Contents · 11/1/2011 · Concrete materials 4 Cement 4 Aggregates 7 Properties of concrete 10 ......
Transcript of Contents · 11/1/2011 · Concrete materials 4 Cement 4 Aggregates 7 Properties of concrete 10 ......
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Contents
Contents 1
Introduction 3
Concrete materials 4
Cement 4
Aggregates 7
Properties of concrete 10
Compressive strength 10
Tensile or flexural strength 10
Durability 10
Workability 11
Cohesiveness 11
Concrete Testing 12
Sampling 12
Slump testing 12
Compression testing 14
Proportioning and mixing 17
Design strength 17
Target strength 17
Specification of concrete 17
Batching 19
Bulking of aggregates 20
Mixing 20
Premixed concrete 20
Slump 21
Admixtures 21
Air entraining admixtures 21
Water reducing agents 22
Transporting and placing of concrete 24
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Transporting concrete 24
Methods of transporting concrete 25
Placing concrete 26
Compacting 27
Curing 27
Reinforced concrete 30
Basic principles 30
Design of reinforced concrete 31
Formwork 34
Basic requirements 34
Supervision 35
Materials 36
Surface treatments 37
Stripping times 38
Back propping 39
Off-form finishes 40
Joints in concrete construction 41
Prestressed and post stressed concrete 44
Finishing concrete 49
Initial finishing 49
Final finishing 49
Floating 49
Concrete finish class 50
Activity 1 51
Activity 2 52
Summary 53
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Introduction
Concrete is one of the world’s most abundant building materials. It use dates
back to Roman times when limestone mortar was produced by heating
limestone and grinding the stone into a powder and then mixed with water
to form a paste that set both hard and quickly. It was during this era of
limestone mortar, that the first concrete was produced when the Romans
added sand, crushed stone or brick or broken tiles to the limestone mortar.
However, this concrete was severely limited since the mortar would dissolve
on contact with water. So it was a great achievement when a ‘sand’ (really a
volcanic ash) was discovered which, when mixed with lime and rubble,
hardened and could be used under water as well as in ordinary building.
This material was called ‘pozzulan’ since it was produced near the village of
Pozzuoli.
This ‘cement’ opened the way to a much greater use of mortars and
concrete; however, with the fall of the Roman Empire, the use of concrete
seems to have declined and not much is recorded about it until the mid
eighteenth century. It was not until 1845 that the real prototype of our
modern Portland cement was made.
So concrete is hardly a new material, but new aspects of concrete
technology are being investigated all the time and indeed the material has
been the source of an enormous amount of research for many years.
The ability of plastic concrete to be moulded into any shape probably makes
it one of our most versatile building materials and it is difficult to imagine a
building project today which does not make use of it in some manner.
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Concrete materials
Concrete is a composite material which consists of a ‘binder’ (Portland
cement and water, commonly referred to as the paste) and aggregate. The
paste will also usually contain some entrapped air.
Aggregates are generally classified into two groups:
fine aggregates which consist of sand with particle sizes less than 5
mm
coarse aggregates—generally crushed rock of varying sizes but
greater than 5 mm
In properly made concrete each particle of aggregate, whether large or
small, is completely surrounded by paste, and all spaces between the
aggregate particles are completely filled with paste. The aggregates may be
considered as inert materials, while the paste (cement and water) is the
active cementing medium which binds the aggregate particles into a solid
mass.
In a given quantity of concrete, aggregate occupies approximately 75 per
cent of the volume while the remaining 25 per cent is taken up by cement
paste and air voids. Air voids will remain in even well compacted concretes
but usually occupy less than 2 per cent of the total volume unless an air
entraining agent has been used.
Fine (sand) Coarse (gravel, crushed
stone, slag etc)
Cement and water Voids (max 1–2%)
Aggregate Paste
Figure 1 - Composition of concrete
The setting or hardening process of concrete takes place through the
chemical reaction of the cement and water. This process is called
‘hydration’ and is characterised by the release of heat.
Cement Portland cements are hydraulic cements manufactured from carefully
selected raw materials under closely controlled conditions to ensure a high
degree of uniformity in their performance. In Australia, all Portland cements
are made to meet the requirements of AS3972–2010 Portland and Blended
Cements.
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This standard covers five types of Portland cements which can be grouped
under the headings general purpose and special purpose.
General purpose cements:
- Type GP - general purpose Portland cement
- Type GB - general purpose blended cement.
Special purpose cements:
- Type HE - high early strength cement
- Type LH - low heat cement
- Type SR - sulphate resisting cement.
In general, Portland cement is produced by grinding together Portland
cement clinker and calcium sulphate.
General purpose cements
Type GP
General purpose cement is suitable for all uses where special properties are
not required. It is used for concrete products and building work where early
stripping for forms is not required.
Type GB
Blended cement consists of a mixture of Portland cement and pozzulands
such as fly ash and blast furnace slag. Blended cements generally have a
slower rate of strength gain and less heat of hydration when compared to
normal Portland cements; however, with continuous curing, they may
achieve higher long-term strength.
Special purpose cements
Type HE
Type HE cement is used where high strength is required at an early stage;
for example, where it is required to move forms as soon as possible or to put
concrete into service as quickly as possible (e.g. vehicle crossings). It is also
used in cold weather construction to reduce the required period of protection
against low temperatures.
Type LH
Type LH cement is intended for use in massive concrete structures such as
dams. In such structures the temperature rise resulting from the heat
generated during hardening of the concrete is likely to be a critical factor
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Type SR
Type SR—sulphate resisting cement has better resistance to attack by
sulphates in ground water than other types because of its special chemical
composition.
White and off-white cements
White and off-white cements are true Portland cements. White cement is
made from selected raw materials and by processes which introduce no
colour, staining or darkening to the finished product. Off-white cement is in
general use in cottage construction but white cement usually proves cost
prohibitive. Portland cement is generally available in 40 kg bags; that is, 25
bags to the tonne.
High alumina cement
High alumina cement is not a Portland cement. If mixed with Portland
cement it can give a rapid or ‘flash’ set. It is characterised by a very high
rate of strength development accompanied by a high heat of hydration and
by a greater resistance to sulphate and weak acid attack than Portland
cements. Curing conditions require very close control for 24 hours after
placement.
Storage of cement
Cement will retain its quality indefinitely if it does not come in contact with
moisture. If it is allowed to absorb appreciable moisture it will set more
slowly and its strength will be reduced. Therefore, storage of bagged cement
requires storage facilities to be as airtight as possible, and the floor should
be above ground level to protect against dampness. The bags should be
tightly packed to reduce air circulation, but they should not be stacked
against outside walls. If they are to be held for a considerable period the
stacks should be covered with tarpaulins or water-proof building paper.
Doors and windows should be kept closed. A ‘first-in-first-out’ rotation of
bags should be maintained at all times.
Setting and hardening
Setting is the initial stiffening of the cement paste during the period in
which the concrete loses its plasticity and before it gains much strength.
This period is affected by the water content of the paste and the temperature.
The more water in the paste the slower the set, and the higher the
temperature the faster the set.
Hardening is the gain in strength which takes place after the paste has set. It
is affected by the type of cement used and the temperature. High
temperatures cause more rapid hardening.
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Water
Water used for mixing good concrete should be free of deleterious amounts
of acids, alkalis and oil. Water containing decayed vegetable matter is
particularly to be avoided, as this may seriously interfere with the setting of
the cement. Water suitable for drinking will generally be suitable for
concrete making.
Aggregates Aggregates used in concrete should consist of clean, hard, durable particles
strong enough to withstand the loads to be imposed upon the concrete. In
general they should consist of either natural sands or gravels or crushed
rocks, although some manufactured aggregates such as blast furnace slag
and expanded shale and clays can be equally satisfactory. Commonly used
crushed rocks include basalt, granite, diorite, quartzite and the harder types
of limestone. Unsatisfactory materials include slate, shale and soft
sandstone.
Materials such as vermiculite and perlite and other lightweight materials are
unsatisfactory as aggregates for structural concrete as they lack strength.
