Module-2 Construction Materials - Concrete cracks, Water - Cement ratio, Hydration of Cement, Strength of Concrete

1. 1
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Department of Civil Engineering
School of Civil and Environmental Engineering
Ambo University, Ethiopia
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2. 2
CONSTRUCTION MATERIALS

3. Module -2

4. PROPERTIES OF
CONCRETE

6. “Selection of Grade of Concrete” is prime
importance – to Over come the
Environmental Exposures, in general.

7. Why do concrete floors crack?
Cracking is part of concrete’s natural tendency
Cracking can also be caused by environmental
changes
Cracking may be due to foundation issues
Problems caused by cracked floors

8. When it’s first poured, concrete is a liquid. Over the
course of a couple of days, it hardens into a solid,
shrinking as it does so. This initial shrinking often
results in hairline cracking within the first few days of
installation.

9. Note-1: Different mixtures and ratios of the
cement may minimize some cracking, but
can’t eliminate it completely.
Note-2: “The chemical and physical reactions
within the concrete don’t fully stop after it
initially settles”. “Occasional cracking due to
shrinkage” can continue for decades”.

10. Note-3: Concrete has very high compressive strength, but
low tensile strength—that is, it stands up very well under
heavy loads, but does not tolerate stretching or bending of
any kind. If exposed to severe changes in temperature (as
in a building without climate control) a concrete floor may
crack as it expands.
Note-4: It may also crack due to the “freeze-thaw cycle”
during the winter, but this is more often a problem for
outdoor sidewalks and patios that are directly exposed to
the elements.
(Freeze-thaw weathering is a process of erosion)

11. “Freeze-thaw weathering” is a process of erosion that
happens in cold areas where ice forms. A crack in a rock can fill
with water which then freezes as the temperature drops. As the
ice expands, it pushes the crack apart, making it larger. When
the temperature rises again, the ice melts, and the water fills
the newer parts of the crack. The water freezes again as the
temperature falls, and the expansion of the ice causes further
expansion to the crack. This process continues until the rock
breaks.
Freeze – thaw Weathering

14. Note-5: The “climate” may be playing a role in floor’s
cracking, consider getting a climate control system installed
in building.
Note-6: A “sudden increase in concrete cracking” may
result from “unusual pressures beneath the foundation”.
For example, after a heavy storm, some types of soil may
swell with water and exert pressure upward against your
concrete foundation, creating more tensile stress than it
can handle.
Note-7: In the same way, unstable soil can cause shifts
beneath the slab (as is possible with a construction site
that hasn’t been graded properly). Especially large and
deep roots from nearby trees can also cause cracking,
though this is more common in residential areas than
industrial ones.

17. Grades of concrete : “The strength and
composition of the concrete”
The “minimum strength
the concrete” should have following “28
days of initial construction”.
The grade of concrete is understood in
measurements of MPa, where M stands
for mix and the MPa denotes the overall
strength.

18. C: Compressive Strength in N
For instance, C10 indicate the
Compressive strength of 10 newtons, C15 has
the strength of 15 newtons, C20 has 20
newtons strength and so on.
M: Mix to achieve a specific compressive
strength
Different mixes (M) come in various mix
proportions of the various ingredients of
cement, sand and coarse aggregates. For
instance, M20 comes in the respective ratio of
1:1:5:3. You can see other examples below in
the table.

19. Concrete Grade
Mix Ratio (cement: sand:
aggregates)
Compressive Strength (Mpa /
psi)
1 Mpa = 145.038 psi
M5 1 : 5 : 10 5 MPa 725 psi
M7.5 1 : 4 : 8 7.5 MPa 1087 psi
M10 1 : 3 : 6 10 MPa 1450 psi
M15 1 : 2 : 4 15 MPa 2175 psi
M20 1 : 1.5 : 3 20 MPa 2900 psi
M25 1:1:2 25 Mpa 3625 psi

20. Generally M10 and M15 grades of
concrete: leveling course, and bedding for
footings.
Standard concrete and concrete of grade
M20 / M25: Reinforced Cement
Concrete (RCC) works for slabs, beams,
columns and so on.

21. Compressive
strength

22. Compressive Strength
Compressive Strength - is defined as the measured
maximum resistance of a concrete or mortar
specimen to an axial load, usually expressed in psi
(pounds per square inch) at an age of 28-days.
Compressive strength is calculated from the “failure load
divided by the cross-sectional area resisting the load” and
reported in units of pound-force per square inch (psi) or
mega pascals (MPa).

