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Comparative study on precast and cast in-situ concrete structures in

Rwanda.

GATETE Seleman

Corresponding author: GATETE Seleman,

Email:selegate@gmail.com

Abstract

The main objective of the present work is to

determine the comparative study between

precast concrete and cast in situ concrete on

some properties of concrete and determine

precast and merits of precast structure where

my part is to identify various factors which

gives signification importance to precast

fabrication with respect to cast in situ by

showing the importance of precast in today

is world in especially with reference to the

results show that the strength of cast in situ

concrete is 21.83 N/mm2 compared to

precast which has 24.53 N/mm2 means there

is a little bit different in strength and is

better to use precast one because the

construction will be quick and curing was

done on factory that led to high compressive

strength. The obtained results will help the

constructors to choose the appropriate

concrete for any structure.

Keywords: Precast concrete, cast in situ

concrete, compressive Strength

1. Introduction

Concrete is a mixture of cement and water

binds together fine and coarse particles of

inert materials known as aggregates, it is

readily seen that by varying the proportions

of the ingredients innumerable combinations

are possible. These combinations result in

concrete of different qualities. When the

cement has hydrated, the plastic mass

changes to a material resembling stone. This

period of hardening is called curing, which

three things are required: time, favorable

temperatures and the continued presence of

water. The durability of concrete is of vital

importance regarding the life cycle cost of

the structure, which includes not only the

initial cost of the material and labour but

also the cost of maintenance and repair. The

durability of concrete is therefore defined as

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its ability to resist weathering action,

chemical attack, abrasion, and other forms

of deterioration. Concrete has much higher

level of fire resistance than other building

materials. It is not combustible and would

not produce smoke or fuel the fire. Concrete

product can be either precast or in situ

concrete. This study seeks to analyze the

elemental cost comparison of precast and

cast-in-place in Rwandan construction

industry.

2. Literature Review

According to Frederick Raina (2001),

precast refers to the process of construction

in which a concrete element is cast

somewhere other than where it is to be used.

The other place may be somewhere else on

the building site or away from the site,

probably in a casting yard or factory. The

precast element may be pre-stressed, may be

of ordinary reinforced concrete, or may even

be without reinforcement. The single precast

elements may be component of a general

precast concrete system, or may serve as

singular purpose in a construction system of

mixed materials or types of elements. In this

project, I am going to research on advantage

and disadvantage of precast and cast in-situ

concrete elements and the problems

encountered in designing precast element.

The use of precast concrete has various

advantages which include the reduction of

the site labours, less wastage, less volume of

building materials, and increased

environmental and construction site

cleanliness. The use of precast products also

provides safety at construction site and

reduces time of completion in construction.

Most concrete buildings are cast in situ; the

wet mix is deposited and formed at the place

where the finished concrete is desired,

generally referred to as site cast concrete,

since the location is usually at a building

site. Precast concrete is defined as the

process of casting of concrete elements

offsite and move them to the actual building

site. Concrete for site cast construction is

typically brought to the site by concrete,

transporting mixer trucks with the large

rotating barrels. The mix is prepared at a

central batching plant, where controls of the

materials may be carefully monitored.

However, the transporting to the site, proper

mixing in truck, discharging from the truck

and depositing in the forms, and handling

for placement, finishing, and curing are all

subject to the level of responsibility and

craft exercised by the people involved. Site

conditions in terms of accessibility and

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weather can be highly critical to the work,

requiring extreme measure in some

situations to control all the stages in the

production process. The precast process

takes place in a controlled environment,

unaffected by weather. Rigorous inspection

before installation removes cause of delay

on site. Some HCC techniques can reduce or

eliminate follow –on trades, e.g. installing

ceilings and finishes. This enables even

faster program times but requires greater co-

ordination and care in detailing and

protection on site (Emmit Stephen, Gorse

Christopher , 2005). The quantity of

concrete in a precast framework is less than

4 percent of the gross volume of the

building, and of this is in the floors. The

precast concrete elements are columns,

beams, floor slabs, staircases and diagonal

bracing (Raina, 2001).