In general, therefore, concrete aggregates should be:
strong and hard enough to produce concrete of the required
compressive strength and to resist abrasion and wear
durable to withstand the effects of weather and the cycles of wetting
and drying
chemically inert so that they will not react with the cement and cause
deterioration of the concrete
clean and free from impurities such as organic matter which can
inhibit the setting and hardening of the cement
free from silt and clay which, if present in excessive quantities, can
weaken the concrete
free from pieces or wood or coal which weaken the concrete and
cause blemishes
free from weak, soft particles which reduce the strength and break
down when exposed to the weather
free from surface coatings of clay or other weak material which
weaken the bond between the aggregate and the cement paste
Grading
Both coarse and fine aggregates should contain a range of particle sizes.
Graded aggregates produce more workable concretes which are less prone to
segregation and bleeding.
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Particle shape and surface texture
The particle shape and surface texture of aggregates affect the workability.
For workability, particles should be smooth and rounded. On the other hand,
angular materials result in greater strength, so that, in the final analysis,
there is little or no difference in effectiveness. The ultimate decision is one
of economics and availability.
Maximum size of aggregates
The greatest economy is achieved when the largest maximum size aggregate
is used. The factors limiting size are the availability, transporting and
placing equipment to handle the larger sizes, and the clear spacing between
reinforcing bars and the clear spacing between the reinforcement and the
formwork.
Manufactured aggregates
Blast furnace slag
If sound and free from excessive quantities of ferrous iron, blast furnace
slags are satisfactory concrete aggregates. Generally they are angular in
shape and require a higher percentage of fines to produce workable
concrete.
Some slags contain quantities of anhydrited lime which, if undetected, can
hydrate and cause cracking of the concrete. Unsound slags can be detected
by soaking in water for two weeks, at which time they will show signs of
disintegration.
Lightweight aggregates
Expanded shale aggregates produce concrete having approximately two-
thirds the density of those made with dense aggregates, but with comparable
strengths. Lightweight aggregates may be smooth and rounded or harsh and
angular, depending on the method of manufacture.
Testing of aggregates
Since aggregates comprise up to 75 per cent of the volume of concrete, their
properties are obviously important. These properties include size and
grading as well as cleanliness.
The testing of concrete aggregates is generally carried out to determine:
the presence of organic or other deleterious material which may
severely limit the strength of the concrete
the resistance to abrasion, which may limit the durability of the
concrete
the presence of any alkalis which may react with the cement and
cause expansion of the aggregate
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Conclusion
Good concrete can be made from a wide variety of aggregates provided
these are clean and free from harmful impurities. As the quality of concrete
becomes higher, the quality of the aggregate becomes more important and
factors such as grading more critical. Good aggregates, although sometimes
higher in initial cost, are generally more economical because of the higher
quality and lower overall cost of the concrete they produce.
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Properties of concrete
There are several properties of concrete which affect its quality. These are:
compressive strength
tensile strength
durability
workability
cohesiveness
Let’s examine these properties in detail.
Compressive strength Compressive strength remains the common criterion of concrete quality and
will frequently form the basis of mix design. For fully compacted concrete
made from sound clean aggregates the strength and other desirable
properties under given job conditions are governed by the net quantity of
mixing water used per bag of cement. This relationship is known as the
water/cement ratio, that is, the quantity of water in the mix to the amount of
cement present.
Example: A concrete mix having a water/cement ratio of 0.5:1 would
require 10 litres (10 kg) of water for each 20 kg bag of cement.
The ultimate strength of concrete depends almost entirely on the
water/cement ratio, for as the ratio increases the strength of the concrete
decreases.
Tensile or flexural strength This is the measure of the concrete’s ability to resist flexural or bending
stresses.
The tensile or flexural strength of concrete is dependent on the nature, shape
and surface texture of the aggregate particles to a much greater degree than
does the compressive strength.
Durability Concrete may be subject to attack by weathering or chemical action. In
either case the damage is caused largely by the penetration of water or
chemical solutions into the concrete and is not confined to action on the
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surface. The resistance to attack may therefore be increased by improving
the watertightness of the concrete. This is achieved by lowering the
water/cement ratio, assuming the concrete is fully compacted.
Workability The workability of concrete, or the effort required to handle and compact it,
depends on several factors, as follows:
Water/cement ratio: The higher the water/cement ratio, the more
workable concrete becomes. However, the water/cement ratio should
be fixed by considerations other than workability (e.g. strength and
durability), and should not be increased beyond the maximum
dictated by these considerations.
Cement content: The cement paste in concrete acts as a lubricant,
and at a fixed water/cement ratio, the higher the cement content, the
more workable the concrete becomes. It follows then that any
adjustments to increase workability should be made by increasing
the cement and the water content at a constant water/cement ratio.
Grading of aggregates: Grading tends to produce more workable
concrete.
Particle shape and size of aggregates: Smooth, rounded aggregates
will produce more workable concrete than rough, angular
aggregates. Also, for a given water/cement ratio and cement content,
workability increases as the maximum size of the aggregate
increases.
Traditionally concrete with a slump of 85mm or so was specified and
ordered. More recently design standards have been changed to permit a
higher slump level, to improve workability, and reduce WHS related issues
for concretors.
Cohesiveness The cohesiveness of concrete means the ability of plastic concrete to remain
uniform, resisting segregation (separation into coarse and fine particles) and
bleeding during placing and compaction.
Concrete in the plastic state should be cohesive to prevent ‘harshness’ of the
mix during compaction, and to avoid segregation of the coarse and fine
components during handling. Segregation may occur during transporting
over long distances, discharging down inclined chutes into a heap, dropping
over the reinforcement or falling freely through a considerable height and
placing in formwork which permits leakage of mortar. Maximum
cohesiveness usually occurs in a fairly dry mix, so as a rule the wetter the
mix the more likely it is to segregate. Segregation can, however, occur in
very dry mixes.
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Concrete Testing
Concrete is tested on the site or in the laboratory to determine its strength
and durability or to control its quality during construction. These tests help
the engineer or job supervisor to determine whether the concrete is as
specified and that it is safe to proceed with the job or whether adjustments
should be made to the mix.
These tests must be carried out carefully and in the correct manner or the
results may be misleading and cause unnecessary delays while they are
being checked. Worse still, faulty tests may result in either substandard
concrete being accepted or even good concrete being rejected.
There are several ways in which testing can be carried out:
by sampling
by slump testing
by compression testing
Sampling To make a composite sample from the discharge of a mixer or truck, three
or more approximately equal portions should be taken from the discharge
and then remixed on a non-absorbent board. The sample portions should be
taken at equal intervals during the discharge and none should be taken at the
beginning or the end. The concrete at these points may not be truly
representative of the whole mix.
When sampling freshly deposited concrete, a number or samples should be
taken from different points and recombined to make a composite sample.
Care should be exercised to make certain the sample is representative by
avoiding places where obvious segregation has occurred or where excessive
bleeding is occurring.
Slump testing The slump test is a measure of the consistency or mobility of concrete and is
the simplest way of ensuring that the concrete on the site is not varying. It
should be done often as an overall control on the various factors that can
affect the result. Chief among these factors is the water content of the mix,
variation of which can result in varying strengths of concrete. A consistent
slump means that the concrete is under control. If the results vary it means
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that something else has varied, usually the water, which can then be
corrected.
Equipment
To carry out the slump test, the following equipment is required:
A standard slump cone.
A bullet pointed steel rod or tamping rod.
A rule.
The slump cone is made from sheet metal and is 300 mm high, 200 mm in
diameter at the bottom and 100 mm in diameter at the top. It should be fitted
with footrests at the bottom and with handles by which it can be lifted.
The tamping rod is 600 mm long, 16 mm in diameter and bullet pointed.
All the equipment must be assembled before your begin testing.
Figure 2 - Slump test equipment
Method
To make the test, you should follow these steps.
1 Moisten the inside of the slump cone and place it large end down on a
clean level surface. Hold it firmly in place with a foot on each footrest.