25. Key Points to Understand
●The strength of concrete is very much dependent upon the
hydration reaction.
● Water plays a critical role, particularly the amount used.
●The strength of concrete increases when less water is used
to make concrete.
●The “hydration reaction” itself consumes a specific amount
of water.
●Concrete is actually mixed with more water than is needed
for the hydration reactions. This extra water is added to give
concrete sufficient workability.
●The water not consumed in the hydration reaction will
remain in the microstructure pore space. These pores make
the concrete weaker due to the lack of strength-forming
calcium silicate hydrate bonds.

26. Characteristic
strength of
concrete

27. ●The characteristic strength of concrete is the
“result of the compressive strength of
the concrete cube test”.
●The design strength is the required strength of
concrete to be designed as per the IS code. ...
●The design strength is 28.5 N/Sqmm, and
the characteristic strength of concrete is 25
N/Sqmm.
(In short, only 5% of chances are there that the concrete will
fail at its characteristic strength. If a block of M25 Concrete
has characteristic strength of 23MPa, means that the chances
of failure is only 5% at that load of 23MPa and thus 955
chance for survive.)

28. Characteristic strength of concrete (fck)
The compressive strength of concrete is given in terms of
the “characteristic compressive strength” of 150 mm
size cubes tested at 28 days (fck) - as per Indian
Standards (ACI standards use cylinder of diameter 150 mm
and height 300 mm).
The characteristic strength is defined as the strength of the
concrete below which not more than 5% of the test results
are expected to fall. This concept assumes a normal
distribution of the strengths of the samples of concrete.

31. The above sketch shows an idealized distribution of the values of
compressive strength for a certain number of test specimens. The
horizontal axis represents the values of compressive strength in MPa.
The vertical axis represents the number of test samples for a
particular compressive strength. This is also termed as frequency.
The average of the values of compressive strength (mean strength)
from the graph is 40 MPa.
The characteristic strength (fck) is the value in the x-axis below which
5% of the total area under the curve falls. From the graph we can
clearly say that 30 MPa is the characteristic strength of the
given concrete mix. The value of fck is lower than fcm (40 MPa- mean
strength) by 1.64σ, where σ is the standard deviation of the normal
distribution.
So we can say the given concrete mix has a characteristic
strength of 30 MPa or it is a M30 grade mix.

32. Target Strength of Concrete
According to the definition, 95% if the specimens should
possess a strength greater than the characteristic
compressive strength (fck) of concrete. According to IS,
the target strength of the concrete mixture is defined
as: Target strength = fck + 1.65 σ, where σ is the
standard deviation.
Note: For a 95% confidence level, k=1.64 , hence k value varies
on the confidence level of the experiment.
Note: Characteristic strength of concrete is the strength of concrete
specimens casted and tested as per given code of practice and cured for
a period of 28 days; 95% of tested cubes should not have a value less
than this value.
The target mean strength of the M20 grade concrete is 27 n/mm2.

33. We have seen that,
Target strength = Fck + 1.65 σ
Where Fck= Characteristics compressive strength of concrete
σ= Standard deviation
So according to your question we have to determine the value of σ .
27= 20+1.65σ
From the above equation,
the value of σ is 4.24.

34. Acceptance criteria according to Indian standards
As per the IS code (Clause 16 of IS 456:2000), for a given set of
tests, the compressive strength is taken as the average of three tests,
no one test differing from the average by more than 15%. The
strength requirements are deemed to meet standards in the following
conditions are satisfied.
Compressive strength:
Mean of 4 test results > fck + 0.825 σ, or fck + 4 Mpa
(whichever is greater)
Individual strength result > fck – 4 Mpa
Flexural strength (ft is the characteristic flexural strength):
Mean of 4 test results > ft + 0.3 Mpa
Individual strength result > ft – 0.3 MPa

35. Different types of Concrete mixes:
Concrete Mixes are primarily divided into the two different types :
Nominal Mix:
Nominal Mix is generally adopted for small scale constructions. In this type of
mix, the mix ratios and concrete constituent proportions are prefixed and
specified. Eg: M20(1:1.5:3); the quantity of cement, sand and aggregate is
batched in volume as per the fixed ratio 1:1.5:3. From the above table till M25
grade, the concrete proportions are called as Nominal mix concrete.
Design Mix:
Design mix concrete is adopted for high rise constructions. In this type of mix,
the mix ratios are decided by an Engineer after analyzing the properties of
individual ingredients of concrete. Like, cement is tested for Fineness
modulus and Specific gravity of cement in the lab while deciding the Design mix
ratio. There is No Pre-fixed ratio, and ingredients are batched in weight.
From the below table, concrete grades more than M25 falls in Design mix.
In Simple, “Design Mix” refers to “the ratios which are decided by the designer.”

