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STRENGTH EVALUATION OF EXISTING REINFORCED

CONCRETE STRUCTURE

Naser Alenezi

Building and Energy Department, Kuwait Institute for Scientific Research

Shuwaik, Kuwait

naserq8@hotmail.com

ABSTRACT

Reinforced concrete is the most commonly used

construction materials not only in Kuwait but slso

worldwide. It has numerous advantages over other

costruction materials, especially in this country. This study

was conducted to evaluate the structural integrity of a villa

located in Kuwait. The buiding consists of basement floor,

ground floor, first floor and second floor. The building was

constructed using skeleton type system composed of

columns, beams and slabs. The two storey building showed

cracks in the columns, beams, and slabs. The recent study

of structural integrity and evaluation included, comparing

the as built with the design drawings, detailed visual

inspection, evaluating the quality of concrete by using field

and laboratory tests and structure analysis to determine the

safety factors. Problems encountered in reinforced concrete

buildings are not limited to those where the concrete was not

designed for durability. It includes also concrete which was

not constructed for good performance and for durability. So

the second cause of problems in reinforced concrete

buildings is related to inadequate construction. In fact,

many of the problems encountered today in reinforced

concrete structures are related to inadequate construction

practices.Keywords-structure, concrete, materials, slab,

analysis.

I. Introduction

Concrete is the main and the basic construction building

material used in Kuwait compared with the use of wood and

steel which are used in building structures. Therefore,

concrete is the primary of building and construction and in

industry in the state of Kuwait of some sort of specialty the

main construction buildings in Kuwait, the life range of all

buildings and structures all over the world is about 50 to 60

years yet this number is lessened in Kuwait to reach the

range of 10 – 20 years.

Concrete production is often well thought-out to be the

preliminary and chief phase in producing durable concrete

structure. It has many features such as superiority of

concrete components, material goods of the concrete mix

design, batching of materials and mixing circumstances,

skillful manual workers, good craftsmanship, and

dependable regulation (ACI 318).

Many buildings which satisfied the minimum requirement

of concrete strength during construction showed early signs

of deterioration. This raised concern about the quality of

some concrete to maintain its strength is adequate its service

life. Hence, it is not enough that concrete strength is

adequate at 28 days. Concrete need to maintain its

properties during its service life. That is why engineers

design concrete not only for strength but also for durability.

In fact, many of the problems encountered today in RC

structures are related to inadequate construction practice.

Concrete hides some of the problems, but not all related to

concrete shrinkage cracking, openings for HVAC ducts,

corrosion etc. are not always hidden. They appear at

different ages in the life of the structure, but they could have

been be easily avoided with adequate construction practices

and engineering supervision.

Concrete structures in Kuwait worsen too early as a

consequence of the aggressive service environment, hot and

dry climate that is distinguished by high rates of water

evaporation during the day time, the coastal area

condensation in during the night, and salt – contaminated

wind borne dust and ground water. The unpleasant effect of

the climate is escalated by the use of unsuitable concrete –

making resources and inexpert laborers.

The result of this study revealed that the improper

supervision quality control procedures undertaken during

concrete production are the major factors in reducing

concrete quality. Due to the absence of enforceable national

concrete construction guidelines, adverse effects of bad

practices on durability of concrete structures have become

difficult to overcome and particularly inevitable.

II. Methodology

A visual inspection was carried to evaluate the building

original drawings to what is done in the site. A plane of

experiments and tests on field and laboratories were carried

out to complete the aims of this limited study, this included

choosing of some concrete cores from columns, slabs,

beams. Also two tests were taken to evaluate the strength of

concrete which are ultrasonic pulse test and Rebound

hammer test.

Field Evaluation

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The condition survey of the structure was taken as important

steps in the process of the structural evaluation. The survey

is used to document important information and data about

the building and about the cause of concern. For field

evaluation detailed sketches of location and width of cracks,

settlement, and deformation are necessary.

Laboratory Evaluation

Laboratory estimation was exploited to judge the features of

the materials used for concrete production. Drilled cores

testing are the most direct method to determine the in-situ

compressive strength of concrete. It is generally used in a

new structure (or old buildings) when concrete did not pass

the acceptance criteria, or when there is a reason to doubt

the performance/ quality of the concrete.

Compressive strength of concrete core

The quality of concrete is often judged by its strength. In

this study, fifteen cores samples were taken. The selection

of the cores were random from the whole building which

are six cores from both columns and slabs, and four from

beam. The core test is according to the standard

specification ASTM C42M.

Rebounds Hammer test

Rebound hammer test is done to find out the compressive of

concrete by using rebound hammer as per ASTM C 805-02.

The underlying principle of the rebound hammer test is the

rebound of an elastic mass depends on the hardness of the

surface against which its mass strikes. When the plunger of

the rebound hammer is pressed against the surface of the

concrete, the pring- controlled mass rebound and the extent

of such a rebound depends upon the surface hardness of the

concrete. The rebound value is read from graduated scale

and is designated as the rebound number or rebound index.

The compressive strength can be read directly from the

graph provided on the body of the hammer.

III Tests Results

Field evaluation

Visual Inspection

The visual inspection was made to examine any construction

defects and after the inspection, there are many defects were

found which are:

Some beams and columns are misplaced.

Many construction defects were cover by mortar (Fig.1).

Some columns don’t a uniform concrete which means they

have been poured in stages (Fig. 2).

Some settlement is some slabs.

Some columns are bigger in width than the beam’s width

on top of it.

