and touches 12 nearest neighbors, each at a distance of 2r:
• There are six atoms in the planar hexagonal array (the central A layer);
• There are three atoms in the B layer above the A layer;
• There are three atoms in the B layer below the A layer.
The six atoms in the two B layers form a trigonal prism around the central atom in the A layer; the length of this prism is 2.CPIS.r.

Suppose the first two layers of hexagonal closest packed planes are stacked in "AB" fashion but the third layer is positioned so that its atoms lie over the three grooves in the A layer which were not covered by the atoms in the B layer. Then the third layer is in a different orientation from either A or B and is labeled "C". If a fourth layer then repeats the A layer orientation, and succeeding layers repeat the pattern ABCABCA... = (ABC), the resulting unit cell is hexagonal with three host atoms (Z = 3), unit cell edge c = 3.CPIS.r and c:a = 1.5.CPIS. Note that for identical atoms in all layers, (ACB) is identical to (ABC).

It can be shown that this is a closest packed structure because the three host atoms occupy 74% of the total hexagonal unit cell volume. Furthermore, the standard reduced cell in this array is as follows: choose the two central atoms in the top and bottom "A" layers, and connect them to the six atoms shown in the "B" and "C" layers . This unit cell is identical to the standard reduced cell chosen for the face centered cubic lattice. Thus, the (ABC) repeat structure is identical to the face centered cubic lattice (CCP = FCC), with the stacking direction along the body diagonal of the cubic unit cell.

In order for the (ABC) layered lattice to be closest packed, the interlayer spacing must be exactly equal to CPIS.r with c:a = 1.5.CPIS. Thus, the standard reduced cell is then the special rhombohedron found in the face centered cubic lattice.
If the c:a ratio differs from the closest packed value, then the standard reduced unit cell is still a rhombohedron (a = b = c and  =  = , but the cell edges need not be of length 2r and/or the inter edge angles need not be 60o. In these quasi-closest packed structures, either the hexagonal
(Z = 3) cell or the primitive (Z = 1) unit cell may be used to describe this lattice, which is called, by convention, rhombohedral.

Note that quasi-closest packed (AB) lattices are also possible if c:a differs from CPIS.

In closest and quasi-closest packings, the only stipulations are
• two adjacent layers of hexagonal closest packed planes must be different (A, B or C);

• the first and last named layers in the repeat unit must be different (because they are adjacent in the whole pattern);

• different letters arranged in the same pattern represent the same lattice.
Thus, while,there are eighteen possible permutations of three letters in four-layers, as illustrated in the cascade diagrams shown here, not all of these patterns are unique. For example, in each cascade one of the three permutations is actually a two-layer (HCP) pattern. Furthermore, many of the remaining twelve patterns are equivalent. For example, pattern (ABAC) is equivalent to pattern (ACAB):
= AC)(ABAC)(AB =
After all coincident patterns are eliminated (using, for example, a spreadsheet string-search function), there are aparently three unique four-layer closest packing patterns: (ABAC), (ABCB), and (ACBC). However, (ABCB) and (ACBC) represent the same lattice with different choices of the unit cell. Thus, there are only two unique four-layer packings shown here.

In the same way, it can be shown that of the 30 possible five-letter permutations, four are apparently unique: (ABABC), (ABACB), (ABCAC) and (ABCBC). However, by rechoosing axes and turning the stack end-for-end (reading the stack backwards), it is seen that (ABABC) and (ABCAC) are equivalent. Thus, only three five-layer closest packed patterns are unique: (ABABC), (ABACB), and (ABCBC).


Electrode potential
A metal in contact with an electrolyte will ionize slightly until reached the equilibrium, e.g.
The metal will then possess a certain electrical charge. Different metals ionize to different extents and will possess different values of electrical charge at equilibrium. The electrical charge or potential difference between a metal electrode and a standard hydrogen electrode is called the “standard electrode potential” for the metal. In general, metals with negative values of electrode potential are more active than metals with positive electrode potentials.

