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Early Planning for the Concrete Work at the Burj Khalifa, Dubai, UAE

Dr. Ahmad Abdelrazaq

Abstract

Soaring over Dubai skyline at 828 meter, the Burj Khalifa Tower stands tallest structure ever built by man. The tower consists of 156 floors of concrete construction that is topped out with steel floor framing systems up to level 162 and with braced construction to the top of the pinnacle. While the early integration of aerodynamic shaping and wind engineering considerations played a major role in the architectural massing and design of this multi-use/residential tower, where mitigating and taming the dynamic wind effects was one of the most important design criteria, the material selection for the structural systems of the tower was also a major consideration and required detailed evaluation of the material technologies and skilled labour available in the market at the time. Several structural and foundation systems where considered, however, Concrete was selected as the primary structural material for the tower because of its strength, stiffness, damping, redundancy, mouldability, free fireproofing, speed of construction, and cost effectiveness. Therefore, this paper will focus on the early planning for the concrete work at Burj Khalifa.

Introduction

A century ago reinforced concrete was used as the primary structural material for a 20 story building. However, concrete use for tall buildings had many limitations because of its low strength, large mass, and the valuable rentable space it takes to make the project viable, and equally important the slow construction cycle per day. Nowadays, concrete is a very cost effective material and versatile and its use is only limited by the imagination of its users.

Figure 1: Burj Dubai Construction Progress Photo

Today, concrete has become composite materials with strength equivalent to low strength steel used at the turn of the century, and with concrete technologies that overcome all of its limitations. Present concrete technologies is not only limited to material technologies such as High Performance Concrete (HPC), but to the high speed construction cycles it offers, which include but limited to advanced formwork systems, concrete pumping technologies, and simplicity in construction planning and design. The use of Concrete on Burj Khalifa Project demonstrates another beginning of new era for concrete use in super tall building structures at the beginning of this century. This paper will focus on the early planning for the concrete work at Burj Khalifa. The Burj Khalifa project is a multi-use development tower with a total floor area of 465,000 square meters that includes residential, hotel, commercial, office, entertainment, shopping, leisure, and parking facilities.

The Burj Khalifa project is designed to be the center piece of the large scale Burj Dubai Development that rises into the sky to an unprecedented height at 828 meters and consists of 160 floors. The tower is topped out with 208 meter spire that tapers to 1.2 meter diameter pipe at the tip of the pinnacle. Figure 1

The design of Burj Khalifa Tower is derived from geometries of the desert flower, which is indigenous to the region, and the patterning systems embodied in Islamic architecture. The tower massing is organized about the central core with three wings, with four bays, that peels off by one bay from each wing at every seven floors in a spiral pattern as it ascends into the sky. Unlike many super-high rise buildings, with deep floor plates, the Y-shape floor plans of Burj Dubai maximize the views of the Persian Gulf and provide plenty of natural light for its tenants. The modular Y-shaped building, with setback of the three wings, every 7 floors, was part of the original design concept that allowed Skidmore, Owings and Merrill to won the invited design competition.

Figure 2: Lateral Load Resisting System of the Tower

The tower superstructure is designed as an all reinforced concrete building with high performance concrete from the foundation level to level 156 and is topped with an all structural steel braced frame structure from level 156 to the tip of the pinnacle. The tower massing is also driven by the wind engineering requirements to reduce the dynamic wind excitation of the tower by reducing the building width and shape as the tower spirals up into the sky, thus reducing wind dynamic effects, movement, and acceleration. Integrating the wind engineering principals and requirements into the architectural design of tower resulted in very stable dynamic response of the structure against the strong wind effects, thus taming the powerful wind forces. Refer Figure 2

Structural System Brief Description

Lateral Load Resisting System

The lateral load resisting system of the Tower provides resistance to wind and seismic forces and consists of high performance, reinforced concrete ductile core walls from the foundation to the roof that are linked to the exterior high performance reinforced concrete columns through a series of high performance reinforced concrete shear wall panels at the mechanical levels.

The core walls vary in thickness from 1300mm to 500mm. The core walls are typically linked through a series of 800mm to 1100mm deep reinforced concrete or composite link beams at every level. Due to the limitation on the link beam depth, ductile composite link beams are provided in certain areas of the core wall system. These composite ductile link beams typically consist of steel shear plates, or structural steel built-up I-shaped beams, with shear studs, embedded in the concrete section and provides for the majority of the shear and moment resistance. The link beam width typically matches the adjacent core wall thickness.

At the top of the centre reinforced concrete core wall a very tall structural steel braced spire rises to the sky to make the building the tallest tower in the world in all categories. The lateral load resisting system of the spire consists of diagonalized structural steel bracing system that is directly founded at the top of the central reinforced concrete core wall system at level 156.

Floor Framing System

Figure 3: Raft Foundation System

The typical residential and hotel floor framing system of the Tower consists typically of 200mm to 300mm two-way reinforced concrete flat plate slabs spanning approximately 9 meters between the exterior columns and the interior core wall. The floor framing system at the tips of the tower floor consists of 225mm to 250mm two-way reinforced concrete flat plate system. The floor framing system within the interior core consists also of two way reinforced concrete flat plate system with beams.