36. Table for Quantity of Cement, Sand, Aggregate, Water required for
Different grades of concrete :-

37. https://civiconcepts.com/2019/01/co
ncrete-mix-design-step-by-step-full-
calculation/#Procedure_for_M_-
_25_Concrete_Mix_Design

38. Curing

39. Curing of Concrete
◘ It should be done 7 days to 14 days
adequately.
◘ Curing facilitates “better interlocking in
concrete” and it “maintains temperature and
moisture”.
◘ Higher the curing, higher is the strength of
concrete.
◘ Curing can be done in many ways like
membrane curing, spray curing, steam curing
and so on.

41. During the first week to 10 days of curing, it is
important that ….the concrete not be permitted to
freeze or dry out
In practical terms, about 90% of its strength is
gained in the first 28 days.
Concrete compressive strength depends upon many
factors:
- Quality and proportions of the ingredients
- The curing environment.

42. Portland cement consists of five major compounds and a
few minor compounds. The composition of a typical
Portland cement is listed by weight percentage in Table.

43. What is hydrated cement?
The water causes the hardening of concrete through a process
called hydration. Hydration is a chemical reaction in which the
major compounds in cement form chemical bonds with water
molecules and become hydrates or hydration products. ...
Aggregates are chemically inert, solid bodies held together by
the cement.
Calcium silicate hydrate (or C-S-H) is the main product of the
hydration of Portland cement and is primarily responsible for the
strength in cement based materials.
Calcium silicate hydrate is produced by reaction of C3S or C2S
with water. It is frequently described as a gel rather than a
crystalline material because no consistent structure is
discernible using X-ray diffraction.

44. Image shown is a two-dimensional slice from a three-dimensional
spherical computational volume
Dark Blue: Unhydrated cement cores
Red: Inner C-S-H product
Yellow: Outer C-S-H project
Light Blue: Water-filled space

45. When water is added to cement, each of the compounds
undergoes hydration and contributes to the final concrete
product.
Tricalcium silicate is responsible for most of the early strength
(first 7 days).
Dicalcium silicate, which reacts more slowly, contributes only
to the strength at later times.
The equation for the hydration of tricalcium silicate is given by:
Tricalcium silicate + Water--->Calcium silicate hydrate+Calcium
hydroxide + heat
2 Ca3SiO5 + 7 H2O ---> 3 CaO.2SiO2
.4H2O + 3 Ca(OH)2 + 173.6kJ
Upon the addition of water, tricalcium silicate rapidly reacts to
release calcium ions, hydroxide ions, and a large
amount of heat. The pH quickly rises to over 12 because of
the release of alkaline hydroxide (OH-) ions. This initial
hydrolysis slows down quickly after it starts resulting in a
decrease in heat evolved.

46. The above diagrams represent the formation of pores as calcium silicate hydrate is formed.
Note in diagram (a) that hydration has not yet occurred and the pores (empty
spaces between grains) are filled with water. Diagram (b) represents the beginning
of hydration. In diagram (c), the hydration continues. Although empty spaces still
exist, they are filled with water and calcium hydroxide. Diagram (d) shows nearly
hardened cement paste. Note that the majority of space is filled with calcium
silicate hydrate. The hydration will continue as long as water is present and there
are still unhydrated compounds in the cement paste.

48. Water –
Cement Ratio

49. The Importance of Water
In concrete, the single most significant influence on most or all
of the properties is the amount of water used in the mix.
In concrete mix design, the ratio of the amount of water to the
amount of cement used (both by weight) is called the water to
cement ratio (w/c). These two ingredients are responsible for
binding everything together.
The water to cement ratio largely determines the strength and
durability of the concrete when it is cured properly. The w/c ratio
refers to the ratio of the weights of water and cement used in the
concrete mix. A w/c ratio of 0.4 means that for every 100 lbs of
cement used in the concrete, 40 lbs of water is added.

50. Implications of Water-Cement Ratio
Variations
The simplest way to think about the w/c ratio is to think that the
greater the amount of water in a concrete mix, the more dilute
the cement paste will be. This not only affects the compressive
strength, it also affects the tensile and flexural strengths, the
porosity, the shrinkage and the color.
The strength is reduced mostly because adding more water
creates a diluted paste that is weaker. Think of it like over-
diluting grape Kool-Aid. The more water you add, the weaker the
Kool-Aid is.

51. Explained more technically, more water
results in larger spacing of the cement
particles. As the crystals grow, they are too
far apart to knit together and form strong
bonds.

52. Typical Water-Cement Ratios in Concrete Mixes
Typical w/c ratios are as follows:
Normal for ordinary concrete (sidewalks and driveways): 0.6
to 0.7
Specified if a higher quality concrete is desired: 0.4
The practical range of the w/c ratio is from about 0.3 to over
0.8.
A ratio of 0.3 is very stiff (unless super plasticizers are used).
A ratio of 0.8 makes a wet and fairly weak concrete.
Typical compressive strengths when concrete is properly
cured are:
0.4 w/c ratio –> 5600 psi
0.8 w/c ratio –> 2000 psi.

