new report anil

8/10/2019 New Report Anil

1/11

1

1. Introduction

The reinforced cement concrete is the most prominent construction material used from decades.

Because of its high strength, economy and durability, it is the first choice to be used as building

material. Reinforced concrete structures have the potential to be durable and capable of

withstanding a variety of adverse environmental conditions but failure of structure often

observed as a result of degradation of concrete, premature corrosion of reinforcement bar or

increment of load.

Thus monitoring of concrete structure like buildings, bridges, architectural structures etc are very

essential to ensure safety, stability and serviceability. The structure should not only being safe,

should also be function in its intended use. Heavy cracking, deflection, corrosion are main

constituent of the degraded properties of a RC structure. Monitoring of such RC structures which

are under functioning since decades is must and by this maintenance and retrofitting can also beensured by structural engineer.

Assessment of structural properties to identify its current situation and future durability problem

is the basic need of maintenance plan. Current status of structure helps in prediction of its

residual life. This assessment also helps in determining the life cost of structure. The life cost

of structure includes construction cost, maintenance cost, operational cost etc. By condition

assessment one can decide the necessity of repair work. The final goal of condition assessment

of a building is to find the urgency of repair, nature of repair and cost associated with repairing.

Fig 1. Importance of condition assessment

ConditionAssessment

Repairurgency

Methodand cost ofrepairing

Change offunction/ownership

Service lifepridiction


2/11

2

Due to deterioration in RC structure, a maintenance program is necessary to run but for this first

and foremost work is to decide repair urgency i.e. Is it necessary to repair a particular element in

building right now? If yes, then which method is to be employed and what will be the overall

cost of repair work? And If no, then when will require repairing?

The above question can only be answer on the basis of condition assessment. Condition

assessment of RC structure includes Visual inspection, Non destructive testing data and other

testing data. Collecting the data helps in decides repair urgency but it is irony that this decision

varies person to person. Many times without using any rational analysis on the basis of visual

assessment and experience, a structure engineer decides repair urgency. View and experience

which cannot be quantified when used for taking such important decision, this may adversely

affect structure health.

The primary focus of technical requirement is on the functioning of concrete structure. The load-

bearing capacity of structures can be violated by the degradation of concrete and corrosion in

reinforcement bar. Structure must be design; constructed and maintained in such a manner that

safety level can be secured during the intended service life despite degradation and ageing of

materials. Defects in material may also leads to poor serviceability or inconvenience in the use of

a structure.

Performance is the behaviour related to use. It can be assumed as quantifiable property. In

principle the performance can be related to bearing capacity, stability, safety in use, visual

appearance etc. Performance is always a function of time. When the time is considered in the

evaluation of performance, various external factors, called degradation factors take on greatsignificance. In this way performance is linked with durability. Durability is the property

pressing the ability to maintain the required performance. Degradation is the gradual decrease in

performance over time.

Fig 2. Life of structure


3/11

3

2. Literature Review

2.1: Existing practice and new idea:

Guidelines for repair urgency include visual inspection, experimental observations and analysis

of historical data and combined knowledge of several experts. But subsequent decisions on

repair urgency are usually made on basis of engineering judgement, personal knowledge and

experience in field. The nature of decision making is often qualitative and this nature leads to

inconsistency and constraints on information sharing. Subjective interpretation of qualitative data

can differ. Due to lack of information or say inconsistent data relating repair urgency makes

subjective decision making an integral part of it. Repair urgency itself is a qualitative measure

one can decide it as low, moderate or highly urgent.

Fig 3. Contributing factor for decision making on repair urgency

Explicit relation between the variety of condition assessment data and the corresponding repair

urgency cannot be achieved and also implicit functions are also difficult to develop. Hence a

rational formulation for comparison of urgency of repair can be a useful and a consistent tool.

A number of factor can be introduce viz. visual inspection, non destructive testing data, other

deteriorating parameters, durability parameters etc. All arrange in a condition indexing say 0-5 or

0-9 and every parameter will be measured as Condition Index. The overall condition index gives

repair urgency of RC element.

