Slider

Science

SCITECH

AMAZING FACTS

NATURE SPACE

Psychology

The Rise and Demise of Egypt’s Largest Pyramids

Posted date: 9:28 PM / comment : 0 ,

My love of Egypt and my first contact with Egyptian construction started in 1994 when I was asked to provide a scheme to strengthen parts of historic Cairo after the devastating earthquake in 1992. This initial contract was to work with the state-owned Arab Contractors to strengthen the Al Ghory Mosque, which had been extensively damaged. It was at that time that I was able to visit the Giza Plateau to see the pyramids.

My first view of the Great Pyramid was in the early evening when I got out of my vehicle at a local hotel. I was expecting to see a pyramid, but I was not prepared for the scale and size of the monument that was in deep shadow at that time of the day, almost obliterating half the sky. Like many other visitors, the first question that inevitably came to my mind was: ‘How could people with primitive tools build these fantastic monuments in such a short time?’ At that time, I did not take any professional interest in the construction of the pyramids, but I did feel a great deal of respect for the constructors.

Although we have been fascinated with how the ancient Egyptians built these incredible monuments, there is still a lot of discussion and mystery surrounding the actual method. The Step Pyramid of Djoser, which is 62 meters (203 feet) high, was the first high-rise structure that the ancient Egyptians built; previously, their structures were no more than 10 meters (33 feet) tall. How did the ancient Egyptians manage to construct the Step Pyramid, having never before erected a structure anywhere near that size?

Figure 1. The burial chamber of the Step Pyramid.
My professional interest in the construction of the pyramids was initially sparked by observations that I made during Cintec’s work restoring the ceiling of the burial chamber of the Step Pyramid. We were called in to restore the ceiling, which was collapsing due to the failure of the timber beam that the ancient Egyptians had used to hold the ceiling stones in place (Figure 1). Our unique Waterwall airbags supported the dangerous hanging stones temporarily, and our patented anchors permanently secured them (Figure 2).

Figure 2. Cintec Waterwall airbags supporting ceiling of Step Pyramid.
While in the burial chamber, I noticed that although we were drilling holes that were 4 meters (13 feet) in length, we never actually drilled through stones that were more than 40 centimeters (16 inches) wide. This appeared to be a direct contradiction of the common belief that the enormous stones on the outside were the same all the way through the pyramid. In some cases, the fill was a great deal smaller. It was this observation that prompted me to question the accepted theories that attempt to explain how the pyramids were built. Having worked in the construction industry for 54 years, I began to analyze these theories from a practical builder’s perspective.

I put myself in the mind of an ancient Egyptian builder faced with limited tools and little experience of large-scale construction. The main problem that I found with the existing theories was that, from a builder’s perspective, they made the process more difficult than it needed to be. Why would the Egyptians haul huge stones from a long distance away unless it was absolutely necessary? The internal core and filling would never be seen, so why fill it with quarried blocks that took time and presumably money to extract and transport to the site? The logistical problems were already enormous -- coordinating all the elements from quarrying, transport, scaffolding, design, setting out and manpower requirements.


A Progression of Knowledge


Cintec has undertaken restoration work in both the Red and Step Pyramids in Egypt, and during this work I have observed the progression of the ancient Egyptians’ knowledge of construction techniques. With every pyramid they built, they became more skilled and corrected previous design defects. One such example is their use of corbelling to create openings in the pyramid for the burial chamber.

At the Step Pyramid, the builders attempted to create an opening for the chamber by using large timber beams. However, the timber buckled and failed, causing stones to fall. It was this failure that Cintec was brought in to correct. When the ancient Egyptians moved on to create the burial chamber ceiling in the next two pyramids, the Meidum and Bent Pyramids, they attempted to use a corbelling technique to overcome the failure of the timber beams. Both of these pyramids have unusual shapes; the Bent Pyramid’s top section sits at a slightly different angle to the main body, giving the structure its ‘bent’ appearance, while the Meidum Pyramid has the appearance of a truncated box sticking out of the ground, rather than the even slopes of the later pyramids.

Corbelling stone and masonry is now a well-known technique in construction. However, the ancient Egyptians were newly using it when building the pyramids. Therefore in both the Meidum and Bent Pyramids, the builders exceeded the overhang needed for the corbel arch to support the weight. This resulted in the burial chamber being squeezed together, and it is this mistake which I believe is the cause of both pyramids’ unusual shapes. The builders rectified it in the construction of the next pyramid, the Red (or North) Pyramid, which is a perfect example of the correct use of corbelling and has a true pyramid shape.


How Were the Pyramids Actually Built?


This progression of knowledge shows that the ancient Egyptian builders were pragmatists, and as such would have always built in the simplest and most efficient way they knew how. As stated earlier, I have found many of the existing theories on how the pyramids were built to be overly complicated and sometimes entirely impractical. I believe that they instead employed much simpler and therefore more viable methods than many current theories propose. It is my opinion that the pyramids were constructed using internal ramps, combined with some additional scaffolding, and not with enormous external ramps, a theory currently favored by many archaeologists.

I believe that the pyramids consist of three different layers (Figure 3). First is the middle core that is visible on every pyramid after the Bent Pyramid. I predict that this layer is only three blocks wide, with the blocks diminishing in size as they near the apex. This layer was used by the Egyptian builders to retain the core filling and would have been a key to connect the outer cladding. The step design of the pyramid meant that the builders were able to connect the cladding to the pyramid while still supporting the weight of the cladding blocks.

Figure 3. Diagram demonstrating the three layers of the pyramid.
From my observations of the burial chamber of the Step Pyramid, I believe that the infill and central core of the pyramid primarily consist of much smaller stones, and any other larger blocks that the builders wanted to conceal. The inner core was used to create internal ramps, which enabled the Egyptians to build the pyramid from the inside out (Figures 4 and 5). The ramps were started at the mid-point of the pyramid and would zigzag across its full internal width, matching the height of the middle-core stones as the pyramid was built. The small number of heavy middle core blocks could have been raised on these internal ramps and positioned at the perimeter of the pyramid. As most of the inner fill stones were much smaller, they could have been easily handled by men and animals.

Figure 4. Stylized diagram of the first stage of construction.
Figure 5. Stylized diagram of the second stage of construction.
The ramps would get steeper as the pyramid grew in height, but they would not exceed the normal angle used to calculate the external ramp gradient. The ramps could have had small palm tree trunks partly embedded into them as a mechanism to slide the heavier core blocks on wooden sledges. As the pyramid reached the apex, more reliance on scaffolding would have been necessary to top out the structure.

The final layer is the outer cladding, which would have been added last and used by the ancient Egyptians to smooth the outer appearance of the pyramid and ensure its ‘true’ pyramid shape using additional stones or tufla grout, like the final icing on a cake.

Some people have been skeptical of any theory involving the use of scaffolding, as they argue that the ancient Egyptians would not have had access to enough timber. My method requires only a small amount of scaffolding in order to attach the outer cladding, and the same scaffolding could have been moved around the pyramid as they worked. In recent restoration work on the pyramids, traditional timber lashed together has been used as scaffolding (Figure 6), which demonstrates that it clearly would have been possible to use scaffolding to construct them in the first place.

