FolicDurabConcrStrFACTA Nis 09

7/29/2019 FolicDurabConcrStrFACTA Nis 09.

1/20

BASES FORPROVIDE DURABILITY ANALYSISOF CONCRETE STRUCTURES

Radomir Foli

Faculty of Technical Sciences, 21000 Novi Sad, Trg D. Obradovia 6,[email protected] Serbia

S U M M A R Y

Abstract. Reinforced concrete structures are designed in accordance with national or internationalcodes and standards such as Model Code Euro international Committee of Concrete (1993), Eurocode2, ACI 318, RILEM etc. Present design procedures are predominantly based on strength principles. Averification procedure is based on limit state formulation. The durability aspect is a natural extensionof the classical resistance verification where deterioration effects are normally neglected. Thereliability is assessed through the given performance which must be delivered within the designedservice life. This approach can be adopted for a performance based on service life design. Structuressuch as pavements and bridges do not achieve the desired service life, and need to be designed anddetailed for long-term durability based on service-life consideration. In the recent years design is

related to durability through the analisys of carbonation, resistance to chloride ingress, improvedfreezing and thawing resistance, and so on. The review of literature and some recommendations, fordesign of structures aimed to attain greater durability, is presented. Federation Internationale du Beton(FIB).

Radomir FOLI

OSNOVE OBEZBEENJA TRAJNOSTI ARMIRANOBETONSKIHKONSTRUKCIJA

Iskustvo sa armiranobetonskim konstrukcijama u skorijoj prolosti uslovio je preispitivanjetehnikih propisa za njihovo projektovanje i graenje. Poslednjih godina u preporukamameunarodnih udruenja i tehnikim propisima nekih zemalja, kao i u Evopskim normama uvodi senovi pristup projektovanju armiranobetonskih konstrukcija. Razlog za to je. Naime, njihova trajnost , - ( 1993.), 2, , . . . - . .

, .
mailto:[email protected]:[email protected]


2/20

.

1. INTRODUCTION AND TERMINOLOGY

Civil engineering structures are complex systems whose components differ in durability.Building contain elements that can last from 100 to 150 years such as foundations, walls and floorslabs, while on the other hand there are components that need frequent replacing. However, somestructures have not satisfactory durability. Zbog toga je znacajno basic understanding of the complexset of multidisciplinary phenomena governing durability and long term performansce of concretestructures as basis for service life design. In order to construct a durable and reliable structure it isnecessary to design it for durability and provided required service life. The structure is considered asdurable in the actual environment as long as its function is acceptable. Performance of structuremeans its combined short and long-term fulfilment of the functional requirements (safety,serviceability and appearance of structure during its time in use). Funcional requerements andcorresponding properties could be: minimum load carrying capacity (concrete and steel

strength, corrosion and spalls of concrete depth); maximum acceptable deformation(E/modulus, shrinkage, creep, thermal movement, and settlements); maximum penetrabilityfor gaseous or liquid substance (concrete permeability, capillarity and diffusivity, and size andarrangement of cracks).

For civil engineering structures reliability is the probability of a structure to fulfil the givenfunction in its service lifetime, i.e. to keep the characteristics in given limits (performance) as definedin accordance to the defined using regime - consists of safety, durability and serviceability with themaintenance abilities. Failure is described in terms of one or more limit states (connected to theimpossibility of further usage of the structure or element). Durability is the capability of maintainingthe serviceability of a structure over specified time.

Serviceability is viewed as the capacity of the structures to perform the functions for which

they are designed and constructed within normal use conditions. Service life is the period of time afterconstruction during which all properties exceed the minimum acceptable values when routinelymaintained. A service life design conditioned that designers choice of fundamental properties to fulfilall the functional requirements during target time. The two phases of deterioration are:

The initial phase (period) in which no noticeable weakening of properties, except protectivebarrier.

The propagation phase with active deterioration mechanisms develop at an increaserate with time.

In practice there are three different types of service life depending on the type of consideredperformance: Technicalservice life (Fig.1) is the time of service until acceptable state is reached

(failure). Functional service life is the expected time in service until the structure no longer fulfilsthe functional requirements. Economicservice life is the time in service until replacement of thestructure is economically more then keeping it in service. Real service life must not be shorter thannominal - normative life.


