Concrete - a composition material that consist of essentially of binding medium which are embedded
particles or fragments of aggregates in hydraulic cement concrete.
Fundamentals of Concrete
l. Aggregates- composed of sand and gravel or crushed stone.
2. Paste- comprises of Portland cement and water that binds the aggregates.
Composition of Paste are:
1. Portland Cement
2. Water
3. Entrapped Air
Quality of Concrete- it depends to a great extent upon the quality of the paste. In proper made concrete,
each particle of aggregates is completely coated with paste and all the spaces between aggregate
particles are completely filled with paste.
For a given materials and condition of curing, the quality of the hardened concrete is determined by the
amount of water used in relation to the amount of cement.
Some advantages of reducing water content are:
1.
increase in compressive and flexural strength
2.
Increase in water tightness
3.
Lower absorption
4. Resistance to weathering
5. Better bond between successive layer
6. Better bond between concrete from wetting and drying
7.
Less volume changes
•
The less water used, the better the quality of concrete provided that can be compacted properly,
Five (5) Basic Components of Concrete are:
1. Cement
2. Water
3. Air
4. Fine aggregates (sand)
5. Coarse aggregates
Engineering Properties of Concrete
•
Setting and hardening
•
Strength
•
Dimension stability
Physical Condition of Concrete
*Concrete must continue to hold enough moisture throughout the curing period in order for the Portland
cement to hydrate.
• Freshly cast concrete has abundance of water, but as drying progresses from the surface inward, strength
gain will cease at each depth as the relative humidity there drops below 80%.
• When concrete dries, it shrinks just as wood, proper and clay. Drying shrinkage is a primary cause of
cracking and the width of cracks is a function of the degree of drying.
• Hardened concrete- also relate to its moisture content, the included elasticity, creep, fire resistance,
abrasion resistance and durability.
Properties of Concrete
1.
Water tightness- when concrete is exposed to weather or other severe exposure condition, it should
be watertight. Test shows that water tightness of paste depends primarily on the ratio of mixing
water to cement and the length of the moist-curing period.
2.
Abrasion resistance- concrete must have a high abrasion resistance, strong concrete resists
abrasion more than a weak concrete.
3. Durability (volume stability)- hardened concrete changes volume slightly due in change of
temperature, moisture content and sustained stress. The volume or the length changes may be
range from about 0.01% to 0.08%. Thermal volume changes of hardened concrete are approximately
the same as those for steel.
SEVERAL FACTORS OF SHRINKAGE
l. Amount of mixing water and aggregates
2. Properties of aggregates
3. Size of specimen
4. Relative humidity and temperature
5. Method of curing
6. Degree of hydration
7. Time
Special Types of Concrete
1. Structural Lightweight Concrete- is concrete made of light weight aggregates that have a density in the
range of 90 to 1 15 #s/cubic foot (1140 to 1850 kg/m3).
-Due to its lower aggregate density, lightweight concrete does not slump as much as normal weight
concrete that has the same workability.
2. Lightweight Insulating Concrete- are used principally for roofs, firewalls, floodwalls, floor wills and
underground thermal conduits lining.
3. Heavy Weight Concrete- are produced with special heavy aggregates and have densities of up to
400#s/ft3 (6400 kg/m3).
-Is used principally for radiation shielding but is also used for counterweight and other applications where
high density is important. As shielding material, heavyweight concrete protect against the harmful effects of
x-rays, gamma rays and neutron radiation.
4. White concrete- is used to produce white concrete.
-Is made with aggregates and water that contain no material that discolour the concrete.
5. Colored Concrete- can be produced by using colored aggregate or by adding color pigments or both.
*High-performance concrete- is a relatively new term used to describe concrete that conforms to a
set of standards above those of the most common applications, but not limited to strength. While all
high strength concrete is also high-performance, not all high-performance concrete is highstrength.
*Roller-compacted concrete- sometimes called rollcrete, is a low-cement-content stiff concrete
placed using techniques borrowed from earthmoving and paving work. The concrete is placed on
the surface to be covered, and is compacted in place using large heavy rollers typically used in
earthwork. The concrete mix achieves a high density and cures over time into a strong monolithic
block.
• Glass concrete- Recent research findings have shown that concrete made with recycled glass
aggregates have shown better long term strength and better thermal insulation due to its better
thermal properties of the glass aggregates.
*Polymer concrete- is concrete which uses polymers to bind the aggregate. Polymer concrete can
gain a lot of strength in a short amount of time. For example, a polymer mix may reach 5000 psi in
only four hours. Polymer concrete is generally more expensive than conventional concretes.
Composition of Concrete
• Cement
Portland cement is the most common type of cement in general usage. It is a basic ingredient of
concrete, mortar, and plaster.
• Water
•
Combining water with a cementitious material forms a cement paste by the process of
hydration. The cement paste glues the aggregate together, fills voids within it, and allows it to flow
more freely. Less water in the cement paste will yield a stronger, more durable concrete; more water
will give an freer-flowing concrete with a higher slump
•
Impure water used to make concrete can cause problems when setting or in causing
premature failure of the structure.