Figure 6. Scaffolding in use on the Step Pyramid.

Conclusions


I acknowledge that these are only my theories and not facts. However, there is a way to prove my theory of the layers of the pyramid, and I volunteer to carry out this work at no charge to the Egyptian Antiquities. We could diamond drill 100-millimeter (4-inch) core holes into the pyramid at varying heights to a depth of 30 meters (100 feet) and provide a drilling log of all the contents of the bored hole to establish the true nature of the fill. The drilling would be done with the latest dry drilling techniques to prevent damage to the pyramid, and the core would be plugged and filled to match the external appearance.

The short period of intensive construction by an ancient civilization who managed to build these wonderful monuments was remarkable. One can only admire the great ingenuity and effort that was required by a team of specialist builders, who from the very start showed great ingenuity and the ability to adapt, overcome problems, and learn from their mistakes.▪

Peter James (peterjames@cintec.co.uk), is the Managing Director of Cintec International in Newport, South Wales, United Kingdom. He has worked on projects across the globe, strengthening and restoring historically significant structures from Windsor Castle to the parliament buildings in Canada.
This article is available in Adobe PDF format:

Why Are Roofing Materials Corrugated or Ribbed?

Posted date: 11:37 PM / comment : 2 , ,

Strength

  • The corrugations in roofing materials are created by a process known as roll-forming, which creates a material that is stronger and more rigid than a flat sheet of the same thickness. Corrugated metal roofing, in particular, has a high strength of material to weight ratio.

Protection

  • Provided the pitch, or slope, of a corrugated roof is sufficient, the corrugations provide more effective channeling for the runoff of rainwater than a plain roof. There is a minimum pitch for corrugated sheeting, often around 5 degrees, but this is much lower than the requirement for other roofing materials, such as tiles.

Weight

  • Corrugated roofing materials are relatively lightweight and easy to handle when compared with other roofing materials. This means that they provide a cheaper, more convenient method of roofing and reduce the weight of material above your head; this can be a consideration if you live in an area prone to earth tremors.

Applications of Nanotechnology In Construction

Posted date: 10:58 PM / comment : 0 , ,

Applications of Nanotechnology In Construction

Nano-Particles used with building materials are currently used for producing durable, anti-bacterial, purified air compound paint and green building materials. However, some applications of nanotechnology in construction still remain as an idea such as for construction of high-rise buildings, intelligent infrastructure systems, long-span systems etc.
Lu et al. (1992) produced samples with compressive strength up to 800 MPa. Richard et al. (1994) developed concrete by Reactive Powder Concrete (RPCs) which attained strength ranging from 200 to 800 MPa using nano-particles.
One of the most beneficial application using nano-particles in concrete is producing high-compressive strength concrete equivalent to rock hardness for special applications such as filling the annular space surround instrumentation packages previously placed in drilled bore-holes.
The nano-particles used with fly ash concrete provides more environment friendly cleaner concrete with early high strength of concrete than normal fly ash concrete, which is also economical (Said et al. 2009). Moreover, mitigating problems that face Ferrocement construction (a thin wall reinforced concrete) is another application by Hosseini et al. (2010).
nanotechnology-in-construction
The main and most most important constructions that are benefitted from nanotechnology are:
1) Construction materials with ultra high performance (high durability, high ductility and high strength) such as steel, concrete, polymers and self healing structural composites
2) Embedded structural sensors that could be used for health and moisture content monitoring such as MEMS and intelligent aggregates.
3) New coatings such as self cleaning and corrosion protection coatings.
4) New structural design for infra structures incorporating strong materials with ultrahigh Ductility.
5) New tools that can be used to recognize nano-structure of construction composites behavior.

17,000-Ton Swing Bridge Built Above Railway in China

Posted date: 9:26 PM / comment : 0 ,

The mammoth overpass section was first built separately by Chinese engineers at a site adjacent to the high speed railway line, before being hoisted onto a 15-metre high axis at right angles to the rest of the elevated motorway.
The section was then deftly swivelled into its correct alignment by engineers, covering 106 degrees in the space of just 90 minutes.
Swing bridges are essentially movable bridges that are held in place by a vertical locating pin and a support ring, which enables the structures to swivel around horizontally.
The construction of the overpass marks the first occasion that this sophisticated engineering technique has been employed in modern China.
china swing bridge
Because the primary structural support bears aloft the bridge section either at or close to its centre of gravity, no counterweights are required to facilitate movement, which makes swing bridges significantly lighter than other moveable bridges structures.
Swing bridges are most commonly used to support roads or rail lines that pass over rivers or canals at right angles, enabling trains or automobiles to traverse the water feature as well as boats go past the bridge by alternating the alignment of its span.
In the case of the Wuhan overpass section, however, the swing bridge has been employed as a one-off construction technique  in order to avoid disrupting the high speed railway line which is situated beneath the elevated motorway and which was deemed too critical a transportation route to be temporarily closed.
The Chinese debut of this ingenious building method certainly provided passengers travelling on the high speed line with a stunning spectacle. One photo captured the moment that a high-speed train passed almost directly beneath the overpass as it was meticulously rotated into place by engineers.