Foundation System

The piles are typically 1500mm diameter, high performance reinforced concrete bored piles, extending approximately 45 meters below the base of the raft. All piles utilized self compacting concrete (SCC) with w/c ratio not exceeding 0.30, and placed in one continuous concrete pour using the tremie method. The final pile elevations are founded at -55 DMD to achieve the assumed pile capacities of 3000Tonnes.

A robust cathodic protection system is also provided for both the bored piles and the raft foundation system to protect the foundation and the reinforced concrete raft against the severe and corrosive environment (chloride and sulphate) of the soil at the Burj Dubai site. Figure 3

Construction of the Tower Superstructure

The foundation system (pile & raft) of the tower were completed in February 2005, including pile foundation and the raft foundation, and the tower superstructure construction started in April 2005.

Since the construction program was very tight and the superstructure, including 160 floors and the structural steel spire and pinnacle, was expected to be completed within 36 months, Samsung put forward the following strategic approach to achieve this aggressive program:

• Three (3) day-cycle for the structural work and reducing the risks of any structural works that is considered complex and requires careful planning works.
• Optimum transportation methods with large capacity & high speed equipment.
• Combination of optimum formwork System for various building shapes changes along the building height.
• Well organized logistic plans throughout the construction period.
• Application of all high-rise construction technologies available at the time of construction.

Since the construction planning is very extensive and cannot be covered in details in this paper, this paper will focus only on the planning and implementation of the concrete works only. The reader may refer to another paper prepared by the author that covers the design and construction planning of the tower in more details.

Planning for the Concrete Work

High performance concrete is specified as the main structural material for the tower from the early design concept of the project considering all the factors discussed above. However, Dubai is very corrosive environment and all concrete works requires high standards of care at every level and for every aspect of the concrete works. The concrete specifications for the Burj Khalifa is somewhat performance based design and required high strength, high modulus of elasticity, low shrinkage, and high demands for durability and serviceability requirements. This type of specification allowed the contractor to take full advantage of the high performance concrete technologies available and allowed for a very innovative approach to concrete works. Table 1 below provides a comparison summary of the concrete materials specified and the actual concrete used.

Table 1: Comparison of concrete grades

Strategy for the Concrete Planning

Prior to the construction of the tower, an extensive concrete testing and quality control programs were put in place by the author to ensure that all concrete works is done in agreement with the contract documents and all parties involved, including the supervision consultant (Hyder), the owner independent testing agency (IVTA), the concrete supplier (Unimix) top design and quality control team, CTL, and Samsung C& T Task force team. Samsung C& T encouraged a team approach, where all the stakeholders participated and agreed to major issues related to concrete works requirements; the design team focused on the design requirements and compliance, while the contractor focused is integrating all the design requirements with the means and method of designing the concrete mix and delivering to its final placement locations in full compliance of the contract documents. Some of the programs that Samsung put in place from the early development of the concrete mix design until the completion of all test and verification programs included, but not limited to the following:

• Trial mix designs for all concrete ll types needed for the project.
• Mechanical properties, which included compressive strength, modulus of elasticity, split tensile strength.
• Durability tests which included initial surface absorption test, 30 minute absorption test, Water penetration tests and rapid chloride permeability test.
• Creep and shrinkage test program for all concrete mix design, see Figure 4 for testing setup.
• Shrinkage test program for all concrete mix designs.
• Pump simulation test for all concrete mix design grades up to at least 600 meters. See Figure 5 for test setup
• Heat of hydration analysis and tests, which included cube analysis and tests, and full- scale heat of hydration mock tests for all the massive concrete elements that have a dimension in access of 1.0 meter. These tests are needed to confirm the construction sequence of these large elements and to develop curing plans that are appropriate for the project and through major daily and seasonal temperature fluctuations. See Figure 6 for test setup.

Fig. 4: Creep Test	Fig. 5: Pump simulation test

 

Fig. 6: Heat of Hydration Mockup Test

Concrete Mix Design

Since the project required more than 300,000 cubic meters of concrete, an optimum concrete mix design proportions are required to achieve high concrete strength, high flow rate, high early strength, high modulus of elasticity, low heat of hydration, and high pumpability. Achieving these requirements required optimum selection of all the raw materials proportions and verification of a comprehensive concrete testing program.

PFA(Fly-Ash) & M/S(Micro silica)

Crushed Sand &  Dune Sand

Table2: Key concrete technologies used for HPC

In addition to achieving high durability concrete, Table 2 depicts some of the key technologies used in the mix design proportions to achieve the required properties that are above the original concrete design intent. In addition, a comprehensive quality control program is strictly put in place, both at the concrete plant and at the site, to warranty consistent concrete production, pumping, and placement of all the high performance concrete works.