•
Hydration involves many different reactions, often occurring at the same time. As the
reactions proceed, the products of the cement hydration process gradually bond together the
individual sand and gravel particles, and other components of the concrete, to form a solid mass.
Aggregates
Fine and coarse aggregates make up the bulk of a concrete mixture. Sand, natural gravel and
crushed stone are mainly used for this purpose. Recycled aggregates (from construction, demolition
and excavation waste) are increasingly used as partial replacements of natural aggregates, while a
number of manufactured aggregates, including air-cooled blast furnace slag and bottom ash are
also permitted.
6. No-Slump Concrete- refers to concrete with a consistency corresponding to a slump of I in. (25mm.) or
less It is used in the precasting industry for fabrication of pipes, hollow- core slabs, railroad ties, concrete
blocks and concrete bricks.
Types of Concrete and their Weight
l. Lightweight Concrete
-Is classified into 3 types depending upon the kind of aggregates used which predetermines
their weight.
❖ Low Density Concrete
-For insulation purposes with the unit weight rarely exceeding 50 pounds/ft3 or 80kg/m3
❖ Moderate-Strength Concrete
-has a unit weight from 360 to 960 kg/m with a compressive strength of 70 to 120 kg/m3 and
is usually used to fill over light gauge steel floor panels.
❖ Structural Concrete
-Somewhat the same characteristics with that of medium stone concrete and weighs from
90 to 120 #s/ft3 or 1440 to 1920 kg/m3
2. Medium Stone Concrete
Is also known as structural concrete weighing from 145 to 154 #s/ft3 generally assumed to be 1 50 #s/ft3 or
330 kg/m3.
3. Heavy Weight Concrete
-Is used as a shield against gamma rays and other similar structure. It is also used as counterweight for a
lift bridge. The contents of heavy weight concrete are cement, heavy iron ores, crushed rock, steel craps,
punching or fine shot aggregates.
Proportioning of Concrete Mixture
The right proportioning of the ingredients of concrete provides as balance between the requirements of:
❖ Economy
❖ Workability
❖ Strength
❖ Durability
❖ Appearance
Two Methods Adopted in Proportioning
Concrete Mixtures
1.
By volume, 1:2:4 (one part cement, two parts aggregates and four parts coarse aggregates)
2.
By weight measure
Various Tests Conducted for concrete are:
1. Slump Test- this method requires a fabricated metal.
2. Compressive Test- is the process applied in determining the strength of concrete.
Three (3) Basic Types of Joints Used in Concrete Construction
1. Control Joints
-Are the most effective method of preventing unsightly cracking.
-Are grooved, scoud, or sawed in sidewalks, driveways, pavements and floors so that any cracking will occur
in this joints.
-Permit horizontal movement in the plane of the slab; they are cut to a depth of approximately one quarter
the slab thickness or a minimum of one-fifth the thickness.
2. Isolation Joints
-Separate a slab from other parts of a structure and permit horizontal and vertical movement of the slab.
They are placed at the junction of floor with walls, columns, footing and other joints where restraint can
occur. They extend the full depth of the slab and include a pre molded joint filler.
3. Construction Joint
-Occur where concreting work is included for the day; they separate areas of concrete at different times.
Workability
The case of placing and consolidating freshly made concrete
Workability can also describe as:
I. Consistency Is the degree of wetness or slump of a concrete mix It varies directly with the amount of
water in the mix,
2. Plasticity Is the fresh concrete can be molded or deformed without segregation
3. Mobility Is the capacity of the concrete for movement of flow particularly during vibration
Workability of Concrete
Concrete is said to be workable under the following considerable.
l. Proportioned for transport and placed without segregation The aggregate particles must be uniformly
distributed.
2. Easily molded into desired shapes and completely fill the space it is to occupy.
3. Easily finished
Concrete should be proportional to produce workability required for a particular structure, For
example, a fairly thick or stiff concrete mixture may be used for pavement because the concrete can be
vibrated and tempered
Concrete for thin wall and a small column structure may be compacted with a minimum vibration. A
semi-fluid mixture is required for concrete in applications where it must now in order to fill all the space it is
to occupy.
Durability of Concrete
Durability- is the ability to resist the forces of deterioration.
The forces that cause deterioration are:
❖ Freezing the thawing water saturated concrete
❖ Expansion caused by the reaction between reactive aggregates and alkali cement
❖ Reaction between soil and water sulphate and the hydrated Portland cement.
❖ Expansion and shrinkage caused by wetting and drying
Things to avoid in placing concrete to its final form.
❖ Segregation of particles
❖ Displacement of forms
❖ Displacement of reinforcement in the form
❖ Poor bond between successive layers of concrete.