OCEA Project Finalists – I-15 Corridor Expansion

Posted date: 8:50 PM / comment : 0

his is the first of a series about the five finalists for ASCE’s Outstanding Civil Engineering Achievement (OCEA) awards. Established in 1960, the OCEA Award recognizes a project that makes a significant contribution to both the civil engineering profession and society as a whole. The winner of this year’s OCEA award will be announced at ASCE’s Outstanding Projects And Leaders (OPAL) Gala, March 20, at the Renaissance Arlington Capital View Hotel in Arlington, Virginia. Today, read about the I-15 Corridor Expansion project.
I-15 Corridor Expansion Project
I-15 Corridor Expansion Project
Like many interstates in the U.S., Interstate 15 in Utah County, Utah, had reached the end of its design life. The infrastructure was deteriorating and needed to be updated; congestion and population growth in the area demanded the freeway be widened.
The Utah Department of Transportation’s (UDOT) $1.725 billion I-15 Corridor Expansion (I-15 CORE) project used innovative procurement, scheduling, and planning techniques to complete the highway expansion project 2 years ahead of schedule while saving taxpayers $260 million. The expanded roadway relieves congestion for motorists who travel to and from Salt Lake City and Provo. By using accelerated bridge construction, wireless paving and grading, and diverging diamond interchanges, the impact to traffic was reduced for the over 130,000 motorists traveling through the I-15 CORE every day.
ASCE News Associate Editor Doug Scott interviewed Brian G. Tolbert, P.E., project manager/infrastructure with the Fluor Corporation, who served as deputy project director of the I-15 CORE project.
1. What is the most innovative or creative aspect of your project?
Accelerated Bridge Construction (ABC) is not entirely new to UDOT projects, on a singular basis. However, PRC [Provo River Contractors, a joint venture of Fluor, Ames Construction, Ralph L. Wadsworth Construction, and Wadsworth Brothers Construction] moved 5 bridges into place using ABC methods over the course of the I-15 CORE project. In particular, our team set a Western Hemisphere record for moving the Sam White Bridge via self-propelled modular transport (SPMT), the longest completed bridge structure and the longest continuous 2-span bridge structure. This bridge is 354 feet long and 80 feet wide, and was coined the “Super Bowl” of bridge moves due to its football-field size and the fact [that] it was moved around the time of the NFL Super Bowl. The Sam White Bridge was only the second multispan bridge structure moved in the Western Hemisphere. The first multispan moved by SPMT was the 200 South bridge, which was moved into place by PRC the prior weekend. The use of large-scale wireless paving was another innovative aspect of this project. The use of cloud computing allowed up-to-date design information to be available nearly real-time for this GPS-guided system.
2. What was the biggest challenge?
Constructing the largest transportation project in Utah history, faster than any other megaproject, while  maintaining all existing lanes of traffic, was the biggest challenge. Essentially, the entire 24-mile stretch was under construction at the same time. Coordinating all of the construction activities, including traffic lane shifts and effective public outreach, was a significant challenge, too. Maintaining the safety of our workers exposed to high levels of traffic adjacent to work zones was also a major issue during construction.
 3. Did your project have any technical issues that you had to overcome? If so, what were they and how did you overcome them?
The entire project area is ancient lake bed. This results in soils that aren’t ideal for construction, particularly when accelerating the project to a record pace. Perhaps the most time-consuming technical issue to resolve relates to the poor subsurface conditions near Utah Lake that affectan approximate 3-mile stretch of the interstate. The proposed design contemplated raising this section of roadway along the lake by approximately 5 feet to exceed the 100-year flood plain elevation. However, this amount of embankment was predicted to induce long-term settlements that would have lasted for years and resulted in significant settlement amounts – up to 2 feet. PRC and UDOT overcame this issue by redesigning the interstate vertical profile to reduce the embankment-inducing settlement, while also designing and constructing earthen berms along the lake border to provide a dam effect and keep the interstate from being submerged in flood events. Also, highway detention areas in the highway alignment were constructed that would function in the event [that] the 100-year flood occurred.
  4. What time and budget challenges did your project have and what did you do to overcome them?
The extremely short construction schedule was the biggest challenge on the project. We had less than 3 years to design and build a $1.1 billion interstate freeway. We brought in 3 of the industry leaders to complete the design using over 300 design engineers in only 14 months. These firms were HDR, Michael Baker, and Jacobs. During the construction phase our team worked double shifts six days [a] week. We also came up with innovative techniques for cold-weather concrete paving and MOT [Maintenance of Traffic] shifts to help facilitate the schedule.
5. Sustainability is one the three initiatives here at ASCE. Describe how your project adheres to being sustainable.
Our project recycled a large majority of the pavement and base materials that were demolished by the project. Instead of going to landfills or other disposal sites, this material was crushed into useable size and composition for embankment and pavement base materials. Slag, a by-product of steel processing, was used extensively throughout the project, which reduced the need for excavation of other sources. A secondary sustainability benefit was the reduction in traffic congestion and delay [through] utilizing innovative MOT strategies that maintained the existing number of lanes open to traffic throughout 90% of the construction schedule. UDOT estimates this reduction in congestion provided over $800 million in user-cost savings.

Timber Tower Research Project

Posted date: 3:28 PM / comment : 0 , ,

Timber Tower Research Project

Benton Johnson, P.E., S.E., David Horos, P.E., S.E., LEED AP and William Baker, P.E., S.E., F. ASCE, FIStructE
The Timber Tower Research Project by Skidmore, Owings & Merrill, LLP (SOM) was publically released in June of 2013, and is available for download at SOM’s website. The goal of the research project was to develop a structural system for tall buildings that uses mass timber as the main structural material and minimizes the embodied carbon footprint of the building. The structural system research was applied to a prototypical building based on an existing concrete benchmark for comparison. The concrete benchmark building is the Dewitt-Chestnut Apartments, a 395-foot tall, 42-story building in Chicago designed by SOM and built in 1966.

SOM’s proposed system is the "Concrete Jointed Timber Frame". This system relies primarily on mass timber for the main structural elements, with supplementary reinforced concrete at the highly stressed locations of the structure: the connecting joints. This system plays to the strengths of both materials and allows the structural engineer to apply sound tall building engineering fundamentals. The result is believed to be an efficient structure that could compete with reinforced concrete and structural steel systems, while reducing the embodied carbon footprint of the structure by 60 to 75%.


Project Basis


The basis of the research project was rooted in sustainable urban development. Recent population projections have estimated the current world population of 7.0 billion people to increase to 11.0 billion people by the year 2050. More importantly, the number of people that will be living in cities has been estimated to double from 3.5 billion people to 7.0 billion people in the same time frame. Tall buildings will likely be needed in order to house that many additional people in growing cities. Tall buildings constructed to meet population demands need to be developed in sustainable ways to limit environmental impacts.

Tall buildings built using current technology and materials pose a challenge to sustainable city development because they offer both positive and negative environmental impacts. Positive impacts include reducing urban sprawl, promoting alternative transportation, and efficient energy use. These benefits come at the cost of emitting more carbon dioxide to produce the materials and to construct the building. These carbon emissions are referred to as the embodied carbon footprint of a building. A tall building’s embodied carbon footprint is significantly higher relative to low-rise buildings on a per square foot basis. This is because the structure is usually responsible for the majority of the building’s embodied carbon footprint, and tall buildings require far more structure to support their height. The structural system chosen for a tall building can have a significant impact on the overall embodied carbon footprint of the building.

Architectural detail of the wood structure proposed in the Timber Tower Research Report.

Design and Sustainability Issues


Structural engineers currently have four primary materials in which to design buildings: steel, concrete, masonry, and wood. Tall buildings currently use steel or concrete almost exclusively, for two reasons. First, with some limited exceptions, non-combustible materials are required by most building codes for buildings greater than four stories tall. Second, steel and concrete have higher material strengths than masonry and wood, making them a natural choice for tall buildings which require support of very large loads. These factors have generally limited wood use to low-rise buildings. Recently, developments in mass timber technology are overcoming these challenges. Mass timber products such as cross-laminated timber (CLT) can be built up using small pieces of dimensional lumber and structural adhesives to achieve panels as large as 1foot thick and 40 feet long. These panels can be used as floors and shear walls with structural sizes necessary to support a tall wooden building. Wood members of this size have an equally important characteristic; they behave like heavy timbers in a fire and form an insulating char layer which protects underlying material. The charring behavior is predictable and preserves a portion of the member’s structural strength, making performance based fire design of mass timber structures possible. Mass timber has made wood a viable choice for multi-story buildings as evidenced by completed projects in Europe and Australia, and many other proposed projects around the globe.

