The document provides information about various topics related to concrete properties and construction, including mixing, transporting, compacting, curing, deterioration mechanisms, types of ready mixed concrete, shotcrete, and methods for placing concrete underwater. It addresses these topics through a series of questions and detailed responses. The key points are: 1) Concrete mixing, transporting, compacting, and curing methods are described, along with factors that affect concrete quality such as mixing time, transport distance, and curing methods. 2) Common concrete deterioration mechanisms like carbonation and alkali-silica reaction are explained, including how they occur and ways to mitigate them. 3) Types of ready mixed concrete based on mixing

1. Properties of Concrete (Theory)
ASSIGNMENT#1
Submitted To:
Dr. Ayub Elahi
Submitted By:
Taseer Raza
17-CE-93
Department of Civil Engineering,
University of Engineering &Technology, Taxila

2. ASSIGNMENT # 1
Q#1) Write Short Notes on the following
a) Mixing Concrete.
b) Transporting Concrete
c) Compaction of Concrete
d) Curing of Concrete.
a) Mixing concrete:
The process of rolling, folding and spreading of particles is known as the mixing of
concrete. The materials of concrete should be mixed thoroughly so that there is uniform
distribution of materials in the mass of concrete. The thorough mixing also ensures that cement
water paste completely covers the surfaces of aggregates. The mixing of materials of concrete
can be done either with hand or with the help of a concrete mixer machine.
 Hand Mixing:
The materials should be thoroughly mixed, at least three times, in dry condition
before water is added. The prepared mix should be consumed in 30 minutes after adding water.
The mixing by hand is allowed in case of small works of unimportant nature where small
quantity of concrete is required.
 Machine Mixing:
For machine mixing, all the materials of concrete, including water, are collected
in a revolving drum and then the drum is rotated for a certain period. It is found that
mixing of concrete materials with the help of machines is more efficient and it produces
concrete of better quality in a short time. The time of mixing concrete materials in the
mixer and the speed of mixer are very important factors in deciding the strength of
concrete which is formed. The mixing time should be rotated at a speed as recommended
by the manufactures of the mixer.
The concrete discharged by the mixer, after thoroughly mixing concrete materials, should
be consumed within 30 minutes.
b) Transporting Concrete:
Transporting the concrete mix is defined as the transferring of concrete from the mixing
plant to the construction site. Concrete produced from a mixer is required to be transported
to the forms for placement. This handling may produce segregation and loss of slump, if
the distance is long.
Methods of transporting and placing concrete which are used widely, are:
 Discharge directly into forms through short chute.
 Dumpers and trucks (agitating or non agitation)

3.  Cranes and hoists
 Belt conveyers and boom conveyers
 Concrete pumps and pneumatic placers
c) Compactionof Concrete:
Compaction is the process that expels entrapped air from freshly placed concrete and
packs the aggregate particles together so as to increase the density of the concrete. It
enable the fresh concrete to reach its potential design strength, density and low
permeability.
It ensures proper filling of formworks and that reinforcement is surrounded completely. It
can be of two types:
 Manual compaction:
Hand compaction is used for ordinary and unimportant structures. Workability should be
decided in such a way that the chances of honeycombing should be minimum. The
various methods of hand compaction are as given below.
Tamping.
Rodding.
Ramming.
 Mechanical compaction:
Vibration is imparted to the concrete by mechanical means. It causes temporary
liquefaction so that air bubbles come on to the top and expelled ultimately. Mechanical
vibration can be of various types as given under.
Internal Vibrations
External Vibrations
Table Vibrations
Form Vibration
Surface Vibration.
d) Curing of Concrete:
Curing can also be described as keeping the concrete moist and warm enough so that
the hydration of cement can continue. As long as free moisture and unhydrated cement
exist inside the concrete, the strength, hardness and density will gradually increase.
As the concrete gets stronger and denser, its porosity decreases.
If the concrete dries out, it stops gaining strength. This is why it is so important to cover
concrete right after casting and keep it moist.
It is better to use hessian cloth for first 24 hours and then curing can be done by
ponding and spraying.

4.  Methods of Curing:
Water Curing
Steam Curing
Application of heat.
Membrane Curing
Curing Compounds.
Q#2) Discuss in detail following deterioration mechanisms in concrete
a) Carbonation.
b) Alkali Silica reaction.
a) Carbonation:
Carbonation is the reaction of carbon dioxide in the environment with the calcium
hydroxide in the cement paste. This reaction produces calcium carbonate and lowers the
pH to around 9. At this value the protective oxide layer surrounding the reinforcing steel
breaks down and corrosion becomes possible. Carbonation is very slow in dry concrete.
In most structures made with good quality concrete, carbonation will take several (or
many) years to reach the level of the reinforcement.
The concrete will carbonate if CO2 from air or from water enters the concrete according to:
Ca(OH)2 + CO2 -> CaCO3 + H2O
Methods to identify Carbonation:
 Carbonation may be recognized in the field by the presence of a discolored zone in the
surface of the concrete. The color may vary from light gray and difficult to recognize to
strong orange and easy to recognize.
 Carbonation can be visualized by phenolphthalein. Uncarbonated regions of concrete will
show pink colour in phenolphthalein test.
b) Alkali Silica Reaction:
Alkali silica reaction is the reaction between alkali content in cement paste and reactive
silica present in aggregates. The reaction product is a hygroscopic gel, which absorbs
moisture and swells. The formation of the gel can cause expansion and, eventually,
cracking of the concrete.
Mitigating ASR:
 It is very difficult, if not impossible, to halt the ASR reaction once it begins. It may be
possible in some instances to limit the ingress of water into the concrete, but this will
only slow down rather than stop the progress of deterioration.
 Limit the alkali content of the concrete by minimizing the amount of alkali contributed by
the portland cement.