Many cracks on most of the structures (beams, slabs,

columns, walls).

Columns are not leveled.

Diagonal cracks on some walls.

There are moisture and spots on some slabs.

Fig. 1: Mortar covered the defects of the slab

Fig. 2: Settlement on the slab of the floor

Laboratory Evaluation

Compressive strength of concrete core

In this study, 15 cores samples were taken, cures at

controlled temperature and moister, and tested for the

official strength at 28 days. The core tests results to show

the strength of the concrete which was used. The study took

six cores for column, four cores for beams and six cores for

slabs from all floors of the building. It’s well understood

that concrete mixes with the same mix magnitude and

treatment time, should obtain the same compressive strength

estimation. However in this study although most of the

concrete in this building has the same mix proportions, there

was a variation between the compressive strength results

among different members (beams, slabs, and columns.)

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This due to the difference in the accuracy of proportioning,

type of concrete to be casted after batching.

In spite of all, all memebers of this building has indicated a

compressive strength of lower than their strength during the

design stage, and one column of the isolated strength

records were under 33.5 kg/cm2. Weak concretes were used

in the absence of supervision or refusal. Concrete proposed

to reach its designed strength when treated at 28 days . ACI

code specified that concrete should continuously treated for

at least 7 days despite this fact construction site in Kuwait,

the concrete is treated by wetting the drying mode for 7 days

in summer and 2 days in winter, though this action is not

severely followed and often not performed at all.

The dissimilarity of concrete’s design strength must be

according to the sort of that for the structural element,

although , this research explained and they reached the

result that this condition was not considered , as most of

design strengths were 250kg/cm2 , a part from the structural

element sort. Structural elements used in this study. Tabel

1 shows the compressive strength for a chosen members in

the building.

According to ACI specification, the compressive strength of

concrete of the cores would be accepted if two conditions

are meet. First, there should be no value from the

compressive strength of the cores less than 85% of the

design compressive strength which is 250 kg/cm2 for both

slabs and beams. And for the columns is 300 kg/cm2.

The second condition is there should be no value for any

single member less than 75% of the design compressive

strength. The cores test did not meet the requirement of

ACI specification as show in table 2.

Table 1.The Results of The Core Tests

Member Average of

Compressive

strength for

(Cube

kg/cm2)

Condition

I Not less

than

Minimum

value for

individual

kg/cm2

Condition

2 Not less

than

Columns 123.5 225 41.9 225

Slabs 156.6 212.5 87.4 187.5

Beams 132.7 212.5 109.4 187.5

Table 1. Compressive strenght test results for the extracted concrete cores

IV. Conclusions

The investigation done under this study have reached the

following results:

1. The weakness of the conrete that was used for this

building which reach to the shortage in 60% of the

columns, 49% fo the beams and 40% of the slab

comparing to its designe that was mention in the

Sample ID

Member Length mm Diameter mm weight (SSD)

gm

Density (SSD),

gm/cm3 Load, kN

Comp Strength,

kg/cm2 L/D

Corr. Comp. Strength,

kg/cm2

Equivalent Cube strength,

kg/cm2

C1 Column 144.8 73.8 1412.1 2.28 75.9 180.9 1.96 180.9 226.2

C2 Column 140.3 73.3 1307.0 2.21 13.9 33.6 1.91 33.5 41.9

C3 Beam 146.6 73.3 1400.6 2.26 49.5 119.6 2.00 119.6 149.5

C4 slab 110.3 73.6 956.2 2.04 60.4 144.8 1.50 139.0 173.7

C5 Column 148.2 73.5 1405.4 2.24 30.7 73.8 2.02 73.8 92.2

C6 Beam 147.0 73.3 1391.9 2.24 52.1 125.9 2.01 125.9 157.4

C7 slab 88.4 73.6 873.6 2.32 75.6 181.2 1.20 166.9 208.7

C8 Column 149.8 73.7 1471.2 2.30 49.3 117.9 2.03 117.9 147.3

C9 Beam 146.0 73.3 1433.8 2.33 37.9 91.6 1.99 91.6 114.5

C10 slab 128.8 73.5 1281.2 2.34 64.3 154.5 1.75 151.5 189.3

C10 slab 101.2 73.7 1133.9 2.63 58.7 140.3 1.37 132.9 166.1

C11 slab 115.7 73.7 1114.3 2.26 30.3 72.4 1.57 69.9 87.4

C12 Column 147.0 73.7 1469.2 2.34 49.4 118.1 1.99 118.1 147.6

C13 Beam 144.2 73.4 1428.6 2.34 36.3 87.5 1.96 87.5 109.4

C14 slab 108.7 73.6 1082.7 2.34 43.2 103.5 1.48 99.2 124.0

C15 Column 142.3 73.6 1364.2 2.25 28.7 68.8 1.93 68.8 86.0

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drawings. So the conrete was not acceptable to

what is suppose to be.

2. Most columns can not take the loads when the

building is finished which will lead to collapse of

the sturcture.

3. There are many huge construction defects which

will weaken the buiding’s members (beams, slabs,

and columns) like spalles .

4. The constructed concrete strength does not match

the codes for concrete strenght which will effects

the durability of concrete.

References

1) BSI 1881: Partt 119. 1983. Method for determination of

compressive strenght of concrete cubes.

2) S. Albahar, and Abdulsalam, S., 2001. Evaluation of

quality of concrete produced on site for residential

housing in kuwait. Kuwait institute for Scientific

Research.

3) A. Neville, Properties of Concrete. Third edition, 1981.

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