cement concrete

Cement Concrete

Prepared By: Ms. Nitika Kabra 1

CEMENT CONCRETE

Cement concrete is a composite building material. It is a mixture of cement, sand, pebbles or crushed rock and water. This when placed in a skeleton of forms (formwork) and allowed to cure,

becomes hard like stone. An old name for concrete is liquid stone. Now days it is a major building

material and is used in all branches of modern construction. This is so because:

1. It can be easily moulded into different shapes and sizes to give a durable structure.

2. There is very less labour expenditure.

3. It is possible to control its properties, within a wide range using appropriate ingredients and

techniques.

4. Complete mechanisation during its preparation and placing is possible.

5. It has adequate plasticity for mechanical working.

Properties of Cement Concrete (from Rangwala) During hydration and hardening, concrete needs to develop certain physical and chemical

properties.

Materials used in R.C.C. work Cement Fine aggregates

Coarse aggregates Steel

Water Admixtures

1. Cement: (from Rangwala).

2. Aggregates: These are chemically inert. Form the bulk of concrete. They should be hard,

durable and clean, completely free from clay lumps, organic and vegetable matter, fine dust,

etc. They should conform to IS 383. Preference should be given to natural aggregates.

i. Fine aggregates: Pass through BIS sieve no. 480. Usually river sand is used. Crushed

stone may be used in its place.

ii. Coarse aggregates: Retained on BIS sieve no. 480. Usually broken stone is used. The

nature of work decides its max. size. For thin slabs, beams and walls, generally 1/3rd

the

thickness of the section is taken.

Choice of Aggregates: The choice of aggregates may depend on several factors, the most

important being availability. Otherwise it depends on the following factors which in turn

influence the performance of concrete.

S. No. Factors Influence on concrete property

1. Specific gravity / porosity Strength / absorption

2. Chemical stability Durability

3. Surface texture Bond grip

4. Shape Water demand (strength / workability)

5. Gradation or particle size

distribution

Water demand (strength), cohesion, bleeding and

segregation

6. Maximum size Strength and water demand

7. Deleterious materials Water demand (strength), bond, cohesion, durability

Quality of Aggregates

Crushed aggregate generally not contain clay, silt or mud but may contain a percentage of

dust or grit. This dust of present as a coating around the larger aggregate will result in drop in

strength of concrete. Similarly grit and dust proportions of the aggregate will cause an

increase in water demand and subsequent drop in concrete strength.

Cement Concrete


Limits of Deleterious Materials

Fine Aggregate % by

Weight, Max.

Coarse Aggregate %

by Weight, Max.

S.

No.

Deleterious Substances Method of Test

Uncrushed

(a)

Crushed

(b)

Uncrushed

(c)

Crushed

(d)

1. Coal and lignite IS 2386 (P2): 63 1.00 1.00 1.00 1.00

2. Clay lumps IS 2386 (P2): 63 1.00 1.00 1.00 1.00

3. Materials finer than 75

IS sieve

IS 2386 (P1): 63 3.00 15.00 3.00 3.00

4. Soft fragments IS 2386 (P2): 63 - - 3.00 -

5. Shale IS 2386 (P2): 63 1.00 - - -

6. Total of %s of all

deleterious materials

(except mica) incl. 1 5

for a, c and d and 1, 2

for b.

5.00 2.00 5.00 5.00

Texture of Aggregates

The surface texture influences the bond between the aggregate and cement. Smooth surface

such as that found on gravels will have poor bond. Crushed aggregates have a rough texture

and give a good mechanical bond with cement.

Shape of Aggregates

The aggregate shape can be broadly classified as follows in order of desirability:

Rounded: Min. surface area for the same mass. Therefore, requires minimum cement

paste for bonding. Less water cement ratio required for the same workability.

Irregular Rounded

Cubical

Flaky angular

Elongated

Flaky elongated

Flaky (least dimension is less than 3/5th

of its mean dimension) and elongated (length is 1.8

times its mean dimension) shapes have larger surface area for the same mass as compared to

rounded or cubical shapes. The cement paste required to coat the surface and hence the water

demand is much more. These shapes are therefore generally not preferred.

Specific Gravity

Specific gravity of the aggregates is generally indicative of its quality. It is very important for

the aggregates to have high specific gravity generally between 2.4 and 2.9 as the concrete

density will generally depend on it.

Surface Moisture Content, Absorption and Porosity

Natural aggregates generally contain moisture. Generally the moisture is present in the

following forms:

Moist or surface wet Surface dry but saturated

Air dry Oven dry

The moisture in the surface determines the free water which is to be considered while

measuring the water cement ratio. The absorbed moisture within the aggregates is not

considered. Porous aggregates will absorb more moisture or water than dense aggregates, so

concrete will lose its workability at a faster rate.