Factors that regulate the strength of concrete
❖ Correct Proportion
❖ Suitable or quality of the materials
❖ Proper methods of mixing
❖ Proper placement or depositing of concrete inside the forms
❖ Adequate protection of concrete during the period of curing
Curing of Concrete
The protection of concrete from loss of surface moisture is 7 days when ordinary Portland cement is used
and 3 days for an early high strength Portland cement:
The methods applied in curing surface concrete are:
❖ Covering of the surface with burlap continuously wet for the required period
❖ Covering of the slab with a layer of wet sand or saw dust I in. or 25mm. thick
❖ Wet straw or layer on top of the slab continuously wet
❖ Continuously sprinkling of water on the slab surface.
❖ Avoid early removal of forms. This well permit under evaporation of moisture in the concrete.
The objectives of curing are:
❖ To prevent (or replenish) the loss of moisture.
❖ To control the concrete temperature for a definite time.
Curing- has a strong influence on properties of hardened concrete such as durability, strength, waterthickness, wear resistance, volume stability and resistance to freezing and thawing.
With proper curing, the concrete will become stronger and more resistant to stress, abrasion and
frost. The improvement is rapid at early ages but continuous more slowly for an indefinite period.
Shrinkage- to contract due to cold or heat.
Concrete keep continually moist will expand slightly, when permitted to dry concrete will shrink.
Concrete under stress will deform elastically. Sustained stress will result in additive deformation called
creep.
Two Basic Causes of Crack in Concrete are:
• Stress due to applied loads
• Stress due to drying shrinkage or temperature change
Concrete shrinkage cracks occur because of restrain. When shrinkage occurs and there is no
restrain, the concrete does not crack.
Thermal Stress- due to fluctuation in temperature can cause cracking.
Restrains come from several sources
❖ Drying shrinkage is always greater near the surface of the concrete so the moist inner portions
restrain.
❖ Other sources of the restrain are reinforcing steel embedded in concrete
❖ The interconnected parts of the concrete structure
❖ The friction of the sub grade on which a concrete slab or wall is placed
Mixing Concrete:
2 Ways in Mixing Concrete:
1. Site job mixing — shall be done in a batch mixer or approved type. The mixer shall be rotated at speed
recommended type. The mixer shall be rotated at speed recommended by the manufacturer and mixing
shall be continued for at least 1 1/2 minimum.
2. Ready mixed concrete — quantity for numerous special purposes can be ordered directly from the ready
mix concrete producer. The desired type and quality of concrete is delivered in the project site very rapidly.
The production of various concrete mixes is programmed by an electric computer and batches of concrete
for any desired sizes are proportioned automatically by electric control. All ingredients are measured by
weight.
A ready-mix concrete is mixed either:
❖ Mobile Mixer— a batch of concrete is place in a mobile mixer at the plant. The mixing take place from
the time mixer leaves the plant until it reaches the job site.
❖ Stationary Mixer — the concrete is mixed before it is placed in the truck mixer, where the concrete is
only agitated.
❖ Hand mixing — a good concrete can be produced by hand mixing.
Admixture of Concrete
Admixture — are those ingredients in concrete other than Portland cements, water and aggregates that are
added to the mixture before and during mixing.
Classification of Admixture by Function:
1. Air-entraining admixture — are used to entrain microscopic air bubbles. Ex. Alkyl benzene sulphate,
polyethylene oxideoxide, detergents or salts of salty acid.
2. Reducing admixture — used to reduce the quality of mixing water required to produce concrete of a
given consistency or increases the slump of the concrete for given water content. Water reducing admixture
also reduced the setting time of concrete.
3. Retarding admixture — used to retard the rate of setting of concrete. Retarders do not decrease the
initial temperature of concrete.
Uses of Retarders in Concrete
• Offset the accelerating affect the hot weather on the setting of concrete.
• Delay the initial set of concrete or grout when difficult or unusual condition placement occur, such as
placing concrete in large pieces and foundations. Cementing oil well or pumping grout.
• It also acts as water reducing
*It may also entrain some air in concrete.
4. Accelerating admixture — is used to accelerate strength development of concrete at early age.
The common accelerators cause an increase in the drying shrinkage of concrete. The strength
development of concrete can be accelerated by:
❖ Using type Ill high-early-strength Portland Cement
❖ Lowering the water cement ratio by increasing the cement content
❖ Curing at higher temperature.
calcium Chloride - is the active materials or most commonly used as an accelerating admixture.
5. Pozzolans — a siliceous and aluminous material.
Uses of Pozzolans:
❖ Sometimes use in concrete to help reduce external temperature
❖ To reduce or eliminate potential expansion from the alkali reactive aggregate
❖ To counter expansion
❖ To improve the sulphate resistance of concrete
6. Workability agent — is entrained air. It acts as a lubricant and is especially effective in
proving the workability of mixture.
Different types of agents:
❖ Bonding admixtures — are usually water emulsion of several organic materials including rubber,
polyvinyl chloride, polyvinyl acetate, acrylics, and butadiene.