2 Fill the cone, in three approximately equal layers, with concrete from
the sample.
Each layer should be tamped down exactly 25 times with the tamping
rod, which must be allowed to penetrate each layer.
3 The strokes must be uniformly distributed over the whole surface of the
layer and not worked up and down continuously in one place.
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4 After the top layer has been compacted, the surface of the concrete is
struck off level with the top of the cone and any surplus concrete is
removed from around the base.
5 The cone should then be lifted, carefully but firmly, straight up so that
the concrete is allowed to subside. Lift the cone smoothly and quickly
but do not jerk, twist or take off at an angle lest a false result be
obtained.
6 To measure the slump, invert the cone and place it alongside the
slumped concrete. Lay the tamping rod on top of the cone and measure
the amount of slump, measuring to the highest point of the concrete.
The slump is recorded to the nearest 10 mm.
Figure 3 - Slump test
Types of slump
In practice, concrete can slump in three ways:
True slump: the concrete subsides but more or less retains its conical
shape.
Shear slump: the concrete subsides but one side shears or falls away.
Collapsed slump: the concrete collapses completely.
If the concrete collapses or shears away, repeat the test.
Compression testing The strength of concrete is determined by making specimens, curing them,
and then crushing them to ascertain their strength. The preparation of
specimens is most important as a badly prepared specimen will nearly
always give a low result.
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Compressive test specimens are normally cylinders 150 mm in diameter and
300 mm high.
Equipment
Moulds in cylindrical shapes
Tamping rod
Rule
Mineral oil
Moulds for the cylinders should be made of metal and be rigid enough to
retain their shape during preparation of the specimen. They should be fitted
with a base plate which can be fitted securely to the mould to prevent loss of
the cement paste.
Method
1 Before filling with concrete, the mould should be clean and coated
inside with a very light film of mineral oil.
2 Place the mould on a level surface and fill with concrete from the
sample in three equal layers. Rod each layer 25 times with a bullet
pointed rod 600 mm long and 16 mm in diameter, allowing each stroke
to penetrate the previous layer.
In this case it is necessary that the concrete be fully compacted and it
may be necessary to rod each layer more than 25 times. The rodding
must be distributed over the whole surface of each layer and not merely
in one place. The concrete in the mould may be compacted by vibration
if suitable vibrators are available.
3 After the specimen has been moulded, it should be stored in a place
where it will be undisturbed for 18–24 hours, kept moist and at a
temperature of between 21°C and 24°C. After 24 hours the specimen
should be removed from the mould and again stored under moist
conditions and at the correct temperature. This is called curing.
4 For transport to the laboratory, the specimens should be packed in moist
sand or hessian so that they will remain moist and be undamaged during
transit.
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Figure 4 - Preparation of a concrete specimen for compression testing
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Proportioning and mixing
Design strength The designer of a concrete structure determines during the design stage, the
concrete properties that are necessary to ensure that the structure performs
in the desired manner. Since compressive strength is usually the most
important property required and since most other desirable properties are
directly related to it, it is usual for the designer to specify the minimum
compressive strength required, usually at 28 days. The ‘design strength’ is
the minimum strength required by the designer.
Target strength The mix designer must design a mix which will produce concrete with a
strength in excess of the design strength for the following reasons:
It is known that when a series of compressive tests are made from
samples of concrete taken from time to time through the course of a
job, the results will be scattered to either side of an average value,
even though all the concrete is made to the same specification. This
means that the concrete produced is never completely uniform in
quality—some is always weaker than the average strength and some
is always stronger.
Since the designer has specified the minimum strength required, the
mix designer must aim at an average strength, between the target
strength and the design strength.
Generally, a target strength 33 per cent higher than the design strength
meets the requirements of the building codes.
Specification of concrete In writing the specification to ensure that the concrete has the properties
required, the designer has two alternatives:
specify the concrete by strength (the usual method)
specify concrete by proportions
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Concrete specified by strength
Figure 5 - Strength development of Cement1
The designer specifies the minimum compressive strength required in the
concrete and the age at which the concrete should have this strength, usually
28 days.
Figure 6 - Water : cement ratio – the effect of adding water to concrete2
The ratio of water to concrete by weight gives a good indication of the likely
final strength concrete. As W/C ratio increases the concretes strength
decreases. (See Figure 6 - Water : cement ratio – the effect of adding water
to concrete)
1 Nikulski B, Materials 1 Subject Notes, 2007, Unpublished
2 Nikulski B, Materials 1 Subject Notes, 2007, Unpublished
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Figure 7 - Effect of curing3
Concrete specified by proportions
In this case, the designer specifies the materials to be used and the
proportions to be used. Designers use knowledge and experience as a basis
for ensuring that concrete of the desired strength is produced, and the job
supervisor is responsible for the correct materials being used in the specified
proportions. The responsibility for the concrete strength and other properties
remains with the designer.
Batching All materials, including water, should be accurately measured to ensure that
concrete of uniform quality is produced.
The method used to measure the quantities of different materials required
for a mix is called batching by mass. Mass batching is very accurate and
reduces the danger of variations of quality of concrete between one batch
and another.
Batch proportions are often specified in relation to the bag of cement; for
example, one 20 kg bag of cement to so many kilograms of coarse aggregate
and so many kilograms of fine aggregate with perhaps 10 L or 10 kg of
water. Even though the solid materials are measured by mass, it is quite
common for water to be measured by volume from a graduated tank above
the mixer. Provided that the tank is accurately graduated there is no loss of
accuracy as 1 L of water has a mass of 1 kg and is not subject to variation.
3 Nikulski B, Materials 1 Subject Notes, 2007, Unpublished
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With mass batching, there is no need to make allowance for the bulking of
damp sand but allowance must be made for the non-absorbed water held by
the aggregates as this moisture forms part of the mixing water.
Equipment for mass batching ranges from simple inexpensive platform
scales to large and elaborate types, while some large types of concrete
mixers have mass batching devices built into them.
Bulking of aggregates Volume proportions are always specified on the assumption that the
aggregates are loose packed and dry. Most aggregates contain some
moisture and sand exhibits a property described as ‘bulking’ when moist;
that is, sand when moistened increases in volume. This property makes sand
difficult to gauge accurately by volume measurement and is, in fact, the
principal reason why batching by mass rather than by volume is the
preferred method.
Mixing The aim of mixing concrete is to obtain a uniform mixing of all the concrete
materials and to ensure that each particle of aggregate is adequately coated
with cement paste.
Mixing time
Short mixing times, although increasing production, produce patchy, non-
uniform concrete.
Excessive mixing is generally uneconomical and may cause undesirable
grinding of the aggregates particularly if they are on the soft side.
The minimum mixing time allowed by AS3600–2001 Concrete Structures is
11
2 minutes.
Premixed concrete Premixed concrete is used almost universally on residential building sites.
The use of premixed concrete has advantages which include:
Better quality control is possible at a large plant than under most site
conditions.
Less labour is required.
Premixed concrete is controlled by AS1379–2007 Specification and
Manufacture of Concrete, which should be referred to for information on
methods of ordering, mixing and delivery.
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Slump The slump of a batch of concrete at the time of discharge should be
expressed as the average of two tests, one on concrete sampled at the one-
quarter point of the batch volume and the other on concrete sampled at the
three-quarter point.
The concrete should be considered to comply with the specified slump if:
when the specified slump does not exceed 75 mm the average of two
tests is within 12 mm of the specified slump; and
when the specified slump exceeds 75 mm the average of two tests is
within 12 mm of the specified slump.
Admixtures An admixture may improve the properties of concrete. Admixtures are
available in both solid and liquid forms. The general nature of the
admixture should be known before adding it to the concrete mixture in case
it may impair strength or durability.
Accelerators
Accelerators increase the rate of reaction between cement and water in the
mix.
Calcium chloride
The amount of calcium chloride accelerator used should not exceed 2% by
weight of cement when its temperature is between 50C and 200C.
Stannous chloride
Stannous chloride is an expensive accelerator that must be fresh and the
concrete thoroughly compacted.