3/20

Fig. 1 Service life of concrete structures a two-phase modelling deterioration

2. REVIEW OF CODESReinforced concrete (RC) structures designed in accordance with national or international codes

and standards such as Model Code Euro international Committee of Concrete (1993), Eurocode 0 and2, ACI 318, RILEM at all. The minimum requirements to be fulfilled are stated in national codes andstandards. Historical and traditional reasons influenced that codes and standards differ considerablyfrom country to country. The deterministic design for durability will govern for some time still, butwith regular updating of the characteristics of the environmental and improvements of the modellingof transport and deterioration mechanisms. Parameters which influenced on durability are: the cementtype and quality, control of early age cracking, limitation of crack with, etc. Their values depend of theenvironmental aggressively. Probabilistic performance based service life design used becausevariation of concrete structures (CS) due different structures properties of the structural part, concretecompositions and different location conditions. Modelling of environment and deteriorationmechanisms is being developed on a probabilistic basis allowing reliability based service life design.Service life design methods are similar to the load and resistance factor design procedure used forstructural design.

Present design procedures dominated based on strength principles, design are increasingly beingrefined to address durability requirements (resistance to chloride ingress, improved freezing andthawing resistance and so on). Inherent with design calculation is a certain level of durability, such asrequirement for concrete cover to protect reinforcement under aggressive action from environment andindustry. Structures such as pavements and bridges have not exhibited the desirability of service life,and needs to be made to design and detail for long - term durability based on service-lifeconsideration.

The Eurocode system has been chosen as the basis for design. The durability of a structure is itsability to remain fit for use during the design working life given appropriate maintenance. Thestructure should designed in such a way that, or provided with protection so that, no significantdeterioration is likely to occur within the period between successive inspections. The need for criticalparts of the structure to be available for inspection, within complicated dismantling, should be a part ofthe design []. Possible evolutions of a structure during its working life using a suitable performanceindicator that is assumed to be a monotonously decreasing function of time. It can be expressed interms of various units: mechanical, financial, reliability, etc. In all cases, after a certain period of time,the performance indicator decreases, for example due to corrosion of steel, carbonation of concrete,repeated opening of cracks in concrete member, spalling, etc. The principal requirement to beconsidered in the overall strategy for achieving durability: in particular, decision with regard to the lifeperformance required from the structural members and whether individual members are to be

replaceable, maintainable or should have a long-term design life.


4/20

The Eurocode are based on the limit state approach in combination with a system ofcharacteristic values and partial factors. In most cases durability concerns the serviceability ofstructures. In this paper some new formulations by adding deterioration processes in serviceabilitylimit state are presented. In the cases where deterioration of concrete structure might go on unobservedthe durability problem can be directly associated with an ultimate limit state. The description of a limitstate may require one or more limit state functions.

Concrete structures (CS) are exposed to different actions of environment and are vulnerable todamage as corrosion and freezing and thawing. The deterioration of CS is affected by theenvironment, and adequate measures need to be examined when considering the strategy to achievedurability. The use of materials that provide increased durability should be considered in the overallstrategy for durability, for example epoxy-coated reinforcing steels or concrete with low permeability.The design should avoid structural systems which are inherently vulnerable and sensitive topredictable damage and deterioration. The shape of members together with their detailing willinfluence the durability of a structure. The increase durability, structural members should be protectedfrom detrimental environments. Maintenance should be considered during the design. Provisionshould be made for inspection, maintenance and possible replacement

Service life depends on structural design and detailing, mixture proportioning, concreteproduction and placement, construction methods and maintenance. Design of RCS to ensure adequatedurability is complicated process []. Water or other fluid is involved concrete degradation concretepermeability is important. It is well known that deterioration of concrete dependent of the presenceand transport water or other fluid, i.e. concrete permeability (concrete pore structure, presence ofcracks and microclimate at the concrete surface). In Model Code presents the relationship between theconcepts of concrete durability and performance. Transportation of heat, moisture and chemicals, bothwithin the concrete and exchange with the surrounding environment constitute main element ofdurability.

In EN 1990:2002 E [] a structure shall be designed to have adequate: structural resistance, anddurability. Durability including the choice of the design service life depends of environmental actions.To prevent potential causes of failure requires reliability levels to be maintained. Design working lifeshould be specified. Design working life (DWL) category 3 with indicative DWL for replaceablestructural parts is 10 to 25 years; building structures and other common structures indicative DWL is50 years: monumental buildings structures, bridges, and other engineering structures.

In most cases durability concerns the serviceability of the structure. However in cases wheredeterioration might go on unobserved the durability problem can be directly associated with anultimate limit state. The description of a limit state may require one or more limit state functions. Oneof the consequences of the required reliability in the service life design of a structure is the fact thatbetween the design service life and the mean service life a margin be present. This margin depends onthe required level of reliability, the type of service life distribution and its mean value and scatter [].

In [] are introduced three consequences of classes (CC3-high consequences for very great loss)

CC2 for medium and CC1 for low consequences. Reliability classes may be defined by the

reliability index. Three reliability classes RC1, RC2 and RC3 may be associated with consequencesclass (CC1, CC2, and CC3). Minimum value for 50 years reference period are (RC3=4.3; RC2=3.8and RC1=3.3).