Grading of Aggregates

Cement Concrete


They can influence various properties of concrete such as cohesion, water demand,

workability and strength. In order to obtain concrete of denser quality, the fine and coarse

aggregates need to be properly graded.

The grading of fine aggregates has a marked effect on the uniformity, workability and

finishing quality of concrete. The grading of the coarse aggregate may be varied through

wider limits without appreciable effect on workability of concrete.

Grading Limits for Fine Aggregates

Percentage Passing For IS Sieve

Zone 1 Zone 2 Zone 3 Zone 4

10 mm 100 100 100 100

4.75 mm 90 100 90 100 90 100 95 100

2.36 mm 60 95 75 100 85 100 95 100

1.18 mm 30 70 55 90 75 100 90 100

600 micron 15 34 35 59 60 79 80 100

300 micron 5 20 8 30 12 40 15 50

150 micron 0 10 0 10 0 10 0 15

If a particular sieve size is not present in aggregates, it is called gap graded.

Size of Coarse Aggregates

For most of work, 20mm aggregate is suitable. Where there is no restriction to the flow of

concrete into sections, 40mm or larger size may be permitted. In concrete elements with thin

sections, closely spaced reinforcement or small cover, 10mm nominal max. size aggregates

should be used.

Other Impurities

Organic: Even a very small fraction of organic matter will delay or prevent the

hardening of concrete.

Chloride: If present in fine aggregates, will not be harmful to concrete or mortar but

will be harmful to the reinforcement or other steel embedments in concrete or mortar.

Alkali Reactivity: Aggregates should not be alkali reactive as they react with cement.

Expansive forces are created because of it which in turn causes cracking and

disintegration of concrete. Such reaction generally takes place after a lapse of 2 to 3

yrs.

Importance of Bulking of Sand

The bulking of sand should be taken into account when volumetric proportioning of the

aggregates is adopted. Otherwise it which will increase the cost.

There would be less quantity of fine aggregates in the concrete mix, which may make

the concrete difficult to place.

Proportioning by weight avoids this difficulty.

3. Steel: It is generally in the form of round bars (mild steel / twisted / ribbed) of mild steel. The

diameter varies from 5mm to 40mm. Sometimes square bars are also used as steel

reinforcement.

Structural requirement of Reinforcing Steel

All reinforcement shall be free from loose mill scales, loose rust and coats of paints, oil, mud

or any other substances which may destroy or reduce bond.

4. Water: It should be potable. That is, it should be free from harmful impurities like oil, alkali,

acid etc. Water that is drinkable and having no pronounced taste or odour is suitable for use

Cement Concrete


in making concrete. The pH value of water shall not be less than 6. The impurities in mixing

water, when beyond the specified limits, may affect the following properties of concrete:

Strength Setting time

Volume stability Efflorescence

Corrosion of reinforcement (durability)

Sea Water for Making Concrete (from Rangwala)

5. Admixtures: These are added to concrete to improve its qualities. Also for changing

different physical properties in fresh and hardened state. Admixtures may improve concrete

w.r.t:

i. Its strength

ii. Hardness

iii. Workability

iv. Water resistance power

v. Resistance to acids and alkalis

vi. Setting time

Admixtures

Admixtures are organic or non-organic materials in form of solids or fluids that are added to the

concrete either before or during its mixing to alter its properties, such as workability, curing

temperature range, set time or colour and to give it certain characteristics. These generally make

up less than 5% of the cement weight and are added to the concrete at the time of batching/mixing.

The most used types of admixtures are:

Accelerators: These speed up the hydration (strengthening) of the concrete, 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.

Retarders: These slow down the hydration of cement, lengthening set time. Used in hot

weather conditions and large masses of concrete on concrete setting time.

Air-entrainers: Add and distributes tiny air bubbles to the concrete, which reduces damage due

to freeze-thaw cycles.

Plasticizers: Used to increase the workability of concrete, allowing it to be placed more easily

with less compactive effort. Alternatively, they can be used to reduce the water content of a

concrete (termed water reducers) yet maintain the original workability.

Pigments: Change the colour of concrete for aesthetics.

Water reducing admixtures: Require less water to make a concrete of equal slump, or increase

the slump of concrete at the same water content. These are used for hot weather concrete

placing and to aid pumping.

Waterproofing and damp proofing admixtures: These are used to decrease the amount of water

penetration into the larger pores of concrete.

Additions

Fly ash: A by-product of coal-fire electric generating plants; it is used to partially replace

Portland cement by up to 40% by weight.

Ground granulated blast-furnace slag (GGBS): A by-product of steel making, it is used to

partially replace Portland cement by up to 80% by weight.

Silica fume: It is used to increase strength and durability of concrete.

