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University of Nairobi

Department of Civil Engineering

TITLE: USE OF NON- DESTRUCTIVE METHODS TO ASSESS THE STRUCTURAL

INTEGRITY OF AN EXISTING CONCRETE ELEMENT

BY

MIBEY SHIPHRAH CHEPKOGEI

F16/28983/2009

PRESENTED TO: DR.S.W.MUMENNYA

This project has been submitted as partial fulfillment of the degree in:

BACHELOR OF SCIENCE IN CIVL ENGINEERING

2014

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ABSTRACTNondestructive testing is used to obtain material properties of in place elements of a structure

without destruction of the specimen. Concrete being an integral part of construction, knowing

the strength of it before commencement of construction is very important. Knowing the

strength is also important throughout the life of the building. For one to be able to know the

specifications of cast- in concrete the 28-day compressive test on cylinders and cubes is

important which is cast from the same concrete mix as the structure. This is a representative of

the cast concrete strength. However, not all times that the cubes will represent in-situ

conditions various factors influence variations including age of curing, type of aggregate and

size of aggregates.

There are various methods of nondestructive testing methods that can be used for determining

the health of an existing structure. The rebound Schmidt hammer was chosen for testing. It is

used to determine the strength of concrete which works with the principle of rebound from an

elastic mass which depends on the hardness of a surface against which the mass hits.

The laboratory test for strength of concrete requires one to load the cast cubes and cylinders to

be loaded to failure. This may result in actual determination of strength of concrete but may

not necessarily depict actual strength of building knowing there I addition of reinforcement for

the same.

The aim of this project is to obtain calibration curves for the rebound Schmidt hammer and

hence determine the strength of the Civil engineering block building in relation.

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ACKNOWLEDGEMENTI would like to express my sincere gratitude to my supervisor Dr.Siphila. W. Mumenya for the

continued guidance and support during this project.

Gratitude to the Institute of Nuclear Science and Technology for the assistance with

information and equipments for testing.

Appreciation goes out to the concrete laboratory technicians for help with the laboratory work.

My Appreciation goes out to A. J. M Kulah from the Ministry of Transport and Infrastructure-

Materials Department for the valuable information.

Support from my family was highly appreciated and for the friends who helped me through this

project not forgetting to thank the Almighty God for graciously providing good health.

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Contents

ABSTRACT...................................................................................................................................................... 1

ACKNOWLEDGEMENT................................................................................................................................... 3

1. INTRODUCTION......................................................................................................................................... 6

1.1 Introduction ........................................................................................................................................ 6

1.2 Background of Study ........................................................................................................................... 7

1.3 Objectives of Non-destructive testing of concrete elements............................................................. 9

Specific Objective....................................................................................................................................10

1.4 Scope of Study...................................................................................................................................10

1.5 Justification .......................................................................................................................................11

2. LITERATURE REVIEW ...............................................................................................................................12

3. THEORETICAL ANALYSIS..........................................................................................................................22

3.1 Concrete composition.......................................................................................................................23

3.2 Concrete mix design..........................................................................................................................26

4. EXPERIMENTATION.................................................................................................................................28

4.1 Laboratory testing.............................................................................................................................28

4.1.1Fresh concrete ................................................................................................................................28

4.1.2 Procedure.......................................................................................................................................29

General procedure..................................................................................................................................29

4.1.3 Slump Test......................................................................................................................................30

Procedure................................................................................................................................................31

4.1.4 Compaction Factor Test .................................................................................................................33

4.1.5 Compressive Strength of Concrete ................................................................................................36

Calculations.........................................................................................................................................38

4.1.6 Correlation between destructive and non-destructive strengths .................................................40

4.1.7 Results........................................................................................................................................41

4.1.8 Testing on building.........................................................................................................................47

Procedure................................................................................................................................................48

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4.1.8.1 Visual Inspection .....................................................................................................................48

4.1.8.2 Schmidt Hammer Test.............................................................................................................48

5. ANALYSIS .................................................................................................................................................52

5.1 Introduction ......................................................................................................................................52

5.2 Visual Inspection ...............................................................................................................................52

5.3 Test on fresh concrete ......................................................................................................................52

5.3.1 Slump test ..................................................................................................................................52

5.3.2 Compaction factor test ..............................................................................................................53

5.4 Experiments on hardened concrete .................................................................................................53

5.4.1 Hardened concrete density........................................................................................................53

5.4.2 Rebound hammer test ...............................................................................................................54

5.4.3 Compressive strength test .........................................................................................................58

6. CONCLUSION...........................................................................................................................................59

REFERENCES................................................................................................................................................60

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1. INTRODUCTION

1.1 Introduction

Due to the current growth of the infrastructure sector in the Kenya in accordance to the Kenya

Vision 2030, urbanization of various counties will widen and the technology being used

improved. This means the buildings coming up and existing buildings need frequent checks to

help monitor the health of the building. Therefore, suitable methods of testing to be used to

check whether the structure is suitable for the designed use and whether the components of

the structure are able to perform their functions well with less maintenance to it. Hence, the

use of non-destructive testing is appropriate to use.

Most importantly, the monitoring of the buildings is not to preserve the building but save the

costs put up for new constructions but to put the buildings to good use.

Tests chosen to be done on the building should be termed suitable. A suitable method should

be chosen where there will be no or insignificant damages to the building. Damages to the

building may cause the strength of the building to decrease or increase cost of maintenance.

As a consequence, Non-Destructive tests are most suited to testing any building.

This project will focus on the use of Non-Destruction methods and the relevance of it in the

Civil Engineering field.

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1.2 Background of Study

Non-destructive Testing (NDT) is a non invasive test where there is no interference in the

structure or elements under testing. The use of NDT is therefore termed cost effective as there

is no repairs needed and the quality of the structure is not tampered with.I

Uses of Non-destructive testing

NDT is used to check and evaluate various flaws of a concrete slab.

To check the health of a structure.

To prevent failure of various element this can be a financial problem and injuries to

occupants of the structure.

To determine the existence of cracks, voids and various defects.

To determine the position, quantity and the condition of reinforcements.

To monitor the strength development of a structure in terms of prestressing, curing and

load application.

There are many methods in non-destructive tests such as the use of;

Profometer,

Ultra Sonic Pulse Velocity

Rebound Hammer.

Each method has its own scope and each is used to determine different defects.

Visual checks. - This is inspection done by just looking at the structure. Check is mostly

to see any visible signs of distress present like cracks, spalling, surface blemishes and

lack of uniformity.

The Profometer is used to locate the reinforcement bars and determine the concrete

cover and the actual diameter of the reinforcement bar.

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Ultrasonic Method. – This method is dependent on the principle of sound waves

passing through the material. This depends upon modulus of elasticity, poison’s ratio

and the density of the material. A pulse of longitudinal vibrations is produced by a

transducer which is held on the surface to be tested. Sound waves travel through

materials by vibrating the particles that make up the material. The pitch of the sound

is determined by the frequency of the wave. The testing is based on acoustics where the

vibrations are time dependent. The atoms oscillate to produce a mechanical wave.

Depending on how the particles oscillate, there are four different waves that propagate

within the material. These four different principles of wave propagation include: shear

waves, longitudinal waves, surface waves and in thin materials plate waves.