❖ -Non-re-emulsifiable types are resistant to water, better suited to exterior application and used in
place where moisture is present.
❖ Grouting agents- are used in a variety of purposes, to stabilize foundation, full crack and joints in
concrete work, cement oil wells, fill core of masonry walls, grout tendons and anchors, bolts and prep
laced aggregate. To alter the properties of grout for specific application, various air-entraining
admixtures, accelerating, retarders and workability agents are often used.
❖ Damp Proofing and Permeability Reducing agents — include in certain soaps, stearates and
petroleum products. They may reduce the permeability of concrete that are low cement content,
high water, cement ratio. Sometimes use to reduce the transmission of moisture through concrete
that is in contact with soap or damp earth.
❖ Gas-forming agent — aluminium powder and other gas-forming materials are sometimes added to
concrete and grout in very small quantity to cause a slight expansion prior to hardening.
Cement
❖ is powder and is one of the main ingredients in concrete
❖ cement and concrete have been used in construction since at least the Roman Empire
❖ modern cement is made of limestone, silicon, calcium, and often aluminum and iron
❖ the type of cement used in almost all concrete is Portland cement. Portland cement has been
around since 1824. The name Portland does not refer to a brand name, as many might think. The
original inventor, Joseph Aspdin, was a British bricklayer and named his new invention "Portland"
because its color reminded him of the color of the natural limestone on the Isle of Portland which is a
peninsula in the English Channel
Type of Cement
1.
Portland cement- is a particular type of hydraulic cement. Portland cement contains hydraulic
calcium silicates. There are eight specific types of Portland cement that fall into categories ranging
from Type I to Type V.
• Type I and Type IA are general purpose cements. Type Il and Type IIA contain tricalcium aluminate, but no
more than 8%. To compare to the hydraulic cement types, some of the Type Il cements meeting the
standard for the moderate heat of hydration type.
• Type Ill and Type IllA are similar to Type I cements. However, they have higher early strengths because they
are ground finer. Type IV cements are used in special types of structures that require a small amount of
heat to be generated from hydration. Type IV cements develop their strength over a longer period of time
when compared to other types. Finally, Type V cement has a high sulfate resistance which means it
contains no more than 5% tricalcium aluminate.
Type
Name
Purpose
I
Normal
General-purpose cement suitable for most purposes.
IA
Normal-Air Entraining
An air-entraining modification of Type l.
II
Moderate Sulfate Resistance
Used as a precaution against moderate sulfate attack. It
will usually generate less heat at a slower rate than Type I
cement.
IIA
Moderate Sulfate Resistance- Air
An air-entraining modification of Type ll.
Entraining
III
High Early Strength
Used when high early strength is needed. It is has more
CJS than Type I cement and has been ground finer to
provide a higher surface-to-volume ratio, both Of which
speed hydration. Strength gain is double that of Type I
cement in the first 24 hours.
IIIA
High Early Strength-Air Entraining
An air-entraining modification of Type Ill.
IV
Low Heat of Hydration
Used when hydration heat must be minimized in large
volume applications such as gravity dams. Contains
about half the C3S and CYA and double the CS of Type I
Cement.
V
High Sulfate Resistance
Used as a precaution against severe sulfate action principally where soils Or groundwaters have a high
sulfate content. It gains strength at a slower rate than
Type I cement. High sulfate resistance is attributable to
low C3A content.
Blended Cement- is also hydraulic cement and is made by mixing two or more materials. Usually, the
primary materials used in blended cement are Portland cement and slag cement. Fly ash, silica fume,
calcined clay, pozzolan, and hydrated lime are also used.
There are two main types of blended cement:
Type IS (X): Portland blast furnace slag cement
Type IP (X): Portland-pozzolan cement
The X represents the amount of the second material that is in the mixture.
Physical Properties of Portland Cement
❖ Fineness - or particle size of Portland cement affects hydration rate and thus the rate of strength
gain. The smaller the particle size, the greater the surface area-to-volume ratio, and thus, the more
area available for water-cement interaction per unit volume. The effects of greater fineness on
strength are generally seen during the first seven days (PCA, 1988).
❖ Soundness - When referring to Portland cement, "soundness" refers to the ability of a hardened
cement paste to retain its volume after setting without delayed destructive expansion (PCA, 1988).
This destructive expansion is caused by excessive amounts of free lime (CaO) or magnesia (MgO).
Most Portland cement specifications limit magnesia content and expansion. The typical expansion
test places a small sample of cement paste into an autoclave (a high pressure steam vessel). The
autoclave is slowly brought to 2.03 MPa (295 psi) then kept at that pressure for 3 hours. The
autoclave is then slowly brought back to room temperature and atmospheric pressure. The change
in specimen length due to its time in the autoclave is measured and reported as a percentage. ASTM
C 150, Standard Specification for Portland Cement specifies a maximum autoclave expansion of 0.80
percent for all Portland cement types
Setting Time cement paste setting time is affected by a number of items including: cement fineness, watercement ratio, chemical content (especially gypsum content) and admixtures. Setting tests are used to
characterize how a particular cement paste sets. For construction purposes, the initial set must not be too
soon and the final set must not be too late. Additionally, setting times can give some indication of whether
or not a cement is undergoing normal hydration (PCA, 1988).
