FOR STUDENTS

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. 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 ofrolling, 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
minutesafter 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 concreteof 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.

3. 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)
 Cranes and hoists
 Belt conveyers and boomconveyers
 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 to increase the density of
the concrete. It enables the fresh concrete to reach its potential design
strength, density and low permeability.
It ensures properfilling 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.
 Mechanicalcompaction:
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.

4. Internal Vibrations
External Vibrations
Table Vibrations
Form Vibration
Surface Vibration.
d) Curing of Concrete:
Curing can also be described as keeping the concrete moistand warm
enough so that the hydration of cement can continue. If 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. Therefore, 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 andspraying.
 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 carbondioxide 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

5. layer surrounding the reinforcing steel breaks down and corrosionbecomes
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 color 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 productis 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.
 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.

6. 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:
 Transit mixed concrete
 Shrink mixed concrete
 Central mixed concrete
 Transit Mixed Concrete:
It is also called dry batched concrete becauseall 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-batchor 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 consequences of remixing concrete?
 Remixing of concrete:
Concrete begins to stiffen as soonas 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.

7. 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.
 Conditions of Remixing:
Under propersupervision 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 harden quickly.
 A cold joint may develop when concrete is poured next to or on remixed
concrete.
Q#5) How concrete can be 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. Forsmaller 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.

8.  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.
Q#6) What is Shotcrete? Explain it 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 concreteto 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 constructwith 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:

9. 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 rocksurfaces, channeling water flows, and installing temporary
supportand permanent linings.
 New Structures:
Shotcrete can be used effectively where thin sections and large areas are
involved.
Some examples of shotcrete use 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.
Shotcrete floorand walls:
Shotcrete floors in tanks and pools on well compacted sub-baseoron
undisturbed earth have generally given excellent service. Vertical
and overhead construction for walls, slabs, columns, and other structural
members has been frequently shotcrete.

10. 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 can be affected by poor
transportation techniques?
Concrete quality can be greatly affected by poortransportation techniques.
Some of them:
 Poortransportation 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 posea 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 trolley 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 wait in a truck for long can
Q#8) Explain 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.

11.  Concrete can be placed up to 18 feet for 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.
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 must 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 minimizes
segregation of concrete and ensure discharging of entire mortar form belt.
Concrete Pumps:
A concretepump 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
concreteaccurately. This type of concretepump 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.

12. Line Pump/Trailer -Mounted Concrete Pumps:
The second main type of concretepump is either mounted on a truck or
placed ona trailer. This pump requires steel or flexible concreteplacing 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? You’re Supposed to Place
Concrete Underwater in bridge pier Which method would you choose
to place concrete in such conditions?
Placing of concrete:
The process of discharging concrete from trucks to the 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.
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
hopperat the surface. The reason to immerse the tremie pipe lower end is to prevent
intermixing of both concrete and water.

13. Placing the concrete:
As soonas 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 Equipment’s 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 consequentstiffening of concretewhile it passes overlong
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 must 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.

14.  Labor costs per unit volume of concrete for this method of handling
are high, although the method remains popular for small owner
placed
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 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 consolidation of concrete? How concrete
can be consolidated mechanically?
Consolidationof concrete:
Consolidation is the process ofinducing 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 properconsolidation.

15. 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.
 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 forevery 2 feet. When in doubt, vibrate more
with well-proportioned concrete there would be no segregation of the
aggregate.
 Shockor drop table is used to consolidate extremely stiff low slump
concrete in making precastunits.
 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. What defects can be
produced from over vibrations and under vibrations?

16. Purpose of consolidation:
Consolidation eliminates rockpockets and air bubbles and brings fine
material both to the surface and against the forms to producedesired finish. The
process involves carefully working around all reinforcing steel with the
compacting device to assure properembedding of reinforcing steel in the concrete.
Defects from Under vibrations:
 Excessive entrappedair 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 left behind
that lacks workability. It is also caused by insufficient fine aggregates.
 Placementlines:
These are dark lines between adjacent layers of concrete batches. it
occurs when vibrators did not penetrate through under lying layers.
Defects from Over Vibrations:
 Segregation:
For most of concretemixes, over-vibrationcreatestheproblem of
segregationinwhich the denser aggregatessettleto thebottom while
the lighter cement pastetends to move upwards.
 Bleeding:
Bleeding in concrete is caused when surplus water rises 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.

17. 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:
 Gluonium 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-Gluonium self-compacting concrete spreads around reinforcing
to fill every corner of the mold 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, colors are more homogeneous, and
efflorescence produced by vibration is eliminated.