Schmidt Rebound hammer. – This is used to test the surface hardness and can be

correlated with the concrete strength. It works on the principle the rebound and elastic

mass depends on the hardness of the surface.

The hammer weighs about 1.8 kg. The main components are the outer body, the hammer

mass, the main spring, the plunger latching that locks the hammer mass to the plunger and

a sliding rider that is used to measure the rebound of the hammer mass. There is also an

arbitrary scale that measures the rebound distance. The rebound distance is corresponding

to the position of the rider on the scale.

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Fig 1-1: Parts of rebound hammer.

1. Concrete surface 5. Hammer guide 9. Housing

2. Main spring 6. Release catch 10. Hammer mass

3. Rider 7. Compressive spring 11. Plunger

4. Scale 8. Latch

Uses of the rebound hammer:

The rebound hammer assesses the uniformity of concrete.

Assesses the quality of the concrete.

Assesses the compressive strength of concrete. This is done with a rebound index and

compressive strength.

1.3 Objectives of Non-destructive testing of concrete elements

a) To come up with a system of how to test the strength and the quality of

workmanship of an existing building.

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b) To assess the structural integrity and adequacy of the element.

c) To understand the principles behind the non-destructive testing machines, focusing

on the performance, application and also their limitations specifically the Schmidt

hammer.

Specific Objective

To determine the existing strength of the Civil Engineering block and how its structural

health.

1.4 Scope of Study

This project is a case study that will take place at The University of Nairobi, Civil Engineering

block.

For this study, Non-Destructive Test method to be used will be the use of the Schmidt Rebound

Hammer.

The Schmidt Rebound Hammer uses BS 1881: Part 202 for the British Standard codes. (III)

To be able to achieve the above objectives, an element of the existing structure will be

chosen within the vicinity of the testing equipment. Considering that there is need to validate

the experimental results with known information, the choice of the test element will be based

on the available technical data.

The Civil Engineering block at The University of Nairobi is a typical reinforced concrete structure

consisting of;

Ground floor.

Suspended first and second floors supported on approximately 330x330mm columns

and approximately 200x600mm beams.

Steel roofing with IT4 iron sheets.

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The building was constructed in two phases. The first phase was built in 1952 and later

renovated in 1999. The total floor space is approximately 2,352 m2. The building has a

hydraulics laboratory, classrooms, computer laboratory and offices which are on the two

suspended floors.

1.5 Justification

The need for quick and reliable methods of determination of structural integrity is necessary.

As opposed to destructive testing, non-destructive testing involves minimum or no destruction

to the existing structure hence suitable for use. The use of it gives quality assurance of the

building. The civil engineering building is currently in service hence destructive testing is not

suitable for testing. Need for data baseline help in monitoring the health of the structure

which is to be done periodically.

For this study, Non-Destructive Test method mainly the Rebound Hammer (Schmidt Hammer)

will be used to determine the strength of the floor beams.

All data collected will be analyzed making comparisons with the existing strength values

present of the building.

Laboratory tests will also be done in order to compare the actual building strength and cubes

made.

The importance of the study is to determine any defects present in Civil block building that

can contribute to damage to the structure. Non destructive testing is valuable method of

testing because it helps us to determine the strength without hindering the usefulness of the

building.

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2. LITERATURE REVIEW

Nondestructive testing is an interdisciplinary field that plays a major role in ensuring that all

structural components of a structure or a system are in proper functioning conditions.

Determination of concrete strength is vital when constructing is done. Engineering most

important goal is to come up with a valuable stable functioning structure of which it will

withstand present conditions like weather changes and loads on it. One can also be able to

determine the state of the same structure in the future from information of the past of the

structure. This is where non-destructive tests fall in place where minimal destruction is needed

in order to ensure the structure has a long span and remains relevant for the intended purpose.

Non destructive testing is not a new method of testing; it has been practiced for many decades.

Early stages of nondestructive testing started in the early 1800. (VI) This was started when they

needed to check various steel components of cracks on railroad carts and axles. They would dip

the element in oil then dust it with powder. In the presence of cracks, oil would ooze out of the

defect area and would be able to know where the defect was and then fix it. This simple way of

checking defects initiated the improvement of methods of checking for defects of great

magnitude hence the introduction of modern liquid penetrating tests.

The improved method of penetrating tests, the test element is coated with a visible or

fluorescent dye solution. After coating the surface, the excess coat is wiped off and a developer

is applied. The developer is used to drawing the penetrant to the surface through the cracks.

This enables one to see the cracks present in the element. With the use of fluorescent dye,

ultraviolet light is used to view the cracks present.

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Fig 2-1: Process of penetrant testing.

In year 1895, Wilhelm Conrad Roentgen from Germany discovered X-rays. He was then able to

produce the first radiograph machine. With this invention, the possibilities of checking flaws on

steel structures were developed based on the same principle.

This in turn led to the development and advancement of ways of detecting defects to ensuring

effectiveness in the lifespan of the structure.

In year 1920, one Dr.H. H. Lester was able to continue with the study of radiography for metals

and was able to develop industrial radiography tests to check castings done on steel

installations and checking the steam power plants.

Radiography uses X-rays and gamma rays penetrating the surface to check for imperfections.

An X-ray generator is used and radiation is passed to the test surface where a radiograph is

produced onto a film. Imperfections are shown by the change in density in the film.

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Fig 2-2: Radiography.

In 1926, the use of eddy current was made possible by the invention of an instrument that was

able to measure the thickness of the element in test.

Immediately after the use of eddy currents, the use of magnetic induction systems was used in

the detection of various flaws in steel elements on railroad tracks.

The magnetic particle testing is accomplished by inducing a magnetic field in a ferromagnetic

material and dusting off the surface with iron particles either in liquid form or dry form. The

surface imperfections are shown by the distortion of the magnetic field and a high

concentration of the iron near the imperfection. This is one way of visual inspection of a defect.

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Fig 2-3: Magnetic particle testing machine.

Fig 2-4: Magnetic particle test illustration

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In the early 1940s the development of eddy current instruments was developed by C. Farrow,

Theo Zuschlag, Fr. F. Foerster and H. C. Knerr. In this method, electrical currents are generated

in a conductive material by an induced magnetic field. The current is referred to as eddy current

because it flows in circles below the surface of the material under test. Interruptions show the

imperfections and dimensional changes. The conductive properties are also detected. The use

of eddy current can therefore be used to determine the position of reinforcement in concrete

and the diameter and if there is visible deterioration in the same. This is shown in (figure 2-5)

below.

Fig 2-5: Eddy current testing.

In 1940, Dr. Floyd Firestone developed the ultrasonic test method that was developed so that

he would be able to check for inhomogenities of density and elasticity in materials. He stated

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that his invention was able to detect where a flaw was and the exact distance it is from the

surface. The principle it used was sending a high frequency vibration of sound waves to the

surface under inspection and the intervals of frequency of vibrations are reflected back to the

receiver where one can see from the output where a defect may be measured. This method

was applicable in concrete structures.

Fig 2-6: Ultrasonic test method.

Further on in the year 1950, the Schmidt Hammer was invented. This was the first time when

the principle of testing concrete was patented to use in testing concrete structures.

This shows the evolution and advancements made from the establishment of the non

destructive testing. This helped with the improvement of industrial quality measures and

showed the importance of the same in the construction industry.

