(Unibeton - U.A.E.) -- Since the invention of Portland cement, there have been many methods put forward for designing concrete mixes. These methods range from simple practical methods where pre-weighed ingredients are added to a mixer and the mix is assessed by eye, and when a suitable looking concrete is obtained, what remains unused is weighed and the mix is easily back calculated; to highly sophisticated methods, where just about every conceivable property of concrete is factored into the design process. Structured mix design systems started to emerge just before the Second World War when Glanville produced the simple system Road Note No. 4. Concrete Mix Design. This used a series of water/cement ratio curves and idealized gradings to produce a range of low to medium strength mixes, which for many years became the mix designer's bible.
In the late 1950s, it was followed by Shacklock & Entroy's Design of High Strength Concrete Mixes and also mix design methods from both the PCI and ACI, in America and Canada. During the 1970s came Phil Owens' Basic Mix Design, and shortly afterwards the Department of the Environment Mix Design, which essentially replaced the much used Road Note 4.
Since then many methods have appeared including one impressive system developed by Ken Day, to whom the OWICS conference has been dedicated.
Many of these mix design methods rely on tried and tested, so called idealized gradings for combining aggregates. These have been widely used for many years, such that they have been accepted as the norm. With the advent of pumped concrete attention was focused on combining aggregates in such a way as to obtain the maximum possible packing. Many people, including the author, did some work in this field, checking different combined gradings of materials with a Voidmeter; others looked at computer generated models to achieve maximum packing.
However what everyone overlooked was that when the maximum packing was achieved the materials locked together, and lacked the necessary rheological properties to produce a concrete with the correct workability and flow characteristics, to enable it to be successfully transported, handled, placed, and compacted on site. A few years ago, an American realized that the rheological properties of concrete were uniquely related to the shape and size of all the particles present in a concrete mix, not just the way they were combined together. He started from scratch to produce a new system of concrete mix design that was radically different from anything that had ever been tried before. The result, which is called iCrete, short for intelligent concrete, represents a quantum leap forward in concrete technology.
When the author was first told about a concrete mix prepared with this technology, understandably he was a little skeptical. However when the concrete emerged from the truck he realized that this was a concrete destined to change the industry. It was so perfectly proportioned and could be placed with a greater ease than traditional concrete. In addition it could be easily compacted. Even visually stiff concretes can become energized and flow with ease under the influence of vibration. The unique packing arrangement also enables a very high standard of uniform color and finish to be obtained.
What sets the mix design system apart from other methods is that it selects a degree of optimization such that other important rheological properties such as cohesion and viscosity are not compromised in the process. It is possible to make a range of concretes from 30 MPa to 100 MPa that have identical workabilities, viscosity and cohesion levels, so that although the fresh concrete properties are virtually identical, they can cover a wide range of strength requirements. Very high strengths can be attained, in excess of 200 MPa, with concretes that still maintain their flow characteristics. However, most commercially produced concretes tend to be at or below the 100 MPa level.
The mix design process
The mix design process is designed to make the most effective use of the cement by carefully optimizing all the components in the mix to make them function more effectively. This can either result in reduced cement contents, whilst maintaining the plastic and hardened concrete properties of the mix, or higher strengths and enhanced performance can be obtained without any reduction in the cement content. Apart from optimising the aggregate proportions, attention is placed on the rheological properties of the fresh concrete to make it easy to pump, place and compact. One of the difficulties is to convince Engineers, who lack continuous professional development, that concrete with a Slump of 200 mm or a flow of 650 mm to 750 mm is both strong and exceedingly durable.
Also, in order to maximize on the technology, it will be necessary to review the codes and specifications to revise the minimum levels of cement content, which were drawn up in the early 1970s with the introduction of CP110, and have not yet even been amended to take into account of the benefits of superplasticizers, as water reducers, in enhancing the durability of concrete. This process is currently ongoing at this time, but may take several years to filter through the industry.
Initially, material tests are carried out on the ingredients of the mix and the results are applied to patented mix algorithms. The variation in materials is monitored via a database and their interrelationships are continuously updated and monitored. The mixes are designed primarily for cohesion and viscosity. Even where the concrete visually appears to have a low workability it can easily be re-energized by vibration. The concrete is prepared using a pre calibrated amount of water addition at the time of mixing and the workability is controlled by the amount of admixture addition. Sensors monitor and control any variations in the physical properties of the aggregates.
The result is a very high level of consistency batch after batch. This enables only slight variations in workability to be achieved.
Optimized packing
The optimised packing increases the modulus of elasticity of the concrete over traditional concrete mixes resulting in improved stress/strain characteristics. Other significant advantages include reductions in the drying shrinkage by as much as 30 percent over a similar conventional concrete where the water/cement ratios are held constant, and major reductions in creep are also obtained. This can result in a considerable saving in the amount of the steel used in the design, especially in tall building construction. Another benefit of the mix design is greatly reduced water contents and hence water/ cement ratios of the concrete. Because this virtually eliminates bleeding of concrete mixes, very careful attention needs to be given to covering and protecting the concrete surface from any water loss, as soon as possible after casting. The concrete does not bleed and it does not segregate. Where high walls or columns are cast, there is coarse aggregate present in the mix right to the top of the lift.