The structural and fire engineering advancements of mass timber have made recent multi-story wood buildings possible. However, the sustainability of wood seems to be an equally important consideration in the resurgence of multi-story timber buildings. Wood has been shown to be more sustainable than other materials because it generally requires less energy to produce compared to structural steel and reinforced concrete. More importantly, wood is approximately 50% carbon by weight, a carbon sink that is the natural result of photosynthesis. These sustainable aspects of wood make mass timber an attractive material from which to construct the sustainable cities of the future. The intersection of increasing urban populations, need for tall buildings, and the sustainability of wood has led to the increasingly popular concept of tall wood buildings. SOM has committed decades of tall building design expertise to furthering this concept, through the Timber Tower Research Project, by identifying key design and construction issues related to tall wood buildings and proposing the "Concrete Jointed Timber Frame" structural system. This system is optimized for tall buildings and could be competitive with existing tall building structural systems. The proposed system balances the requirements of building marketability, economy, and sustainability.

SOM’s Timber Tower Research Project proposes a "Concrete Jointed Timber Frame" system that relies primarily on mass timber for the main structural elements, with supplementary reinforced concrete at the connecting points.

Material Optimization


The primary goal of any structural system is to provide a marketable and valuable building to the owner and occupants. A marketable building must have adequate and flexible floor area to layout useful space for the occupants. The most marketable building layout is an open floor plan which allows a variety of room layouts and maximum flexibility for future changes. An open floor layout requires that the floor structure span the entire distance of the leasable area. This distance in the Benchmark Dewitt-Chestnut building was 28 feet 6 inches, with a clear span of 26 feet 3 inches. The most advantageous system to span this distance is a flat mass timber panel which minimizes floor-to-floor height of the building. The required panel thickness to span the required distance was determined to be 13½ inches. This thickness was thought to be too great compared to the material required for the Reinforced Concrete Benchmark to be economically viable. Therefore, alternative methods to span the required distance were investigated in order to reduce the amount of structural materials used.

The controlling design consideration for the mass timber floors was determined to be vibration due to occupant activity. The floors were analyzed according to American Institute of Steel Construction Design Guide 11, utilizing the velocity-based methodology, which was found to be more useful for flat slab-type floors. Evaluation of the criteria shows that increasing floor stiffness is the most effective way to control vibrations. The floor stiffening effect of end rotation restraint (fixed end condition) was quickly realized as an efficient way to reduce vibrations. It was determined that an 8-inch-thick mass timber floor panel could be used if end restraint was provided. This requires moment connections at the intersection of mass timber floor panels with vertical elements such as mass timber shear walls and structural glued laminated timber perimeter columns. Several connection schemes were investigated to provide the required moment connections. Steel reinforcing epoxy connected to the mass timber and cast-in reinforced concrete joints were determined to be the most reasonable solutions due to the ability of reinforced concrete to resist complex load paths. These reinforced concrete joints are able to resist floor-to-floor compression, shear, bending moments, and torsion, thus creating an efficient composite-timber system.

The reinforced concrete joints also proved to be useful in other tall building aspects. The concrete jointing between timber floors and timber shear walls provides a link beam between individual wall panels. This creates a stiff lateral load resisting system which is required for a tall building. It was also determined that the demands on the link beams were beyond the capacity of a structural glued laminated wooden link beam, requiring the use of a material other than wood. The concrete joints and link beams were also useful in the design of the lateral system to resist net uplift due to lateral loads. The Prototypical Building has approximately 40% of the dead load of the Benchmark Building. This led to net uplift forces at the extremities of the lateral load resisting system. This net uplift would have been exacerbated without the concrete joints which account for over 50% of the entire structure dead load, yet only 20% of the structural material volume for a typical floor.

A comparison of the structural materials required to construct the Benchmark and Prototypical building shows that the proposed system is very efficient in material consumption and could be competitive with reinforced concrete. The goal of minimizing the structural materials used, namely mass timber, will help reduce costs and minimize new demands on forest resources which may become strained due to increasing populations and demands.

The non-structural effects of the proposed system were evaluated and the most notable effect was the acoustic treatment required on top of the mass timber floors in order to achieve a marketable acoustic rating. The most effective treatment was determined to be a 2-inch-thick gypsum concrete topping. This treatment thickness, in addition to potential ceiling finishes, required 3 inches of additional floor-to-floor height in order to maintain the same floor-to-ceiling height as the Benchmark building. This has impacts on wind loads on the building, and non-structural costs such as the exterior wall system.


Conclusion


SOM believes that the proposed system is technically feasible from the standpoint of structural engineering, architecture, interior layouts, and building services. Additional research and physical testing is necessary to verify the actual performance of the structural system relative to the theoretical behavior. SOM has also developed the system with consideration for constructability, cost, and fire protection. Reviews from experts in these fields, and physical testing related to fire, is also required before this system can be fully implemented in the market. Lastly, the design community must continue to work creatively with forward thinking municipalities and code officials using the latest in fire engineering and performance based design to make timber buildings a viable alternative for more sustainable tall buildings.▪
Benton Johnson, P.E., S.E., is an Associate at Skidmore Owings & Merrill LLP, Chicago, IL. He is the Project Engineer on the Timber Tower Research Project and can be reached at benton.johnson@som.com.

David Horos, P.E., S.E., LEED AP, is a Director at Skidmore Owings & Merrill LLP, Chicago, IL. Timber Tower Research Project and can be reached at david.horos@som.com.

William F. Baker, P.E., S.E., F. ASCE, FIStructE is the Structural Engineering Partner for Skidmore, Owings & Merrill LLP. Bill has dedicated himself to structural innovation, most notably developing the "buttressed core" structural system for the Burj Khalifa, the world’s tallest manmade structure. He is a Fellow of both the ASCE and the IStructE and a member of the National Academy of Engineering.
This article is available in Adobe PDF format:

PDF Timber Tower Research Project

Concrete-Eating Robot Recycles Buildings

Posted date: 9:50 PM / comment : 0 , , ,

Concrete-Eating Robot Recycles Buildings

Building demolition demands a lot of heavy machinery to crush concrete and separate valuable materials for reuse. Often, those materials are transferred to offsite locations, which wastes time and resources. The process also wastes a lot of water in order to prevent harmful dust clouds from blooming. However, a Swedish student’s concrete-eating robot aims to change all that.
“The ERO Concrete Recycling Robot was designed to efficiently disassemble concrete structures without any waste, dust or separation and enable reclaimed building materials to be reused for new prefabricated concrete buildings,” explained Omer Haciomeroglu of the Umea Institute of Design of Design. ”It does so by using a water jet to crack the concrete surface, separate the waste and package the cleaned, dust-free material.”
The idea is to send in a fleet of the ERO robots that will scan buildings to determine the best route to execute demolition. Once the robot goes to work, using vacuum suction and electrical power, it erases the building.
“ERO deconstructs with high-pressure water and sucks and separates the mixture of aggregate, cement and water. It then sends aggregate and filtered cement slurry separately down to the packaging unit to be contained,” Haciomeroglu wrote. ”Clean aggregate is packed into big bags, which are labeled and sent to nearby concrete precast stations for reuse. Water is recycled back into the system.”
Turbulence dynamos strategically placed inside air suction chambers even provide a percentage of ERO’s energy needs. Once the last wall has been demolished, essentially nothing has gone to landfills or been sent away for additional processing.
“Even the rebar is cleaned of concrete, dust and rust and is ready to be cut and reused immediately,” Haciomeroglu stated. “Every bit of the load-bearing structure is reusable for new building blocks.”
So far the design remains a concept, but influential organizations are starting to take note. Last year, Haciomeroglu’s concept won in the Student Designs category of the International Design Excellence Awards.