The inventions done were manually based and in the early 1963, Fredrick G. Weigharts and

James McNulty’s went a step further to digitizing the radiography method of testing. This

revolutionized the whole nondestructive method of testing where most methods of testing

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were digitized and simplified testing. Digitization enabled one to receive an interpreted output

after testing immediately and it’s more accurate.

The improvements also enabled one to check the small and smaller flaws that can easily be

ignored.

Having been able to identify the flaws present, principles were developed where one can be

able to predict the rate at which the flaws or cracks will grow under loading conditions. This

helped them to know how long the structure will be of use after the defect was detected or

how to minimize the progression of the flaws for a longer lifespan of the structure.

This in turn shows that checking for flaws is necessarily not enough. One needs to obtain

quantitative information about the flaws or the condition of the structure to determine the

remaining lifespan of the structure. This led to continued research methods so as to come up

with adequate solutions for the problems facing the construction industry.

In 1946 a Swiss engineer, Ernst Schmidt, invented a piece of equipment for the nondestructive

testing of concrete. The instrument was designed in a way to it was able to measure the

rebound of a spring loaded mass impacting against a concrete surface; the harder the concrete

the greater the rebound number. It can be likened to bouncing a ball on a grass surface or soil

surface and then compared with a concrete surface. The ball tends to rebound much more from

a concrete surface because it is harder. Years on the only Schmidt hammer in regular use were

the ‘N’ type hammer. Which was of length 356mm, it was small enough to be carried in a

pocket but the mass that rebounds against the concrete surface is relatively small and the

results obtained is of high coefficient of variation. This is because the test results are influenced

by moderately minor surface variations in the concrete. Later on in the early 1960’s the ‘M’type

hammer was introduced. The instrument was of length 877mm long and has over thirteen

times the impact energy of the ‘N’ type instrument. Having it is that heavy, proves that is of

great disadvantage to have to carry it around. The greater impact energy from the ‘M’ type

hammer gives a far more accurate reflection of the concrete surface quality. From further

investigation, both a traditional ‘N’ type hammer and the ‘M’ type hammer were compared on

in-situ concrete structures. Initially both hammers were calibrated against concrete of known

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strength and then used to compare the strengths of in situ concrete of strengths ranging

between 20MPa and 70Mpa. The lower spread of rebound numbers on the ‘M’ type hammer

combined with a lower coefficient of variation makes it a useful tool for in situ testing. However

it was note that the real values of the instruments is shown to be the ability to compare

concrete on a similar basis rather than trying to relate the rebound number to a precise value

which often is a common mistake people make while using the equipment. The ‘M’ type

hammer weighs almost ten times the ‘N’ type hammer. The main advantage of the ‘M’ type

hammer is that it has impact energy of 29.43Nm which is greater than that of the ‘N’ type

which has impact energy of 2.207 Nm. Both types of hammers can be used in any direction

provided the hammer is held perpendicular to the surface under testing. (II)

Fig 2-7: M-type Schmidt hammer in use.

‘M’ type hammer

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Many years after the invention of the Schmidt rebound hammer, the ‘N’ type of hammer has

been commonly used as a non-destructive testing equipment to check the strength of in-situ

concrete often after cube compressive strength is obtained.

The full procedure is clearly stated in the British Standard BS 1881: Part 202 which is based on

the original procedure described in BS 4408: Part 4: 1971. This shows that a calibration graph

which has been prepared for specified local concrete. Further through the years many

researchers came up with researches showing the correlation between compressive strength

and the rebound number justifying BS 6089: 1981: Clause 5.6.5 where it indicates that the

accuracy of strength obtained from the readings from the hammer is likely to be within +/- 20 %

of the actual strength of concrete.

Initial usage of the hammer was used to test the strength of precast reinforced concrete piles in

order to determine how soon the piles could be lifted for their casting bed without cracking.

Soon after that there was a widespread in the use of the Schmidt hammer.

In this period of time not many people were able to use the Schmidt hammer and assess the

data obtained. The main problem was that the hammer was used without proper calibration.

Calibration is done by obtaining a series of readings taken from a piece of suspect concrete and

a value of the average rebound number is calculated and compared with the graph supplied

with the hammer to find the concrete strength. The graph provided on the side of the

instrument is that of average Swiss concrete of 1947 made from limestone aggregates which in

real sense may have no relation to the concrete under testing. The generation of a calibration

curve is therefore necessary using the local materials used for the concrete.

In the British Standards BS 1881: Part 202 Clause 3.1 states that the use of universal calibration

such as those produced by the manufacturer of rebound hammers can lead to serious errors

and should be avoided.

When a calibration curve is prepared, one should be able to note that reading taken from wet

surfaces have different readings from that taken from dry surfaces.

The Schmidt hammer in general was developed to provide an inexpensive and quick non-

destructive method for testing concrete in the field.

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In Kenya, nondestructive testing was begun in the year 1990. It was started by foreigners in the

country where the interest was basically on metallurgy. Checking defects on steel used mainly

on the railway tracks. This method was not highly widespread because it concentrated on one

industry. After further knowledge acquirement, the knowledge was not further applicable to

the Kenyan industry hence the interest in ways to help better the construction industry in the

country. This lead to various studies in the appropriate nondestructive methods that was

relevant to the growing industry in Kenya. This sparked an interest in the nondestructive

testing methods available. After further exposure and learned methods, it became a

widespread method used.

The interest also brought rise to a nondestructive technical centre with Kenya Bureau of

Standards with the help of the International Atomic Energy Agency (IAEA). The main aim of this

center was to offer an institution which will be able to serve the country by offering

nondestructive services in inspecting buildings and offering training to interested individuals.

The various NDT methods that KEBS offer is visual inspection, ultrasound testing, radiography,

magnetic particle testing, eddy current testing, penetration tests and strength of concrete.

The other institution that carries out the same tests is under the Ministry of Transport and

Infrastructure, The Materials Testing Department.

They use the appropriate equipments for the specified tests. The machines are maintained in

proper working condition and often calibrated for more accurate results.

This makes Kenya a country is conscious of various improvements of the industry and to keep

up with international standards in ensuring the health of structures.

This streamlines the construction industry, be it buildings, bridges, roads, factories and railways

making it comply with international standards set out improving production hence Kenya can

be able to compete internationally heightening our chances of moving out of the third world

country situation.

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3. THEORETICAL ANALYSIS

In Kenya concrete is the most common construction material used in structures. Like all

construction materials, the quality of concrete needs to be established. There are various

experiments that are designed for specific properties of concrete. One important test is test for

compressive strength of concrete. Testing for compressive strength is done by testing of

hardened concrete.

In order for one to determine the compressive strength, one needs to carry out experiments. (V)

This is done by casting concrete samples then crashing them. The samples are either cubes or

cylinders. The cylindrical specimens are of size 150 mm diameters X300mm height. The cube

size is 150mmX150mmX150mm. Though they both vary they are both used in various regions

for the determination of compressive strength of concrete. This test is also governed by the

codes for respective regions. The British Standards Codes and ASTM (American Standards).

The main difference between the cylinder and the cubes is before crushing the cylinder is first

capped and the cubes do not get capped.

Cubes on the other hand show higher compressive strength as compared to the cylinders.