Marco Breccolotti , Santino Gentile , Mauro

Tommasini , Annibale Luigi Materazzi ,

[2016 ] Beam-column joints in continuous

RC frames: Comparison between cast-in-situ

and precast solutions. Nevertheless, cast-in-

situ solutions intrinsically allow building

moment resisting frames, a behaviour that is

usually hard to achieve using precast

elements. In this paper a technical solution

able to offer both high strength and ductility,

simplicity of construction of the

prefabricated elements and ease of assembly

on site is presented. The solution realizes the

continuity between beam and column by

means of loop splices and cast-in-place

concrete with steel fibers to improve the

ductility of the concrete struts in the wet

joint. It is based on prefabricated beams and

columns with protruding bars that are

connected in situ by means of a concrete wet

joint with steel fibres to moderately increase

the ductility properties of the compressed

struts in the joint. The results of these tests

showed that the two solutions exhibited very

similar structural behaviours, with the

proposed solution achieving a slightly

greater strength and stiffness than those of

the cast-in-situ solution without relevant

modifications to the joint ductility. In detail,

the arrangement of the reinforcing steel has

been updated in order to avoid the yielding

of the steel inside the column and to move

the plastic zone inside the beam.

3. Research methodology

The study was conducted in laboratory tests

in order to assess the compressive strength

of precast concrete. The focused tests were

sieve analysis test, water absorption test;

aggregate and sand and both slump cone test

and compression test on concrete cubes

testing in compression machine. In this

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project we preferred to use concrete cubes.

In order to make those cubes’ different

materials had been used. In additional

different tests on these materials had been

done before in civil engineering laboratory,

for getting data to be used in concrete mix

design and of course for making sure that

they will give correct results.

Selection of materials

The materials selected to be used for the

project are:

Cement

The cement that had been used for making

concrete cubes is CIMERWA 42.5 KN

Aggregates: Fine and course

Aggregates used Fine aggregate (sand)) used

in making concrete cylinders were taken

from Kayumbu quarry and coarse

aggregates (gravel) from Ruhango quarry.

Water

Water used in preparing concrete used for

making concrete cylinders is water supplied

by WASAC. It was free from impurities so

that it did not affect the final results Tests on

sand.

Test for sand

Before deciding that sand from Kayumbu

quarry is good quality sand so that it can be

used as concrete ingredient to be used in

making concrete cubes, the following tests

had been early performed:

Sieve analysis

Water absorption

Sieve analysis test

This test was conducted in order to

determine the particle size distribution of

fine aggregate (sand), to know the grade of

each in two types of sand and to determine

the amount of silt and clay composed.

Test procedure

Preparation of sample

Weighting the sample of 1500g was

taken as a sample to be tested or

sieved

Set a series of sieves with the largest

opening on top to the smallest

downward into a pan to receive the

fineness one that passed through out

of those set of sieves

Separate the material into a series of

particle sizes using the sieves

required by the material

Using a mechanical shaker

Establish a shaking time of 10

minutes as a sample shaking time

Brush particles clearly from each

sieve into the next lower sieve with a

bristle brush ensure that no material

is lost

Place an empty pan on the electronic

balance and tare or zero out its

weight

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Determine the individual weights to

the sand retained on each sieve and

record these weight

Re-tare the pan for the next sizes

sieve or tare a new pan if required to

keep the material separate for sure

Repeat this process until the weight

of the material on each sieve and the

retaining pan(typically the material

passing the No 200 sieve has

recorded)

Make the original sieve analysis a ‘’

total retained” analysis by weighting

the material cumulatively.

Fineness modulus was obtained by

adding cumulative percentage of

aggregates retained on each sieve

and dividing the sum by 100.

Water absorption test

This test was conducted in order to know

how the amounts of water were absorbed by

sand. Sample weight was 500g.

