Successfully addressing the durability requirements for the Gulf region would require a more holistic approach in the production of sturdy, low-permeability concrete for general construction purposes, writes Willfried Krieg, technical director of Saudi Readymix Concrete.

01 April 2006

Design criteria and characteristics required to produce durable concrete predominantly depend on the prevailing environmental conditions, and will vary considerably between different geographical locations.

In the northern hemisphere for instance, concrete durability is mainly governed by its resistance to freezing-thawing cycles and de-ice scaling, whereas the major cause for deterioration of concrete structures in the Gulf is not the disintegration of concrete, but chloride-induced corrosion of the reinforcement.
Consequently, the most crucial durability parameter in this region is the chloride permeability of concrete, because it determines the time span after which the concentration of chlorides at the steel bars will reach the so-called “threshold level of activation”; or simply expressed, the time span to the onset of corrosion.
While chloride ingress and the resulting corrosion of reinforcement are by no means indigenous to the Gulf countries, the problems in this region are aggravated because of the abundantly available “tools of destruction”:
• The salt-laden atmosphere along the seaboards provides a high concentration of the most aggressive chloride ions;
• The high humidity provides the water, which serves as the transport vehicle that carries the chlorides into the concrete, and as the electrolyte in the ensuing galvanic process; and
• The high ambient temperatures, which act to accelerate the process of deterioration in a catalytic manner.
Hence, in order to address the local durability requirements, concrete (for general construction purposes) in the Gulf should ideally be designed for low chloride permeability; if not to entirely block, but at least to decelerate the penetration of chloride ions down to the embedded rebars.

Concrete design & materials
For the composition of a low-permeability concrete, the following design and material parameters are applicable:
• Low water-cement (W/C) ratio: Reduces permeability by reducing capillary voids, which form after evaporation of surplus mixing water. At W/C-ratios below 0.38, those voids will be gradually filled by expanding hydration products as hydration proceeds. However, the benefits from lowering the W/C-ratio cannot be extended indefinitely. There is growing evidence that, below a W/C-ratio of 0.35, internal self-desiccation can lead to micro-cracking, which in turn would increase permeability and chloride diffusion. To support low W/C-ratios, addition of high range water-reducing admixtures is usually required, which can also be instrumental in curbing segregation at liquid consistencies.
• High cement content is required to:
 > To form a sufficient volume of hydra tion products to fill capillary voids;
 > To support workability at low W/C-  ratios; and
 > To yield high alkalinity for formation  and maintenance of the protective   oxidative coating on re-bars.
  However, extremely high cement dosages will have an adverse effect on concrete quality due to the well-documented problems associated with the development of excessive hydration heat.
• Cement with high C3A content: Ordinary Portland cement (OPC) containing a minimum of 8 per cent tri-calcium aluminate has the ability to chemically bind a certain amount of chloride ions at the concrete surface, thus slowing down the penetration into the concrete structure.
• Blended cements: The penetration of chlorides into concrete is a diffusion-controlled process as defined in Fick’s Second Law. Apart from the exposure time and surface chloride level, the predominant factor here is the diffusion coefficient. Blends of Portland cement and artificial pozzolans like condensed silica fume (CSF) and pulverised fuel ash (PFA), or slag-based blends with ground granulated blast furnace slag (GGBFS), can reduce the chloride diffusion coefficient of concrete to an order of magnitude lower than that of concrete with Portland cement alone. Not only is the initial diffusion coefficient in concrete with blended cements lower than for PC concrete, but it also continues to reduce in time, thus resulting in service lives, which are reasonably consistent with expectations.
• Aggregates: Sound and hard sedimentary limestone would be the preferable source for aggregates, due to the lower thermal expansion coefficient as compared to igneous rock. The size distribution shall be well graded, especially in the fine aggregate range. Gap gradations should be avoided.
  Under skilful consideration of these parameters, chloride permeability can be curbed to adequate levels.
  In this context it should be noted that, unlike sulphate attack and carbonation, chloride ingress would not pose an immediate threat to the physical or chemical properties of the concrete itself. That means in an environment where chloride penetration is the dominant issue, concerns about the durability of the concrete itself are actually quite irrelevant. The all-important quality parameter of concrete tailored to suit the prevailing local conditions is not its own durability, but rather its ability to prevent premature, chloride-induced corrosion of the steel reinforcement.
  In light of this, it appears that the concept of “durable concrete” as a solution to the durability problems in the Gulf is rather misleading. The term as such wrongly implies that the imminent threat here would be the premature disintegration of concrete, and subsequently leads to the widespread belief, that the use of “durable concrete” alone would guarantee the durability of our structures.
  This misguided reliance on the powers of a single construction component has, in turn, created a dangerously ignorant attitude toward other indispensable durability considerations, because it distracts from the fact that the preventive effect of low chloride permeability is not an exclusive function of the intrinsic concrete quality. This effect can be utilised only as an integral part of a complex quality network combining structural design criteria, materials properties, and professional workmanship. None of these components is dispensable. For instance, inadequate concrete curing alone will render the protective capacities of even the most advanced concrete utterly ineffective.