53. The water–cement ratio is the ratio of the weight of water to
the weight of cement used in a concrete mix.
The single most important indicator of strength is the ratio
of the water used compared to the amount of cement (w/c
ratio)
A lower ratio leads to higher strength and durability, but
may make the mix difficult to work with and form.
This concept was developed by Duff Abrams of The
Portland Cement Association in the early 1920s and is in
worldwide use today.
A minimum w/c ratio (water-to-cement ratio) of about 0.3 by
weight is necessary to ensure that the water comes into
contact with all cement particles (thus assuring complete
hydration).
Typical values are in the 0.4 to 0.6

54. Advantages of low water/cement ratio:
● Increased strength
● Lower permeability
● Increased resistance to weathering
● Better bond between concrete and
reinforcement
● Reduced drying shrinkage and cracking
● Less volume change from wetting and drying

55. Concrete Curing
Curing - maintenance of a satisfactory moisture content and
temperature in concrete for a suitable period of time immediately
following placing & finishing so that the desired properties may
develop.
Factors that effect curing:
◘ Time
◘ Temperature
◘ Moisture
Concrete strength gain versus time for concrete exposed to
outdoor conditions. Concrete continues to gain strength for
many years when moisture is provided by rainfall and other
environmental sources.

56. Demonstration 1:
Making a Silt Test
Objective: The purpose of this demonstration is to determine the viability of an aggregate
based on a silt test.
Materials and Supplies:
Sample aggregate (sand and kitty litter work well for comparison)
Glass container with lid
Water
Ruler
Procedure:
1. Place 5 cm of aggregate in the container.
2. Fill the container with water so the water level is 2 cm above the aggregate.
3. Shake vigorously for 1 minute, making the last few shakes in a swirling motion to level off
the aggregate.
4. It is suggested that this demonstration be done twice, once with sand and once with kitty
litter to obtain various results.
5. Allow the container to stand for an hour, or until the liquid above the aggregate is clear.
6. The layer that appears above the aggregate is referred to as silt. Measure the silt layer. If
this layer is more than 3 mm thick, the aggregate is not suitable for concrete work unless
excess silt is removed by washing.

58. Demonstration 2
Conducting an Organic Matter Test
Objective: The purpose of this demonstration is to determine the viability of an aggregate
based on the amount of organic matter present.
Materials Needed:
Sand
A 50:50 mixture of sand and dirt
Glass container (10 oz. juice jar or similar size) with lid
A 3% solution of sodium hydroxide NaOH (made by dissolving 9 grams of sodium hydroxide,
household lye, or caustic soda, in 300 mL of water, preferably distilled).
Procedure:
1. Fill container with sand to the 150 mL mark.
2. Add 120 ml of 3 % NaOH solution.
3. Shake thoroughly for 1 or 2 minutes and allow to stand for 24 hours.
4. Repeat the procedure using the sand-dirt mixture.
5. Indicate the color of the liquid remaining on the top of the aggregate.
The color of the liquid will indicate whether or not the aggregate contains too much organic
matter. A colorless liquid indicates a clean aggregate, free from organic matter. A straw-
colored solution, not darker than apple-cider vinegar, indicates some organic matter bur not
enough to be seriously objectionable. Darker colors mean that it contains too much organic
matter and should not be used unless it is washed and tested again.
Expected Results:

60. Demonstration 3:
Effect of Aggregate on Workability of Concrete
Objective: The purpose of this demonstration is to determine the effect of different
aggregates on the workability of the resulting concrete.
Materials and Supplies:
cement
two containers labeled:
"limestone chip aggregate"
"sand aggregate"
water
small limestone chips(pea gravel)
sand
Procedure:
Add 1 part of cement and 1/2 part water to each container. Suggested amounts
include 50 grams of concrete and 25 ml of water.
Mix to make the cement paste.
To the appropriate container, add 3 parts (150g) of limestone chips and mix.
To the second container, add 3 parts (150 g) of sand and mix.
Using gloved hands, knead the concrete to determine its workability.
Which concrete mixture is more workable? Why?
Expected Results:
The sand aggregate is more workable, because the smaller particles facilitate flow.

61. References
1. https://info.cpcfloorcoatings.com/why-do-concrete-floors-crack
2. http://www.ssc.education.ed.ac.uk/bsl/geography/freezethawd.html
#:~:text=Definition%3A%20Freeze%2Dthaw%20weathering%20is,c
old%20areas%20where%20ice%20forms.&text=The%20water%20f
reezes%20again%20as,continues%20until%20the%20rock%20bre
aks.