Test Procedure

Preparation of sample

Record the sample weight by balance

Put the sample in water within 24

hours

Removing sample from water

Record the amount of water

absorbed in weight (M1)

Drying the sample in the dry oven in

24 hours

Record the dried sample weight

(M2)

Finally, the water absorption was given by

this formula: ((W1-W2)/W2)100 where;

w1: wet weight

w2: dry weight

Test for aggregates

In order to decide the suitability of

aggregate for concrete the following test has

been carried out:

Aggregate compressive value test

Aggregate compressive value is a value of

compressive forces at which a building

construction stone can crushes less than 400

KN.

ACV we have conducted with Indian

Standard IS 2386 (Part IV) which reports

that aggregate crushing value shall not

exceed 45% of aggregates used for concrete.

Test Procedure

Preparing the sample

Making the weight of the sample that

have passed in a sieve 14mm and

retained in sieve 10 as BS

recommends

Compressive test with 400KN on the

sample through the compressive

machine

Making the weight of the sample

retained in 2.36 sieve after passing

the crushed sample in the sieve of

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2.36 Diameter retained sample as

British Standard requires

Calculating an ACV value as the

formula of BS Requires as IW-

R2.36S*100/IW

The formula say that IW=Initial Weight

R2.36S =Items retained in 2.36 mm Sieve

Tools and materials that were required to

make acv test are:

• Sieves of 14 and 12 mm

• Sieve of 2.36

• Mould

• Plunge

• Pan

• Gravels that have passed sieve 14mm

and retained in sieve 12mm

Test for concrete

The test for concrete is carried out to decide

the strength of the concrete against the loads

applied on it.

Compressive strength

This was a laboratory test conducted in

order to determine the compressive strength

(load bearing capacity) of quarry sand and

rainfall deposited sand, to evaluate the

performance and durability of quarry and

rainfall deposited sand and to check the

uniformity of quarry and rainfall deposited

sand quality.

Apparatus:

• Compression testing machine

• Tape measure

• Electronic Jointing table

• Electric or pneumatic hammer

• Vibrating table

Test procedure

Filling of cube moulds must be done

in three layers each approximately of

50 mm thickness. The concrete must

be placed using a scoop and the

scoop should be moved around the

top edges of the cube mould so that

symmetrical distribution of concrete

is done in each layer as the concrete

slides down from the inclined scoop

into the moulds.

Each layer must be compacted fully

either by using a tamping rod or by

using vibration techniques. If

concrete is compacted by hand

tamping, in 150 mm mould, then 35

strokes are given per layer uniformly

covering the entire surface especially

the corners. If 100mm mould is used

then each concrete layer must be

hand tamped giving 25 strokes.

To avoid entrapment of air in corners

and the sides it is recommended to

tamp the sides of the moulds either

by using the tamping rod or by using

a wooden mallet. Concrete in cube

moulds can also be compacted using

vibrations techniques. Electric or

pneumatic hammer or vibrating table

should be generally used. Never try

to compact cubes using a needle or

poker vibrator.

If the electric or pneumatic hammer

vibrator is used then it is

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recommended that cube moulds,

which have firm bolting arrangement

between the sides and the base

plates, be used. Use of clamps

instead of bolts may not give

adequate fixity. It is necessary that

filling and compacting operations

should be done in an identical

manner in three layers as in the case

of hand tamping.

It is necessary to hold the electronic

or pneumatic hammer down onto a

piece of timber placed over the top

of the mould. When the electric

hammer is used, it is also preferable

to keep the mould on a levelled hard

wooden piece rather than on any

hard surface.

Finally, trowel the surface level with

the top of the mould. Identification

mark, number and/or date can be

lightly scratched on the wet towelled

concrete surface using a matchstick

or a scraper.

Curing the specimen in 14days and

28days for getting to be hardened

After curing the specimen, placing it

with flat faces horizontal in

compression test machine

Apply load axially till failure occurs

and note maximum load at failure

The load at failure is a maximum load at

which the specimen fails to produce any

further increase in the indicator reading on

testing machine.