Other than compressive strength, other factors have been proven to affect the compressive

strength of concrete. These include size and shape of aggregates also the type of class of

concrete used.

Concrete use began in ancient Rome and has become widely used.

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Modern tests show that the Roman concrete had as much compressive strength as modern

Portland-cement concrete (ca. 200 kg/cm2). However, due to the absence of steel

reinforcement, its tensile strength was far lower and its mode of application was also different.

Modern structural concrete differs from Roman concrete in two important aspects.

Mainly, its mix uniformity is fluid and homogeneous, allowing it to be poured into forms

rather than requiring hand-layering together with the placement of aggregate, which, in

Roman practice, often consisted of rubble.

Second, integral reinforcing steel gives modern concrete assemblies great strength in

tension, whereas Roman concrete could depend only upon the strength of the concrete

bonding to resist tension. (Mehta & P.K Paulo, 1999)

3.1 Concrete composition

Over the years, there has been rise in many different types of concrete. It is created by varying

the proportions of the constituents that make them up, that is aggregates, cement and sand. In

this way or by substitution for the cementitious and aggregate phases, the finished product can

be customized to its application with varying strength, density, or chemical and thermal

resistance properties. The mix design depends on the type of structure being built, how the

concrete will be mixed and delivered and how it will be placed to form this structure and also

the specified use of the structure.

The constituents of concrete include the following;

a) Cement

Portland cement is the most common type of cement in general use here in Kenya. It is a basic

ingredient of concrete. It has been used over the years as the paste that binds the constituents

of concrete together (aggregates). Therefore the use of cement in construction is of great

importance in concrete production.

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b) Aggregates

Fine and coarse aggregates make up the bulk of a concrete mixture. Sand, natural gravel and

crushed stone are used mainly for this purpose. One can also adopt the use of recycled

aggregates (from construction, demolition and excavation waste). This substitution of natural

aggregates as partial replacements or full replacements is on the rise. This method is a way of

economizing cost. The presence of aggregate greatly increases the sturdiness of concrete above

that of cement, which otherwise is a brittle material and thus concrete is a true composite

material.

At least three-quarters of the volume of concrete is occupied by aggregate, it is not surprising

that its quality and type is of considerable importance. Not only may the aggregate limit the

strength of concrete, as aggregate with undesirable properties cannot produce strong concrete,

but the properties of aggregate greatly affect the durability and structural performance of

concrete. Also The characteristics of aggregate:

I. The stiffness,

II. shape,

III. texture,

IV. maximum size, and

V. grading of both coarse and fine aggregate is significant.

Aggregates were originally viewed as inert materials dispersed throughout the cement paste

largely for economic reasons. It is possible however to take an opposite view and look on

aggregate as a building material connected into a cohesive whole by cement paste, in a manner

similar to masonry construction, in fact aggregate is not truly inert and its physical, thermal,

and sometimes also chemical properties influence the performance of concrete.

Aggregate is cheaper than cement and it is therefore economical to put into the mix as much of

the former and as little of the latter as possible. But economy is not the only reason for using

aggregate; it confers considerable technical advantages on concrete, which has a higher volume

stability and better durability than hydrated cement paste alone. (Neville, 1981)

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The size of aggregate used in concrete ranges from tens of millimeters down to particles less

than one-tenth of a millimeter in cross-section. The maximum size actually used varies but in

any mix, particles of different sizes are incorporated the particle size distribution being referred

to as grading. In making low-grade concrete, aggregates from deposits containing a whole

range of sizes, from the largest to the smallest, is sometimes used; this is referred to as all-in or

pit run aggregate. The alternative, always used in the manufacture of good quality concrete, is

to obtain the aggregate in at least two size groups, the main division being between fine

aggregate and coarse aggregate.

Many properties of aggregate depend entirely on the parent rock, e.g. chemical and mineral

composition, petrologic character, specific gravity, hardness, strength, physical and chemical

stability, pore structure, and colour. On the other hand, there are some properties possessed

by the aggregate but absent in the parent rock: particle shape and size, surface texture, and

absorption. All these properties may have a considerable influence on the quality of the

concrete, either fresh or in the hardened state.

Though these different properties of aggregate are examined, it is difficult to define a good

aggregate other than saying that it is an aggregate from which good concrete (for the given

conditions) can be made. While aggregate whose properties all appear satisfactory will always

make good concrete, the inverse is not necessarily true and this is why the criterion of

performance in concrete has to be used.

c) Water

Combining water with a cementitious material forms a cement paste by the course of action of

hydration. The cement paste glues the aggregate together, fills voids within it and allows it to

flow more freely. It has been established that less water in the cement paste will yield a

stronger, more durable concrete and more water will give a free-flowing concrete. Impure

water used to make concrete can cause problems when setting or in causing premature failure

of the structure.

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3.2 Concrete mix design

Concrete mix design is constituted by the selection and proportioning the various constituents

of concrete in order to reach the required workability, durability and strength.

BS5328:1981: Methods of Specifying Concrete including Ready-mixed Concrete. (III)

The different types of concrete are;

a) Prescribed mixes. This is where proportions are provided that gives a required strength,

durability and testing in this is not required.

b) Designed mix. This is testing is an essential thing to do in order to achieve a certain

strength and taking into consideration the standards available.

Water- cement ratio is important in any concrete mixture. It has been known that cement

absorbs about 0.23 of its weight of water in normal conditions which is a dry mix. The actual

range of water-to-cement used is 0.45-0.6. This ratio affects workability of concrete. Not

forgetting that also the aggregate–to –cement ratio is also important as in that it also affects

the workability of concrete.

Several methods of mix design are used. The main factors drawn in are discussed briefly for mix

design according to Design of Normal Concrete Mixes.

The table below shows the various ratios of concrete mixes used in the Construction Industry in

Kenya.

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Table 3-1: Concrete mix ratios.

CLASS RATIO

CEMENT SAND COARSE

AGGREGATE

15 1 3 6

20 1 2 4

25 1 1.5 3

30 1 1 2

The selection of the aggregate-to-cement ratio depends on the grading curve for the aggregate.

It is also known that the characteristic strength of concrete is measured by the 28 day cube

strength. BS1881L:1983: Methods of Testing Concrete

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4. EXPERIMENTATIONThis is done in 2 stages, the laboratory casting of concrete cubes to produce a calibration curve

and the actual testing on the Civil Engineering building.

The constituent of traditional vibrated concrete include; water, cement fine aggregates and

coarse aggregates. The tests on constituent materials are only done on the aggregates in

accordance to the Kenya Standard, KS 95:2003 – Specification for natural aggregates for use

in concrete.

4.1 Laboratory testing

This is aimed to obtain a simple correlation used during on-site construction by engineers.

Samples of concrete were made from three different concrete mixes which are highly likely to

be used in the construction industry in Kenya. The aggregates and Portland cement used are of

natural source here in Kenya.

A standard cube size specimen of 150 X159 X 150 mm was casted and the results to be

compared with the Schmidt rebound hammer strength.

24 cubes were casted for each mix design.

4.1.1Fresh concrete

All constituents are to be mixed properly so as to produce a homogenous concrete mix where the

surface of all aggregates particles is coated with the cement paste.

Apparatus

Shovel

Weighing bucket

Weighing machine

Trowel

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Constituents

Fine aggregates (river sand)

Coarse aggregates

Water

Portland cement.