Triechanclamine
Small amounts of triechanclamine accelerator may be used at 0.5% to 0.4%
by weight of cement. It may increase the shrinkage. If used excessively it
can produce rapid setting.
Air entraining admixtures Air entraining admixtures are soluble salts of wood resins, fatty acids,
soluble salts or sulphate or sulphonated hydrocarbons. These are used to
develop microscopic bubble systems by agitation in mixer. This improves
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workability and durability to reduce bleeding and decrease segregation.
Bubbles provide lubricating and plasticising effects which allows for less
water without loss of slump.
Air cells remain separate entities in hardened concrete and act as barriers to
normal entry of water and water-borne salts via capillary pores. They also
provide expansion chambers to withstand extreme temperature changes.
They improve volume of air by 3 to 5%. Excess air entrained can cause
serious loss in strength.
Set retarders
Hydroxylated carboxylic acids and their salts, certain sugars and
carbohydrates can be used to retard the onset of setting. These are useful
additives to extend the time between mixing, placing and finishing from 1 to
3 hours. They leave more water in the concrete for workability until it is
placed when hydration can continue.
Water reducing agents Water reducing agents add strength. Intermixing of cement and water is
minimal due to differing surface temperatures and energies.
Strength is improved at all ages and the strength is due to physico-chemical
effects on hydration rather than to the use of less water.
Super plasticisers
Super plasticisers improve workability. They make concrete almost self-
levelling. The duration of effectiveness of super plasticisers is 20 to 90
minutes and then the concrete returns to its original behaviour.
Waterproofing
Tests have proven that waterproofing admixtures are largely ineffective so
waterproof sheeting is still needed under slabs. Transmission through walls
and upright structures or into concrete floors may be effected to some
degree by these additives. Talc, fullers earth, some silicates, substances
from saps, fatty acids, ammonium and BU + YL stearates are used for
waterproofing. Most cause a reduction of strength
Workability agents
Workability agents improve cohesiveness, for easier placement and better
compaction. They reduce permeability by filling voids between particles.
They can also be used for mixes deficient in fines. They are finely divided
providers and include hydrated lime, bentonite, talc, clay and pulverised
stone.
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Pigments
When pigments are used in concrete, cement content should be increased by
10 to 15% by weight. Colour lightens when concrete is dry. Special curing
is needed for consistency of colour. Either a layer of washed sand or curing
compound containing matching colour should be used.
Expanding agents
To counteract the effects of shrinkage, settlement and bleeding expanding
agents can be used. They are used to provide maximum bearing, base
plates, and steel columns for under-pinning work. In grouting for cavity
joints, ducts containing pre-stressed concrete members that are post-
tensioned. Wax is used to facilitate pumping and reduces bleeding.
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Transporting and placing of concrete
The care taken in the production of good quality concrete is to some extent
nullified unless the mixed concrete is transported from the mixer to the
forms, placed and compacted satisfactorily.
Transporting concrete Irrespective of the methods used to transport, place and compact the freshly
mixed concrete, the following requirements are basic to good practice:
The concrete must be transported, placed and compacted with as
little delay as possible.
The concrete must not be allowed to dry out before compaction.
There must be no segregation of the materials.
The concrete in the forms should be fully compacted.
Dangers of poor transporting practice
Delay
Stiffening of concrete begins as soon as the cement and water are
intermingled. This stiffening increases with time, and therefore, the time
which elapses after mixing has an adverse effect on the workability of the
mix. Under normal conditions, the amount of stiffening which takes place in
the first 30 minutes after mixing is not significant, and if the concrete is kept
agitated, up to one and a half hours can normally be allowed to elapse
between mixing and compacting.
Drying out
Concrete is designed to have a workability which will allow it to be fully
compacted with the equipment available. If it is allowed to dry out during
transportation or placing, it will lose workability and full compaction may
not be possible.
Segregation
Segregation can occur if unsuitable methods are used to transport, place and
compact plastic concrete and results in the hardened concrete being non-
uniform with weak and porous honeycomb patches.
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Inadequate compaction
The strength, durability and permeability of the hardened concrete all
depend on the concrete being fully compacted in the forms. Inadequate
compaction results in an appreciable loss of strength.
Methods of transporting concrete There are several methods of transporting concrete:
barrows
hoists
trucks
chutes
pumps
pipelines
Barrows
These are the most basic of the vehicles used in this country for transporting
concrete but are still in considerable use. They are particularly suited for
smaller jobs and for larger jobs with short hauls.
The number of barrows should be sufficient to take the full mix from the
mixer in order to minimise waste of time and avoid confusion.
Hoists
The hoist is a commonly used means of elevating concrete. Proprietary hoist
towers ranging in height from about 4.5 m to 45 m can be made. These
hoists can operate an elevating platform on to which one or two barrows of
concrete can be wheeled.
Trucks
Trucks are in general use for transporting concrete from a central mixing
plant to scattered jobs or to various parts of a large project. In ordinary
trucks, wet concrete is liable to segregate and dry mixes are liable to
compact.
Premix firms have overcome the problem of segregation during transport by
the use of agitator trucks for wet mixes and by truck-mounted mixers which
transport a dry batch and mix it when approaching the site.
Chutes
Unless special care is taken to ensure that the discharge is vertical at the end
of the chute and that long chutes are adequately protected to prevent drying
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out, this can be one of the most unsatisfactory methods of transporting
concrete.
The slope of chutes should be sufficient to allow the flow of the lowest
slump concrete being used on the job. A baffle at the end of the chute should
direct the concrete into a vertical downpipe at least 600 mm long to prevent
segregation of the concrete on discharge from the chute.
Pumps and pipelines
Pumps and pipelines enable concrete to be transported across congested
sites and where space is limited. The maximum horizontal distance concrete
can be pumped is 500 m. Vertical pumping in excess of 120 m may be
achieved but heights are normally kept below 30 m. Maximum length
cannot be combined with maximum height.
Curves and rises should be limited as they reduce the maximum pumping
distance. A 90° bend, for example, is equivalent to about 10 m of straight
pipe. Each metre rise in elevation is equivalent to about 5 m of straight
horizontal pipe, although this value depends on pipe size and concrete
velocity. With very slow rates of pumping in large pipes this equivalent
value can be as high as 30 m.
The output of a conventional 100 mm pump varies between about 10 and
100 m3
per hour, depending on type of pump and conditions.
Concrete for pumping must be of medium workability with a slump of 70
mm to 120 mm and must be free from any tendency to segregate. The
introduction of fly ash to the concrete improves pumpability and workability
of the mix, and therefore adds appreciably to the distance concrete can be
pumped.
Placing concrete Certain precautions must be taken when placing concrete, to ensure that:
formwork and reinforcement is not damaged or dislodged
the concrete is free from segregation
other qualities of the concrete are not impaired
The following is a summary of some of the most important points of good
placing practice:
Concrete should be placed vertically and as near as possible to its
final position. If spreading is necessary it should be done with
shovels and not by causing the concrete to flow.
Concrete should not be dropped into the forms from an excessive
height as this can cause damage and segregation. The height to fall
should be kept to a minimum and should not exceed 1.8 m unless a
drop chute or a vertical funnel is used.
Placing should start from the corners of formwork and from the
lowest level if the surface is sloping.
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Each load of concrete should be placed against the face of the
previously deposited concrete, not away from it.
If stone pockets occur, the stones should be shovelled from the
pocket and tamped or vibrated a into sandy section.
Concrete should be deposited in horizontal layers and each layer
should be compacted before the next is placed. Each layer should be
placed in one continuous operation and before the previous layer has
hardened.
As the top of a lift is neared, drier mixes should be used to allow for
the water gain which begins to form on the surface.
To minimise the pressure on forms with high lifts, the rate at which
the concrete rises should not exceed 1.5 m per hour in warm weather
and 600 mm per hour in cold weather.
Concrete should not be placed during heavy rain without overhead
shelter to prevent the rain washing the surface of the concrete.
