Durability Design of Concrete Structures
National and International Scenario
P. N. Ojha
Joint Director & Head
Centre for Construction Development and Research National Council for Cement and Building Materials
Ballabhgarh, Haryana
Puneet Kaura
Manager
Centre for Construction Development and Research National Council for Cement and Building Materials Ballabhgarh, Haryana
Brijesh Singh
Group Manager
Centre for Construction Development and Research National Council for Cement and Building Materials
Ballabhgarh, Haryana
The performance of structure is its ability to meet functional and time requirement. Concrete deteriorates with time and results in gradual decrease in performance over time. The time dependent degradation of concrete can be intrinsic to the concrete or induced by the interaction between the service condition and the concrete. Durability design is by nature is a multilevel task: at whole structure level, at a level of structural elements and at a material level.[1] At level of structure as a whole, durability design is about selecting a service life of structure for given environmental action and arriving at rational structural element assemblage and their layout so that the transient performance of the structure can always be maintained to an expected level. At the level of structural element, the durability design is about fulfilling the selected design service life through more specific technical requirements such as section details, concrete cover thickness, material properties etc. Technical requirements from other structural design considerations such as design for structural loads or fire etc also interferes at this level in the selection of section details, concrete cover thickness, material properties etc. Then at third level, concrete mixture is designed appropriately both in order to satisfy the specific material properties transferred from durability consideration and other structural design considerations such as design for structural loads or fire etc.
Concrete Deterioration Phenomena
The deterioration of concrete materials under environmental actions is fundamental knowledge for durability design. Concrete can deteriorate from a variety of reasons, and concrete damage is often the result of a combination of factors. Depending on basic nature of deterioration, concrete deterioration can be classified into three categories: chemical, physical or biogenic in nature. Deterioration such as Alkali Aggregate Reaction (AAR) or Alkali Silica Reaction (ASR), Internal Sulfate attack (ISA), Delayed Ettringite Formation (DEF), External Sulfate attack (ESA), Salt Crystallization, Carbonation, Chloride Ingress, Leaching, Acid attack etc. are generally clubbed into the first category. Reaction products might cause problems, due to dissolution or expansion or adverse chemical reactions between substances and components of the concrete. Carbonation induced reinforcement corrosion and chloride induced reinforcement corrosion are the electrochemical in nature, where chemical reactions at the anode and cathode are combined with an electrical current through the steel and through the concrete. Damages to the concrete due to freezing and thawing, creep, shrinkage, thermal cracking, abrasion, erosion, impact or excessive loading are generally clubbed into the second category which is physical in nature. Damage due to vegetation and biological growth, dirt and rubbish deposits etc. are biogenic in nature and are clubbed into third category.[2-6].
Fig. 1: Principal Factors Involved in Transport Process in Uncracked Concrete[7
The very reason of concrete deterioration stems from the fact that concrete is a porous material and through the pore network, aggressive agents can enter inside the concrete through various transport process. Transport mechanisms (Ionic transport, Moisture transport, Electrochemical potential, Temperature distribution) govern the rate of ingress of the deterioration inducing species in concrete and its understanding is fundamental to developing service life prediction model of reinforced concrete structures.[4] Understanding the transport mechanisms helps in transfer of lab experiments on accelerated durability tests to the actual behaviour of structures under field exposure. Transport mechanism explains and provides ways and means to quantify the deleterious effects, helps in developing mitigation strategies and assists in material selection and their quality control testing. Principal factors involved in transport process in uncracked concrete are summarized in Fig. 1.[7] In cracked concrete, more complex transport mechanisms are involved and are still not clearly understood.