Galvanic cell: anode and cathode
Corrosion of most metals is of the galvanic type. A galvanic cell is set up when two metals, of different electrode potentials, are in contact with a same electrolyte and are also electrically connected external to the electrolyte. An example is copper and zinc. Copper with the higher electrode potential series will become the “cathode” and zinc having the lower electrode potential will become the “anode”. In the circuit external to the electrolyte, electrons will flow from the anode to the cathode. The anode will tend to ionize continuously with this removal of electrons. The galvanic cell becomes a “corrosion cell” where the anode is being corroded away in the electrolyte.
In a carrions osion cell, it is always the anode that is corroded away. That is the metal will go into the solution or electrolyte:
In some cases, (OH)- ions will be attracted to the anode and the metal ions react with (OH)- ions to form oxide, hydroxide or salt of the metal. A common example is the rusting of iron in water, which contains dissolved oxygen. The compound, ferric hydroxide, Fe(OH)3, is “rust”:
Fe^(2+)+1/2 O_2+H_2 O→Fe^(2+)+2OH^-→Fe〖(OH)〗_2
2Fe〖(OH)〗_2+1/2 O_2+H_2 O→2Fe〖(OH)〗_3
At the cathode, a number of possibilities will occur. Metal may be deposited (which is seldom in corrosion):
Hydrogen may be evolved:
Or in the presence of dissolved oxygen, hydroxyl ions may be formed:
O_2+2H_2 O+4e^-→4OH^-

Galvanic corrosion cells: common types
Corrosion cells may be set up in a number of ways. The main types are:
Composition cells
May be set up between two dissimilar metals
E.g. a steel pipe connected to copper plumbing, and a steel properller shaft in a brass bearing. In both cases, steel becomes the anode and corrodes away. Microscopic composition cells can be set up in a two-phase alloy when exposed to an electrolyte. In pearlite (steel), for instance, the ferrite is anodic with respect to the cementite. A pure metal or a single-phase alloy thus possesses better resistance to corrosion than an impure metal or a multi-phase alloy.

Stress cell
Metal atoms within a highly stressed region will have a higher tendency to ionize than their unstressed counterparts.
A stress cell may be established in a component where the stress distribution is uneven. In cold-worked component, residual stresses exist in the cold-worked portion and a stress cell may be set up between the cold-worked portion and the non-worked portion of the metal. The cold-worked portion is the anode subjected to corrosion. The grain boundary zone of a metal is under stresses and tends to be anodic with respect to the crystal grain. A coarse-grained metal is thus more corrosion resistance than a fine-grained sample of the same metal.

Concentration cell
A metal in contact with a concentrated electrolyte solution will ionize to a smaller extent than when it is in contact with a dilute electrolyte. In situations like electrolyte flowing in metal pipes or ducts, the portion in contact with the dilute electrolyte will be anodic with respect to the portion in contact with the more concentrated solution.
Of widespread importance are oxidation-type concentration cells which can occur of there are variations in dissolved oxygen content throughout an electrolyte. For example, when a moist metal surface is exposed to air, corrosion has a tendency to occur in the region where there is a deficiency in dissolved oxygen content. The oxygen rich region becomes the cathode because oxygen is being reduced there:
O_2+2H_2 O+4e^-→4OH^-
The electrons are being removed from the oxygen-deficient region where the metal there becomes the anode and ionizes. Examples of oxygen-deficient areas are cracks or devices, or metal surfaces covered by dirt or other contaminations.