Compacting It is essential that concrete be properly compacted to ensure maximum
density. Air holes must be eradicated, voids between aggregate particles
must be filled and all aggregate particles must be coated with cement paste.
Thorough compaction results in:
maximum strength
watertight concrete
sharp corners
a good bond to reinforcement
protective cover to reinforcement
a good surface appearance
Vibration
Concrete is usually vibrated to achieve good compaction. There are three
types of vibrators:
immersion vibrators
form vibrators
surface or screed vibrators
The immersion vibrator is driven either electrically, mechanically or
pneumatically and is probably the most efficient type of vibrator as it
vibrates the concrete directly by immersion in the concrete. They are
particularly suited to the compaction of large volumes of concrete.
Curing Concrete hardens as a result of the chemical reaction that occurs between
cement and water which is called hydration. Hydration occurs only if water
is available and if the concrete's temperature stays within a suitable range.
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After placing concrete, the concrete surface needs to be kept moist for a
period of time to permit the hydration process. This period is referred to as
the curing period and is usually 5-7 days after placing conventional
concrete.
While it is true that concrete increases in strength and other desirable
properties with age, this is so only so long as drying is prevented. The
hydration of cement is a chemical reaction and this reaction will cease if the
concrete is permitted to dry. Evaporation of water from newly placed
concrete not only stops the process of hydration, but also causes the
concrete to shrink, thus creating tensile stresses at the drying surface; and if
the concrete has not developed sufficient strength to resist these stresses,
surface cracking may result.
As in many other chemical reactions, temperature affects the rate at which
the reaction between the cement and water progresses; the rate is faster at
high temperatures than at lower temperatures.
It follows then that concrete should be protected so that moisture is not lost
during the early hardening period and should also be kept at a temperature
that is favourable to hydration.
Curing methods
Curing methods can be classified as follows:
The supply of additional moisture to the concrete during the early
hardening period.
Sealing the surface to prevent loss of moisture from the concrete.
Ponding
On flat surfaces, concrete can be cured by building an earth or sand dyke
around the perimeter of the concrete surface in which a pond of water is
retained.
Ponding is not only a very efficient method of preventing water loss from
the concrete but also maintains a uniform temperature in the concrete.
Sprinkling
Sprinkling can be either continuous or intermittent. If intermittent, care must
be taken to ensure that the concrete does not dry between applications of
water. A fine spray of water applied continuously through a system of spray
nozzles provides a constant supply of moisture and prevents the possibility
of cracking or crazing caused by alternate cycles of wetting and drying.
Wet coverings
A 50 mm thick layer of earth or sand, straw or hessian or other moisture
retaining material spread over the surface of the concrete and kept
constantly moist so that a film of water remains on the surface of the
concrete throughout the drying period has proved very satisfactory.
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Waterproof paper and plastic sheets
Strips of waterproof paper or plastic sheeting spread over the surface of the
concrete prevents the evaporation of the water from the concrete. The edges
of the sheeting should be overlapped and sealed with sand, tape or by
weighting down with planks or other heavy objects. An important advantage
of this method is that periodic additions of water are not required.
Curing compounds
Liquid membrane forming curing compounds sprayed over the surface of
moist concrete retard or prevent the evaporation of moisture from the
concrete. Some curing compounds prevent the bonding of fresh concrete to
hardened concrete and should not be used for instance on the base slab of a
two-course floor since the top layer may be prevented from bonding. The
adhesion of resilient floor coverings to concrete floors may also be affected
by some curing compounds.
Curing of vertical surfaces
Vertical surfaces can be satisfactorily cured by:
leaving the forms in place. If wooden forms are used, they must be
kept moist by sprinkling
draping hessian over the surface and keeping it moist
constant sprinkling or hosing of the surface
Length of curing period
For most structural purposes, the curing time for concrete varies from a few
days to two weeks according to conditions; for example, lean mixes require
longer curing time than rich mixes and temperature affects the curing time
as does the type of cement used.
Since all the desirable properties of concrete are improved by curing, the
curing period should be as long and as practicable in all cases.
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Reinforced concrete
Basic principles Concrete, like any other building material, has limitations, mainly because
of the fact that while it is strong in compressive strength, it is comparatively
weak in tensile strength. To overcome this weakness in tension, concrete
which is to be subjected to tensile stresses is reinforced with steel bars or
mesh which is so placed that it will resist such stresses.
The designing and detailing of reinforcement is the job of the designing
engineer and will not be dealt with in any great detail here, but it is
important that those who supervise the fixing of reinforcement on the job
have an appreciation of the basic principles of reinforced concrete. They can
then understand why it is necessary that reinforcement be correctly handled
and fixed in the positions indicated on the job drawings.
Figure 8 - Types of stress found in a structure
Reinforced concrete is so designed to combine the concrete and steel into
one structural entity in such a way as to make the best use of the
characteristics of each of these materials.
The aim of reinforced concrete design is to combine the steel reinforcement
with the concrete in such a manner that just enough steel is included to resist
the tensile stresses and excess shear stresses while the concrete is used to
resist the compression stresses.
Steel and concrete combine together successfully because:
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the bond between concrete and steel directly counteracts any
tendency for the concrete to stretch and crack in a region subjected
to tension
with temperature changes, concrete and steel expand and contract the
same amount. If this were not so, the different expansion rates would
break the bond between the two materials and so prevent the transfer
of tensile stresses to the steel
concrete has a high fire-resistance and protects the steel from the
effects of fire
A broad understanding of stresses and the methods of indicating the
particular stress on drawings is essential.
Design of reinforced concrete In order to be effective, the tensile reinforcement must be prevented from
sliding in the concrete. The adhesion or bond between the concrete and the
steel is related to the surface area of the steel embedded in the concrete.
Adequate anchorage is effected by extending the rods past the critical points
(where no longer required to resist tensile and shear stresses) and by the use
of:
standard hooks
plain rods extended into the supports (rarely used)
deformed bars (rolled with lugs or projections)
The three environment phases
In the course of time, the environment surrounding the reinforcement
changes.
Before the concrete is cast, the steel bars are exposed to atmospheric
rusting, which is due to the simultaneous presence of water and
oxygen (air).
The bars are surrounded by freshly mixed concrete which although it
contains water, is normally so alkaline that it prevents further
corrosion of the steel.
For a very long time the bars are encased in solid concrete which is
slightly permeable, may crack, and may itself be modified by
chemical attack.
The surface condition of reinforcement shall comply with the following
requirements.
At the time concrete is placed, reinforcement shall be free from mud,
oil, grease and other non-metallic coatings and loose rust which
would reduce the bond between the concrete and the reinforcement.
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The prevention of corrosion
There are three ways of reducing or preventing the corrosion of the steel in
reinforced concrete.
One is to use more cement with or without a greater thickness of
concrete cover so as to preserve the high alkalinity around the
reinforcement.
Another is to put a protective coating of some additional material on
the reinforcement.
Finally, rust resistant alloy steels or even non-ferrous metals may be
used.
The likelihood of corrosion
If the reinforcement were to be surrounded by a minimum thickness of 60
mm of impermeable uncracked concrete, even a moderately aggressive
environment will cause corrosion in due course. In dry, unpolluted air the
protection of 25 mm of concrete cover should maintain the required
alkalinity of the concrete in contact with the steel. These specifications are,
however at risk due to the effects of workmanship, tensile cracking of the
concrete, and the porosity of the aggregate, and in some circumstances it
may not be possible to meet them. The best protection against corrosion is to
ensure specified cover with well compacted concrete.
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Figure 9 - Positioning of main reinforcement to resist tensile stresses in beams
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Formwork
Basic requirements In its plastic state, concrete can be readily moulded into any desired shape.
As any inaccuracy or blemish in the formwork will be reproduced in the
finished concrete, it is essential that the forms be designed and constructed
so that the desired size, shape, position and finish of the concrete is
obtained. Although the formwork is a temporary structure, it will be
required to carry heavy loads resulting from the mass of the freshly placed
concrete and construction loads of materials, workers and equipment. The
formwork must therefore be substantial enough to carry these loads without
fear of collapse or deflection, and within the confines of AS1509, SAA
Formwork Code.
