CONCRETE

This is a composite mixture of cement, fine aggregates (sand), coarse aggregates (gravel)
with water in the right proportions. OR. A structural material consisting of hard, chemically
inert particulate substance known as aggregate (usually sand and gravel) that is bonded
together by cement and water. The proportions of each material controls the strength and
quality of concrete.

𝑪𝒐𝒏𝒄𝒓𝒆𝒕𝒆 = 𝐶𝑒𝑚𝑒𝑛𝑡 + 𝐹𝑖𝑛𝑒 𝑎𝑔𝑔𝑟𝑒𝑔𝑎𝑡𝑒 + 𝐶𝑜𝑎𝑟𝑠𝑒 𝑎𝑔𝑔𝑟𝑒𝑔𝑎𝑡𝑒 + 𝑊𝑎𝑡𝑒𝑟

+ 𝐴𝑑𝑚𝑖𝑥𝑡𝑢𝑟𝑒(𝑜𝑝𝑡𝑖𝑛𝑎𝑙)
𝑪𝒐𝒏𝒄𝒓𝒆𝒕𝒆 = 𝐶𝑒𝑚𝑒𝑛𝑡 + 𝐴𝑔𝑔𝑟𝑒𝑔𝑎𝑡𝑒𝑠 + 𝑊𝑎𝑡𝑒𝑟 + 𝐴𝑑𝑚𝑖𝑥𝑡𝑢𝑟𝑒(𝑜𝑝𝑡𝑖𝑜𝑛𝑎𝑙)
𝑪𝒐𝒏𝒄𝒓𝒆𝒕𝒆 = 𝐵𝑖𝑛𝑑𝑖𝑛𝑔 𝑎𝑔𝑒𝑛𝑡 + 𝐴𝑔𝑔𝑟𝑒𝑔𝑎𝑡𝑒𝑠(𝑓𝑖𝑛𝑒&𝑐𝑜𝑎𝑟𝑠𝑒) + 𝑊𝑎𝑡𝑒𝑟
+ 𝐴𝑑𝑚𝑖𝑥𝑡𝑢𝑟𝑒(𝑜𝑝𝑡𝑖𝑜𝑛𝑎𝑙)
In the ever evolving world requiring constantly developing construction materials and
methods, Concrete is one of the most widely used construction materials majorly because of
its high strength, durability and ease to work.

Concrete is made in-situ either on site or in a factory into a workable paste that can be
spread or poured into forms or moulds forming a hard mass resembling stone on hardening
over time. The hardening of concrete is a gradual process and influences its strength in a
process known as curing.
Areas of application/ uses of concrete

 Concrete is utilized in all sorts of buildings from residential to multi-storey workplace

blocks
 It isalso used in the construction other forms of structures like bridges, roads,
tunnels, dams for electricity production, tanks for water storage etc.
 It’s used to construct statues for different purposes
 It’s used to construct walls e.g. Retaining walls, boundary walls etc. based on the
required strength
 It is also used to construct canals, drains etc. for irrigation purposes
Advantages/ benefits of concrete

 It is a relatively cheap material with a relatively long life with few maintenance costs
 It is strong in compression
 Before hardening, it is a very pliable material that can be shaped or moulded into any
shape and size
 It is non-combustible
 It is easy to make as its instantly mixed and materials are locally available
 It is very durable and should be designed to face up to earthquakes, hurricanes,
typhoons and tornadoes
 Has less corrosive and weathering effects due to the environment
Disadvantages/ limitations of concrete

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  It is weak in tension. However this can be solved by adding reinforcement
 when it hardens, it is highly brittle
 it has a low strength to weight ratio
 it is susceptible to shrinkage on drying which in turn leads to cracking
 it requires proper curing till hydration to get required amount of strength
Properties of concrete

The properties of concrete are determined by the nature quantity and quality of the
composition materials or the method used in its production:
Strength
This is perhaps the most widely known and most important property of concrete. It is further
subdivided into; Compressive, Tensile, Flexural and Shear strength with compressive
strength being the most important and best structural property it has.
The strength of concrete greatly depends on the age of the concrete. Over the course of the
first 28days after it has been laid, the concrete will gain 70-75% of its final strength. This
increases to around 90-95% over the course of the first year. Compressive strength of
concrete can be tested using the compressive testing machine where loads are applied to
either concrete cubes or cylinders in the crushing test. With modern technology,the
Rebound hammer is used to determine the strength of concrete.
Durability

This property determines whether the concrete will be able to withstand the conditions for
which it is designed without breaking down over a number of years. Durability of concrete is
affected by the following factors; amount of load it carries, the design mix, temperature,
water content and its quality, compaction, curing process and period, cement content and
chemical attack etc.
Workability
The ease with which concrete can be mixed, transported, placed and compacted is termed
as workability of concrete and it highly influences the strength and durability of the final
concrete. Workability of concrete is affected by; mix proportions, size and shape of
aggregates, grading and surface texture of aggregates, water content, time, temperature,
use of admixture etc.