4.1.2 Procedure

In order to make the different concrete mix designs, different batches of concrete were made.

Since in this experiment we chose to do three different classes hence three batches were made.

The classes of concrete made were class 20, 25 and 30.

General procedure

We began by measuring various constituent to specific weight. The unit weight was first

determined. The unit weight test was determined by first recording the volume of moulds to be

used. The sum of volumes was then gotten. After the volume of was found it was then

multiplied by the density of concrete to get the actual volume of concrete that needed to be

mixed for the specific batch. An additional 10 % of the entire volume was added to allow for

wastages and shrinkage as well.

M= D X V

The mass of concrete achieved was then divided according to the ratio of the specified concrete

mix. In this project being;

Class 20, Ratio 1:2:4

Class 25, Ratio 1:1.5:3

Class 30, Ratio 1:1:2

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Table 4-1: Ratio for the mix design.

CLASS MASS(KG) WATER CEMENT SAND COARSE

AGGREGATES

20 48.6 3.82 7.63 14 28

25 48.6 4.86 9.724 14.58 29.16

30 48.6 5.35 10.69 10.69 32.076

After careful measuring of the constituents, it was then mixed in the mechanical concrete

mixer. Aggregates were first put in followed by the sand and cement. This was mixed

thoroughly to form an even mixture. After it was completely mixed water was the added

carefully while the mixture was being checked. This was mixed until all aggregates were coated

and were bound together.

After the right consistency was achieved, the fresh concrete was then subjected to various

tests.

4.1.3 Slump Test

(BS 1881; 102, ASTM C143)

The purpose of the slump test is to measure the consistency of concrete. This test is an

essential test which shows the water content of a batch of fresh concrete. It therefore

determines the workability of concrete dependent on the intended use or placement of

concrete.

When one decided to add water in the case to increase the slump, the extra after has various

effects to the concrete including;

a. The mix may tend to segregate, the coarse aggregate separate from the fine ones.

b. The concrete will shrink more as it hardens increasing the likelihood of cracks appearing.

c. The air content might increase tampering with the strength decreasing it.

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d. The strength of concrete decreases.

e. The concrete will set more slowly.

These are but few of the few effects of a higher slump value. Hence it is very important not to

exceed the water levels while mixing but use adequate amount of water to achieve the

required strength.

Procedure

A representative sample of concrete was obtained from the batch made. The cone was cleaned

and oiled where after it was filled one-third at a time. Each layer was compacted 25 times with

a 5/8 inch diameter tamping rod which is rounded at the end. The overflow was then cleaned

away from the base and the cone top flattened out. The cone mould was then was then lifted

vertically. Within a short period say 5 seconds, the slump was measured to the nearest mm.

The slump is the distance that the concrete has fallen from the original height of the cone. The

slump was then recorded.

Fig 4-1: Slump test.

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Fig 4-2: Measuring slump.

Calculations

Original height –New height= Slump value

SAMPLE 1

300-292= 8 mm

SAMPLE 2

300- 295= 5 mm

SAMPLE 3

300-295= 5mm

The values obtained are within the range (5mm- 10 mm). The results of a slump test are

generally inconclusive hence the compaction factor test is used to further justify if the concrete

is workable.

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4.1.4 Compaction Factor Test

(BS 1881: Part 103 )

The purpose of this test is to find the workability of concrete. It checks the uniformity of

concrete. This test is based on the principle that workability of concrete is indicated by its

compaction by a standard amount of work done to it. By allowing it to fall under gravity

through a standard height.

The standard cylinder ratio of density of fully compacted concrete yields the compaction factor.

Compaction factor = Density of partially compacted concrete

Density of fully compacted concrete

Requirements

I. Compaction factor apparatus. It has two hopper cone below the other and the cylinder

below the lower hopper.

II. Tamping rod.

III. Weighing balance.

IV. Concrete.

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Fig 4-3: Compaction factor apparatus.

Procedure

The internal sides of the hoppers were cleaned and made moist. The sample was then put

in the top hopper having the slider door closed. After the top hopper was filled up, the

slider door was then opened to release the concrete into the second hopper. Later the

slipper door of the lower hopper was opened to release concrete into the cylinder. Excess

concrete was cleaned out of the cylinder and the weight of the partially compacted

concrete was weighed and recorded (w 1). The cylinder was then refilled and compacted it

well. The concrete at this stage was again weighed and value recorded (w 2).

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Calculations

SAMPLE 1

Weight of partially compacted concrete=18 kgs

Weight of fully compacted concrete=21.5 kgs

Compaction factor=w1/w2= 18/21.5= 0.837

SAMPLE 2

Weight of partially compacted concrete= 18.05 kgs

Weight of fully compacted concrete= 19.7 kgs

Compaction factor= w1/w2= 18.05/19.7= 0.916

SAMPLE 3

Weight of partially compacted concrete= 17.6 kgs

Weight of fully compacted concrete= 22 kgs

Compaction factor= w1/w2= 17.6/22= 0.8

Having come to the conclusion the concrete batch was workable; we proceeded to cast them in

moulds of 150 X 150 X 150 mm cubes.

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Fig 4-4: Cast concrete cubes.

The concrete cubes after casting were let to set for at least 24 hours before being de-moulded

and set aside to cure. The cubes were then subjected to compressive tests on day 7 and day 28.

4.1.5 Compressive Strength of Concrete

The strength is determined by the ability of a material to resist stress without failure. Failure of

concrete is evident due to cracking. During compression disintegration often appears hence

crushing. Strength then is generally referred to in the construction industry because it is

relatively easy to measure and other properties related to the strength can be gotten from the

strength data.

The compressive strength is determined by a standard uniaxial compressive strength test that is

accepted universally as an index of concrete strength.

The failure mechanism and in four sections;

1. At the first quarter of the ultimate strength, random cracking starts to appear in the

transitional zone where large aggregates are.

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2. In the second quarter of the ultimate strength, cracks continue to grow further away

from the transitional zone.

3. At the third quarter of the ultimate strength, major cracks enlarge.

4. At the final quarter of the ultimate load, the major cracks link up in the vertical direction

and finally the specimen splits.

Fig 4-5: Illustration of compressive strength test.

Fig 4-6: Cube crushed under compressive test machine.

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The vertical cracks are as a result of expansion of concrete in the lateral direction. If concrete is

confined such that it isn’t allowed to expand freely, cracks will be resisted hence increase of

strength. This is evident on columns where stirrups are placed around vertical reinforcement to

prevent lateral displacement on the interior concrete hence increase of concrete strength.

Specimens that were stored in water for curing were tested immediately on removal from the

water still in their wet condition. The dimensions of the cubes were first noted and the weights

established before testing commenced.

The bearing surface of the machine was wiped clean and loose material removed from the

surface of the specimen which was to be in contact with the compression platens. The cube

was then put into the testing machine in such a manner that load was applied on opposite sides

of the cube as cast. The axis of the cube was then carefully aligned with the centre of thrust of

the spherically seated platen. The platen is then brought to bear on the specimen by rotating

gently by hand so that a uniform seating was achieved. The load was then applied at a steady

increment without shock. It was loaded until the cube breaks down and where no greater load

can be applied to it.