Corrosion control
It is almost impossible to prevent corrosion completely but it is possible to delay or minimize corrosion. The commonest technique is the application of a protective coating to the metal surface so as to isolate the metal from contact with any electrolyte. Surface coatings can be paints, plastics or a layer of another metal or oxide coatings. Composition cells can be avoided by preventing two dissimilar metals from contact. Washers are used when copper plumbing is joined to steel pipes.
Aluminium, chrominum, nickel, tin and zinc are used for coating steels. The coating metal may either be anodic or cathodic with respect to the underlying steel. If the coating is cathodic such as tin on steel, any break on the coating will establish a galvanic cell with the anodic steel being corroded away. On the other hand, if zinc is used as the coating, the galvanic cell formed with a break on the coating will have the zinc as anode. While the zinc coating is corroded, the steel continues to be protected. This is the benefit of galvanizing of steel.
An oxide film can sometimes offer corrosion resistance to the underlying metal. Anodizing of aluminium is a common process that uses an electrical current to force the formation of a thick Al2O3 film on the aluminium. The oxide film has a better corrosion resistance and is itself tightly bounded and also bonds well to the underlying aluminium, stainless steels, formed by adding chromium to steel, is highly resistant to corrosion because the chromium readily oxidizes to form a protective layer of passive chrome-oxide film on the steel surface. However, under conditions where no oxygen is present, the oxide layer may break down and the steel is no longer protected.
Another method of corrosion control is cathodic protection. Galvanic cells are deliberately created where the metal to be protected is made the cathode and the anode is to be sacrificed. In galvanized steels, the zinc coating is the sacrificial anode. Zinc and magnesium are common sacrificial anodes used to protect ships’ hulls and buried steel pipes. An impressed current can also make the

concrete test


Originally published May 1977.

R.F. Feldman

The standard method of evaluating the quality of concrete in buildings or structures is to test specimens cast simultaneously for compressive, flexural and tensile strengths. The main disadvantages are that results are not obtained immediately; that concrete in specimens may differ from that in the actual structure as a result of different curing and compaction conditions; and that strength properties of a concrete specimen depend on its size and shape.

Although there can be no direct measurement of the strength properties of structural concrete for the simple reason that strength determination involves destructive stresses, several non- destructive methods of assessment have been developed1. These depend on the fact that certain physical properties of concrete can be related to strength and can be measured by non-destructive methods. Such properties include hardness, resistance to penetration by projectiles, rebound capacity and ability to transmit ultrasonic pulses and X- and Y-rays. These non-destructive methods may be categorized as penetration tests, rebound tests, pull-out techniques, dynamic tests, radioactive tests, maturity concept. It is the purpose of this Digest to describe these methods briefly, outlining their advantages and disadvantages.

Penetration Tests

The Windsor probe is generally considered to be the best means of testing penetration. Equipment consists of a powder-actuated gun or driver, hardened alloy probes, loaded cartridges, a depth gauge for measuring penetration of probes and other related equipment. A probe, diameter 0.25 in. (6.5 mm) and length 3.125 in. (8.0 cm), is driven into the concrete by means of a precision powder charge. Depth of penetration provides an indication of the compressive strength of the concrete. Although calibration charts are provided by the manufacturer, the instrument should be calibrated for type of concrete and type and size of aggregate used.

Limitations and Advantages. The probe test produces quite variable results and should not be expected to give accurate values of concrete strength. It has, however, the potential for providing a quick means of checking quality and maturity of in situ concrete. It also provides a means of assessing strength development with curing. The test is essentially non-destructive, since concrete and structural members can be tested in situ, with only minor patching of holes on exposed faces.

Rebound Tests

The rebound hammer is a surface hardness tester for which an empirical correlation has been established between strength and rebound number. The only known instrument to make use of the rebound principle for concrete testing is the Schmidt hammer, which weighs about 4 lb (1.8 kg) and is suitable for both laboratory and field work. It consists of a spring-controlled hammer mass that slides on a plunger within a tubular housing. The hammer is forced against the surface of the concrete by the spring and the distance of rebound is measured on a scale. The test surface can be horizontal, vertical or at any angle but the instrument must be calibrated in this position.

Calibration can be done with cylinders (6 by 12 in., 15 by 30 cm) of the same cement and aggregate as will be used on the job. The cylinders are capped and firmly held in a compression machine. Several readings are taken, well distributed and reproducible, the average representing the rebound number for the cylinder. This procedure is repeated with several cylinders, after which compressive strengths are obtained.