Methods of testing workability include; compaction factor test, slump test and vee-bee
consistometer test.
Impermeability (density and porosity)
The less porous the denser the concrete and thus increased strength and durability of
concrete. In addition, porosity leads to penetration of materials, which could affect the
component materials reducing the durability of the concrete.
Other properties of concrete include:

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Fire resistance
Segregation
Bleeding
Thermal and acoustic conductivity
It binds rapidly with steel and since it is weak in tension, the steel reinforcement is placed to
take up tensile forces
COMPOSITION MATERIALS IN CONCRETE AND THEIR FUNCTIONS
Cement
Cement is a binder (binding agent or matrix) or substance used for construction that sets,
hardens and adheres to other materials to bind them. It is the major binding material in
concrete. When water is added to cement, hydration(a chemical reaction in which the major
compounds in cement form chemical bonds with water molecules and become hydrates or
hydration compounds) takes place resulting in the cement setting and hardening afterwards.
Cement occurs as a fine powder supplied in various types to suit different conditions.
Cement can be classified as hydraulic (require water for hydration process to take place) or
non-hydraulic (react with carbon dioxide in the air to asset and harden) based on their
setting and hardening mechanisms.
Manufacture
Either cement is manufactured in the Dry or wet process by burning substances containing
carbonates such as chalk or limestone and those containing silica and alumina such as shale
and clay.
The manufacture of Portland cement begins with quarrying of limestone (CaCO3) which is
then blasted by heavy crushers. The crushed limestone is then mixed with clay/shale, sand
and iron ore and ground together to form a homogeneous powder. The mixture is heated in
a kiln at high temperatures (between 13000-14500 C). The firing zone in the kiln is found on
the lower end of the kiln as the raw materials exit the kiln as cooled marble sized pieces
called a clinker. The clinker is ground and 5% of gypsum is added (to regulate setting)
forming Portland cement.
Components of cement
Cement is a homogenous compound (made of compounds of calcium) each serving a
different function in the cement and affecting the concrete used in construction.

Compound Weight Use in the mixture

(%)
Tricalcium silicate 50 Responsible for the early strength of the concrete
(7days)
Dicalcium silicate 25 Reacts more slowly and is responsible for strength at
later stages of the concrete life

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 Tricalcium Aluminate 10  contributes to early strength because it’s the first
compound to hydrate and turns out higher heat
of hydration
- poor sulphate resistance and increases
volumetric shrinkage upon drying
- Has high heat generation and reactive with soils
and water containing moderate to high sulphate
concentrations.
Tetracalcium 10  acts as a filler in the cement
Aluminoferate  responsible for the grey colour of OPC
Gypsum 5 Regulates setting (slows down setting of cement such
that cement is adequately hardened.

Types of cement

There are various types of cement used in the construction industry. Each type of cement
has its properties, uses, and advantages based on composition materials used in its
manufacture.

 Ordinary Portland Cement (OPC)

This is the most widely produced and used type of cement that is suitable for all
general concrete construction. Portland cement reacts with water to form hydrates
of calcium and aluminium. The initial set of OPC takes place in the first 45 minutes
and the final set within 10hours and develops strength sufficiently for most concrete
work. Several other cements will produce concretes with more specialized
properties.
 Rapid hardening cement (RHC)
This is a type of cement whose setting time is similar to that of OPC but develops
strength more rapidly and enabling the formwork to be struck earlier. It is
advantageous in cold weather. This cement has a higher percentage of lime and
Tricalcium silicate (C3S)
The strength of Rapid Hardening cement at 3 day is similar to that of OPC at 7 days
with the same water to cement ratio. The major advantage of Rapid Hardening
cement is that it enables formwork to be struck earlier and as such increases rate of
construction and reduces cost of construction by saving formwork costs.
It is used for concreting in road works so that traffic can be opened earlier,
manufacturing precast slabs, posts, electric poles and concreting in cold countries.
 Quick setting cement (QSC)
Unlike RHC, which gains, strength earlier than OPC, QSC, gains strength at the same
rate as OPC but sets earlier than OPC. Similarly, formwork can also be removed
earlier for QSC.
It has advantage of requiring less water for hydration, has high resistance to water,
and the ultimate strength gained is similar to that of OPC. However, work done with
QSC must be done very fast otherwise it is hard to remix, it is susceptible to sulphate
attack and development of cracks on due to poor heat distribution.