As the cost of formwork can amount to about one-third of the total cost of a
concrete structure, efficiency in its construction can become an important
factor in the overall economy of the job.
Good formwork
The guiding principles for the production of good formwork are:
quality
safety
economy
Quality
First quality formwork should be:
Accurate: True to the shapes, lines and dimensions required by the
contract drawings.
Rigid: Forms must be sufficiently substantial so as to prevent any
movement, bulging or sagging during the placing of the concrete.
Tight-jointed: If joints are not tight, they will leak mortar. This will
leave blemishes in the shape of fins on the surface of the concrete
and may result in honeycombing of the concrete close to the leaking
joint.
Well-finished: The quality of the finish of the concrete is dependent
on the finish of the forms. Nails, wires, screws and so on should not
be allowed to mar the surface of the finished concrete.
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Safety
Strength: For the safety of the workers and of the structure, the
formwork must be strong enough to withstand not only the mass of
the wet concrete but also the live loads of workers, materials and
equipment. It is impossible to over emphasise how important this
aspect of safety really is.
Soundness: Materials must be of good quality and durable enough
for the job. The time will come, no doubt, when it will be essential to
use for structural load-bearing members, only timber that has been
tested with the mechanical stress grading process.
Economy
For economy, formwork should be:
Simple: Formwork should be designed for simplicity of erection and
removal.
Easily handled: Shutters and units should be light enough to permit
easy handling.
Standardised: Where standardisation of formwork is possible, the
ease of assembly and the possibility of reuse serve to lower the
formwork cost.
Reusable: Formwork should be designed for easy removal and in
sections that are reusable. This will minimise the amount of waste
material and thus decrease the cost of the formwork.
Supervision The field supervisor’s work falls into four categories:
Control: The supervisor must ensure that formwork is constructed in
accordance with the specifications and working drawings and must
check that all dimensions are within the allowable tolerances.
Planning: The supervisor might also play a part in planning the work
so as to achieve an efficient cyclic program of assembly, concreting,
removal and restoring.
Safety: The supervisor must play a leading role in ensuring adequate
safety precautions to protect workers. There will be many occasions
where she or he should seek the counsel of the site engineer.
Workmanship: The supervisor must ensure that formwork is
constructed to a high standard of quality.
Some of the deficiencies which can lead to form failures are:
Premature removal of forms or props.
Inadequate bracing and poor splicing of multiple storey timber
props. Splices should have long cleats at the joint on all four sides
and be well nailed.
Failure to control the rate of placing concrete in deep forms without
regard to the effect of temperature changes.
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Failure to regulate properly the placing of concrete on horizontal
forms and prevent unbalanced loadings.
Failure to check the adequacy of footings for falsework to prevent
settlement in unstable ground.
Failure to inspect formwork during concreting to detect any
abnormal deflections or signs of imminent failure.
Failure to provide adequately for lateral pressure on formwork.
Props not plumb.
Locking devices on metal props not locked or inoperative.
Overturning by wind.
Damage in excavations by reason of embankment failure.
Failure to check that the drawings are being interpreted correctly.
Points which are related to workmanship are:
Joints or splices in sheathing, plywood panels and bracing should be
staggered.
Tie rods or clamps should be in the correct numbers and locations.
Tie rods or clamps should be properly tightened.
The connections of props and stays to joists, stringers and wales
must be adequate to resist any uplifts or twisting at joints.
Form coatings should be applied before placing of reinforcement and
should not be used in such quantities as to run onto bars.
Bulkheads for control and construction joints should preferably be
left undisturbed when forms are stripped, and removed only after the
concrete has cured sufficiently.
Bevelled inserts to form keyways at contraction joints should be left
undisturbed when forms are stripped, and removed only after the
concrete has cured sufficiently.
Wood inserts for architectural treatment should be partially split by
sawing to permit swelling without applying pressure to the concrete.
The loading of new slabs should be avoided in the first few days
after concreting.
Formwork must not be treated roughly or overloaded if reuse is
desired.
Materials Formwork can be constructed in many different types of materials. Details
about each type follow.
Timber
Partially seasoned softwoods, such as Oregon or pine, dressed where in
contact with the concrete, make good formwork. Fully seasoned timber will
swell excessively when wet and green timber will warp and shrink during
hot weather.
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Plywood
Varying in thickness from 5 mm to 20 mm, plywoods give a large area of
joint-free surface. Plastic coated plywood (plasply) can be used to give a
smooth grainless surface to the finished concrete. Plywood can be bent to
produce curved surfaces.
Hardboard (Masonite formboard)
Hardboard has many of the features of plywood but requires more support
and cannot be curved so easily.
Steel
Steel is relatively costly but it can withstand repetitive reuse. Steel framing
and bracing can be used in conjunction with timber and plywood panel
systems. There are a number of proprietary steel formwork systems
available.
Surface treatments
Preparation of forms for concreting
All debris, particularly chippings, shavings and sawdust, must be removed
before the concrete is placed and the surfaces which are to be in contact with
the concrete must be cleaned and thoroughly wetted or, alternatively, treated
with a suitable composition. Compositions that have not been approved by
the engineer or architect must not be used.
Temporary openings must be provided at the bases of columns and wall
forms and at other points where necessary to allow cleaning and inspection
immediately before the placing of the concrete.
Surface coatings for forms
Any material used as a surface coating for forms must:
act as a separating agent to allow the release of the forms without the
concrete sticking to their surfaces
act as a sealer to prevent the forms absorbing water from the
concrete
not stain or disfigure the finished concrete surface
not prevent the adhesion of render or other similar surface finishes
not reduce the active life of the forms
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Wood and plywood forms
A number of form oils suitable for timber forms are marketed commercially.
These are designed to penetrate the surface to some extent and leave the
surface of the form only slightly greasy to the touch. For plywood, apart
from the commercially produced oils, a mixture of linseed oil and kerosene
is satisfactory. Plywood may also be coated with shellac, lacquer, resin-
based products or plastic compounds which almost totally exclude water
from the plywood, thus preventing the grain from rising. Such coatings
require little or no oiling.
Metal forms
Form oils suitable for timber forms are not always suitable for metal forms.
Paraffin-based form oils and petroleum-based oils blended with synthetic
castor oil, silicone or graphite have proved successful on metal forms.
Stripping times The time of the removal of forms is generally specified by the architect or
engineer in the contract documents or made subject to this person’s approval
because of the danger to the structure if forms are stripped before the
concrete has developed sufficient strength. Forms can usually be safely
stripped when the concrete has developed about two-thirds of its 28-day
strength. However, the earliest possible removal of forms is desirable for the
following reasons:
To allow the reuse of forms as planned.
In hot weather, to permit curing to begin.
To permit any surface repair work to be done while the concrete is
still ‘green’ and favourable to good bonding.
Additional information is available in AS
Remember safety is paramount, and it is much better to be sure than sorry.
Vertical forms can generally be removed before the forms to the soffits of
beams and slabs.
Where stripping times have not been specified, Table 1 - Times for stripping
formwork and supports may be used as a guide to appropriate stripping
times when using normal Portland cement.
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Table 1 - Times for stripping formwork and supports
Location and type of formwork
Average temperature of concrete during the period before stripping
21°C to 32°C 4°C to 21°C
Days Days
Beam sides, walls and
unloaded columns
1–2 2–5
Heavily loaded columns,
tunnel linings supporting
unstable material, and other
heavily loaded structures
7–10 10–14
Slabs, including flat slabs
and flat plates, with props
left under
3–7 7–20
Removal of props from
under slabs
7–14 14–21
Beam and girder soffits
(with props left under) and
arch soffits
7–10 10–14
Removal of props from
under beams
10–14 14–28
Information on required formwork stripping times for reinforced concrete
slabs continuous over formwork supports not supporting structures above
is provided in AS 3600:2009 Table 17.6.2.4 if not provided in project
documentation.