Crushed Glass: Recycled, crushed glass can also be added in the production of concrete for an

aesthetic effect in the construction of walkways.

Water Cement Ratio The water in concrete has to perform the following two functions:

1. The water enters into chemical action with cement and this action causes the setting and

Cement Concrete


hardening of concrete.

2. The water lubricates the aggregates and it facilitates the passage of cement through voids of aggregates. This means that water makes the concrete workable.

Water required for these two functions is about 0.5 to 0.6 times the weight of cement. This ratio of

the amount of water to the amount of cement by weight is termed as the water cement ratio. The

strength and quality of concrete primarily depends upon this ratio.

The quantity of water is usually expressed in litres per bag of cement.

Important points to be observed in

connection with the water-cement ratio

are as follows:

1. The minimum quantity of water should be used to have reasonable

degree of workability.

2. The water-cement ratio for structures which are exposed to

weather should be carefully

decided.

3. Some rules-of-thumb are developed for deciding the

quantity of water in concrete. The

two such rules are:

a. Weight of water = 28% of the

weight of cement + 4% of the

weight of total aggregates.

b. Weight of water = 30% of the

weight of cement + 5% of the

weight of total aggregates.

Water cement ratio is important

because:

1. Water cement ratio and strength of concrete: The strength of

concrete is inversely proportional

to the water-cement ratio.

2. Water cement ratio and durability of concrete: W/C governs the porosity of the hydrated cement paste. Thus the value

of W/C is relevant to many aspects of durability.

Workability The term workability (or consistence, as it is known in Europe) is used to describe the ease or

difficulty with which the concrete is handled, transported and placed between the forms with

minimum loss of homogeneity. If the concrete mixture is too wet, the coarse aggregates settle at the

bottom of concrete mass. On the other hand, if the concrete mixture is too dry, it will be difficult to

handle and place it in position and have more entrapped air within the concrete. Both these

conflicting conditions should be correlated by proportioning carefully various components of

concrete mixture.

The important facts in connection with workability are as follows:

1. If more water is added to attain the required degree of workmanship, it results into concrete of low strength and poor durability.

2. If the strength of concrete is not to be affected, the degree of workability can be obtained: a. By slightly changing the proportions of fine and coarse aggregates, in case the concrete

mixture is too wet; and

b. By adding a small quantity of water cement paste in the proportion of original mix, in

Cement Concrete


case the concrete mixture is too dry.

3. A concrete mixture for one work may prove to be too stiff or too wet for another work.

4. The workability of concrete is affected mainly by water content, water-cement ratio and aggregate-cement ratio.

5. The workability of concrete is also affected by the grading, shape, texture and maximum size of the coarse aggregates to be used in the mixture.

The slump test (from Rangwala)

Recommended Slumps of Concrete (from Rangwala)

Classification of Concrete Mixes (from Rangwala)

Proportioning of Concrete The process of selection of relative proportions of cement, sand, coarse aggregate and water, so as

to obtain a concrete of desired quality is known as the proportioning concrete.

It is observed that if a vessel is taken and filled with stones of equal size, the voids to the extent of

about 45% are formed. This result is independent of the size of stones. The theory of formation of

concrete is based on this phenomenon of formation of voids. When coarse aggregate is placed, such

voids are formed. When fine aggregate (sand) is added, it occupies these voids. Further when finely

powered cement is added, it occupies the voids of sand particles. Finally, when water is added, it

occupies very fine voids between cement particles. During the process of setting, a chemical

reaction takes place between water and cement. This results in an absolutely solid substance,

known as the concrete.

In general, the proportions of coarse aggregate, fine aggregate, cement and water should be such

that the resulting concrete has the following properties:

1. When concrete is fresh, it should have enough workability so that it can be placed in the

formwork economically.

2. The concrete must possess maximum density or in other words, it should be the strongest and

most watertight.

3. The cost of materials and labour required to form the concrete should be minimum.

4. The concrete should be cohesive.

5. The concrete should be homogeneous, so that the properties of concrete are uniform.

The different methods of proportioning concrete (from Rangwala)

Batching of Concrete The concrete batching is done using the following methods:

Volumetric Batching: Fermas or boxes of certain size are made and with the help of these the

mixing is done. The bulking of sand needs to be taken into consideration while using this method.

Weigh Batching: The concrete proportioning is done according to the weights of the material. The

mix is properly designed in this case. The bulking of sand does not have a marked effect on

concrete while using this method.

Mixing the Materials of Concrete The process of rolling, folding, and spreading of particles is known as the mixing of concrete.

Thorough mixing is essential for the production of uniform, high quality concrete. It ensures that

cement water paste completely covers the surface of aggregates and fills all the spaces between

them with cement paste. Concrete can be mixed either by hand or by machine.