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 It is used for underwater (static or running) construction, in high temperatures where
water evaporates easily, in cold and rainy weather conditions. It is also used in
grouting especially in repair of structures and fixing concrete steps.
 Low heat cement
It is usually referred to low heat Portland cement. It is produced with less Tricalcium
Silicate and Tricalcium Aluminate and it sets, hardens and evolves heat much less
than OPC. It is used in large structures requiring huge amounts of concrete where the
heat generated cannot be easily dissipated and early strength is not usually required
such as dams, massive bridge abutments, gravity retaining walls and other massive
concrete structures.
 Sulphate resisting cement
It has reduced amounts of Tricalcium Aluminate and Tetracalcium Aluminoferate.
Its major advantage is high resistance to sulphate-bearing waters/soils and weak
acids.
It is used in underground seawater structures and soils with high sulphate
concentrations.
 High alumina cement
High alumina cement is obtained by melting a mixture of bauxite and lime and
grinding with the clinker. It is a rapid hardening cement with initial and final setting
time of about 3.5 and 5 hours, respectively
 Blast furnace slag cement
It is obtained by grinding the clinkers with about 60% slag and resembles more or less
in properties of Portland cement but evolves less heat and can be used in for massive
concreting. It can be used for works where economic considerations are predominant
 Portland Pozzolana cement (PPC)
This is produced by combining Portland cement or the clinker with pozzolanic
material (natural or artificial). The natural pozzolanas are mainly volcanic materials
such as pumice or volcanic ash while artificial pozzolanas include fly ash, burned clays
etc.
It has a high resistance to chemical attack but the rate of heat evolution and strength
gain is reduced, thus, it is used in general construction and construction of hydraulic
structures, marine works, sewage works,and pre-stressed and post-tensioning works
and mass concreting such as dams, dykes, retaining walls, foundations and piers.
 White and coloured cements
White cement is produced by reducing the content of iron compounds in cement
while coloured cements are produced by mixing 5-10% of mineral pigments to either
white or ordinary cements.
They are used for architectural purposes such as precast curtain wall and facing
panels, terrazzo surface, etc. and for interior and exterior decorative work like
external renderings of buildings, facing slabs, floorings, ornamental concrete
products, paths of gardens, swimming pools, etc.
 Hydrophobic cements
 Expansive cements

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Supply and storage of cement

In Uganda, cement is supplied in paper or polyethene bags and the major precaution for
storage is prevention of contact and subsequent absorption of moisture. Water absorption
leads to loss of strength and partial initiation of hydration process.

The bags of cement should be stored in a water tight shed with a sound and dry floor. If the
floor is not sound and dry, then the floor should be raised by building blocks and the cement
piled on wooden pallets and covered with plastic/polyethene sheeting or tarpaulin. Care
should be taken to avoid over stacking of the cement bags.
On smaller jobs, the cement bags should be stored in the open placed on raised timber and
covered with plastic/polyethene sheeting or tarpaulin.
Aggregates

Aggregates is a broad category of coarse to medium grained particulate material used in

construction. Aggregates may be natural, manufactured or recycled for example sand,
gravel, crushed stone, slag, glass and recycled concrete. Aggregates are the most mined
materials in the world with a number of applications in the construction industry including,
concrete, septic tanks, road bases, retaining wall drains etc.
Aggregates make up 60-80% of the concrete mix. They provide compressive strength and
bulk of the concrete. For any particular mix of concrete, aggregates are selected for their
durability, strength, workability, and ability to receive finishes.
For a good concrete mix, aggregates need to be clean, hard and strong particles free of
absorbed chemicals or coatings of clay and other fine materials that could cause the
deterioration of concrete such as organic impurities.
Most aggregates in use are extracted from rock quarries. The quarrying usually involves use
of explosives to break the rock from the main rock. The pieces that fall off are further broken
down by either hand or machine depending on the type of rock. Machine crushed
aggregates are stronger then hand crushed aggregates.
Aggregates should be tipped on a hard, clean surface on the site.
Aggregates are divided into two types i.e. fine and coarse aggregates