In [12] are introduced five degrees of aggressiveness of the environmental exposure accordingto Model-Code CEB-FIP (1993). In ISO based on the principles given in [10] is presentedclassification of environmental conditions [6]. In [7] is discussed environmental management-lifecycle assessment, and in [8] service life planning. Document [9] is also important for performancestandard in building. These documents are broadly discussed in [1].

The durability of a structure means its resistance against the actions from theenvironment surrounding the structure. The element of design, material selection, execution

and curing which determine the quality of concrete is illustrated in Fig. 1.


5/20

Figure Relationship between durability and performance, after[CEB 1992]

The rate of deterioration may be estimated and consequently the prediction of design servicelife, in the context of durability including: the use of knowledge and experience acquired fromlaboratory and field investigations; estimates based on the performance of similar materials in a

similar environment, modelling degrading processes, and use of accelerated testing. The long termcapacity depends on the degradation of concrete and steel. The minimum acceptable values forperformance, or maximum acceptable values for degradation, are called durability limit states.Severalmathematical models have been developed to predict service life of concrete subjected to degradationprocesses as describe in Fig. 2.

Fig. 2-Aggressive substance influence on concrete and reinforcement

3. RELIABILITY AND METHOD OF DURABILITY DESIGN


6/20

The theory of durability design based on the theory of safety traditionally is used in structuraldesign. The strengthR and actions (load)Sare functions of a large number of stochastic variables, andgeneral is function of time [11]. The main criterion for reliability can be written as:

FAILURE =R < S (1)

The probability of failurePfdefined as:

Pf (t)= P{R(t) < S(t)} (2)The failure probability increases continuously with time as schematically presented in Fig

3. The function Pf (t) has the character of a distribution function. Considering continuousdistributions the failure probability Pf at a certain moment of time can be determined by usingthe convolution integral:

Pf(t) =

dssfsF SR )()( (3)

where FR(s)= the distribution function of R,

fs(s)= the probability density function of S,s = the common quantity or measure of R and S.

Fig. 3-Increase of failure probability

The steightforward solution of the integral (4) is only available in a few cases, i. e. when thedistributions of R and S are normal, but the integral can be solved by approximative numericalmethods. The distribution of service life can be obtained by calculating the failure probability values atdifferent moments of time.

In stochastic durability design, not only target service life but also the definition of maximumallowable probability of not reaching the target service life is necessary. It is called the probability offailure []. When failure is caused by degradation of materials the term "durability failure" is used.Theory of durability design is based on the theory of structural reliability. The basic formulae ofdurability design can be written according to two optional principles:

performance (actions S are set into relationship with the performance), and service life principle (the service life tL evaluated by a service life model must be greater than

the required target service life tg).

In deterministic durability design approach, actions (loads), resistance, and service life are used asdeterministic quantities, and distribution of this function is not considered. The design formula:

R(tg) - S(tg)>0 (4)

where tg is the target service life.

In reality S and R are time dependent functions, while the service life principle design formula is:tL-tg>0 (5)

where tL is the service life function


7/20

In stochastic design method the distributions of actions, response and service life are taken intoaccount. The condition that the probability that the service life of a structure would be shorter than thetarget life is smaller than a certain allowable failure probability is written as:

P{failure}tg =P{tL


8/20

place in the concrete surrounding the reinforcement caused by: carbonation of concrete due o carbondioxide in air, penetration of aggressive anions (especially chlorides) into concrete. Steel will corrodefreely when exposed to moisture and oxygen under ambient conditions. The passive ferric oxide filmon embedded steel may be disrupted by a reduction in the alkalinity of the concrete by carbonation orby the presence of aggressive ions such as chlorides and sulphates. If de-passivating condition exists inconcrete, either by a reduction in alkalinity (pH


9/20

The term Cc depends on the type of cement as well as on the presence of additions. Thediffusion coefficient Dc is controlled primarily by the pore structure and by the moisture content of theconcrete.

As a result, the alkalinity of the concrete is lowered and the passivation can no longer bepreserved, so that corrosion of the reinforcement is then possible. Carbonation is diffusion process

[14], and depth of carbonation dc can be expressed as a function of time:

tad c =2 (15)

Constant a is dependent on permeability of concrete (water-cement ratio, type of cement andits content, particle size of aggregate, curing of concrete and humidity) and the content of carbondioxide in the air. The constant a is expressed by:

KRw

a

=7.2

6.1746(16)

where w is water cement ratio (< 0.6),

R is influence of the cement, andKis the influence of the climatic conditions.

R varies between 1.0 2.2 depending on cement class, and K from 0.3 1.0 [14].

Depth of carbonation is:

tKRw

d

=7.2

6.1746(17)

The formula (16) gives an average value. The maximum depth of carbonation is 5 to 10 mm(Fig. 6). The forms of carbonation profile encountered in practice presented in Fig. 7.