5.  Another solution is to limit or prohibit the use of reactive aggregates.
 Another alternative is to specify slag cement to prevent ASR when reactive aggregates are
used.
Q#3) Write a note on Types of ready Mixed Concrete.
Ready Mixed Concrete is a concrete that is manufactured in a factory or within a batching
plant based on the standard required specifications.
There are three types of ready mix concrete (RMC) depending upon the mixing of the various
ingredients as given below:
o Transit mixed concrete
o Shrink mixed concrete
o Central mixed concrete
 Transit MixedConcrete:
It is also called dry batched concrete because all the basic ingredients including water are
charged directly into the truck mixer. The mixer drum is revolved fast at charging speed
during the loading of the material and after that it continues rotating at a normal agitating
speed.
 Shrink Mixed Concrete:
The concrete is partially mixed in the plant mixer and then balance mixing is done in the
truck mounted drum mixer during transit time. The amount of mixing in transit mixer
depends upon the extent of mixing done in the central mixing plant. Tests should be
conducted to establish the requirement of mixing the drum mixer.
 Central Mixed Concrete:
It is also called central batching plant where the concrete is thoroughly mixed before
loading into the truck mixer. Sometimes the plant is also referred as wet-batch or pre-mix
plants. While transporting the concrete, the truck mixer acts as agitator only. Sometimes,
when workability requirement is low or the lead is less, non-agitating units or dump trucks
can also be used.
Q#4) What is remixing of concrete? Under what conditions remixing is
allowed? What are consequencesofremixing concrete?
Remixing of concrete:
Concrete begins to stiffen as soon as water and cement is mixed. The stiffness that
occurs within first 30 minute is generally not a problem. If concrete is kept agitated it can be
used within one and a half hour of mixing.
Fresh concrete that is left to agitate in the mixer drum may be used upon remixing if it
becomes sufficiently plastic to be consolidated in the forms.

6. Conditions of Remixing:
Under proper supervision a small amount of water may be added to remix the concrete
provided the following conditions are met:
 Maximum allowable water-cement ratio is not exceeded.
 Maximum allowable slump is not exceeded.
 Maximum allowable agitating and mixing time is not exceeded.
 Concrete is remixed for half the minimum required mixing time or number of revolutions.
ConsequencesofRemixing:
 Adding too much water to make concrete more fluid can lower its strength.
 Adding too much water can also cause segregation.
 Remixed concrete tends to hardens quickly.
 A cold joint may develop when concrete is poured next to or on remixed concrete.
Q#5) How concrete canbe mixed using stationary mixers?
A Concrete mixer is a device that uniformly combines cement, aggregate such as sand or
gravel and water to form concrete . A typical concrete mixer uses a revolving drum to mix the
components. For smaller volume works, portable concrete mixers are often used so that the
concrete can be made at the construction site, giving the workers sufficient time to use the
concrete before it hardens.
The rotating drum allows users to mix their materials with ease and using a very limited
amount of energy.
Stationary mixers include both mixers at batching plant and portable mixers at site . In
batching plant mixer cement is added through pipes, sand and aggregate are added through
conveyor belt.
 In stationary mixers at site pour water, 1 cement bag, sand and aggregate according to
specified mix design.
 Turn the mixer on and mix properly.
 Pour the mixed concrete in wheel barrow to transport it to desired place.
Mixing time:
Once the appropriate mixer has been chosen, it is necessary to determine the mixing time. This
is the duration of time it takes to mix concrete, once the mixer is fully charged with all the
materials. This is not same for all batches. a drum mixer with a small diameter creates a greater
velocity than a drum mixer with a large diameter, therefore, the mixing time would be decreased.
However, if the goal is stiffer concrete, a longer mixing time is required.

7. Q#6) What is Shotcrete? Explainit with the help of examples.
Shotcrete:
Shotcrete is a concrete conveyed through a hose and pneumatically projected at high
velocity onto a surface. It is the force of this spraying action that leads to compaction of
the concrete which then forms layers of concrete to the required thickness.
Shotcreting has proved to be the best method for construction of curved surfaces. Tunnel
linings and domes are now much easier to construct with the advent of with shotcrete
technology. Shotcrete is a widely accepted and used way of placing material that is cementitious
in nature for a vast variety of applications.
Examples of Shotcrete applications:
The success of the shotcrete application depends upon proper planning and supervision, plus
the skill and continuous attention provided by the shotcrete applicator.
 Repair:
Shotcrete can be used to repair the damaged surface of concrete, timber,
or steel structures provided there is access to the surface needing repair. The following
examples indicate a few ways in which shotcrete can be used in repairs:
Bridges:
Shotcrete repair can be used for bridge deck rehabilitation, but it has generally been
uneconomical for major full-thickness repairs.
Buildings:
In building repairs, shotcrete is commonly used for repair of fire and
earthquake damage and deterioration, strengthening walls, and encasing structural
steel for fireproofing. The repair of structural members such as beams, columns, and
connections is common for structures damaged by an earthquake.
Spillway Surfaces:
Surfaces subject to high velocity flows may be damaged by cavitation erosion or
abrasion erosion. Shotcrete repairs are advantageous to repair such damage.
 Underground Excavation:
For the most part, shotcrete is used in underground excavations in rock. Typical
underground shotcrete applications range from supplementing or replacing conventional
support materials such as lagging and steel sets, sealing rock surfaces,
channeling water flows, and installing temporary support and permanent linings.
 New Structures:
Shotcrete can be used effectively where thin sections and large areas are involved.
Some examples of shotcrete uses in such cases are:
Pools and tanks:
Shotcrete has been used extensively to construct concrete swimming pools. More
recently, large aquariums have been constructed using shotcrete.