Performance Deterioration and Durability Limit States
Reliable transient performance of the structure is primary requirement. Performance of structure has three components: safety, serviceability and durability. reliability also has three components: time (specific), condition (prescribed) and function (predetermined). Therefore, Reliable structural performance has three domains: structural safety, structural serviceability and structural durability. Structural durability which is related to material damage is not independent from structural safety and serviceability. Structural durability is a special part of structural safety or structural serviceability. For time-domain reliable performance of a structure, structural safety and structural serviceability both includes structural durability and corresponding limit states are ultimate limit state (ULS) and serviceability limit state (SLS) and these both limit states includes the condition limit state into them.[8-10] Damage of concrete and associated physicochemical phenomenon and mechanism of deterioration is the core content whereas the effect of degradation on structural performance and associated various limit states is main content of structural durability. In the long run the load carrying capacity of structure depends on the degradation of concrete and reinforcement inside the concrete. The performance of structural elements is required to be evaluated by first analyzing the rate of change in performance on the material level. Concrete as a material degradation process modeling describing the interaction of the material and environment with accuracy (trueness and precision) is essential for prediction of durability. The minimum acceptable values for performance (or maximum acceptable value for degradation) are called durability limit state. [8-9]
Durability of concrete structure exposed to its environment shall be such that it remains fit for use during its design service life.
These requirements are satisfied in one, or a combination, of the following ways [11, 13]:
– by using materials that, if well maintained, will not degenerate during the design service life;
– by designing protective and mitigating systems;
– by providing such dimensions that deterioration during the design service life is compensated for;
– by choosing a shorter lifetime for structural elements that when necessary are replaced one or more times during the design life;
- in combination with appropriate inspection at fixed or condition-dependent intervals and appropriate maintenance activities.
Standards and Codes for Durability Design
At International level, ISO 2394, General principles on reliability for structures,[12] provides conceptual framework for risk and reliability informed decision making for design and assessment of structures over their entire service life and serves as a basis for the task of preparing international standards, national standards or codes of practice. ISO 13823:2008, general principles on the design of structures for durability,[13] has been prepared in accordance with the guidelines of ISO 2394, provides conceptual framework for limit-states based method of evaluation and design of structures for durability. This standard also provides framework for the development of mathematical models to predict the service life of components of the structure. The goal of this international standard is to ensure that all analytical models are incorporated into the limit-states method, the same as currently being used for the verification and design of structures for gravity, wind, snow and earthquake actions. This standard advocates taking the cause-and-effect process into account in developing methods for the prediction of service life. However, this International Standard does not address design procedures for durability. The fib Bulletin 34: Model Code for Service Life Design (MC SLD) (2006),[14] addresses Service Life Design (SLD) for plain concrete, reinforced concrete and pre-stressed concrete structures, for standardization of performance based design approaches. ISO 16204:2012, Durability-Service life design of concrete structures, is material specific standard based on the principles given in ISO 2394, ISO 13823 and fib MC SLD. This international standard treats design for environmental actions leading to deterioration of concrete and embedded steel. Concrete structures are designed in accordance with national or international codes and standards such as Indian standard IS 456, Eurocode EN 1992-2004, Eurocode EN 206, ACI 318-2011, RILEM, JSCE etc. The structural design focuses on the structure’s ability to resist the mechanical load imposed on the structure whereas durability design focuses on structure’s ability to resist the environmental impact and associated deterioration and degradation imposed on the structure during its entire life span. Service life design methods are similar to the load and resistance-factor design procedure used for structural design. Modeling of environment and deterioration mechanisms is being developed on a probabilistic basis allowing reliability based service life design.
Approaches to Durability Design of Concrete Structures
The service life can be designed by using two principles: deem-to-satisfy rules, and performance-based design. The deem-to satisfy rules are based on specifying a certain concrete composition and concrete caver, but the result is not a specified service life. Traditionally, national and international concrete standards give requirements to achieve the desired design service life based on the ‘deemed to-satisfy’ and the ‘avoidance of deterioration’ approach.
The performance–based design is based on requirements of performance of the structure, and the result will be a long specified service life with limit states. Service Life Design (SLD) or Limit State Design (LSD), also known as Load and Resistance Factor Design (LRFD) is based on approach to resist (not to avoid) deterioration caused by environmental actions. For SLD, quantifiable models should be available on the Load side (Environmental Actions) and Resistance side (Resistance of Concrete) i.e. Deterioration of concrete, its impact on physical performance of structure and associated uncertainty and variability should be quantifiable. [11, 14] Limit State of Durability is defined as:The theory of durability design is in principle based on the theory of safety (or structural reliability) used in structural design.