Fig. 6a-Irregular carbonation front in concrete, after[]

Fig. 6b- The forms of carbonation profile encountered in practice [Error: Reference source notfound]

Due to the non-uniform front of the carbonization, the time of its development will be:

2

22

2

6.1746

7.2

=wKR

dtd

(18)

The effect of wetting and drying cycles on carbanation is illustrated in Fig.


10/20

Figure 6c- Influence of wetting and drying cycles on the rate of carbonation

5. CHLORIDE CORROSION OF REINFORCEMENT

The chloride induced corrosion of the reinforcement caused by high concentrations of chlorideions into concrete. In unsaturated concrete, the adsorption and diffusion of gases and liquid absorptionare important, whereas in saturated concrete the predominant transport process is likely to bemolecular or ionic diffusion. Only in case of higher pressures the fluid flow may occur []. Thedeterioration of concrete element due to reinforcement corrosion can be divided into two phases (Fig.7):

The initiation phase starts with the construction of the structure and end with the de-passivation of the reinforcing steel. During this phase chlorides penetrate from the surface of theconcrete to the reinforcement.

During the propagation phase reinforcement actively corrodes, and duration of these phasesdepends of corrosion rate and finishes with the structural failure.

The commonest sources of chlorides are see water and de-icing salts, and admixed chloride isnot considered. A durable concrete structures should have both a long initiation phase and slowcorrosion rate. The initiation period is usually much longer than propagation period. Some simplifiedmodels such as Fick`s second law of diffusion has been used for life prediction in combination withso-called critical chloride levels, the actual processes are much more complex than such simplisticmodels []. According to Tuutti`s model the corrosion rate in the propagation period controlled by therate of oxygen diffusion to the chloride and temperature. A conservative estimate of the service life isusually made by only considering the initiation period. If the concrete is continuously saturated withwater, the model predicts that corrosion processes active in the propagation period become the rate -controlling processes because of the extremely low diffusion rate of oxygen through the water.Chloride ions can exist in concrete in two forms: dissolved in water and as chemically bondedchlorides that have reacted with the cement paste. The ratio between the free chloride ion content inthe pore solution and the bond chloride content in the porous body varies between 0.4 and 1.0.


11/20

Fig 7 - Schematic presentation of conceptual model (Tuutti) of corrosion of steel reinforcement inconcrete, after[]

Low permeability increases the electrical resistivity of concrete which impends the flow ofelectrochemical corrosion currents. The intrusion of chloride ions in reinforcement concrete can causesteel corrosion if oxygen and moisture are also available to sustain the reaction. Chloride ions may be

introduced into concrete in a variety of ways (penetration by de-icing salt, industrial brines, marinespray, fog, or mist). Chloride transport in water and diffusion of chloride ions are formulated withFick`s second low described in [].

The one-dimensional diffusion process follows Fick`s second law of diffusion (Fig. 5) and theerror function solution by Crank used, and must be solved:

2

2

x

CD

t

C

=

(19)

with boundary conditions of:

Cx=0 at t=0 and 0 < x < (20)

Cx=Cs at x=0 and 0 < t < (21)Where: Cx = chloride concentration at depthx at time t,

Cs = surface chloride concentration,D = diffusion coefficient;x = distance from the concrete surface to steel reinforcement,t= time of exposure, anderf= error function.

Crank's solution of Fick`s second law of diffusion can be stated as follows [14], using apparentdiffusion coefficient:

5.0

)(21

tD

xerf

C

C

caS

x = (22)

Where:Dca=apparent diffusion coefficient

Solving this second order parabolic parallel differential equation for specific boundaryconditions allows relationships to be found between time tthe chloride diffusion coefficientD and theconcentration of chloride Cat a depthx from Eq. (22) can determined initiation time.

During the 1980s Schiessl examined the use of finite difference models to take account of thecriticisms of using a fixed value of diffusion coefficient alone without allowing for other effects. Thefollowing approximation of Fick`s second law was achieved, in the case of the one-dimensionalmodel, assuming central difference from for spatial derivatives:

211 2

xcccD

tC iii

+=

+ (23)

Temporal derivatives and spatial derivatives were determined and a linear set of equations was set up.These were solved for each time step and a concentration profile was determined.

The formula may be simplified by using a parabola function [14]:

=5.0)3(2

1tD

xCC

Sx (24)

The formula for the initiation time of corrosion can then be written in the following form:


12/20

2

5.00

)(112

1

=

S

I

C

C

x

Dt (25)

A simplified form of the equation for design is:

caAD

xt

2

= (26)

where t= time to de-passivating,x=depth of cover,A= constant dependent on the surface chloride level (CS) and the critical chloride corrosionthreshold (Ccr),Cca =diffusion coefficient.