Maximum load at failure dived by average

area of bed face gives compressive strength

Bed face area=150mm*150mm= 22500mm2

4. Results

Results for sand tests

Sieve analysis test results of dry sand

The dry sieve analysis was conducted to

bank sand we take 500g, liver sand from

NYABARONGO River (Kayumbu) and

crushed stone sand.

Sieve

size

accordi

ng to

British

Standar

d

Mass of

fine

aggregat

es

retained

(g)

Percentag

e of fine

aggregates

retained

(%)

Cumulat

ive

percenta

ge of

fine

aggregat

es

retained

(%)

Cumul

ative

percen

tage of

fine

aggreg

ates

passing

(%)

10.0mm 0 0 0 100

4.75mm 10 2 2 98

2.36mm 60 12 14 86

1.18mm 120 24 38 62

600μm 80 16 54 46

300μm 120 24 7

8

22

150μm 80 16 94 6

75μm 20 4 98 2

Pan 10 2 100 0

TOTAL 500 100

Conclusion: The our sand is of good quality

because there is no many clay

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By the sieve, analysis diagrams above, the

visible difference varies depends on the

sieves arrangement (from the bigger to the

smaller sizes) on x- axis and percentages of

passing on y-axis. Therefore, our sand is

finer as shown. By ISI 383 grading graph

compared with the percentage of passing

sample below

SNo Sieve Size(mm) %Passing Grading zone III

1 10 100 100

2 4.75 98 95-100

3 2.36 86 85-90

4 1.18 62 60-65

5 0.6 46 45-60

6 0.3 22 20-25

7 0.15 6 0-5

8 0.075 2 3-5

Water absorption test result

The weight of moist(wet) sand sample

(M1 =2500g)

The weight of dry sand sample

(M2=2461.5g)

Water absorption test results of our sand(w)

W = 100=

= 1.56 %

Our sand absorbs water at the rate of 1.56%,

Therefore, by ASTM values of water

absorption test should not exceed 2.3% for

concrete purpose. Therefore, the above used

type of sand is good for high quality project

of concrete.

4.2 Test for aggregate

Aggregate compressive value test (ACV)

An aggregate compressive value is a value

of compressive forces at which a building

construction stone can crushes under

400KN.

We have used two samples

The first sample was contained of 3541.5g

of aggregates that have passed in sieve of

14mm and retained in sieve of 12mm.

With this sample, we made the test as the

procedures say and we got the Aggregate

compressive value as 14 KN/mm2

In our test we have conducted with

IW=3541.5g

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R2.36mm=3030.5g

After making an ACV we got =

During the project we have also used

another acv test of 3891g of stones sample

the results as the formula says IW-

R2.36S*100/IW we got

From this we have choose to use the sample

14% due to the Indian standard that says the

lower value of ACV the higher strength of

the aggregates.

5. Discussion

Slump test result

The height of the cone = 30cm

Block-test height = 21.7cm

Slump = 7.5cm which falls in the range of

medium workability

Concrete workability

Degree of workability Slump (cm)

Extremely low 0

Very low 0-2.5

Low 2-2.5

Medium 5-10

High 10-12.5

Very high 15

According to The degree of workability, the

workability concrete of our concrete is

medium this is good to be used for

manufacturing of concrete.