 Fine aggregates
These particulates mainly pass through the British Standard 5mm sieve.
Natural sand is the most common type of fine aggregate in use today. It is composed
of fine rock material and mineral particles. Its composition is variable depending on
the source i.e. swamps, lakes etc.
Other types of fine aggregates include crushed stone and crushed gravel.
Fine aggregates assist in producing workability and uniformity in the mixture, fills the
voids existent in the coarse aggregates and thus increases the density of concrete. It
also assist in the hardening of cement by allowing the penetration of water through
its voids, helps the cement paste to stick on the coarse aggregates, helps prevent

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 possible segregation of the concrete during transportation over long distances and
reduces shrinkage of the cement.
 Coarse aggregates
These are particulates that are retained on the standard 5mm sieve.
They are normally natural gravel, or crushed gravel or stone. Artificial coarse
aggregates such as clinker and slag should only be used in lightweight concrete.
The type of work determines the maximum size of coarse aggregates but size usually
ranges from 20mm for reinforced concrete to 40mm for mass concrete.
Coarse aggregates provide the bulk in the concrete increases the crushing strength of
concrete and reduces the cost of the concrete.

For purposes of removal of organic impurities and other particles attached to the
aggregates, they are usually washed with clean water.

The amount of water in the aggregates directly affects the quality of the concrete. A wet mix
of concrete produces a weak product and as such, the water contained in the sand should be
watched.
Grading of aggregates
Is the determination of particle size distribution of aggregates in a load of aggregates.
ALTERNATIVELY, determination of the average grain size of the aggregates before they are
used in construction. It applies to both coarse and fine aggregates. The aggregate sample is
sieved through a set of sieves and weights retained on each sieve in percentages are
summed. This process is known as sieve analysis.
Poorly graded aggregates contain particles of the same size while well-graded aggregates
contain particles of different sizes. The purposes of grading is to produce a concrete mix that
is relatively free from voids.
Bulking of sand

This refers to the percentage increase in the volume of sand due to increase in moisture
content. Sand with a moisture content (mc) of 10% of dry weigh will increase in volume up
to 30% and the rate of bulking is inversely proportional to the size of the aggregates.

The major purpose of adding sand to concrete is to reduce segregation and to fill the pores
in the concrete. Therefore, testing of bulking of sand is important to ensure that the right
proportion of sand is in cooperated in the mixture.
Water
Water in concreting enacts the chemical reaction of cement resulting in setting and
subsequent hardening of the concrete. It also enables the concrete to become sufficiently
plastic for easy mixing, placing and compacting.

Water used for concreting should be clean or generally, water that is fit for drinking and
should be kept in containers that do not pollute it. However, non-drinkable water can be
used provided its source does not negatively affect the properties of concrete.

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Water to cement ratio; - this is the ratio of weight of water to weight of cement in a
concrete mix. A lower ratio leads to higher strength and durability but may make the mix
difficult to work and form and vice versa. Workability can be resolved by using plasticizers
and super plasticisers.
Admixtures/ Additives
These are often chemicals added in small amounts before or during mixing of concrete to
improve certain properties of freshly mixed concrete.
Some admixtures and their functions

Type Function
Air entraining improves durability, workability, reduces bleeding, reduces
freezing/thawing problems (e.g. special detergents)
Accelerators speeds setting time and hardening in cold weather (e.g. calcium
chloride, hydrogen chloride)
Superplasticizers increase strength by decreasing water needed for workable
concrete (e.g. special polymers)
Retarders delays setting time and are useful in hot weather where normal
setting time is shortened by high temperature.(e.g. sugar solute,
zinc salt)
Mineral admixtures improves workability, plasticity, strength (e.g. fly ash)
Pigments Add colour to the concrete (e.g. Metal oxides)
Pore fillers Increase the cohesiveness of concrete thereby improving
resistance to bleeding
Water repellents Prevent absorption of water into concrete e.g. in a concrete roof
(e.g. usually vegetable and mineral oils)

CONCRETING OPERATIONS
These include batching, mixing, handling and transportation, placing and compaction and
curing.
(a) Batching
This is the process of measuring or proportioning ingredients or materials in the right
proportions to prepare concrete. Prior to batching, cement should be kept in a damp proof
and draught-proof structure while aggregates should be stockpiled on a hard and clean
surface. Batching can be by either mass or volume.
(i) Batching by volume
It is carried out using constructed open boxes called gauge boxes for proportioning the
materials according to various mix proportions. The general capacity of a gauge box is equal
to the volume of one bag of cement.
It is a less precise method that does not require skilled labour and is more economical. It is
used for nominal mixes (e.g. 1:2:4) especially on small projects.