The maximum load was then recorded and the type of failure noted.

Calculations

The compressive strength of the cube was calculated by dividing the maximum load applied to

it by the cross sectional area of the cube.

An average value was the taken to be the representative.

DAY 7

SAMPLE 1

395, 400, 400

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Av=398 KN

(398 X1000)/ (150 X150) = 17.69 N/mm2

SAMPLE 2

430, 440, 440

Av = 436.7 KN

(436.7 X1000)/ (150 X150) = 19.4 N/mm2

SAMPLE 3

510, 490, 510

Av =503.3 KN

(503.3 X1000) / (150 X150) = 22.4 N/mm2

DAY 28

SAMPLE 1

680, 600, 650

Av = 643.3 KN

(643.3 X1000) / (150 X150)= 28.6 N/mm2

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SAMPLE 2

730,730,730

Av = 730 KN

(730X1000) /(150X 150) = 32.4N/mm2

SAMPLE 3

750, 780, 770

Av =766.6 KN

(766.6 X1000)/ (150X150) = 34.1N/mm2

Having satisfied that the compressive strength is within range of values of specified actual

measurement of the specified class of concrete; further test in correlation to the Schmidt

hammer was therefore considered.

4.1.6 Correlation between destructive and non-destructive strengths

This laboratory work is meant to find the simple correlation used by engineers. The cubes that

were cast earlier were used for this test. The 28 day cubes in this case were chosen for it

represents the matured compressive strength of concrete. (X)

The concrete cubes were held in position by the compressive testing machine under a fixed

load. Measurements of the rebound are in accordance to BS 1881. An average of 5 readings

was taken one face of the cube. The hammer was held horizontally to test opposite sides of the

cube. The average of the recorded rebound number was then obtained. After which the cube

was then crushed to obtain compressive strength of it. (IV)

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Fig 4-7: Cube specimen in testing machine under test using Schmidt hammer.

4.1.7 Results

The test results for the rebound number are recorded in the table that follows.

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Table 4-2: Rebound number on 7-day concrete.

Rebound Number Average

C20 20 22 20 21 20 20.6

18 19 18 20 22 19.4

22 19 19 24 19 20.6

22 22 19 20 22 21

C25 20 24 23 22 20 21.8

18 16 20 18 22 18.8

26 20 18 20 16 26

20 18 20 22 24 20.8

C30 28 26 28 20 26 25.6

27 20 30 26 24 25.4

28 22 26 26 22 24.8

32 26 28 20 24 26

Table 4-3: Cube compressive strength of 7-day concrete cubes.

Rebound Number Cube Compressive Strength (N/mm2)

20.6 11.77

19.4 9.81

20.6 11.77

21 11.77

21.8 12.75

18.8 9.81

26 19.61

20.8 11.77

25.6 19.61

25.4 17.65

24.8 17.65

26 19.65

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Table 4-5: Cube compressive strength of 28-day concrete cubes.

Table 4-4: Rebound number on 28-day concrete.

C Rebound Number Average

C20 26 24 30 28 30 27.6

30 26 28 30 30 28.8

30 26 26 30 31 28.6

31 28 30 32 30 30.2

C25 32 30 30 31 26 29.8

28 30 26 34 28 29

30 32 30 33 28 30.6

32 28 30 26 33 29.8

C30 28 33 34 26 30 30.2

30 32 26 28 30 29.2

34 32 33 34 30 32.6

30 32 30 26 28 29.2

Rebound Number Cube Compressive Strength (N/mm2)

27.6 21.57

28.8 23.53

28.6 23.53

30.2 25.5

29.8 25.5

29 23.53

30.6 27.46

29.8 25.5

30.2 25.5

29.2 23.53

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Fig 4-8: Calibration curve of 7-day concrete.

32.6 29.42

29.2 23.53

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Fig 4-9: Calibration curve of 28-day concrete.

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Fig 4-10: Quadratic calibration curve of 28-day concrete.

f(x)=0.023966x2+0.128425x-0.00117

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Fig 4-11: Cubic calibration curve of 28-day concrete.

f(x)=0.01446x3-1.09734x2+28.16894x-214.64151

Linear nonlinear line regression analysis is important in the study of correlation between the

rebound number and the crushing strength of the standard concrete cubes.

4.1.8 Testing on building

The civil engineering block building was the test subject. The building has three floors; the

ground floor, first and second floor suspended. The ground floor has offices and hydraulics

laboratories. The first floor and second floors has offices and classrooms. The building has

hollow-pot floor slabs and columns of size of 330 X330 mm. The testing of the building is done

in various steps. Knowing the use of the building, we chose a classroom on the first floor for

testing. This was chosen because it is a reflection of what goes on in the entire building.

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Procedure

4.1.8.1 Visual Inspection

Visual inspection is important because it often provides valuable information regarding the

building. Visuals are able to show the type of workmanship, material deteriorations and

structural serviceability. Things that can be noticed by the eye are like, spallings, cracks, pop-

outs disintegration, surface colouring and lack of uniformity. Visual inspection was pursued

after relevant structural drawing, plans and elevations were gotten. After proper studying of

the drawings, I was able to establish the section of the building that was subjected to tests. The

methods and dates of construction provide vital information about the building. Visual

inspection was done on the entire building checking the condition of the building by looking for

specific test points, check for surface cracks, the extent of it present and probable cause of the

damage. The environs of the building were also checked. Other relevant observations were also

taken note of such as the size of the building. No special equipments were used for this

inspection. (III)

Having identified the condition of the building visually, the building was then divided to various

homogeneous areas that is noticeable having different rooms on the respective floors.

Having decided that the room for test, being the classroom on the first floor nears the

computer room; the points selected for the test was marked for identification purposes. The

members were then checked if they were in dry conditions. This is because testing is favourable

in this condition.

4.1.8.2 Schmidt Hammer Test

The room has a total of seven columns and it is at the corner of the building. The column size is

330 x 330 mm. All columns were then subjected to testing. The size of the room is 11.28 m X

11.28 m.

Smooth surfaces on the columns were subjected to the test using the Schmidt hammer. Having

selected various points on the column, it was then made sure that the surfaces were smooth

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enough and then six rebound readings were taken. The hammer was held horizontal to surface.

The readings were taken at least 20 mm away from the edge of the column. This process was

repeated for all the columns in the room. Then the rebound numbers obtained was then

recorded in the table below.

Table 4-6: Rebound number of columns.