Hand Mixing: The materials are stacked on a watertight platform, and mixed thoroughly in dry

condition at least three times, before water is added. Then it is again thoroughly mixed. The mix

should be used within 30mins after the addition of water. It is done in case of small unimportant

works, where small quantity of concrete is required.

Machine Mixing: Machine mixing makes more uniform batches. The methods for mixing concrete

by machines vary. The concrete may be mixed by machines at the place where the concrete will be

Cement Concrete


used. Ready-mix companies make huge batches of concrete at mixing plants, and take it to the work

site in trucks.

All the materials of concrete including water are collected in a revolving drum and then the drum is

rotated for a certain period. The resulting mix is then taken out of the drum.

The mixing of materials of concrete with the help of machines is more efficient and it

produces concrete of better quality.

The mixtures of various types and capacities are available. They may be either of tilting or non

tilting drum type. They are generally provided with power operated loading hoppers.

The water should enter the mixer at the same time or before the other materials are places, to

ensure even distribution of water.

The mixing should be carried out till the mix is observed to be of uniform colour and

consistency.

The drum should not be overloaded.

There should be some additional sand and cement in the first batch, as some of it sticks to the

sides and the blades. It would require more water, but the W/C should remain unchanged.

The mixer should be thoroughly washed and cleansed after use.

The damaged or broken blades should be removed.

The mixing time should preferably be 2 mins.

The concrete discharged by the mixer should be used within 30 mins.

Transportation and Placing of Concrete The mixed concrete is to be transported from the mixing point to the placing point on the formwork

in the shortest possible time so that it does not lose its workability. Concrete is required to be

transported in both horizontal and vertical direction.

The type of equipment used for this purpose depends on the nature of work, height above ground

level and distance between the point of preparation and placing of concrete.

There are several modes of transportation of concrete mixes. They can be classified as:

1. Fully Manual: Concrete is transported by head load. This is done for ordinary building works.

A human ladder is formed and the concrete is conveyed in pans from hand to hand. Proper

walkways and staging should be provided for the worker to work quickly, safely, and

comfortably.

2. Semi Manual and Semi Mechanised: The concrete is transported horizontally by manual

means and vertically by mechanical means such as a builders hoist, cranes etc. It is done in

most of the building works.

3. Fully Mechanised: The concrete is transported both vertically and horizontally by mechanical

means such as dumpers and truck mixers for horizontal transportation, hoists for vertical

transportation and conveyors, concrete pumps, cranes, helicopters with buckets (in hilly areas)

for both vertical and horizontal transportation. It is used for important works.

Precautions while transporting concrete:

a. There should be no segregation of aggregates or bleeding of concrete during transportation as

it can result in the loss of strength, non uniformity, porosity and poor durability.

b. Water should not be added to the concrete during its passage from mixer to formwork.

Placing. Workers pour the wet concrete into forms made of wood, plywood, or steel. The forms

hold the concrete in shape until it hardens. The concrete may be dumped directly into the forms, or

poured down chutes.

Precautions: (from Rangwala)

a. Formwork should be properly cleaned and well watered.

b. Concrete should be deposited as near as practicable to its final position.

c. Large quantities of concrete should not be deposited at a time.

d. The concrete should be dropped vertically from a reasonable height.

e. The concrete should be deposited in horizontal layers of about 150mm in height.

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f. Concrete should be placed in single thickness.

g. Cold joints should be avoided by placing the layer of concrete over the previous layer which

can still be compacted.

h. The concrete should be thoroughly worked around the reinforcement and tapped in such a way

that no honeycombed surface appears on the removal of formwork.

i. The concrete should be placed on formwork as soon as possible.

j. During placing, it should be seen that all the edges and corners of concrete surface remain

unbroken, sharp, and straight in line.

k. There should not be any obstruction along the path of placing concrete as it will cause

segregation.

l. The location at which the concrete is being placed should be visible to the naked eye.

m. For good and uniform finish in wall and columns, it is necessary to fill the forms at a rate

greater than 2.0 to 3.0 m height per hour.

Consolidation of Concrete It is used to mean the compaction between aggregate and aggregate, between aggregate and

reinforcement, and between aggregate and forms. Compacting process consolidates fresh concrete

within the mould of formworks and around the embedded parts and steel reinforcement.

The main aim of consolidation is to eliminate air bubbles and thus give maximum density to

concrete. It is important as a presence of 5% of voids reduces 30% of strength of concrete. The

amount of entrapped air is directly related to workability of concrete. Lower the workability, higher

the percentage of entrapped air.

The process of consolidation of concrete can be carried out either by hand or with the help of

vibrators.