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The quality of the concrete may differ because of the bulkage of fine aggregates (sand) and
as such concrete is not uniform and consistent.
(ii) Batching by mass/weight
This is a more precise method of bathing which in-cooperates the use of a weighing balance
to get the exact weight or mass of the ingredients in the concrete mix. It is a more expensive
method compared to volume batching and requires skilled labour.

It is normally used for design mixes and on sites where mechanical plant is used for the
mixing projects e.g. large projects.
(b) Mixing concrete
After proportioning the ingredients, the process of mixing proceeds. There are basically two
methods of mixing concrete i.e. hand and machine mixing
(i) Hand mixing

This method of mixing concrete should be used for small quantities of concrete on small
projects where quality control is less important with the proportion of cement increased by
10%. It is cheap, does not require highly skilled labour and shovels / spades are the only
tools required.
Process of hand mixing

 Measured quantities of fine aggregates (sand) are spread on a clean hard surface/
platform.
 Cement is added and the materials are mixed dry by turning them from one point to
another with a spade/shovel until a uniform colour is obtained.
 The above mixture is spread out and coarse aggregates are spread over the mixture
 Water is then added to the mixture and it is turned using a spade/shovel until a
plastic stage is reached.
 The concrete is now ready for testing and casting.
Note
1. If there’s any dirt or debris in the aggregates, they should be washed before use
2. The base platform must be hard, clean, watertight and big enough for the hand
mixing procedure.
3. The working platform should be thoroughly cleaned at the end of the day.
(ii) Machine mixing

This method is faster, saves on material, produces a better mix, requires skilled operatives
both working the machine and proportioning and is used on large projects where large
amounts of concrete are required and quality control is paramount.
Process of machine mixing

 The inner surface of the mixer is wetted

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  Coarse aggregates are poured/placed in the mixer followed by sand and finally
cement.
 The materials are mixed dry until a uniform colour is obtained
 Water is then added to the mixer and it is powered until the concrete becomes
plastic and attains a uniform colour.
Note
1. The mixer operator should visibly monitor the workability of the concrete,
homogeneity and cohesiveness of the mix and the consistency of the concrete
mixture.
2. The usual mixing time is usually more or less a minute after all ingredients including
water have been poured into the drum
3. After mixing the concrete, the mixer should be thoroughly cleaned and regularly
checked for damages, shatter or corrosion.
4. Concrete should be used within 30minutes after mixing and discharging concrete
from the mixer.
There are two types of mixers commonly used in concrete production i.e. power mixers and
freefall mixers. Freefall mixers consist of a rotating drum with blades on the interior. As the
drum rotates, the concrete keeps on being dropped as it is being mixed and it is emptied by
either tilting or changing the direction of the rotation. On the other hand, the power mixers
consist of rotating arms on the interior of the drum, which mix the concrete.
Characteristics of well-mixed concrete
 It should be of a uniform colour
 All concrete materials should be homogeneously mixed
 Cement paste should cover all the surfaces of the aggregates
 Segregation and bleeding should not occur after concrete mixing
Workability of concrete
Is the ease with which concrete mix can be handled after mixing. It has a great bearing on
the quality, durability and strength of concrete.
Factors affecting workability of concrete

 Water to cement ratio (should be between 0.4-0.6 i.e. weight of water divided by
weight of cement in a concrete mix)
 Method of mixing
 Size and shape of the aggregates
 Proportioning of the ingredients
 Transportation method used and the place of deposition.
 Admixtures added to the concrete
Methods of testing workability of fresh concrete
These include; the slump test, compaction factor test and Vee-Bee Consistometer test.

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 (i) Slump test

This is a test used on site to test the consistency of concrete before it sets. It helps to
examine the right quantities of cement, fine and coarse aggregates that are used to make
the concrete. It is not suitable for very wet or very dry concrete and the maximum size of
aggregates should not exceed 38mm.
It is a cheapest method that does not require highly skilled labour and results are instant.
However, Care should be taken to avoid any vibrations or shake in the process of carrying
out the test.
The apparatus for the slump test include:

- Slump cone
Also known as a frustum 300mm high open on both ends. Base diameter 200mm and
top diameter 100mm with handles on the sites.
- Tamping rod
This is a steel or metal rod 600mm high and 16mm diameter used to compact the
concrete
- Plate
This is a nonporous and inelastic flat metal sheet on which to conduct the slump test
- Measuring tape
This is used to measure the drop in height (slump) in the test
- Trowel
The trowel in this test is used to fill concrete in the slump cone.
Slump test apparatus