Reliability and failures are addressed either in probabilistic terms or through partial factor of safety.[11, 14] Common step in durability design are:
Fig. 2: Summary of Service Life Design Approaches (ISO 13823, 2008)
Step-1 Selection of realistic model describing the process (to quantify the deterioration and resistance provided by the concrete): Model should incorporate the environmental actions with statistically quantified environmental parameters (e.g. temperature, relative humidity, splash rain events etc.). From resistance side, model should be able to be validated by realistic laboratory experiments and by practice in the field.
Step-2 Definition of limit states: against which the structure should be designed for. Such as depassivation of reinforcement caused by carbonation, cracking due to reinforce cement corrosion, spalling of concrete cover due to reinforcement corrosion, collapse due to loss of cross section of the reinforcement.
Step-3 Apply the models: described in step-1 to calculate the probability that the limit states defined in step-2 above occur (determination of the probability of occurrence) or use of appropriate partial factor of safety on environmental loads and resistance provided by the concrete.
Step-4 Define the type of limit states: (SLS, ULS) of the limit states described in step 2. Depending on the type of limit state (SLS, ULS) and the consequences of a failure.
The flow of decisions and the design activities needed in a rational service life design process are given in fig. 2.
Partial Factor Method The partial safety factor approach is a deterministic approach where the probabilistic nature of the problem (scatter of material resistance and environmental load) is taken into account by partial safety factors. [11, 14]
To take care of possibility of unfavourable deviations of action, design value of an action is obtained by multiplying the representative value by the partial factor. To take care of possibility of unfavourable deviations of materials and product properties from the representative values, design value of material or product property is obtained by dividing the characteristic value by a partial factor.
Deterioration Mechanism, Mathematical Modeling and Service Life Design for Durability A mathematical model is suitable for SLD when parameters of the models applied and their associated uncertainty is quantifiable by means of tests, observations and/or experience.[11, 14]
Widely accepted mathematical models exist for following deterioration mechanisms and therefore, Service Life Design (SLD) approach of durability design is possible and have been dealt in ISO 16204 and fib MC SLD [11, 14]:
– Caronation induced corrosion
– Chloride induced corrosion
– Freeze/thaw attack
Widely accepted mathematical models do not exist for following deterioration mechanisms and therefore, the problem may not be possible to address using Service Life Design (SLD) approach.[11, 14]’
- Chemical attack;
– Alkali aggregate reactions;
– Fatigue caused by dynamic loading and leading to time dependent material degradation and corrosion;
– Fatigue caused by dynamic loading and simultaneous corrosion caused by environmental action is not treated.
In national and international concrete standards and codes, presently, durability design for these deterioration mechanisms can only be possible through traditional way of design i.e. “deemed tosatisfy” or “avoidance of deterioration” approach.
Provisions in IS 456 in Comparison to International Standard for Durability Design
So far in IS 456:2000,[15] the standard practice is to design concrete structures on the basis of deemed to satisfy approach. The codal provisions/clauses in the code ensures durability on the basis of parameters like minimum cover, crack width control, maximum spacing of rebars, minimum concrete grade, minimum cement content, maximum w/c ratio, selection of cement and cementitious material etc. Most of these limiting values largely based on short-term experience obtained for significantly less severe exposure conditions, considering a reference design life of 50 years. In IS 456:2000, the general environment to which the concrete will be exposed during its working life is called ‘exposure condition’ and classified as mild, moderate, severe, very severe and extreme. Concrete mix designs are carried out in line with the prescriptive provisions of Table 3 of IS 456:2000. This classification of exposure condition is according to aggressiveness in general, irrespective of aggressive agent characteristics and type of deterioration of concrete. Unlike to IS 456:2000, The fib Bulletin 34: Model Code for Service Life Design,(EN 1990:2002) and other International standards have made provisions to select the design service life as produced in Table 1.
