2. Concrete work Reciprocity involves giving back to the community, whether through sharing findings, providing resources, or contributing in meaningful ways. Transparency about research goals, processes, and data use builds trust.pdf

Ethical considerations are paramount in ethnographic fieldwork. Informed consent ensures that participants are fully aware of the research purpose, methods, and potential impacts, and agree to participate voluntarily. Confidentiality protects participants’ identity and privacy, fostering a trusting environment. Adhering to local norms and avoiding harm are crucial for maintaining respect and minimizing any negative impact on the community. Reciprocity involves giving back to the community, whether through sharing findings, providing resources, or contributing in meaningful ways. Transparency about research goals, processes, and data use builds trust and supports ethical practices.

### Conclusion

Ethnographic fieldwork’s immersive and holistic approach allows re

1. 1
ADDIS ABABA SCINECE AND TECHNOLOGE UNIVERSITY
College of Engineering
Department of Civil Engineering
General workshop practice (CEng2110)
2.Concrete work
Learning Outcomes(CLO):
After completion of this chapter, students will be able to:
Identify and Select appropriate tools and materials required for concreting activity
Define the activity sequence of concreting works
Identify the fresh and hardened state property of concrete
Lecture material by:
Zemu Gina
AASTU, Addis Ababa Ethiopia
March 2024

2. Introduction to Concrete

2
Concrete is an artificial stone like composite construction material which is made
up of mainly:
Cement
Water
Coarse Aggregate (Gravel)
Fine Aggregate (sand) &
Admixture (if required).
Now days, concrete has become the most common
construction material due to its :
Strength
Durability &
Economy (as compared to steel)

3. Advantages of Concrete

Concrete once cast and cured does not require any maintenance and can hold up
against any weather condition
Concrete is a non-combustible or Non-decaying material that makes it inert material
that doesn’t easily burn or rot.
It`s superior structural integrity provides an added degree of protection from the
severe weather as well as an earthquake.
Concrete is produced from locally available materials and leaves a small
environmental footprint while still providing high-level durability
Concrete can be shaped in various forms when freshly mixed.
It is a durable and cost-effective material which is a necessity for underground use.
Use of concrete
For building construction
For road construction especially in bridge and rigid pavement(Portland Cement
Concrete Pavement)
For construction of reinforced concrete dam
As a bedding material of different substructures
It is almost in all foundation or substructure works



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4. Concrete composition
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Portland Cement : 7% to 15% by Vol.
Water :14% to 21% by Vol.
Aggregates : 60% to 75%
- coarse aggregates
- Fine aggregates
Up to 2% air (depending on top size of coarse aggregate)

5. Concrete composition
5
Cement: we are dealing with Portland Cement, a hydraulic cement, the most widely used
binding agent in the construction industry.
Portland Cement manufactured in Ethiopia mainly of two grades under different brands
1. Ordinary Portland Cement:
Usually Graded 42.5 R
Its compressive strength is 42.5MPa (strength class 42.5R)
Manufactured purely using clinker with 2-5% gypsum
2. Portland Pozzolana Cement(PPC):
Usually graded as 32.5 R
its compressive strength is 32.5MPa (strength class 32.5R)
Manufactured using around 70% by weight clinker &
30% Pozzolanic materials(eg. Pumice, volcanic ash)
with 2-5% gypsum

6. CONCRETING PRODUCTION PROCESSE
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Batching Mixing Transporting
Placing Compacting Curing and Finishing

7. CONCRETE PRODUCTION AND CASTING PROCESSES
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Batching: Measuring required quantities of ingredients
Mixing
Transporting
Placing
Compaction (Consolidation)
Curing

Fig. Property of concrete as a function of time

8. Fresh Concrete

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Fresh concrete or plastic concrete is a freshly mixed material which can
be molded in to any shape.

At fresh state, consistency of
concrete classified as:
I. Stiff
II. Plastic
III. Flowing
-Plastic consistence is best required
property.