Hand Compaction: It is done for unimportant works. The methods include ramming, tamping,

spading and slicing with suitable tools. It requires fairly wet concrete. Wherever feasible, hand

compaction should be preferred as the use of vibrator may lead to segregation of aggregates. The

concrete mixes that can be hand compacted should not be compacted using vibrators.

Vibrators: Compaction is best done by vibration. As hand compaction requires very sincere

manual effort, good supervision, and high workability, under present circumstances it is not

possible to achieve this, and hence compaction by mechanical means is preferred.

Advantages:

1. It is possible by means of vibrators to make a harsh and stiff concrete mix, with a slump of

about 40 mm or less, workable.

2. The quality of concrete can be improved by use of vibrators as less water will be required or

in other way, economy can be achieved by adopting a leaner mix when vibrators are used.

3. The use of vibrators results in the reduction of consolidation time. Hence the vibrators are

used where the rapid progress of work is of great importance.

4. With the help of vibrators, it is possible to deposit concrete in small openings or places where

it will be difficult to deposit concrete by hand methods.

Types of vibrators: (from Rangwala)

Over vibration should be avoided. A good cohesive mix will withstand over vibration but lean

concrete mix will segregate and result in concrete of non uniform strength, quality and durability.

Curing of Concrete Curing is the process of keeping concrete under a specific environmental condition, i.e. to maintain

an environment of humidity around freshly placed concrete till the process of hydration is relatively

complete. Good curing is typically considered to provide a moist environment and control

temperature. A moist environment promotes hydration resulting in a higher quality material. It

makes concrete harden properly. Allowing the concrete surface to dry out excessively can cause the

concrete to crack.

Improper curing can lead to several serviceability problems including cracking, increased scaling,

and reduced abrasion resistance.

Cement Concrete


After the concrete becomes firm enough to resist marring, should be sprinkled with water, and then

covered with wet canvas, wet burlap, or wet sand. This cover keeps the concrete from drying too

rapidly.

Purpose of curing (from Rangwala)

Period of curing (from Rangwala)

Effects of improper curing (from Rangwala)

Factors affecting evaporation of water from concrete (from Rangwala)

Methods of Curing:

Selection of method of curing is done according to the following factors:

Specifications

Availability of curing materials

Economics

Type of concrete structure (pre-cast / cast in situ)

Shape and size of concrete surface

Aesthetic appearance

All the methods of curing are derived from the basic principle of lowering of the surface

temperatures and prevention of water evaporation.

Several specialized curing techniques are employed in the modern construction work, but the most

commonly employed methods of curing are as follows:

1. Ponding with water: It is the best of the methods. It consists of little earthen dams which are

built over the entire surface to be cured. 1he squares thus formed are then flooded with water

to a depth of about 50 mm or so. This is an effective method for flat horizontal surfaces. But

it is not practicable on vertical surfaces.

2. Covering concrete with wet jute bags: Covering of concrete surfaces with wet jute bags has

also practical limitations on account of difficulty in maintaining them close to the surface

especially in localities having winds blowing at high speed.

3. Covering concrete with wet sand, saw dust, etc.: They can be used only under specific

circumstances.

4. Covering concrete with water-proof paper or polythelene sheets and holding it in position

5. Intermittent spraying with water and continuous sprinkling of water: It is the most common

method of curing under Indian conditions. It is however observed that the intermittent

spraying of water takes place after the surface water has dried out. It results into harmful

effects as young concrete is constantly subjected to multiple wetting and drying leading to

early deterioration.

6. Applying curing compounds: It is a simple operation and it can be brought about by praying

while the concrete is wet. The application is generally carried out by tree sprayers which are

employed for spraying of insecticides.

Formwork It is a temporary construction to contain wet concrete in the required shape while it is cast and

setting. Or

It is the term given to either temporary or permanent moulds into which concrete or similar

materials are poured. Or

Formwork is a complete system of temporary structure to support a construction.

In the context of concrete construction, the falsework supports the shuttering moulds.

Falsework is the temporary structure erected to support work in progress of construction. It is

composed of shores, formwork for beams or slabs (or both), lateral bracings, and that part of

formwork that supports the forms usually for a large structure such as bridge.

The concrete is contained in a timber or steel casing for a certain period after its placing. This

casing is known as the shuttering, centering, form work or moulds and it is to be removed when

concrete has hardened sufficiently to support its own weight.

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Shuttering

Centering is the specified formwork for soffit used in construction of arches, shells, space

structures, or any continuous structure where the entire falsework is lowered decentered or struck

out in the predetermined phases, as a unit, to avoid introducing injurious tress in any part of the

structure. Loosely the term is also used for the parts of formwork required for soffit of beams and

slabs.

