advance concrete technology Carleton university

1. QUIZ 1 (RAJENDRA SINGH)
1. Why concrete is the most usedconstructionmaterial worldwide? Cite advantages and
disadvantages of concrete.
ANS- Concrete is used in buildings and other structures around the globe, and it is the only
building material that can give us benefits such as weather protection and toughness while
being affordable at the same time. There are some following points which will prove why
concrete is widely used material which as follows:-
1. Concrete dates back thousands of years, with some of the greatest civilizations in
history using the ingredients found in concrete to build their structures. The ancient
Egyptians mixed together sand and gravel with cement-like binder and water to create
a basic form of concrete that they used to build many of their structures.
2. The second biggest reason why concrete is so popular around the world is that it is
extremely durable. Concrete is weatherproof. It is also resistant to erosion, meaning
that it will last longer than other building materials.
3. It is made largely of cement, and cement is made from limestone. Limestone is the
most abundant mineral on earth, so there is no shortage of cement being made on a
daily basis. It can also be produced from many other materials.
4. Lots of concrete is locally produced which means that it provides many jobs and
stability for the residents of those areas.
But everything has there on advantages and disadvantages and concrete is no exception.
Following are some advantages of concrete-
 Concrete is very durable.
 It can be cast into any desired shape.
 Its casting can be done on working site which makes it more economical compared to
other materials.
 It can withstand high Temperatures.
 It can be used in storm withstand structures since it is resistant to wind and water.
 The deterioration of concrete is not appreciable with age.

2. Following are some disadvantages of concrete-
 Concrete has low tensile strength as compared to its compressive strength which is
very high.
 It is less ductile.
 Sometime concrete contains some salts which can cause damage to the structure.
 The weight of concrete is high as compared to other materials.
2. Why the selectionof a proper particle size distribution (PSD; also called grading) is so
important in concrete mixtures? Which properties of the aggregates may influence
concrete behaviour? Please cite at least 5 aggregate properties.
ANS- The results showed that an increase in the P.S.D of the coarse aggregate particle
distribution resulted in an increase in the water demand of the mix, this was further
substantiated by a decrease in the resulting compressive strength. A suitable gradation of
the combined aggregate in a concrete mix is described in order to secure workability and to
secure economy in the use of cement. A well-graded mixture produces strong concrete
than a harsh or poorly graded one. For constant mix proportions, the increase in specific
surface of the aggregate, causes a decrease in the amount of cement relative to the surface
of the aggregate, thus causing more voids around the surfaceof the aggregateparticles and
decrease in strength.
But there are properties or characteristics of aggregate which influence the properties of
resulting concrete mix. These are as follow.
1.COMPOSITION-
Aggregates consisting of materials that can react with alkalies in cement and cause
excessive expansion, cracking and deterioration of concrete mix should never be used.
Therefore it is required to test aggregates to know whether there is presence of any such
constituents in aggregate or not.
2. SIZE & SHAPE-
The size and shapeof the aggregate particles greatly influence the quantity of cement
required in concrete mix and hence ultimately economy of concrete. For the preparation of
economical concrete mix on should uselargest coarseaggregates feasible for the structure.

3. 3. SURFACETEXTURE-
The development of hard bond strength between aggregate particles and cement paste
depends upon the surfacetexture, surfaceroughness and surfaceporosity of theaggregate
particles.If the surfaceis rough but porous, maximumbond strength develops. In porous
surfaceaggregates, the bond strength increases due to setting of cement paste in the
pores.
4. SPECIFIC GRAVITY-
The ratio of weight of oven dried aggregates maintained for 24 hours at a temperature of
100 to 1100
C, to the weight of equal volume of water displaced by saturated dry surface
aggregate is known as specific gravity of aggregates.
5. POROSITY & ABSORPTION-
The minute holes formed in rocks during solidification of the molten magma, due to air
bubbles, are known as pores. Rocks containing pores are called porous rocks. Water
absorption may be defined as the difference between the weight of very dry aggregates
and the weight of the saturated aggregates with surfacedry conditions.
(3.) What are the raw materials used to produce Portland cement? What’s the final
compositionof Portlandcement? Once Portlandcement is hydrated, which are the three
main components formed?
ANS- Raw Materials contain appropriate amounts of calcium compounds, silica, alumina
and iron oxide are crushed and screened and placed in a rotating cement kiln. Ingredients
used in this process are typically materials such as limestone, sandstone, marl, shale, iron,
clay, and fly ash.
Portland cement composition-
Portland cement is made up of four main compounds: tricalcium silicate
(3CaO·SiO2), dicalcium silicate (2CaO·SiO2), tricalcium aluminate (3CaO·Al2O3), and a tetra-
calcium aluminoferrite (4CaO·Al2O3Fe2O3).Small amounts of uncombined lime and
magnesia also are present, along with alkalies and minor amounts of other elements.
The most important hydraulic constituents are the calcium silicates, C2S and C3S. Upon
mixing with water, the calcium silicates react with water molecules to form calcium silicate
hydrate (3CaO·2SiO2·3H2O) and calcium hydroxide (Ca[OH]2), and the hydration reaction
can be crudely represented by the following reactions:2C3S + 6H = C3S2H3 + 3CH2C2S + 4H =

