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Lecture 4
CONCRETE MATERIALS

CIEN 30023
S.Y. 2ND SEM 2019-2020
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⚫ Concrete Materials
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• Concrete is the mixture of fine and coarse
aggregates with cement and water and thereafter
observing and implementing proper curing
requirements.

The two major components of concrete are the PASTE

and MINERAL AGGREGATE which is the fine and coarse.

Paste: combination of water and cement.

Mortar: combination of paste and fine aggregates.

Concrete: main material present in rigid pavements.

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Materials Composition
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 A Typical Mix
lb/CY Wt % Vol %

Cement 667 18% 12%

Water 300 8% 18%

Agg 2835 74% 64%

Air 0% 6%
Classes and Uses of Structural Concrete

Class A - All superstructures and heavily reinforced

substructures such as slabs, beams, girders,
columns, arch ribs, box culverts, reinforced
abutments, retaining walls and reinforced
footings.
Class B - Footings, pedestals, massive pier shafts, pipe
bedding, and gravity walls, unreinforced or
with only a small reinforcement
Class C - Thin reinforced sections, railings, precast R.C.
piles and cribbing and for filler in steel grid
floors
Class P - Prestressed concrete structures and members
Seal - Concrete deposited in water
Cement 9

 The function of cement in the concrete is to bind the coarse

and fine aggregate particles together by setting and
hardening around such particles. There are different types of
cement and each type is used under certain conditions due
to its special properties. However, for ordinary construction,
generally Ordinary Portland Cement is used. When water is
mixed with cement, a chemical reaction takes place because
of which the cement paste first losses its plasticity and
becomes stiff, at the same time it acquires hardness and
strength.
PHASE SHIFT FROM FRESH
CONCRETE TO HARDENED ONE
3 Hydration of the silicate

 C3S
 2(3CaO·SiO2) + 6H2O [H]→ 3CaO·2SiO2·3H2O (C3S2H3) + 3Ca(OH)2
[CH]
 C-S-H : product of hydro-reaction : effect on strength
 Calcium hydroxide : no relationship with strength, melt when it meets
water.
 Calcium Hydroxide
 Weak,
 Add silica (pozzolan material)-> C-S-H
 Fly ash(SiO2) + Ca(OH)2 + H2O → C-S-H
 C2S
 2C2S + 4H → C3S2H3 + CH
Hydration of cement
Structure of hydrated product over time

After 1 hr After 3 hrs

After 1d After 3d After 7d

 3-Hydration
 Hydration: reaction of cement and water
leading to hardening of the paste.
 It is an exothermic process (generates heat)
and consists of 4 stages:
Heat
1 Mixing stage
1 2 3 4 5 2 Dormancy

3 Hardening

4 Cooling

5 Densification
Time

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 Cont.

3-Hydration

 Stages:

Stage Characteristics
Reaction between tricalcium aluminate (C3A)
1 Mixing and water to produce calcium aluminate
hydrate (CAH)
Mix remains plastic with no heat generation
2
Dormancy
(concrete should be placed and finished)
Mix starts to stiffen, stage dominated by
silicate reactions producing calcium silicate
Hardening
(C3S) hydrated (great contributor to concrete
3
strength and low permeability)

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 Cont.

3-Hydration

 Stages:

Stage Characteristics
Concrete shrinks. Shrinkage is restrained
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by friction from underlying layers: tensile
Cooling
stresses in concrete slabs. Sawing the
concrete will help avoiding cracking.
Hydration continues if cement and water
are still present in the mixture. Curing
Densifi-
5 compounds can be early used here to
cation
keep the concrete moist and assist the
continuation of hydration processes.

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Early-age strength gain
Later-age strength gain
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 4-Chemical Admixtures

 Admixtures: materials added to concrete mixtures

to modify their properties (e.g., air content, water
requirement, setting time, etc.).
 Are used to create small air bubbles in the paste
and improve concrete resistance to freezing-
thawing cycles.
 Specifications: ASTM C 260
 Water-reducing admixtures and admixtures to
decrease or increase rate of hydration: specified
in ASTM C 494 and AASHTO M 194.

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Concrete Properties Commonly Modified By
Admixtures
1. Rate of hydration or setting time
2. Workability
3. Dispersion
4. Air-entrainment
Commonly Used Admixtures In Concrete Construction
• Accelerators - used to speed up the initial set of concrete
These are added to concrete either:
a. to increase the rate of hydration of cement,
and hence to increase the rate of development
of strength, or
b. to shorten the setting time
Commonly Used Admixtures In Concrete
Construction

• Retarders
➢ Used to slow down the setting of concrete and can delay
initial setting time up to 4 hours.

➢ Useful in concreting during hot weather, when the normal

setting time is shortened by the higher temperature, and in
preventing the formation of cold joints.