• Strength - cement paste strength is typically defined in three ways: compressive, tensile and flexural.
These strengths can be affected by a number of items including: water- cement ratio, cement-fine
aggregate ratio, type and grading of fine aggregate, manner of mixing and molding specimens, curing
conditions, size and shape of specimen, moisture content at time of test, loading conditions and age. Since
cement gains strength over time, the time at which astrength test is to be conducted must be specified.
Typically times are 1 day (for high early strength cement), 3 days, 7 days, 28 days and 90 days (for low heat
of hydration cement).
• Specific Gravity Test- is normally used in mixture proportioning calculations, The specific gravity of
Portland cement is generally around 3.15 while the specific gravity of Portland-blast-furnace-slag and
Portland- pozzolan cements may have specific gravities near 2.90 (PCA, 1988).
• Heat of Hydration- is the heat generated when water and Portland cement react. Heat of hydration is
most influenced by the proportion of C3S and C3A in the cement, but is also influenced by water-cement
ratio, fineness and curing temperature. As each one of these factors is increased, heat of hydration
increases. In large mass concrete structures such as gravity dams, hydration heat is produced significantly
faster than it can be dissipated (especially in the center of large concrete masses), which can create high
temperatures in the center of these large concrete masses that, in turn, may cause undesirable stresses as
the concrete cools to ambient temperature.
2. Non-Portland hydraulic cements
o
Pozzolan-lime cements- mixtures of ground pozzolan and lime are the cements used by the
Romans, and can be found in Roman structures still standing (e.g. the Pantheon in Rome).
They develop strength slowly, but their ultimate strength can be very high. The hydration
products that produce strength are essentially the same as those produced by Portland
cement.
o
Slag-lime cements- ground granulated blast furnace slag is not hydraulic on its own, but is
"activated" by addition of alkalis, most economically using lime. They are similar to pozzolan
lime cements in their properties. Only granulated slag (i.e. water-quenched, glassy slag) is
effective as a cement component.
o
Super sulfated cements- these contain about 80% ground granulated blast furnace slag, 15
% gypsum or anhydrite and a little Portland clinker or lime as an activator. They produce
strength by formation of ettringite, with strength growth similar to a slow Portland cement.
They exhibit good resistance to aggressive agents, including sulfate.
o
Calcium aluminate cements- are hydraulic cements made primarily from limestone and
bauxite. The active ingredients are monocalcium aluminate CaA1204 (CaO • A1203 or CA in
Cement chemist notation, CCN) and mayenite Ca12A114033 (12 CaO • 7 A1203 , or C12A7 in
CCN). Strength forms by hydration to calcium aluminate hydrates. They are well-adapted for
use in refractory (high-temperature resistant) concretes, e.g for furnace
o
Calcium sulfoaluminate cements -are made from clinkers that include ye'elimite or S in
Cement chemist's notation) as a primary phase. They are used in expansive cements, in
ultra-high early strength cements, and in "low-energy" cements. Hydration produces
ettringite, and specialized physical properties (such as expansion or rapid reaction) are
obtained by adjustment of the availability of calcium and sulfate ions. Their use as a lowenergy alternative to Portland cement has been pioneered in China, where several million
tons per year are produced. Energy requirements are lower because of the lower kiln
temperatures required for reaction, and the lower amount of limestone in the mix. In addition,
the lower limestone content and lower fuel consumption leads to a C02 emission around half
that associated with Portland clinker. However, S02 emissions are usually significantly higher.
o
Natural cements- correspond to certain cements of the pre-Portland era, produced by
burning argillaceous limestones at moderate temperatures. The level of clay components in
the limestone (around 30-35 %) is such that large amounts of belite (the low-early strength,
high-late strength mineral in Portland cement) are formed without the formation of excessive
amounts of free lime. As with any natural material, such cements have highly variable
properties.
Special Cements
o
White Portland Cement- is similar in all respects to normal Portland except in color. It is made from
specially selected raw materials containing negligible amounts of iron and manganese oxide, and
the manufacturing process is controlled to produce a pure white, non staining cement. It is used
primarily for architectural purposes such as curtain-wall and facing panels, decorative concrete,
stucco, tile grout or wherever white or colored concrete or mortar is specified.
o
Masonry Cement- has been specially designed to produce better mortar than that made with
normal Portland Cement or with a lime-cement combination. It is made by grinding together a
carefully proportioned mixture of normal Portland cement clinker and high- calcium limestone.
o
Air-Entraining Portland Cement- sometimes small amount of certain air-entraining agents are
added to the clinker and ground with to produce air-entraining cements. Concrete made with them
contains millions of minute well-distributed and completely separated air bubbles. It has proved to
be more resistant to severe frost and to the effects of salt applied to sidewalks and pavements for
ice and snow removal.
o
Oil- Well Cement — this is a special Portland Cement used for sealing oils wells. It must be slowsetting and resistant to high temperatures and pressures. There are specifications for oil-well
cements (API Standard IOA) which cover requirements for six classes of cement, each applicable for
use at a certain range of depths.
o
Waterproofed Portland Cement- is normally produced by adding a small amount of a stearate,
usually calcium or aluminum, the cement clinker during the final grinding. It is made in both white
and gray color.