Information on required formwork stripping times for reinforced concrete
slabs and beams not supporting structures is provided in AS 3600:2009
Table 17.6.2.5 if not provided in project documentation.
Also as a guide only information on for multistorey formwork stripping
times with and without back propping is provided in AS 3610:1995 Table
5.4.3 and clause 5.4.4 if not provided in project documentation.
Back propping Builders must consider a number of issues when planning the construction
of multistorey reinforced concrete buildings. On the one hand concrete
curing to achieve desired strength must be achieved prior to formwork
removal. However builders will want to reuse formwork as soon as possible
on upper levels to minimise cost. Careful consideration must be given to
suitable formwork stripping times. Advice from a structural engineer about
the suitable formwork stripping time is recommended. Options are available
and include the implementation of back propping to accelerate formwork
reuse. AS3610 states that undisturbed support requirements for multistorey
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formwork systems are to be in accordance with project documentation and
formwork documentation.
Formwork is supported by false work, and typically in multistorey
formwork, ply sheets are supported by joists which are supported by
bearers. Once a new slab has been cast, and after the required number of
days delay, back propping can be implemented by:
Fixing the required number of back props to the underside of the
plywood directly (not to joists or bearers supporting ply) before any
supports or ply or anything else is removed.
Then, and only then, remove the rest of the supports, beams, joists and
ply with no back props holding it up.
New back props are then placed under the bare concrete.
Then the props that are left holding up just the ply sheets are removed
one at a time, the ply is removed and the prop is put back against the
concrete before the moving on to the next one.
In this way the slab is always supported.
Activity 1
Look up AS3610:1995 Formwork for concrete and particularly Table
5.4.3 and clause 5.4.4. Determine the impact that back propping has
on formwork removal by answering the following questions:
1. If back propping is used should the minimum number of levels
of undisturbed supports be increased or decreased?
2. If back propping is used can the maximum number of levels of
support which may be back propped exceed half of the total
number of levels of support at the time of the pour?
3. Can stacked materials be placed on any of the supported floors?
4. What is the minimum time between pause of successive floors?
5. What are the minimum temperature requirements?
Off-form finishes It is economical for the structural concrete to form the surface finish. Where
special characteristics such as smoothness, pattern, texture, intricate detail
and so on are required, extra special care must be taken in the selection of
form materials and in the form construction.
Smooth surfaces
Most sheathing and lining materials are available in grades smooth enough
to produce a blemish free concrete surface. The correct choice of form oil is
important in achieving the desired smoothness.
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Wood grain finishes
A surface simulating wood grain can be produced by casting the concrete
against a plywood form liner which has had the grain revealed by wire-
brushing or sand-blasting. Sometimes an exposed grain plywood is available
ready-made for this purpose. To produce a rough board marked surface,
sawn boards are used for sheathing. These boards may be sprayed with
ammonia to raise the wood fibres and accentuate the grain markings.
Textured and patterned surfaces
These finishes are obtained by lining the forms with liners such as striated
plywood, rubber matting and moulded plastic. The liners are either nailed or
fixed with a waterproof glue to the inside surfaces of the forms.
Joints in concrete construction Interruptions to the placing of concrete will inevitably occur when pouring
large quantities. Irrespective of the length of these interruptions, if the
concrete is allowed to stiffen to the extent that it cannot be worked, then a
joint must be made. Other cases will occur when it is necessary, for
structural reasons, to break the continuity of placing and to form a joint.
Joints can be of two general types:
Construction joints: These aim at bonding the new concrete to the
hardened concrete in such a manner that the concrete appears to be
monolithic and homogenous across the joint and allows for no
relative movement of the concrete on either side of the joint.
Control joints: These allow for relative movement on either side of
the joint, thus they can be either construction joints or expansion
joints.
Construction joints
In practice, it is very difficult to obtain a perfect bond at a joint and a plane
of weakness will always occur at a construction joint. For this reason, they
should be avoided wherever possible.
While unscheduled interruptions are often unavoidable during placing,
making an unplanned construction joint necessary, some breaks in the
continuity of placing may be foreseen either in the design stage or just
before commencement of construction, thus allowing the position of many
joints to be planned. Good planning will aim to interrupt placing in a
position suitable for a control joint and so eliminate the need for a
construction joint.
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Location of construction joints
Where construction joints are necessary in structural members they should
be made where the shear forces are at a minimum. The joint should be at
right angles to the axis of the member so that axial forces act normally to the
joint and do not tend to cause sliding along a weakened plane.
Concrete for columns should be poured continuously to just below the soffit
of the beam, drop panel or capital, and the concrete left for at least two
hours to settle before fresh concrete is placed. The whole floor system
around the head of the column should then be cast in one operation after
suitable preparation of the joint.
Construction joints in beams should be made in the middle third of the span
and on no account should they be made at or near the supports or over any
other beam, column or wall since shearing stresses are usually very high at
these positions.
When a construction joint is required in a floor slab it should be made near
the middle of the span.
Making vertical construction joints
When making a construction joint in a beam or slab, the concrete must not
be allowed to assume its natural angle of repose, but should be taken up to a
suitable stop board so as to form a vertical joint. To assist the transfer of
load across the joint, either dowels or a keyway to aid mechanical bonding
may be used at about mid-depth of the beam or slab. This is recommended
in sections over 150 mm deep. Reinforcement must not be cut at a
construction joint but must be left continuous in the member.
Figure 10 - Making a vertical construction joint
Preparation of construction joints
The correct method of preparation and making of construction joints is
detailed in AS3600 1994 Concrete Structures Code.
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Watertight construction joints
A correctly made horizontal construction joint in a wall should not require
sealing, but if the joint is to be in contact with water and particularly if
subjected to hydraulic pressure, effective sealing will be necessary because
of the tendency of the joint to open up as the concrete shrinks. This can best
be carried out by using a water stop. PVC water stop membranes extending
into the concrete equally each side of the joint and welded or glued together
at the ends to form a continuous diaphragm are commonly used.
Contraction joints
A contraction joint is a concrete joint made so that the concrete is free to
shrink away from the joint while all other relative movement across the joint
face is prevented.
As concrete sets, hardens and dries out, it shrinks. If no provision is made to
relieve the drying-shrinkage tensile stresses within the concrete, cracking
will occur when these stresses exceed the tensile strength of the concrete. If
the concrete is completely unrestrained, cracking will not occur, but very
few structures are completely unrestrained.
Contraction joints are most needed in unreinforced concrete structures
because reinforcement considerably increases the tensile strength of
concrete, restrains overall shrinkage movement and prevents the formation
of large shrinkage cracks.
Location of joints
Contraction joints should be located where it can be expected that the
severest concentration of tensile stresses will occur, such as:
Where abrupt changes in cross section occur.
On irregularly shaped floors and slabs (e.g. T, H, L and U shapes), to
divide them into rectangular shapes.
Where structures are weakened by openings.
In long structures such as walls and road pavements, which are not
sufficiently reinforced to prevent the formation of shrinkage cracks.
In large areas of pavement or slab on the ground.
Construction of joints
A vertical plane of weakness is purposely formed in the slab or wall.
Vertical movement is controlled by forming a keyed joint or by using non-
ferrous dowels with one end capped and coated so that they are free to slide.
The bond between new and existing concrete at a contraction joint must be
broken.
Dummy contraction joints
A dummy contraction joint is a plane of weakness built into a structure by
means of a groove, either sawn or formed with a grooving tool.
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This joint functions as a contraction joint by localising shrinkage cracks to
beneath the groove. The irregularity of the crack serves to transfer loads
across the joint and prevents relative movement in the plane of the joint.
Since this type of joint is an alternative to a full depth contraction joint, the
location should be the same as for contraction joints.
Expansion joints
An expansion joint is formed by creating a gap between the two surfaces of
the concrete to allow for expansion. The gap is usually filled with a
compressible filler and all relative movement in the plane of the joint is
prevented.
Expansion joints are generally provided in structures exceeding 30 m length,
in unreinforced or lightly reinforced road pavements and as sliding joints
between a roof slab and a supporting wall.