Concrete Pumping

Present concrete pumping technologies has made the concrete works very competitive with the structural steel construction thus allowing for fast construction cycles. In addition, concrete pumping technologies has become one of the major construction equipment needed for concrete, composite and steel building construction. Placing concrete nowadays is no longer dependent on the crane usage. However, designing the optimum concrete mix design to allow for concrete pumping requires considerations for at least the following factors:

• The selection of the optimum Concrete mix design with excellent flow characteristics to minimize/avoid blockages;
• The equipment should have enough capacity to deliver concrete to the highest level, more than 160 floors;
• The pipe line should be designed and installed for maximum output efficiency and the least friction losses
• Selection of equipment and Pipe line system that integrates well with the site logistic and construction planning throughout the period of construction; and
• Maintaining quality control by monitoring all components of the system and concrete properties.
• Systematic coordination of all concrete works from the plant to the job site, including batching interval, delivery, discharging,, pumping, placement

Preliminary Concrete Pumping Simulation

Preliminary horizontal concrete pump simulation test, Figure 5, was performed at the early concrete planning works to estimate the friction losses on the 600m pipe length, to confirm the concrete pump capacity, and the behaviour of the concrete pumping system, including pumping system details, hydraulic pressure, concrete delivery pressure and number of strokes per minute. The purpose of this test is to also verify the concrete mechanical properties and characteristics before and after concrete pumping. Table 3 below provided a summary of the horizontal concrete pumping system and the concrete testing programs.

Table3: Preliminary Concrete Pumping simulation and concrete testing program

Final Concrete Pumping and Equipment

While the horizontal concrete pump simulation test was successful and indicative that re-pumping was not required, pumping the concrete vertically and under different environmental conditions may present itself with unexpected conditions. Therefore, a secondary pump was planned for emergency situation at level 124.

Table4: Summary of Pumping Monitoring Program and Concrete Testing

Three major pumps are placed at the ground level and can potentially connect to any of the five (5) pump lines installed at the ground level, that can potentially connects in turn to four (4) concrete placing booms. See Figure 7 for Tower pump Equipment and major pipe lines. Pumping line 1 is placed at the centre core, pumping line 2, 3 & 4 were placed at the south, west, and east wings of the core wall respectively. An additional pumping line 5 was used at the centre core area for emergency use. As of the time of writing this paper, the secondary pump was not used and most of the concrete has been pumped directly to the highest elevation that in excess of 585m, which makes the highest concrete pumping ever done. 

The concrete pumping system is monitoring closely to verify the expected behaviour and to make adjustment to the pumping system and concrete types as deemed necessary. The pressure in the pumping system is monitoring every 50m, and the concrete characteristics and mechanical properties are also tested. Table 4 shows a summary of the monitoring and testing programs incorporated to ensure the successful completion of the direct concrete pumping to the highest at elevation 585.7m and beyond.

Concrete Creep and Shrinkage Test

An extensive Creep and shrinkage testing program was performed at CTL laboratory in the Skokie, Illinois, and USA and at Samsung C & T Own Testing Laboratory in Seoul, Korea. The testing programs were performed in accordance with ASTM C512 Creep and Shrinkage Test. The concrete was tested for both sealed and unsealed conditions, and loaded at 7 days and 28 days at 25% and 40% the tested concrete compressive strength. Because the high performance concrete mixes are designed for high strength, high durability, low water cement ratio, low shrinkage, and pumpability, most of the concrete mixes revealed very low shrinkage values and most of the shrinkage tend to occur very early. See Figure 4 for a typical Creep and Shrinkage test setup.

Fig. 7: Tower Pump Equipment & Pipe Lines

Analysis of the creep and shrinkage test results also revealed that the Burj Dubai creep and shrinkage characteristics tend to slightly differ from those predicted by international model codes. Due to the extensive nature of this issue, this topic cannot be covered in detail in this paper.

Conclusion

At the turn of the century, concrete construction was at its infancy and the present concrete construction technologies have made concrete a reality for super tall building design. The Burj Dubai project presents a testimony that tall building system development is always directly related to the latest development in material technologies, structural engineering theories, wind engineering, seismic engineering, advancement in computer technologies, and construction methods. The Burj Khalifa Project capitalizes on these technologies to result in the latest advancement in the development of super tall buildings and the art of structural engineering.

As of today, the Burj Khalifa project has become the tallest man made structure in the world in all categories and it has become a catalyst to the explosive use of high performance concrete in highrise building construction in the Middle East in particular and to the whole world in general. I believe that the Burj Dubai project will also serve as another beginning for new technological challenges that high performance concrete can offer for tall building industry.

About Author:

Dr. Ahmad Abdelrazaq, Lecturer at Seoul National University Korea. He is an Executive Vice President of the chief of High rise Building and Structural Engineering Divisions at Samsung C & T Corporation. Research interests include the development of innovative structural systems in concrete/steel/composite structures, and in shaping super tall buildings to control their dynamic response to wind excitatio. He is now serving as the Chair of the ASCE/SE Tall Buildings committee.

Acknowledgement: This paper was published in the proceedings of the R. N. Raikar Memorial International Conference and Dr. Suru Shah Symposium on 20-21, December 2013 at Hotel Hyatt Regency, Mumbai- Advances In Science & Technology Of Concrete, organised by the India Chapter of American Concrete Institute.