8. Shotcrete floor and walls:
Shotcrete floors in tanks and pools on well compacted sub-base or on undisturbed earth
have generally given excellent service. Vertical
and overhead construction for walls, slabs, columns, and other structural members has
been frequently shotcreted.
Shotcrete Domes.
Construction techniques using inflatable air-forming systems have made
the construction of shotcrete shells or domes practical.
Q#7) How quality of finished concrete canbe affectedby poor transportation
techniques?
Concrete quality can be greatly affected by poor transportation techniques. Some of them:
 Poor transportation technique can cause segregation of aggregates with poor surface
finish.
 Concrete tends to become harden when agitated truck is not used for delivery of concrete.
 This can pose a difficulty in handling and finishing concrete. Water is added on site to
remix concrete to gain fluidity and workability which in turn if exceeds designed water-
cement ratio would result in loss of strength.
 Movement of hand trolly or trucks on rough road surface makes vibrations.
 This results in heavy aggregates to deposit at bottom of truck.
 Water and cement slurry come on top.
 Concrete can waits in a truck for long can
Q#8) Expalin the use of Chutes, Belt conveyors and concrete pumps with
suitable examples.
Chutes:
Concrete Chute is a long metal trough with rounded bottom and open ends used to transport
concrete at lower elevation. Normally chute is made of metal or lined with metal and provided
with uniform slope. Generally a 1V:2.5H slope is adopted.
The slope of the chute must not be flatter than it.
 They are mostly use for placing concrete at lower level.
 They pour concrete with precision and maintain strength and prevents segregation.
 Concrete can be placed upto 18 feet far using Chute. Initial setting time should be
considered while using chute for transporting concrete. The height of chute should be less
than 3m to prevent segregations.

9. Belt Conveyors:
Belt conveyors are used to discharge concrete from truck mixer to point of discharge. But
belt conveyors are used with extent. The main objection in this type of equipment is segregation
which is not allowed in structural concrete.
 Modern conveyors have adjustable reach, variable speed towards back and forth, travelling
diverter.
 This system extremely useful when enormous amount of concrete has to be transferred
quickly through a relatively less accessible area.
 They can be used to transport both to lower grounds and higher grounds.
 The discharge end must be designed such that the arrangement minimize segregation of
concrete and ensure discharging of entire mortar form belt.
Concrete Pumps:
A concrete pump is a machine used for transferring liquid concrete by pumping.
Boom Concrete Pumps:
It uses a remote-controlled articulating robotic arm (called a boom) to place concrete
accurately. This type of concrete pump is attached to a truck or longer units are on semi-trailers.
 Boom pumps are used on most of the larger construction projects as they are
capable of pumping at very high volumes
 They have labor saving nature.
 They are a revolutionary alternative to line-concrete pumps.
Line Pump/Trailer -Mounted concrete Pumps:
The second main type of concrete pump is either mounted on a truck or placed on a
trailer. This pump requires steel or flexible concrete placing hoses to be manually attached to the
outlet of the machine.
 Line pumps normally pump concrete at lower volumes than boom pumps.
 They are used for smaller volume concrete placing applications such as swimming
pools, sidewalks, and single family home concrete slabs and most ground slabs.
Q#9) What is meant by placing of concrete?Youre Supposedto Place concrete
Underwater in bridge pier Which method would you choose to place concretin
such conditions?
Placing of concrete:
The process of discharging concrete from trucks to the final destination point is placing of
concrete. Before placing concrete, the formwork and reinforcement should be checked to make
sure that they are clean and free of any debris, such as ends of tying wire. The fresh concrete should
be deposited as close as possible to its final position.