Col 1 2 3 4 Av

1 40 46 42 33 34 40 33 40 46 42 40 48

40.33333

40 42 40 40 46 34 40 40 42 40 40 60

42

46 40 40 36 40 44 36 46 40 40 46 48

41.83333

42 40 40 38 40 42 38 42 40 40 42 46

40.83333

2 40 38 40 27 30 31 27 40 38 40 40 52

36.91667

36 32 38 36 40 34 36 36 32 38 36 46

36.66667

42 38 38 31 40 36 31 42 38 38 42 48

38.66667

40 38 38 34 38 33 34 40 38 38 40 60

39.25

3 48 48 54 32 34 34 32 48 48 54 48 48

44

60 44 50 38 35 36 38 60 44 50 60 46

46.75

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48 46 47 36 36 38 36 48 46 47 48 52

44

46 41 50 36 37 37 36 40 46 42 40 46

41.41667

4 52 58 47 33 34 40 33 40 42 40 40 48

42.25

46 54 42 40 46 34 40 46 40 40 46 60

44.5

51 48 50 36 40 44 36 42 40 40 42 48

43.08333

54 49 44 38 40 42 38 40 38 40 40 46

42.41667

5 33 34 40 27 30 31 27 36 32 38 36 52

34.66667

40 46 34 36 40 34 36 42 38 38 42 46

39.33333

36 40 44 31 40 36 31 40 38 38 40 48

38.5

38 40 42 34 38 33 34 40 46 42 40 46

39.41667

6 27 30 31 32 34 34 32 40 42 40 40 42

35.33333

36 40 34 40 46 42 40 46 42 40 46 42

41.16667

31 40 36 40 42 40 40 42 40 40 42 40

39.41667

34 38 33 46 40 40 46 40 40 46 40 40

40.25

7 32 34 34 42 40 40 42 40 40 42 40 40

38.83333

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38 35 36 40 38 40 40 38 40 40 38 40

38.58333

36 36 38 36 32 38 36 32 38 36 32 38

35.66667

36 37 37 42 38 38 42 38 38 42 38 38

38.66667

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5. ANALYSIS

5.1 Introduction

The objective of this project was to determine the strength of the Civil Engineering block in

relation to rebound number obtained from the Schmidt hammer by performing necessary

experiments to correlate the values obtained.

The rebound test is solely based on the principle that the rebound of an elastic mass depends

on the hardness of the surface subjected to tests. This in turn will provide information of the

surface of the element being tested.

In this chapter the results obtained from the experiments will be analyzed followed by

discussions of the results. The experiments carried out include slump test and compaction

factor test for the fresh concrete, rebound hammer and compressive test on hardened

concrete. (VII)

For each test the results in the previous chapter will be discussed.

5.2 Visual Inspection

After inspection was done, various observations were made. There was presence of surface

wearing off on columns were seen. These indicate the amount differential movements from

persons. It was so minimal that it does not tamper with the strength of the columns. Other

cracks were seen on walls. The cracks were not extensive hence it being insignificant. The

cracks might be plaster cracks. Check of cracks was the only significant visual test significant in

regards to the objective of this project to determine the strength of the building.

Visual inspection generally provides an initial indication of the condition of concrete. This

enables one to formulate a successive testing procedure. This also helps one with a trained

visual eye document the defects and damages.

5.3 Test on fresh concrete

5.3.1 Slump test

For each mix design a slump test was conducted and results recorded.

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Slumps are of a specific description; true, shear or collapse slumps. Slump values are influenced

by the water/cement ratio. If the slump value is high, it indicates that the water/cement value

is large. Low water/cement ratio inhibits low workability. Having chosen a water/cement ratio

of 0.5, the slump values ranged from 5mm- 8 mm. This is in the general range of workable

concrete hence the concrete mix made was ready for use.

5.3.2 Compaction factor test

This test is carried out after the slump test because slump test is not fully reliable in

determining the workability of concrete hence this test to confirm the workability of the

concrete.

The values obtained were 0.837, 0.916, and 0.8 from the three mix designs. For a normal range

of concrete the values range from 0.8-0.92. This enables us to finally establish that the concrete

mix made was workable hence continuation of the experiment.

Few of the factors that generally affect the workability include;

Water content:-Too much water reduces the cohesiveness of the particles hence will

lead to bleeding and segregation.

Aggregate properties: -The shape and texture. More spherical and smoother aggregates

are more workable.

Cement:- Fine cements reduce the fluidity at a specified water/cement ratio but

increase cohesion. High cement content the better the workability of concrete.

5.4 Experiments on hardened concrete

5.4.1 Hardened concrete density

The concrete was then cast in 150x150x150 mm cube moulds. According to the British code

standards, the weight of the concrete was then measured after curing. The weights were taken

from the 7-day concrete cubes and the 28-day concrete cubes. From the weight of concrete we

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were able to determine the average density of the concrete cubes. The results are recorded in

the table below.

Table 5-1: Relative density of hardened concrete for the mix designs.

Age Mix Design Weight Density(kg/m3)

7 days

20 8 2370

8 2370

25 7.8 2311.11

7.9 2340.74

30 8 2370

8 2370

28 days

20 8.2 2429.63

8.1 2400

25 8.1 2400

8 2370

30 8.2 2429.63

8.2 2429.63

The concrete cubes were cured under water for the specified number of days. The densities are

a clear indication that curing cubes in water is of great importance because hydration reaction

is not stopped. Hydration of cement is responsible for concrete strength. (Safiuddin, Raman, &

25

Zain, 2007) Air cured concrete will have weaker concrete bonds hence low densities and lower

concrete strength.

5.4.2 Rebound hammer test

The hardened concrete was tested, both the 7-day and 28-day concrete cubes. The results

obtained include graphs of the rebound number against compressive strength of the cubes.

This is to provide a correlation between the two parameters.

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From a guidebook (Concrete Test Hammer Mod N, Toni Technik), a calibration linear graph is

proposed for the correlation purpose. The results obtained are in tables 4-2, 4-3, 4-4and 4-5and

the graphs generated for each being fig. 4-8, 4-9, 4-10 and 4-11.

It is then observed that the relationship between the compressive strength and the rebound

number is observed to be fairly linear according to the curves generated. The rebound hammer

results can be attributed to aggregate grading. According to researchers Li and Zheng (2002),

aggregate density has specific peak point to near surface section. This means that larger cubic

specimens will tend to have large rebound number.

Factors that generally affect the rebound number include:

1. The surface smoothness. (BS 1881: Part 202).

2. Age of concrete. In old and dry concrete, the surface is harder than the interior hence

high rebound numbers will be observed. Moist surfaces will result in lower rebound

numbers.

3. Carbonation of concrete. In dry concrete the carbonation area can extend over 20 mm

thick. This makes the strength estimation to be over estimated by almost 50%. When

significant carbonation occurs, the surface layer ceases to be a representative of the

concrete.

4. Type of cement used. Concrete that is made from super sulphated cement gives an

estimated 50 % decrease of strength as compared to Portland cement and high alumina

cement gives a higher strength of up to 100%.

5. Compaction. Variations of internal compaction give variations in strength. All

calibrations done assume that full compaction was achieved.

6. Rate of hardening. Variations in the initial rate of hardening, ensuing curing influence

the strength of concrete which in turn influences the rebound number.

Calibration curve that generally relates the rebound number to the strength is always provided

by the manufacturer of the Schmidt hammer. It is not of practical use depending on the type of

concrete made hence it is important for one to generate their own calibration curve to suit the

concrete mix design to be used. Correct functioning of the rebound number was key hence

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frequent checks are important to establish its state. This is checked with the standard anvil

provided by the manufacturer. Calibration curves generated for one hammer is not to be used

with a different hammer because each hammer has different specifications.

Cubic specimens show strong wall effects. Wall effect indicates the amount of mortar that is

required to fill spaces between concrete aggregates is less than the amount of mortar needed

to fill the space between aggregates and the mould’s wall (Neville, 2002). This causes an

increase in compressive strength of specimens. Tests have been then done to prove that the

wall effect influence can be eliminated. The researched showed that the size of aggregates

majorly influences the rebound number.