The Progression of High Strength Concrete

Posted date: 6:00 PM / comment : 0 , ,

The Progression of High Strength Concrete

The definition of high strength concrete continues to change. This change occurs as the art of achieving a particular strength is reduced to practice, and the structural requirements push at the edge with needs for higher strength. One such example is the CN Tower in Toronto, with its required strength in 1976 of 5000 psi. At that time, this was difficult to achieve. Today 5000 psi concrete is routinely used and produced without special precautions.

In the manufacturing of high strength concrete, there are significant differences from those seen in practice just a few years ago. Cementitious components and content, admixtures aggregates and curing have changed. Once the province of high cement contents and silica fume, much of the developments over the last decade have revolved around a better understanding of, and attention paid to, the microstructure of the concrete. High strength concrete can be modeled as a three phase system -- the paste, the aggregate and the interface between them (Figures 1 and 2). By taking this approach, an engineered composite material can be designed.

Figure 1: Thin section of concrete showing microstructure of paste. Red arrows are typical flyash particles.
Figure 2: Thin section of concrete showing microstructure of paste. Note the relatively low degree of hydration of the slag particles (red arrows).
Actions taken to modify the interfacial transition zone between the aggregate and paste have increased the load transfer between the paste and aggregate; thereby increasing the strength of the concrete. It is the action of meta-kaolin, silica fume and other finely divided materials in modifying this interfacial transition zone that originally led to significant increases in strength. These materials were once used at high replacement levels, frequently greater than 10%. While high strength was often achieved, the workability and susceptibility to fracture of the concrete were problems that ultimately limited the strength.

In modern high strength concrete, blends of smaller quantities and fractions of silica fume result in large increases in strength without compromising the ability of the mixtures to be placed. In many cases, ternary or even quaternary blends of pozzolanic material with Portland cement are seen in practice.

One of the reasons for these blends is that the heat generated during the hydration process can cause residual stress within the paste, and reduce the strength of the concrete. While these types of effects can readily be removed in metals by annealing, no such process is available for concrete. The curing process must be engineered to control the hydration reaction so residual stresses are minimized. Temperature monitoring or other devices are used to track the progress and monitor the reaction that produces the binder.

The literature at the turn of the 20th century often referred to the curing process as annealing. While having very little to do with the concept of heat treating of metals, this curing, when performed properly, is a critical factor in the performance of higher strength concrete. Strengths up to 20,000 psi have been realized using pozzolanic materials, dispersants, limestone modified cements and careful attention to aggregate materials selection. The limestone acts to nucleate the reaction and reduces the quantity of unhydrated-cement.

Admixture technology has progressed. Stabilizing admixtures and dispersants with a low affinity for the solid surface, where a large fraction of dispersant remains in solution, has allowed mixtures to be held in a state of "suspended animation" while the concrete is placed. They can perform predictably, allowing scheduling of the construction process. Retarders, dispersants and stabilizers will increase the strength.

One of the difficulties that the designer of high strength concrete mixtures encounters is the ability to have workable concrete with very low water/cement ratios. Use of modern high-efficiency dispersants (super plasticizers) has led to observed autogeneous drying of materials due to hydration or vaporation due to high temperature. As a consequence, some very low water/cement ratio concrete have shown good performance in the laboratory but poor performance in the larger structural members, where the heat of hydration is not as readily dissipated and where the sample is not immersed in water for 28 or even 56 days. These limits are advancing by innovations such as the use of lightweight aggregate for internal curing and steel whiskers to distribute stresses.

Aggregate materials are no different. As the paste strength increases, and the interfacial transition zone densifies, the strength of the aggregate or the presence of fractures therein become a limiting factor. Reducing the maximum particle size and carefully selecting the geological origin of the materials can lead to significant improvements in strength.

Recognition by the design and construction team that the concrete strength does not need to be achieved at seven or 28 or even 56 days, but only as the structure is loaded, allows mixtures that have relatively low cement contents and very high pozzolanic replacement to achieve compressive strengths in excess of 15,000 psi.

Care must be taken in the production of high strength concrete in order to ensure that the performance of the concrete in situ is what was intended in the design. Raw materials, batching and handling at the plant and at the installation site must be controlled. Without an understanding of the importance of high strength concrete at the production plant and by the individual vehicle drivers, it will be more difficult to achieve performance of the structure.▪

Kevin A. MacDonald, FACI, is President and Principal Engineer for Beton Consulting Engineers LLC, Mendota Heights, MN. He was named a Fellow of ACI in 2004. Kevin can be reached at kmacdonald@betonconsulting.com.
This article is available in Adobe PDF format:

Prefabricated Construction Method

Posted date: 11:01 AM / comment : 0 , , ,

Prefabricated Construction Method

Prefabricated Construction Method
Prefabricated construction is a building process in which elements or modules of the structure are prefabricated at plants, then transported to the construction site for installation. Using this method can reduce  the time of building, also saving construction cost. Prefabricated construction is now widely applied for new houses or other building structures like bridge, tunnels,  culverts, water supply system…
The benefits of prefabricated construction method is from the  fabrication of standard components on factory floor. This  production is less time consumption compared to actual condition of construction process. The prefabricated elements are transported to the site for installing process. At the site, the modules are unloaded, moved into position with the support of heavy cranes, and assembled to form a designed building.
Together with the fast assembly, prefabricated construction also saves a lot of money on the construction project. By using standard patterns, the building materials are saved at the manufacturing factories. This help to reduce the waste in formwork and other materials that can occur during traditional building procedures.
Another considerable  profit using prefabricated construction method is the energy efficiency. Because the prefab elements of a panelized home are precut, they fit snugly together, making for a tighter edifice. This means less effort for heating and cooling, resulted  in lower energy bills.
The rapid development of prefabricated houses has led to the increasing of construction templates that homeowners have more choice for designs of their houses. By combining these templates, it is possible to design the layout of the house, specify the dimensions of each room, and build a home that is exactly to the specification of the owners. There are also complex building plans for prefabricated construction that can be adjusted slightly and still have the benefit of using materials of standard lengths, widths, and textures.
Prefabricated houses are not the only type of construction structures that can be produced using prefabrication construction method. As mentioned above, this method is widely used in many types of constructions like bridges, culverts or even swimming pools.

Reactive Powder Concrete.

Posted date: 10:37 PM / comment : 0 , ,

Reactive Powder Concrete (RPC) is a developing composite material that will allow the concrete industry to optimize material use, generate economic benefits, and build structures that are strong, durable, and sensitive to environment. A comparison of the physical, mechanical, and durability properties of RPC and HPC (High Performance Concrete) shows that RPC possesses better strength (both compressive and flexural) and lower permeability compared to HPC. This page reviews the available literature on RPC, and also presents the results of laboratory investigations comparing RPC with HPC. Specific benefits and potential applications of RPC have also been described.
High-Performance Concrete (HPC) is not just a simple mixture of cement, water, and aggregates. It contains mineral components and chemical admixtures having very specific characteristics, which give specific properties to the concrete. The development of HPC results from the materialization of a new science of concrete, a new science of admixtures and the use of advanced scientific equipments to monitor concrete microstructure.


