17 Jan PRACTICAL APPROACH TOWARDS CONCRETE SPECIFICATIONS IN CONSTRUCTION INDUSTRY FOR BETTER DURABILITY (PART I)
Hello Everyone………. In this blog, second in the series for our portal, nothing comes to my mind than DURABILITY of Concrete structures to be spoken of, though the topic in sense is quite wide.
Lots of Papers have been published in various Construction Journals, lots of presentations have been done in various seminars on DURABILITY OF CONCRETE STRUCTURES. Still, an attempt is being made here, from my two decades of experience in Cement/concrete industry, to write on CHANGES NEEDED ON PRACTICAL APPROACH TOWARDS CONCRETE MANUFACTURED/SPECIFIED IN CONSTRUCTION INDUSTRY for DURABLE CONCRETE STRUCTURES and thereby for better SUSTAINABILITY.
“Durability may be defined as the ability of the material or structure to withstand the service conditions for which it has been designed over a prolonged period without significant deterioration”
Concrete is the most widely used material in the world next to water. Despite the availability of know-how and equipment for quality concrete construction, in the recent past many concrete structures have undergone pre-mature deterioration resulting in early reconstruction or major repairs involving huge expenditures. This could be due to inadequate anticipation of severity in exposure conditions, poor quality of concreting practises, increased severity due to environmental pollution etc…
Besides DURABILITY of STRUCTURES, SUSTAINABILITY is yet another important issue confronting the construction sector. A steep increase in population during the last few decades combined with industrialization and urbanization have resulted in unlimited exploitation of non-renewable natural resources. It is believed that if this trend continues unabated, very little resources will be left for the future generations. Besides, the emission from GREEN HOUSE GASES – especially CO2 and NOx- have reached an alarming level, resulting in an unprecedented rise in ambient temperatures throughout the globe.
Since the construction sector is the largest user of natural resources it is widely believed that this sector has to play a major role towards achieving the sustainable development of our society. It is now established beyond doubt that use of Blended/supplementary cementitious materials like Fly Ash, Ground Granulated Blast Furnace Slag (GGBS), silica Fumes, Blended cements like PPC (Portland Pozzolana Cement) and PSC (Portland Slag Cement) etc not only helps in minimizing the Portland Cement content – thereby reducing the Green House Gas emissions – but also improves a host of Properties of Concrete, thereby its durability.
Deterioration of concrete begins almost immediately after casting as the hardened properties are influenced by phenomena which occur at an early stage, such as plastic cracking, bleeding, segregation and thermal effects. In the hardened state, concrete may be affected by a variety of internal and external factors which cause damage by physical and/or chemical mechanisms.
The main reasons for all concrete durability problems are the interconnected porous nature of the hydrated cement paste and the heterogeneity of concrete. Hence, the key to concrete durability is the achievement of tight impermeable concrete pore structure.
DURABILITY DESIGN APPROACHES IN MAJOR COUNTRIES:
Structural concrete is designed to meet specific criteria for workability and strength. Conventionally, Durability requirements are incorporated as specific material requirements in the design process, particularly with respect to the maximum water/cement ratio, minimum cement content, minimum grade of concrete, cover to concrete, etc appropriate for a particular service environment. In other words, the design approach is prescriptive regarding mix and material parameters or properties which are deemed to ensure durable concrete. Once the concrete has been mixed and placed, only the compressive strength is measured to ensure compliance with the design requirements and specifications. This is done on specially prepared samples, made, cured and loaded under conditions that bear little resemblance to those in the actual structure.
The fact remains that strength and durability are not necessarily directly related. While compressive strength generally correlates with the response of the material to load or stress, prescriptive specifications in effect extrapolate acceptable strength performance to also indicate suitable durability performance of the concrete. It is not difficult to recognize the inadequacy of this approach, particularly because the compressive strength test is not able to account for the physical-chemical nature of the concrete and its ability to resist the deteriorative effect of its environment. Furthermore, while both strength and durability are related to water/cement ratio of the concrete, strength is governed by the internal bulk of the concrete but durability is primarily controlled by the relatively thin surface cover zone over the reinforcement.
Concrete quality in the cover zone is often dictated by factors, in addition to good mix design, like quality of the materials actually used in project sites ( in some cases have observed varies than the one used during concrete mix design ) and construction practices such as handling, placement, consolidation and curing of the concrete. Thus, a reliable measure of the quality cover concrete zone can be obtained by assessing the concrete after hardening in the structure, rather on the cube specimens cast during pour, cured under standard room temperature continuously at the age of determining compressive strength.
Codes and specifications followed in major countries play an important role in ensuring the durability of concrete structures. The vast majority of international codes on structural concrete are basically “Prescriptive” in nature, that they specify the limiting values of the following four parameters for code-defined exposure conditions:
- Minimum cement (cementitious) content.
- Maximum free water/binder ratio.
- Minimum Grade of concrete.
- Cover to Reinforcement.
Since clear cut definition of exposure classes is not available, the use of Indian standards recommendations at times becomes confusions and leads to differing interpretation. For a better approach, the classification could be based on deterioration mechanism and their severity in line with the existing international codes.
This is primarily the basis for the call to develop suitable strategies to implement what are commonly known as “Performance Based Specifications”.
The concept of performance specification, developed in the late 1990’s, is a promising solution to the durability related problems of concrete. This is based on the performance criteria defined for the constructed structure.
According to National Ready mixed concrete association (NRMCA), USA, “Performance Specification is a set of instructions that outlines the functional requirements for hardened concrete depending on the application. The instructions should be clear, achievable, measurable and enforceable. Performance specifications should avoid requirements for means and methods and should avoid limitations on the ingredients or proportions of the concrete mixture”.
Many countries like USA, South Africa, Canada etc have been experimenting the concept of performance specification for more than a decade and results from the field are promising.
Advantages of Performance specifications are:
- Performance specifications clearly defines end-result requirements and owners expectations.
- Concrete mixes are optimized and field operations performed to ensure that owner’s expectations are met.
- Specifiers focus on what is needed, rather than on how to get it.
- Benefits can be obtained from the use of:
- Unique materials.
- Material combinations.
- Technology expertise.
- Knowledge of local materials and conditions.
- By concentrating on end results, performance specifications will foster innovations.
- More durable product lead to lower life cycle costs and hence to enhanced sustainability.
- Durable concrete can be obtained by proper concrete mix design, adopting good concreting practice at site, proper compaction and curing, adopting adequate cover to reinforcement and anticipating the proper severity of exposure during the life time of the concrete structure.
- One of the surest ways of enhancing the sustainability of concrete constructions is by improving its long term durability and thus the useful SERVICE LIFE of the CONCRETE STRUCTURE. However, the currently available codal provisions and specifications practiced in several countries are not conducive in achieving these objectives. The Current approach of “assuming” that long service life can be ensured following certain ”PRESCRIPTIVE” specifications has many limitations. It does not exactly define what service life is and what constitutes the end of service life. This highlights the need for adopting performance based SPECIFICATIONS and a holistic approach based on life-cycle cost.
In the forthcoming series on the same subject, we will see Durability specifications of various International Standards including Indian Standards, Role of Blended cement like PPC and PSC in improving Sustainability through ENHANCED DURABILITY, Check Lists / Procedures to ensure Enhanced Durability at Project sites. See you, until then…..