7-day compressive test calibration curve is not good for use because by the seventh day, the

strength gained by the cube is about 65% to 70% of the 28-day strength hence the use of 28-

day calibration curve for the correlation.

Having generated the calibration curve of the 28-day strength, extrapolation of the curve was

done in order to cater for the rebound number obtained from testing the columns in the

building. This is shown in fig 5-1 below.

Fig 5-1: Compressive strength vs. rebound number of 28-day concrete.

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From this curve we were be able to get the compressive strength value from the rebound

number obtained from the tests and correlate the same to the manufacturers curve. Table 5-2

shows the rebound number of the columns and compressive strengths from the curves given

and from the calibration curve generated.

Table 5-1: Average rebound number of columns and their compressive strength.

Col. Rebound Number Average ReboundCompressive Strength(N/mm2)

CalibrationCompressive Strength(N/mm2)

Col1

40.33 42 41.83 40.83 41.2475 44.13 44

Col2

36.92 36.67 38.67 39.25 37.8775 39.23 38.50

Col3

44 46.75 44 41.42 44.0425 50 48.20

Col4

42.25 44.5 43.08 42.42 43.0625 48.06 48

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Col5

34.67 39.33 38.5 39.42 37.98 38.25 38

Col6

35.33 41.17 39.42 40.25 39.0425 40.21 40.20

Col7

38.83 38.67 35.67 38.67 37.96 38.25 38.20

The results obtained that the calibration yields relatively the same result as the manufacturers

curve. The main reason for that may be the type of cement used for construction (Portland

cement) has the same properties as that used by the manufacturer.

The results also show that the strength of the concrete increased from the 28-day concrete

cube samples. This shows generally that strength of concrete increases with the age of the

concrete.

5.4.3 Compressive strength test

Compressive strength test was the most essential part of this project. Determination of

compressive strength was done by crushing the set concrete cubes using the standard

compressive machine. The results were recorded in table 4-3 and 4-5.

Having the load being applied steadily with an increment until no greater load was sustained.

The type of failure was also recorded. Failure modes of crushing can be termed as satisfactory

or unsatisfactory. Satisfactory mode of crushing is indicated when the four exposed surfaces

crack approximately equally with little or no damage to faces in contact with the platens.

Unsatisfactory failure is established by formation of tensile cracks on the surfaces. This is seen

by uneven cracks on the faces exposed or splitting. (VIII)

In this particular experiment it was noted that the failure mode was satisfactory.

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6. CONCLUSION

The main objective of this project was to put methods of non destructive testing to use to

determine the structural health of buildings and the reliability of the method of test chosen for

the test. The test chosen for the test was the use of the Schmidt hammer to determine the

compressive strength.

The results obtained from the laboratory were used to come up with a calibration curve relating

rebound number and compressive strength. The results obtain a linear curve hence the

conclusion that the higher the rebound numbers the higher the compressive strength. The

results are also influence by the size of aggregates used, type of curing, rate of hardening ,

carbonation and type of cement in use. Hence the need to generate calibration curves for every

mix of concrete.

This project has been able to:

1. Determine the usefulness of checking uniformity of concrete quality in construction.

2. The estimation of strength of concrete.

Having gotten positives of this project, there are also limitations to the same.

1. The use of rebound hammer gives high variations. This may make it difficult to

accurately establish the actual internal state of concrete being used and the building

under test. The accuracy of absolute strength prediction is 0f about +/_ 25%.

2. Since different factors influence the strength of concrete, correction factors need to be

used to allow the effect for the existing concrete.

The use of the Schmidt hammer is a viable non-destructive method of testing method to test

the compressive strength of concrete but cannot be used alone as a conclusive method of

testing. It should be used with other methods of testing to accurately determine the strength of

concrete.

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REFERENCESI. V. Mohan Malhotra. (2004) -Handbook on Nondestructive Testing of Concrete.

Department of Natural Resources Canada, Ottawa.

II. Christopher Stanley. – An Assessment of the Schmidt Type ‘M’ Rebound

Hammer for Non- Destructive Testing of In Situ Concrete Strength.

http://cipremier.com/100035052.

III. BS 6089:1981," Guide to Assessment of Concrete Strength in Existing Structures',

British Standard Institution.

IV. BS 1881: Testing Concrete, Part 201, 1986." Guide to the use of Non-destructive

methods tests of test Hardened Concrete'.

V. Amasaki S (1991) Estimation of strength of concrete structures by the rebound

hammer, CAJ Proc Cem Conc 45: 345-351. ASTM C 805-85. (1993) Test for

Rebound Number of Hardened Concrete. ASTM, U.S.A.

VI. Guidebook on Non-Destructive Testing of Concrete Structures.-International

Atomic Energy Agency, Vienna, (2002).

VII. Schmidt, E. (1951). A non-destructive concrete tester. Concrete, 59(8): 34–5.

VIII. Suresh Chandra Pattanaik.- Ultrasonic Pulse Velocity and Rebound Hammer as

NDT Tools For Structural Health Monitoring. [Paper published in the conference

proceedings of International Conference NUiCONE 2010 at Institute of

Technology, Nirma University, Ahmedabad from December 09-11, 2010].

IX. Ramboll ( 2006) -Non Destructive Testing and Inspection Manual.- Central

Railway.

X. Schmidt Hammer Operating Manual.

61 | P a g e

APPENDIXLIST OF FIGURES

Fig 1-1: Parts of rebound hammer. Page8

Fig 2-1: Process of penetrant testing. Page 12

Fig 2-2: Radiography. Page 13

Fig 2-3: Magnetic particle testing machine. Page 14

Fig 2-4: Magnetic particle test illustration. Page 14

Fig 2-5: Eddy current testing. Page 15

Fig 2-6: Ultrasonic test method. Page 16

Fig 2-7: M-type Schmidt hammer in use. Page 18

Fig 4-1: Slump test. Page 30

Fig 4-2: Measuring slump. Page 31

Fig 4-3: Compaction factor apparatus. Page 33

Fig 4-4: Cast concrete cubes. Page 35

Fig 4-5: Illustration of compressive strength test. Page 36

Fig 4-6: Cube crushed under compressive test machine. Page 36

Fig 4-7: Cube specimen in testing machine under test using Schmidt hammer. Page 40

Fig 4-8: Calibration curve of 7-day concrete. Page 43

Fig 4-9: Calibration curve of 28-day concrete. Page 44

Fig 4-10: Quadratic calibration curve of 28-day concrete. Page 45

Fig 4-11: Cubic calibration curve of 28-day concrete. Page 46

Fig 5-1: Compressive strength vs. rebound number of 28-day concrete. Page 55

62 | P a g e

LIST OF TABLES

Table 3-1: Concrete mix ratios. Page 26

Table 4-1: Ratio for the mix design. Page 29

Table 4-2: Rebound number on 7-day concrete. Page 41

Table 4-3: Cube compressive strength of 7-day concrete cubes. Page 41

Table 4-4: Rebound number on 28-day concrete. Page 42

Table 4-5: Cube compressive strength of 28-day concrete cubes. Page 42

Table 4-6: Rebound number of columns. Page 48

Table 5-1: Relative density of hardened concrete for the mix designs. Page 53

Table 5-1: Average rebound number of columns and their compressive strength. Page 56

XI.