HPC has achieved the maximum compressive strength in its existing form of microstructure. However, at such a level of strength, the coarse aggregate becomes the weakest link in concrete. In order to increase the compressive strength of concrete even further, the only way is to remove the coarse aggregate. This philosophy has been employed in Reactive Powder Concrete (RPC)1.

Reactive Powder Concrete (RPC) was developed in France in the early 1990s and the world’s first Reactive Powder Concrete structure, the Sherbrooke Bridge in Canada, was erected in July 1997. Reactive Powder Concrete (RPC) is an ultra high-strength and high ductility cementitious composite with advanced mechanical and physical properties. It consists of a special concrete where the microstructure is optimized by precise gradation of all particles in the mix to yield maximum density. It uses extensively the pozzolanic properties of highly refined silica fume and optimization of the Portland cement chemistry to produce the highest strength hydrates1.
The concept of reactive powder concrete was first developed by P. Richard and M. Cheyrezy and RPC was first produced in the early 1990s by researchers at Bouygues’ laboratory in France2. A field application of RPC was done on the Pedestrian/Bikeway Bridge in the city of Sherbrooke, Quebec, Canada3. RPC was nominated for the 1999 Nova Awards from the Construction Innovation Forum. RPC has been used successfully for isolation and containment of nuclear wastes in Europe due to its excellent impermeability4.
The requirements for HPC used for the nuclear waste containment structures of Indian Nuclear Power Plants are normal compressive strength, moderate E value, uniform density, good workability, and high durability5. There is a need to evaluate RPC regarding its strength and durability to suggest its use for nuclear waste containment structures in Indian context. 
Composition of Reactive Powder Concrete
RPC is composed of very fine powders (cement, sand, quartz powder and silica fume), steel fibres (optional) and superplasticizer. The superplasticizer, used at its optimal dosage, decreases the water to cement ratio (w/c) while improving the workability of the concrete. A very dense matrix is achieved by optimizing the granular packing of the dry fine powders. This compactness gives RPC ultra-high strength and durability6. Reactive Powder Concretes have compressive strengths ranging from 200 MPa to 800 MPa.
Richard and Cheyrezy1 indicate the following principles for developing RPC:
  1. Elimination of coarse aggregates for enhancement of homogeneity
  2. Utilization of the pozzolanic properties of silica fume
  3. Optimization of the granular mixture for the enhancement of compacted density
  4. The optimal usage of superplasticizer to reduce w/c and improve workability
  5. Application of pressure (before and during setting) to improve compaction
  6. Post-set heat-treatment for the enhancement of the microstructure
  7. Addition of small-sized steel fibres to improve ductility
Table 1 lists salient properties of RPC, along with suggestions on how to achieve them. Table 2 describes the different ingredients of RPC and their selection parameters. The mixture design of RPC primarily involves the creation of a dense granular skeleton. Optimization of the granular mixture can be achieved either by the use of packing models7 or by particle size distribution software, such as LISA8 [developed by Elkem ASA Materials]. For RPC mixture design an experimental method has been preferred thus far. Table 3 presents various mixture proportions for RPC obtained from available literature1,3,9,10.

Table 1: Properties of RPC enhancing its homogeneity and strength
Property of
RPC		DescriptionRecommended ValuesTypes of failure eliminated

Reduction in
aggregate size		Coarse aggregates are replaced by fine sand, with a reduction in the size of the coarsest aggregate by a factor of about 50.Maximum size of fine sand is 600 µm
Mechanical,
Chemical &
Thermo-mechanical 
Enhanced mechanical propertiesImproved mechanical properties of the paste by the addition of silica fumeYoung’s modulus values in 50 GPa – 75 Gpa rangeDisturbance of the mechanical stress field.
Reduction in aggregate to matrix ratioLimitation of sand contentVolume of the paste is at least 20% greater than the voids index of non-compacted sand.
By any external source (e.g., formwork).
Table 2: Selection Parameters for RPC components
ComponentsSelection ParametersFunctionParticle SizeTypes
SandGood hardness
Readily available and low cost.
Give strength,
Aggregate		150 µm
to
600 µm		Natural,
Crushed
CementC3 S : 60%;
C2S : 22%;
C3A : 3.8%;
C4AF: 7.4%.
(optimum)		Binding material,
Production of primary hydrates		1 µm
to
100 µm		OPC,
Medium
fineness
Quartz PowderfinenessMax. reactivity during heat-treating5 µm
to
25 µm		Crystalline
Silica fumeVery less quantity of impuritiesFilling the voids,
Enhance rheology,
Production of secondary hydrates		0.1 µm
to
1 µm		Procured from Ferrosilicon industry
(highly refined)
Steel fibresGood aspect ratioImprove ductilityL : 13 – 25 mm
Ø : 0.15 – 0.2 mm		Straight
SuperplasticizerLess retarding characteristicReduce w/c_Polyacrylate based
Table 3: RPC mixture designs from literature
P. Richard and M. Cheyrezy1S. A. Bouygues3V. Matte9S. Staquet10
[1995][1997][1999][2000]
Non fibred12 mm fibres25 mm fibresFibredFibred
Portland Cement1111111
Silica fume0.250.230.250.230.3240.3250.324
Sand1.11.11.11.11.4231.431.43
Quartz Powder--0.39--0.390.2960.30.3
Superplasticizer0.0160.0190.0160.0190.0270.0180.021
Steel fibre----0.1750.1750.2680.2750.218
Water0.150.170.170.190.2820.20.23
Compacting pressure--------------
Heat treatment temperature20ºC90ºC20ºC90ºC90ºC90ºC90ºC

The major parameter that decides the quality of the mixture is its water demand (quantity of water for minimum flow of concrete). In fact, the voids index of the mixture is related to the sum of water demand and entrapped air. After selecting a mixture design according to minimum water demand, optimum water content is analyzed using the parameter relative density (d0/dS). Here d0 and dS represent the density of the concrete and the compacted density of the mixture (no water or air) respectively. Relative density indicates the level of packing of the concrete and its maximum value is one. For RPC, the mixture design should be such that the packing density is maximized.
Microstructure enhancement of RPC is done by heat curing. Heat curing is performed by simply heating (normally at 90°C) the concrete at normal pressure after it has set properly. This considerably accelerates the pozzolanic reaction, while modifying the microstructure of the hydrates that have formed1. Pre-setting pressurization has also been suggested as a means of achieving high strength1.
The high strength of RPC makes it highly brittle. Steel fibres are generally added to RPC to enhance its ductility. Straight steel fibres used typically are about 13 mm long, with a diameter of 0.15 mm. The fibres are introduced into the mixture at a ratio of between 1.5 and 3% by volume1. The cost-effective optimal dosage is equivalent to a ratio of 2% by volume, or about 155 kg/m3.
Mechanical Performance and Durability of RPC
The RPC family includes two types of concrete, designated RPC 200 and RPC 800, which offer interesting implicational possibilities in different areas. Mechanical properties for the two types of RPC are given in Table 4. The high flexural strength of RPC is due to the addition of steel fibres.