Limitations and Advantages. The Schmidt hammer provides an inexpensive, simple and quick method of obtaining an indication of concrete strength, but accuracy of ±15 to ±20 per cent is possible only for specimens cast cured and tested under conditions for which calibration curves have been established. The results are affected by factors such as smoothness of surface, size and shape of specimen, moisture condition of the concrete, type of cement and coarse aggregate, and extent of carbonation of surface.

Pull-Out Tests

A pull-out test measures, with a special ram, the force required to pull from the concrete a specially shaped steel rod whose enlarged end has been cast into the concrete to a depth of 3 in. (7.6 cm). The concrete is simultaneously in tension and in shear, but the force required to pull the concrete out can be related to its compressive strength. The pull-out technique can thus measure quantitatively the in-situ strength of concrete when proper correlations have been made. It has been found, over a wide range of strengths, that pull-out strengths have a coefficient of variation comparable to that of compressive strength2.

Limitations and Advantages. Although pullout tests do not measure the interior strength of mass concrete, they do give information on the maturity and development of strength of a representative part of it. Such tests have the advantage of measuring quantitatively the strength of concrete in place. Their main disadvantage is that they have to be planned in advance and pull-out assemblies set into the formwork before the concrete is placed. The pull-out, of course, creates some minor damage. The test can be non-destructive, however, if a minimum pull-out force is applied that stops short of failure but makes certain that a minimum strength has been reached. This is information of distinct value in determining when forms can be removed safely.

Dynamic Tests

At present the ultrasonic pulse velocity method is the only one of this type that shows potential for testing concrete strength in situ. It measures the time of travel of an ultrasonic pulse passing through the concrete. The fundamental design features of all commercially available units are very similar, consisting of a pulse generator and a pulse receiver. Pulses are generated by shock-exciting piezo-electric crystals, with similar crystals used in the receiver3. The time taken for the pulse to pass through the concrete is measured by electronic measuring circuits.

Pulse velocity tests can be carried out on both laboratory-sized specimens and completed concrete structures, but some factors affect measurement:


  1. There must be smooth contact with the surface under test; a coupling medium such as a thin film of oil is mandatory.
  2. It is desirable for path-lengths to be at least 12 in. (30 cm) in order to avoid any errors introduced by heterogeneity.
  3. It must be recognized that there is an increase in pulse velocity at below-freezing temperature owing to freezing of water; from 5 to 30°C (41 - 86°F) pulse velocities are not temperature dependent.
  4. The presence of reinforcing steel in concrete has an appreciable effect on pulse velocity. It is therefore desirable and often mandatory to choose pulse paths that avoid the influence of reinforcing steel or to make corrections if steel is in the pulse path.

Applications and Limitations. The pulse velocity method is an ideal tool for establishing whether concrete is uniform. It can be used on both existing structures and those under construction. Usually, if large differences in pulse velocity are found within a structure for no apparent reason, there is strong reason to presume that defective or deteriorated concrete is present.

High pulse velocity readings are generally indicative of good quality concrete. A general relation between concrete quality and pulse velocity is given in Table I4.

Table I. Quality of Concrete and Pulse Velocity

General Conditions

Pulse Velocity ft/sec


Above 15,000







Very Poor

below 7,000

Fairly good correlation can be obtained between cube compressive strength and pulse velocity. These relations enable the strength of structural concrete to be predicted within ±20 per cent, provided the types of aggregate and mix proportions are constant.

The pulse velocity method has been used to study the effects on concrete of freeze-thaw action, sulphate attack, and acidic waters. Generally, the degree of damage is related to a reduction in pulse velocity. Cracks can also be detected. Great care should be exercised, however, in using pulse velocity measurements for these purposes since it is often difficult to interpret results. Sometimes the pulse does not travel through the damaged portion of the concrete.