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Procedure

The slump cone is place on the plate. Fresh concrete whose workability is to be measured is
poured in the cone with the help of a trowel in four layers. Each layer is compacted by
tamping evenly 25times using the tamping rod. The cone is the rotated gently and lifted
using the handles. The decrease in the height of the concrete in the cone is observed and
measured.
The shape and decrease in the height of the concrete decides the type of slump i.e. collapse
slump, shear slump and true slump.
- True slump
This occurs when there is low subsidence in the height of the concrete.
- Shear slump
With shear slump, the concrete slides down in the form an inclined plane.
- Collapse slump
Here, the concrete collapses as soon as the slump cone is removed and this usually
occurs in wet mixes

(ii) Compaction factor test

The apparatus for the compaction factor test consist of two conical hoppers (known mass)
adopted from the field hoppers used in the field mounted on a cylindrical container of
known mass, a tamping rod, trowel and weighing scale for determining the weights.
Compaction factor test apparatus

Trap door

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Procedure

Concrete whose workability is to be measured is carefully poured into the top hopper with
the help of a trowel. Care should be taken to avoid any extra work or force and compaction
in the process of pouring.

The bottom of the top hopper is opened to allow concrete to flow into the second hopper. A
tamping rod is used to remove any extra concrete that could have remained in the hopper
above. The sides of the hoppers should be smooth or lubricated to allow the concrete to
fully flow to prevent extra force in removing or clearing the hoppers.
The above procedure is repeated to allow the concrete to flow into the cylinder below the
second hopper.
The cylinder is cleaned and evened out with a trowel to remove any excess or overflown
concrete. The cylinder is then measured and its weight recorded as weight of partially
compacted concrete.
The cylinder is filled with concrete in four layers each evenly compacted 25 times with a
taping rod. Its weight is measured and recorded as weight of fully compacted concrete.
The compaction factor is the ratio of partially compacted to fully compacted concrete and
usually lies within a range of 0.7-0.9
(iii) Vee-Bee Consistometer Test.
This test measures the relative effort (expressed as time(s)) required to change a mass of
concrete from one definite shape to another I.e. from conical to cylindrical. This method is
suitable for concrete with low amounts of water to cement ratio and slump values less than
50mm otherwise, the process would be too fast and the time no measureable.
Procedure
The slump test is performed placing the slump cone inside the sheet metal cylinder of the
consistometer
The glass disk attached to the swivel arm shall be moved and placed just on top of the slump
cone in the pot and before the cone is lifted, the position of the slump cone shall be noted
by adjusting the disk attached to the swivel arm. The cone shall then be lifted and the slump
measured by lowering the glass disk attached to the swivel arm.

The electrical vibrator shall then be turned on and the concrete allowed to spread out in the
pot and stop clock started simultaneously. The vibration is continued until such a time that
the conical shape disappears and attains a cylindrical shape. This can be observed through
the glass disk by disappearance of transparency.
Immediately the concrete attains a cylindrical shape, the stop clock is turned off. The time
taken for the concrete to change shape from conical to cylindrical is noted in seconds known
as Vee-Bee seconds.
Vee-Bee test apparatus

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NB. Do not copy table.

Slump Degree of Consistency Compaction Vee-Bee Uses for which

value workability factor time (s) concrete is suitable
(mm)
0 Extremely Moist earth 0.70 20-(15-10) Precast paving slab
low
0-25 Very low Very dry 0.75 10-(7-5) Roads (power
vibrator)
25-50 Low Dry 0.85 7-(4-3) Mass concreting, light
reinforcement section
(hand vibrator)
50-100 Medium Plastic 0.90 3-(2-1) Heavily reinforced
section (manual
vibrator)
100-150 High Fluid 0.95 More fluid RCC with congested
than 1 reinforcement (can’t
be vibrated)

(c) Transportation

This is the process of moving concrete from the mixer or mixing area to the point where it is
to be placed. Ideally, concrete should be placed in its final position within 30minutes of
leaving the mixer. Therefore, concrete should not be transported over long distances to
reduce the number of vibrations that may lead to segregation of the concrete.
Factors determining the method of transportation
- Nature of the site

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 - Type and size of the job
- Distance to be covered below, above or along the ground
- The equipment available
- Cost
Methods of transportation
- Head/mortar pans
- Wheel barrows or motorised buggy
- Mechanical plant e.g. belt conveyor, concrete pump, pneumatic placer, chute, bucket
and steel skip etc
(d) Placing and compacting of fresh concrete

This is the laying of concrete in its final position on the structure e.g. foundation, column,
wall, etc. soon after placing. Concrete should be compacted to take full form or shape it is
intended before it starts setting. The purpose of compaction is to make concrete as dense as
possible by removing/eliminating the air bubbles or voids in it.
Compaction may be achieved either manually by use of a spade, piece of iron bar or timber
or mechanically by use of a poker vibrator and other mechanical plant.
The levelling of concrete is carried out by tamping the concrete with a straight edge board.
The tamping compacts the concrete and brings the excess water to the surface so that it can
be evaporated.
Compacting and levelling activities.