The constant A is determined through Fick s second law. According [13] the use of Eq. (27)with the following values of the constant, assuming a critical corrosion threshold of 1 per cent byweight of cement A=1.8, for CS = 3% (by weight of cement), and A=2.65, for CS = 4%. In many

standards require threshold values not higher than 0.4% (CI -) by weight of cement for reinforcedconcrete (RC), and 0.2% for prestressed concrete.

6. SERVICE LIFE PREDICTION

If the degradation process at a proportional rate by the same mechanism in both acceleratedaging and long-term in service tests, an acceleration factorKcan be obtained from:

K=RAT/RLT (27)

Where: RAT is the rate of degradation in accelerated tests, and RLT is the rate of degradation inlong-term in service testing. If the relationship between the rates is non-linear, then mathematicalmodelling of the degradation mechanism is recommended to establish the relationship [].

One approach to predicting the structures reliability or its service life under future operatingconditions is through probability-based techniques involving time-dependent reliability analyses.

For service - life prediction and reliability assessment, the probability of non-failure over someperiod of time, (0,t), is more important than the reliability of the structure at the particular timeprovided by (7). The probability that a structure survives during interval of time (0,t) is defined byreliability function L(0,t). If n discrete actions S1, S2,.... Sn occur at time t1, t2, ...tn during (0,t), thereliability function becomes

L(0,t = P {R(t1) > S1,...,R(tn) > Sn (28)

The conditional probability of failure within time interval (t,t+t), given that the componenthas survived during (0,t), is defined by the hazard function

h(t) = -d(ln L(o,t)/dtwhich is especially useful for analyzing structural failures due to aging or deterioration. For

example, the probability that time to structural failure, Tf, occurs before a future maintenanceoperation at t+t, given that the structure has survived to t, can be evaluated as

G (t)=P{Tf t+t Tf>t}= 1 exp[+ tt

t

dxxh )( ] (29)

The hazard function for pure change failures is constant. However, when structural agingoccurs and strength deteriorates, h (t) characteristically increases with time.

Four measures of reliability have been considered: conventional factor of safety (FS), thecentral factor of safety (CFS), the safety margin (S), and the reliability index . The reliability index


13/20

concept is very popular indicator for probabilistically based design in structural engineering.Assessment of reliability is made entirely by comparing the calculated reliability index with thosefound to be adequate on the basis of previous experience with the structure under consideration. Theprocess begins with a mathematical model that relates the capacity (strength) and demand (actions) fora limit state of interest. The lifetime safety factor depends on the maximum allowable failureprobability the smaller the maximum allowable failure probability the greater is the lifetime safetyfactor. The lifetime safety also depends on the form of service life distribution.

Fig.8 - The meaning of lifetime safety factor in a performance problem []

Fig. 8. illustrates the meaning of lifetime safety factor when the design is carried out accordingto the performance principle. The function R(t) - S is called the margin of safety. The crossing point ofthe R(t) curve with the minimum load effect S given the mean service life which equals the designservice life.

Performance behaviour can always be translated into degradation behaviour, because

degradation is a decrease in performance. The transformation is performed by the followingsubstitution:

R0 - R(t) = D(t) (8)

R0 - S = R0 - Rmin = Dmax (9)

Fig. 9. shows the principle of design in a degradation problem. D(t) is degradation effects ofenvironmental loading on the performance of the structure. The curve D(t) crosses the maximumdegradation at the design service life, which must be longer than the service life (Fig. 7). The rangeDmax - D(T) is the safety margin. The diagram of the member forces extreme values represents theforces envelope for the them the envelope of the possible influences for all the registrations.

Fig.9- The meaning of lifetime safety factor in degradation problem []


14/20

7. REFERENCES

1. Durability of Concrete Structures-Investigation, repair, protection, Ed. by G. Mays, E&EN Spon,London, 1992.

2. CEB-FIP: Model Code 1990, T. Thelford, London, 1993.3. CEB-FIP: Durable of Concrete Structures, Design Guide, T. Thelford, London, 1992.

4. DuraCrete: General Guidelines for Durability Design and Redisgn, The Europien Union BriteEuRam III, Project No. BE95-1347, Performance based Durability Design of Concrete Structures,Report No. T7-01, 1999.

5. FIB (CEB-FIP), Bulletin 3 Structural Concrete Textbook on behaviour, Design andPerformance (Updated knowledge of the CEB/FIP Model Code 1990), Vol. 3, December 1999.

6. FIB (CEB-FIP), Bulletin 34 Model Code for Service Life Design, fib, Lausanne, Switzerland,2006, str. 116

7. FIB (CEB-FIP), Bulletin 47 Environmental design of concrete structures general principles,August 2008.

8. HERON Vol. 52, No 4, Special Issue on Durabity of Concret Strucrures, Delft, 2007.9. HERON Vol. 54, No 1, Special Issue: Adapting to climate change, Delft, 2009.10. EN 1990-Eurocode- Basis of structural design, CEN, Brussels, 2002.