Formwork Types

Formwork comes in three main types:

Traditional timber formwork. The formwork is built on site out of timber and plywood or

moisture resistant particleboard. It is easy to produce but time consuming for larger structures,

and the plywood facing has a relatively short lifespan. It is still used extensively where the

labour costs are lower than the costs for procuring re-usable formwork. It is also the most

flexible type of formwork

Engineered Formwork systems or Modular Formwork. This formwork is built out of

prefabricated modules with a metal frame (usually steel) and covered on the application

(concrete) side with material having the wanted surface structure (steel, timber, etc.). The two

major advantages of formwork systems, compared to traditional timber formwork, are speed

of construction (modular systems clip or screw together quickly) and lower life-cycle costs

Stay-In-Place Formwork systems. This formwork is assembled on site, usually out of

prefabricated insulating concrete forms. The formwork stays in place (or is simply covered

with earth in case of buried structures) after the concrete has cured, and may provide thermal

and acoustic insulation, space to run utilities within, or backing for finishes.

Following precautions should be taken for form work of concrete:

1. The formwork should be designed in such a way that it can be easily removed and used again.

2. The formwork should be fixed in such a way that the least hammering is required for its

removal. Otherwise it may injure the concrete.

3. The inside surface of formwork should be coated with crude oil or soft soap solution or grease.

This will make removal of form work easy.

4. The formwork should be sufficiently strong to bear the dead load of wet concrete as well as

the impact of ramming or vibrating the concrete. The over-estimation of loads results in

expensive form work and the under-estimation of loads results in the failure of formwork.

5. It is desirable to bring down the cost of form work to a minimum consistent with safety.

6. The formwork should be so arranged that there is minimum of leakage through the joints. This

is achieved by providing tight joints between adjacent sections of the formwork.

Joints in Concrete Structure The types of joints provided in concrete structures are:

1. Construction joints: The construction joints are

provided at locations where the construction is stopped

either at the end of day or for any other reason, where

the placement of concrete is stopped for more than the

initial setting time of concrete. Here the two successive

placements of concrete meet. The provision of a

construction joint becomes necessary to ensure proper

bond between the old work and the new one.

The construction joints may be horizontal or vertical.

For inclined or curve member, the joint should be at

right angle to the axis of the member.

(See Rangwala also)

Cement Concrete


Expansion and contraction Joints: These joints are provided in all the concrete structures of length

exceeding 12 m, mainly for two purposes:

To allow changes in volume of concrete due to temperature

To preserve the appearance and the original shape of the concrete structures

These joints generally consist of some elastic material, known as the joint filler and dowels or keys.

2. Expansion or isolation joints: These are used to separate or isolate slabs from other parts of

the structure, such as walls, footings, or columns etc. They permit independent horizontal and

vertical movement between adjoining parts of the structure and help minimise cracking when

such movements are restrained.

3. Contraction joints: These are intended to create weak places in concrete and regulate the

locations where cracks resulting from dimensional changes, will occur. These are installed to

allow for shrinkage movement in the structure. It can either be a complete contraction joint

(where both steel and concrete are completely discontinuous) or partial contraction joint

(there is discontinuity of concrete but reinforcements continue across joint). No gap is

provided for the joint.

Guniting (from Rangwala)

Quality Control of Concrete The term quality control in civil engineering parlance is used to mean that the work is done

according to the specifications provided in a contract document.

For preparing a high quality concrete, the field organisation, may broadly be divided into the

following three divisions:

1. The engineering division which provides lines and grades and makes the initial inspection of

all forms, reinforcement and installation of all embedded parts.

2. The manufacturing division which exercises control over concrete materials, batching and

mixing.

3. The placing division which is concerned with the control of concrete placing and of related

operations prior and subsequent thereto.

The concrete produced at site should be strongest, densest, most workable and most economical for

the job for which it is prepared. The amount of cement should be low and that of aggregates should

be high.

The most economical concrete with the highest possible density is obtained by observing the

following general requirements

1. The air bubbles should be eliminated from the body of the concrete.

2. The cement particles should be of the smallest size.

3. The concrete would be compacted fully so as to remove voids.

4. The concrete should be cured sufficiently and adequately, say for 28 days.

5. The cubical particles of the aggregates should be used so that good interlocking is gained.

6. The water-cement ratio should be kept low.

The quality control of concrete demands a high degree of awareness

The quality control, if properly achieved to, grants the following advantages:

1. It helps in improved utilization of scarce resources and in extended utilization of low-grade

materials.

2. It helps to minimize failures.

3. It results into lower costs of construction as higher stresses can be assumed.

4. The structure becomes durable with lower costs of maintenance.

Concrete Testing Engineers usually specify the required compressive strength of concrete which is normally given as

the 28 day compressive strength in megapascals (Mpa) or pounds per square inch (psi). Slump test

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of concrete is done using the slump cone to check the workability of concrete. The concrete test

cubes are cast to find out the compressive strength of the concrete.