Repairurgency

Functionalrequirement

Condition

aseessmentdata

Experience,intelligence

andexperience


4/11

4

2.2: Detailed Literature Review:

1. Nikhil P. Tirpude; Kamal Kant Jain; and Bishwajit Bhattacharjee:

Decision Model for Repair Prioritization of Reinforced Concrete Structures

(Journal of performance of constructed facilities

ASCE/ March 2014)

In this paper, authors presented


5/11

5

3. Objective of study

To study life cycle cost of structure

To design and estimate construction cost of steel and RCC tank of same capacity

To study various degradation model for RCC and steel

To investigate maintenance program required for tanks during their life cycle.

To calculate average maintenance cost per year.

To calculate and compare final life cycle cost of both tanks for different service life


6/11

6

4. Problem Formulation

4.1 Model for deterioration of RCC:

# Deterioration mechanism and parameters:

Reduction in performance level of a material may simply called as its deterioration. For RC

structural component, deterioration depends on number of factors, mainly of them are tabulated

below:

S.No. Degradation parameters Environmental exposure

1. Corrosion Marine, Industrial

2. Cracking All

3. Shrinkage All

4. Alkali silica reaction Soil moisture

5 Freeze thaw Temperature cycle6. Sulphate attack Soil, heat and water

7. Decalcification Natural water, acid, sea water

8. Structural fatigue Loading

Besides the above, there are many more parameters affecting the performance of concrete. In this

report the main focus is on the corrosion mechanism in concrete which ultimately degrades the

structural performance.

Corrosion mechanism-

The co-operation of concrete and steel is based on the fact that concrete gives the reinforcement

both chemical and physical protection against corrosion. The chemical effect of concrete is

attributed to its alkalinity, which causes an oxide layer to form on the surface of steel;

phenomenon is called as Passivation, as the oxide layer prevents propagation of corrosion. The

concrete also provides physical barrier against agents that promote corrosion, such as water,

oxygen and chlorides. If there is some change in concrete surrounding steel. The changes may be

physical, such as cracking and disintegration, exposing part of the steel surface to open air and

leaving it without the physical and chemical protection of concrete.

Chemical changes also take place in the concrete surrounding the reinforcement; the most

important are following:

1. Carbonation of concrete due to carbon dioxide in air.

2. Penetration of aggressive anions (especially chlorides) into concrete.

Carbonation is the reaction of carbon dioxide with hydrated cement minerals in concrete. This

phenomenon occurs in all concrete surfaces exposed to air, resulting in lowered pH in the

carbonated zone. In carbonated concrete the protective passive film on steel surfaces is destroyed

and corrosion is free to proceed. The effect of chlorides is not based on the decrease in pH but on

their ability otherwise to break the passive film.

On loading the concrete get some micro-cracking so as to distribute load to reinforcement bar.

Surrounding environment may contain chloride and carbonation which on due course of timereaches to reinforcement and corrode it. After that the corrosion increases the volume of bar


7/11

7

hence form more cracking and spalling of concrete. Also causes reduction in bar diameter, thus

causing insufficient section to carry load. This lead to heavy deflection and cracking which

decreases the serviceability of building.

The service life ends when the steel is depassivated. This rule is usually applied to all chloride

induced corrosion as the local attack penetration rate is still not safely quantified anduncertainties governing the propagation period are high. Thus the service life is limited to the

initiation period only.

Corrosion mechanism can be represented in two phases:

1.

Initiation period to

2. Propagation of corrosion t1

In the case where no corrosion is allowed the following formula for service life can be used

tl= to

where

tl = the service life to= the initiation time

The limit state is based on cracking of concrete cover due to oxides generated during corrosionduring which the cross sectional area of steel is progressively decreased, the bond between steel

and concrete is reduced and the effective cross sectional area of concrete is diminished due to

spalling of cover. This approach is applied in case of corrosion due to carbonation.

The service life based on cracking of concrete cover is defined as the sum of the initiation period

and time for cracking of the concrete cover to a given limit.

tL= t0+ t1

The propagation time t1 ends when a certain maximum allowable loss of cross sectional area or

loss of bond or crackwidth is reached. If crack originated from the beginning of service life, the

initiation time much shorter (to0).