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 (e) Curing
This is the process of maintaining freshly laid concrete under uniform conditions of
temperature and moisture to facilitate complete or full hydration of cement compounds.
When the above conditions are maintained, the final properties of concrete will be archive
especially final strength.

Curing is highly dependent on water and as such, concrete should be protected from sun and
drying winds for at least 7days.
Methods of curing

 Plastic sheeting
 Sand layer
 Water spraying
 Curing compounds
 Formwork curing
 Matting / Hessian method
Methods of casting concrete

There are two ways by which concrete structures (walls, beams, paving stones, posts, slabs
etc.) can be made i.e. in-situ or precast.

(i) Cast-in-situ / In-situ / Cast-in-place

This is where the concrete is mixed, placed and left to cure in its final position on
the structure. Alternatively, where the concrete is mixed on site and place for
curing in its final position on the structure.
(ii) Precast concrete
This is when concrete is mixed and cast from elsewhere (factory) and later
transported to site (in hardened state) for placement in its final position on the
structure/job.
Advantages of precast concrete

 Speeds up the construction work

 Construction is not interrupted by bad weather
 There is less risk of the reinforcements being displaced
 Concrete has a better finish
 Concrete has uniform and accurate components or units since mixing, placing and
curing are carried out under controlled conditions in the factory
Disadvantages of precast concrete

 Transportation and handling costs are increased

 Requires skilled labour to join the building units
 Structural connections between the precast units may be difficult

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  Construction is less flexible
Types of concrete
There are two major types of concrete i.e. plain concrete and reinforced concrete.
Plain/Mass concrete
This is a mixture of cement, fine and coarse aggregates without reinforcement. Mass
concrete is used when the structural member being constructed is only subjected to
compressive forces and not bending for example foundations, pavements and oversite
concrete.
Reinforced concrete
This is a type of concrete in which is embedded with steel reinforcements which act
together with the concrete to resist forces. Generally, mass concrete has a tensile
strength that is just 10% of the total compressive strength and as such, incorporation of
reinforcement serves to archive this purpose.
The reinforcement may be in the form of bars (plain, ribbed or twisted), rods or mesh.
These absorb the tensile, shear and at times the compressive forces increasing the
general strength of the concrete.
Reinforcements in common use.
Plain mild steel

As suggested by their name, these are plain steel bars which are used on areas where
economy supersedes the strength component. They are also used in small diameters to
make stirrups for binding the reinforcement for columns, beams etc. reinforcement.
Ribbed high tensile steel

These are hot rolled steel bars that are majorly used as main and distribution bars in
reinforcement. They have ribs that help to form a tight bond with the concrete.
Mild steel ribbed bars
These are cold worked steel bars that serve the same purpose as the ribbed steel bars
using the twisted format to form the joints with the concrete. Their application has
greatly gone down with introduction of the ribbed bars.
Mesh reinforcement

These exist in many forms forexample expanded metal, wire mesh, welded wire mesh
etc.
Other types of concrete
Besides the major plain and reinforced concrete, other types of concrete are classified
basing on the other variations in the basic ingredients that are used in the concrete i.e.
binders like lime, cement, asphalt, polymer etc. Aggregates forms like glass, stone, and

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 other light metals and the different types of reinforcement e.g. FRP reinforcement, steel
reinforcement, cables etc. some of these type of concrete are explained below;
 Lime concrete
In lime concrete, cement is replaced with lime as the binder. It is used as a toping and
basing for floors and ceilings respectively, domes, vaults, foundations and terrace
roofing. It was a widely used type of concrete before it was replaced by cement
especially when mixed with volcanic ash.

The lime concrete is cheap, has high workability and plasticity, and is environmental
friendly since it requires less energy in the production of the lime. It also enables
early curing since it retains water for a long time facilitating early curing. However, it
needs more time to set, has low strength and inconsistent quality.
 Asphalt concrete

AC is a mixture of aggregates and asphalt/bitumen as the binding material. Asphalt is

a sticky, black, highly viscous form of petroleum that also occurs naturally. AC is used
in the construction and maintenance of surface roads, parking lots, airports,
pavements, dams etc.
 Prestressed concrete

This is a type of reinforced concrete in which a predetermined engineering stresses

are placed in a structural member to counteract the stresses when the member is
subject to loading. It is used in members with spans exceeding 9m in members like
floor beams, piles, railways sleepers, bridges, roofs and runways.
Special alloy steel wires or tendons that can be grouped to form cables are used are
used to either pre-tension or post-tension the concrete. In pre-tensioning, hydraulic
jacks stretch the cables; the concrete is cast in the forms, allowed to cure to transfer
the stress from the jacks to the concrete that counteracts the forces after casting the
member on the structure.