Uses of Cement
✓
Strengthened with iron bars, or meshed wire, placed in it when it is being molded to shape, it is
known as re-enforced concrete, and will thus form bridge floors, bridge spans, and the upper floors
of buildings which must support great weight.
✓
In marine use, concrete is limited because of its weight, It may be used as permanent ballast in the
bilges of steel ships, and is an effective protection from corrosion when applied to absolutely clean
iron or to iron surfaces covered with closely adhering red rust. When so used, cement may be mixed
with water and applied with a brush, or it may be mixed in the proportion of about two pans sand
and one part cement and applied wet, with a trowel, in a layer varying from 1/4 inch to any thickness
desired. In this way ships' tanks, bunkers, and bilges are protected, as the mixture forms a close bond
with the iron. In no case will this bond form if the iron is oil coated.
Aggregates
✓
is a broad category of coarse particulate material used in construction, including sand, gravel,
crushed stone, slag, recycled concrete and geosynthetic aggregates
✓
are a component of composite materials such as concrete and asphalt concrete
✓
serves as reinforcement to add strength to the overall composite material
✓
due to the relatively high hydraulic conductivity value as compared to most soils, aggregates are
widely used in drainage applications such as foundation and french drains, septic drain fields,
retaining wall drains, and road side edge drains
✓
are also used as base material under foundations, roads, and railroads.
Types of Aggregates
• Crumb. Porous aggregates with more or less spheroidal shapes.
• In angular blocks. Aggregates formed Of two more or less flat faces, which when cut form edges and
these lead to vertices. In short, their shape is similar to irregular geometrical polyhedrons. The faces of the
aggregates fit well with the faces of their adjacent aggregates.
• In sub-angular blocks. Similar to the above, but the blocks are less defined. The faces are not as flat, the
edges are blunt and there are hardly any vertices. Neither do the aggregates fit so well in the microstructure
of angular blocks.
• Prismatic. Angular blocks, like a prism, in which the vertical dimension predominates with regard to the
other two. They are normally too large to be able to observe them in a microscope.
• Platy. Aggregates with a leafy shape, in which the vertical dimension is much shorter than the other two.
Admixtures are those ingredients in concrete other than portland cement, water, and aggregates that are
added to the mixture immediately before or during mixing.
Classification of Admixture by Function
✓
Water-reducing admixtures usually reduce the required water content for a concrete mixture by
about 5 to 10 percent. Consequently, concrete containing a water-reducing admixture needs less
water to reach a required slump than untreated concrete. The treated concrete can have a lower
water-cement ratio. This usually indicates that a higher strength concrete can be produced without
increasing the amount of cement. Recent advancements in admixture technology have led to the
development of mid-range water reducers. These admixtures reduce water content by at least 8
percent and tend to be more stable over a wider range of temperatures. Mid-range water reducers
provide more consistent setting times than standard water reducers.
✓
Retarding admixtures, which slow the setting rate of concrete, are used to counteract the
accelerating effect of hot weather on concrete setting. High temperatures often cause an increased
rate of hardening which makes placing and finishing difficult. Retarders keep concrete workable
during placement and delay the initial set of concrete. Most retarders also function as water
reducers and may entrain some air in concrete.
✓
Accelerating admixtures increase the rate of early strength development, reduce the time required
for proper curing and protection, and speed up the start of finishing operations. Accelerating
admixtures are especially useful for modifying the properties of concrete in cold weather.
✓
Superplasticizers, also known as plasticizers or high-range water reducers (HRWR), reduce water
content by 12 to 30 percent and can be added to concrete with a low-to-normal slump and watercement ratio to make high-slump flowing concrete. Flowing concrete is a highly fluid but workable
concrete that can be placed with little or no vibration or compaction. The effect of superplasticizers
lasts only 30 to 60 minutes, depending on the brand and dosage rate, and is followed by a rapid loss
in workability. As a result of the slump loss, superplasticizers are usually added to concrete at the
jobsite.
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Corrosion-inhibiting admixtures fall into the specialty admixture category and are used to slow
corrosion of reinforcing steel in concrete. Corrosion inhibitors can be used as a defensive strategy for
concrete structures, such as marine facilities, highway bridges, and parking garages, that will be
exposed to high concentrations of chloride. Other specialty admixtures include shrinkage-reducing
admixtures and alkali-silica reactivity inhibitors. The shrinkage reducers are used to control drying
shrinkage and minimize cracking, while ASR inhibitors control durability problems associated with
alkali-silica reactivity.