The pulse velocity method can also be used to estimate the rate of hardening and strength development of concrete in the early stages to determine when to remove formwork. Holes have to be cut in the formwork so that transducers can be in direct contact with the concrete surface. As concrete ages, the rate of increase of pulse velocity slows down much more rapidly than the rate of development of strength, so that beyond a strength of 2,000 to 3,000 psi (13.6 to 20.4 MPa) accuracy in determining strength is less than ±20%. Accuracy depends on careful calibration and use of the same concrete mix proportions and aggregate in the test samples used for calibration as in the structure.

In summary, ultrasonic pulse velocity tests have a great potential for concrete control, particularly for establishing uniformity and detecting cracks or defects. Its use for predicting strength is much more limited, owing to the large number of variables affecting the relation between strength and pulse velocity.

Radioactive Methods

Radioactive methods of testing concrete can be used to detect the location of reinforcement, measure density and perhaps establish whether honeycombing has occurred in structural concrete units. Gamma radiography is increasingly accepted in England and Europe. The equipment is quite simple and running costs are small, although the initial price can be high. Concrete up to 18 in. (45 cm) thick can be examined without difficulty.

Maturity Concept

The basic principle of concrete maturity is that increase in strength is a function of time and temperature, and that any standard of maturity in terms of strength must include both temperature and period of curing. The maturity of the concrete at any instant can be calculated by integration of temperature as a function of time if complete records of time-temperature changes are kept. The datum temperature is usually taken as -10°C (14°F). The technique can be of great use in winter concreting operations where monitoring of strength at early ages is very important. It must be emphasized, however, that in measuring maturity no property of the concrete itself is measured. If the concrete design and placing are good, the test will tell when the concrete has been adequately cured; it will not indicate the quality of the concrete.

Concluding Remarks

Although efforts are continuing to improve non-destructive testing methods and the tests themselves are not difficult to perform, test data are not always easy to interpret because concrete is a most complex material. The tests must not, therefore, be regarded as a substitute for standard compression tests. What they do provide are data on actual structures that would not be possible with standard tests; and they should be valuable during winter concreting for ensuring safety and determining time for the possible early removal of forms. They are excellent, also, for determining relative strengths of concrete in different parts of the same structure. If used properly they can provide a very important link in the chain of testing and evaluating concrete and concrete structures.



  1. Malhotra, V.M. Testing hardened concrete: non-destructive methods. Amer. Concrete Inst., Monograph No. 9, 1976.
  2. Malhotra, V.M. and G. Carette. Comparison of pull-out strength of concrete with compressive strength of cylinders and cores, pulse velocity and rebound hammer. Canmet Report 76-8, Nov. 1975.
  3. Jones, R. and I. Facaoaru. Testing of concrete by the ultrasonic pulse method. Materials and Structures, Vol. 2, No. 10, July-August 1969, p. 253-661.
  4. Leslie, J.R. and W.J. Cheeseman. An ultrasonic method for studying deterioration and cracking in concrete structures. Amer. Concrete Inst., Proceedings, Vol. 46, Sept. 1949, p. 17-36.






  1. Distinguish between cement and concrete.
  2. Name at least three items you have encountered today which are concrete.
  3. What are the major ingredients for concrete, and what role do they play?
  4. What is meant by "workable?" Why is it important for concrete to be workable?
  5. Give an example of an aggregate. What is the practical use for this aggregate in making concrete?

NAME _____________________________



  1. What can be used to slow the hardening of concrete (give example?) Why would slowing this process be desirable?
  2. What can be used to speed the hardening of concrete (give example?) Why would speeding up this process be desirable?
  3. Suppose you were in charge of building a skyscraper. What would be your choice for aggregate and why?




  1. What will happen to concrete if it dries out too quickly?
  2. Suppose you were to be the chief designer in charge of building a concrete ship to carry people overseas. What aggregate might you choose to put in your concrete and why?
  3. Explain what the dormancy period of fresh concrete is. How do contractors make use of the dormancy period?
  4. Explain how can you measure the consistency of freshly mixed concrete?