9. 9
Batching
The measurement of materials for making concrete is known as batching.
There are two methods of batching:
1.Volume batching
2.Weight batching
Weight batching Volume batching

10. 10
Weight Batching
•Ingredients are measured by weight depending upon the mix design.
•Preferred from volume method due to accuracy.
•Usually applicable in batch plants & Large projects.
•To prepare 1:2:3 concrete mix using weigh batching, measured quantity of materials
are 50 kg of cement, 100 kg of fine aggregate and 150 kg of coarse aggregate.
Concrete weigh batcher may be manual or semi-automatic or fully
automatic .
Manual type is used for small concrete production job and all the process will be
done manually while other two types are used for large concrete production.
semi-automatic weight batching, aggregate container gates are lifted
manually and it is automatically closed after reaching required quantity in the
weighing machine.
In fully automatic weigh batcher, all the process will be done automatically.

11. Volume batching
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In volume batching, materials are measured on the basis of
volume. It is less precise method of batching.
Measurement boxes or gauge boxes of known volume are used to measure materials.
Cement is taken in the form of bags, where volume of one bag of cement (50 kg) is taken
as 35 liters.
Volume of Gauge box used is made equal to the volume of one bag of cement which is 35
liters or multiple thereof.
Standard Size: 40cm*50cm*18cm
:30cm*30cm*40cm

12. 12
Cont…
Volumes of different sized fine aggregate and coarse aggregate are measured
individually by these gauge boxes.
Water is measured using water cans of known volume.
To make 1:2:3 ratio concrete mix according to volume batching, one should
take one bag of cement (35 liters), 2 box of fine aggregate (75 liters) and 3
boxes of fine aggregate (105 liters).
If the water-cement ratio is 0.5, then half of the volume of cement which is 25
liters of water should be taken.

13. 13
Mixing
Concrete mixing is a process of mixing the ingredient of concrete such as
cement, sand, aggregate, water, and admixture together to make concrete of suitable
grade. To make the different grades of concrete, mixing of concrete materials should
be done properly as per the standard achieve the design strength of concrete. A
concrete should be mixed for a minimum of 2 minutes to get the required result.
The following methods are used for concrete mixing
1.Hand Mixing of Concrete
2.Machine Mixing of Concrete
Hand Mixing of Concrete: is practiced for small-scale unimportant concrete
works. In this method, as material mixing is not sufficient compared to machine mix it
is desirable to add 10 percent more cement to cater to the inferior concrete produced
by this method. Dry mix the ingredients first and add water slowly.

14. 14
Mixing Concrete


Machine Mixing of Concrete: For large size construction, the
concrete mixing is done by machine only, for reinforced concrete work,
and for medium or large scale mass concrete work.
Mixing of Concrete in a mixer is not only efficient but also economical when the
quantity of concrete to be produced is large.

15. 15
Mixing Concrete…

It is recommended that to make the batch of concrete having
50 kg of cement. If you are going for the purchase/rental of mixer you
should ask for such a capacity mixer that should hold all the materials for
one bag of cement.
For getting a good quality concrete mix to follow the below steps
Dry mix the ingredients (sand, coarse aggregate and cement) first
Add 25 percent of the total quantity of water required for mixing and start
the mixing.
Add the remaining amount in two of three phase.
The mixer is generally designed to run at a speed of 15 to 20 revolutions per
minute. For well mix concrete, it is seen that about 25 to 30 revolutions are
required in a well-designed mixer.

16. 16
Transportation of Concrete
Transportation of Concrete is done by various methods and equipment on site.
The only care that should be taken is that the homogeneity of concrete obtained
at the time of mixing should be maintained until it reached its final destination.
Following are the methods adopted for transportation are:

Mortar Pan
Wheel Barrow
Crane
Bucket
Truck Mixer
Belt Conveyors
Chute
Skip and Hoist
Pump and Pipe Line
Helicopter

17. 17
Placing and compacting of concrete
Concrete should immediately be placed in the structure and should be compacted
before placing the next layer.
Purpose of compaction is to eliminate entrapped air from the concrete so that
concrete is less permeable and good bond between reinforcement and concrete is
achieved.
1% entrapped air cause 5–6 % reduction in concrete strength.
Two methods of vibration: Internal and External
Internal vibration: Roding – for thin sections by pushing iron bar inside and outside of
concrete.