11. Foli, R.:Durability and service life of concrete structures-Design modelling, PAM, Bull. forAppl. and Comp. Math.(BAM), Budapest, Nr. 2195, 2004, pp. 33-44.

12. Foli, R. (2004): Chloride corrosion of Reinforcement in Concrete Structure - Mathematicalmodelling, Budapest, BAM-CVII/04, No 2233, pp 93-104.

13. Foli, R.: Some aspects of deterioration and maintenance of concrete bridges, The 6th

International Conference of Danube Bridges, Budapest, 12-14 September 2007, Ed. M. Ivanyi &R. Bancila, pp. 377- 388.

14. Gulvanessian, H., Calgaro, J-A., Holickz,M.: Designers Guide to EN 1990, T. Telford, London,2002.

15. Kevern, J.T. et al. Previous concrete in servere exposures-Development of pollution-reducingpavement for nothern cities, Concrete International, July 2008, pp. 43-49.

16. Kraker, A. at. all.: Safety, Reliability and Service Life of Structures. Heron, Vol.27. No 1. Delft1982.

17. Mitchell, D., Frohnsdorff, G.: Sevice-Life Modeling and Design of Concrete Structures forDurability, Concrete International, December 2004, pp. 57-63.

18. Pravilnik o tehnikim normativima za beton i armirani beton u objektima izloenih agresivnomdejstvu sredine (Sl. list SFRJ 18/92) i Dejstva materija agresivnih prema betonu i zatita od njih

19. Richardson, M. G.:Fundamentals of Durable Reinforced Concrete, Spon Press, London, 2002.20. RILEM Report 14: Durability Design of Concrete Structures (Ed. A.Sarja and E. Vesikari), Spon,

London, 1996.21. Service-Life Prediction-State of the Art report, ACI Committee 365. 1R-42, 2000. pp. 44.22. Siems, A., Wrouwenvelder, A., Beukel,A. : Durability of Buildings-A Reliability Analysis,

Herron, Vol. 30, No 3, Delft, 1985., pp 3-48.

23. Siems, T., Polder, R., de Vries, H.: Design of Concrete Structures for durability, HERON, Delft,Vol. 43, No 4, 1998. pp 227-244.24. The Application and Measurement of Protective Coatings for Concrete, the Concrete Repair

A ssociation, 2001.25. Vrouwenvelder, A., Schiessel, P.: Durability aspects of probabilistic ULS design, Heron, Vol. 44,

No 1, Delft, 1999, pp. 10-20.

26. IEC 60721: Classification of environmental conditions27. ISO 14040:1997, Environmental management- Life cycle assessment Principles and framework28. ISO 15686, Buildings and construction assets Service life planning29. ISO/DIS-2349, "General Principles on Reliability for Structures".


15/20

Concrete structure shall be designed so that it can satisfy requirements regarding safety,serviceability, durability and environmentality of the struture throughout its design servicelife.

Beside design for durability (selection of materials and geometrical value) f or prolongedservice life, i.e. increase durability it is very important limitation: of CO2 emission, water

pollution, soil contamination, dust, chemical substances, and so on. Environmentalperformances....

ENVIRONMENTAL DESIGN

The environmental design system of buildings and civil engineering structures, in which theselection of materials and structural shape, construction work, maintenance, and demolitionand recycling, should be established in aim to minimized the used and resources and energy,to decrease hazardous substances, and control construction and demolition waste [].

Design of durable structures contributed to the realization of a sustainable society.Environmental design contributes for sustainability and to reduce detrimental impacts on

nature, society and humans. Some of FIP reports [] and [] provide general principles for allphase of design, construction, use, management, demolishes and reuse after demolition ofconcrete structures. In ASO 14004 environment define as surrounding of structures includingair, water, land, natural resources, human and their interaction. Environment risk managementobtain activities which integrate recognition of risk, assessment of environmental risk,development of strategies to manage it, and mitigation of environmental risk. Environmental

performance related to the organisations control of its environmental aspects based onenvironmental policy, objectives and targets.

In conceptual design must make good decision in the early phase of a project. The structuraldesign focuses on the structures ability to resist the environmental impact imposed on the

structure. Durability design comprises the design concerning the structures ability to resist ofminimizing the environmental impact imposed on the structure. Environmental designcomprises the design of minimizing the environment impact that the structure imposes on theenvironment during its entire life span, provided that structural and durability requirementsare fulfilled [fibEnv].

The interaction between the concrete and the environment determines the possibledeterioration mechanisms.