Types of Admixtures
1.Retarding admixtures - slow down the hydration of cement, lengthening set time. Retarders are
beneficially used in hot weather conditions in order to overcome accelerating effects of higher
temperatures and large masses of concrete on concrete setting time. Because most retarders also act as
water reducers, they are frequently called water-reducing retarders.
2. Accelerating admixtures- Accelerators shorten the set time of concrete, allowing a cold- weather pour,
early removal of forms, early surface finishing, and in some cases, early load application. Proper care must
be taken while choosing the type and proportion of accelerators, as under most conditions, commonly used
accelerators cause an increase in the drying shrinkage of concrete.
3. Super plasticizers - also known as plasticizers, include water-reducing admixtures. Compared to what is
commonly referred to as a "water reducer" or "mid-range water reducer", superplasticizers are "high-range
water reducers". High range water reducers are admixtures that allow large water reduction or greater flow
ability (as defined by the manufacturers, concrete suppliers and industry standards) without substantially
slowing set time or increasing air entrainment.
Each type of super plasticizer has defined ranges for the required quantities of concrete mix ingredients,
along with the corresponding effects. They can maintain a specific consistency and workability at a greatly
reduced amount of water. Dosages needed vary by the particular concrete mix and type of super plasticizer
used. They can also produce a high strength concrete. As with most types of admixtures, super plasticizers
can affect other concrete properties as well. The specific effects, however, should be found from the
manufacturer or concrete supplier.
4. Water reducing admixtures - require less water to make a concrete of equal slump, or increase the
slump of concrete at the same water content. They can have the side effect of changing initial set time.
Water reducers are mostly used for hot weather concrete placing and to aid pumping. A water-reducer
plasticizer, however, is a hygroscopic powder, which can entrain air into the concrete mix via its effect on
water's surface tension, thereby also, obtaining some of the benefits of air-entrainment.
5. Air-entraining admixtures - air-entraining agents entrain small air bubbles in the concrete. The major
benefit of this is enhanced durability in freeze-thaw cycles, especially relevant in cold climates. While some
strength loss typically accompanies increased air in concrete, it generally can be overcome by reducing
the water-cement ratio via improved workability (due to the air- entraining agent itself) or through the use
of other appropriate admixtures. As always, admixtures should only be combined in a concrete mix by a
competent professional because some of them can interact in undesirable ways.
Purpose of Admixture in Concrete
• producers use admixtures primarily to reduce the cost of concrete construction
• to modify the properties of hardened concrete
• to ensure the quality of concrete during mixing, transporting, placing, and curing
• to overcome certain emergencies during concrete operations
• certain admixtures, such as pigments, expansive agents, and pumping aids are used only in extremely
small amounts and are usually batched by hand from premeasured containers
Pre stressing/ Pre stressed concrete
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is a method for overcoming concrete's natural weakness in tension
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It can be used to produce beams, floors or bridges with a longer span than is practical with ordinary
reinforced concrete.
Uses of Concrete
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Concrete has long been used for the foundations of structures of all kinds, and for filling in the span
drills of arches or the hearting and backs of walls.
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Of late years, as the material has improved, it has been employed for many other purposes, a few
only of which can now be mentioned,
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The walls of ordinary houses, as well as the more massive walls of engineering structures, are now
frequently built in concrete, either in continuous mass or in blocks.
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Concrete is also used for walls in the form of slabs fitted into timber quartering: and in hollow blocks,
something like those Of terra cotta, filled in With inferaor material
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This material is also adapted for arches, for stairs, for flooring of different kinds and even for roofs.
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It can easily be made in slabs well fitted for paving and by the use of wooden moulds can readily be
cast in the form of window sills, lintels, dressings of all kinds, steps, etc., and can even be used for
troughs and cisterns.
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Drain pipes and segments of sewers are also sometimes made of concrete. It was thought that the
acids in sewers might act upon the cement, but this has been found practically not to be the case.
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This material has been largely used in making the Paris sewers, and also occasionally in this country.
Construction Joints- during the concrete pour, any break in the work can cause the already poured
concrete to harden enough to prevent new Concrete from bonding adequately. This requires the use of
construction joints, also known as pour joints.
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Reinforcement is placed in the old concrete prior to hardening, which extend into the space where
the new concrete will be poured. This ensures that the entire structure acts as one piece rather than
moving independently over time. Construction joints may also be used to provide additional support
between floor levels or at corners where they will be hidden in the final product. It is important to
understand how to place construction joints, especially at stress points, to ensure that structural
integrity is not compromised.
Three Types of Concrete According to Weight
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Light Weight Concrete
•
Low Density Concrete- is used for insulation purposes. Its unit weight would rarely exceed 50
pounds per cubic foot or 800 kg per cubic meter.