10. Underwater concreting:
To place concrete underwater such as in bridge piers Methods such as tremie method is used
extensively:
Tremie Method:
Underwater concreting using tremie method is convenient for pouring large amount of high
flowable concrete. The concrete is moved to the hopper by either pumping, belt conveyer or skips.
Tremie pipe, which upper end connected to a hopper and lower end continuously submerged in
fresh concrete, is used to place concrete at the exact location from a hopper at the surface. The
reason to immerse the tremie pipe lower end is to prevent intermixing of both concrete and water.
Placing the concrete:
As soon as concreting began the pipe mouth should be submerged up to 1- 1.5 m into fresh
concrete to prevent water entering the pipe. The concrete flow rate is controlled by lowering and
raising the pipe and either decrease or increase in concrete discharge indicates the loss of the
seal, therefore flow of concrete should be continuous and carefully monitored.
Q#10) What are Limitations of Equipments Belt Conveyors, cranes and
buckets, Wheel barrows, Concrete Pumps and tremies.?
Limitations of Belt conveyors:
 Vibration of rubber belt is responsible for concrete segregation.
 Drying and consequent stiffening of concrete while it passes over long distance
exposed to hostile ambient environment.
 Continuous or periodic monitoring of belt is necessary
 Heat affects the material of belt.
Limitations of Cranes and buckets:
 When larger amounts of concrete have to be installed it is unfavorable to have
only limited bucket capacity, since the concrete can only be transported step by
step.
 Avoid great falling heights during concrete placing, especially in case of fair-
faced concrete, highly flowable concrete and self-compacting concrete. Great
falling heights can result in segregation of the fresh concrete.
 Careful operation between trades and operation is needed to keep crane busy.
Limitations of Wheelbarrows:
 They are slow and labor is required to operate that.
 Hand carts (buggies) concrete short distances for a high rise building project.
 Labor costs per unit volume of concrete for this method of handling are high,
although the method remains popular for small owner placed

11. Limitations of Concrete pumps:
 Constant supply of freshly-mixed concrete is needed with average consistency
and without any tendency to segregate.
 Care must be taken in operating pipeline to ensure an even flow and to clean out
at conclusion of each operation.
 If high production rates in cubic yards per hour are required, the smaller pumps
may not have enough capacity.
Limitations of tremies:
 It is necessary to use a high slump, 7 to 8 inches, since it is not practical to
vibrate tremie concrete.
 It is necessary to add extra cement to tremie concrete for safety. This results in
increased costs and greater internal heat development. However, heat
development is generally dissipated fairly quickly because of the temperature of
the water above the concrete.
 In many cases it is difficult or impossible to inspect the results of the job.
Q#11) What is meant by consolidationof concrete? How concrete canbe
consolidatedmechanically?
Consolidationof concrete:
Consolidation is the process of inducing a closer arrangement of the solid particles in
freshly mixed concrete or mortar during placement by the reduction of voids, usually by
vibration
Centrifugation
Rodding
Tamping
or some combination of these actions
Drier and stiffer mixtures require greater effort to achieve proper consolidation.
When good consolidation practices are combined with good formwork, concrete surfaces have a
highly pleasing appearance.
MechanicalConsolidationofConcrete:
The most widely used consolidation method is vibration. Vibration may be either internal,
external, or both.
 Centrifugation (spinning) is used to consolidate concrete in concrete pipe, piles, poles,
and other hollow sections.
 Many types of surface vibrators are available for slab construction, including vibrating
screeds, vibratory roller screeds, plate and grid vibratory tampers, and vibratory finishing
tools.

12.  For most cast-in-place concrete placed in forms, consolidation is accomplished with an
internal vibrator with a vibrating head on the end of a shaft and usually a flexible shaft.
The operator controls the power unit and a spinning cable rotates an eccentric weight in
the head to create the vibration.
 The vibrator should be allowed to sink under its own weight to the proper depth then be
moved up and down for five to 15 seconds. It is then pulled out slowly, about 15 seconds
for every 2 feet. When in doubt, vibrate more with well-proportioned concrete there
would be no segregation of the aggregate.
 Shock or drop table is used to consolidate extremely stiff low slump concrete in making
precast units.
 The Table vibrator is widely suitable for consolidating concrete surfaces such as floors
and slabs. It adequately consolidates slabs up to 20 cm thickness; internal vibration is
required for higher slab thicknesses.
 The form vibrator is attached to the exterior face of mold or forms properly otherwise
energy will be lost because of improper attachments. Moreover, form vibrator is the
proper choice for compacting concrete in thin and heavily congested forms, consolidating
stiff mixtures, and supplementing external vibrators.
Q#12) State the purpose of consolidation. Whatdefects can be produced
from over vibrations and under vibrations?
Purpose of consolidation:
Consolidation eliminates rock pockets and air bubbles and brings fine material both to
the surface and against the forms to produce desired finish. The process involves carefully
working around all reinforcing steel with the compacting device to assure proper embedding of
reinforcing steel in the concrete.
Defects from Under vibrations:
 Excessive entrapped air voids;
When vibrations do not reach in all directions or done for less time, Under vibration
leaves a lot of entrapped air in concrete which reduces strength
 Sand streaks:
When heavy bleeding washes mortar then a harsh mixture lefts behind that lacks
workability. It is also caused by insufficient fine aggregates.
 Placement lines:
These are dark lines between adjacent layers of concrete batches. it occurs when
vibrators did not penetrate through under lying layers.