  1. Briefly discuss the importance of a proper water to cement ratio.
  2. Explain the purpose of a superplasticizer in making concrete.
  3. Why should gloves be worn when mixing concrete? Be specific.
  4. Water is important in making concrete, however, it can also be harmful to concrete. Explain this statement.



  1. Distinguish between cement and concrete.

Cement is a component of concrete. Cement and water make the "glue" which holds concrete together.

  1. Name at least three items you have encountered today which are concrete.

Answers will vary.

  1. What are the major ingredients for concrete, and what role do they play?

cement- reacts with water to form "glue"

water- reacts with cement, the amount also determines strength

aggregate- makes concrete stronger, more durable, and less costly

  1. What is meant by "workable?" Why is it important for concrete to be workable?

Cement which is workable is able to be poured into forms without difficulty. A slump test is used to measure workability.

  1. Give an example of an aggregate. What is the practical use for this aggregate in making concrete?

gravel, sand, vermiculite, perlite,

Aggregate makes the concrete stronger and cheaper.



  1. What can be used to slow the hardening of concrete (give example?) Why would slowing this process be desirable?

Sugar can be added to the concrete to retard hardening. For example, if the concrete needs to be transported a long distance, then a retarding admixture would be desired.

  1. What can be used to speed the hardening of concrete (give example?) Why would speeding up this process be desirable?

Calcium chloride solution can be added to speed the hardening of concrete. For example, in cold weather it is desirable to speed up the hardening process and produced higher heat of hydration.

  1. Suppose you were in charge of building a skyscraper. What would be your choice for aggregate and why?

The aggregate would depend upon how the concrete is to be used in the building. Lightweight aggregates like shale are used for insulating properties. However, normal weight aggregate would be required for strength. Availability and economy of aggregate are important, too.



  1. What will happen to concrete if it dries out too quickly?

Concrete will most likely crack due to drying shrinkage. The hydration reaction which strengthens concrete will be halted from lack of water resulting in weaker concrete.

  1. Suppose you were to be the chief designer in charge of building a concrete ship to carry people overseas. What aggregate might you choose to put in your concrete and why?

A lightweight aggregate would be desirable for building a ship needing to float. However, the boat would be dangerous because of poor tensile properties of concrete. It would have to be reinforced to be safe.

  1. Explain what the dormancy period of fresh concrete is. How do contractors make use of the dormancy period?

The dormancy period of fresh concrete is the period during which the concrete is in a plastic state and the reaction is very, very slow. This state lasts from 1 to 3 hours and allows contractors to transport concrete to the job site and consolidate it before it hardens. After the dormancy period, the hydration reaction accelerates, and the concrete sets and becomes hard.

  1. Explain how can you measure the consistency of freshly mixed concrete?

A slump test can be performed on freshly mixed concrete to determine its consistency. This is done by pouring it into an inverted cup with the bottom cut out. Once the cup is removed, the concrete is observed. It is desirable that the concrete stay 50-75% of its original height for good workability.



  1. Briefly discuss the importance of a proper water to cement ratio.

The water to cement ratio determines the strength of concrete. The less water that is used to obtain a workable concrete, the more strength the resulting hardened concrete will have. However, remember that workability is lost if water to cement ratio is too low.

  1. Explain the purpose of a superplasticizer in making concrete.

A superplasticizer is an admixture which is used to make concrete more workable with the use of less water. Using a superplasticizer will result in a stronger concrete because less water is used.

  1. Why should gloves be worn when mixing concrete? Be specific.

Gloves should be worn while mixing concrete because one of the products of the hydration reaction is calcium hydroxide, a base. In fact, upon mixing concrete, the pH rises to 12 which means the solution is strongly basic. This can burn, irritate, and dry out the skin.

  1. Water is important in making concrete, however, it can also be harmful to concrete. Explain this statement.

Water transports harmful substances that lead to concrete degradation. Water is the central issue in freeze-thaw damage of concrete.