The structure interacted with the environment. Classification of environmental exposure ...EN 1990. Complex set of multidisciplinary phenomena governing durability and long term

performance of concrete structures as a basis for service life design. The focus is on the

structure and its interaction with the environment [CEB 1992].Action (load) must be resisted by the structure through selecting a combination a combinationof structural systems, element geometry, and material properties [fib 3]. With durabilitydesign we can verified that the intended service life can be achieved with an acceptable levelof reliability. The codes provide only qualitative definitions of exposure and they fail todefine the design life in relation to durability.


16/20

Fig. Importance of the protective concrete layer to protect the structure against deterioration

Fig. Time to corrosion initiation due to carbonation or chloride penetration depending on thereal concrete cover as achieved in the finished structure. The example corresponds to high

quality uncracked concrete

Aim of a design is to achieve an acceptable probability that the structure being designed willperform satisfactory during its intended life. In first step designer must define actions/loads


17/20

and asses safety factor as multiplier. These design actions/loads must be resist by the structurethrough selecting a combination of structural systems, element geometry and types ofmaterials. With durability design we must provide some structural measure and calculation toverify that the intended life can be achieved with ban acceptable level of reliability.

Designers` guide EN 1990-he degree of reliability should be adopted so as to take intoaccount: the cause and mode of failure (suddenly collapse, low ductility- britle element)should be designed for a higher degree of reliability; the possible consequences of failure interm of risk to life and economic consequences; the expense, level of effort and social andenvironmental conditions; the expense level of effort and procedure necessary to reduce therisk of failure.

In design working life this procedure are used:

the selecon of design actions and the consideration of material property deterioration,

comparison f different design solutions and choice of materials (balance between theinitial cost and cost over an agreed period, i.e. life cycle cost,

management procedures for systematic uctures.

Climate change is great concern about the origin of the change in global temperature..Indesign climate effects are taken into account by applying design codes, or on bases of pastobservation of behaviour.

Channges in climate will have an effect on the design loads. Structures are design to have aminimum resistance to the actions (loads) on the structures or their parts. Climatic actions andtheir intensities on structures such as wind, temperature, rain and snow vary in time.

The European standard for structural saafety EN 1990 use 50 years for buidings and 100yearsfor monumental building structures, bridges and other civil engineering structures.

The reliabilty of structure depends on both actions (loading) and properties (performance).For structural safety design is based on reliability analysisusing probabalistic model for boththe loads and resistance of the structure, and treated stochastically. The probability densitiesof the resistanceR of a structure, and of the load effect Sarepredicted with adequate models.

acceptablef PRSPP = )(

Durind time value SandR changed and an be described using probabilistic distributionfunctions. Using statistical methods and models it is possible to deternine the proability of

failureP

f. The probability (Those value) is relative to a defined sevice life (lifetime) ofstructures. Structure is safe ifPf smaller then a prefdefined value that depends on theimportance of the building, the type of loss of performance taken nto account and the

potential cosequences Cof failure. The risk is defined as a porbability P that failure will occurmultiplied by te consequences caused by the failure.

Risk=P C

Acceptable risk is social problem. The larger the consequences the smaller the acceptedprbability of failure will be. The desig load has a very small probability of exidence of about10-4 antil 10-5.To establish design loads statistical distribution of extrime loads having longretern periods are used.


18/20

Chaange of climate paraameers, such as higher temperature, amount and intensity ofprecipitation, different wind regime, will affect te durbility of materials (final&protective)HERON, 2009.

Models for discribing the deterioration mechanisms must integrate knowledge form a widerange of different disciplines, such as statics, statistics, materials technology, design,construction and economy.

Experience form past used similar existing structure used to identify critical parameters toformulated mathematical models in aim enshure a rational and coherent service life design.

Inificant deterioration machanisms are reinforcement corrosio and subsequently cracks andspallings of concrete. Main causes of corrosion (with present of water) are chemical attacks,alkali-aggregate reactions, and frezz-thaw bursting.

Mitchell

Prediction of durability (or service life) gives clearer view on extending service life, reducinglife-cycle and enhancing environmental sustainability. State of practice in design concretestructure (CS) prediction of service life routinely is considered with oriented degree ofreliability. To achieve highest level of reliability and desired durability designers must useddeveloped mathematical models. Traditionally designers estimate service life using judgment

based on experience with similar structures in similar environments. The lack of experienceand no standard guidelines caused bed solution for some important structures. Low durabilityof CS, especially in aggressive environments, conditions development of mathematicalmodels for prediction of the service life, or follow guidelines from codes. Recently all

professional association focused and active work on development guidelines and models fordesigning of durability of CS. deterioration (degradation)