Compression test

The result of compression test which

differentiates compressive strength of

precast and cast in situ concrete

TARGET GRADE: M25

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Compressive Strength of cast in situ concrete at 14 days and 28 days

CAST IN SITU CONCRETE

No of

specimen

Water

cement ratio

Ratio Area of each

specimen(mm2)

Load of each

specimen(N)

Compressive

strength after

14 days

curing(N/mm2)

Load of each

specimen(N)

Compressive

strength after

28 days

curing

1 0.5 1:1:2 22500 407475 18.11 496125 22.05

2 0.5 1:1:2 22500 398025 17.69 478575 21.27

3 0.5 1:1:2 22500 407925 18.13 498825 22.17

Average 17.97 21.83

Compressive Strength of Precast concrete after 7 days and 28 days

PRECAST CONCRETE

No of

specimen

Water cement

ratio

Ratio Area of each

specimen(mm2)

Load of each

specimen(N)

Compressive

strength after 7

days

curing(N/mm2)

Load of each

specimen(N)

Compressive

strength after

28 days curing

1 0.5 1:1:2 22500 470250 20.9 537750 23.9

2 0.5 1:1:2 22500 473175 21.03 558000 24.80

3 0.5 1:1:2 22500 554625 22 560475 24.91

Average 21.31 24.53

The curing was conducted on the following

way in this test:

For precast

•We used 6 cubes of 15*15cm each were

cured in the bucket full of water for 28 days

For cast in situ

•We used 6 cubes also of 15*15cm each

were cured every day repeatedly in period of

14 days and 28 days, the results of the

compressive strength for every test are

shown in the table above.

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Conclusion

The main goals of the work have been

achieved.

As conclusion the result of test shows that

the strength of cast in situ concrete is 21.83

N/mm2 compared to precast which has 24.53

N/mm2 means there is a little bit different in

strength and is better to use precast one

because the construction will be quick and

curing was done on factory that led to high

compressive strength.

The study also revealed that the Rwandan

construction industry based on cast in situ

concrete more that on precast concrete

structures.

After the completion of different laboratory

test, analyzing and reading, consulting and

browsing on several source of information

including books, publications, adverts and

website as stated in references there are

number of conclusion which we are able to

make:

1. With using a precast concrete

structures, the strength of concrete

increases and also last longer

compared to cast in place concrete

2. With using a precast concrete

structures, the construction is very

quick

3. With using a precast concrete, the

future expansion is easy

4. With using precast concrete, the

curing is preserved

5. With using precast concrete, the

destruction is easy

6. With precast concrete we get

alternative economic use of

materials.

Limitation and recommendation

While conducting this study I met some of

the challenges. I was unable to purchase the

abundant materials of concrete for making

many samples of concrete because of lack of

enough funding. Again it was not an easy

task for me to get soil and concrete

laboratory where to perform different tests

as time of project was rushing. After seeing

the challenges met during this project, I

recommend IST Burkinafaso to support the

final year’s student projects; if possible they

should have MoUs with Institutions having

soil laboratory such as in Rwanda.

Based on the research works, it is highly

recommended to adopt precast concrete in

building construction.

I also recommend the institution to increase

the period reserved for academic

dissertation.

Acknowledgement

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It is my pleasure to thank Prof. Naon

Betabole for accepting me to work under his

guidance. I thank him cordially for the help

extended to me during the entire time of

doing the assessment. I remain ever grateful

to him. I also thank all professors, lecturers

of IST Burkinafaso who gave me technical

and professional supported while conducting

the assessment. I finally thank my family

members who funded this research; I

acknowledge their services with thanks.

List of abbreviations

TVET: Technical and

Vocational Education and

Training

WDA: Workforce

Development

M: Grade of concrete

GOR: Government of

Rwanda

R2.36: Materials retained in

sieve 2.36 mm

ASTM: American Society

for Testing Material

FCK: Characteristic strength

IW: Initial Weight

References

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Deek, O. (1985). Precast concrete structure

fabrication revised edition 2.

Emmit Stephen, Gorse Christopher . (2005).

Precast structural Analysis.

F.A.MNSE, E. A. (2015). Comparison of

compressive strength of concrete. Ilaro:

HRMARS.

J., A. (february 2012). Precast Concrete

Product.

Neville. (1987). Type of slump test.

Pumnia, D. B. (1967). Building

construction.

Raina, F. (2001). Concrete technology.

London.

Wiley, J. (1991). Engineering Material. Sons

Inc.