Use of Concrete in Structures Mass concrete structures: These include gravity dams such as the Hoover Dam and large

breakwaters.

Reinforced concrete: Reinforced concrete contains steel reinforcing that is designed and placed in

structural members at specific positions to cater for all the stress conditions that the member is

required to accommodate.

Post-tensioned concrete structures: Buildings with monostrand post-tensioned slabs are a widely

used application of prestressed concrete. This method achieves performance and construction

improvements over other construction methods. Post-tensioned slabs are a preferred method for

industrial, commercial and residential floor slab construction. Prestressed floor systems using

monostrand cables may be designed as either one or two way slab systems, and may be flat plate,

flat slab waffle slab, or other slab sections.

Precast concrete: Ancient Roman builders made use of concrete and soon poured the material into

moulds to build their complex network of aqueducts, culverts and tunnels. Modern uses for precast

technology include a variety of architectural applications including free-standing walls used for

landscaping, soundproofing and security walls. Precast architectural panels are also used to clad all

or part of a building facade. Storm-water drainage, water and sewage pipes and tunnels make use of

precast concrete units.

Varieties of Concrete

Regular Concrete: It is the lay term describing concrete that is produced by mixing cement, with

sand or coarse aggregate. This concrete can be produced to yield a varying strength from about 10

MPa to about 40 MPa, depending on the purpose, ranging from blinding to structural concrete

respectively.

Reinforced Concrete: It is made by casting concrete around steel rods or bars. The steel

strengthens the concrete. Almost all large structures require this extra strong type of concrete.

Prestressed Concrete: It is usually is made by casting concrete around steel cables stretched by

hydraulic jacks. After the concrete hardens, the jacks are released and the cables compress the

concrete. Concrete is strongest when it is compressed.

Pervious Concrete or No Fines Concrete (from Rangwala)

Coloured Concrete (from Rangwala)

Lightweight Concrete (from Rangwala)

Precast Concrete (from Rangwala)

Aerated Concrete: It contains tiny air bubbles. These bubbles are formed by adding soaplike

resinous or fatty materials to the cement, or to the concrete when it is mixed. The bubbles give the

water in concrete enough room to expand as it freezes. Such qualities make aerated concrete a good

material for roads and airport runways.

High-Early-Strength Concrete: It is chiefly used in cold weather. This concrete is made with

high-early-strength Portland cement, and hardens much more quickly than ordinary concrete. It

costs more than ordinary concrete. But it is often cheaper to use, because it cuts the amount of time

the concrete must be protected in cold weather.

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High Strength Concrete: It has a compressive strength generally greater than 40 MPa. High-

strength concrete is made by lowering the water-cement (w/c) 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. Super plasticizers are commonly

added to high-strength mixtures to increase its workability.

High Performance Concrete (HPC): It can be defined as A concrete, meeting special

performance requirements that may involve enhancement of placement, and compaction without

segregation, early age strength, toughness, volume stability, or service life in severe

environment. The concrete has high workability, high strength and high durability.

Proportioning of materials, limiting total cement paste content to 1/3rd

by volume of concrete

achieves dimensional stability. Substituting Portland cement by pozzolanic or cementitious

admixture helps improving the properties of both fresh and hardened concrete in addition to making

HPC more economical.

While all high-strength concrete is also high-performance, not all high-performance concrete is

high-strength.

Self Compacting Concrete These self-compacting concretes (SCCs) are characterized by:

extreme fluidity as measured by flow

Self-compacting concretes are also known as self-consolidating ally between 700-750 mm,

rather than slump

no need for vibrators to compact the concrete

placement is simpler

no bleeding of water, or aggregate segregation

SCC can save up to 50% in labor costs due to 80% faster pouring and reduced wear and tear on

formwork.

Roller Compacted Concrete: Roller-compacted concrete 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. Roller-compacted concrete is typically used for concrete pavement.

Glass Concrete: The use of recycled glass as aggregate in concrete. This greatly enhances the

aesthetic appeal of the concrete.

Shotcrete: Shotcrete uses compressed air to shoot (cast) concrete onto (or into) a frame or

structure. Shotcrete is frequently used against vertical soil or rock surfaces, as it eliminates the need

for formwork. It is sometimes used for rock support, especially in tunnelling.

The term Gunite is occasionally used for shotcrete, but properly refers only to dry-mix shotcrete,

and once was a proprietary name.

Fibre Reinforced Concrete In conventional concrete, micro cracks develop even before loading because of drying shrinkage

and other causes of volume change. When the structure is loaded, the micro-cracks open up and

propagate.