In [Michell] service-life models classified in three main categories: empirical, mechanistic(physic-chemical), and semi-empirical. Empirical models based on previously observedrelationships among service life, concrete compositions, and exposure condition depend ofenvironmental conditions, without invoking and understanding of the scientific reasons forthee relationships. This category includes neural network models. The physic-chemicalmodel provides predictions of service life based on mathematical descriptions of the

phenomena involved in concrete degradation, understanding of the microstructure of concretebefore and during degradation process, for instance. Semi-empirical models tend to usesimpler mathematical expressions then mechanistic one. Predictions are made using fitting

parameters that are calibrated on the basis of data on the performance in the field or in

laboratory of concrete.Service-life prediction models may be probabilistic, when service-life expressed in the formof probabilistic distribution functions. The quality of service life predictions depend ofcapability of the models used and quality of the input data. Ensuring viability of appropriatemethods for characterization of concrete to provide data for the testing and use of modelsDeveloping the ability to specify performance requirements for concrete Providingguidelines for selection of limits for tests for relevant degradation mechanisms, and allowingfor scatter in test results. For performance tests use of some prescriptive tests may still benecessary a limit on chloride content, for instance. In [Michell] indicates nine steps, includingservice life predictions, that should be taken in the design and detailing of reinforced concretestructures to achieve a required service life:


19/20

Step 1: Defining the problem (the type of structure, the required service life and the exposureconditions) in the beginning of a project. He exposure conditions for individualelements of the structure depend of natural, and imposed due to its use, environment.The exposure conditions are likely to differ at different points on the surface (waterdue to excessive deflection, thermal effects, i.e.). Each type of degradation involves:

transport of water and ions thru the concrete; reaction involving a volume change,degradation of the internal structure of the concrete. Degradation may involve one ormore mechanical, chemical, thermal, and electrochemical processes.

Step 2: Proportioning the mixture, considering the surface, and providing for drainage. Whenproportioning the concrete additives and chemical admixtures should be considered,because can influence o service life (reduced permeability of concrete). Surfacetreatment and overlays can increase service life. Drainage slope and camber mustdesign.

Step 3: Selection the reinforcement. Different level of protection (using stainless steel, epoxi-coated steel reinforcement, cathodic protection, controlling the tensile stresses with

prestressing) can increase service life.Step 4: Designing the cover. The quality and thickness of concrete caver over the

reinforcement one of the most important factor effecting on service life. Coverdeterioration and corrosion of the reinforcement can lead to reduced bond integrityand spalling and delamination of cover.

Step 5: Controlling cracking. Early age cracking developed tensile stresses and causingchanges in the concrete material properties as is elastic modulus, tensile strength, andearly age creep. Construction joints and pour strips must be carefully located tominimize restraint effects.

Step6: Using models in estimating service life. When designing for durability the manyfactors is considered and usually iterative process performed. In each iteration, thedesign parameters adjusted until an estimate (based of a model-based prediction)service life made in step 6, is judged to be acceptable. Service-life modelling requiresan understanding of chemical reactions and transport processes within concrete, aswell as thermal, mechanical, and electrochemical effects. It is very important tochoose model for a specific application: the exposure conditions, validation model bynumerical methods, laboratory tests and field observations; and what assumption doesthe model depend upon? Influence of variation in cover thickness, placing andmaterial variation tolerances, mixture proportions, curing and exposure conditionsneed to investigate for important structures. In case when service life not meet or

exceed the required service life it will be necessary to return to Step 2, and repeatsubsequent step with some of the adjust design parameters.

Step 7: Selecting, in the design phase, appropriate procedures for quality assurance andquality control of construction (control of locations of the reinforcement, periodic overmeasurement before and concreting), duration and type of curing.

Step 8: Assuring quality construction. Provide effective measures of control.

Step 9: Developing a plan. For achieving durable structures in all phase of design,construction, used, inspection and maintenance must provide adequate quality.

Lifetime model depend on the quality of condition model for material. Condition

model, i.e. the condition of material is described with following function:Condition=start condition damage


20/20

Condition function, as % of undamaged (start condition c) batcy ; a is rate ofdeterioration, and b-is power constant (determined experimentally).

Reinforcement corrosion results in spalling of the concrete. To determine the risk ofcorrosion of the reinforcement Ficks second low of diffudion has been used:

with: C- chloride penetration,D-diffusion coefficient,x-distance from he surface, t-time.

Designing for durability is the process in which following assessments are made [Rendell, etal.]: ZA POSTOJEE OBJEKTE

define the service life of the structure,

plan the finances of the project,

investigate the site,

identify,

mechanisms of attack that will cause material damage,

assess the rate of which material properties will deteriorate,

assign exposure classification, according EN 206,

select material and/or provide protection for achieving design service life,

ensure the system of control of material and construction process,

proposed an inspection programme

Documents

FolicDurabConcrStrFACTA Nis 09