8/11

8

Service life of RC structurebased on corrosion model

Initiation time ofcorrosion

Chloride inducedcorrosion

Carbonationinduced corrosion

Propagation time

1. Initiation time of corrosion

Carbonation induced corrosion- Corrosion initiation period can be defined as time required for

carbonation front to reach the rebar level depth i.e. carbonation depth becomes equal to the

concrete cover, or when chloride concentration of the concrete surrounding the rebar reaches to

its threshold value (0.2% by the weight of concrete). Usually carbonation models relates

carbonation depth with the age of structures, mostly carbonation models are based on Ficks first

law of diffusion

Where-

Cdis carbonation depth, t is age of the structure and K is the coefficient of carbonation.

Value of K based on concrete quality, aggregate types, exposure conditions, and moisture

content. By selecting best suitable value of K from the several proposed values or by

evaluating value of K by performing curve fitting of experimental data, corrosion initiation

time can be evaluated by following equation

(

)

Chloride induced corrosion:

Chloride concentration above threshold value initiates corrosion process, chloride induced

corrosion process are mainly modelled using Ficks second law of diffusion presented in

following equation

[

]

Where C is the chloride content at depth x and timet, D is the diffusion coefficient.

Corrosion initiation time which is the time required by chloride content at rebar depth to reachthreshold value, can be evaluated using well known solution of above law


9/11

9

[ ( )]

Where c= concrete cover, cs= surface chloride content, cth= threshold chloride content.

erfis the well known error function

Carbonation depth and chloride content can be modelled with respect to time by using Ficks law

of diffusion by applying other diffusion parameter.

2. Propagation time:

Corrosion begins when the passive film is destroyed as a result of falling pH due to carbonation,

or as a result of chloride content rising above threshold close to reinforcement. The volume of

corrosion products in many times that of the original metal. The greater need for volume causes

tensile stress in concrete around the steel bar, leading to cracking or spalling of the concrete

cover.

When corrosion develops three main phenomena appear:

1. A decrease in the steel cross section.

2. A decrease in steel-concrete bond.

3.

Cracking of concrete cover (therefore decrease in concrete load bearingcross section)

In order to determine the propagation time, a critical threshold value of load bearing capacity has

to be defined. Higher the corrosion, lesser will be bar diameter hence decrease in load bearing

capacity.

So propagation time may be quantified as follow (Alonso and Andrade 1993)-

Where

RmaxMaximum allowable loss of radius of steel bar; r= rate of corrosion

Cracking time of concrete cover- In the case of generalized the critical loss of bar radius is based

on cracking of concrete cover. The propagation time can be approximated by following (Seimes,

Vrouwenvelder and van den Beukel 1985):

Where ; C= thickness of concrete cover(mm)


10/11

10

D= diameter of the rebar(mm)

r= rate of corrosion(m/mm)

Factor affecting the rate of corrosion-

1. Temperature

2.

Humidity3. Carbonation/ chloride contamination in concrete

4. Water cement ratio

5. Types of cement and admixture present in concrete

6. Wetting and drying cycle on concrete surface

Propagation time of corrosion at cracks:

If concrete cover cracked from the beginning and the crack width is larger than 0.1-0.3,

corrosion normally starts without any initiation period. A structure engineer may set a limit for

the minimum diameter of steel bar and maximum depth of corrosion.

The propagation time at crack of calculated from the following formulae:

Where; Smax= Maximum allowable depth of corrosion Dmin= Minimum diameter of bar

The rate of corrosion in cracked concrete may be taken almost equal to that of in un-cracked one.


11/11

11

5. Proposed Methodology

Proposed methodology includes:

1.

Design of steel and RCC rectangular water tank for capacity = 200000lit

2.

Estimation of construction coast of both tank

3.

Mentainace requirement of RCC and Steel tank during their service life = 10year, 20year,

50 year, 75year, 100years

4. Maintenance cost for each tank during their service life

5. Finding the final LIFE CYCLE COST of both tank and comparing both.

new report anil

Documents