In addition, where the concrete is vibrated, the lack of segregation also results in little or no segregation discoloration, thereby resulting in very uniform colored concrete surfaces that also tend to be free of blowholes. This makes the concrete very useful to architects who like to use as-struck concrete surfaces. In addition, its ability to be molded into complex shapes whilst still maintaining a high quality surface makes it an ideal material for use in the production of precast concrete.
Where it is necessary to control the temperature rise in large concrete pours, the use of reduced cement contents, combined with partial cement replacements can result in a significant reduction in temperature rise of between 14°C to 20°C with optimized mixes.
Environmental aspects
Environmental aspects of the concrete include a reduction in greenhouse gases. The cement industry produces about six percent of the annual carbon dioxide emissions in the world. For every one kg of cement produced at least 700 g of carbon dioxide are given off in the production process. While this does not damage the environment as much as indiscriminate cutting down of the Amazon Rain Forest, clearly if the amount of CO2 in the environment can be reduced, it is of benefit to us all.
The optimized mixes produce on average about 27 percent of the emissions as compared with traditional concrete, representing a considerable saving in greenhouse gases. A kilogram of carbon dioxide produces about five m3 of carbon dioxide gas and a truck loaded with normal concrete contributes a significant amount of carbon dioxide to the environment.
However most of this originates from the cement manufacturer rather than the actual concrete production. The UAE produces 2.9 tons of carbon dioxide per year per capita, due mainly to the surge in construction activity, whereas the world average is 0.6 tons per year per capita. The UAE has embarked on building a low carbon city and iCrete mixes are being used as part of this development.
In New York these revolutionary concrete mixes are being used on the 9/11 site for the construction of the Freedom Tower, which is replacing the towers of the World Trade Centre. It is also being used in the Beekman Tower in New York. The architect for the Beekman Tower, Frank Gehry, said he had never seen concrete like this before, and would like to use it on all his future projects.
Technical advantages
Within a decade it is likely that all concrete will be produced using this type of system, because there are significant advantages for all parties in the construction process:
Client
• Reduced construction period
• Time and cost saving
• Low carbon footprint (27% of normal concrete)
• Good publicity – new and innovative technology
• Green alternative at no extra cost
• Less maintenance – longer service life
Contractor
• Designed for workability cohesion and viscosity
• Easy to place and compact and finish
• No bleeding
• No segregation
• Less variability batch after batch
• Reduced finishing times for slabs and floors
• Ease of handling and placing – less labor required
• Optimizes available materials
• Reduced contract period
• User friendly.
• Savings at every stage of the project
Engineer
• Green concrete – effective use of cement, better not lower quality.
• Increased strength for same cement content
• Lower standard deviation for strength.
• Higher modulus of elasticity – reduced section sizes
• Reduced creep – less reinforcement required.
• 30% lower shrinkage
• Lower …
- RCP
- Permeability
- Absorption
• Lower temperature rise
• Less heat of hydration
• Consistent quality batch after batch
• Improved cohesion and workability
• Minimises the amount of steel reinforcement required.
Architect
• High quality finish
• Uniform color
• Good appearance
• LEED and Estidama points for use
• Environmentally friendly.
• Freedom for design and application
• Consistent quality batch after batch
Strength and durability
Compressive strengths are increased over conventional concretes with the same water/cement ratio and flexural strengths of the concrete are about 16 percent of the compressive strength, compared with seven to 10 percent for traditional concrete. Rapid Chloride Permeability tests on plain OPC concretes even without the addition of either slag or microsilica produced values ranging from 503 – 860 coulombs. These are considered to indicate low chloride permeability.
This test is widely used in the Middle East but is really an indication of the ability of the concrete to conduct current and is so variable even with the same concrete that the results need to be assessed with caution. Where slag and/or silica fume are used as partial cement replacements, even lower RCP values can be obtained.
The results from in service production of an iCrete mix over a similar control mix of 40/20-grade result in +five MPa at seven days and +seven MPa at 28 days. Use of the sensors to control non-modified concrete improve the 28day strength by an average of four MPa over standard production concrete. The tighter production control improves the process standard deviation by between one to three MPa.
Conclusion
This mix design technology is an important development that will revolutionize the concrete industry of the future. The consistent quality combined with ease of placing and compaction, together with enhanced finished surface quality, give the technology a very distinct advantage over any of the traditional concretes currently being produced using other systems of mix design.
After an extensive evaluation of the technology, Unibeton took the decision to implement the technology throughout its extensive series of plants throughout the United Arab Emirates. Its customers are highly impressed with the concrete handling and quality.
By: Christopher Stanley
Christopher Stanley is technical director for Unibeton, the largest ready mix concrete producer in the United Arab Emirates. This paper was presented at the OWICS 2009 conference.
Nenhum comentário:
Postar um comentário