Design & workmanship
That means we must eventually arrive at the insight that durability in the Gulf region cannot depend on the potentials of a single component. It is rather a matter of the composite quality of a concrete structure as a whole, including the following structural considerations:
• Structural design: Design with a minimum concrete cover over rebars of 50 mm in dry conditions. For marine structures, a minimum of 75 mm is recommended. Slender structural components should be avoided. For instance walls, columns, or slabs of 20 cm thickness do not lend themselves favourably to a concrete cover of 75 mm.
• Formwork: Non-absorbent, watertight formwork lined with CPF(controlled permeability formwork)-sheeting which acts as a filter to allow air and water to pass through it in a controlled manner whilst retaining cement. This leads to a reduction of the W/C-ratio in the near surface area of the concrete, with consequent improvements in surface quality.
• Reinforcement: Proper positioning of the reinforcement in the formwork to ensure that the specified concrete cover can be maintained for all rebars without exception. Corroded or soiled rebars should be cleaned prior to pouring concrete, to enable formation of a protective oxidative coating on the clean, hard rebar surface after being embedded in the alkaline concrete. Hot rebars should be cooled down by means of spraying chilled water prior to pouring concrete, in order to avoid “flash setting” of concrete in contact with hot rebars.
• Concrete placing: Thorough planning and preparation of concrete pours in order to avoid formation of “cold joints”, air entrapment between vertical layers, and segregation due to excessive free dropping-heights or use of flat chutes. Concrete should be placed nearest to its final position in the formwork. “Current best practice” should be strictly adhered to.
• Concrete compaction & surface finishing: Providing vibrators in suitable sizes and numbers, and strictly adhering to “current best practice” in order to achieve compaction to the lowest possible air content. Sufficient manpower should be provided for surface finishing to cope with the speed of placing. The finishing steps should be precisely timed in relation to the development and subsequent evaporation of bleeding water.
• Concrete curing: Timely commencement and uninterrupted application of accepted curing methods for at least seven days, to enable consolidation of cement hydration, and to avoid premature carbonation. “Hot weather concreting” precautions should be strictly adhered to in order to prevent formation of plastic shrinkage cracking.

Conclusion
The term “durable concrete” is misleading, as it implicitly contains the false promise of an ultimate, stand-alone cure against premature deterioration of concrete structures. It promotes a dangerous reliance on the powers of a single component and consequently, the negligence of the many other indispensable durability considerations.
The name “durable concrete” is also entirely concealing the fact that the most crucial property for extended structural service life in the Gulf region is not the durability of the concrete itself but its ability to curb chloride diffusion, in order to prevent premature, chloride-induced corrosion of the steel reinforcement. However, this preventive ability is not a sole function of the intrinsic concrete quality; it can be utilised only if the concrete forms an integral part of a complex network encompassing structural design criteria, materials properties and professional workmanship on site.
It is therefore strongly suggested to replace the term “durable concrete” with the expression “durable structures”, as a constant reminder that it requires the concerted efforts of architects, concrete suppliers, contractors, site engineers, and inspectors alike to achieve structural service life spans, which are reasonably consistent with the owner’s expectations.

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