13. Defects from Over Vibrations:
 Segregation:
For most of concrete mixes, over-vibration creates the problem of segregation in which
the denser aggregates settle to the bottom while the lighter cement paste tends to move
upwards.
 Bleeding:
Bleeding in concrete is caused when surplus water rise to the surface when concrete is
vibrated. The bleed water brings with it a small amount of cement and fines. Such
particles are left on the surface of the concrete casting after the water has evaporated.
 Form damages
Over vibration may damage the formwork. Formwork should be strong enough to bear
the vibrations .
Q#13) What are superplasticizers? How they help in producing self-
compacting concrete?
Super Plasticizers:
Superplasticizers, also known as high range water reducers, are chemical
admixtures used where well-dispersed particle suspension is required. These polymers
are used as dispersants to avoid particle segregation, and to improve the flow
characteristics of suspensions such as in concrete applications.
Self-compacting concrete:
 Glenium C303SCC is a high-performance superplasticizer for self-compacting
concrete.
 It has a polycarboxylic ether base that provokes electrostatic repulsion in the
cement particles and prevents their flocculation even after hydration has begun.
 The high dispersion capacity of the additive produces self-levelling concretes with
a water/cement ratio of around 0.3.
 These low ratios provide an important increase in structural strength and reduced
porosity.
 Since all the water in the mix reacts with the cement and none is free to evaporate.
This results in greater impermeability and durability.
 HAC-Glenium self-compacting concrete spreads around reinforcing to fill every
corner of the mould or formwork, avoiding blockage and segregation of large
aggregate, without the aid of vibration or other compaction techniques.
 Its low occluded air content results in fewer surface pores and thus a better finish.
In addition, colours are more homogeneous and efflorescence produced by
vibration is eliminated.

14. Q#14) What is Passivation?How passive layer in concrete is deteriorated?
Passivation:
Passivation, in physical chemistry and engineering, refers to a material becoming
"passive," that is, less affected or corroded by the environment of future use. Passivation
involves creation of an outer layer of shield material that is applied as a microcoating, created by
chemical reaction with the base material, or allowed to build from spontaneous oxidation in the
air.
Passive layeron steelin concrete:
 Carbonation process in concrete initiates the corrosion process of steel bars by
demolishing the protective oxide layer around steel bars.
 The alkaline conditions that prevail in concrete (pH > 13.0) favor the formation of a
passive layer on the embedded steel reinforcement.
 At the high pH, a thin oxide layer forms on the steel and prevents metal atoms from
dissolving. This passive film does not actually stop corrosion; it reduces the corrosion
rate to an insignificant level.
 For steel in concrete, the passive corrosion rate is typically 0.1 µm per year. Without
the passive film, the steel would corrode at rates at least 1,000 times higher
2Fe → 2Fe2+
+ 4e-
2H2O + O2 + 4e-
→ 4OH
2Fe2+
+ 4OH-
→ 2Fe(OH)
 The amount of carbonation is significantly increased in concrete with a high water-to-
cement ratio, low cement content, short curing period, low strength, and highly
permeable or porous paste.
 When pH is lowered, chloride threshold for corrosion is significantly lowered at or
below 100 ppm. Like chloride ions, however, carbonation destroys the passive film of
the reinforcement, but does not influence the rate of corrosion.
Q#15) How do you define durability of concrete? Discussthe mechanism of
deteriorationof concrete due to carbonation. What defects are produced in
concrete due to carbonation?
Durability of Concrete:
 Durability is the ability of a concrete or any material to last a long time without
significant deterioration.
 Concrete resists weathering action, chemical attack, and abrasion while maintaining
its desired engineering properties.
 Different concretes require different degrees of durability depending on the exposure
environment and the properties desired.

15.  Concrete ingredients, their proportioning, interactions between them, placing and
curing practices, and the service environment determine the ultimate durability and
life of the concrete.
Carbonationof Concrete:
Carbonation is the reaction of carbon dioxide in the environment with the calcium
hydroxide in the cement paste. This reaction produces calcium carbonate and lowers the pH to
around 9. At this value the protective oxide layer surrounding the reinforcing steel breaks down
and corrosion becomes possible.
Conditions for Carbonation:
 The reaction of carbon dioxide and calcium hydroxide only occurs in solution and so in
very dry concrete carbonation will be slow.
 In saturated concrete the moisture presents a barrier to the penetration of carbon dioxide
and again carbonation will be slow.
 The most favourable condition for the carbonation reaction is when there is sufficient
moisture for the reaction but not enough to act as a barrier.
Mechanism:
 The first reaction is in the pores where carbon dioxide (CO2) and water (H2O) react to
form carbonic acid (H2CO3):
CO2 + H2O H2CO3
 The carbonic acid then reacts with the calcium phases:
H2CO3 + Ca(OH)2 CaCO3 + 2H2O
Defects in Concrete due to carbonation:
 It reduces the quality of concrete significantly.
 Carbonation changes alkaline nature to acidic nature.
 Reduces ability of concrete to protect steel reinforcement from rusting.
 Concrete may become porous and impermeable.
Q#16) Explain Carbonationbriefly. State the method to measure the depth of
carbonationin laboratory.
Carbonation of concrete:
Carbonation is the reaction of carbon dioxide in the environment with the calcium
hydroxide in the cement paste. This reaction produces calcium carbonate and lowers the pH to
around 9. At this value the protective oxide layer surrounding the reinforcing steel breaks down
and corrosion becomes possible.
 The reaction of carbon dioxide and calcium hydroxide only occurs in solution and so in
very dry concrete carbonation will be slow.