•
Moderate Strength Concrete- has a unit weight of 360 to 960 kg, per cubic meter with a
compressive strength of 70 to 176 kg. per square centimeter commonly used to fill light gauge
steel floor panels.
•
Structural Concrete- has similarity in characteristics with that of medium stone concrete. It
weighs 90 to 120 pounds per cubic foot or 1440 to 1920 kg. per cubic meter used in buildings
roads, bridges, etc.
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Medium Stone Concrete - is an structural concrete. It weighs from 145 to 152 pounds per cubic foot
of 2325 to 2435 kg. per cubic meter.
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Heavy Weight Concrete -is used as shield against gamma rays reactor and other similar structures.
It is also used as counter weight for lift bridges. The contents of heavy weight concrete are cement,
heavy iron ores, crushed rock, steel scraps, punching or shot as fine aggregate. The weight of
heavyweight concrete depends upon the kind of aggregate used in mixing.
concrete
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is a rocklike or stone like material produced by combining coarse and fine aggregates, Portland
cement and water and allowing the mixture to harden in forms of the shape and dimensions of the
desired structures
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the word concrete comes from the Latin word "concretus" meaning compact or condensed, the past
participle of "concresco", from "com-Il (together) and "cresco"
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is the universal material of construction
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in a wide range of properties can be obtained by appropriate adjustment of the proportions of the
constituent materials
Properties of Concrete
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Concrete has relatively high compressive strength, but significantly lower tensile strength, and as
such is usually reinforced with materials that are strong in tension (often steel). The elasticity of
concrete is relatively constant at low stress levels but starts decreasing at higher stress levels as
matrix cracking develops.
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Concrete has a very low coefficient of thermal expansion, and as it matures concrete shrinks. All
concrete structures will crack to some extent, due to shrinkage and tension. Concrete which is
subjected to long-duration forces is prone to creep.
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Tests can be made to ensure the properties of concrete correspond to specifications for the
application.
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The density of concrete varies, but is around 150 pounds per cubic foot (2,400 kg/m3 or 4,050
lb/yd3).
Types of Concrete
• Regular concrete-is the lay term describing concrete that is produced by following the mixing instructions
that are commonly published on packets of cement, typically using sand or other common material as the
aggregate, and often mixed in improvised containers. This concrete can be produced to yield a varying
strength from about 10 MPa (1450 psi) to about 40 MPa (5800 psi), depending on the purpose, ranging from
blinding to structural concrete respectively. Many types of pre-mixed concrete are available which include
powdered cement mixed with an aggregate, needing only water
• High-strength concrete- has a compressive strength generally greater than 6,000 pounds per square
inch (40 MPa = 5800 psi). High-strength concrete is made by lowering the water-cement (WIC) ratio to 0.35
or lower. Often silica fume is added to prevent the formation of free calcium hydroxide crystals in the
cement matrix, which might reduce the strength at the cement-aggregate bond.
• Stamped concrete- is an architectural concrete which has a superior surface finish. After a concrete floor
has been laid, floor hardeners (can be pigmented) are impregnated on the surface and a mold which may
be textured to replicate a stone / brick or even wood is stamped on to give a attractive textured surface
finish. After sufficient hardening the surface is cleaned and generally sealed to give a protection. The wear
resistance of stamped concrete is generally excellent and hence found in applications like parking lots,
pavements, walkways etc.
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Decorative stones such as quartzite, small river stones or crushed glass are sometimes added to the
surface of concrete for a decorative "exposed aggregate" finish, popular among landscape
designers.
The Three Types of Joints
Control Joints -these joints are also called contraction joints, which adequately describes their
purpose. Control joints are strategically placed throughout concrete members or slabs to provide room for
movement due to weather and time, such as temperature changes, shrinkage and deformation.
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The joint is not a complete break in the concrete. Instead, it is a joint that goes one-third of the way
through the concrete. This weakens the surface of the concrete while maintaining the structural
integrity. The idea is that the joint will allow any cracks that may form to occur along the control joint
and not in other areas of the surface.
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Control joints are usually placed partially through the underlying foundation, whether it is a slab, wall
or other, in order to ensure that the concrete retains its structural integrity and water tightness.
Because of the naturally uneven nature of concrete cracks and the steel that is used to reinforce the
joint, a control joint ensures that no movement occurs along the joint as time progresses.
Isolation Joints- also known as expansion joints, isolation joints are similar to control joints except that the
joint goes straight through the concrete. Isolation joints allow the concrete to move along the joint without
compromising the strength of the structure.
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Generally, these joints are positioned at stress points within a concrete slab or at junctures, such as
where walls meet the concrete. When construction involves the use of columns, isolation joints are
necessary at the base of the column where it meets the concrete slab below. As time passes and the
pressure of the structure as well as weathering impacts the column, it will naturally rotate and settle.
The isolation joint endures that the column can move while remaining in place and providing the
required support to the overall structure.