Unlike to IS 456, Eurocodes(EN 1992, EN206-1) prescribes the durability related requirements for concrete both on the material level and on structural level. The environmental actions are defined into different environmental classes and intensity degrees according to the respective deterioration mechanism as illustrated in Table 2.
In its most recent version of ACI 318 -11, environmental actions are classified into several categories. Eurocode and ACI 318 strictly follow a prescriptive approach of durability design, however certain provisions for performance based design is also there. Unlike Eurocode and ACI 318, the JSCE guidelines adopts mainly a performance based approach for durability design. ISO 16204: 2012 which is based on the principles given in ISO 2394, General principles on reliability for structures, ISO 13823, General principles on the design of structures for durability and fib “Model Code for Service Life Design” treats design for environmental actions leading to deterioration of concrete and embedded steel.
Durability Design Concrete against Carbonation Induced Reinforcement Corrosion
The decrease in pH of the concrete interstitial solution linked to the penetration of atmospheric CO2 into the porosity of the concrete is carbonation and the induced corrosion is carbonation induced reinforcement corrosion. For the purpose of durability design, the ingress of the carbonation front is assumed to obey the following equation: C (t) = W. K. tx
Where C is Depth of carbonation in mm at a given time t (in years) W is weather coefficient takes into account the varying meso-climatic conditions for the specific concrete member during the design service life, such as humidity and temperature.
K is carbonation coefficient, a factor reflecting the basic resistance of the chosen concrete mix (like water/cement ratio, cement type, additions) under reference conditions and the influence of the basic environmental conditions (like mean relative humidity and CO2 concentration) on ingress of carbonation. It also reflects the influence of the execution.
X is the exponent indicating change of rate of carbonation w.r.t binder type.
For the design of a new structure, the weather coefficient W can be taken in between from 0.9 to 1.25 depending upon how the environmental action is conducive in the progress of carbonation front. The carbonation coefficient K might be derived from literature data or existing structures where the concrete composition, execution and exposure conditions have been similar to those expected for the new structure. A relationship between carbonation coefficient K and electrical resistivity of concrete made with different type of cement may be developed experimentally and can be used for further design purpose. For very important structures (Service life more than 50 year), carbonation coefficient K shall be determined through accelerated carbonation test during the concrete mix design stage. Time for initiation of corrosion (ti) in years, shall be calculated using the model equation. The propagation time (tp), which is time from initiation of corrosion to initiation of crack shall be taken appropriately. It shall be ensured that, tsl (design service life)≤( ti+tp).
Durability Design against Chloride Induced Reinforcement Corrosion
The three most important variables that govern the chloride intrusion into concrete and the corrosion of the reinforcement are concentration of chlorides at the surface (Cs), concentration threshold value which initiates corrosion of steel, and the transport rate of chloride ions in the concrete cover layer[16,17,18].
The penetration of chloride ions into concrete is a complex nonlinear dynamic phenomenon including several transport mechanisms (ionic diffusion, capillary sorption, permeation, dispersion, etc. The ionic diffusion is considered to have the most dominant effect under the assumption that concrete cover is fully saturated.
The ingress of chlorides in a marine environment may be assumed to obey modified Fick’s second law of diffusion.
Where: C(x, t) is the content of chlorides in the concrete at a depth x (structure surface: x = 0 mm) and at time t [% by mass of binder]; Cs is the chloride content at the concrete surface [% by mass of binder]; Ci is the initial chloride content of the concrete [% by mass of binder]; x is the depth with a corresponding content of chlorides C(x, t) [mm]; Dapp(t) is the apparent coefficient of chloride diffusion through concrete [mm2 /year] at time t;
t is the time [years] of exposure; erf is the error function
Dapp(t0) is the apparent diffusion coefficient measured at a reference time of t 0;
α is the ageing factor giving the decrease over time of the apparent diffusion coefficient.