18. 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 bythe environment of future
use. Passivation involves creation of an outer layer of shield material that is
applied as a micro coating, created by chemical reaction with the basematerial, or
allowed to build from spontaneous oxidation in the air.
Passive layeron steelin concrete:
 Carbonation process in concrete initiates the corrosionprocess ofsteel
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 corrosionrate to an insignificant level.
 Forsteel in concrete, the passive corrosionrate is typically 0.1 µm per
year. Without the passive film, the steel would corrodeat 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 corrosionis 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.

19. Q#15) How do you define durability of concrete? Discuss the
mechanism of deterioration of 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.
 Concrete ingredients, their proportioning, interactions between them,
placing and curing practices, and the service environment determine the
ultimate durability and life of the concrete.
Carbonation of Concrete:
Carbonation is the reaction of carbondioxide 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 corrosionbecomes possible.
Conditions for Carbonation:
 The reaction of carbondioxide 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
carbondioxide and again carbonation will be slow.
 The most favorable 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

20.  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 concreteto protectsteel reinforcement from rusting.
 Concrete may become porous and impermeable.
Q#16) Explain Carbonation briefly. State the method to measure the
depth of carbonation in laboratory.
Carbonation of concrete:
Carbonation is the reaction of carbondioxide 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 corrosionbecomes possible.
 The reaction of carbondioxide 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
carbondioxide and again carbonation will be slow.
 The most favorable 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
several years or in Carbonation Chamber for several 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.

21.  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 causes deterioration in
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 reaction:
 Alkali silica reaction is a heterogeneous chemical reaction which
takes place in aggregate particles between the alkaline poresolution
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 occupymore spacethan 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:
Step 1:
Silica + alkali alkali-silica gel (sodium silicate)
SiO2 + 2NaOH + H2O Na2SiO3.2H2O (2KOH can replace 2NaOH)

22. Step 2:
Gel reaction product + water expansion
Q#18) State the defects produced in concrete due to ASR?
What are the mitigation measures for ASR?
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 becausethey 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 meet
the system.
 The alkali-silica reaction will not take place in a concretestructure 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.

23.  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
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 costof no agitating 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 stiffen.
3. Belt
conveyors
 It is used to place concrete on site. For conveying
concrete horizontally or to a higher or lower level.

24.  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 secondarydischarge 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 costand 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.
 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.

25.  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 between dry 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 concretewhich 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
producemortar 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

26. Q#21) What is compaction of 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 to increase the density of the
concrete.
Importance of Compaction:
 Compaction significantly increases the ultimate strength of concreteand
enhances the bond with reinforcement.
 It also increases the abrasion resistance and general durability of the
concrete, decreases the permeability.
 It helps to minimize its shrinkage and creep characteristics.
 Propercompaction also ensures that the reinforcement, tendons, inserts and
fixings are surrounded by dense concrete, the formwork is filled i.e. there are
no pockets of honey-combed material.
 It is important to compactthe 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 compactioncan lose as much as one third of
its strength.
Q#22) Why curing of concrete is important? What are the factors that may
affect duration 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 controlthe 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.

27.  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 to occupythe spacein the gel pores.
Duration of Curing:
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.
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 controlthe 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.

28.  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.

29.  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 till concrete gains the desired strength.
Q#24) What steps are involved in the concreting process? Explain
The concreting process consists ofthe following steps:
 Batching
 Mixing.
 Delivery.
 Placing
 Compaction
 Curing
Batching:
 Batching is the process ofmeasuring concrete mix ingredients by either
mass or volume and introducing them into the mixer. To produceconcrete 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 concreteshould 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.

30. 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:
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 costof these trucks may be lower than agitating mixers. Concrete
tends to stiffen more rapidly in non-agitating trucks therefore slump should
be more.
 Transpiration techniques adopted must be good as poortransportation can
greatly impair the quality of fresh concrete.
 Poortransportation techniques can cause stiffening, segregation and slump
loss.

31. Placing:
 Concrete can be placed using different equipments that can handle concrete
effectively on site.
 As soonas concrete is brought to site by transit mixers, they should be
discharged as soonas 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 concreteshould 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 porosity and water
permeability of concrete.
 It produces fine finishes of concrete surface.
 Compaction can be done either manually or mechanically. Manual
compaction 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 concretesurface
using vibrators. Vibrators can be external or internal. External vibrators can
be attached to 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 because of the chemical reaction of the mix
water and the cement, a reaction that starts at the instant the two materials
first meet each other and can continue for long time.

32.  Curing is done to controlthe 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 to occupythe spacein 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 between Truck 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 rotates 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
are 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.