16.  In saturated concrete the moisture presents a barrier to the penetration of carbon dioxide
and again carbonation will be slow.
 The most favourable condition for the carbonation reaction is when there is sufficient
moisture for the reaction but not enough to act as a barrier.
Methods to measure depth of carbonationin laboratory:
Using pH indicator:
 In this method, first concrete specimen is kept in an open environment for a number of
years or in Carbonation Chamber for a number of months.
 Generally, conditions of 5% CO2, 50% Relative Humidity, and 20-22ºC is maintained in
a carbonation chamber.
 Then sample is broken and is sprayed with a pH indicator.
 Popularly a standard solution of 1% phenolphthalein in 70% ethyl alcohol is used.
 In the noncarbonated region with pH values above 9.2, the phenolphthalein indicator
turns purple-red; and in the carbonated portion with pH less than 9.2, the solution
remained colorless.
Q#17) What is meant by AAR. How ASR causesdeteriorationin concrete?
Alkali Aggregate Reaction(AAR):
AAR is a reaction between alkali content present in cement and aggregates containing
either carbonates or silica. While mostly inert, some concrete aggregates, can react in the highly
alkaline environment in concrete resulting in internal expansion that causes deleterious cracking.
Expansion due to AAR is a slow process and results in deterioration 10 to 15 years after
the concrete structure has been built.
Deteriorationdue to Alkali Silica reation:
 Alkali silica reaction is a heterogeneous chemical reaction which takes place in
aggregate particles between the alkaline pore solution of the cement paste and
silica in the aggregate particles.
 Hydroxyl ions penetrate the surface regions of the aggregate and break the
silicon-oxygen bonds. Positive sodium, potassium and calcium ions in the pore
liquid follow the hydroxyl ions so that electro neutrality is maintained.
 Water is imbibed into the reaction sites and eventually alkali-calcium silica gel
is formed.
 The reaction products occupy more space than the original silica so the surface
reaction sites are put under pressure.
 At a certain point in time the tensile stresses may exceed the tensile strength
and brittle cracks propagate. The cracks radiate from the interior of the
aggregate out into the surrounding paste.
In general, the reaction can be viewed as a two-step process :

17. Step 1:
Silica + alkali alkali-silica gel (sodium silicate)
SiO2 + 2NaOH + H2O Na2SiO3.2H2O(2KOH can replace 2NaOH)
Step 2:
Gel reaction product + water expansion
Q#18) State the defects produced in concrete due to ASR? What are the
mitigation measures forASR?
Defects due to ASR:
The cracking caused by ASR can have several negative impacts on concrete, including:
 Expansion: The swelling nature of ASR gel increases the chance of expansion in
concrete elements.
 Compressive Strength: The effect of ASR on compressive strength can be minor for
low expansion levels, to relatively higher degrees at larger expansions compressive
strength is not very accurate parameter to study the severity of ASR; however, the test is
done because of its simplicity.
 Fatigue: ASR reduces the load bearing capacity and the fatigue life of concrete.
ASR Mitigation techniques:
 Limit the alkali metal content of the cement. Many standards impose limits on the
"Equivalent Na2O" content of cement.
 Limit the reactive silica content of the aggregate. Certain volcanic rocks are particularly
susceptible to ASR because they contain volcanic glass (obsidian) and should not be used
as aggregate.
 The use of calcium carbonate aggregates is sometimes envisaged as an ultimate solution
to avoid any problem.
 Another method to reduce the ASR is to limit the external alkalis that come in contact
with the system.
 The alkali-silica reaction will not take place in a concrete structure if the internal relative
humidity of the concrete is lower than 80%. As a result, keeping the concrete dry will
prevent the reaction from occurring. However, this is practically impossible for exterior
structures.
 Lowering the permeability of concrete by reducing the water-cement ratio reduces the
internal moisture and delays the reaction.
 Effective mineral admixtures include fly ash, silica fume, ground granulated slag, and
calcined clay reduce ASR expansions by either reducing the alkali content of the concrete
mix or reducing the pH of the pore solution

18. Q#19) Describe Briefly methods and equipment for transporting and placing
concrete.
Concrete transportation and placing equipments.
EQUIPMENT USES, ADVANTAGES AND DISADVANTAGES
1.Truck mixers  Both mixing and transporting equipment
 The mixing process is usually useful to maintain the consistency
of concrete.
 Available in 1m3 to 12m3 drum and interior depends upon
external conditions.
 The rotation speed of the drum is controlled and is also a very
important factor for the mixture control.
2. Non Agitating
truck
 Used to transport concrete on short hauls over smooth
Roadways.
 Capital cost of nonagitating equipment
is lower than that of truck agitators or mixers.
 Concrete slump should be limited. Possibility of segregation.
Height is needed for high lift of truck body upon discharge.
Concrete tends to stiffens.
3. Belt coveyors  It is used to place concrete on site. For conveying concrete
horizontally or to a higher or lower level.
 Belt conveyors have adjustable reach, traveling diverter, and
variable speed. It Can place large volumes of concrete quickly
when access is limited.
 End-discharge arrangements needed to prevent segregation. In
hot weather, belt needs cover
4. Buckets and
cranes
 They are used to transport and place concrete on upper floors of
high rise buildings.
 Convey concrete directly from central discharge point to
formwork or to secondary discharge point.
 Bucket capacity should conform to the size of the concrete
batch. Discharge should be controllable
5. Chutes on truck
mixers
 For conveying concrete to a lower level, usually below ground
level, on all types of concrete construction.
 Low cost and easy to plan. No power required; gravity does
most of the work.
 End-discharge arrangements needed to prevent segregation.
6. Pneumatic Guns  Used where concrete is to be placed in difficult locations and
where thin sections and large areas are needed.