18. 18
Placing and compacting of concrete
External vibration: Vibrators are clamped to formwork or placed on the surface.
Surface vibration: Concrete is vibrated from the surface when screeding (striking
off) the concrete is carried out. (For pavements and slabs).
Table vibration For precast concrete Surface vibration

19. 19
Concrete curing
The process of keeping the fresh concrete under moist condition and favourable
temperatures so that hydration may continue till concrete gains strength.
Curing starts after final set.
Due to inadequate curing, concrete develops plastic shrinkage cracks, thermal
cracks, along with a considerable loss in the strength of the surface layer.
Methods of curing
I. Water Curing Ponding and Spraying

20. 20
1.4. Construction Site Care and Precaution…
II. Membrane Curing: When there is a shortage of water in the area.
III. Miscellaneous Curing: a chemical that forms a layer of membrane (film)
that prevents evaporation.
Membrane curing
Miscellaneous curing

21. Desired Properties of Concrete
At Fresh state:
Workability: Easy of mixing, transporting, placing and vibrating
No segregation or bleeding
How can we justify the fresh concrete is workable or not? At least by
using Slump Test: measures concrete consistency or stiffness of concrete
at fresh state
At Hardened state:
Adequate compressive and flexural strengths
Durability
Good surface finishes
Minimum entrained air
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22. Concrete grades
Concrete grade is defined as the minimum compressive strength rating of a
concrete mix cured for 28 days after it was poured.
 According to Compulsory Ethiopian Standards(CES) 149, 1-1, The
compressive strength of concrete is denoted by concrete strength classes which
relate to the characteristic (5%) cylinder strength fck, or the cube strength fck,
cube, in accordance with EN 206-1.
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23. Cont…
In Ethiopian construction industry, the letter “C” is used to denote the
characteristic cube strength of concrete. C-30 meaning characteristic cube
strength is 30Mpa.
C-5,C-7, C-10 for lean concrete on well compacted soil before re-bars laid
C-15, C-20 for non-load bearing structural elements and general purposes
C-25, C-30, ,…C-50, mainly used in structural elements like beams,
columns and slabs

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24. Concrete Mix designs
The objective of mix design is to decide the proportions of materials which will
produce concrete of required properties(true slump at fresh state, compressive and
flexural strength as well as durability at hardened state.
Categories of Slump test results:
1. True Slump: Mix has high stiff consistency. In a true slump concrete just subsides
shortly and more or less maintain the mould shape. This type of slump is most desirable
2. Zero Slump: If concrete maintains the actual shape of the mould, it is called zero
slump which represents stiff, consistent and almost no workability
3. Collapse slump: In this case, fresh concrete collapses completely. Mix is too wet or
high workability mix slump test isn’t appropriate for such mix.
4. Shear Slump: If one-half of the cone slides down in an inclined plane, it is called a
shear slump. It is an indication of lack of cohesion of the mix. Again perform the
experiment to avoid shear slump


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25. Categories of Slump test results…
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True slump test Collapse slump
Potential non
cohesive
Shear slump
No displaced
center
Zero slump
Potential non
plastic

26. Concrete Mix designs
Concrete Mix Design Calculator (Based on the ACI Code)
Concrete mix design is the process of selecting the right amount of constituent
materials to produce a mixture with the desired properties.
 ACI method of Concrete Mix Design is one of the commonly used mix design
methods. At end , normally, mix ratio that results in workable concrete and target
strength values is selected.
Ingredients specification(quality test for assurance will be done if necessary), target
concrete strength (C-20, C-25, etc) for the structure selected , target slump value,
recommended water to cement ratio as well as cement-sand-aggregate ratio are
some of concrete mix parameters considered as input.

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27. Concrete Mix designs
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