Table 5 shows typical mechanical properties of RPC compared to a conventional HPC of compressive strength 80 MPa11. As fracture toughness, which is a measure of energy absorbed per unit volume of material to fracture, is higher for RPC, it exhibits high ductility. Apart from their exceptional mechanical properties, RPCs have an ultra-dense microstructure, giving advantageous waterproofing and durability characteristics. These materials can therefore be used for industrial and nuclear waste storage facilities1.
RPC has ultra-high durability characteristics resulting from its extremely low porosity, low permeability, limited shrinkage and increased corrosion resistance. In comparison to HPC, there is no penetration of liquid and/or gas through RPC4. The characteristics of RPC given in Table 6, enable its use in chemically aggressive environments and where physical wear greatly limits the life of other concretes12.

Table 4: Comparison of RPC 200 and RPC 800
RPC 200RPC 800
Pre-setting pressurizationNone50 MPa
Heat-treating20 to 90°C250 to 400°C
Compressive strength (using quartz sand)170 to 230 MPa490 to 680 MPa
Compressive strength (using steel aggregate)--650 to 810 MPa
Flexural strength30 to 60 MPa45 to 141 MPa
Table 5: Comparison of HPC (80 MPa) and RPC 2009

PropertyHPC (80 MPa)
RPC 200
Compressive strength80 MPa200 MPa
Flexural strength7 MPa40 MPa
Modulus of Elasticity40 GPa60 GPa
Fracture Toughness<10³ J/m²30*10³ J/m²

Table 6: Durability of RPC Compared to HPC10

Abrasive Wear2.5 times lower
Water Absorption7 times lower
Rate of Corrosion8 times lower
Chloride ions diffusion25 times lower

Limitations of RPC
In a typical RPC mixture design, the least costly components of conventional concrete are basically eliminated or replaced by more expensive elements. The fine sand used in RPC becomes equivalent to the coarse aggregate of conventional concrete, the Portland cement plays the role of the fine aggregate and the silica fume that of the cement. The mineral component optimization alone results in a substantial increase in cost over and above that of conventional concrete (5 to 10 times higher than HPC). RPC should be used in areas where substantial weight savings can be realized and where some of the remarkable characteristics of the material can be fully utilized2. Owing to its high durability, RPC can even replace steel in compression members where durability issues are at stake (e.g. in marine condition). Since RPC is in its developing stage, the long-term properties are not known.
Experimental study at IIT Madras
Materials Used
The materials used for the study, their IS specifications and properties have been presented in Table 7.
Mixture Design of RPC and HPC
  • Considerable numbers of trial mixtures were prepared to obtain good RPC and HPC mixture proportions.
  • Particle size optimization software, LISA8 [developed by Elkem ASA Materials] was used for the preparation of RPC and HPC trial mixtures.
  • Various mixture proportions obtained from the available literature were also studied.
  • The selection of best mixture proportions was on the basis of good workability and ideal mixing time.
  • Finalized mixture proportions of RPC and HPC are shown in Table 8.
Table 7: Materials used in the study and their properties
Sl. No.SampleSpecific GravityParticle size range
1Cement, OPC, 53-grade
[IS. 12269 – 1987]		3.1531 µm – 7.5 µm
2Micro Silica
[ASTM C1240 – 97b]		2.25.3 µm – 1.8 µm
3Quartz Powder2.75.3 µm – 1.3 µm
4Standard sand, grade-1
[IS. 650 – 1991]		2.652.36 mm – 0.6 mm
5Standard sand, grade-2
[IS. 650 – 1991]		2.650.6 mm – 0.3 mm
6Standard sand, grade-3
[IS. 650 – 1991]		2.650.5 mm – 0.15 mm
7Steel fibres (30 mm)
[ASTM A 820 – 96]		7.1length: 30 mm & dia: 0.4 mm
8Steel fibres (36 mm)
[ASTM A 820 – 96]		7.1length: 36 mm & dia: 0.5 mm
920 mm Aggregate
[IS. 383 – 1970]		2.7825 mm – 10 mm
1010 mm Aggregate
[IS. 383 – 1970]		2.7812.5 mm – 4.75 mm
11River Sand
[IS. 383 – 1970]		2.612.36 mm – 0.15 mm
Table 8: Mixture Proportions of RPC and HPC
MaterialsMixture Proportions
RPCRPC-F*HPCHPC-F**
Cement1.001.001.001.00
Silica fume0.250.250.120.12
Quartz powder0.310.31--
Standard sand grade 21.091.09--
Standard sand grade 30.580.58--
River Sand--2.402.40
20 mm aggregate--1.401.40
10 mm aggregate--1.501.50
30 mm steel fibres-0.20--
36 mm steel fibres---0.20
Admixture (Polyacrylate based)0.030.030.0230.023
Water0.250.250.40.4
* Fibre RPC     ** Fibre HPC
Workability and density were recorded for the fresh concrete mixtures. Some RPC specimens were heat cured by heating in a water bath at 90°C after setting until the time of testing. Specimens of RPC and HPC were also cured in water at room temperature.
The performance of RPC and HPC was monitored over time with respect to the following parameters:
Compressive Strength (as per IS 51613 on 5 cm cubes for RPC, 10 cm cubes for HPC), Flexural Strength (as per IS 516 on 4 x 4 x 16 cm prisms for RPC, 10 x 10 x 50 cm beams for HPC),
Water Absorption (on 15 cm cubes for both RPC and HPC),
Non destructive water permeability test using Germann Instruments (on 15 cm cubes for both RPC and HPC),
Resistance to Chloride ions Penetration test (on discs of diameter 10 cm and length 5 cm as per ASTM C 120214).
Results
Fresh concrete properties
The workability of RPC mixtures (with and without fibres), measured using the mortar flow table test as per ASTM C10915, was in the range of 120 – 140%. On the other hand, the workability of HPC mixtures (with and without fibres), measured using the slump test as per ASTM C23116, was in the range of 120 – 150 mm. The density of fresh RPC and HPC mixtures was found to be in the range of 2500 – 2650 kg/m3.
Compressive strength
The compressive strength analysis throughout the study shows that RPC has higher compressive strength than HPC, as shown in Fig. 1. Compressive strength at early ages is also very high for RPC. Compressive strength is one of the factors linked with the durability of a material. In the context of nuclear waste containment materials, the compressive strength of RPC is higher than required.
Fig 1: Compressive strength of RPC and HPC
he maximum compressive strength of RPC obtained from this study is as high as 200 MPa, while the maximum strength obtained for HPC is 75 MPa. The incorporation of fibres and use of heat curing was seen to enhance the compressive strength of RPC by 30 – 50%. The incorporation of fibres did not affect the compressive strength of HPC significantly.
Flexural strength
Plain RPC was found to possess marginally higher flexural strength than HPC. Table 9 clearly explains the variation in flexural strength of RPC and HPC with the addition of steel fibres. Here the increase of flexural strength of RPC with the addition of fibres is higher than that of HPC.
Table 9: Flexural strength (as per IS 516) at 28 days (MPa)
RPCRPC-FHPCHPC-F
NC*HWC**NC*HWC**NC*NC*
11121822810
*Normal Curing    **Hot Water Curing
As per literature3, RPC 200 should have an approximate flexural strength of 40 MPa. The reason for low flexural strength obtained in this study could be that the fibres used (30 mm) were long. Fibre reinforced RPC (with appropriate fibres) has the potential to be used in structures without any additional steel reinforcement. This cost reduction in reinforcement can compensate the increase in the cost by the elimination of coarse aggregates in RPC to a little extent.