19.  Ideal for placing concrete in freeform shapes, for repairing
structures, for protective coatings, thin linings, and building
walls with one-sided forms.
 Only experienced nozzle men should be employed.
7. Wheel barrows  Used to transport concrete on short flat hauls on all types of
onsite concrete construction, especially where accessibility to
work area is restricted.
 Ideal inside and on jobsites where placing conditions are
constantly changing but labor intensive.
8. Pumps  Used to convey concrete directly from central discharge point at
jobsite to formwork or to secondary discharge point.
 Delivers concrete in continuous stream. Pump can move
concrete both vertically and horizontally.
 Truck-mounted pumps can be delivered when necessary to small
or large projects.
 Tower-crane mounted pump provides continuous concrete for
tall building construction.
Q#20) Define Shotcrete. Differentiate betweendry shotcrete and wet
shotcrete.
Shotcrete:
Shotcrete is a concrete conveyed through a hose and pneumatically projected at high
velocity onto a surface. It is the force of this spraying action that leads to compaction of
the concrete which then forms layers of concrete to the required thickness.
DRY SHOTCRETE WET SHOTCRETE
 In this process, cement and moist
aggregate are mixed and then placed
into a device that meters the mixed
material into a stream of compressed
air
 In the wet-mix process, all ingredients
are first mixed to produce mortar or
concrete. The mortar or concrete is
then placed into delivery equipment.
 Water is added at end of the nozzle  Water is added with all ingredients
and compressed air is added at end of
nozzle.
 Bonding capability is excellent due to
low water cement ratio
 Bonding capacity is low due to high
water cement ratio.
 Amount water added can be variable  Amount of water can be controlled
and is constant

20. Q#21) What is compactionof concrete?Explain briefly importance of
compaction.
Compactionof Concrete:
Compaction is the process that expels entrapped air from freshly placed concrete and
packs the aggregate particles together so as to increase the density of the concrete.
Importance of Compaction:
 Compaction significantly increases the ultimate strength of concrete and enhances the
bond with reinforcement.
 It also increases the abrasion resistance and general durability of the concrete, decreases
the permeability.
 It helps to minimise its shrinkage and creep characteristics.
 Proper compaction also ensures that the reinforcement, tendons, inserts and fixings are
completely surrounded by dense concrete, the formwork is completely filled – i.e. there
are no pockets of honey-combed material.
 It is important to compact the concrete fully because, Air voids reduce the strength of the
concrete. For every 1% of entrapped air, the strength falls by somewhere between 5 and
7%. This means that concrete containing about 5% air voids due to incomplete
compaction can lose as much as one third of its strength.
Q#22) Why curing of concrete is important? What are the factors that may
affectduration of curing.
Importance of Curing of concrete:
 Concrete derives its strength as a result of the chemical reaction of the mix water and
the cement, a reaction that starts at the instant the two materials first come in contact with
each other and can continue for long time.
 Curing is done to control the rate and extent of moisture loss from concrete to ensure an
uninterrupted hydration of Portland cement after concrete has been placed and finished in
its final position.
 Concrete that “dries” out will not reach its design strength or meet specifications. The
longer the cure, the better the concrete.
 Cement requires a water/cement ratio about 0.23 for hydration and a water/cement
ratio of 0.15 for filling the voids in the gel pores. In other words, a water/cement
ratio of about 0.38 would be required to hydrate all the particles of cement and also
to occupy the space in the gel pores.
Duration of Curing:
The duration of curing of concrete depends on
 The grade & type of cement
 Mix proportion
 Desired concrete strength

21.  Shape and size of the concrete member
 Environmental & exposure conditions.
The duration may vary from few days to a month.
In case of Ordinary Portland Cement (OPC), Exposed surfaces of concrete shall be kept
continuously damp or in a wet condition by ponding or by covering with sacks, hessian cloth
or other similar material and kept continuously wet for at least 7 days from the date of
placing. In case of concrete where mineral admixtures or blended cements are used, it is
recommended that the above minimum periods may be extended to 10-14 days, for assisting
the secondary reaction.
Q#23) What role is played by curing in strength development process of
concrete? State the method of curing in extreme weather.
Role of curing in strength development:
Curing is done to control the rate and extent of moisture loss from concrete to ensure an
uninterrupted hydration of Portland cement after concrete has been placed and finished in its
final position. Concrete strength increase with age as moisture and a favorable temperature is
present for hydration of cement.
 An experimental investigation was
conducted by "Cement, Concrete &
Aggregates Australia" (CCAA) on the
importance of curing.
 A comparison of the strength of
concrete at 180 days of moist curing
with various periods of moist curing
(0, 3, 7, 14 & 28 days) and then
allowing it to dry out. From the graph , it can be observed that concrete allowed to
dry out immediately, achieves only 40% of the strength of the same concrete water
cured for the full period of 180 days.
Curing in hot weather:
 Curing materials which reflect sunlight to reduce concrete temperature must be
used.
 Water curing is recommended and care should be taken to prevent excessive stress
caused by alternative wetting and drying or by cold water on warm concrete.
 Framed enclosures of canvas tarpaulins or sun shades may be used to protect the
concrete from direct sunlight.
Curing in cold weather:
 In cold weather, some procedures like heated enclosures, insulating blankets &
curing compounds may be used.