4. C3S2H3 + CH During the initial stage of hydration, the parent compounds dissolve, and the
dissolution of their chemical bonds generates a significant amount of heat. The cement
begins to harden, as CH and C-S-H are produced. This is the cementitious material that
binds cement and concrete together. As hydration proceeds, water and cement are
continuously consumed. Fortunately, the C-S-H and CH products occupy almost the same
volume as the original cement and water; volume is approximately conserved, and
shrinkage is manageable. Thus the following are main component formed while hydration-
Cement Compound Weight Percentage Chemical Formula
Tricalcium silicate 50 % Ca3SiO5 or 3CaO.
SiO2
Dicalcium silicate 25 % Ca2SiO4 or 2CaO.
SiO2
Tricalcium aluminate 10 % Ca3Al2O6 or 3CaO.
Al2O3
4. What are the three main types of chemical admixtures? When should they be used in
practice?
ANS-Following are three main types of chemical admixtures used nowadays in industry-
Air-Entrainment-
In terms of increasing freeze-thaw durability, air-entrained concrete is preferred to use.
This particular admixture typically produces a much moreworkableconcrete compare to
the concrete that is non-entrained. Using this admixture, the resistanceof the concrete
tends to improve, which is really necessary. Hereare somebenefits that this admixture
provides:
 High-degreeof workability
 High-degreeof durability
 High-resistanceto rounding cycles of drying and wetting
The entrained air bubbles tend to act as a physicalbuffer caused by the stresses dueto the
augmentation of water volumein freezing temperatures. Admixtures like this are
compatible with almostevery single admixture available for mixing. Furthermore,
compressivestrength will mostlikely minimize by about5%, usually for every 1% of
entrained air.

5. Set-Retarding Admixture-
Concrete admixtures such as set retarding are utilized to delay the chemical reaction that is
taking place during the start of the concrete setting process. This type of concrete
admixture is used to lessen the effect of high temperatures that can producea quicker
initial concrete setting. Set-retarding mixtures are also being used in the concrete
pavement construction for a few reasons:
 Reducing additional costs to put up new concrete batch plants onsite
 Allows more time to complete concrete pavements
 Helps in eliminating cold joints in concrete
Furthermore, retarders can be used as well in resisting cracking due to the deflection of
concrete. Concrete contractors musttake note that concrete deflection occurs when
horizontalslabs are being placed in different sections. Mostretarders function as water
reducers and might entrain air in concrete. For contractors, using builder trends software
can really make a huge difference when it comes to handling procedures like this.
Accelerating-
This type of admixture is used to increase further the rate of strength development for
concrete. Itis also used to lessen concrete setting time. The most common accelerator
component in this admixture is calcium chloride. However, the componentitself can
possibly promotecorrosion activities of steel reinforcement. Additionally, concrete best
practices like an adequate cover, proper consolidation and concrete mix design can prevent
any corrosion issues. Thus, accelerating admixtures are particularly usefulfor altering the
properties of concrete during cold weather.
As we know concrete workability, strength, and water tightness mustbe achieved by
choosing preferableaggregates, Portland cement, and better water-cement ratio
maintenance. When this is not possibledue to unusualsituations such as hot temperatures,
prolonged deicing salt exposure, increasewear or other chemicals, it is believed that
admixtures are helpful. Ingredients added to the concrete other than aggregates, water,
and Portland cement are commonly referred to as admixtures. While it is not usually
required, there are severalcases whereadmixtures are being used selectively in order to
improvethe concrete mix.

6. 5. What’s the main purpose of a proper concrete mix-proportioning? Which are the
aspects that should be targeted while mix-proportioning concrete?
ANS- Concrete is the single most important material in modern building and concrete mix
proportions are the most important aspects of working with concrete. There are only four
fundamental ingredients in concrete: cement, aggregate, sand, and water, but each serves
a particular purpose. Understanding the purpose of each ingredient is key to determining
what concrete mix proportions are best in a given situation. Concrete mix design is
economically proportioning of concrete ingredients for better strength and durability based
on construction site. While the nominal concrete mix may have a higher amount of cement,
when it is designed mix,the cement requirement may below for the same grade of concrete
for a given site. The proportions resulting from concrete mix design are tested for their
strength with the help of compressive strength tests on concrete cubes and cylinders.
A well-designed concrete mix will ensure that the finished structureis as strong as it can be.
It also maximizes the efficiency of ingredients by reducing wastage and getting the most
concrete out of the wet and dry ingredients.
The following aspects are targeted while mix-proportioning concrete-
1. Compressive strength
It is one of the most important properties of concrete and influences many other
describable properties of the hardened concrete. The mean compressive strength required
at a specific age, usually 28 days, determines the nominal water-cement ratio of the mix.
The other factor affecting the strength of concrete at a given age and cured at a prescribed
temperature is the degree of compaction. According to Abraham’s law the strength of fully
compacted concrete is inversely proportional to the water-cement ratio.
2. Workability
The degree of workability required depends on three factors. These are the size of the
section to be concreted, the amount of reinforcement, and the method of compaction to
be used. For the narrow and complicated section with numerous corners or inaccessible
parts, the concrete must have a high workability so that full compaction can be achieved
with a reasonable amount of effort. This also applies to the embedded steel sections. The
desired workability depends on the compacting equipment available at the site.

7. 3. Durability
The durability of concrete is its resistance to the aggressive environmental conditions. High
strength concrete is generally more durable than low strength concrete. In the situations
when the high strength is not necessary but the conditions of exposure are such that high
durability is vital, the durability requirement will determine the water-cement ratio to be
used.
4. Quality Control
The degree of control can be estimated statistically by the variations in test results. The
variation in strength results from the variations in the properties of the mix ingredients and
lack of control of accuracy in batching, mixing, placing, curing and testing. The lower the
difference between the mean and minimum strengths of the mix lower will be the cement-
content required. The factor controlling this difference is termed as quality control.