Water absorption
Fig. 2 presents a comparison of water absorption of RPC and HPC. A common trend of decrease in the water absorption with age is seen here both for RPC and HPC. The percentage of water absorption of RPC, however, is very low compared to that of HPC. This quality of RPC is one among the desired properties of nuclear waste containment materials.
Fig. 2: Water absorption of RPC and HPC

The incorporation of fibres and the use of heat curing is seen to marginally increase the water absorption. The presence of fibres possibly leads to the creation of channels at the interface between the fibre and paste that promote the uptake of water. Heat curing , on the other hand, leads to the development of a more open microstructure (compared to normal curing) that could result in an increased absorption.
Water permeability
The non-destructive assessment of water permeability using the Germann Instruments equipment actually only measures the surface permeability, and not the bulk permeability like in conventional test methods. A comparison of the surface water permeability of RPC and HPC is shown in Fig. 3.
It can be seen from the data that water permeability decreases with age for all mixtures. 28th day water permeability of RPC is negligible when compared to that of HPC (almost 7 times lower). As in the case of water absorption, the use of fibres increases the surface permeability of both types of concrete.
Fig. 3: Surface Water Permeability of RPC and HPC

Resistance to chloride ion penetration
Results of rapid chloride permeability test conducted after 28 days of curing are presented in Table 10. Data indicate that penetration of chloride increases when heat curing is done in concrete. Total charge passed for normal-cured RPC is negligible compared to the other mixtures. Even though heat-cured RPC shows a higher value than normal-cured RPC, in absolute terms, it is still extremely low or even negligible (<100 Coulombs). This property of RPC enhances its suitability for use in nuclear waste containment structures.
The data also indicate that addition of steel fibres leads to an increase in the permeability, possibly due to increase in conductance of the concrete. The HPC mixtures also showed very low permeability, although higher compared to RPC.
Table 10: Rapid Chloride Permeability Test (as per ASTM C 1202)
RPCRPC with fibresHPC
NC*HWC**NC*HWC**NC*HWC*
Cumulative Charge passed in Coulombs4
(less than 10)		94140400250850
ASTM C1202 classificationNegligibleNegligibleVery lowVery lowVery lowVery low
*Normal Curing     **Hot Water Curing
Summary
Reactive Powder Concrete (RPC) is an emerging technology that lends a new dimension to the term ‘high performance concrete’. It has immense potential in construction due to its superior mechanical and durability properties compared to conventional high performance concrete, and could even replace steel in some applications.
The development of RPC is based on the application of some basic principles to achieve enhanced homogeneity, very good workability, high compaction, improved microstructure, and high ductility. RPC has an ultra-dense microstructure, giving advantageous waterproofing and durability characteristics. It could, therefore, be a suitable choice for industrial and nuclear waste storage facilities.
A laboratory investigation comparing RPC and HPC led to the following conclusions:
  • A maximum compressive strength of 198 MPa was obtained. This is in the RPC 200 range (175 MPa – 225 MPa).
  • The maximum flexural strength of RPC obtained was 22 MPa, lower than the values quoted in literature (~ 40 MPa). A possible reason for this could be the higher length of fibres used in this study.
  • A comparison of the measurements of the physical, mechanical, and durability properties of RPC and HPC shows that RPC possesses better strength (both compressive and flexural) and lower permeability compared to HPC.
  • The extremely low levels of water and chloride ion permeability indicate the potential of RPC as a good material for storage of nuclear waste. However, RPC needs to be studied with respect to its resistance to the penetration of heavy metals and other toxic wastes emanating from nuclear plants (such as Cesium 137 ion in alkaline medium) to qualify for use in nuclear waste containment structures.
References
  1. Richard P, and Cheyrezy M, “Composition of Reactive Powder Concrete”, Cement and Concrete Research, Vol. 25, No.7, (1995), pp. 1501 – 1511.
  2. Aitcin P.C, “Cements of yesterday and today Concrete of tomorrow”, Cement and Concrete Research, Vol. 30, (2000), pp 1349 - 1359.
  3. Blais P. Y, and Couture M, “Precast, Prestressed Pedestrian Bridge - World’s first reactive powder concrete structure”, PCI Journal, Vol. 44, (1999), pp. 60 - 71.
  4. Dauriac C, “Special Concrete may give steel stiff competition, Building with Cincrete”, The Seattle Daily Journal of Commerce, May 9, 1997.
  5. Basu P.C, “Performance Requirements of HPC for Indian NPP Structures”, The Indian Concrete Journal, Sep. 1999, pp. 539 – 546.
  6. Bonneau O, Vernet C, Moranville M, and Aitcin P. C, “Characterization of the granular packing and percolation threshold of reactive powder concrete”, Cement and Concrete Research, Vol. 30 (2000) pp. 1861 – 1867.
  7. Goltermann P, Johansen V, and Palbol L, “Packing of Aggregates: An Alternative Tool to Determine the Optimal Aggregate Mix”, ACI Materials Journal, Sep-Oct. 1997, pp. 435 – 443.
  8. Elkem AS website – http://www.silicafume.net/
  9. Matte V and Moranville M, “Durability of Reactive Powder Composites: Influence of Silica Fume on the leaching properties of very low water/binder pastes”, Cement and Concrete Composites, 21 (1999) pp. 1 - 9.
  10. Staquet S, and Espion B, “Influence of Cement and Silica Fume Type on Compressive Strength of Reactive Powder Concrete”, 6th International Symposium on HPC, University of Brussels, Belgium, (2000), pp. 1 – 14.
  11. Bickley J. A, and Mitchell D, “A State-of-the-Art Review of High Performance Concrete Structures Built in Canada: 1990-2000”, (2001), pp. 96 – 102.
  12. HDR Engineering Website on Reactive Powder Concrete, Last modified Nov. 1999, http://www.hdrinc.com/engineering/engres.htm
  13. Indian Standard Designation IS 516-1959, “Methods of Test for Strength of Concrete,” BIS, New Delhi, 2002.
  14. ASTM Standard Designation C1202-97, “Standard Test Method for Electrical Indication of Concrete’s Ability to Resist Chloride Ion Penetration,” ASTM, Pennsylvania, 2001.
  15. ASTM Standard Designation C109-99, “Standard Test Method for Compressive Strength of Hydraulic Cement Mortars,” ASTM, Pennsylvania, 2001.
  16. ASTM Standard Designation C143-00, “Standard Test Method for Slump of Hydraulic Cement Concrete,” ASTM, Pennsylvania, 2001.
Subscribe to: Posts (Atom)
Copyright © 2014 Science and Nature
⬆
Top