22.  The temperature of fresh concrete must be kept above 100C by using heated
raw materials and the curing shall be continued for a longer period of time till
concrete gains the desired strength.
Q#24) What steps are involved in the concreting process?Explain
The concreting process consists of the following steps:
 Batching
 Mixing.
 Delivery.
 Placing
 Compaction
 Curing
Batching:
 Batching is the process of measuring concrete mix ingredients by either mass or volume
and introducing them into the mixer. To produce concrete of uniform quality, the
ingredients must be measured accurately for each batch.
 Water and liquid admixtures can be measured accurately by either volume or mass.
Volumetric batching is used for concrete mixed in continuous mixers.
 Specifications generally require that materials be measured for individual batches within
the following percentages of accuracy: cementitious material ±1%, aggregates ± 2%,
water ±1%, and admixtures ± 3%.
 Admixtures that cannot be added in solution can be either batched by mass or volume as
directed by manufacturer.
Mixing:
 After batching of concrete ingredients, they are mixed in equipments specified by
standard codes of practices (ASTM).
 All concrete should be mixed thoroughly until it is uniform in appearance, with all
ingredients evenly distributed.
 Increased output should be obtained by using a larger mixer or additional mixers, rather
than by speeding up or overloading the equipment on hand.
Stationary mixing:
Concrete is sometimes mixed at the jobsite in a stationary mixer or a paving mixer.
Stationary mixers include both onsite mixers and central mixers in ready mix plants.
They are available in sizes up to 9.0 m3 (12 yd3) and can be of the tilting or non-tilting type
or the open-top revolving blade.
All types may be equipped with loading skips and some are equipped with a swinging
discharge chute.
Ready Mix concrete:
This concrete that is manufactured in a batch plant, according to a set engineered mix
design. It can be manufactured by any of the following methods:

23. Central-mixed concrete is mixed completely in a stationary mixer and is delivered
either in a truck agitator, a truck mixer operating at agitating speed, or a non-agitating
truck.
Shrink-mixed concrete is mixed partially in a
stationary mixer and completed in a truck mixer.
Truck-mixed concrete is mixed completely in a truck
mixer.
Transportation:
 First is the barrel truck or in–transit mixers. This type of truck delivers concrete in a
plastic state to the site.
 Second is the volumetric concrete mixer. This delivers the ready mix in a dry state and
then mixes the concrete on site.
 Non agitating trucks used to transport concrete on smooth haul roads. Capital cost of
these trucks may be lower than agitating mixers. Concrete tends to stiffen more rapidly in
non agitating trucks therefore slump should be more.
 Transporation techniques adopted must be good as poor transportation can greatly impair
the quality of fresh concrete.
 Poor transportation techniques can cause stiffening, segregation and slump loss.
Placing:
 Concrete can be placed using different equipments that can handle concrete effectively on
site.
 As soon as concrete is brought to site by transit mixers, they should be discharged as
soon as possible.
 Different equipments such as conveyor belts, chutes, concrete pumps, pneumatic guns are
used for concrete placement.
 Placing concrete should be avoided during noon or in very hot day.
 Materials that touches concrete should be kept cool.
Compaction:
 Compaction is done to remove air voids from freshly placed concrete and to increase
density of concrete to pack particles together effectively.
 Compaction increase durability of concrete by decreasing pororsity and water
permeability of concrete.
 It produces fine finishes of concrete surface.
 Compaction can be done either manually or mechanically. Manual compction is done by
tamping, ramming and rodding and is usually performed in small quantity of works and
unimportant structures.
 Mechanical compaction is done by imparting vibration to concrete surface using
vibrators. Vibrators can be external or internal. External vibrators can be attached to

24. formwork or formwork is placed on vibrators. They include internal vibrators, form
vibrators, vibrating tables, vibratory screeds or surface vibrators.
Curing of Concrete:
 Curing is name given to the procedures used for promoting the hydration of cement ,and
consists of a control of temperature and of the moisture movement from and into the
concrete.
 It has important role on strength development and durability of concrete.
 Concrete derives its strength as a result of the chemical reaction of the mix water and
the cement, a reaction that starts at the instant the two materials first come in contact with
each other and can continue for long time.
 Curing is done to control the rate and extent of moisture loss from concrete to ensure an
uninterrupted hydration of Portland cement after concrete has been placed and finished in
its final position.
 Cement requires a water/cement ratio about 0.23 for hydration and a water/cement
ratio of 0.15 for filling the voids in the gel pores. In other words, a water/cement
ratio of about 0.38 would be required to hydrate all the particles of cement and also
to occupy the space in the gel pores.
The duration of curing of concrete depends on
 The grade & type of cement, Mix proportion, Desired concrete strength, Shape and size
of the concrete member, Environmental & exposure conditions. The duration may vary
from few days to a month.
 Curing is usually done by water curing ponding, hessian clothes, impermeable
membranes, curing compounds.
Q#25) What is the difference betweenTruck mixers and Truck Agitator?
Truck Mixers Truck Agitator
 Trucks mixers are used to mix
concrete completely in truck and when
mixing is complete this truck then
roattes at agitating speed
 This type of truck is only used to carry
concrete from batching plants to job
site while rotating at agitating speed to
prevent ready mix concrete from
getting harden.
 70 to 100 revolutions of drum is
required to ensure proper mixing of
concrete material
 Agitating speed is 2 to 6 rpm
 When truck mixers are used, ASTM C 94 also limits the time between batching and
complete discharge of the concrete at the job site; this time is 1.5 hours or before the
drum has revolved 300 times after introduction of water to the cement and aggregates.