➢ Also used to reduce the incidence of thermal cracking

and to maintain concrete workability during large pours
or long ready-mix hauls.
Commonly Used Admixtures In Concrete
Construction
Water - Reducers
➢ Water reducers or plasticizers are used to :
a. Obtain a higher strength by decreasing the water-
cement ratio at the same workability as an ordinary
mix;
b. Achieve the same workability by decreasing the
cement content; and
c. Increase the workability so as to ease placing of
mix in inaccessible locations.
➢ The reduction in the quantity of mixing water varies
between 5 and 15 percent.
 Commonly Used Admixtures In Concrete
Construction
Superplasticizers
➢ New type of water-reducing admixtures that are used to
produce flowing concrete which is useful for placing in
very heavily reinforced sections, inaccessible locations, or
where very rapid placing is required.

➢ Another use of superplasticizers is in the production of

concrete of normal workability but with an extremely
high strength due to a very low water/cement ratio.

➢ Superplasticizers can reduce the water content by 25 to

35 percent, and increase the 24-hour strength by 50 to
70 percent.
Commonly Used Admixtures In Concrete
Construction

Air – entraining Admixtures

➢ These admixtures help incorporate a controlled amount
of air in the form of minute bubbles (approximately
0.01 to 0.25 mm diameter) in concrete during mixing,
without significantly altering the setting or the rate of
hardening characteristics of concrete.

➢ Air-entrained concrete is used where improved

workability, easier placing, and increased durability
are required and also useful in making lightweight
aggregate concrete.
Commonly Used Admixtures In Concrete
Construction

➢ Air-entrainment, while improving the durability and

plasticity, may have an adverse effect on the strength of
concrete. Within the normal range of air content, the
maximum reduction in compressive strength and flexural
strength rarely exceeds 15 to 10 percent, respectively.
Curing Materials

• Curing means preventing or delaying drying up of intrinsic

moisture inside the capillaries of concrete for adequate
hydration of cement in the mix in order that it may
continue to gain strength.

• Curing materials for concrete such as cotton mats,

burlap cloth, water-proof paper, liquid membrane
forming compounds or sheeting (film) materials shall
conform to the requirements of AASHTO M 182,
AASHTO M 171 and AASHTO M 148, respectively.
Curing Materials

• Immediately after the finishing operations have been

completed and the concrete has sufficiently set, the surface
of the newly placed concrete shall be cured for a period of
72 hours with any of the methods as in Item 311-Portland
Cement Concrete Pavement.
Factors affecting the strength of
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concrete :
1. Quality of aggregate and cement.
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2. Quantity of mixing (water and cement)
(Water-Cement ration) -. The lower the
water-cement ratio is, the higher the
strength
3. Time of mixing – Concrete batches of up to
one cu.m shall be mixed at not more than
one and one-half (1 ½ minutes). Mixing
time shall be increased from ½ to 1 min. for
every cubic meter, but in no case shall
batch be mixed more than 5 min. For this
reason, the biggest mixes as yet, is 8 cu.m
capacity.
4. Curing Conditions – Occasionally treated
with water or with curing compound.
Physical Properties of Concrete
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:
1. Workability – When there is so much fines or excess42of fine
aggregates or what we call over-sanded mixtures – will lessen
the strength of resulting concrete. However, if the water-
cement ratio is maintain, and need additional cement to make
it more workable, it will entail higher cost. Grading and
maximum size of aggregate also affect the physical feature of
concrete. Fine or very coarse sands should not be used or it is
objectionable.
2. Durability – This is important in concrete because it will be able
to withstand deterioration due to exposure to weathering
action (water-cement ratio is 0.58 by weight).
3. Strength – Depends on the water-cement ration. Proper curing
methods should be observed.
4. Impermeability – Imperviousness or water tightness is an
essential ingredient of concrete particularly when it is exposed
to weather with less water used in the mixture, the concrete
product can be made dense and will not leak.
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6-Properties of Concrete

 Effects on workability
Property/ Increase/
Effect on workability
characteristic decrease
Aggregate-to- Decrease
Increase
cement ratio (at a fixed w/c ratio)
Aggregate texture,
angularity and Increase Decrease
elongation
Moderate Increase
Entrained air
Excessive Decrease

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 Cont.

6-Properties of Concrete

 Effects on workability
Property/ Increase/ Effect on
characteristic decrease workability
Cement fineness Increase Decrease
Water-to-cement
Increase Increase
ratio (w/c)
Fine aggregates Deficiency Decrease
Increase
Water content Increase
(up to a point)

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Quality Control Measures
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:
In order to produce 4. Concrete 46
satisfactory Production
concrete, the Control
following should 5. Control in
be observed: Transporting and
1. Selection of Placing
Materials 6. Control in
2. Design of Consistency
Concrete Mixture 7. Sampling and
3. Aggregate Testing of Mixture
Production 8. Curing and
Control Protection
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Procedure:
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- Immerse vibrator quickly through full depth of
freshly deposited concrete
- If there is another layer below, vibrator should
penetrate into that as well to ensure monolithic
construction
- Concrete should be vibrated for between 5 and 15
seconds
- Put the vibrator into the concrete in a definite
pattern to cover the whole area of the structure
_ Gradually withdraw vibrator, giving time for the hole
to close by itself without trapping air.
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