In post-tensioning, the concrete is first cast in the forms or on site leaving space
where the cables will pass during the tensioning process by means of a flexible duct
or a rubber sheath that can be deflated and removed after curing. After the concrete
has cured, the cables are past in the holes and stretched by hydraulic jacks and
anchored after gaining the required strength and the ends are sealed.

Advantages of prestressed concrete

 The in-built compressive strength of concrete is used to the fullest
 Free from cracks and offers higher resistance to stresses than normal RCC
 Since it offers larger spans for members, fewer joints are used as
compared to normal RCC
 Longer span also ensures an increased untroubled floor space and parking
facilities.
Disadvantages of prestressed concrete

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  It requires a high degree of workmanship and control
 It is very expensive both in the making and transportation of the members
from the factory to site for the case of pre-tensioned type
 It requires additional special equipment like jacks, anchorage etc.
 Glass concrete
In this type of concrete, broken glass pieces are used as concrete, which gives a
glittery finish to the finished member.
 Gypsum concrete
 Shotcrete etc.
Defects in concrete
It is very important for construction experts or consultants to be on the lookout for some of
the defects on structures because they affect the lifespan of the building and can lead to the
building becoming a hazard to life occupying or using the building. Some of the defects are
explained below.

 Cracking
This is the complete or partial separation of a concrete member into two or more
parts. It can occur in both fresh and hardened concrete when the tensile stresses
applied on a member exceed the maximum tensile strength. It is caused by use of an
improper mix design, insufficient curing, lack of expansion joints in the member etc.
It can be prevented by use of low water-cement ratio, increasing the aggregate
content in the mix, loads should be applied on the members only after reaching its
full strength and rapid evaporation of water from the concrete should be prevented.
 Efflorescence
This is the formation of white deposits of salts on the surface of the concrete. Its
formed as a result of presence of soluble salts in the water that is used in the mixing
of the concrete. It can be prevented by using clean and pure water in the mixing of
the concrete.
 Curling
This is the distortion into curved shape by either upward or downward movement of
the edges or corners of the concrete slab surface. It is caused my moisture difference
between the top and bottom surface due to shrinkage after drying.
 Crazing
This is the formation of closely spaced uneven shallow cracks on the surface of the
concrete. Rapid hardening of the top surface of the concrete due to high
temperatures, presence of excess water in the mix and insufficient curing cause it.
It can be prevented by carrying out proper curing and dampening the area where the
concrete is to be cast to prevent loss of water by absorption.
 Blistering and Delamination
This is the formation of hollow bumps of different sizes on the surface of the
concrete due to entrapped air under the finished concrete surface. It is majorly
caused by improper finishing of the concrete leading to excessive air and water being
entrapped in the mix. Severe blistering will lead to delamination.

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  Honeycombing
This occurs where improper or incomplete vibration is done leaving voids in the
concrete where the mortar failed to fill the spaces between the aggregates
 Segregation
This is the separation of the cement paste and aggregates by virtual of difference in
size, density, shape and other properties during handling and placement. It is caused
by over vibration or compaction of concrete that pushes the aggregates to the base
and the paste to the top. It can also be caused by poor mixing of the concrete with
excess water, dropping the concrete from heights as in column construction and
transporting the concrete over long distances.
 Bleeding
This a form of segregation in which the water in the concrete raises to the surface of
the concrete forming a paste as the heavier particles settle downward. It is caused by
presence of excessive water in the concrete, improper proportioning of ingredients,
insufficient mixing and concreting in sunny.
 Alkali-silica reaction
 Carbonation
Concrete mixes

Some of the most commonly used mixes are shown below and their applications on the
building. Although it should be noted that there are some limitations to these prescribed
mixes.
a) 1:3:6 for plain concrete foundations, oversite concrete etc.
b) 1:2:4 for normal reinforced concrete like columns, beams, foundations etc.
𝟏
c) 1:1𝟐:3 for strong reinforced concrete like floor slabs, columns etc.
d) 1:1:2 for strong works like slabs columns, retaining walls etc.

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