However, the addition of small, closely spaced and uniformly dispersed fibres in concrete

substantially improves its static and dynamic properties. These fibres offer increased resistance to

crack growth, through a crack arresting mechanism and improve tensile strength and ductility of

concrete.

Fibre reinforced concrete (FRC) can be defined as a composite material consisting of cement

mortar or concrete and discontinuous, discrete, uniformly dispersed fibres.

The inclusion of fibres in concrete and shotcrete generally improves material properties including

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ductility, toughness, flexural strength, impact resistance, fatigue resistance, and to a small degree,

compressive strength. The type and amount of improvement is dependent upon the fibre type, size,

strength and configuration and amount of fibre.

Type of Fibres

The types of fibres that can be used are:

Steel Fibres: Used in roads, pavements, airfields, bridge deck etc.

Glass Fibres: Used for decorative applications.

Plastic Fibres

Carbon Fibres

Mineral Fibres: Asbestos fibre is generally used. The composite has considerably high

flexural strength.

Advantages of Fibre Reinforced Concrete

Reduction in Shrinkage and Cracking.

Improved Bond Strength.

Enhancement of Fatigue Strength and Endurance Limit.

Better Toughness.

Lowers Permeability of Concrete as fibres reduce micro-crack formation.

Reduces Rebound or scattering in Concrete.

Applications of FRC

Fibre reinforced concrete finds application in situations given below.

Pavements and Floors.

Blast Resistant Structures.

Water Retaining Structures.

Wearing Surface to Existing Bridges / Culverts.

Repairs and Rehabilitation Works.

Hot Weather Concreting

The concrete during the hot weather requires special care in its preparation, transportation,

placement and curing. The problems faced due to the hot weather, impact on quality of concrete, so

precautions need to be taken.

Effects: The control of temperature of concrete is very important as temperature has major effects

(useful and harmful) on its workability, delivery, placing and other properties. The major effects

are:

a. Useful:

i. Accelerates setting of concrete

ii. Rapid initial gain of strength (up to 7 days)

b. Harmful:

i. Faster evaporation of mixing water

ii. Loss of workability

iii. Formation of cold joints

iv. Excessive plastic shrinkage

v. Rapid evaporation of water during curing period

vi. Rapid hardening makes finishing difficult

vii. Reduction in 28 days compressive strength

viii. Increased tendency to crack

The ideal temperature of fresh concrete for placing is between 15 0C to 25

0C. It otherwise needs

heating or cooling.

Remedial measures or precautions that can be taken during hot weather concreting:

a. The most direct way to keep the concrete temperature down is by controlling the temperature

of its ingredients esp. aggregates and mixing water. This can be done by:

i. Keeping aggregates under shade

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ii. Cooling aggregates by sprinkling water

iii. Using cooled water or ice

b. Mix should be designed to have min. cement content consistent with the other functional

requirements such as durability.

c. Use of plasticizers / super plasticizers and retarders.

d. Ambient temperature shall be below 40 0C at the time of placement.

e. The period between mixing and placing shall be kept an absolutely minimum.

f. Formwork, reinforcement and sub grade shall be sprinkled with cool water just prior to

placement of concrete.

g. The area around the work shall be kept wet to the extent.

h. The speed of placement and finishing should be maximum.

i. Immediately after compaction and surface finish, concrete shall be protected from

evaporation of moisture (3 to 4 hrs. after placing). It shall be covered with wet (not dripping)

gunny bags, etc.

j. Once the concrete has attained hardening sufficient to withstand surface damage (approx. 12

hrs. after mixing), moist curing shall commence.

k. Moist curing for hot weather shall not be less than for 10 days.

l. Cure continuously.

The climatic factors affecting concrete in hot weather are high ambient temperature and reduced

relative humidity. The effects may be more pronounced with increased wind velocity.

Cold Weather Concreting

The concrete during the cold weather requires special care in its preparation, transportation,

placement and curing. The problems faced due to the cold weather, impact on quality of concrete,

so precautions need to be taken.

Effects: The control of temperature of concrete is very important as temperature has major effects

(useful and harmful) on its workability, delivery, placing and other properties. The major effects

are:

a. Useful:

i. Slower evaporation of mixing water

ii. Less loss of workability

iii. Less plastic shrinkage

b. Harmful:

i. Decreases setting of concrete

ii. Slow initial gain of strength

iii. Reduction in 28 days compressive strength

Remedial measures or precautions that can be taken during cold weather concreting:

a. The most direct way to keep the concrete temperature high is by controlling the temperature

of its ingredients esp. aggregates and mixing water. This can be done by:

i. Keeping aggregates under in sun

ii. Using hot water or steam

b. Use of accelerators

c. Ambient temperature shall be above 50C at the time of placement.

d. The period between mixing and placing shall be on the higher side.

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