Prestressed and post stressed concrete
Prestressing
The basic principle of prestressing concrete is very simple. If a material has
little tensile strength it will fracture immediately its own tensile strength is
exceeded, but if such a material is given an initial compression, then, when
load-creating tension is applied, the material will be able to withstand the
force of this load as long as the initial compression is not exceeded. You are
already familiar with the properties of concrete that result in a material of
high compressive strength but low tensile strength. By inserting steel
reinforcing bars of the correct area into a concrete member, and fixed in a
predetermined pattern, ordinary concrete can be given an acceptable amount
of tensile strength. Prestressing techniques are applied to concrete in an
endeavour to make full use of the material’s high compressive strength.
Tendons of strand can be used singly or in groups to form a multi-strand
cable. The two major advantages of using strand are:
providing a large prestressing force in a restricted area
the production of long flexible lengths that can be stored in drums
thus saving site space and reducing site labour requirements by
eliminating site fabrication self help exercise.
A prestressing force inducing precompression into a concrete member can
be achieved by anchoring a suitable tendon at one end of the member and
applying an extension force at the other end which can be anchored when
the desired extension has been reached. Upon release, the anchored tendon,
in trying to regain its original length, will induce a compressive force into
the member. Figure 8 shows a typical arrangement in which the tendon
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including the compressive force is acting about the neutral axis and is
stressed so that it will cancel out the tension induced by the imposed load
W. The stress diagrams show that the combined or final stress will result in
a compressive stress in the upper fibres equal to twice that of the imposed
load. The final stress must not exceed the characteristic strength of the
concrete as recommended and if the arrangement given in the figure is
adopted the stress induced by the imposed load will only be half its
maximum.
Figure 11 - Prestressing principles
To obtain a better economic balance the arrangement shown in figure y is
normally adopted where the stressing tendon is placed within the lower third
of the section. The basic aim is to select a stress that, when combined with
the dead load, will result in a compressive stress in the lower fibres equal to
the characteristic strength of the concrete and a zero stress in the bottom
fibres. Note, however, that this is the pure theoretical case and is almost
impossible to achieve in practice, but provided any induced tension
occurring in the lower fibres is not in excess of the tensile strength of the
concrete used, and acceptable condition will exit.
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Figure 12 - Alternative prestressing arrangement
Post-stressing
Concrete is cast around ducts in which the stressing tendons can be housed.
The stressing is carried out later. When the stress required has been
reached, the tendons are anchored at their ends to prevent them returning to
their original length thus inducing the compressive force. The anchors used
form part of the finished component. The ducts for housing the stressing
tendons can be formed by using flexible steel tubing or inflatable rubber
tubes. The void created by the duct will enable the stressing cables to be
threaded prior to placing the concrete, or they can be positioned after the
casting and curing of the concrete has been completed. In both cases the
remaining space within the duct should be filled with grout to stop any
moisture present setting up a corrosive action and to assist in stress
distribution. A typical arrangement is shown in Figure 10.
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Figure 13 - Post-stressing principles
Poststressing is the method usually employed where:
stressing is to be carried out on site
curved tendons are required
the complete member is to be formed by joining together a series of
precast concrete units
where negative bending moments are encountered
Figure 11 shows various methods of overcoming negative bending moments
at fixed ends and for continuous spans.
Figure 14 - Overcoming negative bending moments by using post-stressing
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Figure 12 shows a typical example of the use of curved tendons in the cross
members of a girder bridge. Another application of post-tensioning is in the
installation of ground anchors.
Figure 15 - Structural uses of prestressed concrete
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Finishing concrete
Initial finishing Immediately after placing and vibrating a screed is used to quickly level the
concrete. The screed board is moved forward with a sawing motion, and
concrete shovelled up to and away from the front of the screed as necessary.
After initial screed the area should be checked for level and adjusted where
necessary. Overworking the surface should be avoided.
Final finishing Edging, jointing, floating, trowelling and brooming should be delayed as
long as possible, within reason, before final set. The correct timing is
determined by a variety of factors such as concrete temperature and age,
type of cement, admixture type, and quantities of water, cement and
admixtures used. Weather conditions, depth of pour, type of aggregate, type
of substrate and the like also influence the time for final finishing.
Excessive surface moisture: Cement should not be used to dry up surface
moisture as this will cause surface cracking later. Instead mopping or
dragging with hessian are preferable.
Dry and windy conditions resulting in cracking: accelerated evaporation
due to hot windy weather can result in setting that is too rapid for
satisfactory finishing, and even surface cracking. Due to the amount of time
it takes to finish concrete, and the impact adverse weather can have,
typically builders pour concrete slabs early in the morning. RC concrete
piers on the other hand are often poured in the afternoon where such issues
are less critical.
Floating After necessary delay the surface is floated with a wood float. This smooths
irregularities in the surface following screeding, by pushing large aggregate
below the surface and removing imperfections.
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Concrete finish class
AS3610:2010 Formwork for concrete at Table 3.2.1 sets out the applicable
surface classes for finished concrete. These concrete classes are often
referred to in specifications, and must be achieved by builders.
Class 1 – is the highest class that is recommended for use in special features
of buildings of a monumental nature.
Class 2 – has a consistently good quality that is intended to be viewed in
detail.
Class 3 – has good visual quality that is intended to be viewed as a whole.
Class 4 – had good general alignment and where texture is not important.
Class 5 – where alignment and texture are not important.
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Activity 1
In this activity you will inspect the concrete finish of a number of
nearby reinforced concrete structures. Find three different reinforced
concrete structures which have some visible concrete elements, such
as slabs columns or beams
1. Determine the concrete class achieved for each of the visible
concrete elements, using the class groups from AS3610
Table 3.2.1. For example inspect the finish in a reinforced
concrete car park in a big shopping centre, to that in the
columns or façade in an office building, to that in block of
units. Which has the highest standard of finish, and what is
its corresponding finish class?
2. AS3610 section 5.2.1 states that the surface appearance of
concrete should be evaluated by assessment of the extent of
blowholes, grout loss, honey combing and surface treatment
with a viewing distance of at least 2m or more if that is the
items normal viewing distance. The standard gives other
measures for undulations, flatness, out of plumb, stepping
and whether out of plumb. Now reconsider your evaluation
of the concrete finish of inspected concrete above based on
these characteristics. Would you make any changes to your
classification?
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Activity 2
Obtain copies of the following documents and review in relation to
topics covered.
www.concrete.net.au/publications/pdf/Long-span%20Floors.pdf
http://www.rta.nsw.gov.au/doingbusinesswithus/downloads/contractor-
ohs/tipsheets_dl1.html and obtain Formwork PDF
www.construct.org.uk/bpg/BPGEarly_Striking.pdf
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Summary
Concrete is a composite material, comprised of Portland cement and water
(known as the paste) and aggregate. Aggregate occupies approximately 75
per cent of the volume of the concrete while the paste and voids occupy the
remainder. General purpose (type GP) is the most commonly used cement in
the building industry.
Water and aggregates used in concrete should be free of any deleterious
materials, and aggregates should also be hard and durable.
Compressive strength is the common criteria of concrete quality and is
dependent on the water/cement ratio. Concrete is tested on site for
consistency (the slump test) and off site, following strict curing procedures,
to determine the compressive strengths at 28 days (the compression test).
In residential building, concrete is delivered to the site ‘ready mixed’ in
nearly all cases except where only a small quantity is required and then will
usually be mixed on site using bags of premixed cement and aggregate.
Good practice for the transport and placing of concrete must be followed to
ensure a strong, dense and watertight product. It must be properly cured to
allow an increase in strength with age. The first seven days are particularly
important in allowing the chemical process of hydration to proceed
unheeded.
Reinforced concrete combines steel and concrete, making use of the best
properties of both materials to produce a product used universally on
virtually all building projects. The tensile strength of the steel is combined
with the compressive strength of concrete as a building material. This
strength, combined with its ability to assume any desired shape and its
resistance to fire, makes concrete a very valuable and adaptable material for
the building industry.