Depending on the type of binder and the micro-environmental conditions, the ageing factor is likely to lie between 0.2 and 0.8. For the design of a new structure, the parameters Cs, Ci, α and Dapp (t 0 ) may be derived from existing structures where the concrete composition, execution and exposure conditions have been similar to those relevant for the new structure.
Design value of surface chloride content, Csd shall be taken for different exposure classes from the existing literature. Design value of threshold chloride content/critical chloride content may be taken 0.4% by mass of cement for concrete made with OPC and 0.3% by mass of cement for PPC and PSC as recommended in various existing literature. Initial Diffusion coefficient Da(t0 ) can be determined from established equation between Da(t0 ) and W/C ratio for OPC, PPC and PSC and then a design value of chloride diffusion coefficients (Dd(t)) for service life (exposure period) t will be calculated. At concrete mix design stage, for a known w/c or w/cm ratio, performance based accelerated test such as RCPT/Electrical Resistivity shall be conducted as applicable. For important structures, Da(t0) shall be determined through unidirectional diffusion test, which is a long duration test. RCPT/Electrical Resistivity shall be conducted as for determining the values to be considered for acceptance during quality control. The service life is determined on the basis of critical surface chloride content and threshold chloride content value. It will be ensured that Cx,t ≤ Cth ( threshold chloride content).
Durability Design against Sulphate Attack
Sulphate attack is the most common chemical form of concrete deterioration. Sulphates are commonly found in soil, aggregates, sea water and cements. Sulphate attack on concrete has been reported to be the cause of severe damage to concrete for over a century. Sulphate attack can lead to expansion, cracking, strength loss, and disintegration of the concrete Sulphate attack is generally attributed to the reaction of sulphate ions with calcium hydroxide and calcium aluminate hydrate to form gypsum and ettringite. IS 456 gives recommendations for the type of cement to be used, maximum free water/cement ratio and minimum cement content required at different sulphate concentrations in near-neutral ground water having pH of 6 to 9. IS: 456-2000, provides conditions as well as requirement to safeguard concrete against sulphate attack. In case of very high sulphate concentrations i.e. Class 5 conditions, use of lining such as polyethylene or polychloroprene sheet; or surface coating based on asphalt, chlorinated rubber, epoxy; or polyurethane materials have been recommended so as used to prevent access by the sulphate solution. For durability design against sulphate attack, national and international concrete standards and codes, presently adopts traditional way of design i.e. “deemed to-satisfy” or “avoidance of deterioration” approach.
Durability Design against Alkali Aggregate Reactivity (AAR)
The chemical reaction between alkali hydroxides from Portland cement and specific category of aggregates is known as alkali aggregate reaction. The swelling and cracking due to aggregate alkali reaction appears after many years of construction. Even though the deterioration due to it is a slow process but is progressive and combined with other causes can lead to complete failure of structure. The AAR can be subdivided to two types viz, Alkali silica reaction (ASR) and alkali carbonate reaction (ACR). Currently, in IS: 456-2000 “deemed to-satisfy” or “avoidance of deterioration” approach is recommended for durability design against AAR.
Durability Design against Freeze Thaw
Deterioration of concrete exposed to freezing generally occurs when there is sufficient amount of internal moisture. The transition of water to ice results an increase in volume. When the saturated pores in the concrete freezes, it can cause severe cracking and disruption. Widely accepted mathematical models exist for freeze thaw and Service Life Design (SLD) approach of durability design against freeze thaw can be adopted in IS: 456-2000 in similar lines to ISO 16204 and fib MC SLD.
Conclusions
n the recent decade, owing to the ageing and often premature deterioration of infrastructure in India, the durability design of concrete structures has increasingly received attention. There is need for suitable and reliable performance based approaches which can relate to the shortcomings of the traditionally prescriptive design methods for concrete durability. The prescriptive approach therefore often fails to offer a rational basis for the selection of suitable materials, design procedures, quality assurance etc. which hampers the service life of structure. The application of a performance approach for concrete durability shifts a large portion of the responsibility from the design engineer to the concrete supplier and contractor, who have to work as a team to produce a structure that meets the required durability characteristics.
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